Dairy wastewater polluting load and treatment performances of an industrial three-cascade-reactor plant

•Dairy wastewater was treated in an industrial three-cascade-reactor plant.•The plant maintained high treatment performances over a wide range of influent loads.•The large majority of the organic matter depletion occurred in the first reactor.•At high organic loads of the influent DO and ORP were very low in the first reactor.•Values of the sludge volume index indicated good settling properties of the flocs.

An industrial three-cascade-reactor plant treating 45 m3 d−1 of dairy wastewater (DW) was monitored for approx. one year to investigate the effect of variable daily influent loads. It removed more than 85% COD, NH4-N and non-ionic and anionic surfactants from DW within the loads 7–24, 0.4–2.3, 0.4–0.7 and 0.1–0.5 kg d−1, respectively; NH4-N removal, in particular, was almost quantitative. Although the degradation of the above parameters below the lower load thresholds declined to 78.7, 87.5, 50.2 and 64.7%, respectively, their residual concentrations met effluent discharge standards. The biomass settling properties, assessed as sludge volume index (SVI), were satisfactory (generally lower than 150 ml g−1) regardless of the organic load of the influent. The depletion of the pollutant load took mainly place in the first reactor albeit a significant contribution to the removal of the slowly degradable organic matter fraction was given by the two subsequent reactors.