City Converts Wastewater Treatment Facility to Sequencing Batch Reactors
The City of Lenoir, North Carolina, has recently completed an expansion of its Gunpowder Creek Water Reclamation Facility (WRF), that included the conversion of the four existing process treatment tanks to a four basin sequencing batch reactor (SBR) facility. The plant, owned and operated by the City, was originally constructed during 1976 as a 1.0 million gallon per day (MGD) dual-train contact stabilization wastewater treatment plant. It was later increased to a capacity of 1.2 MGD through changes in operation. By 1994, the plant influent flow was nearing plant capacity, so the City and its consulting engineer, McGill Associates, P. A., of Asheville, North Carolina, began plans to expand the plant to a capacity of 2.0 MGD. Due to site restrictions, high peak influent flows, and a stringent seasonal ammonia effluent limit of 2.0 mg/l, the City's engineer recommended that the walls of the four existing circular concrete tanks be raised to provide additional capacity and the four tanks be converted to SBRs.
Unlike conventional flow-through treatment processes, the SBR process incorporates all the separate stages of activated sludge treatment within the same basin. At the Gunpowder Creek WRF, each SBR cycle includes an anoxic selector stage, aeration stages, a clarification stage, and a decant stage to discharge the treated effluent. The advantages of an SBR facility are influent flow equalization within the basins, nutrient removal capability, and the inclusion of a selector stage to improve solids settablity and control filamentous organism growth. SBR plants are a cost-effective treatment alternative for small and medium-sized municipal and industrial wastewater treatment plants that require nutrient removal or have hydraulic or organic influent fluctuations. Automatic operation is provided with a computerized control system. These features allowed the City to increase the capacity of the plant without constructing new tanks for flow equalization and biological treatment, and provide the high degree of nitrification required to meet the effluent discharge ammonia limit.
Project Design and Bidding
The Gunpowder Creek WRF receives approximately 20 percent of its influent from local industries, with the remaining 80 percent consisting of domestic flow from the southeast part of Lenoir and from the nearby Town of Hudson. As part of the City's pretreatment program, industries are required to provide pretreatment if the strengths of their waste exceed certain limits. This results in an overall influent to the plant that can generally be characterized as medium strength. The facility's current National Pollutant Discharge Elimination (NPDES) permit requires that the effluent meet limits of 30 mg/l for BOD, 30 mg/l for Total Suspended Solids (TSS), 2 mg/l for ammonia nitrogen (NH3) between the months of April and October, and a minimum dissolved oxygen concentration of 5 mg/l. In order to ensure that the limits would be met and in anticipation of more stringent limits in the future, the design for the facility expansion was based on achieving an effluent with a maximum of 10 mg/l BOD, 10 mg/l TSS, 1 mg/l ammonia nitrogen and a minimum dissolved oxygen concentration of 6 mg/l. Table I shows the influent and effluent design criteria for the project.
The conversion of the existing treatment tanks to SBRs was part of a $2.2 million construction project that also included an expansion of the influent pump station, installation of a new mechanical bar screen, installation of a vortex grit removal system, conversion of a chlorine contact basin to a post flow equalization basin, construction of a new dual-train chlorine contact basin with chlorination and sulfur dioxide feed equipment, construction of a step aeration facility for post aeration, and construction of a 120,000 gallon bolted steel sludge holding tank. Schematics of the existing and new plant layouts are shown in Figures 1 and 2.
The SBR portion of the project was bid with two alternative SBR systems, one for a jet aeration SBR and one for a fine bubble diffused aeration SBR. Prospective SBR equipment suppliers submitted information to the engineer during bidding for prequalification. Contractors were required to list one of the prequalified SBR equipment suppliers for each alternate in their bid proposals. This bidding format created a competitive bidding atmosphere for the SBR equipment, while maintaining a high level of quality assurance. North Carolina law required that the project be bid in separate prime contracts for the general and electrical work. The fine bubble diffused aeration alternative proved to be the least costly alternative and the low bidder for the general contract, Hickory Construction Company of Hickory, North Carolina, elected to use equipment manufactured by Aqua-Aerobic Systems, Incorporated located in Rockford, Illinois. The electrical contract was awarded to Robertson Controls of Shelby, North Carolina.
