A critical analysis of mass and energy balances for external sludge hydrolysis process configurations in mesophilic anaerobic digesters intensification

Uma análise crítica dos balanços de massa e energia para configurações de hidrólise externa de lodo para intensificação de digestor mesofílico anaeróbio

ABSTRACT

This article examines the application of external sludge hydrolysis processes (ESHP), such as thermal hydrolysis process (THP), biological hydrolysis process (BHP), and thermal alkaline hydrolysis (TAH), in the treatment of solids at wastewater treatment plants. The aim of this study was to evaluate the benefits and challenges associated with ESHP, focusing on its impact on the mass and energy balances of the solid phase. This evaluation is carried out suing a simplified mathematical model fed with data from the literature. This study uses simplified modeling of the solid phase, considering ESHP, anaerobic digestion, sludge dewatering, and thermal drying. The design parameters are based on conservative estimates from the literature. The findings indicate that enhanced process performance can significantly reduce energy demand, especially by lowering the water content in solid-phase processes, which is a major contributor to thermal energy consumption. The results show that ESHP offers significant advantages, including increased infrastructure capacity and energy optimization. This process allows plants to thermally dry all sludge without relying on external energy sources such as natural gas. Moreover, it facilitates greater energy extraction from dried sludge using cleaner thermal technologies like pyrolysis. The study also emphasizes the importance of adjusting the solids content in sludge or, alternatively, directing the energy flow first to the waste activated sludge (WAS) to perform hydrolysis. This adjustment aims to improve the overall mass and energy balances. In conclusion, ESHP enhances sludge treatment performance, leading to more than a 20% reduction in sludge cake production and a 7–25% decrease in thermally dried sludge.

Keywords
anaerobic digestion; energy balance; external sludge hydrolysis process; process intensification; solid-phase sludge; thermal hydrolysis process; resource recovery; biological hydrolysis process; thermal alkaline hydrolysis; circular economy; water resource recovery facility; process intensification

Resumo

Este artigo examina a aplicação do processo de hidrólise externa de lodo (ESHP), incluindo o processo de hidrólise térmica, o processo de hidrólise biológica e a hidrólise termoalcalina, no tratamento de sólidos em estações de tratamento de águas residuais. O objetivo é avaliar os benefícios e desafios associados ao ESHP, com foco em seu impacto nos balanços de massa e energia da fase sólida, obtidos por meio da aplicação de um modelo matemático simplificado, alimentado com dados da literatura. Este estudo emprega uma modelagem simplificada da fase sólida, considerando o ESHP, a digestão anaeróbia, a desidratação do lodo e a secagem térmica. Os parâmetros de projeto são baseados em estimativas conservadoras da literatura. As análises indicam que a otimização do processo pode reduzir significativamente a demanda energética, especialmente por meio da diminuição do teor de água nos processos da fase sólida, que são grandes consumidores de energia térmica. Os resultados mostram que o ESHP oferece vantagens significativas, incluindo o aumento da capacidade da infraestrutura e a otimização do consumo energético. Esse processo permite que as plantas sequem termicamente todo o lodo sem depender de fontes externas de energia, como o gás natural. Além disso, viabiliza uma maior extração de energia do lodo seco por meio de tecnologias térmicas mais limpas, como a pirólise. O estudo também destaca a importância do ajuste do teor de sólidos no lodo para aprimorar os balanços de massa e energia. Em conclusão, o ESHP melhora o desempenho do tratamento de lodo, resultando em uma redução de mais de 20% na produção de torta de lodo e em uma diminuição de 7 a 25% no volume de lodo seco termicamente. Essas descobertas fornecem insights valiosos para concessionárias e demais partes interessadas envolvidas no projeto e na operação de estações de tratamento de águas residuais, com foco na otimização dos balanços de massa e energia, além da eficiência de custos.

Palavras-chave:
digestão anaerobia; balanço energético; processo de hidrólise externa de lodo; intensificação de processo; fase sólida do lodo; processo de hidrólise térmica; recuperação de recursos; processo de hidrólise biológica; hidrólise termo alcalina; economia circular; estação recuperadora de recursos; intensificação de processo

INTRODUCTION

The traditional role of wastewater treatment plants (WWTPs) is being redefined into that of water resource recovery facilities (WRRFs) through innovations in process intensification. WWTPs are no longer seen solely as treatment centers for wastewater and sludge but also as key players in resource and energy recovery (Neczaj; Grosser, 2018). Verstraete and Vlaeminck (2010) introduced the ZeroWasteWater concept, which emphasizes recovering significant resources while minimizing costs, fossil fuel usage, and CO₂ emissions. This model involves the initial separation of particulate matter, which is sent to anaerobic digestion, while the liquid fraction undergoes aerobic polishing and heat recovery, and is upgraded to potable water, underscoring the critical role of anaerobic digestion in achieving self-sustainable wastewater treatment.

One of the most promising advancements in WRRFs is the integration of an external sludge hydrolysis process (ESHP) with a mesophilic anaerobic digester (MAD). The primary aim of ESHP is to enhance energy extraction through better energy and solids management. Though primarily used in the UK, ESHP technologies are gaining traction in the US, Chile, Germany, China, and beyond. Among these, the thermal hydrolysis process (THP) is widely adopted, with over 60 full-scale applications worldwide using mixed and secondary sludge (Mágrová; Jenícek, 2021). Despite its success, O’Callaghan, Adapa and Buisman (2021) rated THP as Level 3, the lowest innovation level, even after 23 years on the market. Level 3 signifies that a technology is used in fewer than 100 plants across fewer than 10 countries, with annual sales under $100M. Nevertheless, THP has effectively addressed key challenges in the water sector.

Achieving energy self-sufficiency in WWTPs is possible, though it faces technological and financial barriers, especially in developing countries (Gu et al., 2017). To reach this goal, it’s vital to tackle operational challenges, particularly in solids treatment, to improve energy efficiency. Although anaerobic digestion is a leading technology for self-sustaining wastewater treatment, discussions on mass and energy balances in solids treatment remain scarce. For example, THP literature rarely addresses the energy requirements of the process (Barber, 2016). Among the few existing studies, Mills et al. (2014) stand out for exploring the energy and mass balances in various THP configurations.

ESHP technology not only improves energy efficiency but also reduces sludge production. This study uses simplified modeling of the solid phase, considering ESHP, anaerobic digestion, sludge dewatering, and thermal drying. The design parameters are based on conservative estimates from the literature. The findings indicate that enhanced process performance can significantly reduce energy demand, especially by lowering the water content in solid-phase processes, a major thermal energy consumer.

This research aims to shed light on how different approaches can optimize overall performance. For example, increasing the solids content of sludge entering the anaerobic digester reduces the energy required to heat the sludge to the mesophilic range of 35°C. However, an upper limit exists, which remains underexplored in the literature. Diluted sludge leads to significant energy losses, as excess free water absorbs heat without contributing to the digestion process and increases reactor size. Additionally, prioritizing ESHP pretreatment for waste activated sludge (WAS) over a WAS and primary sludge (PS) mix enhances energy efficiency. WAS is more challenging to stabilize, and directing thermal energy there first ensures optimal conditions for digestion. Mixing ambient temperature PS with the properly heated hydrolyzed WAS aids in its cooling, and the temperature can be further adjusted with minimal energy demand. Besides reducing energy demand, ESHP leads to higher biogas production and lower generation of dry sludge, with a higher proportion of fixed solids (FS) favoring the sludge dewatering stage. Sludge cakes with higher solids content require less energy for thermal drying, and with the use of ESHP, there is no need for external fuel such as natural gas to assist in this process. Ultimately, in the pursuit of a self-sustainable WWTP, the exploration of ESHP technology in various configurations can utilize existing infrastructure to optimize overall WWTP performance.

OBJECTIVE

The objective is to evaluate the benefits and challenges associated with ESHP, focusing on its impact on the mass and energy balances of the solid phase in WWTPs, obtained through the application of a simplified mathematical model fed with data from the literature. This study included technologies evaluated beyond THP, incorporating configurations such as the biological hydrolysis process (BHP) and thermal alkaline hydrolysis (TAH). While THP is more widely recognized, it is essential to evaluate other ESHP technologies, including BHP and TAH, both of which have demonstrated successful full-scale applications. These technologies offer reduced complexity by operating at lower temperatures, without the need for pressurized reactors, and by utilizing simpler heat reuse mechanisms. Evaluating these technologies is critical for gaining a comprehensive understanding of the available options to enhance energy extraction in wastewater treatment processes.

MATERIALS AND METHODS

Initially, this study conducted a literature review of ESHP technologies to identify the key process performance characteristics, which were then used in the simplified mass and energy balance mathematical model.

The investigation is based on the results generated by using a simplified model of the solid phase for mass and energy balances, considering ESHP, anaerobic digestion, sludge dewatering, and thermal drying. The design parameters are based on conservative estimates from the literature review.

External sludge hydrolysis processes process descriptions

The ESHP technologies evaluated are those most widely applied in full-scale operations, including THP, BHP, and TAH. Each technology was described to provide a clear understanding of their mechanisms and benefits.

Thermal hydrolysis process

THP was initially developed to improve sludge dewatering rather than enhance anaerobic digestion. Historically, systems like the Zimpro heat treatment were used for stabilization and conditioning, but their drawbacks—such as odor issues and high concentrations of organics and ammonia in side streams—limited their widespread adoption (Barber, 2020). Cambi is the leading company in this field of technology.

According to Sahu et al. (2022), the classic pretreatment before MAD process is as follows (Figure 1):

• Raw sludge is pre-dewatered to 16–18% dry solids (DS) and is continuously fed into the pulper to be mixed and heated by recycled steam;

• Process gases released in the THP are contained and compressed for safe treatment to remove odor;

• Thermal hydrolysis takes place in reactors at 165°C for 20–30 min, killing all pathogens and denaturing extracellular polymeric substances (EPS), giving a significant viscosity reduction;

• The sterilized sludge is then passed rapidly to the flash tank, resulting in cell destruction from the pressure drop and further liquifying the sludge. The released steam is recycled to the pulper to preheat incoming raw sludge;

• The sludge is cooled to the required digestion temperature partly by adding dilution water and partly in heat exchangers;

• The sludge from THP is followed by MAD for biogas production, and biogas is used for combined heat and power or upgraded to biomethane for vehicles, and the like. The heat from engines or turbines is used to produce steam consumed in the THP.

