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.

Storage cases

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.
Initially treated water was stored just to have water ready for irrigation, but then it was realized that storage had positive effects on the water quality, depending on tanks characteristics, operational methods and climatic conditions. The average storage time varies from 50 to 180 days, depending on managing methods.
In Italy some small storage tanks are present near industries, especially in Sicily. They are almost completely filled in during winter and the water is used during the arid season for irrigation purposes.

Purification processes

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.
On the free surface of tanks when exposed to sunlight, algae grow. They can produce oxygen and increase pH level (up to 11). Some predators contained in the tanks swallow these algae and bacteria producing CO₂.
The sun radiation cannot reach the bottom of the tanks and algae cannot grow, so from the intermediate level of the tanks the conditions are anaerobic, with the consequent growth of anaerobic and optional bacteria. If anaerobic microorganisms float form the bottom to the surface of the tank, algae oxidize them.
During the winter a lot of organic matter accumulates on the bottom of storage tanks. In spring some anaerobic reactions develop due to the rise of temperature. These reactions go on until green algae grow. The tank is not homogeneous anymore, but an aerobic and an anaerobic layer can be located. This thermal stratification causes an increasing aerobic activity on the surface and anaerobic on the bottom, because of the algae growth, which block the penetration of solar radiation.
The main factors, which influence microorganisms’ removal, are the solar radiation intensity and the algae activity. Storage time and working methods are important factors too. Tests show that water storage for 60 days in a 15-20 m deep tank allows having clean water for irrigation.

Purification yield

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.
In continuous storage tanks wastewater supply to the tank is constant, while the effluent depends on irrigational needs. In some periods water is simultaneously taken and supplied, thus decreasing its quality.
In batch storage tanks wastewater is supplied during a short time and it is maintained in stationary conditions until a fixed taken time. This assures that water can reach a high purification level before it is taken. The wastewater is usually stored for 15-60 days depending on climate conditions, tank depth, and wastewater composition. A good alternative is to use two tanks in series. The wastewater of the first tank can be stored for a shorter time if it is used for purposes requiring lower quality.
Storage tanks should never be emptied completely in order to save impermeably layer from the solar radiation and to maintain in tanks some living microorganisms and algae. Mud disposal is not dangerous for stored primary treated water.

Check also the other natural wastewater treatment techniques: lagooning and phytodepuration.