Managing concentrate streams

Because RO and NF are separation processes, they generate a concentrate stream consisting of components of the feed water in concentrate form, as well as possible additives, such as antiscalants and salts. The management of these concentrate streams is of great importance given the increasing application of these advanced treatment processes.

Dealing with concentrate streams in NF/RO treatment techniques

One can clearly observe that, for a variety of reasons, the interest in the application of membrane techniques in drinking water treatment is generally increasing. For this reason, and for socio-developmental reasons, the management of the concentrate streams is becoming more and more important. It is also frequently the case that concentrate management can be an obstacle to the application of membrane techniques. Commissioned by the drinking water utilities, KWR therefore surveyed and produced a report on the current status of concentrate stream management in drinking water treatment processes.

Survey as basis for practice-relevant innovation

This survey, conducted in 2019, of NF/RO concentrate stream management practice revealed that four of the ten Dutch drinking water utilities – namely, Oasen, PWN, Vitens and WMD – have integrated NF and/or RO into at least one of their drinking water production sites. The residual stream generated at these sites by the respective membrane filtration technique is discharged into surface water, into a harbour / the sea, or directed to a wastewater treatment plant (WWTP).

The number of production sites applying NF and/or RO technology is expected to increase in the 2020-2030 period by 47% compared to the 2019 level. The discharge of the generated residual streams, considering the total societal costs of the drinking water provision and of wastewater treatment, is a satisfactory form of residual stream management, which can explain the current NF/RO concentrate management practice.

Legal and regulatory framework for concentrate management

The survey shows that the authorisation of these discharges is not based on standardised water-quality parameters, and that the requirements are stated in the form of concentrations, not in terms of the total load. Individual agreements are made for each discharge with the relevant, typically local, regulatory authorities. The over-arching ambition of the Dutch government, besides maintaining an impeccable drinking-water quality, is also directed at reusing residuals and minimising the use of primary raw materials (including water). It is expected that these ambitions will play a more prominent role in the new authorisations issued in the future.

Focus parameters for concentrate stream treatment

Despite the site-specific differences in discharge requirements, in order to get a sense of a typical residual stream composition from drinking water production using NF/RO membrane technology and from conventional drinking water production, general scenarios were created for brackish groundwater, bank-filtered water, surface water and fresh groundwater. On the basis of European environmental quality requirements for surface water that is used to produce water for human consumption, critical parameters were identified in the elaborated scenarios for the discharge of these streams into surface water. For every scenario the standard of 0.1 µg/L for plant protection products, biocides and their human toxicologically-relevant degradation products was quickly exceeded when these substances were present in the feedwater. In addition, salt compounds, such as ammonium, arsenic, chloride, phosphorus, iron, manganese, sodium, nitrate and sulphate were identified in (some of) the elaborated scenarios as possible critical components for the discharge into surface water. A list with selective removal techniques shows that there is a selective removal technique available for each of these salt compounds.