Denitrification Reactor Design

A denitrification reactor in RAS is an anoxic vessel where heterotrophic bacteria reduce dissolved nitrate to nitrogen gas, using a soluble carbon source such as methanol, ethanol, glycerol, or reactor-grown biofloc as electron donor. Design centers on three variables: empty-bed hydraulic retention time of 1–4 hours, a carbon-to-nitrate dosing ratio of roughly 2.5–3 g COD per g NO3-N removed (with online nitrate or ORP feedback to avoid under- or over-dosing), and dissolved oxygen held below 0.3–0.5 mg/L so the biofilm's outer layer consumes any incoming oxygen while the inner layer stays anoxic. Sulfur-based autotrophic denitrification suits sites wanting to avoid carbon handling. The process removes about 4.5 g alkalinity per g nitrate-N, so pH buffering must be planned. Positioned after nitrification, the reactor converts the daily nitrate load (roughly 0.3–0.5 kg NO3-N per 100 kg feed) into harmless N2, cutting make-up water demand dramatically — often from 10% to under 1% of system volume per day — and shrinking effluent volume and permits accordingly. Methanol overdosing leaves residual BOD and is an operational hazard, making control instrumentation a worthwhile investment.