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Iranian Journal of Environmental Health Science & Engineering,Vol. 3, No. 1, 2006, pp.53-64 DETERMINATION OF DESIGN CRITERIA OF AN H-IFAS REACTOR IN COMPARISON WITH AN EXTENDED AERATION ACTIVATED SLUDGE PROCESS N. Mehrdadi , A. A. Azimi , G. R. Nabi Bidhendi, *B. Hooshyari
Faculty of Environment, Tehran University, Tehran, Iran
Received 7 July 2005 Code Number: se06009 ABSTRACT Advanced compact wastewater treatment processes are being looked for by cities all over the world as effluent standards are becoming more stringent and land available for treatment plants more scarce. In this investigation, a new biofilm process for this purpose was studied. The design and operational criteria of a full scale extended aeration activated sludge system was compared with an H-IFAS reactor which has been operated at a pilot scale. The objective was to define the feasibility of using the H-IFAS (Hybrid Integrated Fixed Film Activated Sludge) reactor for upgrading the existing wastewater treatment plants with conventional processes. The results showed that besides the considerable difference between the organic loading of the two processes, H-IFAS reactor has a very good capability to reduce simultaneously the concentration of nitrogen and phosphorus. Organic degradation rate in extended aeration and H-IFAS systems were 0.3 and 6.22 kgCOD/m3.day at 23.48 °C, respectively. Nitrification, denitrification and phosphorus removal rate for the H-IFAS reactor were 343.28 g N/m3.day, 338.17 gN/m3.day, and 204.78gPO4 -P/m3.day, respectively. At the same conditions, these criteria for extended aeration activated sludge processes were obtained as 75gN/m3.day, 28.5 gN/m3.day and 7 gPO4 -P/m3.day), respectively. Key words: IFAS, MBBR, H-IFAS, biofilm, nitrification, denitrification, biological phosphrous removal, extended aeration INTRODUCTION The activated sludge process is a suspended culture system that has been in use since the early 1900s. The process derives its name from the fact that settled sludge containing living, or active, microorganisms is returned to the reactor to increase the available biomass and speed up the reactions. It may be either a completely mixed or plug flow process. The process is aerobic, with oxygen being supplied by dissolution from entrained air Extended Aeration (EA) is one of the modifications of the activated sludge process in which aeration time is increased to about 24 hours. The major advantage of this configuration is that the rate at which waste activated sludge is removed from the process approaches zero. The treatment efficiency decreases, however, and the power costs for aeration are higher (Asano, 1998). Primary treatment is usually eliminated. Somedisadvantages of EA activated sludge process can be considered as follows:
MATERIALS AND METHODS A: Ekbatan Wastewater Treatment Plant Ekbatan wastewater treatment plant which was selected to setup the H-IFAS reactor and was working with extended aeration activated sludge process, is situated at the south west of Tehran. The general information of the system are summarized in Table 1. In Table 2, some process parameters of aeration basins are given. B: H-IFAS Pilot Plant System The pilot plant of H-IFAS reactor was built according to the scheme in Fig. 1. The system comprised an anaerobic (300 L), an aerobic (850 L) two anoxic zones (850 L), and a clarifier. The raw wastewater was fed to an elevated storage tank by a centrifuge pump from the grit chamber downstream and fed to the reactor in an upflow continuous manner. In order to control the content of media in each zone some stainless steel meshes were installed between these parts. The aerobic and anoxic zones were separated with an impermeable metallic wall. The upper part of the walls consisted of some meshes which directed flow to the anoxic zones. All anaerobic, aerobic and anoxic zones were filled up with moving media, and in anoxic zones two mixers were used to provide an appropriate mixing. The wastewater and the air flow were supplied in a counter current manner to ensure proper contact time. Aeration was supplied with a side channel blower and aeration rate was measured and monitored by an air flow meter. In the meantime a wastewater flow meter was used as well to define the flow rate of influent raw wastewater which was fed to the reactor in each operation phase. The rate of return sludge from the clarifier was controlled and monitored by a full automatic timer which controlled the flow and time of sludge returning in each phase. Two centrifugal pumps were used for transferring raw wastewater to the storage tank and another one for returning sludge to the anaerobic zone. In aerobic processes, the biofilm carrier movement was caused by the agitation set up by the air keeping the carriers moving. In the aerobic reactors, fine bubble aeration system was installed. The media fill fraction in anaerobic, aerobic and anoxic zones were selected as 60%, 55% and 55% , respectively.A recycling flow from anoxic zone was directed to aerobic zone for alkalinity recovery and pH adjustment. The experiments were implemented in eight phases based on media locations