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.