Nitrogen and DOC removal from wastewater streams of the metal-working industry

In pilot plant studies, permeates of ultrafiltration of boron- and ethanolamine-containing coolants were used as a carbon source for denitrification of NO2–N and NO3–N in the wastewater from electrochemical deburring after metal ion precipitation. Treatment of the two wastewater streams in a cascade of submerged fixed bed reactors at increasing pH-values revealed that a more alkaline pH resulted in a lower denitrification rate and an increasing nitrite concentration in the effluent. The DOC-removal efficiency was about 70% for pH values of 7.8–9. It was not possible to establish a nitrification. The biomass in the treated wastewater was fine-particulate and could hardly be separated. For evaluation of the maximal biodegradability of the DOC of the permeates in batch experiments, the long-term BOD instead of the BOD5 was more suitable. Results of BOD21-determinations corresponded with the DOC-removal efficiency of the immobilized population in the pilot plant at HRTs ranging from 3.9–13.5 d. Whereas the nitrate content of the EC-wastewater could be reduced completely, even at high loading rates, about 30% of the COD of the permeates remained undegraded. A COD of <300 mg O2/l could only be obtained after chemical oxidation of the effluent by UV-activated H2O2. Production-related variations of the boron- and DOC-concentrations in the permeates of ultrafiltration of cooling lubricants apparently prevented the establishment of a nitrification of ammonia, that was released during denitrification with ethanolamines as a carbon source. Only when both wastewater streams were co-treated with a, only slightly polluted third, non-toxic wastewater stream of the factory, elimination of nitrate and ammonia from EC-wastewater and permeate, respectively, was successful.