Treating Water Plant Residuals at a Wastewater Treatment Plant – The WSSC Experience

The Parkway Plant located in Laurel, Maryland was designed to handle mostly domestic wastewater from the area. It is owned and operated by the Washington Suburban Sanitary Commission – WSSC. Typical plant layout is shown in Figure I. The Parkway Wastewater Treatment Plant, a 7.5 mgd design capacity Biological Nutrient Removal (BNR) system, presently processing 6.0 million gallons of wastewater daily, has been effectively treating water plant sludges for twenty-four years.The Patuxent Water Filtration Plant, in Laurel, Maryland, is also owned and operated by the WSSC providing quality drinking water to its customers in the Montgomery and Prince George County areas. Prior to October 1, 1975, the water plants residuals were discharged into the Walker Branch of the Patuxent River. The Maryland Water Resources Administration indicated that the issuance of an NPDES Permit would restrict discharge of this residual to the stream. In order to comply with this requirement, WSSC undertook a study to determine the best method of disposal. One option was to discharge the residual to the Parkway Wastewater Treatment Plant for treatment. In anticipation of this requirement, laboratory investigations were begun in November of 1974 to determine the effects of the water plant's alum residual on the Parkway WWTP. The preliminary study showed that this residual would significantly increase the suspended solids in the primary clarifier effluent. This situation does not necessarily mean that the effect will be detrimental to the entire treatment process; however, at that time, there was some suspicion that it might be. Therefore, further and more comprehensive studies were needed to establish what effect the Patuxent residual would cause at Parkway.The Maryland Water Resources Administration then issued an NPDES permit for the water plant which directed that sludge discharges into the Walker Branch be discontinued after October l, 1975. Construction was undertaken to tie in the discharge line from the residual holding basins to a sanitary sewer leading to the Parkway Plant. This was found to be the most economical and expedient method of complying with the discharge permit requirements. The plant was to discharge its alum sludge to the sewer and allow it to mix with the wastewater in the line. The time of concentration from the water plant to the wastewater plant was determined at that time to be approximately 5 hours. Currently that time is estimated to be 3 to 4 hours with increases in daily flows. Upon reaching Parkway, the residuals would be handled and removed in the typical operating scheme of the plant. Since most of the residuals coming from the water plant would be composed of inorganics, they should be captured in the primaries. They would then be sent to the thickeners along with the primary and waste activated sludge for further processing. It was anticipated that 75,000 gallons per day of alum sludge with 6,900 pounds of dry solids – approximately one percent – would have to be treated. No additional modifications to the Parkway facility were planned. This was slightly off the mark. Check this data.