GB777041A - Improvements in coagulation and sludge conditioning - Google Patents

Abstract

A modification of the process of the parent Specification comprises mixing with a sewage medium a solution of a basic chloride of aluminium, iron or chromium, the ratio of the number of chloride ions in this solution to the total number of chloride ions which would be present in an equal volume of an equimolar solution of the normal chloride salt alone lying in the range 17 to 40 per cent. The minimum quantity of basic aluminium chloride solution necessary to produce coagulation, and calculated as Al2O3, amounts to approximately 0,6 per cent of the dry solids content where the sewage medium is a sludge. The basic chloride solution may contain approximately 15 per cent by weight of the basic chloride calculated as R2O3, but the amount is not critical. Chlorides having a "chloride-ion ratio" (as above defined) of 25 per cent are particularly suitable, and solutions of chlorides corresponding to formulae ranging from R2Cl2(OH)4 to R2Cl(OH)5 (the aforementioned ratio ranging from 34 to 17 per cent) may be economically prepared. Aluminium chlorohydrate having a ratio lying in the above range is preferred. Examples are given of the application of the process to raw primary sludge, cold digested primary sludge, mesophilic digested primary sludge, elutriated digested primary sludge, activated sludge, humus sludge and mixtures. It may be applied to thermophilic digested primary sludge. The age of the sludge or mixture varied from nil to seven days. An improvement in filtration rate was observed in all cases. Specification 516,706 is referred to.