Figure 1
Sludge line showing the different unit operations with Cambi thermal hydrolysis process + mesophilic anaerobic digester.

In collaboration with Cambi, utilities, and universities, significant research and development efforts have been made, resulting in the proposal of different configurations for the use of the THP. These configurations (Figure 2) include:

• Conventional THP (before MAD): Pretreatment of combined sludge (PS + WAS);

• WAS-only THP (before MAD): Pretreatment of WAS;

• Intermediate THP, I-THP (Inter MADs): Treatment of combined sludge after the first MAD, followed by feeding it into a second MAD.

Figure 2
Thermal hydrolysis process configurations.

After MAD THP, Solids Stream: Treatment of sludge after MAD, with the liquor from sludge dewatering returned to the MAD.

The application of THP in “Before MAD” configurations can significantly enhance the destruction of total volatile solids (VS), achieving up to 65% VS destruction (Barber, 2020). It is generally reported that WAS is less biodegradable than PS (Parkin; Owen, 1986). Shana et al. (2013) notes that the destruction of VS in PS is around 60%. Oosterhuis et al. (2014) observed that VS destruction in WAS increased from 26% in conventional MAD to 42% with the addition of thermal hydrolysis, marking a 62% improvement.

The benefits of THP on sludge dewaterability are well-documented in the literature. It is widely agreed that THP significantly enhances sludge dewatering characteristics, typically improving dewaterability by up to 10 percentage points in mesophilically digested sludge, depending on the influent sludge composition and the dewatering device used (Barber, 2020). Regardless of the type of dewatering equipment, sludge treated with THP consistently shows superior dewatering performance compared to untreated sludge.

Neyens, Baeyens and Dewil (2004) reported that the improvement in sludge dewatering with advanced sludge treatment (AST) technologies, such as THP and TAH, occurs through two primary mechanisms: i. degradation of EPS proteins and polysaccharides, reducing the EPS water retention properties; and ii. promotion of flocculation, which reduces the amount of fine flocs.

Svennevik et al. (2019) examined the performance of full-scale plants with and without THP by sampling 22 full-scale facilities, some of which also employed co-digestion. Their study found an improvement of approximately 7 percentage points when THP was added before digestion. These empirical correlations may serve as a useful tool for predicting the impact of pre- or post-treatment.

THP enables significant process intensification, with Cambi recommending a retention time of 18 days for PS and WAS in MAD. This process increases the loading rate by a factor of 2.3 compared to standard anaerobic digestion, with typical design loading rates ranging from 4.4 to 5.75 kg VS/m³/day (Oosterhuis et al., 2014; Barber, 2016).

Shana (2015) reported that sludge treated with the I-THP configuration exhibited slightly higher average DS content compared to the average cake DS content obtained from the conventional THP configuration.

Barber, Nilsen and Christy (2017) evaluated the performance of THP after MAD and reported the following results:

• VS destruction: 75%;

• Increase in biogas production: 44% compared to conventional MAD;

• Dewaterability: 38–42% (baseline at 22% with conventional MAD).

Table 1 outlines the advantages and disadvantages of THP.

Table 1
Advantages and disadvantages of thermal hydrolysis process.

Advantages

Disadvantages

Widely implemented in full-scale installations, providing extensive performance data. Produces sludge with a high solids concentration after dewatering. Can be integrated into various configurations in relation to MAD, such as before MAD, after MAD, and between MADs (Inter MAD). Even operating at high temperatures, it offers configurations with lower thermal demand compared to conventional MAD systems.

Operates at high temperatures and pressures, requiring robust equipment and safety measures. Requires sludge with a high solids concentration achieved through pre-dewatering, which reduces thermal energy demand but introduces additional process stages and associated operational costs, including labor and polymer chemical use. More commonly applied in large WWTPs or centralized sludge treatment facilities, making it less suitable for medium and small WWTPs due to its operational complexity.

• MAD: mesophilic anaerobic digester; WWTPs: wastewater treatment plants.
• Source: the author (2025).

Biological hydrolysis process

The original name for the technology demonstration carried out by the companies United Utilities and Monsal in the Blackburn Sludge Treatment Scheme, UK, was “enhanced enzymic hydrolysis.” The primary objective of this project was to produce highly treated sludge suitable for agricultural recycling. For the purpose of this text, the term “biological hydrolysis process” (BHP) will be used. The demonstration project received support from the European Commission LIFE Programme—EU LIFE Project/LIFE05 ENV/UK/00124. The Blackburn Sludge Treatment facility processes approximately 13,500 tons per year, serving a population equivalent of half a million people from the Blackburn and South Lancashire area (Le et al., 2006).

The original concept developed by Monsal did not include pathogen destruction as a primary objective. However, to comply with UK regulations, particularly the requirement for a 6-log reduction in pathogens, a batch process step was introduced, operating at a temperature of 55°C with a detention time of 5 hours. The hydrolysis step, on the other hand, operates continuously at 42°C with a detention time of 3 days. Operating at 55°C allows the plant to achieve the minimum 6-log reduction in Escherichia coli using low-grade heat, in accordance with UK regulations (Le et al., 2006). The BHP flowcharts are presented in Figure 3, showing two different temperature configurations.

Figure 3
Biological hydrolysis process processes flowcharts.

The BHP has demonstrated an enhanced VS destruction rate of approximately 10%, resulting in a total VS removal of 50% compared to the previous 41%. This improvement corresponds to a 24% increase in biogas production at the Blackburn facility (Werker et al., 2007).

By incorporating BHP, wastewater treatment plants can effectively increase the organic loading rate (OLR) in anaerobic digesters by reducing the hydraulic retention time (HRT) and operating at higher feed sludge DS concentrations. Some sites have achieved digester OLRs as high as 4 kg VS/m³-day. The observed biogas yield at these sites regularly exceeded 350 m³ per tonne of DS (tds), with multiple plants reaching biogas yields above 400 m³/tds. Several wastewater treatment plants with BHP systems operate their anaerobic digesters with an HRT of 12–14 days. At Great Billing (operated by Anglian Water), the HRT for the plant remains consistently around 16 days (Theodoulou; Bonkoski; Harrison, 2017).

The dewatered digested sludge cake exhibited a higher-than-expected DS content, averaging approximately 28% over the last 6 months of operation, with values ranging from 25 to 30% (Riches et al., 2010). When comparing the dewaterability of biosolids from two types of full-scale plants, Theodoulou et al. (2016) observed that THP plants demonstrated slightly superior performance compared to BHP plants.

While the company United Utilities was an early adopter of BHP, the company Anglian Water continued its utilization and development. Based on various experiences with BHP (Figure 3), the expert team at Anglian Water introduced a variation of the original Monsal development called HpH. The main difference between Monsal 55 and HpH lies in the order of the processes. In Monsal 55, the pasteurization step at 55°C occurs after the hydrolysis process, while in HpH, pasteurization is the first step. This change in the process sequence may potentially influence the selection of microorganisms. To meet the sludge pasteurization criteria, Monsal 55 must be carried out in batch reactors and not continuously, as can be seen in Figure 3B, where there must be at least two batch reactors (RV5 and RV6) to allow a continuous flow of sludge. The requirement for having a batch process is to ensure that all sludge particles will be subjected to the time and temperature conditions of the reactor, thus ensuring sanitation. The sludge heating at 55°C in both processes, Monsal and HpH, is achieved using high-temperature exhaust gas from combined heat and power (CHP) systems. The use of exhaust gas from a cogeneration engine normally involves significant heat losses and the use of heat exchanger devices. However, at both Monsal and HpH, thermal energy is recovered through an unconventional method of direct injection of this exhaust gas into the sludge. It is assumed that this method is possible to be carried out in practice, since BHP reactors are large enough to absorb the high flow rates and temperatures of the exhaust gas from a CHP, thus allowing greater energy recovery. One observation in Figure 3 is that the blue line coming out of the BHP and digester reactors would be the biogas line. However, it should be noted that in the BHP reactors the biogas production is minimal, since theoretically in the hydrolysis phase the formation of methane would not occur.

In April 2022, Royal Haskoning DHV reached an agreement with Anglian Water to transfer the HpH patent, and the technology is now being commercialized under the name “Helea.”

In the United States and other countries that follow EPA 503 regulations, the implementation of BHP must be adapted to meet the criteria for biosolids treated in a process equivalent to a process to further reduce pathogens (PFRP). This involves complying with specific time–temperature requirements based on hold time, temperature, and sludge solids concentration (below or equal to/above 7%).

As a result, the application of BHP in the US must include a pasteurization step carried out in batches with a duration of 5 hours at a temperature of 63°C. This step ensures compliance with the pathogen reduction standards mandated by regulatory guidelines (Theodoulou; Bonkoski; Harrison, 2017). Table 2 outlines the advantages and disadvantages of BHP.

Table 2
Advantages and disadvantages of biological hydrolysis process.

Advantages	Disadvantages
Requires a lower temperature increase for sludge compared to other ESHP technologies. Does not require a high-pressure reactor. Allows for simple energy recovery by using exhaust gas from CHP systems to heat the sludge, thanks to the large volume of reactors.	There are currently no full-scale applications of BHP for WAS. Requires larger reactor volumes compared to other ESHP technologies, leading to a greater area requirement for implementation. Necessitates a more reliable and complex pump system to transfer sludge between reactors.

• ESHP: external sludge hydrolysis process; CHP: combined heat and power; BHP: biological hydrolysis process; WAS: waste activated sludge.
• Source: the author (2025).

