Two-stage methanotrophic bioreactor for the treatment of chlorinated organic wastewater
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A two-stage continuous methanotrophic bioreactor was developed for the treatment of wastewater contaminated by chlorinated organic solvents. The chosen design eliminated the problem of competitive inhibition during cometabolic biodegradations by separating the consumption of growth substrate (methane) and the degradation of chlorinated organics into two stages. In the first stage, a mixed methanotrophic culture was grown in a dispersed-growth continuous flow stirred tank reactor (CFSTR). In the second stage, trichloroethylene (TCE) and/or cis-1,2-dichloroethylene (cDCE) contaminated wastewater was mixed with the suspended cells from the growth reactor and fed into a plug-flow reactor (PFR) where the cometabolic degradation occurred. The mixed methanotrophic culture in the CFSTR was grown in a copper-free, iron-enriched nitrate mineral salts medium to induce methanotrophic cells to produce soluble methane monooxygenase enzymes which are highly active in cometabolic degradations. Formate and oxygen were added prior to the PFR to enhance the chlorinated organic degradation rates and capacities. A kinetic model that incorporates chlorinated organic transformation capacity and competitive inhibition was used to develop the reactor design and to predict treatment performance for TCE and/or cDCE. Model predictions were verified by comparisons with experimental data. A bench-scale two-stage reactor (with a 4-h wastewater retention time) was demonstrated to be capable of treating wastewater mixtures containing TCE (4.7 mg/l) and cDCE (4.8 mg/l) to below the maximum contaminant levels (MCLs, 5 g/l each) continuously for at least 31 d. The optimal overall material cost of methanotrophic cell growth, formate amendment, and oxygen amendment for the treatment of TCE and cDCE wastewater (5 mg/l each) to the MCLs was estimated to be $0.17 per 1000 liters of wastewater.
