Effect of Hydraulic Retention Time on Nitrification in an AirLift Biological Reactor

EFFECT OF HYDRAULIC RETENTION TIME ON NITRIFICATION IN AN AIRLIFT BIOLOGICAL REACTOR

A.A.L. Furtado¹, R.T. Albuquerque², S.G.F. Leite² and R.P. Peçanha²

¹Centro Nacional de Pesquisa e Tecnologia Agroindustrial de Alimentos/EMBRAPA - Av. das Américas, 29501, Pedra de Guaratiba - 23020-470 - Rio de Janeiro,RJ, Brazil

Phone: [phone] - Fax: [phone] email: [e-mail]

² Escola de Química - Centro de Tecnologia - Ilha do Fundão - Universidade Federal do Rio de Janeiro - 21949-900 Rio de Janeiro, RJ, Brazil - Phone: [phone]

Fax: [phone] email: [e-mail]

(Received: July 1, 1997; Accepted: May 14, 1998)

Abstract - The occurrence of nitrogenous compounds in industrial effluents at concentration levels above legal limits, is a well-known and serious pollution problem for the receiving body. The biological process for the removal of these substances, commonly referred to as ammoniacal nitrogen, is known as nitrification. Bacteria involved are mainly of the genuses Nitrosomonas and Nitrobacter. The aim of the present work was to study the effect of the hydraulic retention time (HRT) on the efficiency of ammonia removal from a petroleum refinery effluent using activated carbon particles as a biofilm support in an airlift bioreactor. The experiments were carried out using HRTs, equal to six, eight and ten hours. The results show that HRT equal to 8 and 10 hours were enough to reduce ammoniacal nitrogen concentration to levels below permited legal limits (5mg/L NH₃-N). The reactor nitrifying performance was maximized at 85% removal of ammoniacal nitrogen, for a HRT equal to 10 hours.

Keywords: Nitrification, airlift, HRT

INTRODUCTION

Industrial effluents usually contain dissolved chemicals known to threaten the aquatic fauna and flora of receiving bodies. Common examples are nitrogen compounds, mainly those presenting ammoniacal nitrogen. Excessive amounts of these chemicals can cause serious environmental problems.

The biological process known as nitrification convertes ammonia to nitrite and nitrate via organisms generally refered to as ammonia oxidizers and nitrite oxidizers, respectively (Tijhuis et al., 1992).

The conventional biological treatment systems, in general, represent a problem for industries as they are space-demanding. For this reason, alternative compact and efficient systems have been in development for some time. Three-phase fluidized bed of the air-lift type is just one of them.

The effluent is forced upwards through a bed of particles, with enough velocity to fluidize it. The particles were previously covered with a suitable biofilm which in continuous contact with the effluent, removes the pollutants. The air required for the process is introduced at the bottom of the reactor. The air is responsible for the circulation of particles inside the reactor.

The main advantages of this type of reactor are: high removal rates, compact structure, small hydraulic retention time and high biomass concentration. Furthermore, the intense mixing and turbulence favors all interphase mass transfer involved in the process (Sutton and Mishra ,1994 and Tavares, 1992).

The support material has great influence on the biofilm growth, oxygen transfer and reactor’s hydrodynamics. The most important physical characteristics of the carrier particle in such wastewater treatment processes are its size, density, shape and surface roughness (Lazarova and Manen, 1994 and Jian-an and Nieuwstad, 1992).

The aim of the present work was to study the effect of the hydraulic retention time on the efficiency of ammonia removal from a petroleum refinery effluent using activated carbon particles as a biofilm support in a three-phase hybrid fluidized bed - airlift reactor.

EXPERIMENTS

Equipment

The airlift reactor main vessel was comprised of two concentric plexiglass vertical tubes as shown with other details in Figure 1. The reactor had a volume of 5 liters and was operated continuously. The ratio between the free cross sections of downcomer and riser was 2.25.

Reactor’s Inflow Characterization

The effluent used in this work was the wastewater output of a secondary treatment plant at Duque de Caxias petroleum refinery (REDUC), PETROBRÁS, RJ, Brazil. The feed hydraulic load was around 0.1Kg of ammoniacal N per cubic meter per day, corresponding to 30mg/L of NH₃ per liter. Other pollutants were also present as usual, giving rise to a COD of approximately 70 mg/L.

The amount of pollutants fed to the reactor was not fully constant for all experiments. This fact was related to the natural fluctuations of the industrial scale waste treatment process at REDUC, from where all effluent samples came. Also, sampling was always done on the same feed stream of a local aeration lagoon.

Additionally, it was decided to keep the amount of ammoniacal-N constant at the reactor´s feed, namely 25mg/L. The necessary corrections were made using NH₄Cl.

Reactor´s Operation

The reactor was operated in continuous regime using 4% (volume) of packing. The support used was activated carbon of vegetable origin with an average particle size of 2mm and density of 1350 Kg/m³. The superficial air velocity was 0.66 cm/s. The reactor was inoculated with 200 mL of a previously acclimatized sludge, originally from a treatment plant at Paraná petroleum refinery (REPAR) - Petrobrás, PR, Brazil.

