Development of a Site Specific Toxicity-Based Operating Guideline for Nitrite Nitrogen in a Petroleum Refinery Wastewater Discharge

96-hour static renewal and 96-hour flow-through bioassays were performed to measure the acute toxicity of nitrite nitrogen (NO2-N) to juvenile rainbow trout in biologically treated process wastewater discharged at a California petroleum refinery. In four static renewal tests using spiked effluent samples, the NO2-N concentration determined to be lethal to 50% of the test organisms (LC50) ranged between 70.1 milligrams per liter (mg/L) and 123.8 mg/L at a median chloride (Cl−) concentration of 410 mg/L. The median LC50 was 113.3 mg/L NO2-N. Fourteen flow-through bioassays indicated no significant mortality when juvenile rainbow trout were exposed to approximately 55 mg/L or less NO2-N and background Cl−-averaged 450-500 mg/L.This work demonstrated that NO2-N in the treated refinery wastewater is much less toxic to rainbow trout than would be expected based on LC50 data reported in the open literature. Others have shown a linear relationship between NO2-N lethality to rainbow trout and Cl− concentration, though the previously published data were obtained using dilution water containing substantially less Cl− than the refinery effluent. The data presented in this paper support the hypothesis that Cl− at typical petroleum refinery discharge concentrations suppresses NO2-N toxicity to juvenile rainbow trout. In the absence of other toxicants, NO2-N in the range of 50-60 mg/L should not result in failure of a flow-through bioassay test when Cl− is >350 mg/L. A self-imposed process control guideline of 30 mg/L NO2-N is recommended to provide an ample margin of operating safety for the refinery discharge.