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.

The impact of sidestreams on liquid treatment processes varies with the source of the sidestream. One process that can result in nutrient-rich sidestreams is anaerobic digestion (or similar anaerobic processes). Anaerobic digestion releases nutrients that are returned to the liquid stream in the filtrate/centrate from dewatering processes. For example, ammonia concentrations in dewatering sidestreams can range from 900 to 1,500 mg/L as nitrogen (N) or more, which can increase the ammonia concentration in the plant influent by 3 to 5 mg/L on an average day basis. If such a sidestream is returned over one shift (8 hours), it can have a profound impact on oxygen uptake and aeration system design. It can also result in poorly settling mixed liquor suspended solids (MLSS).

At many plants, phosphorus returned in the sidestream has not been an issue. However, in plants using biological phosphorus removal (Bio-P), the return of phosphorus released in the anaerobic digester can stress the Bio-P process. Biological phosphorus removal concentrates the majority of the plant influent phosphorus in the biosolids, unlike the biosolids generated at carbon or nitrogen removal plants. During anaerobic digestion, approximately 60 percent of the stored total phosphorus in the feed sludge is released as ortho-phosphorus. If a regional plant processes biosolids from multiple Bio-P plants, the phosphorus that is released and returned to the liquid treatment facility may double or even triple the phosphorus load entering in the raw influent. In such situations, sidestream treatment may be required.

While nutrient-rich sidestreams are normally a concern only with anaerobic digestion, nitrates generated during aerobic digestion can also have an impact on plants for which total nitrogen (TN) limits have been established in their National Pollutant Discharge and Elimination System (NPDES) permits. Normally, the nitrates are not a concern unless the load is unusually high (from a regional biosolids facility), or the dewatering schedule is limited to only a few shifts per week. Unless the flow is equalized, the centrate/filtrate generated by intermittent dewatering can “shock load” the liquid treatment facility.

If nitrogen and phosphorus in sidestreams are a concern, there are a number of ways to treat the sidestreams before returning them to the liquid treatment facility. If phosphorus is the only concern, metal salts can be added directly to the digester, to the digested solids prior to dewatering, or to a separate sidestream treatment flocculation basin followed by a small scale clarifier to remove the precipitated solids. If nitrogen is the primary concern, small-scale activated sludge facilities can be designed to nitrify and/or denitrify the sidestreams. Carbon and alkalinity supplementation is typically required for total nitrogen removal. Several proprietary processes are available to remove nitrogen, including the SHARON®, InNitri®, BABE®, and ANAMMOX® processes.

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