The reactor was operated in closed circuit for a week in order to immobilize the microorganisms on the support. The presence of the biofilm was confirmed via electronic microscopy as well as via growth on solid medium (Furtado et al., 1996). The experiments were carried out keeping all variables involved constant except the hydraulic retention time. The operational conditions used are described in Table 1.

Table 1:
Experimental operational conditions

Run	HRT (h)	Load N-NH3 (Kg N-NH3/m3.d)	Operating time (d)	Liquid superficial velocity (m/h)
1	6	0.1	15	0.66
2	8	0.07	15	0.50
3	10	0.06	15	0.40

Figure 1:
Biological reactor and ancillaries.

Process Monitoring

The process’ removal efficiency was followed by determining the concentration of ammonia, nitrite and nitrate by means of ion selective electrodes. The chemical oxygen demand was quantified in accordance with a standard method (APHA, 1995). The pH and the level of dissolved oxygen were also monitored during experiments.

RESULTS AND DISCUSSION

Table 2 shows the average results of duplicate determinations for the steady operation of the reactor.

Dissolved oxygen concentration was fairly constant at 5mg/L in all experiments. This level is considered sufficient for the occurrence of nitrification (Garrido et al., 1997). The temperature oscilated between 28 and 30⁰C, which is considered an optimum range for the growth of both ammonia and nitrite oxidizing bacteria (Ford, 1981).

The drop in pH associated with nitrite and nitrate detection is unequivocal evidence of nitrification. The pH values are not within the optimal growth range for nitrifying bacteria, 7.5 to 8.5. In this respect our aim was to get as close as possible to the conditions that would prevail in an industrial scale treatment plant.

The results displayed in Table 2 show that HRT equal to 8 and 10 hours were enough to reduce ammoniacal nitrogen concentration to levels below permited legal limits (5mg/L NH₄⁺-N). In all experiments the outflow concentrations of nitrite and nitrate increased consistently as compared with the inflow values, a well known evidence for nitrification. Furthermore, there had been a slight reduction in the amount of organic matter originally present at feed, revealing the presence of heterotrophic microorganisms on the biofilm. Growth on adequate solid medium has made the existence of mixed nitrifying flora evident.

The minute removal of organic matter is probably associated with a preferential growth of nitrifying biofilm in detriment of heterotrophic ones. Also the process´ conditions were adjusted to favor nitrification.

Van Benthum et al. (1997), studied the relationship between biofilm growth and hydraulic retention time. They concluded that the optimum time range for the development of the nitrifying biofilm was from 3 to 12 hours, much longer than the time required for heterotrophic ones.

Figure 2 shows the ammoniacal nitrogen removal profile for the various hydraulic retention times used. Clearly for an HTR of 10 hours, the reactor achieved steady regime in five days, while HRTs of 8 and 6 hours required, respectively, nine and eleven days. Also, the final concentration of nitrogen for an HRT equal to 10 hours was the smallest.

Table 2:
Results for the reactor´s steady operation

Variables	Hydraulic Retention Time (h)
	6	8	10
NH4+-N reactor’s inflow (mg/L)	24.7	24.7	24.7
NH4+-N reactor’s outflow (mg/L)	5.7	4.0	3.6
NO2--N reactor’s inflow (mg/L)	0.22	0.21	0.21
NO2--N reactor’s outflow (mg/L)	0.44	0.26	0.35
NO3--N reactor’s inflow (mg/L)	0.86	0.84	0.90
NO3--N reactor’s outflow (mg/L)	2.71	2.78	4.27
pH reactor’s inflow	7.1	7.2	7.0
pH reactor’s outflow	6.2	6.0	6.0
COD reactor’s inflow (mg/L)	75	102	85
COD reactor’s outflow (mg/L)	61	86	77
%removal N-NH3	69.4	79.8	81.7

Figure 2:
Time profile for the removal of ammoniacal nitrogen during experiments.

The reactor has shown not only good performance in the nitrification of petroleum refinery effluent but has also show potential for the optimization of operating conditions. In the course of future work, certain modifications will be made on certain parameters: the pH will be raised to 8.5 which is ideal for nitrification, and the amount of support material inside of the reactor will be increased allowing reduced operational HRT values.

CONCLUSION

It can be concluded that, in spite of the presence of organic matter in the effluent, the action of the nitrifying bacteria was significative. For an HRT of 10 hours, the reactor nitrifying performance was maximized with 82% removal of ammoniacal nitrogen. For this HRT value, the reactor achieved steady regime on the fifth day with ammoniacal-N concentration on the outlet stream below maximum legally permited value.

ACKNOWLEDGEMENT

The authors are grateful to FUJB and FAPERJ for the financial support as well as to CENPES/PETROBRÁS, PAM/PEQ/COPPE, IM/UFRJ and IG/UFRJ for the tecnical expertise.

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Publication Dates

• Publication in this collection
  27 Oct 1998
• Date of issue
  Sept 1998

History

• Received
  01 July 1997
• Accepted
  14 May 1998