Thermal alkaline hydrolysis

TAH, commercially known as Pondus, was developed by engineer Andreas Dünnebeil in Germany. The initial studies on Pondus began at the University of Berlin as part of Niendorf’s 2003 thesis titled “Chemisch-thermische Desintegration von Schwimmschlamm auf der Kläranlage Waßmannsdorf” (Chemical-thermal disintegration of float sludge at the Waßmannsdorf wastewater treatment plant). TAH is specifically designed to work with WAS rather than PS, as hydrolysis pre-treatments are found to be more effective on WAS. Figures 4 and 5 illustrate the TAH process.

Figure 4
Thermal alkaline process Pondus scheme.

Figure 5
Configuration of thermal alkaline process in the solid phase.

The design parameters for TAH (Czarnecki, 2016) are as follows:

• HRT: The HRT for TAH is 2 hours, which refers to the duration that the sludge remains in the TAH system for treatment;

• Alkaline Dosage: The alkaline dosage for TAH typically ranges from 1.5 to 2.0 L of sodium hydroxide (NaOH) per cubic meter of WAS to adjust pH and enhance organic matter breakdown;

• Temperature: TAH operates at a temperature range of 65–70°C, promoting hydrolysis reactions and process efficiency.

The Kenosha Water Utility underwent a solids phase upgrade, which included the installation of TAH before MAD and the addition of a thermal dryer. Following the retrofit, the full-scale plant experienced a 15–25% increase in biogas production and a 2–4% increase in dewatered sludge solids concentration (Hughes; Dünnebeil, 2017). The solids concentration in the sludge cake increased from approximately 26% (ranging between 22 and 28%) to a maximum of 31%, with an operational range consistently maintained at 27% (Tan; Li, 2017).

Toutian et al. (2021) conducted a critical review of TAH, with the following key conclusions:

• With higher initial biodegradability of untreated WAS, the effect of TAH on biomethane yield (BY) decreases. Depending on initial biodegradability, a BY increase of 22–97% is expected;

• Treatment temperatures below 100°C were shown to be as effective as temperatures above 100°C in terms of increasing BY;

• Alkali dosage and the resulting initial pH have a significant effect on BY increase, with an optimal range of 40–60 mg NaOH per gram of total solids (TS) in the sludge;

• Further investigations are needed to clarify the effect of TAH on digester volume and sludge dewaterability.

Modeling

section provides a description of the methodology employed in the simplified model of mass and energy balance, including the assumptions made, equations used, and scenarios analyzed. The goal is to evaluate the mass and energy balances of WWTPs incorporating ESHP technology using modeling techniques.

Scenarios descriptions

The study evaluated different scenarios to assess the impact of various ESHP configurations on mass and energy balances. The process configurations were categorized based on their positioning relative to the MAD. The scenarios are summarized in Table 3. The flowcharts of the scenario’s configurations are shown in Figures 6, 7, 8, 9, and 10.

Table 3
Scenarios

Scenarios	ESHP position in relation to the MAD	Sludge type	Stabilization process	Processes after stabilization
1	-	PS + WAS	Conventional MAD	Dewatering + thermal drying
2	Before MAD	PS + WAS	THP + MAD	Dewatering + thermal drying
3			BHP + MAD	Dewatering + thermal drying
4		PS + hydrolyzed WAS	THP (WAS only) + MAD	Dewatering + thermal drying
5			TAH (WAS only) +MAD	Dewatering + thermal drying
6	After MAD	Anaerobic digested sludge	MAD + THP	Dewatering + thermal drying
7	Inter MAD	Anaerobic digested sludge	MAD + THP + MAD	Dewatering + thermal drying

• BHP: biological hydrolysis process; ESHP: external sludge hydrolysis process; MAD: mesophilic anaerobic digester; PS: primary sludge; TAH: thermal alkaline process; THP: thermal hydrolysis process; WAS: waste activated sludge.
• Source: the author (2025).

Figure 6
Conventional mesophilic anaerobic digester (baseline) flowchart.

Figure 7
Before mesophilic anaerobic digester external sludge hydrolysis process (mixed primary sludge + waste activated sludge) flowchart.

Figure 8
Before mesophilic anaerobic digester external sludge hydrolysis process (waste activated sludge-only) flowchart. (A) Before mesophilic anaerobic digester thermal hydrolysis process (waste activated sludge-only) flowchart. (B) Before mesophilic anaerobic digester thermal alkaline hydrolysis (waste activated sludge-only) flowchart.

Figure 9
Inter mesophilic anaerobic digester external sludge hydrolysis process flowchart.

Figure 10
After mesophilic anaerobic digester external sludge hydrolysis process flowchart.

General premises for the model

This study builds upon the work conducted by Mills (2015), which focused on THP. As a result, the same reference values for sludge input data and design parameters for THP have been adopted. Table 4 presents the general premises for sludge input used in the mass and energy balance calculations.

Table 4
General premises of sludge input.

Parameter	Value
Total solids load	1,000 g/h
Sludge type composition	PS, 40%; WAS, 60%
VS/TS ratio for both PS and WAS	77%
Specific gravity of mixed thickened sludge	1.1 ton/m3
PS thickened sludge dry solids/DS	6.0%
WAS thickened sludge DS	5.5%

• DS: dry solids; MAD: mesophilic anaerobic digester; PS: primary sludge; THP: thermal hydrolysis process; VS/TS ratio: volatile solids/total solids; WAS: waste activated sludge.
• Source: the author (2025).

Since the actual solids temperature of the sludge is rarely recorded (WEF, 2018), influent or effluent temperatures can be used as representative of the raw solids temperature (Metcalf & Eddy et al., 2014). For this reason, the initial thickened sludge temperature is considered to be 15°C.

Table 5 shows the solid and water loads for the thickened sludge based on the sludge source (PS or WAS).

Table 5
Sludge input data for a total dry sludge input of 1,000 g/h.

Sludge type	Load contributions						Thickened sludge
	%	Dry load (g/h)	FS (%)	FS (g/h)	VS (%)	VS (g/h)	%	Wet load (g/h)	Water load (g/h)	Specific gravity (kg/L)
PS	60	600	23	138	77	462	6.0	10,000	9,400	1.10
WAS	40	400	23	92	77	308	5.5	7,273	6,873	1.10
PS + WAS	100	1000	23	230	77	770	5.8	17,273	16,273	1.10

• PS: primary sludge; WAS: waste activated sludge; FS: fixed solids; VS: volatile solids.
• Source: the author (2025).

Adopting performance parameters for ESHP processes to feed a mathematical model presents a challenge, as it can lead to misleading conclusions about the advantages and disadvantages of these technologies. The literature does not offer direct comparisons between these technologies in terms of biogas production and VS destruction. However, it can be stated that, among the existing ESHP technologies, THP has the most comprehensive data from full-scale installations. Therefore, the premise adopted in using performance parameters is that the data available for THP from Mills (2015) will be taken as a reference, due to the extensive practical experience accumulated over the years.

Performance parameters for BHP and TAH technologies will be considered similar to, but slightly lower than, those of THP, as they have fewer practical applications, and the data available in the literature often appear more optimistic compared to those for THP. The parameters for the ESHP processes used in the model are presented in Table 6.

Table 6
Parameters for the external sludge hydrolysis processes used in the model.

Scenarios	ESHP position	Configuration	MAD				Sludge dewatering DS (%)
			SRT	VSR (%)
			(days)	PS	WAS	MIXED
1	--	Conventional MAD	20	60	26	46	23
2	Before MAD	THP + MAD	18	65	50	59	30
3		BHP + MAD	18	62.5	50	57.5	27
4		THP (WAS only) + MAD	18	60	50	56	28
5		TAH (WAS only) + MAD	18	60	42	53	26
6	Inter MADs	MAD1 + THP + MAD2	16 + 16	-	-	62	30
7	After MAD	MAD + THP + liquor recycle	18	-	-	67.5	40

• BHP: biological hydrolysis process; DS: dry solids; MAD: mesophilic anaerobic digester; MAD1: first mesophilic anaerobic digester; MAD2: second mesophilic anaerobic digester; PS: primary sludge; SRT: sludge retention time; THP: thermal hydrolysis process; VSR: volatile solids reduction; WAS: waste activated sludge.
• Source: the author (2025).

According to Barber (2020), the energy demand necessary in thermal processes like MAD and THP may be calculated from elementary heat transfer theory as follows (Equation 1):

(1)$$Q=\left(m_{\mathrm{s}}{C}_{\mathrm{ps}}△T_{\mathrm{s}}+m_{\mathrm{w}}{C}_{\mathrm{pw}}△T_{\mathrm{w}}\right)/\mathrm{\eta }$$

Where:

Q = energy required (kJ);

m = mass (kg);

C_p = specific heat capacity (kJ/kg/°C) = 1.5 kJ/kg/°C for sludge (DS) and 4.2 kJ/kg/°C for water;

ΔT = temperature difference required (°C) = (T_out – T_in);

ƞ = efficiency to account for head losses by convection and radiation;

The subscripts s and w refer to sludge and water, respectively.

Losses in the thermal process, equal to 65% of the energy demand, are considered for both MAD and ESHP, which is the same value adopted by Barber (2020).

The solids capture in processes like thickening, pre-dewatering, and dewatering is adopted equal to 100% (ideal value), to simplify calculations. The capture rates of various dewatering processes typically range from 90 to 99% (WEF, 2018). Figure 11 illustrates this premise.

Figure 11
Solids concentration process.

Gas from anaerobic digestions contains about 65–70% CH₄ and typical values of biogas production vary from 0.75 to 1.12 Nm³/kg of VS reduction (Metcalf & Eddy et al., 2014). It is assumed for the model a specific biogas production of 1.0 Nm³/kg of VS reduction with calorific value of 23.000 MJ/Nm³ (6.39 kWh/Nm³) was needed.

For thermal drying, the following assumptions are made: a. specific energy demand for thermal drying of 1.10 kWh/kg of water removed (US EPA, 2006) and b. sludge pellet DS% after thermal drying: 90%.

