AWWA MTC95009

Surface discharge is often the most cost effective option for disposal of membrane concentrates. Permitting of discharges to surface waters through State and Federal regulatory programs requires, among other things, testing and analysis of toxicity in the discharge. The objective of this paper is to describe a confounding factor (ionic imbalance) in toxicity testing (bioassays) which was discovered in a reverse osmosis concentrate in Jupiter, Florida. The paper also discusses techniques used to alleviate the ionic imbalance so that the concentrate could be tested for "classic" toxicants. Bioassay testing of RO concentrate conducted with the invertebrate Mysidopsis bahia (possum shrimp) showed consistent mortality during 96 hour tests. Extensive chemical analysis ruled out most typical "toxicants" in the concentrate. Analysis of ionic composition of the concentrate indicated that the major ions present were in proportions which were different from those of natural sea water. Literature review and communications with individuals in the field indicated that certain major ions must be in proportional ranges for the survival of Mysids. Through application of these principles and experimentation with addition of specific ions, the toxicity which had occurred as a result of ionic imbalance, was eliminated.