A SUBMERGED ATTACHED GROWTH BIOREACTOR FOR DECENTRALIZED WASTEWATER TREATMENT

Transcription

1 A SUBMERGED ATTACHED GROWTH BIOREACTOR FOR DECENTRALIZED WASTEWATER TREATMENT P.B. Pedros * and W.K. Dobie F.R. Mahony & Associates, Inc. 273 Weymouth Street Rockland, MA ABSTRACT The Amphidrome process is a submerged attached growth bioreactor (SAGB) that has been used in Massachusetts, Connecticut, Rhode Island and New Jersey for decentralized wastewater treatment systems ranging in size from 440 gpd to 150,000 gpd. The two primary advantages of a SAGB are the small volume requirement and the elimination of downstream clarification. Five years of data from four plants operating in Massachusetts are presented below. Each facility was designed to treat a domestic wastewater to an effluent BOD 5 30 mg/l, TSS 30 mg/l, and total nitrogen 10 mg/l. The process is described and the performance, including loading and removal rates, at the four treatment plants is presented. The results indicate 1) 97% nitrification at an organic loading of 2.5 kg-bod 5 /m 3 -day, and 2) a nitrification rate of kg-n/m 3 -day and a denitrification rate of kg-n/m 3 -day each at a total ammonia loading of kg- N/m 3 -day. KEYWORDS SAGB, BNR, biological filter, decentralized system INTRODUCTION Due to the severe impact of eutrophication on water resources, the removal of inorganic nutrients, nitrogen and phosphorus, from wastewater has become an increasingly important consideration and has imposed new challenges in the design of wastewater treatment plants. Nitrogen discharge limits for many coastal regions and tidal estuaries have become especially stringent in recent years and biological nutrient removal (BNR) processes have been developed to meet the challenge. One such technology is the submerged attached growth bioreactor (SAGB), which derives its name from the fact that the media is always submerged in the process flow. The two primary advantages of a SAGB are the small volume requirement and the elimination of downstream clarification (Grady et al. 1999). A submerged biofilter allows for a high biomass concentration leading to a short hydraulic retention time and, thus, a significantly reduced reactor volume as compared to a different fixed film reactor or a suspended growth reactor. In addition, the media in a SAGB may be fine enough to provide physical filtration for solids separation. During the last twenty years, different configurations of SAGBs have been conceived and advances in the understanding of these systems have been made. SAGBs have been used to achieve complete nitrogen removal by combining the aerobic oxidation of soluble organic matter 4608

2 (SOM) and nitrification into one operational unit and denitrification into a separate unit operation (Andersen et al and Holbrook et al. 1998). A pilot study of a biological aerated filter (BAF), a specific type of SAGB, installed in series and downstream of a denitrification unit demonstrated that a recirculation of 300% of the inflow, from the filter back to the denitrification unit, removed organics and nitrogen to the required levels (Yoshinobu et al. 1997). Combined removal of organics and nitrogen was demonstrated in a single BAF with a separate anoxic zone created within the reactor (Rogalla and Bourbigot 1990). However, the use of a single-unit, single-zone SAGB for achieving the combined removal of organics and nitrogen is an innovative process variation. THE SAGB PROCESS This particular SAGB process was specifically designed for the combined oxidation of carbonaceous matter, nitrogen removal and suspended solids removal in a single-unit single-zone biofilter. The system includes one anoxic/equalization tank, one clear well and one SAGB. (See figure 1.) and operates as a sequencing batch reactor in which the wastewater is cycled back and forth through the filter. The biofilter is intermittently aerated to achieve both the aerobic environment required for the oxidation of organics and nitrification and the anoxic environment required for denitrification. It provides low visibility, since all tanks are typically installed underground, compact footprint, effective nutrient removal and minimal effect from cold air temperatures. Figure 1 Cross Section of the System The influent wastewater enters the system through the anoxic/equalization tank, which has an a a sludge storage zone and settling zone, which serves as a primary clarifier before the SAGB. From the anoxic/equalization tank the wastewater flows by gravity into the SAGB. The driving force of the forward flow is the hydrostatic pressure created by the differential liquid levels within the tanks. The reactor consists of: 1) an underdrain, 2) support gravel, 3) filter media, and 4) a backwash trough. The underdrain, located at the bottom of the reactor, provides support for the media and even distribution of air and water into the reactor. The design allows for both the air and water to be delivered simultaneously or separately via individual pathways to the bottom of the reactor. 4609

