Coliform bacteria and nitrogen fixation in pulp and paper mill effluent treatment systems

The majority of pulp and paper mills now biotreat their combined effluents using activated sludge, On the assumption that their wood-based efflueuts h ave negligible fixed N, and that activated-sludge microorganisms will not f ix significant N, these mills routinely spend large amounts adding ammonia or urea to their aeration tanks (bioreactors) to permit normal biomass grow th. N-2 fixation in seven Eastern Canadian pulp and paper mill effluent tre atment systems was analyzed using acetylene reduction assays, quantitative nitrogenase (nifH) gene probing, and bacterial isolations. In situ N-2 fixa tion was undetectable in all seven bioreactors but was present in six assoc iated primary clarifiers. One primary clarifier was studied in greater deta il, Approximately 50% of ail culturable cells in the clarifier contained ni fH, of which >90% were Klebsiella strains, All primary-clarifier coliform b acteria growing on MacConkey agar were identified as klebsiellas, and all t hose probed contained nifH. In contrast, analysis of 48 random coliform iso lates from other mill water system locations showed that only 24 (50%) poss essed the nifH gene, and only 13 (27%) showed inducible N-2-fixing activity . Thus, all the pulp and paper mill primary clarifiers tested appeared to b e sites of active N-2 fixation (0.87 to 4.90 mg of N liter(-1) day(-1)) and a microbial community strongly biased toward this activity. This may also explain why coliform bacteria, especially klebsiellas, are indigenous in pu lp and paper mill water systems.