This step in the purification process is needed to prevent biofouling of the membranes during reverse osmosis. The addition of (liquid) oxygen creates favourable conditions for bacteria in the activated carbon filter, such that they convert nutrients into water and carbon dioxide so that there is a considerable reduction in the biofouling of the membranes. Excess bacteria (biomass) and the backwash water are returned to the rwzi. There are two stages in the carbon filter: a pre-filter and a second filter. These filters have a filtering speed of 10 m³ per hour and 5 m3 per hour. A screen removes the small activated carbon particles before the water goes to the buffer storage. This water is almost drinking-water quality.