EPA TECHNOLOGY TRANSFER EFFICIENT PREPARED BY TREATMENT OF U.S. SMALL MUNICIPAL ENVIRONMENTAL FLOWS AT PROTECTION DAWSON.MINN AGENCY ------- ------- Increasingly rigid water pollution control stan- dards require, in many cases, higher degrees of sewage treatment than often provided by conven- tional processes. The performance of many small sewage treatment plants is frequently degraded by the discharge of large quantities of solids in the plant effluent. The solids discharges may be caused by improper plant operation, surges of raw sewage flow, or mechanical failures within the plant. These problems are particularly common to small plants which often receive limited operator attention and which are frequently subjected to severe flow varia- tions. A survey in 1960 of extended aeration plants throughout the U.S. showed that they removed an average of 86 percent of the BOD but only 62 percent of the suspended solids found in the raw sewage. There are also an increasing number of cases where nitrification or nitrogen removal is required. The Dawson, Minnesota, project was placed in operation in October, 1974 and has demonstrated the ability of a small plant (260,000 gpd capacity) to provide highly reliable and efficient removal of BOD suspended solids, and consistent nitrification. Under controlled conditions, nitrogen removals of up to 90% were also attained without the use of any chemicals. These desirable goals were achieved at reasonable costs and without any extraordinary operator attention. The Dawson project has demon- strated that small plants can meet very stringent effluent goals (BOD and suspended solids less than 5 mg/l, ammonia concentrations of 0.1 mg/l) with- out the use of expensive chemical treatment processes. ------- Figures 1 and 2 illustrate the process. Following screening, the raw wastewater from Dawson is pumped to an aeration channel or "oxidation ditch." The process is a modification of the activated sludge process. Aeration is provided by two floating aera- tors which also keep the liquid moving around the aeration channel at a velocity high enough to pre- vent deposition of the solids. The aeration channel at Dawson is designed so that the depth of mixed liquor can be varied from 3 feet to 4.1 feet, provid- ing 83,000 gallons of surge storage capacity within the aeration channel. At design flows, the aeration time in the channel is 17.7 to 25.4 hours, depending upon the depth. Under the actual flows received, aeration times averaged 35 hours during a 300-day test period. The mixed liquor flows from the chan- nel to a chamber which controls the flow to the downstream clarifiers. By use of the storage volume in the aeration channel, peak flows were reduced about 31%, stabilizing the operation of the secondary clarifiers. The mixed liquor flows to two down- stream clarifiers in series. Design overflow rates are 580 and 400 gpd/sq ft for clarifiers 1 and 2 respectively. Provisions were made to feed chemi- cals and provide flocculation between the clarifiers to assure the stringent effluent suspended solids goal of 5 mg/l would be met. It has not proven neces- sary to feed chemicals. Chlorination is provided prior to discharge. Excess sludge is trucked to nearby farmland where it is spread. /deration Channel ------- RAW SEWAGE SUPERNATANT RETURN WASTE SLUDGE INFLUENT MANHOLE TRUCK LOADING RETURN SLUDGE CLARIFIER #7 FLOCCULATOR CLARIFIER #2 CHLORINATION EFFLUENT Figure 7. Process Flow Sheet ------- INDICATES SAMPLING OR TESTING POINT FLOATING AERATOR NO. 2 FLOATING AERATOR NO. 1 SUBMERGENCE ADJUSTMENT MECHANISM SLUDGE PUMP NO. 2 SLUDGE PUMP NO. 1 CLARIFIER NO. 1 CLARIFIER NO. 2 EFFLUENT MANHOLE CHLOKINt CONTACT TANK CHEMICAL TRANSFCR MANHOLC CHEMICAL STORAGE Figure 2. Dawson Treatment Plant ------- The performance of the Dawson plant over the first 300 days of operation may be summarized as follows: BOD, mg/l Influent Effluent Suspended Solids, mg/l Influent Effluent Average 155 3 200 8 60-250 1-7 70-1621 1-30 If only 2 days of the suspended solids data are eliminated, the average suspended solids content of the final effluent becomes 5 mg/l. Throughout the entire study, essentially complete nitrification was achieved as ammonia concentrations in the effluent were generally 0.1 mg/l or less. Nitrifica- tion was achieved even during severely cold weather when water temperatures in the aeration channel were very near 0°C. Temperatures were below 6°C for the first 100 of the 300-day study period. Sludge ages were always 20 days or more. It was found that denitrification could be achieved by careful control of aeration so that dissolved oxygen levels went to zero before the mixed liquor completed traveling around the aeration channel to the aerator. Over a 65-day