Societal benefit from the valorisation of reverse osmosis concentrate?

The ‘Opportunities for Concentrate’ project is part of the WiCE (Water in the Circular Economy) programme of the Joint Research Programme of KWR and the water utilities (BTO). The project investigates how circular processing of concentrated residual streams can contribute to a future-proof freshwater supply and possibly to a more socially responsible treatment of residual streams. Reverse osmosis (RO) membrane filtration is an effective technique, for instance, for the use of brackish seepage water for the production of drinking water, or for the treatment of brackish groundwater to produce good irrigation water for horticultural crops. But the technique generates concentrate as a waste stream and it is the processing of this stream that might hinder application. Management options for this concentrate stream of increased salt concentration, which remains after the treatment, can only be returned to deeper subsurface layers with municipal authorisation or, in some instances, discharged into the sea. In the years ahead, the ‘Opportunities for Concentrate’ project will explore how to valorise the water and the components from these residual streams.

From discharge to valorisation

‘In many places brackish groundwater is too saline to be used without first being desalinated,’ says KWR researcher Luuk de Waal. ‘Reverse osmosis is a good treatment technique, provided, among other things, that a suitable management method is found for the concentrate. By simplifying the main stream, and exchanging its components with other components and subsequent recovery the water, one creates opportunities for its valorisation. This means that the concentrate does not need to be a residual stream, if we put some energy into it, both literally and figuratively speaking.’ In four work packages, De Waal and other researchers are working on a technical proof of concept, and studying how this can be implemented, from both economic and social-responsibility perspectives. This is relevant not only for horticultural companies, but also for drinking water utilities and other user groups interested in producing qualitatively good water using RO. ‘We are also taking a close look at other residual streams, such as organic residual streams, that offer prospects for the concentration and valorisation of these components,’ adds the KWR researcher.

Kitchen salt and the rest

In the short term, it is the first work package that is most relevant for the ‘Kennis in je Kas/Glastuinbouw Nederland’ Foundation of the Dutch greenhouse horticultural sector. This work package focusses on the valorisation of brackish groundwater concentrate. The greatest portion of the salts contained in brackish groundwater – about 65-85 percent – is made up of ‘natural’ sodium and chloride. De Waal: ‘It would be great if we succeeded in exchanging the remaining components, so that we would have a main stream consisting almost exclusively of these two elements. With the full recovery of high-quality freshwater, there would be no water stream remaining that would need to be sent underground or into the sea; only kitchen salt would be left, which could potentially be used by the water sector itself or other (industrial) sectors.’ It will however not be easy to valorise the kitchen salt, because it can be collected in its pure form and is not expensive. The exchanged salt components, such as calcium, magnesium, sulphate and carbonate, could find useful applications at horticultural companies or elsewhere.

Ion exchange and evaporation

The treatment technologies needed to achieve the desired objective are available in principle, but have not yet been tested on brackish groundwater. De Waal explains: ‘During the whole process, the brackish groundwater is processed into three streams: clean irrigation or drinking water, a concentrate solution with sodium chloride, and a residual stream with other salt components, which could ultimately be valorised via a crystallisation process. We are now busy drawing up an inventory of methods that enable ion exchange under brackisch groundwater conditions. This involves using resin columns that can bind the ions. Agents are then used to selectively unload the saturated resin columns, with the possibility of creating separate residual streams. This is followed by the thickening of the main stream via RO, after which the remaining volume should be dried in an energy-efficient manner.’

The more salt, the more difficult

This method sounds quite simple, because ion exchange works well with relatively clean, fresh water. But this is not the case with seawater. ‘Brackish groundwater actually is positioned between the two,’ says De Waal. ‘That’s why the question concerns the extent to which the different resins continue functioning with rising electrical conductivity. This we will be determining for a variety of brackish water streams over the next six months.’ Subsequently, the existing and innovative residual stream treatment scenarios will also be assessed on their societal value, including costs/benefits, energy and sustainability. This will provide the basis for the stakeholders to make well-informed decisions.

The Chances for Concentrate project (2022-2024) is being conducted by KWR in partnership with the Dutch drinking water utilities, De Watergroep, AquaMinerals, Glastuinbouw Nederland/Kennis in je Kas, Aquafin, SCW Systems, Rivierenland Water Authority, HVC, and Schieland and Krimpenerwaard Water Authority.