Packaged domestic waste water treatment systems operate on the principle of active sludge with long term ventilation. They are complete units that separate biologically aerobic microorganisms and organic material causing pollution in the water. The ventilated waste water is overflowed to the settling tank for the precipitation of the biomass. The treated water is chlorinated after passing through the settling tank before being discharged so that waste water is disinfected against microorganisms that have escaped precipitation. The settled biomass is recycled by the use of a reverse circulation pump to provide sufficient microorganism concentration. Tanks used in packaged domestic waste water systems are produced from carbon steel or ferroconcrete material and can be embedded or installed above ground. BT (ferroconcrete) and ST (carbon steel) range domestic waste water treatment plants are designed to meet the needs of sites with a capacity between 100 - 750 people, hotels, holiday villages, schools, dormitories, plants and other locations where waste water created by domestic use has to be treated. Parallel systems can be designed to suit seasonal changes in occupancy or for high capacities