Primary Chemical Treatment Minimizing Dependence on Bioprocess in Small Treatment Plants

Abstract

In wastewater from small communities, institutions, hotels, camps and tourist areas the major pollutants such as organic matter, oil and grease and organic nitrogen are mainly in particulate form, while the soluble fractions of organics and nitrogen are low. This is due to the short residence times in the sewerage systems preventing decomposition and lysis. Such wastewaters are also characterized by high variations in pollution load, mostly in the particulate fraction. Small plants, treating wastewater for discharge to lakes or rivers, have to remove organic pollutants and nutrients, mainly phosphorus. Chemical flocculation-sedimentation, as primary treatment, was considered a promising process combination to obtain reliable operation and for minimizing effects of variations in flow and load. Two possible process combinations were studied: a. chemical flocculation-sedimentation as primary stage, followed by biotreatment (CBT); b. biotreatment of raw wastewater, followed by chemical flocculation-sedimentation (BCT). It was found that primary chemical treatment by alum, or lime, removes more than 90 percent of the suspended solids, colloids, phosphates, oil and grease; the organic matter as BOD and organic nitrogen removals were about 70 percent. By removing the particulate organic matter and other disturbing factors the successive bioprocess in CBT was substantially improved: the growth rate constants for general biomass and for nitrifiers as well as the removal rates were higher for CBT than for BCT; the critical sludge age for nitrification in CBT was 4 days, as compared to 10 days in BCT. The reduction of organic load by about 70 percent, producing a more uniform influent and the better bio-kinetics achieved in CBT, enable a considerable economy in the bioreactor volume and lower energy requirement. Experimental results showed that the CBT sequence enables lower sludge production containing higher concentrations of solids, most of them inorganic.