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

Miki, Marcelo Kenji

doi:10.1590/S1413-415220240018

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

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Figure 2 Thermal hydrolysis process configurations.

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Figure 3 Biological hydrolysis process processes flowcharts.

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Figure 4 Thermal alkaline process Pondus scheme.

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

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Figure 6 Conventional mesophilic anaerobic digester (baseline) flowchart.

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

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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.

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

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

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Figure 11 Solids concentration process.

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

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

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

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

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

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

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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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Table 1 Advantages and disadvantages of thermal hydrolysis process.

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Table 2 Advantages and disadvantages of biological hydrolysis process.

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Table 3 Scenarios

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Table 4 General premises of sludge input.

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

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

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

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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).

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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(1)

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.

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.

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		PS + WAS	BHP + MAD	Dewatering + thermal drying
4		PS + hydrolyzed WAS	THP (WAS only) + MAD	Dewatering + thermal drying
5		PS + hydrolyzed WAS	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

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%

Sludge type	Load contributions						Thickened sludge
Sludge type	%	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

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

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

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

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	Balance	Specific digester volume (m³/kg DS)
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

[T1FN1] MAD: mesophilic anaerobic digester; WWTPs: wastewater treatment plants.

[T1FN2] Source: the author (2025).

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