Specific premises for the model

The specific premises for ESHP processes are the following:

Initial temperature of the sludge in thermal hydrolysis process

It is assumed that the initial temperature of the input sludge (mixed raw sludge/Full Cambi, WAS only, digested sludge/I-THP/solid stream) is 90°C, due to the heat recovery in the pulper, which receives waste steam from flash tank and the reactor. The amount of heat recovered by the thermal hydrolysis plant is also a very significant factor, and it is common to assume a 75°C (165°C – 90°C) temperature difference (Barber, 2020).

Steam demand in thermal hydrolysis process

THP utilizes steam to heat sludge instead of heat exchangers and for this reason the DS of the processed sludge is slightly different from the initial value due to the dilution of water from the steam.

At operating temperature of 165°C, specific enthalpy is 2,673 kJ/kg from steam tables (Barber, 2020).

Therefore, based on thermal energy demand for THP, it is possible to calculate the steam requirement in kg/h and the new DS % of the sludge with the content of water as vapor.

Temperature of waste activated sludge hydrolyzed through thermal hydrolysis process

The temperature of the WAS hydrolyzed is assumed to be 102°C (Barber, 2020), which is then mixed with primary sludge. The final temperature of the mixed sludge should be adjusted to 37.5°C.

Return of the liquor from final sludge dewatering to mesophilic anaerobic digester in after mesophilic anaerobic digester external sludge hydrolysis processes (solids stream)

Temperature of liquor from final dewatering = 75°C (Barber; Nilsen; Christy , 2017).

Energy demand for intermediate-thermal hydrolysis process

For I-THP, this configuration requires a more sophisticated procedure to calculate the energy demand for thermal recovery systems. Therefore, rather than performing the basic calculations, the numbers proposed by Mills (2015) were adopted, according to Table 7.

Table 7
Parameters for intermediate thermal hydrolysis process for a total dry sludge input of 1,000 g/h.

Parameter	Value
Energy demand for MAD1	0.480 kW
Energy demand for I-THP	0.490 kW
Energy demand for MAD2	0 kW
Biogas production MAD1	2,160 kW
Biogas production MAD2	1,030 kW
Sludge retention time for each digester	16 days

• I-THP: intermediate thermal hydrolysis process; MAD1: first mesophilic anaerobic digester; MAD2: second mesophilic anaerobic digester.
• Source: Mills (2015).

For input data to MAD2 it is assumed that MAD1 works equally as a conventional MAD.

Temperature adjustment before entering mesophilic anaerobic digester

For WAS-only hydrolysis, the hydrolyzed sludge should be mixed with PS, resulting in a new temperature. When all the sludge is processed through the external hydrolysis process, the final temperature exceeds the conventional MAD temperature range.

Therefore, the temperature of the processed sludge should be adjusted to match the MAD temperature. In some cases, it will be necessary to increase the final temperature because the mixing between PS and WAS hydrolyzed sludge hasn’t reached 37.5°C.

In other cases where the temperature of all the sludge exceeds 37.5°C, it is necessary to cool down the temperature. In such cases, any potential gains from thermal energy recovery are not considered.

Thermal energy balance between biological hydrolysis process and mesophilic anaerobic digester

For BHP, the pasteurization step at 55°C for 5 hours was not considered. Only the hydrolysis step at 42°C for 3 days was included. This decision was made because the goal was not to produce a pathogen-free sludge cake but rather to assess whether the biogas production could meet the energy demand for thermal drying. In theory, it would be necessary to cool the sludge temperature from 42 to 37.5°C. This potential gain from thermal energy recovery is not considered.

All calculations of the simplified model are shown in Appendix A.

RESULTS AND DISCUSSION

The model results for each configuration are presented through figures and tables in Appendix B.

The summary of results is presented in Tables 8 and 9.

Table 8
Water path in conventional mesophilic anaerobic digester and external sludge hydrolysis process configurations for 1,000 g/h dry solids (60% primary sludge and 40% waste activated sludge).

Type of configuration process		Water path
		MAD input	ESHP	Dewatering			Thermal drying
		Water load (kg/h)	Process water (kg/h)	FS/TS (%) ratio	Cake DS (%)	Cake production (kg/h)	Water removed (kg/h)	Sludge pellet (kg/h)
Conventional MAD		16.27	-	36%	23%	2.79	2,08	0.71
Before MAD	THP (PS + WAS)	9.00	5.06	42%	30%	1.82	1.21	0.61
	BHP (PS +WAS)	11.50	11.50	41%	27%	2.06	1.44	0.59
	THP (WAS only)	11.82	2.42	40%	28%	2.03	1.40	0.63
	TAH (WAS only)	16.27	6.87	39%	26%	2.28	1.62	0.66
Inter MAD	THP	(16.27) + (5.79)	3.25	44%	30%	1.74	1.22	0.58
After MAD	THP	18.45	2.43	48%	40%	1.20	0.67	0.53

• Source: the author (2025).
• BHP: biological hydrolysis process; DS: dry solids; ESHP: external sludge hydrolysis process; MAD: mesophilic anaerobic digester; PS: primary sludge; TAH: thermal alkaline hydrolysis; THP: thermal hydrolysis process; WAS: waste activated sludge.

Table 9
Energy balance and specific digester volume for 1,000 g/h dry solids (60% primary sludge and 40% waste activated sludge).

Type of configuration process		Energy					MAD parameter
		Production	Demand			Balance	Specific digester volume (m3/kg DS)
		Biogas kW	MAD kW	ESHP kW	Thermal drying kW
Conventional MAD		2.28	0.67	-	2.29	- 0.68	7.54
Before MAD	THP (PS + WAS)	2.90	-	0.73	1.33	0.84	3.93
	BHP (PS +WAS)	2.83	-	0.58	1.59	0.66	4.90
	THP (WAS only)	2.75	0.07	0.29	1.39	1.00	5.03
	TAH (WAS only)	2.75	-	0.69	1.79	0.27	6.84
Inter MAD	THP	3.05	0.67	0.49	1.28	0.61	6.08
After MAD	THP	3.32	0.67	0.35	0.73	1.57	7.64

• BHP: biological hydrolysis process; ESHP: external sludge hydrolysis process; FS/TS: fixed solids/total solids; MAD: mesophilic anaerobic digester; PS: primary sludge; SRT: sludge retention time; TAH: thermal alkaline hydrolysis; THP: thermal hydrolysis process; VSR: volatile solids reduction; WAS: waste activated sludge.
• Source: the author (2025).

Energy demand for sludge stabilization

From Table 9, it can be observed that the balance between the energy produced through biogas and the energy demand for sludge stabilization via anaerobic digestion is greater in the conventional MAD process compared to any MAD + ESHP configuration.

Figure 12 illustrates the energy balance between the energy produced through biogas and the energy demand for sludge stabilization.

Figure 12
Energy balance between the energy produced through biogas and the energy demand for mesophilic anaerobic digester + external sludge hydrolysis process.

At first glance, the high process temperature of 165°C in THP might suggest a significant energy demand. However, to achieve a favorable energy balance in the THP process, the sludge should be processed with a high solids content of 16.5%. Studies indicate that a higher DS content reduces energy requirements for pretreatment and increases the net profits of the facility linearly, due to the additional electricity yield (Kor-Bicakci; Eskicioglu, 2019). This condition reduces the amount of free water, which is a major consumer of thermal energy when increasing the temperature.

The effect of hydrolysis on raw sludge results in the solubilization of organic matter, making the sludge much more fluid. The significant decrease in bulk viscosity presents an opportunity to increase mixing speed, thereby enhancing gas production and biogas yields (WEF, 2019). Given the reduction in sludge viscosity, there is an opportunity to operate with the highest possible solids content in the anaerobic digester to optimize reactor volume utilization, without compromising sludge homogenization, especially with existing mixing systems.

Adjusting the DS (%) of the input sludge to the anaerobic digester is one of the initial steps toward improving mass and energy balances, even without implementing an ESHP. High solids digestion can reduce the required digester volume or effectively increase the capacity of an existing digester. This approach has been used with feed concentrations of 7–8% TS at several facilities, primarily in Europe (WEF, 2017). In this type of technology, special attention must be given to sludge mixing and anti-foam devices.

A key consideration when heating sludge at high concentrations for ESHP is the method used. THP employs direct steam injection, while BHP utilizes exhaust gas from CHP, meaning that neither method relies on indirect techniques through conventional surface-contact heat exchangers, such as shell and tube, plate and frame, tube in tube, and the like.

An alternative approach to enhance the energy balance in before MAD ESHP is to direct thermal energy initially to WAS, creating the WAS-only configuration. In the case of before MAD THP WAS only, the thermal energy demand (0.36 kW) for sludge stabilization (THP WAS only + MAD) is 46.3% lower compared to conventional MAD (0.67 kW). This significant reduction is attributed to the positive combined effect of applying thermal energy exclusively to WAS at a high solids concentration. Before MAD THP WAS only achieves a better energy balance compared to before MAD THP (PS + WAS) (0.73 kW). If the goal is to produce pathogen-free sludge cake, it would be worth considering a combination of before MAD THP WAS only and a separate pasteurization process for the mixed sludge, either before or after MAD. In terms of energy balance, this approach would be more efficient, as hydrolysis has a lesser impact on energy extraction from PS. However, it is important to note that implementing an additional unit process would increase labor for operation and maintenance, as well as require additional investment.

It is also important to clarify that pre-dewatering is a process that demands a significant amount of polymer. When conducting an economic analysis to compare different technologies, it is essential to account for the costs associated with these chemicals, as they impact operational expenses (OPEX). On one hand, TAH is often associated with high chemical costs due to the demand for alkalinity in the process; on the other hand, the chemical costs associated with THP are rarely cited as a disadvantage.