3 Operation of the SAGB alternates between down-flow (forward flow) and up-flow (reverse flow) modes. The up-flow is accomplished by pumping from the clear well back up through the filter, following the exact same path through the reactor as it did in the forward flow cycle. However, a check valve in the influent line of the reactor prevents the flow from returning to the anoxic/equalization tank via that line. Instead, the flow fills the reactor until it overflows into the return flow/backwash trough and flows back to the front of the anoxic/equalization tank by gravity. The recycled flow, which contains nitrates, mixes with the incoming raw influent, which contains organic carbon, and starts to flow forward again when the pump shuts off. To achieve the required aerobic and anoxic conditions within the biofilm, process air to the reactor is supplied intermittently via the underdrain at the bottom of the reactor and is independent of the return flow cycles. A typical aeration sequence would be three minutes with the process blower on and then fifteen minutes with the blower off. The cyclical forward and reverse flow of the waste stream and the intermittent aeration of the filter achieve the required hydraulic retention time and create the necessary aerobic and anoxic conditions to achieve the required level of biological nitrogen removal. PERFORMANCE RESULTS The results presented are from four SAGB treatment plants operating in Massachusetts. The design criteria were based on the typical effluent limits applied to wastewater treatment plants that discharge into the ground in Massachusetts and are listed in Table 1. The data analyzed were from Amphidrome Plus systems, which included a separate, smaller filter for denitrification to insure low nitrate levels and therefore, an effluent total nitrogen limit 10 mg/l. Table 1. Effluent Standards for Discharge to Groundwater Constituent Effluent Limit Regulatory Basis BOD 30 mg/l Massachusetts DEP TSS 30 mg/l Massachusetts DEP Total Nitrogen 10 mg/l Massachusetts DEP Treatment plants 1, 2, and 3 were condominium complexes and treatment plant 4 was a combined middle school and high school. The reactor sizes, media volumes and methanol use for each system are shown in Table

4 Table 2. Reactor Sizes and Media Volume Plant/Flow (gpd) Reactors Media Vol. Plus Reactor Media Methanol Vol. 1 36, ft.x10.5 ft. 900 ft ft. dia. 63 ft. 3 0 gpd 2 25, ft.x13 ft. 740 ft ft. dia. 56 ft gpd 3 36, ft.x10.5 ft. 1,300 ft ft. dia. 94 ft gpd 4 14, ft. dia. 400 ft ft. dia. 50 ft gpd Process aeration of the SAGBs was based on 0.7 cfm/ft. 2 of filter area and was typically governed by the physical requirements for an even air pattern and not the biological requirements. Therefore, the process aeration ranged from 35 cfm to 86 cfm. The total aeration time per day ranged from 3 to 5 hours. Each of the systems operated with recycle flows from two to three times the daily flow. The ammonia loading varied between kg-n/m 3 -day and the organic loading from kg-bod 5 /m 3 -day. The effect of total organic loading (TOL) on nitrification is illustrated in Figure 2. The results indicate that a nitrification efficiency of 97% was achieved at a TOL of 2.5 kg-bod 5 /m 3 day, which corresponds to the 98% efficiency at 3.5 kg-bcod/m 3 day reported by Rogalla et al. (1990) with a BAF designed for oxidation of organics and nitrification. The results also indicate that near complete nitrification is possible at a TOL up to 1 kg-bod 5 /m 3 day. This is higher than the TOL limit suggested in the USEPA Nitrogen Control Manual (1993), which suggests that to achieve 90% nitrification in a single BAF, the TOL should not exceed 1 kg-bod 5 /m 3 day. The effect of total ammonia loading (TAL) on reactor performance is illustrated in Figure 3. Excluding the elevated total nitrogen (TN) effluent values, which were due to operational problems, the results indicate that during the five-year period the effluent total nitrogen concentration at each of the plants was below the required 10 mg/l. For the highest loading of kg-n/m 3 day the effluent total nitrogen was 5.4 mg/l. The ammonia loading and nitrification rates presented in Figure 4 illustrate a nitrification rate of kg-n/m 3 day at an ammonia loading of kg-n/m 3 day. This is in agreement with Terayama et al. (1997), who reported a nitrification rate of 0.18 kg-n/m 3 day at an ammonia loading of 0.19 kg-n/m 3 day. 4611