Conversion to SBR BasinsWhile performing the work in the existing treatment tanks, the contractor was required to maintain operation of one of the existing two process trains at all times. This construction sequence permitted simultaneous work on two of the four existing tanks while the remaining two tanks remained in operation to treat incoming wastewater. Each existing concrete tank included interior steel walls, sand support bracing, walkway bridges, clarifier equipment, air piping and diffusers, concrete fillet in the clarifiers, and other process piping. All these items were removed by the general contractor and the tank interiors were cleaned.
In order to increase the treatment capacity of the plant to 2.0 MGD, the walls of all four existing basins were raised approximately 5 feet. This provided a maximum side water depth of 20 feet plus 2-feet of freeboard. A reinforced concrete grade beam was poured around each tank to strengthen the walls. This was needed for the additional 5 feet of hydrostatic pressure. The sizes of the existing tanks complicated the hydraulic designs in that two of the tanks were 68 feet in diameter, and the other two tanks were 55 feet in diameter. This required that the influent flow be split with 60 percent going to the larger tanks and 40 percent to the smaller tanks. To achieve this 60p;40 split, the computer-based control system was programmed to periodically close the influent valves to the smaller tanks.
In addition, the 68-foot-diameter tanks were approximately 4 feet higher in elevation than the 55-foot-diameter tanks. Therefore, two separate effluent lines were run from the SBR tanks, one for the larger tanks and one for the smaller tanks. Since no hydraulic connection exists, the possibility of the higher tanks overflowing the lower tanks is eliminated.
The SBR equipment for each basin includes an electric influent control valve, a floating mixer, stainless steel air piping, fine bubble tube diffusers, a solid excluding decanter, a submersible waste sludge pump and an electric effluent control valve. Five positive displacement blowers were provided by the SBR equipment supplier to provide air to all four tanks. Two blowers are dedicated to the larger tanks, two are dedicated to the smaller tanks, and the fifth blower serves as a backup to both systems. A control panel with a programmable logic controller (PLC) automatically controls all the equipment, including the control valves. In addition, a personal computer was provided with full monitoring capability to allow the operator to monitor the plant's operation from the office. The control software automatically opens and closes valves, and turns the SBR equipment on and off in sequence according to the different stages in the SBR cycle. A jib crane and hoist were also installed on top of each wall to allow the operators to remove equipment for maintenance.
The SBR Cycle
The SBR cycle at the Gunpowder Creek WRF includes six treatment stages and an idle stage. The treatment stages include anoxic fill, aerated fill, aeration, settle, decant and waste sludge periods. Each of the four basins will go through four of these cycles each day at the design flow of 2.0 MGD.
The purpose of the anoxic stage is to improve settling characteristics of the solids and control the growth of filamentous organisms that can cause sludge bulking. The anoxic stage can be compared to what is commonly referred to as a "selector" in a conventional treatment facility. This stage also plays an important role in phosphorous removal by creating an atmosphere in which phosphorous is released by the cell mass.
In any activated sludge process, it is important to control the growth of filamentous microorganisms that tend to cause poor settling sludge. In the anoxic-fill stage of the SBR, a floating mixer automatically turns on as influent wastewater is entering the tank, completely mixing the influent with the settled sludge that is present in the bottom of the tank. This results in an anoxic to anaerobic condition as the dissolved oxygen (D.O.) that was present in the top supernatant layer is consumed by the microbial respiration. The high food-to-microorganism (F/M) ratio at the extremely low D.O. level permits the soluble organics to be rapidly adsorbed into organisms with good floc-forming characteristics. Consequently, this leaves very little soluble organics for filamentous organisms, limiting their growth. The duration of the anoxic fill stage at design flow is 27 minutes per cycle.
