THE NECESSITY OF ABSOLUTE RELIABILITY IN CHLORINE DISINFECTION AND DECHLORINATION SYSTEMS
Abstract:Recent enactment of endangered species habitat and classification of many watercourses flowing to the Pacific Ocean has heightened the necessity of providing redundancy and absolute reliability in wastewater effluent chlorine disinfection and dechlorination systems. Many rivers and estuaries are now classified by the Federal Fish and Wildlife Agency as habitat for Coho salmon, which is an endangered species in eastern Pacific waters and tributary streams.
Water quality jurisdictions, which include the State of California, have recently enacted and are levying mandatory fines for any incidence of a Wastewater Discharge Permit violation. Continuous monitoring of effluent residual chlorine concentration is required, and permitted limits are virtually zero or, in some instances of larger waterways, up to 0.1 mg/L. A residual chlorine violation is considered very serious and usually fines are levied considerably above the 3,000 minimum under law. The condition that compliance is based upon continuous monitoring and reporting provides no margin for slight error or control inaccuracy.
Additionally, most dischargers in these rivers and bays have maximum coliform concentrations much more stringent than federal secondary limits to provide for edible shellfish harvesting, aquaculture activities, or Indirect Potable Water Reuse. Typically, discharges must meet a total coliform concentration limit of less than a 5-day running average of 2.2/100 ml with a maximum of 23/100 ml. The discharge permits require sampling for coliform at peak daily flow conditions, when contact time is at a minimum.
Deficiencies of absolute measurement and control reliability of commercial continuous chlorine amperometric analyzers are common. Redox analyzers may not be used for final effluent chlorine concentration measurement, but frequently offer better control and much less maintenance. Air binding of hypochlorite feed systems and crystallization blockage of sodium bilsulfite feed lines are all too common, when no stoppage of chemical feed can be tolerated. Limits on ammonia concentrations for aquatic toxicity to less than 1 mg/L has lead to almost complete nitrification in wastewater treatment, which in turn often results in excessive chlorine demand as a result of variable break point conditions and insufficient disinfection strength to always attain the low total coliform limits. This is a particular problem, as the effluent coliform sampling is often of wastewater treated when flows through biological treatment units were low, and ammonia concentration is low as well as excessive nitrite and other chlorine interfering nitrogen components.
Several examples of chlorination-dechlorination difficulties and solutions to provide absolute reliability are described for two wastewater treatment facilities at Oroville (SCOR) and Yuba City discharging into California's Feather River, Willows in a tributary stream of the Sacramento River, and at Palo Alto discharging into San Francisco Bay.
Alternative disinfection systems, which include ultraviolet light irradiation, also need to have redundancy and reliability. However, where seasonal flows ranges are large, and in consideration of capital investment and localized electric energy costs, it may be expected that many wastewater systems will continue to use chemical disinfectants and dechlorinating agents.
The costs of retrofitting chlorine disinfection and dechlorination facilities are described, as well as the ongoing operation and maintenance costs. Still, these costs are a small fraction of what other waste treatment processes cost or of multiple discharge limit exceedance fines. Now there are more serious consequences for any permit violation, particularly when acute toxicity and public health are the issues.
Document Type: Research Article
Publication date: 2002-01-01
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