NY town chooses the FAST option
After receiving four state discharge violation notices following rainstorms, the town of Dexter decided to incorporate Smith & Loveless' FAST® system into its WwTP writes James A.Bell, Vice President of Technical Services for Smith and Loveless.
Dexter's WwTP is divided into two FAST treatment trains, complete with Kraus-Fall Clarifiers
The Village of Dexter, (pop.1500) located 120 km north of Syracuse, NY on the Black River, has been plagued with sewer collection system infiltration and inflow (I/I) since the sewer network was completed in 1927.
The previous extended aeration (CMAS) WwT facility often experienced overflow bypass and solids washouts during wet weather periods. A sewer rehabilitation program was not successful in reducing the I/I to an acceptable level. Between 1981 and 1990, four consent orders were issued by the DEC to bring the system into compliance.
Therefore, consulting engineers Bernier, Carr & Associates chose the submerged fixed film aeration process because of its ability to maintain bacterial growth during I/I events. The plant upgrade also included retrofitting some of the existing tanks for the aerated sludge holding and the elimination of a raw sewage overflow bypass pipe. The group also included a peripheral feed clarifier system capable of handling the high clarifier surface overflow rates that occurred during the I/I events.
The FAST® process, manufactured by Smith & Loveless, combines attached bacterial growth on the surface of a submerged media with suspended growth. High surface area media is installed in a submerged condition within the aeration zone. Coarse bubble aeration diffusers provide the oxygen transfer to the wastewater and provide the pumping of the wastewater through the submerged media. Excess biological solids are transported into the separate clarification system.
During normal operation, solids transported to the clarifiers are wasted to the sludge holding tank. Because a consistently large biological population is retained within the aeration zone, variations in loadings do not require the operator to adjust the mixed liquor suspended solids concentration. In fact, returning sludge to the aeration zone is not required for normal operation.
The real advantage of the FAST® process is its ability to maintain the bacterial population during extreme I/I flows.
During periods of low flow, all of the wastewater is directed to one unit to reduce the time sludge is held in the clarifier. Sludge pumped from these clarifiers can be wasted to an aerobic digester or returned to the splitter box. A drain valve was provided to allow one clarifier to be decommissioned during times of low flow. The contents of the clarifier can be drained into the return activated sludge pumping system for return to the head of the treatment plant or to be wasted to the aerobic digester.
The new WwTP was started-up in the summer of 1992. In order to evaluate its performance, a comparison was made of the five years preceding 1992 and the five years following 1992.
The comparative data includes the average monthly flow and the maximum daily flow for each month. The operating procedure prior to the system upgrade was to bypass flow in excess of the design flow directly into the Black River.
A review of the BOD data for the average monthly influent and effluent from the period of 1987 through 1991 indicates that in many cases the effluent BODs are quite similar to the influent BODs demonstrating poor performance of the WwT system. In addition to the bypassing of wastewater, the treatment system was also providing higher BOD concentrations following I/I events as a result of the effects of the loss of biological solids during high flows.
The data for the average monthly influent and effluent total suspended solids from 1987 through 1991 shows that in some cases the effluent suspended solids are actually higher than the influent suspended solids indicating the washout of the biological solids.
Data for 1992 is not included in this report because of the questionable data recorded during the time of the new system's construction and start-up. However, the new system demonstrated a rapid start-up, providing effluent data far exceeding the permit limitations within one month after the commissioning of the system.
Figures for maximum daily flow show the new wastewater system's capabilities in providing continuous treatment even during extreme I/I events. The highest average monthly flow occurred in April 1993 when the new wastewater system treated 3750 m3/d.The highest maximum daily flow occurred in January 1996 when the flow exceeded 7080 m3/d. In February 1994 the Village of Dexter plugged the bypass system, allowing all the I/I to be treated by the WwTP.
A review of the influent and effluent BOD data from 1993 to 1997 indicates that the effluent BOD from the WwT system never exceeded the 30 mg/l effluent limitation.
A review of the data for total suspended solids measured in the influent and effluent from 1993 through 1997 indicates the total suspended solids in the effluent never exceeded 15 mg/l.
These results demonstrate the effectiveness of the FAST® process combined with the Kraus-Fall clarifier in maintaining high effluent qualities over varying flow conditions and in maintaining the biological solids within the wastewater treatment system during I/I events.
The efforts of Bernier, Carr & Associates and Smith & Loveless provided the Village of Dexter with a wastewater system that would handle any I/I event. This provided the Village of Dexter with significant savings compared to the expense of repairing its I/I problems caused by the old sewer system. As a result, a small community faced with solving an I/I problem should consider and compare the cost of designing a new wastewater treatment plant with the cost of replacing an aged system.