The influence of cations on activated sludge behavior

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Virginia Polytechnic Institute and State University


This study investigated the influence of cations on biopolymer characteristics and sludge properties. Settling and dewatering properties of sludges were measured and correlated to the biopolymer characteristics. In addition, effects of cations on sludge conditioning with polymer were studied.

Experiment mainly consisted of two parts, reactor study and batch study. In re- actor study continuous-flow reactors were operated. Variables used included varying types of substrate, addition of magnesium or sodium, and changes in pH. The batch study included conditioning tests on the sludges with combinations of cationic polymer and salts.

Biopolymers were extracted using alkali extraction followed by gel filtration and subsequent carbohydrate and protein analysis. The sludge settling and dewatering were measured in terms of SVI and specific resistance, respectively. Sludge filtering rate (TIF) was used to measure the conditioning efficiency.

The results indicated that the influence of cations depended on the type and concentration of salt. An optimal concentration of Mg was found to improve biopolymer binding. The organic composition of feed also affected biopolymer characteristics. A higher pH combined with a high amount of sodium released biopolymer and resulted in sludge deflocculation. A relationship between unbound biopolymer and soluble effluent COD was observed but no discernible relation between biopolymer binding and sludge settling and dewatering properties was found. Cations were found to reduce polymer requirement during sludge conditioning.

In addition, the amount of extractable biopolymers by alkali extraction was greatly influenced by salt. Magnesium inhibited the release of biopolymer, but sodium im- proved the efficiency of biopolymer extraction.