and quantities and COD/TP/TKN ratios. The Bee-Cell2000 media was chosen as the fixed film media for this investigation which was made from Polystyrene and was specifically designed with a high internal (protected) specific surface area to maximize attached growth potential but with sufficient void space to minimize plugging. The media consisted of two concentric cylinders connected by internal walls along the length of the media. The overall dimensions were 22 mm in diameter and 15 mm in length. The internal surface area was 388 m2/m3 and total surface area was 650 m2/m3. Only the protected internal surface area was considered for design purposes (Table 3). Raw wastewater characteristics which were adopted from Ekbatan Wastewater Company’s Laboratory Archive are presented in Table 3. The reactor was operated during 8 phases in 8 months (from June 19th, 2005 to March 18th, 2006) and the samples were collected , tested and analyzed for measuring physico-chemical and bacteriological parameters. Both systems (H-IFAS and EA activated sludge) were operated and monitored for over 8 months to allow reactor to reach steady state conditions, and then at least three runs of steady state data were collected from each compartment during each phase of experiments. At steady state, mixed liquor suspended solid (MLSS), mixed liquor volatile suspended solid (MLVSS), chemical oxygen demand (COD), biochemical oxygen demand (BOD), total kjeldahl nitrogen (TKN) and total phosphorus (TP) were measured. The MLSS, MLVSS, TKN, TP, and COD were analyzed in accordance with Standard Methods for the Examination of Water and Wastewater, 19th Edition, 1995. RESULTS BOD and COD Removal BOD and COD removal in H-IFAS and extended meter of this media was held in aerobic zone) with aeration systems have been shown in Fig. 3 and an average temperature of 23.48 °C, HRT=1.26 Fig. 4. Organic degradation rate of anaerobic zone hr and OLR=8.66 kgCOD/m3.day was equal to (0.18 cubic meter of this media was held in 6.22 kg COD/m3.day. With regarding to Fig. 5 and anaerobic zone) with an average temperature of 6, organic degradation rate of the extended 23.48 °C, HRT=27 min and OLR=22.5 kgCOD/ aeration activated sludge process at temperature m3.day was equal to 3.56 kgCOD/m3.day. Organic of 23.48 °C and OLR=0.3 kgCOD/m3.day was degradation rate of aerobic zone (0.4675 cubic equal to 0.26 kgCOD/m3.day. phosphorus removal Biological phosphorus removal rate of the reactor at optimized operational conditions with a phosphorous loading rate of 243.57 gP-PO4/ m3.day , an average temperature of 23.48 °C and HRT= 27 min was equal to 204.78 gP-PO4/ m3.day. Since the rate of return sludge from the clarifier was controlled and adjusted by an automatic timer, it was tried to test and study the effect of returning sludge rate on the efficiency of phosphorous removal in the reactor. It was shown that the best result for P removal efficiency occurred when the rate sludge return was about 15% of the influent flow rate. With regarding to Fig. 8 phosphorus removal rate in Ekbatan WWTP, was obtained 7 g PO4-P/m3.day Nitrification Efficiency In optimized conditions, nitrification rate of media (0.4675 cubic meter of this media was held in aerobic zone) with an average temperature of 23.48 °C, HRT=1.26 hr and nitrogenous organic loading rate of 628.56 gTKN/m3.day was equal to 343.28 gTKN/m3.day (Fig. 9 and Fig. 10). Regarding to Fig. 9 nitrification rate of E.A. activated sludge process at Ekbatan WWTP with the same conditions was obtained as 75 gN/m3.day. Denitrification Efficiency Considering Fig. 11, denitrification rate of media 23.48 °C, HRT=1.26 hr and nitrogenous organic (0.4675 cubic meter of this media was held in loading rate of 347 gN-NO3/m3.day was equal to anoxic zones) with an average temperature of 338.17 gN-NO3/m3.day. DISCUSSION As shown in Fig. 3 and Fig. 4, BOD and COD removal efficiencies in H-IFAS plant were constant with changes in influent flow. In fact H-IFAS reactor had more flexibility on flow fluctuations. The results of this investigation clearly showed that H-IFAS can be successfully accomplished in three stage (aerobic-anaerobic-anoxic) systems. With regarding to the obtained results, the H-IFAS reactor’s carbonaous organic removal efficiency is much higher (at least 5-6 times) than EA activated sludge process efficiency. Consequently for a constant influent flow we can have a much more compact system. This matter provides the conditions of making a good economical saving in construction and operation of the system. Phosphorus appears in wastewater as orthophosphate, polyphosphate and organically bound phosphorus.The last two components accounting usually for up to 70 percent of the influent phosphorus. Microbes utilize phosphorus during cell synthesis and energy transport. As a result, 10 to 30 percent of the influent phosphorus is removed during traditional mechanical/biological treatment (Barlindhaug et al., 1996). When enhanced phosphorus removal is desired, the process is modified, so that the sludge is exposed to both anaerobic and aerobic conditions. Then certain microorganisms, capable of storing phosphorus (in the form of polyphosphates), metabolize