The aerated fill stage is next. The mixer continues to operate and the blowers automatically turn on to provide aeration as wastewater continues to enter the basin. During this phase the ammonia nitrogen and BOD concentrations decline as biological carbonation and nitrification occurs. To achieve denitrification for total nitrogen removal, the blowers can be programmed to cycle on and off to alternately achieve aerobic and anoxic environments. The Gunpowder Creek WRF does not currently require denitrification, so the blowers remain on throughout the entire aerated fill cycle. At design flow, the duration of the aerated fill stage is 117 minutes.
The aeration stage is the first true "batch" treatment stage in the cycle. The influent valve automatically closes as flow is directed to another basin and the mixer and blowers remain on to continue to supply air to the completely mixed basin. This is an important phase in the SBR cycle in that it allows the ammonia and BOD removal to continue in an atmosphere that prevents new influent wastewater from entering the basin. Since oxygen demand from new influent is not added to the basin, all oxygen supplied by the blowers is utilized in driving the ammonia and BOD levels to even lower limits. The duration of the aeration stage at the Gunpowder Creek WRF is 27 minutes at design flow.
Phosphorous removal in an SBR is achieved through phosphorous release by the cell mass during the anoxic fill stage, and phosphorus uptake during the aeration stages. Although phosphorous removal is not currently required at the Gunpowder Creek WRF, it can be refined by adjusting the duration of the stages to achieve optimum results.
After the aeration stage is completed, all equipment turns off, the influent and effluent valves remain closed, and the tank sits idle to allow the solids to settle. This stage creates a completely static clarifier. There is no inflow or outflow to disrupt the settling of suspended solids. After the 45 minute settle stage is complete, the effluent valve opens and the floating mechanical decanter automatically begins drawing the supernatant to be discharged to the post flow equalization basin. The decanter is designed to exclude solids by drawing water from below the surface scum layer. The decanter floats with the water level from a maximum level of 20 feet to a minimum of approximately 11 feet. This decanting stage takes 72 minutes at the design flow of 2.0 MGD. When determining the low water level in an SBR, it is important to provide sufficient volume in the lower level for storage of the settled sludge to avoid the discharge of solids with the supernatant effluent. Following the decant stage, sludge from the bottom of the tank is pumped to the sludge holding tank.
At the design flow of 2.0 MGD, one cycle will take six hours for each of the four tanks, with each tank going through four cycles per day. The treatment stages take approximately five hours, and an idle stage of approximately one hour follows the waste sludge stage. Table 2 shows the typical duration of each stage in the SBR cycle at the design flow of 2.0 MGD
The SBR system is designed to treat up to two times the average flow without adjusting cycle times for the different treatment stages. When flows increase above twice the average design flow, the control system enters a storm flow mode and automatically begins shortening cycle times. However, the system preserves the duration of the settle stage as long as possible to prevent discharge of solids. The Gunpowder Creek WRF was designed to treat a peak hourly flow of 7.0 MGD (sustained for a maximum of three hours). Adjustments to cycle times vary for the individual basins during a storm flow event depending on which stage the basin is in when the storm flow enters the plant. At the maximum peak flow of 7 MGD, at least 42 minutes of aeration and 30 minutes of settling are provided each cycle to maintain adequate treatment.
The Gunpowder Creek WRF occasionally experiences peak hourly influent flows in excess of three times the average daily flow. One of the appealing features of the SBR is its inherent ability to dampen these peak influent flows. This characteristic is due to available storage in the basins resulting from the low water level at the beginning of the anoxic fill stage, and the idle period at the end of each cycle. This flow equalization feature is especially valuable to the Gunpowder Creek WRF due to the high surges in influent flow.
Since the SBR is an intermittent discharge system, a higher flow is discharged in a shorter time period when compared to a conventional flow through treatment system. A post flow equalization basin can be provided to dampen the surges in flow and to reduce the sizes of downstream treatment systems. Fortunately, the City was able to utilize an existing basin to save costs.
