NITRITE REDUCTION EVALUATION FOR THE MIAMI-DADE WASD SOUTH DISTRICT WASTEWATER TREATMENT PLANT

The Miami-Dade Water and Sewer Department (Department) operates the 112.5 mgd average daily flow South District Wastewater Treatment Plant (SDWWTP) which utilizes the conventional pure oxygen activated sludge treatment process followed by deep well disposal to comply with cBOD5 of 20 mg/L and TSS of 20 mg/L permit limits. The SDWWTP consists of two plants operating in parallel see Figure 1. Plant 1 (75 mgd) consists of pretreatment facilities followed by four oxygenation reactors and four secondary clarifiers. Plant 2 (37.5 mgd) consists of pretreatment facilities, two oxygenation reactors and two secondary clarifiers.A Consent Order requires that the SDWWTP effluent meet both FDEP high level disinfection standards and EPA primary drinking water standards including a nitrite maximum contaminate limit (MCL) of 1.0 mg/L; which had occasional exceedences. To control the effluent nitrate concentrations, the historical data received, source control of nitrite and/or nitrifying bacteria, and operational modifications were investigated. Potential sources of nitrifying bacteria included treated leachate and hauled aerobically digested sludges. Source control options were reviewed. Operational controls evaluated included reducing the sludge age, chlorination to oxidize the nitrite and formation of an anoxic zone. The anoxic zone approach was selected and implemented with six months of successful operation.