Towards a sustainable approach to concentrate treatment

During the online WiCE Roadshow (a live event was not possible because of corona), on Friday, 4 February 2022, there was a lively discussion among the almost 50 participants. It became clear that ‘handling concentrate residual streams’ is considered a very pertinent issue, both within and outside the water sector. The conclusion: all stakeholders in principle share the ambition of redirecting the current practice of concentrate stream management (discharge) towards recovery and reuse. The participants see the WiCE programme (Water in the Circular Economy) as a perfect environment in which to conduct the required integrated research.

Membrane filtration

A variety of challenges ensure that water utilities will be making more and more use of membrane filtration techniques. Membrane filtration is a useful application in the quest for alternative water sources, the control of salinisation, for opportunities for freshwater reuse, and as a barrier against the increase of very worrisome compounds. However, since membrane filtration is a separation technique, a concentrate stream is left over. In the current situation, this stream is directly or indirectly discharged into surface water or into a saltwater body.

Growing residual stream

Discharge is undesirable within the framework of the WiCE ambitions. And not only that: it can be expected that the regulatory authorities will be supervising surface water quality more stringently in the near future. Moreover, the Circular Water 2050 vision, which was formulated for and by this joint research programme of the water utilities, specifies that water, energy and residual streams are to be (re)used and/or valorised. Driven by the necessary implementation of membrane filtration techniques, and the future vision of a circular, intersectoral freshwater system, we need to deal more effectively with a growing residual stream volume. But how? During the Roadshow, an overview of concentrate streams in the drinking water, industrial and wastewater sectors, and presentations of operational examples of concentrate stream treatment, provided plenty of material for reflexion and discussion.

Laws and regulations

One thing all the participants agreed on, is that the ‘front-end valve’ needs to be turned off. This means that the anthropogenic pollution can no longer be allowed to cause problems for other water users or nature downstream from the discharge point. Today’s legal and regulatory framework will have to be adjusted. But it would be utopian to think that this will eliminate pollution in the (aquatic) environment – because, for example, other rules apply outside our country’s borders. And a tightening of laws and regulations does not mean that existing pollution will just disappear. This is why a commitment is needed to the further development of (new) treatment and purification techniques for concentrate streams. But how the water sector goes about this varies quite a bit.

Different ambitions

Stakeholders in the drinking and wastewater, agricultural and industrial sectors show different levels of ambition when it comes to dealing with residual streams. These vary from the combined discharge with a fresh(er)water and/or diluted stream, to the (selective) removal of discharge-critical and/or valuable components. According to the participants, the biggest opportunities for concentrate lie in three directions:

• freshwater reuse,
• the recovery of inorganic material with ‘ore-grade’ purity,
• the recovery or valorisation of organic material from the concentrate.

Value from concentrate stream

To make reuse possible, there are still a number of challenges that need to be met. For instance, the concentrations of valuable components in the feed water are generally low, and the application of reverse osmosis membrane filtration produces concentrations that are four to five times greater in the concentrate stream. However, it is not the case that the component concentrations in the concentrate stream are by definition suitable for profitable reuse. Nevertheless, the participants generally look favourably at investigations into alternatives to discharge. Even though, for them, it is part of daily practice and nothing in the regulations obliges them to change this. For each concentrate stream, the biggest challenge is the extraction of maximum value. The problems that arise in this regard are mostly local in nature: the specific source composition, the operational management of the membrane installation, the properties of the discharge-receiving water body, and the policy of the local regulatory authority.

Missing policy

What the Roadshow participants highlighted was the absence, on the part of Rijkswaterstaat and the Water Authorities, of a clear approach to assessing the discharge of a residual stream. The lack of a clear policy from the authority with overall responsibility, that is, the Ministry of Infrastructure and Water Management, was pointed to as the possible reason for this. People also come across cases where secondary raw materials often have to meet stricter requirements than linear materials.

Circular ambition

The current regulatory framework is designed in such a way that the discharge of a residual stream has become commonly accepted, which also applies with regard to the social costs involved. This is however not compatible with the drinking water sector’s stipulated circular ambition for 2050. Furthermore, the European Union has set itself the objective of having half of all feedstock of circular origin in 2030, and of reaching full circularity in 2050. For this reason, the ambition level for these streams has to be set higher than where we see it is set in today’s practice. The comprehensive overview is often lacking in the local legal and regulatory bodies that are responsible for implementing the policy on the ground.  To achieve the set objectives and ambitions, investment decisions should not only consider the costs, but also the possible sustainability benefits. Moreover, when dealing with residual stream treatment techniques, the water sector should also dare to think and act beyond its own borders.

Energy, emissions and societal values

What became apparent during the discussions about sustainability is that we need to keep the bigger picture in mind. Because, even though the valorisation of concentrate streams might require relatively high energy inputs, it produces benefits in terms of emission reduction. We often equate sustainability with energy-neutrality, but the reuse and/or recovery of raw materials can, despite the required energy input, constitute a sustainable practice. Energy, emissions, but also societal values and future values play a role. The WiCE programme is very suited for the cross-water-sector research required to give this shape.