Reuse of coagulant from aquafer (HerCauWer)

In the Netherlands drinking water treatment almost always generates aquafer as a residual. By treating this material with acid, we can again transform it into iron salts, which can then be applied as a flocculant in drinking water or wastewater treatment.

Reuse of iron salts

The abstraction and treatment of drinking water almost always involves the formation of aquafer (iron sludge), either through the aeration of ferrous groundwater, or through the addition of iron salts as a flocculant. By treating this iron sludge with acid one can again produce iron salts, which can then be reused as a flocculant in wastewater or drinking water treatment. In this way we can reuse the material and close the resource loop.

Research into technical and economic feasibility

Attempts were made in the recent past to reuse iron salts in this manner. In the first instance, we focussed on possible problems that might arise in the process. Then lab experiments were conducted to establish the composition of aquafer from different sites, and the best way of extracting the iron. Research was then carried out on the performance of the extracted iron as a flocculant compared to commercial flocculants, and how possible foaming problems might be prevented. The results showed that, even if the iron-extraction performance depends on the sludge type, in some cases efficient extraction levels (reaching 99%) are certainly possible, and that the extract in coagulation experiments produces the same results as commercial iron salts. A pilot research project has now been conducted in which extracted iron is used as a flocculant in wastewater treatment at the WWTP in Bath (its application in drinking water processes requires certification). The outcome was that the HerCauWer flocculant produced the same water quality as did the commercial flocculant; moreover, no difference was reported in the resulting sludges. In fact, per gram of iron, a slightly higher rate of phosphorus removal was observed: the Fe/phosphorus ratio for HerCauWer flocculant was 0.19, while that of the commercial product was 0.28.

Circular process promotes sustainability

The use of flocculants accounts for a large part of the footprint of drinking water treatment. Moreover, drinking water utilities are in search of application possibilities for the formed sludge. LCA studies have shown that making flocculants from aquafer and their reuse can significantly shrink this footprint. The HerCauWer flocculant is moreover cost-competitive in relation to the commercial iron salts. The use of aquafer from drinking water treatment as a flocculant agent in wastewater treatment therefore represents an appealing practical example of a circular application.