In the case of before MAD TAH WAS only, it has been demonstrated that there is no need to manipulate the solids content of the sludge to achieve a favorable mass and energy balance. In other words, TAH can be implemented without additional facilities, such as pre-dewatering. The thermal energy demand in the TAH configuration with MAD (0.69 kW) is approximately the same as that of conventional MAD (0.67 kW). This is because all the thermal energy is initially directed toward the WAS to reach 70°C. After hydrolysis under alkaline conditions, the hydrolyzed WAS is mixed with PS, resulting in a temperature close to the operational range of MAD, typically between 35 and 40°C (Dünnebeil, 2018). In practice, this has been demonstrated in wastewater treatment plants such as Gifhorn and Wolfsburg in Germany. However, further improvement in mass and energy balance is still possible through the use of innovative sludge thickening technology with low polymer consumption and high solids content (DS = 7%), as observed at the Kenosha Wastewater Treatment Plant in the USA (Czarnecki, 2016).

To date, before MAD TBH (PS + WAS) has primarily been used for treating a mixture of PS and WAS. However, exploring the potential of a WAS-only TBH approach requires further practical investigation (Ding; Chang; Liu, 2017; Liu; Chang, 2023), particularly if the primary emphasis is on energy extraction rather than sludge sanitization.

Another improvement in the energy balance with the adoption of ESHP is due to enhanced MAD performance, resulting in improved biodegradability, greater biogas production, and lower sludge production, characterized by a higher content of FS. The impact of this improved digestion is reflected in the reduced VS content, as the volatile solids parameter is a strong indicator of the dewatering behavior of sewage sludges (Skinner et al., 2015).

Dewaterability improvement

The effects of adopting ESHP on sludge dewatering have been studied by various researchers, with the most extensive research focusing on THP, which is the predominant process.

As a result, the selected DS content values in sludge dewatering for this study are not absolute, given the challenges in selecting appropriate values and the absence of a comprehensive comparative study involving various ESHP configurations for the same type of sludge and their impact on sludge dewaterability. It is not feasible to extract data from full-scale mass balances due to numerous site-specific conditions (Barber, 2016). The general guideline followed is that a higher degree of sludge stabilization, as indicated by the FS/TS ratio, tends to suggest better sludge dewaterability. The values of the FS/TS ratio and cake DS (%) presented in Table 9 are consistent with this guideline. For the lowest FS/TS ratio value of 36% in conventional MAD, the cake DS is 23%, while for the highest value of 48%, the cake DS in the after MAD configuration is 40%.

Figure 13 illustrates the water content in the sludge cake for the different ESHP configurations compared to conventional MAD. The reduction in water content in the cake is more than 20% compared to the conventional MAD baseline.

Figure 13
Water content in sludge cake of external sludge hydrolysis process configurations relative to conventional mesophilic anaerobic digester.

Drivers for external sludge hydrolysis processes implementation

One facilitator for the implementation of ESHP is the economy of scale resulting from the local arrangement of plants. In the UK, a common configuration for sludge processing involves a Central Facility, typically the largest wastewater treatment plant in the region, along with satellite plants located 10–20 km away. Examples include the Davyhulme sludge treatment facility operated by United Utilities with THP/Cambi (Edgington et al., 2014) and the Great Billing Advanced Digestion Plant operated by Anglian Water with BHP/Monsal (Riches et al., 2010). Satellite plants transport sludge cake to the Central Facility via trucks, and in some cases, liquid sludge is pumped through pipelines from the plants to the Central Facility. The concentration of plants under a single owner, such as Thames Water, United Utilities, Anglian Water, and the like, allows for economies of scale in finding technical solutions for sludge disposal.

When adopting a Central Facility, the manipulation of solids from the sludge to be processed may involve both dilution and pre-dewatering, depending on the loads received internally and externally. Typically, BHP applications have been limited to Central Facilities. In these cases, external sludge in cake form is diluted and mixed with the original thickened sludge generated at the WWTP, with the goal of achieving a solids content between 8% and 10%. In such Central Facility scenarios, there is no pre-dewatering step for the sludge to have a high solids concentration before entering BHP. For BHP applications in WWTPs without the contribution of external sludge, care must be taken to ensure that the sludge has a higher solids content than is traditionally achieved by conventional thickening methods.

Another critical factor in the widespread adoption of THP in the UK was the ability to beneficially reuse sanitized cake sludge in soil, leading to reduced volume generation due to improved sludge dewatering. This enabled the decommissioning of thermal drying facilities and incinerators (Barber; Christy, 2018). However, it is important to recognize that legal and local restrictions may sometimes hinder the beneficial use of sanitized sludge on land, particularly in metropolitan areas where there are significant distances between sludge generation points and land disposal sites.

Another interesting point relates to the optimization of infrastructure usage. By increasing the OLR, less MAD volume is required for sludge processing in practice, as indicated by the specific digester volume values in Table 9. In the case of capacity expansion projects, often limited by the availability of space for expansion, the use of ESHP can be a critical factor in the decision-making process when choosing among alternatives.

Energy balance favorable with sludge thermal drying

Thermally dried sludge opens up various possibilities for end use, including the beneficial application of sanitized sludge in the form of pellets instead of cakes, leading to a significant reduction in volume. It can serve as a fuel source in cement kilns or be utilized in other thermal processes.

By adopting ESHP in conjunction with MAD, it becomes possible to process all the generated sludge using thermal drying, without the need for an external energy source such as natural gas, as shown in the balance column of Table 9. In many practical cases, the performance parameters adopted in this study are often surpassed, allowing the use of CHP to generate both electrical energy and heat. Despite efficiency losses in heat recovery, it is feasible to design a configuration with ESHP and thermal drying without relying on external fuel. This technological pathway also makes thermal processes, like pyrolysis, more accessible, as it relies on dried sludge for extracting additional energy (Pearce; Mills; Winter, 2014; McNamara et al., 2023). Pyrolysis has been identified as a promising sludge treatment method, primarily because it is a zero-waste method with greater potential for solving wastewater challenges compared to other methods (such as incineration and gasification), and it is characterized by lower and acceptable gas emissions (Samolada; Zabanioutou, 2014).

An important aspect to highlight about BHP technology is its ability to recover thermal energy from CHP systems by reusing exhaust gas with high flow rates and temperatures in large sludge reactors. However, in the adopted model, this specific consideration was not factored in. Instead, the premise of heating through a heat exchanger was used, representing a more conservative approach.

As shown in the energy balances in Table 9, the thermal drying demand—except for the after MAD configuration—exceeds the energy required for sludge stabilization (MAD + ESHP). Therefore, as illustrated in Figure 12, the reduction in water content in the sludge cake achieved through ESHP use leads to a corresponding decrease in energy demand for thermal drying.

The reduction in pellet production ranges from 7 to 25% compared to the baseline.

CONCLUSIONS

Based on the analysis of the simplified mass and energy balance models with MAD + ESHP configurations, the following conclusions can be drawn:

• The use of MAD with ESHP offers an energy gain compared to conventional MAD when considering the balance of energy produced through biogas, minus the combined thermal energy demands of both MAD and ESHP. This is due to (a) increased biogas production and (b) reduced thermal energy demand resulting from higher sludge solids concentration and/or the initial direction of thermal energy toward the WAS;

• The THP, BHP, and TAH processes of ESHP are patented technologies, or supplier-dominated technologies. Consequently, the sizing parameters of their units, as well as those of upstream and downstream units, such as MAD and thickening/pre-dewatering, are defined by the technology supplier. Implementing these types of technologies requires integration between the utility, the design company, the construction company, and the technology supplier. This type of technology is more commonly integrated into an existing system rather than as a greenfield installation, thus allowing for a well-established baseline for comparison. It is recommended that these technologies be considered in retrofitting or capacity expansion studies, accounting for both CAPEX and OPEX, to enable a more rational and economical use of resources in wastewater treatment through process intensification;

• A promising technological route for sludge treatment is the use of pyrolysis. To implement this technology, thermally dried sludge is required, which can be achieved through ESHP. One of the key benefits of ESHP is the reduction in water content in the sludge cake, leading to improved dewatering and, consequently, less water to be removed during thermal drying, eliminating the need for external fuel such as natural gas;

• The key question in circular economy is no longer what but rather how. The shift in terminology from “wastewater treatment plant” to “water resource recovery facility” aligns with the concept of “process intensification,” where processes are carried out more efficiently in compact, specialized reactors. One of the technological approaches in solid-phase treatment that is gaining increasing importance is ESHP, due to its technical and economic benefits, which are still often unfamiliar to many in the technical field.

Appendix A

A.1 Baseline Calculations for Conventional MAD

a) Energy demand for MAD

Energy to heat sludge = (0.6 kg/h + 0.4 kg/h) × 1.5 kJ/kg/°C × (37.5°C – 15°C) = 37.5 kJ/h

Energy to heat water = (9.4 + 6.873 kg/h water) × 4.2 kJ/kg/°C × (37.5°C – 15°C) = 1,538 kJ/h

Energy to heat thickened sludge before losses = 37.5 + 1.538 = 1,572 kJ/h

Therefore, after losses becomes 1,572/65% = 2,418.5 kJ/h = 2,588.3 kJ/h × 0.000278 = 0.67 kW

b) Specific digester volume

SRT = 20 days

Wet load (kg/h) = 1.0 kg/h + 16.873 kg/h = 17.873 kg/h = 428.952 kg/day = (428.952 kg)/(1,100 kg/m³)/day = 0.38 m³/day

Specific digester volume = 20 days × 0.38 m³/day = 7.54 m³ (for 1,000 g/h)

c) Energy production from biogas

PS FS (IN) = 138 g/h ⇒		⇒ PS FS (OUT) = 138 g/h
PS VS (IN) = 462 g/h ⇒	(60 VSR %)	⇒ PS VS (OUT) = 185 g/h

