Acid solubilization of sewage sludge and ash constituents for possible recovery

Many of the trace elements and phosphorus in digested sewage sludges can be solubilized with acid at an average acid cost of $41 and $77 per ton of dry solids for H₂SO₄ and HCl, respectively ($46 and $86/metric ton). But an economic gain accrued by the recovery of metals and phosphates would be minimal since processing of the large volumes of dilute acid leachate would be expensive. Based on acid costs alone, sludge detoxification does not seem to be economically feasible, so the elimination of metal discharges at source seems to be the only practicable solution to reducing heavy metal levels in digested sludges.

Sulfuric acid can be used to non-selectively solubilize metals and phosphates from sludge incinerator ash for about $21 per ton of ash treated ($24/metric ton). In most instances, separation and recovery of the constituents in the leachate would be too costly to make profitable recycle possible. Optimization of incineration parameters such as temperature can be used to minimize the environmental impact of landfilling sludge incinerator ash.