Washington County, Md.,’s Smithsburg Wastewater Treatment Plant (WWTP) used 6 mm influent screening, two sequencing batch reactors (SBRs), cloth filtration and ultraviolet disinfection. The plant was designed for an average daily flow of 0.33 MGD.
The Smithsburg WWTP faced inflow/infiltration problems, which resulted in routinely shortened SBR cycles. The facility also had settling issues and problems with operating above 2,500-3,000 mg/L mixed liquor suspended solids (MLSS) in the winter months. From November through April, the plant’s effluent ammonia limit is 3.9 mg/L. From May through October, the limit lowers to 2.3 mg/L. The plant normally nitrified in summer when they could operate at a lower MLSS concentration of 2,000 mg/L. However, problems always returned in the winter (water temperature of 43.7˚F/6.5˚C) when they increased the MLSS to nitrify and meet effluent ammonia limits. Without a high enough MLSS concentration, the plant could not maintain nitrification and exceeded its ammonia limits.