An economic assessment of coagulant recovery from water treatment residuals

Coagulant recovery from waterworks sludge for re-use is a key option towards the reduction of chemical usage in the water industry. Whilst this concept is not novel, process economics and recovered product quality issues have limited its implementation. Ion selective membranes have recently been shown to satisfactorily address the latter, but economic feasibility remains a key issue which has been largely overlooked.This study used empirical data taken from bench-scale tests of coagulant recovery using Donnan dialysis (DD) with bulk chemical prices to determine the operational expenditure (OPEX) for full-scale recovery. Calculated values were compared with existing coagulant dosing procedures, as well as potential alternative recovery technologies based on electrodialysis (ED) and ultrafiltration (UF), to determine the cost benefit.It was determined that under current commodity and technology prices, coagulant recovery by DD offers no cost benefit in comparison to conventional practice. Process improvements, such as incorporating acid recovery, identifying alternative waste disposal routes and improving membrane performance, can significantly increase economic viability. UF was shown to provide OPEX reductions of around 40% when compared to conventional practice, and ED was found to be cost neutral. None of the assessed technologies are currently able to offer cost benefit for ferric coagulant.

► Acid recycling is mandatory for economic coagulant recovery.
► ED coagulant recovery has the same OPEX as conventional dosing of virgin coagulant.
► UF coagulant recovery reduces OPEX for alum dosing in potable treatment by 40%.
► DD associated OPEX must be lowered by >40% to equal or reduce virgin alum costs.
► OPEX for selective recovery of ferric coagulants exceeds those for virgin ferric.