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. Besides the fact that the recovery of metals would generate possible earnings, the metals’ removal would also improve effluent quality, sludge quality and residual ash, and therefore open alternative disposal possibilities. For the first time in the Netherlands – in a TKI Water Technology project – the concentrations of 66 elements (notably metals) in water, sludge and fly ash are being measured, and the potential for their recovery determined.
Research into metals and other elements in the watercycle
At the start of the research, knowledge about the load and concentration, together 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 using Inductively Coupled Plasma Mass Spectrometry (ICP-MS); for more information, see the report ‘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 occur in different streams in the drinking water and wastewater cycle and in sludge final-processing. This concerned heavy metals, rare metals and rare-earth metals.
These monitoring results have been used to assess the potential of the various technologies for the removal and/or recovery of the selected metals 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’).
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.