THE EFFECT OF CATIONIC SALT ADDITION ON THE SETTLING AND DEWATERING PROPERTIES OF AN INDUSTRIAL ACTIVATED-SLUDGE

Cations have been found to influence the settling and dewatering prope rties of activated sludge, especially for industrial wastewaters in wh ich very high concentrations of monovalent cations are often found. Be cause the cation content of wastewaters is often influenced by upstrea m processes, an understanding of the influence of various cations can be an important consideration in pinpointing operational problems in w astewater treatment plants. An industrial activated-sludge treatment p lant was studied to determine whether variations in settling and dewat ering properties of the mixed liquor and waste solids could be caused by changes in the cation content of the wastewater. Laboratory reactor s received both return activated sludge and wastewater from the indust rial treatment plant, and the feed was supplemented with either sodium (Na+), potassium (K+), or magnesium (Mg+). it was found that when the monovalent-to-divalent (M:D) cation ratio on a milliequivalent basis was increased to greater than approximately 2:1 by either Na+ or K+ ad dition, dewatering properties became poorer and polymer conditioning r equirements increased. The soluble protein content also increased as t he mixed liquor M:D ratio increased, indicating release of biopolymer from the flocs. Magnesium addition at low doses caused a decrease in t he dewatering rate, but at higher doses both the settling and dewateri ng properties of waste solids improved substantially. Conversely, when Na+ and K+ concentrations in the raw wastewater decreased significant ly to less than 10 and 0.1 milliequivalents (meq), respectively, settl ing and dewatering properties improved substantially and addition of M g2+ did not improve conditions beyond those of the unamended control. Each of the cations studied caused unique changes in the properties of activated-sludge solids that could not be correlated with the M:D rat io, suggesting that some of the effects are not simply physical/chemic al but may be physiological as well.