A Tale of Two Filaments: BNR System Recovery from a Major Process Upset

Abstract

Organic overloading from an industrial source and a blower malfunction combined to cause a BNR nitrogen removal plant to self-destruct. This is a story about the Glendale WWTP at Lakeland Florida, a 13.7 mgd facility that suffered a major process upset. Unlike other activated sludge plants experiencing organic overloading, this plant resisted conventional recovery efforts. Chlorinating RAS and re-establishing a good dissolved oxygen profile in the oxic zone did not result in system recovery. Quite to the contrary, filaments continued to flourish.After three weeks of frustration and only transitory successes in improving mixed liquor settleability (SVI), nitrification was lost. Staff had to ask "What are we doing wrong?" The facility should have been well on the road to recovery but it wasn't. A sample of MLSS was sent out for analysis and the report indicated O21N predominated, with Thiothrix I also present. These two organisms, O21N and Thiothrix I, both thrive under septic conditions. Many times, the definition for septic conditions means only a lack of dissolved oxygen. Restoring the DO concentration was not working. After more investigation, the definition of septicity was found to also include high concentrations of acetic acid or volatile fatty acids (VFA). It was found that the VFA concentration in the influent to the activated sludge process was high.A two-pronged remedy was developed. A nitrate solution was fed to the anoxic zones in the activated sludge basins to use the nitrate oxygen source to consume VFA under anoxic conditions. Reducing the VFA concentration in the anoxic zone could sufficiently lower the aerated zone influent VFA concentrations to allow the conventional floc forming bacteria to out-compete the filaments for food. Chlorination of RAS was continued to kill the accumulated filaments. KEYWORDSO21N, Thiothrix I, volatile fatty acids, anoxic zones, nitrate reduction 8428 WEFTEC®.07