Sustainability
Three streams arrive at the press together, and the biological one is usually the largest.
— t/day
What actually leaves the site
The biological yield is quoted per kilogram of COD removed, which is the convention in textile effluent work and slightly overstates growth because part of the COD removed is stripped or adsorbed rather than metabolised; where the plant has a long sludge age, endogenous decay reduces the net yield substantially and the figure should be lowered rather than the model changed. A plant running extended aeration will generate materially less sludge than this default suggests, and that is one of the main reasons to run it. The coagulant sludge factor covers the metal hydroxide the coagulant precipitates as and not the coagulant mass itself; it varies with the product and with pH, and alum, ferric and polyaluminium chloride are not interchangeable in it. Cake moisture is the single largest lever on the number that gets paid for: moving from 78 to 72 percent cuts the wet tonnage by more than a fifth without changing a gram of dry solids, which is why press performance and conditioning polymer are worth more attention than they usually get. Nothing here addresses sludge classification, which in most jurisdictions is what actually sets the disposal cost - a sludge carrying heavy metals from a dyestuff or a fixing agent is a different waste stream at a different price.
ETP Sludge from Solids, Coagulant and Biomass — free, with the formula and a worked example, at Textile School.