BDT-water filters, also known as gray water filters, are compact facilities for cleaning of the so called bathing, dishing and washing water, in other words all the water from a household except the toilet water. The grey water from the household flows in to and through the system naturally. The function of the filters is based on both mechanic and biological cleaning. Organic filtering material is used in the filter that catches dirt from the water, the micro organisms on the filter materials surface uses this as nutrition. The air flow to the micro organisms plays a significant role for the function of the facility. This works with help from natural flow, in through the lower vent and out through the upper vent. Processed sewage water from the treatment plant can then be led out into the ground.