12 Primary treatment
1. Primary treatment 1.1 Objectives • To eliminate the coarse material normally present in the raw wastewater that could clog/block pumps, pipes and possibly sewer lines. • To mix and balance well different tannery streams and thus produce homogenized “raw material” that can be treated in a consistent manner. • To adjust pH and eliminate toxic substances (sulphides) and avoid shock loads that can negatively affect the rather sensitive biological treatment. • To significantly decrease the BOD/COD load and thus simplify the biological treatment phase and reduce its cost. To summarize, the purpose is to eliminate the coarse matter, remove almost completely Cr and sulphides, remove the major part of suspended solids, and considerably reduce the BOD and COD content. Basic steps: Screening (bar, self-cleaning)1 Pumping/lifting Fine screening Equalization and sulphide oxidation Chemical treatment (coagulation, flocculation) Settling Sludge dewatering2 1.2 Screening 1.2.1 Bar screening, removal of larger solids
Figure 5. Rough bar screen, side view
1Quite often a grease-and-sand trap is also included, usually before fine screening and pumping. 2 Primary and secondary sludge are handled/dewatered together; however, since some ETPs have only primary treatment and the main part of sludge is generated during that stage, sludge dewatering is covered here.