An anaerobic-aerobic sequential batch process with simultaneous methanogenesis and short-cut denitrification for the treatment of marine biofoulings

Highlights: • Combined denitritation–methanogenesis was examined to treat intermittent wastes. • Achieved high NO₂⁻ accumulation of 96.4% on average in nitritation. • Achieved high substrate COD removal of 96% in denitritation–methanogenesis. • Intensive effluent exchange enhanced nitrogen removal and decreased CH₄ content. • Daily effluent exchange of 80% could maintain both effluent and biogas qualities. - Abstract: Although combination of denitritation and methanogenesis for wastewater treatment has been widely investigated, an application of this technology to solid waste treatment has been rarely studied. This study investigated an anaerobic-aerobic batch system with simultaneous denitritation-methanogenesis as an effective treatment for marine biofoulings, which is a major source of intermittently discharged organic solid wastes. Preliminary NO₂⁻-exposed sludge was inoculated to achieve stable methanogenesis process without NO₂⁻ inhibition. Both high NH₄⁺-N removal of 99.5% and high NO₂⁻-N accumulation of 96.4% were achieved on average during the nitritation step. Sufficient CH₄ recovery of 101 L-CH₄ kg-COD⁻¹ was achieved, indicating that the use of NO₂⁻-exposed sludge is effective to avoid NO₂⁻ inhibition on methanogenesis. Methanogenesis was the main COD utilization pathway when the substrate solubilization occurred actively, while denitritation was the main when solubilization was limited because of substrate shortage. The results showed a high COD removal efficiency of 96.0% and a relatively low nitrogen removal efficiency of 64.4%. Fitting equations were developed to optimize the effluent exchange ratio. The estimated results showed that the increase of effluent exchange ratio during the active solubilization period increased the nitrogen removal efficiency but decreased CH₄ content in biogas. An appropriate effluent exchange ratio with high anaerobic effluent quality below approximately 120 mg-N L⁻¹ as well as sufficient CH₄ gas quality which can be used as fuel for gas engine generator was achieved by daily effluent exchange of 80% during the first week and 5% during the subsequent 8 days.