Storage in tanks
Storage in tanks is a natural wastewater treatment technique that consists in wastewater storage in large basins. In these basins purification processes take place. This technique is applied to primary urban wastewater treatment. Storage tanks are made of soil, their storage capacity varies up to 12 millions of m3 and they are about 6-8 m deep. Storage tanks are very used where it is necessary to have a high quantity of water available in a short period (i.e. for irrigation). Storage tanks can be daily or monthly regulated: the objective is to have a constant and continuous water supply.
In many Mediterranean countries wastewater accumulation in tanks is very common. Reusing systems in which treated water is stored in large tanks so as to be available for irrigation, were recently developed, especially in Israel and Jordan.
Chemical-physical processes developing in storage tanks are the typical ones for aquatic ecosystems. The exchanging processes between the anaerobic bottom layer and the aerobic top layer are very similar to the ones that develop in optional ponds. The main difference between ponds and storage tanks is that ponds have a constant hydraulic volume and organic charge, being stationary systems. Storage tanks are not stationary systems, because the storage time, the percentage of fresh streams stored and the organic charge vary during the year meaningfully.
Storage tank purification yield is very variable depending on wastewater, climate, tank dimensions and operational characteristics. From experience it is known that BOD5, COD and suspended solid particles reduction is influenced by algae biomass presence: suspended solids removal is lower than 70% and BOD5 removal is included between 30 and 50%. Microorganisms’ removal is very influenced by the storage time: it is higher during winter, when water is not taken out from the tank for irrigation purposes.
Building and managing features
Storage tanks should have an elongated shape and the inlet stream has to be as far as possible from the outlet stream. Withdrawal channels should be built in a way to always take water from the surface. Storage tanks should be placed far from urban areas, because of the possibility of bad odours exhalation. The optimum tank depth is between 6 and 10 m.