WAS FS (IN) = 92 g/h ⇒		⇒ WAS FS (OUT) = 92 g/h
WAS VS (IN) = 308 g/h ⇒	(26 VSR %)	⇒ WAS VS (OUT) = 228 g/h

PS FS _(IN) + WAS FS _(IN) = FS _(IN) = FS _(OUT) = 230 g/h

PS VS _(IN) + WAS VS _(IN) = VS _(IN) = 770 g/h

PS VS _(OUT) + WAS VS _(OUT) = VS _(OUT) = 413 g/h

Total Solids _(OUT) = 230 + 413 = 643 g/h

Volatile solids destruction = 770 – 413 = 357 g/h = 0.357 kg/h

Volumetric biogas production = 0.357 Nm³/h

Energy biogas production = 0.357 × 6.39 = 2.28 kW

d) Energy demand for thermal dryer

Digested sludge production

Total water load _{(IN) (OUT)} = 16.273 kg/h

Total solids _(OUT) = 0.643 kg/h

DS (%) = (0.643)/(16.273/0.643) = 3.8%

Digested sludge dewatered production

DS (%) = 23%

Total solids _(OUT) = 0.643 kg/h (solids capture of 100%)

Total wet sludge _(OUT) = 0.643/23% = 2.794 kg/h

Total water load = 2.151 kg/h

Thermal dried sludge production

DS (%) = 90%

Total solids _(OUT) = 0.643 kg/h

Total wet sludge _(OUT) = 0.643/90% = 0.714 kg/h

Total water load = 0.071 kg/h

Total water removed from sludge dewatered to thermal dried sludge

Total water removed = 2.0152 – 0.071 = 2.080 kg/h

Energy demand for thermal drying

Energy demand = 2.080 kg/h × 1.10 kWh/kg = 2.29 kW

A.2 Calculations for Before MAD ESHP (Mixed PS + WAS)

A.2.1 Before MAD THP (Mixed PS + WAS)

a) Pre-dewatering for THP

TS _(IN) = 600 +400 = 1,000 g/h =>16,5%

Wet sludge _(OUT) = 1,000/16,5% = 6061 g/h =>

Water load _(OUT) = 6,061 g/h – 1,000 = 5061 g/h

b) Demand energy for THP

Sludge temperatures

T₀ = 90°C (energy recovery)

T_F = 165°C

Energy to heat sludge = (1.0 kg/h) × 1.5 kJ/kg/°C × (165°C – 90°C) = 113 kJ/h

Energy to heat water = (5.061 kg/h water) × 4.2 kJ/kg/°C × (165°C – 90°C) = 1,594 kJ/h

Energy to heat pre-dewatered sludge before losses = 113 + 1594 = 1,707 kJ/h

Therefore, after losses becomes 1707/65% = 2626 kJ/h = 2626 kJ/h × 0.000278 = 0.73 kW

c) DS and temperature adjustments before MAD (after THP)

Sludge heating through steam injection

Specific enthalpy = 2,673 kJ/kg

Steam demand = 2,626 kJ/h / 2673 kJ/kg = 0.982 kg/h = 982 g/h

Sludge DS (%) after THP (steam dilution)

Water load _(OUT) = 982 + 5,061 = 6,043 g/h

Wet sludge _(OUT) = 1,000 + 6,043 = 7,043 g/h

DS (%) = 1000/7073 = 14.2%

Sludge DS (%) before MAD

DS (%) = 10%

DS (%) = 1,000/wet sludge _(IN)

Wet sludge _(IN) = 1,000/10% = 10,000 g/h

Water load _(IN) = 10,000 – 1,000 = 9,000 g/h

d) Energy demand for MAD

Temperature of thermo hydrolyzed sludge = 102°C

T after DS (adjustment)

Wet sludge × 102°C + dilution water × 15°C

(7043 g/h × 102°C + (9,000 – 7,043) × 15°C) / (7,043 + 1,957) = 83.1°C >>>> 37.5°C

There is no energy demand to MAD. Thermal energy recovery is not considered.

e) Specific digester volume

SRT = 18 days

Wet load (kg/h) = 10.0 kg/h = 240 kg/day = (240 kg)/(1,100 kg/m³) /day = 0.22 m³/day

Specific digester volume = 18 days × 0.22 m³/day = 3.93 m³ (for 1,000 g/h)

f) Energy production from biogas

PS FS (IN) = 138 g/h ⇒		⇒ PS FS (OUT) = 138 g/h
PS VS (IN) = 462 g/h ⇒	(65 VSR %)	⇒ PS VS (OUT) = 162 g/h

WAS FS (IN) = 92 g/h ⇒		⇒ WAS FS (OUT) = 92 g/h
WAS VS (IN) = 308 g/h ⇒	(50 VSR %)	⇒ WAS VS (OUT) = 154 g/h

PS FS _(IN) + WAS FS _(IN) = FS _(IN) = FS _(OUT) = 230 g/h

PS VS _(OUT) + WAS VS _(OUT) = VS _(OUT) = 162 + 154 = 316 g/h

Total solids _(OUT) = 230 + 316 = 546 g/h

Volatile solids destruction = 770 – 316 = 454 g/h = 0.454 kg/h

Volumetric biogas production = 0.454 Nm³/h

Energy biogas production = 0.454 × 6.39 = 2.90 kW

g) Energy demand for thermal dryer

Digested sludge production

Total water load _{(IN) (OUT)} = 9.0 kg/h

Total solids _(OUT) = 0.546 kg/h

DS (%) = (0.546)/(0.546+ 9.0) = 5.7%

Digested sludge dewatered production

DS (%) = 30%

Total solids _(OUT) = 0.546 kg/h (solids capture of 100%)

Total wet sludge _(OUT) = 0.546/30% = 1.82 kg/h

Total water load = 1.82 – 0.546 = 1.274 kg/h

Thermal dried sludge production

DS (%) = 90%

Total solids _(OUT) = 0.546 kg/h

Total wet sludge _(OUT) = 0.546/90% = 0.607 kg/h

Total water load = 0.061 kg/h

Total water removed from sludge dewatered to thermal dried sludge

Total water removed = 1.274 – 0.061 = 1.213 kg/h

Energy demand for thermal drying

Energy demand = 1.213 kg/h × 1.10 kWh/kg = 1.33 kW

A.2.1 Before MAD BHP (Mixed PS + WAS)

a) Pre-dewatering for THP

TS _(IN) = 600 +400 = 1,000 g/h =>8,0%

Wet sludge _(OUT) = 1,000/8,0% = 12,500 g/h =>

Water load _(OUT) = 12,500 g/h – 1,000 = 11,500 g/h

b) Demand energy for BHP

Sludge temperatures

T₀ = 15°C

T_F = 42°C

Energy to heat sludge = (1.0 kg/h) × 1.5 kJ/kg/°C × (42°C – 15°C) = 40.5 kJ/h

Energy to heat water = (11.5 kg/h water) × 4.2 kJ/kg/°C × (42°C – 15°C) = 1,304.1 kJ/h

Energy to heat pre-dewatered sludge before losses = 40.5 + 1,304.1 = 1,345 kJ/h

Therefore, after losses becomes 1,345/65% = 2,069 kJ/h = 2626 kJ/h × 0.000278 = 0.58 kW

c) Energy demand for MAD

Temperature of biological hydrolyzed sludge = 42°C >>> 37.5°C

There is no energy demand to MAD. Thermal energy recovery is not considered.

d) Specific digester volume

SRT = 18 days

Wet load (kg/h) = 12.5 kg/h = 300 kg/day = (300 kg)/(1,100 kg/m³) /day = 0.27 m³/day

Specific digester volume = 18 days × 0.27 m³/day = 4.90 m³ (for 1,000 g/h)

e) Energy production from biogas

PS FS (IN) = 138 g/h ⇒		⇒ PS FS (OUT) = 138 g/h
PS VS (IN) = 462 g/h ⇒	(62.5 VSR %)	⇒ PS VS (OUT) = 173 g/h

WAS FS (IN) = 92 g/h ⇒		⇒ WAS FS (OUT) = 92 g/h
WAS VS (IN) = 308 g/h ⇒	(50 VSR %)	⇒ WAS VS (OUT) = 154 g/h

PS FS _(IN) + WAS FS _(IN) = FS _(IN) = FS _(OUT) = 230 g/h

PS VS _(OUT) + WAS VS _(OUT) = VS _(OUT) = 173 + 154 = 327 g/h

Total solids _(OUT) = 230 + 327 = 557 g/h

Volatile solids destruction = 770 – 327 = 443 g/h = 0.443 kg/h

Volumetric biogas production = 0.443 Nm³/h

Energy biogas production = 0.443 × 6.39 = 2.83 kW

f) Energy demand for thermal dryer

Digested sludge production

Total water load _{(IN) (OUT)} = 11.5 kg/h

Total solids _(OUT) = 0.557 kg/h

DS (%) = (0.557)/(0.557+ 11.5) = 4.6%

Digested sludge dewatered production

DS (%) = 27%

Total solids _(OUT) = 0.557 kg/h (solids capture of 100%)

Total wet sludge _(OUT) = 0.557/27% = 2.062 kg/h

Total water load = 2.062 – 0.557 = 1.505 kg/h

Thermal dried sludge production

DS (%) = 90%

Total solids _(OUT) = 0.557 kg/h

Total wet sludge _(OUT) = 0.557/90% = 0.619 kg/h

Total water load = 0.061 kg/h

Total water removed from sludge dewatered to thermal dried sludge

Total water removed = 1.505 – 0.061 = 1.444 kg/h

Energy demand for thermal drying

Energy demand = 1.444 kg/h × 1.10 kWh/kg = 1.59 kW

A.3 Calculations for Before MAD ESHP (WAS Only)

A.3.1 Before MAD THP (WAS Only)

a) WAS pre-dewatering for THP

WAS TS _(IN) = 400 g/h =>16,5%

WAS wet sludge _(OUT) = 1,000/16,5% = 2,424 g/h =>

WAS water load _(OUT) = 2,424 g/h – 400 = 2,024 g/h

b) Demand energy for THP

Sludge temperatures

T₀ = 90°C (energy recovery)

T_F = 165°C

Energy to heat sludge = (0.4 kg/h) × 1.5 kJ/kg/°C × (165°C – 90°C) = 45 kJ/h

Energy to heat water = (2.024 kg/h water) × 4.2 kJ/kg/°C × (165°C – 90°C) = 638 kJ/h