period when such control was practiced, nitrogen removals of 60-80% were typically achieved with some values as high as 90%. The wastewater itself provided the carbon source for the denitrify- ing organisms so that no additions of methanol or other carbon sources were required. It was found that the plant could be operated for as long as 90 days without the need for sludge wast- ing by allowing the mixed liquor solids to build up to 10,000 mg/l before wasting sludge. The mixed liquor solids readily settled to solids concentrations of 2.5 to 3.5%. The high degree of treatment noted above has been consistently achieved without any extra- ordinary operator skills or attention. About 4 to 6 manhours per day are spent at the plant. A typical day in the life of a plant operator at Dawson includes the following duties: 1. Read influent flow meter. 2. Clean bar screen. 3. Skim settling tanks. 4. Make a visual check of all equipment to make sure it is operating. 5. Measure temperature of air, channel and settling tank contents. 6. Collect necessary samples for analysis (pri- marily raw sewage, MLSS and effluent). 7. Check DO of channel. 8. Check chlorine residual and set feed rate. 9. Wash down the walkways and building walls. 10. Sludge hauling (when required). 11. Cutting the grass or shoveling the snow as necessary. ------- Floating Aerators ------- Table 1 presents the construction costs of the Dawson plant both at the time of the actual bid (1971) and at an EPA sewage treatment plant index reflecting second quarter, 1976 cost levels. Operation and maintenance costs are given in Table 2. Power costs, of course, are related pri- marily to the electric motors that drive the various items of process equipment. The total connected horsepower is 53 hp, not all of which is in service continuously. Heating costs relate to an oil fired furnace and hot air blower system within the con- trol building and laboratory. This is the only struc- ture on the plant site that is equipped with a per- manent heating system. Due to freezing problems in the clarifier building, portable heater units are used at certain times during the winter months. This cost is included under miscellaneous supplies and replacement parts. Miscellaneous supplies and replacement parts included such items as a space heater for providing temporary heat in the clarifier building, three furnace repairs, replacement of a hydraulic hose on the aerators, grease and oil, clean- ing supplies, lawn mower parts and similar items. There were no major repairs necessary to the process equipment items. The transportation item reflects the cost of disposing of sludge on nearby fields. The sludge is disposed of in liquid form as weather conditions permit. ------- Table 1 DAWSO4N CONSTRUCTION COSTS Approximate Unit Process or Item Costs Raw Sewage Pumps and Related Items Channel Aerators and Related Items Clarifiers and Housing Return Sludge Pumping System Control Building and Laboratory Other (Chlorination, Chemical Feed System) ^^ Bid Date November 30, 1971 Estimated Costs @EPA STP Index = 166 @ EPA STP Index = 255 $ 18,000 100,000 65,000 12,000 43,000 12,000 $250,000 Table 2 $ 28,000 153,000 100,000 18,000 66,000 18,000 $383,000 DAWSON OPERATION AND MAINTENANCE COSTS a ** ~" ir* Raw Cost Sewage Channel & Item Pumping Aerators Power 0.5 3.6 Heating Costs Chlorine Misc. Supplies & Replacement Parts Labor 0.5 1.5 Transportation Subtotals 1.0 5.1 Return Control Clarifier Clarifier Sludge Bldg. & No. 1 No. 2 Pumping Laboratory 0.1 0.1 1.2 0.5 0.7 2.0 1.0 1.0 1.2 6.0 1.1 1.1 2.4 9.2 Other Totals 0.2 6. 0. 1.0 1. 0.6 2. 2.3 13. 0.2 0. 4.3 24. 2 7 0 ,6 5 2 2 a Average flow of 174,650 gallons per day Cents/1,000 gallons treated ------- ------- The technology demonstrated at Dawson provides a reliable, flexible, and highly efficient treatment approach applicable to many small treatment plants. This is particularly significant since about 70% of the municipal plants in the United States are less than 1 mgd in capacity. The high degrees of nitrification and of removal of suspended solids and BOD achieved without the need for chemical treat- ment techniques may make the approach cost- effective in many water quality limited areas. The simple, stable sludge handling aspects also con- tribute to the attractiveness of the process for small plants. The potential for nitrogen removal demon- strated at Dawson, although not yet fully defined, may make the process attractive for application to instances where nitrogen removal is required. For Further Information: Detailed information on this project is available in the form of a Final Report. This Report can be obtained by writing: Benjamin Lykins, Jr. U.S. EPA-MERL 26 W. St. Clair Cincinnati, Ohio 45268 ------- |