It was hypothesized that nutrient removal from wastewater could be achieved by using methane oxidizing bacteria (methanotrophs). Because methane is inexpensive. it can be used as an energy source to encourage bacterial growth to assimilate nitrogen and phosphorus and other trace elements. This initial feasibility study used synthetic nutrient mixtures and secondary sewage effluent as feed to a laboratory-scale methanotrophic attached-film expanded bed (MAFEB) reactor operated at 35°C and 20°C. The MAFEB system operated successfully at low nutrient concentrations under a variety of nutrient-limited conditions. Using a synthetic nutrient mixture with a nitrogen:phosphorus feed ratio (w/w) of 9:1, phosphate concentrations were reduced from 1.3 mg P/ L to below 0.1 mg P/ L, and ammonia was reduced from 12 mg N/L to approximately 1 mg N/L on a continuous flow basis, with a bed hydraulic retention time of 4.8 hours. The average nutrient uptake rates from synthetic nutrient mixtures were 100 mg nitrogen and 10 mg phosphorus/L of expanded bed/d. Nutrient assimilation rates increased with increasing growth rate and with increasing temperature. Nitrogen/phosphorus uptake ratios varied from 8 to 13, and the observed yield varied from 0.11 to 0.16 g volatile solids (VS)/g chemical oxygen demand (COD). Nutrient removal from secondary sewage effluent was successfully demonstrated using sewage effluent from two local treatment plants. Nutrient concentrations of 10-15 mg N/L and 1.0-1.8 mg P/L were reduced consistently below 1 mg N/L and 0.1 mg P/L. No supplemental nutrients were added to the sewage to attain these removal efficiencies since the nutrient mass ratios were similar to that required by the methanotrophs. Removal rates were lower at 20°C than at 35°C, but high removal efficiencies were maintained at both temperatures. Effluent suspended solids concentrations ranged from 8 to 30 mg volatile suspended solids (VSS)/L, and the effluent soluble COD concentration averaged 30 mg/L.
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