Iron recovery from a representative water treatment plant sludge

Water treatment plant sludge disposal has become a timely problem to consider. While many methods of sludge treatment and disposal have been suggested, coagulant recovery may prove to be the most economical. This investigation was concerned with recovery of iron coagulant by chemical addition. Sulfuric acid, hydrochloric acid, and sodium hydroxide were added to a water treatment plant sludge containing precipitated iron. The acidic method of recovery proved to be technically feasible.

Iron dissolved upon addition of acid to the sludge. The concentration of soluble iron increased substantially when the pH was lowered to a level of 2.0 and then diminished at pH values below 1.0. Iron recovery appeared to generally follow the stoichiometry predicted.

Acid addition to the sludge resulted in an improvement in sludge dewatering properties. Settling rates increased consistently with greater amounts of acid added, causing substantial reductions in sludge volume. The filterability of the sludge was also improved. Specific resistance decreased as the pH was lowered, reaching a minimum value at a pH level of about 3.0, and then increased due to compaction.

Sludge volume reduction and iron recovery did not occur simultaneously at a given pH. It appeared that the acid acted to either solubilize the iron or to destabilize the colloidal particles, depending on the system pH. The recovered iron proved to be superior to fresh iron for coagulation of low turbidity waters. The suspended solids in the recovered iron solutions appeared to aid in nucleation. While coagulation with fresh iron resulted in a lower raw water residual turbidity, the critical coagulation concentration was less when recovered coagulant was used.