One of the existing chlorination basins was converted to a post flow equalization basin to reduce the instantaneous flow that would be discharged to the chlorine contact basin, dechlorination facility, step aeration facility, and Gunpowder Creek. The decanters are designed to decant at an average flow of 4.8 MGD for each of the larger basins and 3.1 MGD for each of the smaller basins. During the storm flow mode, simultaneous decanting by more than one basin can occur, creating the need for the flow equalization basin to dampen the peak flows to the downstream final treatment facilities. The post flow equalization basin includes two submersible pumps with variable speed drives to pump the SBR effluent to the chlorine contact basin for chlorination, dechlorination and post aeration. Chlorine and sulfur dioxide feed rates are automatically adjusted based on effluent flow.
Start-up of the SBR basins began in late April 1996, with one large and one small basin initially being put into operation. As with most facilities, the start-up period was not trouble free. An average of approximately 0.9 MGD of influent wastewater was diverted to the two SBR basins during May 1996, and the plant failed to meet its discharge permit limits during this first month of operation. Several factors could have contributed to the initial poor performance of the SBRs. For example, in an effort to achieve a higher level of treatment as quickly as possible, the two new SBR basins were brought on line before the concrete grade beam around the tank was completed. For this reason, they could not be operated at their normal water level. This decreased the treatment capacity of the units. Another contributing factor could have been insufficient time for the microorganisms to become acclimated to the new environment.
Performance of the facility steadily improved during the first few months of operation, and the facility met the design effluent limits for BOD, TSS and ammonia by July 1996, achieving less than 1 mg/l of ammonia. It is noted that with only half of the SBRs in operation, the two SBR units were treating approximately their design capacity of 1.0 MGD for the months of May, June and July. Near the end of August, the other two SBR basins were put into operation. Table 3 shows the results of the plant effluent performance during the first five full months of operation as an SBR facility. The results show that the plant was in full compliance with its BOD, TSS, ammonia, and dissolved oxygen limits between July and September of 1996.
Besides the conversion to SBRs, two other plant additions have proved beneficial to the City. A 120,000 gallon bolted steel sludge holding tank was constructed to contain and thicken the sludge that is wasted during waste sludge phases of the SBR cycles. The City of Lenoir hauls the sludge in a liquid form from the Gunpowder Creek WRF to its larger Lower Creek WRF approximately 10 miles away for dewatering and lime pasteurization. Prior to the construction of the sludge holding tank, the City was hauling liquid sludge at approximately 0.8 percent solids between three and five times a day. By allowing the sludge to thicken in the new holding tank to approximately 2 percent solids, the City will be able to significantly reduce the number of trips and, consequently, reduce their operational costs.
Another important addition to the plant is the step aeration facility located downstream of the sulfur dioxide mixing zone. The extended detention time in the post flow equalization basin and chlorine contact basin results in a low dissolved oxygen concentration in the treated wastewater by the time it reaches the dechlorination chamber. A concrete step aeration facility was constructed immediately downstream of the dechlorination facility to allow the plant to meet its minimum D.O. permit requirement of 5 mg/l. The step aeration facility includes 16 concrete steps that permit the effluent to cascade down through approximately 13 vertical feet. The facility operator has reported that the D.O. concentration has increased from 2 mg/l at the beginning of step aeration to 8 mg/l after step aeration.
One of the most important factors in evaluating the effectiveness of a project is the satisfaction of the Owner. The City of Lenoir's staff has been impressed with the quality of the effluent produced by the SBRs. The computer-based automation requires few operator adjustments and the City is planning to install a telemetry system to remotely monitor the facility from their larger wastewater treatment facility. This installation will allow the City to significantly reduce the number of hours spent by the operators at the Gunpowder Creek WRF. The City now has an expanded facility with influent flow equalization to dampen the high peak flows that have historically occurred. It has also proven its ability to produce effluent of a higher quality than required by their NPDES permit. In addition, the SBR system will allow the Gunpowder Creek WRF to achieve phosphorous and total nitrogen removal if required in the future with only minor modifications to the system.
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