Sublethal Toxicity Identification Evaluation Studies – Potential Impacts of conductive Dissolved Solids


Treated effluent from the Firmenich wastewater treatment plant in Plainsboro, New Jersey (two staged activated sludge process designed for nitrogen and phosphorus removal) is discharged to the Millstone River. The New Jersey Department of Environmental Protection (NJDEP) permit requires a 96 hour acute toxicity test with Pimephales promelas (fathead minnows), with an LC50 value of 50%. The permit also requires seven day chronic toxicity testing with Ceriodaphnia dubia (water fleas) and Pimephales promelas. These requirements are narrative criteria for testing and reporting IC25 values for survival, growth, and reproduction. Firmenich has successfully passed all of the acute toxicity testing requirements with Pimephales promelas. The chronic toxicity tests have determined that Ceriodaphnia dubia is the most sensitive species, with the most sensitive endpoint being reproduction versus survival. The existing permit requires reporting of the chronic toxicity IC25 values. Even though no IC25 limits have been established in the existing permit, the NJDEP recommended that Firmenich determine the cause/source of chronic sublethal toxicity.

The scope of services performed was to determine the cause/source of sublethal toxicity and develop a recommended solution including the following:
• Process engineering and functional operational evaluation of the wastewater treatment plant
• Toxicity screening tests with Ceriodaphnia dubia
• Toxicity Identification Evaluation (TIE) and Modified TIEs
• Specific chemical analyses (organics and inorganics)
• Whole effluent and mock TDS studies
The results of all of these studies are summarized in this paper along with the data and detailed data analyses.

The sublethal toxicity was determined to be water quality related due to residual organic compounds and inorganic compounds (conductive dissolved solids) that cannot be successfully treated/removed by the conventional treatment technologies employed at the Firmenich wastewater treatment plant. The complex nature of the Firmenich wastewater causes apparent synergistic interactions with the organics and inorganics in the treated effluent. Due to the complex nature of the Firmenich wastewater and the fact that the existing treatment plant cannot reliably remove/control the sublethal toxicity to the Ceriodaphnia dubia, a modification/upgrade of the treatment plant would be required to control/eliminate the potentially toxic constituents in the treated effluent. The use of membrane technology, specifically reverse osmosis (RO), for effluent polishing treatment appeared to be the best technical alternative for resolving the
effluent toxicity to Ceriodaphnia dubia. The best technical approach to accomplish this objective appears to be to convert the existing biological system to a membrane biological reactor (MBR) treatment system using ultrafiltration (UF) membranes. Therefore, it was recommended that Firmenich pursue detailed investigations for modifying/upgrading the treatment plant to a MBR system using UF membranes for solid/liquid separation followed by RO membranes for removing the residual organic and inorganic constituents causing potential toxicity in the biologically treated effluent (if future permit requirements dictate sublethal criteria).

Sublethal toxicity, Ceriodaphnia dubia, conductive dissolved solids, toxicity identification evaluation, toxicity reduction evaluation

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