Distribution system
The sewage is distributed evenly over the surface of the bed by a network of pressurised distribution pipes. The distribution pipes should have a diameter of 32–45 mm and should have 5–7 mm holes placed in the bottom of the pipes for every 0.4–0.7 m. It is important that the whole distribution system is placed under pressure for a period that is long enough to secure an even distribution of water over the entire bed surface. In practice, the volume pumped should be at least three times the volume of the distribution pipe system. If 32 mm diameter distribution pipes are used for a single household system, the volume of the distribution system will be approximately 20 l. Therefore, the volume of water pumped to the system at every pulse should be at least 60 l. This means that the loading frequency at a normal loading rate for a household will be 8–12 pulses/day, and when half of the effluent water is recirculated in the system, 16–24 pulses/day. The distribution pipes are insulated against freezing by a 0.2 m layer of coarse wood chips or sea shells placed on the surface of the filter.