Energy to heat pre-dewatered sludge before losses = 45 + 638 = 683 kJ/h

Therefore, after losses becomes 683/65% = 1,051 kJ/h = 1,051 kJ/h × 0.000278 = 0.292 kW

c) DS and temperature after mixing PS + THP hydrolyzed WAS

Sludge heating through steam injection

Specific enthalpy = 2,673 kJ/kg

Steam demand = 1,051 kJ/h / 2,673 kJ/kg = 0.393 kg/h = 393 g/h

WAS sludge DS (%) after THP (steam dilution)

WAS water load _(OUT) = 393 + 2,024 = 2,417 g/h

WAS wet sludge _(OUT) = 400 + 2,417 = 2,817 g/h

WAS DS (%) = 400/2,817 = 14,2%

Mixing PS + THP hydrolyzed WAS

PS TS (g/h) = 600 g/h

Thickened PS DS (%) = 6%

T₀ = 15°C

PS wet sludge = 600/6% = 10,000 g/h

PS water load = 10,000 600 = 9,400 g/h

T after Mixing

(PS wet sludge × 15°C + WAS wet sludge × 102°C)/( PS wet sludge + WAS wet sludge) = (10,000 × 15 + 2,817 × 102)/(10,000 +2,817) = 34.1°C < 37.5°C (below MAD operational temperature)

DS (%) after Mixing

= TS/(TS + PS water load + WAS water load) = (1,000)/(1,000 + 9,400+2,417) = 7,8% (OK < 10%)

d) Energy demand for MAD

Sludge temperatures

T₀ = 34.1°C (after PS + THP WAS)

T_F = 37.5°C

Energy to heat sludge = (1.0 kg/h) × 1.5 kJ/kg/°C × (37.5°C – 34.1°C) = 5.1kJ/h

Energy to heat water = (9.400 + 2.417 kg/h water) × 4.2 kJ/kg/°C × (37.5°C – 34.1°C) = 168.7 kJ/h

Energy to heat thickened sludge before losses = 5.1 + 168.7 = 173.8 kJ/h

Therefore, after losses becomes 173.8/65% = 267.4 kJ/h = 267.4 kJ/h × 0.000278 = 0,07 kW

e) Specific digester volume

SRT = 18 days

Wet load (kg/h) = (10.0 + 2.817) = 12.817 kg/h = 307.6 kg/day = 307.6 (kg)/(1,100 kg/m³) /day = 0.28 m³/day

Specific digester volume = 18 days × 0.28 m³/day = 5.03 m³ (for 1,000 g/h)

f) Energy production from biogas

PS FS (IN) = 138 g/h ⇒		⇒ PS FS (OUT) = 138 g/h
PS VS (IN) = 462 g/h ⇒	(60 VSR %)	⇒ PS VS (OUT) = 185 g/h

WAS FS (IN) = 92 g/h ⇒		⇒ WAS FS (OUT) = 92 g/h
WAS VS (IN) = 308 g/h ⇒	(50 VSR %)	⇒ WAS VS (OUT) = 154 g/h

PS FS _(IN) + WAS FS _(IN) = FS _(IN) = FS _(OUT) = 230 g/h

PS VS _(OUT) + WAS VS _(OUT) = VS _(OUT) = 185 + 154 = 339 g/h

Total solids _(OUT) = 230 + 339 = 569 g/h

Volatile solids destruction = 770 – 339 = 431 g/h = 0.431 kg/h

Volumetric biogas production = 0.431 Nm³/h

Energy biogas production = 0.431 × 6.39 = 2.75 kW

g) Energy demand for thermal dryer

Digested sludge production

Total water load _{(IN) (OUT)} = 9.400+2.417 = 11.817 kg/h

Total solids _(OUT) = 0.569 kg/h

DS (%) = (0.569)/(0.569+ 11.817) = 4.6%

Digested sludge dewatered production

DS (%) = 28%

Total solids _(OUT) = 0.569 kg/h (solids capture of 100%)

Total wet sludge _(OUT) = 0.569/30% = 1.897 kg/h

Total water load = 1.897 – 0.569 = 1.328 kg/h

Thermal dried sludge production

DS (%) = 90%

Total solids _(OUT) = 0.569 kg/h

Total wet sludge _(OUT) = 0.569/90% = 0.632 kg/h

Total water load = 0.632 – 0.569 = 0.063 kg/h

Total water removed from sludge dewatered to thermal dried sludge

Total water removed = 1.328 – 0.063 = 1.265 kg/h

Energy demand for thermal drying

Energy demand = 1.265 kg/h × 1.10 kWh/kg = 1.39 kW

A.3.2 Before MAD TAH (WAS Only)

a) WAS thickened for TAH

WAS TS _(IN) = 400 g/h => 5,5%

WAS wet sludge _(OUT) = 400/5,5% = 7,273 g/h =>

WAS water load _(OUT) = 7,273 g/h – 400 = 6,873 g/h

b) Demand energy for TAH

Sludge temperatures

T₀ = 15°C

T_F = 70°C

Energy to heat sludge = (0.4 kg/h) × 1.5 kJ/kg/°C × (70°C – 15°C) = 33 kJ/h

Energy to heat water = (6.873 kg/h water) × 4.2 kJ/kg/°C × (70°C – 15°C) = 1,588 kJ/h

Energy to heat pre-dewatered sludge before losses = 33 + 1,588 = 1,621 kJ/h

Therefore, after losses becomes 1,621/65% = 2,494 kJ/h = 2,494 kJ/h × 0.000278 = 0.693 kW

c) DS and temperature after mixing PS + TAH hydrolyzed WAS

Mixing PS + TAH hydrolyzed WAS

PS TS (g/h) = 600 g/h

Thickened PS DS (%) = 6%

T₀ = 15°C

PS wet sludge = 600/6% = 10,000 g/h

PS water load = 10,000 600 = 9,400 g/h

T after mixing

(PS wet sludge × 15°C + WAS wet sludge × 70°C)/(PS wet sludge + WAS wet sludge) = (10,000 × 15 + 7,273 × 70)/(10,000 + 7,273) = 38.2°C > 37.5°C (above MAD operational temperature)

DS (%) after mixing

= TS/(TS + PS water load + WAS water load) = (1,000)/(1,000 + 9,400 + 6,873) = 5,8% (OK < 10%)

d) Energy demand for MAD

T after mixing PS + TAH hydrolyzed WAS

38.2°C > 37.5°C

There is no energy demand to MAD. Thermal energy recovery is not considered.

e) Specific digester volume

SRT = 18 days

Wet load (kg/h) = (10.0 + 7.273) = 17.273 kg/h = 414.6 kg/day = 414.6 (kg)/(1,100 kg/m³) /day = 0.38 m³/day

Specific digester volume = 18 days × 0.38 m³/day = 6.84 m³ (for 1,000 g/h)

f) Energy production from biogas

PS FS (IN) = 138 g/h ⇒		⇒ PS FS (OUT) = 138 g/h
PS VS (IN) = 462 g/h ⇒	(60 VSR %)	⇒ PS VS (OUT) = 185 g/h

WAS FS (IN) = 92 g/h ⇒		⇒ WAS FS (OUT) = 92 g/h
WAS VS (IN) = 308 g/h ⇒	(42 VSR %)	⇒ WAS VS (OUT) = 179 g/h

PS FS _(IN) + WAS FS _(IN) = FS _(IN) = FS _(OUT) = 230 g/h

PS VS _(OUT) + WAS VS _(OUT) = VS _(OUT) = 185 + 179 = 364 g/h

Total solids _(OUT) = 230 + 364 = 594 g/h

Volatile solids destruction = 770 – 364 = 406 g/h = 0.406 kg/h

Volumetric biogas production = 0.406 Nm³/h

Energy biogas production = 0.406 × 6.39 = 2.59 kW

g) Energy demand for thermal dryer

Digested sludge production

Total water load _{(IN) (OUT)} = 9,400 + 6,873 = 16,273 kg/h

Total solids _(OUT) = 0.594 kg/h

DS (%) = (0.594)/(0.594+ 16.273) = 3.5%

Digested sludge dewatered production

DS (%) = 26%

Total solids _(OUT) = 0.594 kg/h (solids capture of 100%)

Total wet sludge _(OUT) = 0.594/26% = 2.285 kg/h

Total water load = 2.285 – 0.594 = 1.691 kg/h

Thermal dried sludge production

DS (%) = 90%

Total solids _(OUT) = 0.594 kg/h

Total wet sludge _(OUT) = 0.594/90% = 0.66 kg/h

Total water load = 0.66 – 0.594 = 0.066 kg/h

Total water removed from sludge dewatered to thermal dried sludge

Total water removed = 1.691 – 0.066 = 1.625 kg/h

Energy demand for thermal drying

Energy demand = 1.625 kg/h × 1.10 kWh/kg = 1.79 kW

A.4 Calculations for Inter MADs ESHP

a) Energy demand for MAD1

Energy to heat sludge = (0.6 kg/h + 0.4 kg/h) × 1.5 kJ/kg/°C × (37.5°C – 15°C) = 37.5 kJ/h

Energy to heat water = (9.4 + 6.873 kg/h water) × 4.2 kJ/kg/°C × (37.5°C – 15°C) = 1,538 kJ/h

Energy to heat thickened sludge before losses = 37.5 + 1.538 = 1,572 kJ/h

Therefore, after losses becomes 1,572/65% = 2,418.5 kJ/h = 2,588.3 kJ/h × 0.000278 = 0.67 kW

According to Mills (2015) the energy demand for MAD1 is equal to 0.48 kW. It will be considered the calculated number of 0.67 kW.

b) Specific digester volume for MAD1

SRT = 16 days

Wet load (kg/h) = 0.38 m³/day

Specific digester volume = 16 days × 0.38 m³/day = 6.08 m³ (for 1,000 g/h)

c) Energy production from biogas of MAD1

It is considered equal to conventional MAD.

