Recovering metals from water, sludge and fly ash

The focus of water treatment is increasingly shifting to the recovery of raw materials, partly stimulated by the quest for a circular economy. In the same way as drinking water companies and Water Authorities, sludge final-processors and waste-to-energy plants are also concerning themselves with the recovery and useful reuse of residuals. For the time being, in the wastewater and sludge cycle the investment is being directed mainly at phosphorus recovery.

Technology

The research project ‘Terugwinnen zware metalen en zeldzame aardmetalen uit afvalwater en slib-eindverwerking’ (KWR 2016.021) (‘Recovery of heavy metals and rare earth metals from wastewater and sludge final-processing’) showed that there are opportunities for the recovery of (rare) heavy metals and earth metals, including copper, zinc, gold and palladium. Besides the recovery of the metal/metals, the removal of metals improves effluent quality, sludge quality and residual ash, and therefore opens alternative disposal possibilities.

Challenge

At the start of the research, knowledge about the load and concentration, in combination with the speciation (appearance), of metals in the water and sludge processing cycle was not sufficient to provide the basis for a selection of a suitable recovery technology. Phase 1 of this project began with the development of a method for sample pre-treatment and metal-concentration measurement based on inductively coupled plasma mass spectrometry (ICP-MS); see ‘Terugwinnen van metalen uit water, slib en vliegas: ICP-MS methodeontwikkeling’ (KWR 2017.066) (‘Recovery of metals from water, sludge and fly ash: ICP-MS method development’). On the basis of this method, we researched what concentrations and speciation (i.e., total, dissolved/colloidal, residue) of 66 elements – including heavy, but also rare metal and earth metals – occur in different streams in drinking water and wastewater cycles and sludge final-processing.

These monitoring results have been used to assess the potential of the various technologies (new and newly-applied) for the removal and/or recovery of the selected metal(s) from water – i.e., (sources of) drinking water, membrane concentrate, WWTP influent and effluent – wastewater treatment sludge and fly ash. The results have been included in the report ‘Terugwinnen van metalen uit water, slib en vliegas: monitoringsresultaten & potentie’ (KWR 2018.019) (‘Recovery of metals from water, sludge and fly ash: monitoring results & potential’).

Solution

In Phase 2 of the project the effectiveness of the selected technology will be tested on a small scale for one or more of the streams with a view to underpinning its feasibility. This research will focus on the application of different ion exchange resins on wastewater and released fly ash; among the metals to be looked at are copper, zinc, cobalt and perhaps iron, lithium, titanium, wolfram, palladium, silver and gold. The results have been included in the report ‘‘Terugwinnen van metalen uit water, slib en vliegas. Resultaten fase 2 – Experimenten’ (KWR 2019.48).