Process Descriptions

1 INTRODUCTION The USBF process is a modification of conventional activated sludge process that incorporates an anoxic selector zone and an upflow sludge blanket clarifier. The USBF process may be designed for • carbonaceous (BOD) removal  • BOD removal and nitrification • BOD removal, nitrification, and denitrification • BOD removal, nitrification/denitrification and phosphorus removal For carbonaceous removal, the anoxic zone serves as a “selector zone” that conditions the mixed liquor to improve settleability and to control filamentous organism growth. For nitrification, denitrification and phosphorus removal designs, the anoxic zone provides the necessary conditions for dissimilarity nitrate reduction and phosphorus removal by “luxury uptake”. In this process, ammonia nitrogen is oxidized to nitrite and then to nitrate by Nitrosomonas and Nitrobacter bacteria, respectively in the aeration zone. The nitrate is then recycled to the anoxic zone where the nitrate is reduced by dissimilarity nitrate reduction. In this reaction, the incoming BOD serves as the carbon source or electron donor for the reduction of nitrate to elemental nitrogen. The phosphorus removal mechanism in this process is the same as that employed in the Phostrip and modified Bardenpho processes. In the USBF process, fermentation of soluble BOD occurs in the anaerobic or anoxic zone. The fermentation products are selectively used or assimilated by a special group of microorganisms that are capable of storing phosphorus. During the aerobic stage of treatment, soluble phosphorus is taken up by the population of the phosphorus storing bacteria (Acinetabacter) that was developed in the anoxic zone. The assimilated phosphorus is then removed from the system as excess biomass or waste sludge. The amount and rate of phosphorus removal depends primarily on the BOD/P ratio of the influent wastewater.  2 PROCESS DESIGN The Ecofluid Design Program for the USBF process is based on the Lawrence and McCarty kinetic models for BOD removal, nitrification and denitrification. The process model equations along with the kinetic coefficients and related critical design parameters are presented in the attached VBR guide (the nomenclature as shown in the VBR guide is somewhat different than the standard U.S. texts). The USBF process is capable of removal of BOD5 to less than 5 mg/l, TSS removal to less than 10 mg/l without filtration, total nitrogen removal to less than 10.0 mg/l and total phosphorus removal to a range of 1.5 to 2.5 mg/l. Higher levels of phosphorus removal down to 0.1 to 0.5 mg/l can be achieved by metal salt addition to the aeration zone immediately prior to the mixed liquor entering the clarifier. A number of metal salts may be used including Alum (Al2(SO4)3.14H2O), Sodium Aluminate (Na2O.Al2O3), Ferric Chloride (FeCl3), Ferrous Chloride (FeCl2), Ferrous Sulfate (FeSO4.& H2O) or Ferric Sulfate (Fe2(SO4)3). Since the bulk of phosphorus (over 80%) in the USBF process is accomplished by biological uptake, the small polish dosages of a metal salt coagulant do not significantly increase sludge production. For example, removal of phosphorus by FeSO4 is given as by the two following reactions: Phosphorus Precipitation 3FeSO4 + 2PO4-3 ---------> Fe3 (PO4)2 + 3SO4-2 Alkalinity Reduction and Hydroxide Precipitation Fe+++ + 3HCO-3 -----------> Fe(OH)3 According to the above two reactions, removal of 2 mg/l of PO4-3, would theoretically produce 6 mg/l of additional sludge. In actual practice, a value of 5 mg/l of sludge per mg/l of PO4-3 removed provides a conservative design value. For an influent wastewater having 240 mg/l of incoming BOD and a sludge yield of 0.6 lbs TSS/lb BOD removal, and the use of FeSO4 to remove 2 mg/l of PO4-3, the total increase in sludge production would be about 7%. The USBF process utilizes a unique patented upflow sludge blanket clarifier. The upflow blanket clarifier utilizes a trapezoidal shape where the mixed liquor enters the bottom of the clarifier through a specially designed baffle where hydraulically induced flocculation occurs. The trapezoidal clarifier shape provides for a steadily increasing surface area from the bottom to the top of the clarifier. This permits a gradually decreasing vertical velocity gradient within the clarifier. The "top surface area" clarifier overflow rate is 150 to 250 gpd/ft2 (6 to 10 m3/d/m2) at average daily design flow. The clarifier is typically designed