Effluent reuse – Harnaschpolder


The greenhouse horticultural sector in the Netherlands requires large quantities of high-quality irrigation water at an attractive rate. This project examined the possibilities of storing treated effluent in the subsurface, and then using it for greenhouse horticulture irrigation during water shortages in dry periods.

The project focused on the possibilities of using Delft Blue Water (DBW) – treated and reprocessed urban water from the Harnaschpolder and Nieuwe Waterweg WWTPs – as irrigation water for a cluster of horticultural companies in Westland after its subsurface storage.


Besides rainwater, many horticulturalists currently use brackish groundwater, which first needs to be desalinated to make it suitable for irrigation. This produces a saline residual stream, also known as brine or concentrate, which is then injected into the deeper subsurface. These discharges are, in principle, not permitted; though a dispensation has been granted until 2022. This is why alternative sources of water are being sought. The national ‘good irrigation water’ policy aims to achieve a sustainable supply of irrigation water and has, as one of its priorities, the collective supply of irrigation water through the reuse of effluent.

The direct supply of Delft Blue Water to the horticulturalists involves high costs because of the capacity required to satisfy demand peaks over a relatively short period of a few months. This problem does not arise if, in periods of surplus, the water can be stored in the subsurface for later reuse. Specifically, a smaller installation capacity would be enough to meet the horticulturalists’ demand peaks, compared to the option of extending the entire distribution network to meet the demand peaks. A significantly lower capacity would then suffice for a longer supply period.

This collaborative project demonstrated that it is technically possible to supply water using subsurface storage in the vicinity of the greenhouses. One issue is the percentage of the stored water that can be recovered, also known as the recovery yield. This yield must be at least 60 percent to make the supply of Delft Blue Water attractive. In the current situation, the subsurface storage trial involves the use of rainwater. Because of a leak in the confining layers the yield is still too low. Techniques are being worked on to seal this leak and thus increase the yield.


There is a possible positive business case for supply volumes of about 400,000 m3/year. This means that DBW could be an attractive alternative, with costs comparable to those of existing reverse osmosis provisions (0.80-1.00 €/m3), for large clusters of companies (> 200 hectares with high water demand). For smaller clusters, the initial investment in the required distribution network would outweigh the savings.