Solving a Big Disinfection Problem at a Small Water Reclamation Plant

Abstract

The 4.5 MGD Yucaipa Valley Water District Water Reclamation Plant was experiencing chronic difficulties in meeting a stringent coliform limit, despite high chlorine doses (up to 35 mg/l), 10-15 mg/l residuals, and low effluent turbidity (<2NTU). NPDES permit limits for ammonianitrogen and Total Coliform are 5mg/l (mo.avg.) and 2.2 MPN/100ml, respectively. Historically, breakpoint (free) chlorination was practiced, but consistently disinfecting the fully nitrified effluent proved to be unsuccessful. Ongoing attempts to solve the issue, including the implementation of chloramination in lieu of free chlorination, resulted in significantly improved disinfection results, but chronic coliform violations, typically 4, 6, and 8 MPN, continued to be common. Thus, the District assembled a 'brainstorming" team, consisting of plant personnel and a consultant, to evaluate the issues.Despite continuing low turbidity readings (<2NTU) the team surmised that the disinfection issues may be related to the presence of large size particles, and began an investigation of particle size distribution (PSD) of solids in the chlorine contact tank. Test results showed that a significant reduction of large particles (>30 mkm) was occurring through the chlorine contact tanks, indicating that solids were settling in the contact tanks. Concurrently, particle testing conducted at the start of tertiary filter backwashes indicated that large particles from the bottom of the contact tanks were "stirred up" by backwash supply pumping. Interestingly enough, poor coliform results often coincided with the occurrence of tertiary filter backwashes. The team postulated that the occurrence of large particle solids settling in the contact tanks and the subsequent "stirring up" of these solids during tertiary filter backwashes could be a significant cause of inadequate disinfection.The team hypothesized that an increase of the number of small particles in the filter effluent may be a consequence of biological growth within the gravity filters. To prevent biological growth and to reduce the number of small particles, the injection point of ammonia for chloramination was relocated from the filter influent to the filter effluent. Subsequent testing revealed that the number of small particles leaving the filters no longer exceeded the number of particles entering the filters, and accumulation of large particles in the chlorine contact tank was significantly reduced. To date, effluent turbidities consistently average 0.5 NTU, and consistent coliform compliance has been met.