it for energy production and cell synthesis, resulting in the removal of phosphorus from the system through the waste activated sludge (Heukelekian et al.,1999) operated A2O and modified UCT processes, respectively; to treat a municipal wastewater. In this research an A2O process was simulated in the form of an H-IFAS reactor with this difference that anaerobic, anoxic and oxic zones were designed and constructed in a single reactor. Having separated different zones in the reactor, good efficiencies of pollutants removal have been achieved. Application of biofilm carriers in the reactor increased the population of autotrophic microorganisms which have an important role in nitrification, denitrification and biological Premoval. Phosphorus removal in this reactor was provided by returning settled sludge from the clarifier to the lower part of the reactor which is an AMBR . As it was shown, the process of P-removal in this reactor had a good stability in spite of increasing the HLR in the process of time. On the other hand the efficiency of the reactor in comparison with the other conventional processes was quite different. Namely, although the volume of the reactor was limited and the compactness of the system caused a very small footprint, but the capability of the process in this reactor was reliable and it had a very good efficiency for removing phosphorus from raw wastewater. There are several major factors that influence the kinetics of nitrification. These are organic loading, hydraulic loading, temperature, pH, dissolved oxygen concentration, and media. In this study, nitrification was studied in the pilot scale biofilm reactor (H-IFAS) equipped with a fine bubble aeration system. With regarding to technical specifications of the media and also similar studies with kaldnes-k1 media by Halvaard Odegaard in 1989 (Ødegaard et al., 1998), the media fill fraction of aerobic zone was 55 %. The biofilm system offers the achievement of high biomass age that is very important for the nitrification process. Raw wastewater was used as the source of N-NH4. Figure 9 shows nitrification rate per support area with respect to effluent N-NH4 concentration with 4-6 mg/L dissolved oxygen (DO) range. There was a gradual transition from a first order nitrification at low concentrations of ammonia to half order and to zero-order at higher concentrations of ammonia. Similar results for nitrification in biofilm reactors were obtained. It was found that nitrification is clearly oxygen limited at higher ammonia concentrations. For the unlimited conditions the DO/[N-NH4] ratio in the reactor should be at least 4. Oxygen concentration as well as the velocity of the air and consequently the hydrodynamic conditions in the reactor was controlled by the air supply. With increasing air supply, the concentrations of oxygen and air velocity in the reactor increased and external mass transport resistance decreased. Levine et al.,1985 presented a simplified design of bio film processes using normalized loading curves. Design was based on the parameters that are characteristic for the biological process and parameters that define hydrodynamic conditions in the reactor. This concept can be used for the design of bio film reactors for nitrification (Munch et al., 1990). Higher concentration of organic compounds in the nitrification zone leads to a competition for oxygen in the bio film between heterotrophic (COD elimination) and autotrophic (nitrification) organisms. In this research, nitrification rate was obtained at an optimum range by using a well designed media with high specific surface area and designing an anaerobic zone at the lower part of the reactor. Effluent of aerobic zone was used as the source of N-NO3 to the anoxic zone and methanol with the dosage of 5-10 mg/L served as the readily biodegradable carbon source as the electron acceptor. Anoxic zones consisted of two separated parts with equal volumes which were situated on both lateral sides of the reactor. Denitrification rate was of half-order in the N-NO3 concentration range of 15-30 mg/L when COD was in excess. Some similar results for denitrification in biofilm reactors with dispersed media were obtained by Hallvard Ødegaard (Ødegaard et al., 2002). Nutrient removal (P & N) in H-IFAS reactor with efficiency of 8090% was much more desirable than the efficiency of EA activated sludge process. ACKNOWLEDGEMENTS This research was supported by grant of Presidency’s Researchers Supporting Organization. The authors would like to acknowledge Tehran Water and Wastewater Co. managing director for sincere cooperation in this research and thank Mr. A. Sabzali and Mr. M. J. Vafaee for their helps in theoretical and practical aspects of the research. REFERENCES
© 2006 Tehran University of Medical Sciences Publications The following images related to this document are available:Photo images[se06009t4.jpg] [se06009f3.jpg] [se06009f11.jpg] [se06009f1.jpg] [se06009f4.jpg] [se06009f12.jpg] [se06009f2.jpg] [se06009f7.jpg] [se06009f6.jpg] [se06009f5.jpg] [se06009f9.jpg] [se06009t2.jpg] [se06009t3.jpg] [se06009t1.jpg] [se06009f10.jpg] [se06009f8.jpg] |
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