for a daily peak flow rate of 3 times the average flow ratio which translates to a peak "top surface" clarifier overflow rate of 450 to 750 gpd/ft2 (18 to 31 m3/d/m2) which is very conservative. The clarifier also includes a unique baffle arrangement to allow sludge withdrawal at the bottom of the clarifier. The sludge withdrawal design also incorporates the internal recycle between the aerobic and anoxic zone. The normal design recycle/sludge withdrawal rate is 4 times the average daily flow. This high sludge withdrawal rate from the clarifier bottom creates a downward velocity gradient within the clarifier that significantly improves the hydraulic efficiency of the clarifier compared to conventional clarifier. The internal recycle between the aeration zone and the anoxic zone provides BOD recycle that is required for endogenously supported nitrate reduction. This internal recycle of mixed liquor also provides for recycle of phosphorus removal organisms developed in the anoxic zone that are then carried into the aeration zone for phosphorus uptake. The recycle ratio is established based on the influent BOD/total phosphorus/ammonia nitrogen ratio. The recycle ratio of 4 provides for a 25% - 35% safety factor for domestic wastewater. The major process design parameters for this process depend on (1) wastewater strength and biodegradability (2) wastewater temperature, influent and effluent BOD, N, and P concentrations. Typical HRT’s for the aeration zone range from 6 to 30 hrs. The HRT’s for the anoxic zone typically range from 1 to 2 hrs for a selector zone used for carbonaceous removal and 2-8 hrs for biological phosphorus removal and denitrification. The design SRT is controlled by the temperature dependent nitrification and BOD removal kinetics and the design effluent N-NH4 requirements. The operating SRT is normally maintained at 50% to 100% greater than the design SRT at an operating temperature to provide a safety factor and to accommodate changes in influent wastewater characteristics. (Please note that SRT is both a design parameter and a process control parameter). 3 OPERATING PARAMETERS The dissolved oxygen (DO) concentration should be maintained at 2.0 to 4.0 mg/l in the aeration zone, and less than 0.5 mg/l in the anoxic zone. Under influent loading conditions less than the design values, the HRT in both the aeration zone and in the anoxic zone will be greater than the design value. Under these conditions, the mixed liquor volatile solids concentration in the system will normally be reduced to meet the process requirements. The DO may be maintained at optimum levels by reducing air supply. The increased HRT in the anoxic zone permits more time for exertion of DO demand and production of anoxic conditions needed for fermentation. The operating SRT is controlled by controlling the sludge wasting rate. SRT is normally calculated based on aeration zone volume and MLVSS concentration, since BOD removal and nitrification kinetics control the aeration zone volume. Provision is made in the Ecofluid design for measurement of both the internal recycle and sludge wasting. The operating SRT of the USBF process may be increased significantly above the design requirements without sacrificing effluent quality since the "anoxic selector" zone conditions the mixed liquor solids and the upflow sludge blanket clarifier provides a "filtration/flocculation" mechanism to prevent the discharge of pin-point floc normally associated with high SRT systems. 4 ALKALINITY AND PH If the influent wastewater is not properly buffered it is necessary to add alkalinity to the influent wastewater for the USBF process designed for nitrification and denitrification. The nitrification reaction consumes 7.1 mg/l of alkalinity as CaCO3 for each mg/l of ammonia nitrogen oxidized. The denitrification reaction produces 3.57 mg/l of hydroxide alkalinity as CaCO3 for each mg/l of nitrate-nitrogen reduced. For an influent wastewater having 40 mg/l of NH4-N, the total alkalinity should be 150-200 mg/l to insure adequate buffering. The pH of the system should always be maintained between 7.5 to 8.5 S.U. by the addition of alkalinity when required. The original text of the Description was prepared by Mr. John M. Smith of J.M. Smith & Associates of Cincinnati, Ohio. Mr. Smith has 17 years experience in wastewater treatment research and process design for USEPA’s office of Research and Development plus 18 years as an independent consultant