This paper presents suggested guidelines based on the results of research on several IFAS (Integrated Fixed Film Activated Sludge) and MBBR (Moving Bed Bioreactor) full scale and pilot plants which were retrofitted into existing municipal activated sludge systems. The research has identified methods to compare the operating thresholds of each of four systems:(1) activated sludge, (2) IFAS with fixed media, (3) IFAS with moving bed, and (4) MBBR.A three-step method is presented that may be used to determine the type of system (IFAS / MBBR) and the type of media to apply in the upgrade of the wastewater treatment plant. Operating thresholds in terms of the aerobic HRT and MCRT and the MLSS levels that the wastewater treatment plant can support are presented. For comparing various systems, this paper keeps the loading and temperature constant, based primary effluent with 250 mg/L COD and 25 mg/L TKN, and a mixed liquor temperature of 12 C.The first operating threshold for the four systems can be expressed in terms of the aerobic mixed liquor MCRT (Aer MLSS MCRT). If the Aerobic MLSS MCRT is less than the washout MCRT of nitrifiers, the biofilm surface area must be adequate for maintaining nitrification on the biofilm. For the municipal wastewater with no significant inhibition of nitrification, the washout MCRT was computed to be 5 days. Above the washout MCRT, the amount of biofilm surface area decreases until the ratio of MCRT to washout MCRT is adequate to handle the diurnal peak loads.Based on an threshold on MLSS of 2,500 to 3,000 mg/L and a hydraulic retention time of 4 hours, the washout MCRT can be related to the specific surface area of media that needs to be applied in the system. IFAS and MBBR systems with media installed at specific surface areas less than 200 m 2 /m 3 need to operate at Aerobic MLSS MCRT operate at greater than the washout MCRT of nitrifiers. This includes cord media and sponge media. IFAS systems and MBBR systems that have specific surface area greater than 200 m 2 /m 3 can be operated at Aerobic MLSS MCRTs that are less than the washout MCRT of nitrifiers.A second threshold identified was the MLSS for operating each of the four systems. In activated sludge and IFAS fixed bed systems, this threshold is determined by the secondary clarifier capacity and Sludge Volume Index (SVI). For high rate clarifiers operating at 500 to 600 gpd/sf (20 to 25 m 3 /m 2 /d), the operating threshold for MLSS in an activated sludge system is 3,000 mg/L. For IFAS moving bed systems and MBBR systems, the operating threshold is also determined by the screens installed in the activated sludge basin. Three 6217 factors influence the operation of the screens: the amount of stalked ciliates in the mixed liquor, the Nocardia foam, and the prescreening.Observations of IFAS moving bed systems showed that when the MLSS exceeded 2,000 mg/L at 20 C or 2,500 mg/L at 15 C, and the Aerobic MLSS MCRT was above 5 days, long tailed stalked ciliates could form a mat on the screens if wedge-wire screens are used and the shear...
Research was conducted in IFAS systems to identify problems that arose with the biofilm when the intensity of mixing and aeration fall below or rise above certain thresholds. The types of IFAS media evaluated include fixed bed cord media (Ringlace and Bioweb), moving bed sponge (sponge mediaLinpor and Captor; and plastic media -Kaldnes) and RBC media. An aeration process design model and operating strategies have been developed to address these problems. This paper presents the IFAS and MBBR design methodology and aeration process design component of the Aquifas Unified Model for Activated Sludge, IFAS and MBBR systems. The paper references specific instances in full-scale IFAS plants where problems occurred because of insufficient or improper mixing patterns and documents how they were resolved. The problems observed included nuisance predator development and plugging of media and screens. The IFAS model was upgraded to incorporate computation of biofilm thickness and the impact of the increase in biofilm thickness on the reduction in effective surface area available on a various types of media. Effective surface area is the m 2 of biofilm surface per m 3 of tank volume. The model uses the effective surface area and the amount of media in the tank to compute the substrate (COD, NH4N and Oxidized-N) profiles. Additional research is being conducted to establish certain thresholds for mixing necessary above and beyond the aeration requirements to maintain a thin biofilm.The paper shows how the process design model is applied to improve the design of IFAS and MBBR systems. In instances where the aeration requirement to satisfy the DO set-point is satisfied but the mixing is below the threshold to maintain a thin biofilm, the media fill volume fraction in an aerobic cell can be increased and the fraction of tank volume occupied by the aerobic cell can be increased. By increasing the media fill volume fraction in a situation where the biofilm in an aerobic cell is too thick, the total amount of biofilm surface area present in the aerobic cell is increased and the soluble COD concentration is decreased. This decreases the biofilm thickness observed for the same intensity of mixing and increases the effective surface area of biofilm. This improves the performance of the system. When the media fill volume fraction is increased, the air flow per unit of reactor volume has to be increased to satisfy the additional oxygen demand. This increase in air flow helps satisfy the mixing requirements to mix the media and induce the requisite amount of biofilm shear. In some instances with fixed bed media, supplemental aeration system is installed below the frames to increase the air flow during diurnal peak load hours and thin out the biofilm.
The performance of ten full scale wastewater treatment facilities applying IFAS and MBBR systems was evaluated using the Unified Model for Activated Sludge, IFAS and MBBR systems. At each facility, the model was applied to compute the biofilm surface area required to achieve nitrification based on the plant loading, mixed liquor temperature and aerobic mixed liquor MCRT. The effluent ammonium-N was then computed for the amount of biofilm surface area installed and compared to the plant effluent. The analysis showed that the model was able to accurately predict the performance of the facilities (9 out of 9 for which data exists for average loads, 6 out of 6 for peak loads). When the quantity of media installed exceeded the amount computed by the model (% of Media Required that is Installed is > 100%), the observed data showed that the plant was able to achieve less than 2 mg/L under average flow and less than 3 mg/L under diurnal peak load. What is significant is that when the quantity of media installed was less than the amount computed by the model, the plant was not able to nitrify adequately. This was observed at two out of the ten facilities where the model showed that the effluent ammonium-N would exceed 5 mg/L. The results of the model can be applied to determine the additional amount of biofilm surface area (and media) that is required to achieve satisfactory nitrification. Alternatively, for IFAS systems, the model shows how much the aerobic mixed liquor MCRT would have to be increased in lieu of increasing the amount of biofilm surface area. A series of nomographs were generated to enable a designer to determine the biofilm surface area based on temperatures, 4 hour peak to diurnal average loadings and BOD 5 :TKN ratios.
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