NITROGEN REMOVAL FROM DEWATERING SIDESTREAMS

The Long Island Sound (LIS) is naturally susceptible to hypoxia, a low dissolved oxygen condition. Discharge of nitrogen from wastewater treatment plants aggravates this condition by fueling the growth of planktonic algae; as algae die and decay, dissolved oxygen is depleted. Six of the fourteen water pollution control plants (WPCPs) operated by New York City (NYC) discharge to the East River. The East River is a tidal straight connected to the LIS, and thus influences the hypoxia condition in LIS.In order to meet proposed nitrogen reduction requirements, NYC is evaluating a number of alternatives for controlling nitrogen discharges. One of the options is a process that removes nitrogen from centrate. Centrate, a sidestream of sludge dewatering facilities, contains up to 1,000 mg/L NH3-N. It represents less than one percent of the total volume of flow, but contributes more than 20 percent of the nitrogen load; nitrogen removal from centrate can significantly reduce the nitrogen discharged from these facilities.The proposed process consists of separate centrate nitrification in a dedicated reactor followed by denitrification of the nitrified centrate in the first pass of the existing aeration tanks using endogenous respiration of the return activated sludge (RAS). This paper presents the findings of a one-year pilot project testing this process at NYC's Wards Island Water Pollution Control Plant. The pilot project also included testing of two types of membrane filters for solids separation from centrate mixed liquor as well as six types of aeration diffusers for centrate mixed liquor aeration.