Nanofiltration concentrate disposal: experiences in The Netherlands

Vitens has six full-scale nanofiltration plants running successfully. Another one is under construction. Five of these plants are fed with anaerobic ground water with high levels of hardness, colour and/or sulphate. One plant treats bank filtrated IJssel River water, which is hard and contains organic micropollutants. A full-scale reverse osmosis plant is under construction for a water treatment plant where hard groundwater with organic micropollutants is to be treated. In all of these cases disposal of the resulting concentrate is a serious problem, especially in those cases where no large surface water is present. In general, the concentrate disposal as such is not the problem, but the discussion is focused on a limited number of parameters such as sulphate, chloride, phosphate, iron and anti-scalant. For different reasons the water authorities put restrictions on the disposal of these compounds. Iron is a problem because it will cause a brown (visible) plume in the surface water. Most water authorities have quality objectives for sulphate and chloride in their surface water. The anti-scalant contributes to the total-P content in the case of polyphosphonates, and is considered to be a compound that promotes algae growth and which is not present in a natural environment and is therefore not acceptable. It is necessary to discuss disposal options with the water authority before engineering the installation. It is important to explain why a specific membrane filtration concept is chosen instead of conventional techniques. In the past, several solutions for concentrate disposal were developed by Vitens: treatment of the concentrate before discharging by rapid sand filtration or continuous filtration; selection of a nanofiltration membrane with a lower rejection for sulphate; selection of less contaminated groundwater wells that are used as feed water; discharging the concentrate in the influent or near the effluent of a wastewater treatment plant; transport over several kilometers in order to discharge the concentrate in a larger surface water. Despite these creative solutions it is expected that concentrate disposal will become more difficult. Water authorities use the stand-still principle (concentrations may not be increased), and European legislation may result in more severe restrictions. Therefore, there is a need for technologies that either remove specific compounds from the concentrate before discharge or technologies that make concentrate disposal unnecessary. Else, the concentrate problem may kill the application of membrane filtration in those areas where no large surface water is present.