RETURN STREAM TREATMENT – NOT AS SIMPLE AS IT LOOKS

Water treatment plants have historically disposed of water treatment residuals through discharge to rivers or lagooning, either permanently or semi-permanently. Consequently, little thought was given to the residuals treatment processes during plant design. However, as a result of more stringent discharge limitations and decreasing land availability, mechanical dewatering systems are becoming more common, often with the objective of zero discharge plant operation.At the same time that the number of installations generating residuals return streams has grown, an increasing number of states have established guidelines that limit the turbidity in plant return streams to less than 2 NTU to minimize possible recycle of Cryptosporidium to the plant headworks. Meeting these guidelines has forced the design of water treatment plants to include evaluation of return stream quality and possible treatment process options.The number of return streams generated at a water treatment plant complicates the return stream treatment process. These return flows, which include spent filter backwash, thickening decant, and dewatering filtrate, have varying solids concentrations, chemistry, and are often generated on an intermittent basis. In addition, installations that use membrane separation technologies as part of their water treatment trains can be affected by the presence of polymer in return streams. All of these conditions present challenges to combined treatment. As a result, a variety of treatment systems have emerged for residuals return streams. These systems include processes that range from conventional clarification, thickening, and lagoon treatment to ballasted flocculation, high rate clarification, and membrane treatment.This paper investigates the state of the practice of return stream treatment implemented to meet the 2 NTU turbidity guideline. Since the selection of return stream treatment systems is based on a number of factors, including the type and operation of the raw water treatment processes, space availability, and plant staff preferences, different treatment systems are discussed, along with discussion of the selection, design criteria, and available pilot data from a high rate clarification process.