5 Nitrification Efficiency (%) and Rate (100 kg-n/m3-day) Nitrification Rate Nitrification Efficiency Figure 2. TOL and Nitrification Efficiency Total Organic Loading (kg-bod5/m3-day) Effluent Total Nitrogen (mg/l) Plant 1 Plant 2 Plant 3 Plant Figure 3. TAL and Effluent TN Concentration Ammonia Loading (kg-n/m3-day) 4612

6 Nitrification Rate (kg-n/m3-day) Figure 4. TAL and Nitrification Rate Ammonia Loading (kg-n/m3-day) The relationship between denitrification rate and ammonia loading is illustrated in Figure 5. The denitrification rate ranged from kg-n/m 3 day at a ammonia loading of kg-n/ m 3 day to kg-n/m 3 day at an ammonia loading of kg-n/m 3 day. These rates are low in comparison with rates (0.29 to 1.6 kg-n/m 3 day) reported in the EPA Manual for Nitrogen Control (1993) for full-scale denitrification filters. Two operating conditions likely contributed to the higher rates reported by the EPA: 1) the cited filters were separate unit processes dedicated strictly to denitrification and thus had no aerobic cycle and 2) methanol was used as a supplemental organic carbon source. In this SAGB process, the influent organic carbon was utilized for much of the denitrification process. Although small quantities of methanol were added, the use of sewage as the organic carbon source would tend to reduce the denitrification rate. 4613

7 Denitrification Rate (kg-n/m3-day) Ammonia Loading (kg-n/m3-day) Figure 5. TAL and Denitrification Rate CONCLUSIONS The SAGB technology is an effective biological nitrogen removal process that offers low visibility (all tanks are underground) and compact footprint. As with most SAGBs, it requires less area than many other types of biological treatment processes because of the high concentration of viable biomass within and because downstream clarification is eliminated. In addition, the system can be constructed in concrete tanks, below grade, resulting in a small control building and much lower construction costs. The effluent requirements of BOD5 30 mg/l, TSS 30 mg/l were achieved at all four plants during the five year period (data not presented). Excluding operational problems, the effluent total nitrogen limit 10 mg/l was also achieved. At a total ammonia loading of kg-n/m 3 -day an effluent total nitrogen of 5.4 mg/l was obtained. The nitrification rates at the four SAGB plants examined were comparable to those reported in the literature. REFERENCES Andersen, K. L., E. Bundgaard, V. R. Andersen, S. N. Hong and J. F. Heist, Nutrient Removal in Fixed-Film Systems. Water Environment Federation, 68 th Annual Conference & Exposition, 1, Grady, C. P. L. Jr., G. T. Daigger and H. C. Lim, Biological Wastewater Treatment, Second Edition, Marcel Dekker, Inc., New York, Holbrook, R. D., S. N. Hong, S. M. Heise and V. R. Andersen, Pilot and Full-Scale Experience with Nutrient Removal in a Fixed Film System. Water Environment Federation, 71 st Annual Conference & Exposition, 1,

8 Rogalla, F. and M. Bourbigot, New Developments In Complete Nitrogen Removal With Biological Aerated Filters. Wat. Sci. Tech., 22(1-2), Rogalla, F., M. Payraudeau, G. Bacquet, M. Bourbigot, J. Sibony and P. Filles, Nitrification and Phosphorus Precipitation With Biological Aerated Filters. Research Journal, Water Pollution Control Federation, 62, United States Environmental Protection Agency, Office of Research and Development and Office of Water Manual for Nitrogen Control. EPA/625/R-93/010. Yoshinobu, T., N. Takashi, I. Masumi and H. Morio, Feasibility Study on Nitrogen Removal Process By Biological Aerated Filter. Water Environment Federation, 70th Annual Conference & Exposition, 1,