Abstract: A combination of a biological dry toilet and a biological waste water purifying plant. The combination comprises a tank for composting waste, a means for change of air in the tank, and a sludge sedimentation tank. A biofilter is provided through which the water to be purified is circulated. The combination comprises in addition a means for change of air in the biolfilter, a means for circulating the water to be purified in the biofilter, a means for removing the purified water from the sludge sedimentation tank, a means for removing the sludge from the lower part of the sedimentation tank, a means for feeding the water to be purified through the dry toilet, and a means for mechanically purifying said water in the dry toilet. An equalizing basin is included in the waste water purifying plant, the dry toilet being at least partly submerged in said basin.