Performance of a continuous stirred tank reactor with immobilized denitrifiers and methanogens

Water denitrification by a coimmobilized mixed culture of denitrifiers and methanogens to simultaneously remove nitrate and excessive carbon source, methanol, in a continuous stirred tank reactor was investigated. Complete denitrification occurred at a methanol to nitrate nitrogen (M:N) ratio of greater than or equal to 2.62. Ratios beyond 2.62 methane formation took place resulting in the depletion of excess methanol. The quality of the effluent solution was found to be markedly dependent on nitrate loading. At a constant M:N ratio similar to 3.2, a complete nitrate and methanol removal could be attained when the influent nitrate concentration was <385 mg N/L; otherwise, the hydraulic retention time (HRT) was operated >2 hr (at a constant influent nitrate around 100 mg N/L). The surplus methanol was verified to be converted directly into methane when complete denitrification was achieved. Moreover, a sufficient correlation was found between the biological activity and the redox potential.