Toxicity screening tests using the Reserve Electron Transfer (RET) and Electron Transfer (ETr) assays were performed with five wastewater samples amended with trickling filter (TF) or activated sludge (AS) biomass. In the case of untreated samples, Home Life domestic wastewater (HLD/W) showed the lowest inhibition, followed by domestic sewage (DS), hospital wastewater (H/W), East Straus wastewater (ES/W), and Mills wastewater (M/W) from both ETr and RET assays. After 12 hours of treatment at 20°C, DS with AS biomass had the lowest % inhibition from the RET assay, followed by DS with TF, HLD/W with TF, H/W with TF, M/W with TF, and M/W with AS. AS biomass reduced more toxicity from DS than TF biomass whereas acclimated TF biomass reduced significantly more toxicity than AS biomass, indicating the importance of acclimation. M/W was most toxic and resistant to biodegradation among six wastewater samples. No nitrification occurred with M/W and ES/W. While there was significant nitrification with DS treated by AS biomass, little nitrification by TF biomass occurred even with DS, HLD/W, and H/W. It appears that nitrification is significantly inhibited by M/W and ES/W even when mixed with domestic sewage. It appears that there is a strong relationship between the TIC/TC ratio and % inhibition.