BIOLOGICAL NUTRIENT REMOVAL PROCESS ENHANCEMENT ANALYSIS OF THE MARSHALL STREET ADVANCED POLLUTION CONTROL FACILITY, CLEARWATER, FLORIDA

The City of Clearwater's Marshall Street Advanced Pollution Control Facility (APCF) utilizes a five-stage Bardenpho© process to achieve biological nutrient removal (BNR). This facility a permitted treatment capacity of 10.0 million gallon per day (MGD), and discharges excess reclaimed water to Stevenson's Creek, a tidal creek to Clearwater Harbor, a Class III Marine Water. Surface water discharge requirements are 5.0 mg/L CBOD5, 5.0 mg/L total suspended solids (TSS), 3.0 mg/L total nitrogen (TN), and 1.0 mg/L total phosphorus (TP) on an annual average daily basis. The five-stage Bardenpho© process is known to achieve excellent biological nitrogen removal and moderate biological phosphorus removal. The City's Marshall Street APCF has a history of exceeding the effluent TN limit of 3.0 mg/L. In addition, the APCF must rely on chemical precipitation with alum and iron salts to comply with the TP effluent limit of 1.0 mg/L. The purpose of this project was to identify performance limiting factors at the Marshall Street APCF, and develop recommended improvements. An area of concern is the low organic strength of the influent. The lack of sufficient amount of readily degradable organic substrate negatively affects denitrification. Additional reduction in CBOD5 through the primary clarifiers compounds this problem. Effluent TN is mainly in the form of nitrate-N, suggesting that insufficient denitrification is occurring. In addition, the mixed liquor recycle (MLR) system was designed to cascade through an elevated Parshall flume, stepped channels, and through a channel and elevated port openings to discharge into and above the water surface of the first anoxic basins. The excessive mixing and agitation of the MLR flow entrains air and provides dissolved oxygen (DO) to the anoxic basins. To assess the impacts identified, a Biowin® Model of the APCF's biological processes was constructed and calibrated. The model simulations suggest that the APCF's inability to remove nitrogen is restricted by the low influent organic strength, limiting substrate to drive the denitrification process. In addition, the extraneous oxygen input from the MLR flow diminishes denitrification occurring in the anoxic basins. The recommended physical and operational modifications include the following: conversion of the fermentation basins to first anoxic basins by redirecting the MLR flows to the fermentation basin; eliminate the oxygen input from the MLR flows; and conduct interim operational improvements prior to the upgrades including maintaining low residual DO concentrations in the first of several aerobic basins in series to promote denitrification.