The feasibility of coagulant recycling by alkaline reaction of aluminium hydroxide sludges

Coagulant recycling appears to be economically attractive as much by the reduction of fresh chemical dosing as by the simplification of the sludge disposal. The present paper has studied the feasibility of aluminium coagulant recovery by an alkaline treatment of sludge. The study was carried out on sludge obtained from the drinking-water treatment plant of the Brussels' Intercommunal Waterboard located at Tailfer on the river Meuse. The general conditions are summarized as follows and given as guidelines:

• to limit the starting sludge concentration to a threshold below 20 g l⁻¹ dry wt concentration;

• to optimize the alkalisation process which means a pH between 11.4–11.8 for NaOH and 11.2–11.6 for Ca(OH)₂;

• to choose between NaOH and Ca(OH)₂ is optional but heavy metals contained in the sludge are reduced by a factor of 10–15 by using lime instead of NaOH;

• to use a settling process to recover the supernant before recycling and therefore best limit the recovery to 25–30% of the coagulant even though when using sodium hydroxide, an 80% recovery is possible.

When restricting the recycling to less than 50% of the coagulant (for instance 25–30%) no detrimental effect on the treatment performance was detected in conditions of this laboratory investigation.