PS FS (IN) = 138 g/h ⇒		⇒ PS FS (OUT) = 138 g/h
PS VS (IN) = 462 g/h ⇒	(60 VSR %)	⇒ PS VS (OUT) = 185 g/h

WAS FS (IN) = 92 g/h ⇒		⇒ WAS FS (OUT) = 92 g/h
WAS VS (IN) = 308 g/h ⇒	(26 VSR %)	⇒ WAS VS (OUT) = 228 g/h

PS FS _(IN) + WAS FS _(IN) = FS _(IN) = FS _(OUT) = 230 g/h

PS VS _(IN) + WAS VS _(IN) = VS _(IN) = 770 g/h

PS VS _(OUT) + WAS VS _(OUT) = VS _(OUT) = 413 g/h

Total solids _(OUT) = 230 + 413 = 643 g/h

Volatile solids destruction = 770 – 413 = 357 g/h

Total solids _(OUT) = 230 + 413 = 643 g/h

Volatile solids destruction = 0.357 kg/h

Volumetric biogas production = 0.357 Nm³/h

Energy biogas production of MAD1= 0.357 × 6.39 = 2.28 kW

d) Energy demand for I-THP

Energy demand = 0.49 kW (Mills, 2015)

e) Pre-dewatering for I-THP after MAD1

TS _(IN) = 643 =>16,5%

Wet sludge _(OUT) = 643/16,5% = 3,897 g/h =>

Water load _(OUT) = 3,897 g/h – 643 = 3,254 g/h

e) DS and temperature adjustments before MAD2 (after I-THP)

Steam injection dilution will be considered equal to zero.

Sludge DS (%) before MAD2

DS (%) = 10%

DS (%) = 1,000/WET SLUDGE _(IN)

Wet sludge _(IN) = 643/10% = 6,430 g/h

Water load _(IN) = 6,430 – 643 = 5,787 g/h

f) Specific digester volume for MAD2

SRT = 16 days

Wet load = 6.43 kg/h = 154 kg/day = 154 kg/day/(1,100 kg/m³) = 0.14 m³/day

Specific digester volume = 16 days × 0.14 m³/day = 2.24 m³ (for 1,000 g/h)

g) Energy demand for MAD2

Energy demand = 0.0 kW (Mills, 2015)

h) Total energy production from biogas of MAD1 + MAD2

Total VSR (%) of MAD1 + MAD2 = 62%

VS _(IN) = 770 g/h =>

VS _(OUT) = 293 g/h

Volatile solids destruction = 477 g/h = 0.477 kg/h

Volumetric biogas production = 0.477 Nm³/h

Energy biogas production = 0.477 × 6.39 = 3.05 kW

Energy production from biogas of MAD2

3.05 2.28 = 0.77 kW

i) Energy demand for thermal dryer

Digested sludge production

VS _(OUT) = 293 g/h

FS _(IN) = FS _(OUT) = 230 g/h

TS _(OUT) = 293 + 230 = 523 g/h

Total water load _(OUT) = 5,787 g/h

DS (%) = (0.523)/(0.523+ 5.787) = 8.3%

Digested sludge dewatered production

DS (%) = 30%

Total solids _(OUT) = 0.523 kg/h (solids capture of 100%)

Total wet sludge _(OUT) = 0.523/30% = 1.743 kg/h

Total water load = 1.743 – 0.523 = 1.220 kg/h

Thermal dried sludge production

DS (%) = 90%

Total solids _(OUT) = 0.523 kg/h

Total wet sludge _(OUT) = 0.523/90% = 0.581 kg/h

Total water load = 0.581 – 0.523 = 0.058 kg/h

Total water removed from sludge dewatered to thermal dried sludge

Total water removed = 1.220 – 0.058 = 1.162 kg/h

Energy demand for thermal drying

Energy demand = 1.162 kg/h × 1.10 kWh/kg = 1.28 kW

A.5 Calculations for After MAD ESHP

a) Energy demand for MAD—first iteration (without liquor dewatering return)

Energy to heat sludge = (0.6 kg/h + 0.4 kg/h) × 1.5 kJ/kg/°C × (37.5°C – 15°C) = 34 kJ/h

Energy to heat water = (9.4 + 6.873 kg/h water) × 4.2 kJ/kg/°C × (37.5°C – 15°C) = 1,538 kJ/h

Energy to heat thickened sludge before losses = 34 + 1.538 = 1,572 kJ/h

Therefore, after losses becomes 1,572/65% = 2,418.5 kJ/h = 2,588.3 kJ/h × 0.000278 = 0.67 kW

b) Specific digester volume for MAD—first iteration (without liquor dewatering return)

SRT = 18 days

Wet load (kg/h) = 0.38 m³/day

Specific digester volume = 18 days × 0.38 m³/day = 6.84 m³ (for 1,000 g/h)

c) Energy production from biogas of MAD

PS FS _(IN) = 138 g/h

PS VS _(IN) = 462 g/h

WAS FS _(IN) = 92 g/h

WAS VS _(IN) = 308 g/h

PS FS _(IN) + WAS FS _(IN) = FS _(IN) = FS _(OUT) = 230 g/h

PS VS _(IN) + WAS VS _(IN) = VS _(IN) = 770 g/h

Total VSR (%) = 67.5%

Volatile solids destruction = 520 g/h

PS VS _(OUT) + WAS VS _(OUT) = VS _(OUT) = 770 – 520 = 250 g/h

Volatile solids destruction = 0.520 kg/h

Volumetric biogas production = 0.520 Nm³/h

Energy biogas production = 0.520 × 6.39 = 3.32 kW

d) Pre-dewatering for THP

TS _(IN) = 230 + 250 = 480 g/h =>16,5%

Wet sludge _(OUT) = 480/16,5% = 2,909 g/h =>

Water load _(OUT) = 2,909 g/h – 480 = 2,429 g/h

Pre-dewatering liquor = (9.4 + 6.873) 2,429 = 13.844 kg/h

Pre-dewatering liquor returns to headwork => This organic return load to the activated sludge system is not considered.

e) Demand energy for THP

Sludge temperatures

T₀ = 90°C (energy recovery)

T_F = 165°C

Energy to heat sludge = (0.48 kg/h) × 1.5 kJ/kg/°C × (165°C – 90°C) = 54 kJ/h

Energy to heat water = (2.429 kg/h water) × 4.2 kJ/kg/°C × (165°C – 90°C) = 765 kJ/h

Energy to heat pre-dewatered sludge before losses = 54 + 765 = 819 kJ/h

Therefore, after losses becomes 819/65% = 1,260 kJ/h = 1,260 kJ/h × 0.000278 = 0.350 kW

f) DS and temperature after THP

Sludge heating through steam injection

Specific enthalpy = 2,673 kJ/kg

Steam demand = 1,260 kJ/h / 2673 kJ/kg = 0.471 kg/h = 471 g/h

Sludge DS (%) after THP (steam dilution)

Water load _(OUT) = 2,429 + 471 = 2,900 g/h

Wet sludge _(OUT) = 2,900 + 480 = 3,380 g/h

DS (%) = 480/3,380 = 14.2%

g) Energy demand for thermal dryer

After MAD THP dewatered production

DS (%) = 40%

Total solids _(OUT) = 0.480 kg/h (solids capture of 100%)

Total wet sludge _(OUT) = 0.480/40% = 1.2 kg/h

Total water load = 1.2 – 0.48 = 0.72 kg/h

Final dewatering liquor return flow to MAD = 2.900 – 0.72 = 2.18 kg/h.

It considered this return flow with 100% water.

Thermal dried sludge production

DS (%) = 90%

Total solids _(OUT) = 0.480 kg/h

Total wet sludge _(OUT) = 0.480/90% = 0.533 kg/h

Total water load = 0.533 – 0.480 = 0.053 kg/h

Total water removed from sludge dewatered to thermal dried sludge

Total water removed = 0.72 – 0.053 = 0.667 kg/h

Energy demand for thermal drying

Energy demand = 0.667 kg/h × 1.10 kWh/kg = 0.73 kW

h) Energy demand for MAD—second iteration (with final dewatering liquor return)

Final dewatering liquor return flow to MAD = 2.18 kg/h.

Temperature of final dewatering liquor = 75°C

Total original wet sludge = 17.273 kg/h

T = 15°C

Mixing temperature and flow of original thickened sludge and final dewatering liquor

Temperature = (2.18 × 75 + 17.273 × 15)/(2.18 + 17.273) = 21.7°C

Energy to heat sludge = (0.6 kg/h + 0.4 kg/h) × 1.5 kJ/kg/°C × (37.5°C – 21.7°C) = 24 kJ/h

Energy to heat water = (9.4 + 6.873 + 2.18) × 4.2 kJ/kg/°C × (37.5°C – 21.7°C) = 1,225 kJ/h

Energy to heat thickened sludge before losses = 24 + 1,225 = 1,249 kJ/h

Therefore, after losses becomes 1,249/65% = 1,922 kJ/h = 1,922 kJ/h × 0.000278 = 0.53 kW

i) Specific digester volume for MAD—second iteration (without dewatering liquor return)

SRT = 18 days

Wet load (kg/h) = (2.18 + 17.273) = 19.453 kg/h = 19.453 × 24 /1,100 kg/m³ = 0.42 m³/day

Specific digester volume = 18 days × 0.42 m³/day = 7.64 m³ (for 1,000 g/h)

Appendix B

Figure B.1
Mass and energy balance for conventional MAD with input of 1,000 g/h of dry solids.

Figure B.2
Mass and energy balance for before MAD ESHP (mixed PS + WAS) with input of 1,000 g/h of dry solids.

Figure B.3
Mass and energy balance for before MAD ESHP (WAS only) with input of 1,000 g/h of dry solids.

Figure B.4
Mass and energy balance for inter MADs ESHP with input of 1,000 g/h of dry solids.

Figure B.5
Mass and energy balance for after MAD ESHP (THP solids stream) with input of 1,000 g/h of dry solids.

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• Funding:
  none.

Publication Dates

• Publication in this collection
  31 Mar 2025
• Date of issue
  2025

History

• Received
  30 Oct 2024
• Accepted
  10 Nov 2024