An ultrafiltration plant removes the undissolved components of the effluent. Modules with very strong tubular membranes are submerged in the dirty water. A vacuum pump sucks the water out of the basin through the walls of the tubes. The dirt collects on the outside wall of the tubular membranes. The clean water moves on to the next stage. A layer of compacted dirt is left in the basin which goes back to the rwzi through a buffer. The tubes are made of pvdf (microfiltration). This material is resistant to sodium hypochlorite, a substance which is used to clean the tubes. A stream of air is pushed through the tubes from below to keep them in motion and prevent the dirt from settling. Another advantage is the low level of flux: 20 liter per m² per hour. No chemicals are needed during this process, such as flocculant and additives for pH adjustment.