In the south-East of the Province of Drenthe, in the municipality of Emmen, there are two large areas with greenhouse horticulture. In the early days surface water, taken directly out of canals nearby, was used as greenhouse water, but since the 1960＇s, the quality of the surface water became worse (especially salination). Hence, the surface water became less suitable for greenhouse purposes. The greenhouse, operaters switched over to water taken from a former sandpit, but during the last three years the iron-concentration in that water rose, This immediately led to major problems as the drip tubes were clogged bio ironhydroxide. The local water supply company WMD (NV Waterleidingmaatschappij Drenthe) was asked to solve the problem associated with the supply of greenhouse water. The greenhouse operators asked for the iron concentration in the irrigation water to be reduced to below 0.25 mg/l. WMD decided to introduce a new technology for the production of greenhouse water: Ultrafiltration. A UP-installation with a capacity of 200 m(3)/h was designed, engineered and constructed within a period of only 12 months. In the same time ultrafiltration was tested and optimized on pilot-scale. From that study it was concluded that without flocculant dosing a rather low gross flux had to be used for stable operation. A much higher flux could be attained with minor flocculant dosing.