INVESTIGATION OF “REAPPEARING” CHLORINE RESIDUAL IN DECHLORINATED EFFLUENT AT DELTA DIABLO SANITATION DISTRICT

“Conventional wisdom” has held that dechlorination with sulfites is “complete” and “instantaneous” upon mixing of the dechlorinating agent with typical municipal wastewater treatment plant effluents. Further, it is generally believed that sulfite residuals are stable and not subject to degradation once added to the effluent. Delta Diablo Sanitation District in Antioch, CA, experienced conditions in its dechlorination sampling system in which these widely-held beliefs were found to not hold true. After studying the unusual conditions at the District, the study team came to the following conclusions as applied their special case:The reaction between sulfites and chlorine residual is not “instantaneous” for all compounds titratable as “chlorine residual”.The presence of a sulfite residual is not prima facie evidence that dechlorination is complete.Slimes in the piping system appear to be capable of consuming bisulfite residual and dissolved O2.There continues to be no evidence to indicate that dissolved sulfite residuals can have significant direct effects on effluent pH or dissolved oxygen at plant scale. The stoichiometric requirement for 6.5 parts of sodium bisulfite for reduction of one part of O2 limits the capability of dissolved O2 to be significantly affected by bisulfite.The paper documents that very low concentrations of “slowly dechlorinated” chlorine residuals may be present in the effluent. Problems with conventional analysis techniques are identified, and modified analysis techniques are described to allow for more complete evaluation of dechlorination process performance. Toxicity implications of these findings are also briefly addressed.