As a technique of disposing and recycling a large amount of sludge generated by the pre-coagulation process, which is literally one of the advanced sewage disposal system, we focused on the method of recovering phosphate and coagulant in sludge. Characteristics of extraction and recovery of phosphate were particularly examined in this process, where iron and phosphate were separated by the addition of chemical compounds. Targeting the pre-coagulated sludge with Ferric Chloride, we evaluated these following points; 1) the influence of S/Fe molar ratio and pH on the phosphorus extraction rate, 2) the influence of the amount of added calcium hydroxide on the extracted phosphate recovery rate, and 3) the composition of the material recovered.
As a result, when using potassium hydrosulfide in minimum addition amount (S/Fe=1-1.5) as its reducing agent, maximum extraction rate of around 70% was obtained, which is effectively higher than the result when hydrogen sulfide was used before. Moreover, the extracted phosphate recovery rate hit 90% when Ca/P molar ratio was about 1.5. As for the material recovered, 15% of phosphorus was contained with quite low content of heavy metals and Al.