ACUTE TOXICITY OF AMMONIA (NH3-N) IN SEWAGE EFFLUENT TO CHIRONOMUS-RIPARIUS .2. USING A GENERALIZED LINEAR-MODEL

Toxicity of un-ionized ammonia (NH3-N) to the midge, Chironomus ripari us was compared, using laboratory culture (well) water and sewage effl uent (approximate to 0.4 mg/L NH3-N) in two 96-h, static-renewal toxic ity experiments. A generalized linear model was used for data analysis . For the first and second experiments, respectively, LC(50) values we re 9.4 mg/L (Test 1A) and 6.6 mg/L (Test 2A) for ammonia in well water , and 7.8 mg/L (Test 1B) and 4.1 mg/L (Test 2B) for ammonia in sewage effluent. Slopes of dose-response curves for Tests 1A and 2A were equa l, but mortality occurred at lower NH3-N concentrations in Test 2A (un equal intercepts). Response of C. riparius to NH3 in effluent was not consistent; dose-response curves for tests 1B and 2B differed in slope and intercept. Nevertheless, C. riparius was more sensitive to ammoni a in effluent than in well water in both experiments, indicating a syn ergistic effect of ammonia in sewage effluent. These results demonstra te the advantages of analyzing the organisms entire range of response, as opposed to generating LC(50) values, which represent only one poin t on the dose-response curve.