An inherent burden with the improved SS removal by chemical treatment is the production of chemical sludge. The thickening and dewatering properties of chemical-sewage sludge are worse than those of the sewage sludge alone because of the presence of hydroxide sludges and the increased amounts of colloidal pollutants.
The addition of alum to a basic wastewater containing alkalinity produces a chemical floc. Generally, 1 lb of SS (chemical floc) is produced for each 0.25 to 0.40 lb of aluminum added. Figure 7.32.2 is a typical example of the sludge production and SS reduction by alum treatment.
The chemical sludge produced by adding lime to municipal wastewater depends on the chemical characteristics of the water, the pH level, and the method of operation. The net chemical sludge produced is a result of an interaction of these three parameters.
Lime treatment of municipal wastewater with low hardness (<200 mg/l as CaCO3) and low alkalinity (< 150 mg/l as CaCO3) should be accomplished in two stages, with the corresponding pH levels at 11 to 11.5 and 9.5 to 10, respectively. The chemical sludge produced under these conditions is about 4000 to 5500 lb per mg. For municipal wastewater, with high hardness (>350 mg/l as CaCO3), and high alkalinity (>250 mg/l as CaCO3), single-stage treatment at a pH of 10.5 to 11.0 is recommended. The chemical sludge produced under these conditions is about 5500 to 6500 lb per mg.
The use of organic polyelectrolytes does not produce significant amounts of chemical sludge.
The nature and quantity of the chemical sludge produced by adjusting the pH of industrial waste depends mainly on the initial concentration of the chemical pollutants and the efficiency of the liquid-solids separation step. Generating the data shown in Figure 7.32.2 provides data on sludge production.
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