Aluminium recovered coagulant from water treatment sludge as an alternative for improving the primary treatment of domestic wastewater

Material recovery from water treatment sludge (WTS) is essential to the circular economy in the water sector. Coagulant recovery from WTS can be an alternative for enhancing wastewater treatment. This study evaluated the chemical improvement in the primary treatment (CEPT) of domestic wastewater using recovered coagulants (RCs) from polyaluminium-chloride-based WTS. The recovery process was made from wet and dry WTS by acidification with sulfuric acid and hydrochloric acid. The performance of the RCs in CEPT was compared with the performance of commercial coagulants (ferric sulphate and polyaluminium chloride). In the wastewater treatment, a RC from the wet sludge with 40 mL/L of sulfuric acid (3.5 N) showed better turbidity removal (96%), with minimum cost. Therefore, it was selected as the optimal RC and used as a reference to be compared to the costs of the other RCs. With this optimal condition, 76.3% recovery of aluminium and a 67% volume reduction in WTS were achieved. Moreover, with the optimal RC, 89% total suspended solids (TSS), 62% total chemical oxygen demand (tCOD), 90% total phosphorous (tP), and 97% soluble phosphorous (sP) were removed from the wastewater. These removals imply a rise in primary sludge production and its organic matter content with CEPT, which increases energy recovery by anaerobic digestion. Concerning the commercial coagulants, these removals were similar, with maximum differences of up to 8% in favour of the commercial coagulants. However, RC is significantly better than commercial coagulants from an environmental impact point of view because it allows the reuse of WTS, reduces the final volume of sludge to be disposed of and saves on the use of commercial coagulants.