Nitrogen and Phosphorus-Rich Sidestreams: Managing the Nutrient Merry-Go-Round

Many wastewater treatment plants (WWTPs) in the Midwest are facing nutrient removal for the first time, while plants in other parts of the US and the world are struggling with permits that become more stringent with each cycle. Depending on the permit, in-plant sidestreams generated during biosolids processing can greatly affect plant operations and performance, particularly when these sidestreams are recycled intermittently to the liquid treatment facility. Many facilities dewater digested solids eight hours per day, five days per week, which sends slug loads of nutrients back to the liquid treatment facilities. This introduces intermittent demands on the blowers (for nitrification) and chemical feed systems (alkalinity for nitrification, carbon for denitrification). Plants that have biological phosphorus removal and digestion (particularly anaerobic digestion) must carefully monitor sidestream phosphorus loads. Plants that “import” biosolids from other plants for regional processing are also importing nutrients within these biosolids which can become a problem. If sidestreams are an issue, WWTPs have three choices: (1) export the sidestream, (2) manage the sidestream at the liquid treatment facility, or (3) implement sidestream treatment. This paper presents two sidestream case studies: one WWTP that chose sidestream treatment, and another that chose sidestream management. Both examples involve biological phosphorus removal with anaerobic digestion and “imported” biosolids. Several sidestream management strategies (including equalization), and sidestream treatment processes are discussed.