Dual-chemical conditioning and dewatering of anaerobically digested biosolids: laboratory evaluations

Dual-chemical conditioning of anaerobically digested biosolids was evaluated using either ferric chloride or a cationic surfactant, hexadecyltrimethyl ammonium bromide (HDTMA), followed by a cationic polymer (Percol 757). Significant reductions in the optimal doses of these additives were observed in dual-chemical conditioning when compared to the dose requirements for either chemical when used individually. Adding a proportion of one chemical's optimum dose reduced the requirement for the other by roughly the same proportion. The use of ferric chloride or HDTMA to reduce the polymer requirement was not cost effective at the facilities involved with this study, but their cost-effective use will depend, in general, on the polymer in use and the chemical costs relevant to a particular site. Regardless of cost, use of at least some polymer was required to obtain optimal dewaterability. Thus, neutralization of surface charge is necessary in the chemical conditioning of biosolids, but an additional mechanism, likely to be interparticle bridging, also plays a critical role in obtaining the most effective dewatering.