Constructing settling tanks

Low cost wastewater treatment for the world

Constructing the settling tanks

Why settling tanks?

A series of settling tanks provides a buffer between the vermifilter and the irrigation drippers. Without settling tanks, worms and detritus that exit the vermifilter can block irrigation lines. Unlike filters, settling tanks don't require regular maintenance. Settling tanks not only settle out any remaining solids from the effluent, but they also provide additional treatment to the wastewater. They are simple to construct, virtually maintenance free and are required for recirculation.

Constructing a series of settling tanks or "baffled reactors" is very simple using 200 litre plastic drums and tank fittings. The drums are joined together using one tank fitting to  join two drums. Six 200 litre settling tanks in series should settle sufficient sediment so drippers never block. The final tank 

Plastic tank fitting. Two drums can be joined together with one tank fitting

Water exits from inside one tank through here...

...and enters the next tank through here.

Black HDPE (alkathene) pipe fitted into the tank fitting. Water travels down the pipe to enter the bottom of the next tank.

Two tanks joined together with tank fitting. Once the tank fitting is screwed up the tanks will be tight against each other and waterproof.

The final tank in the series is the pump-out tank. The outlet into the pump-out tank (see red arrow below) should be 5cm higher than in the preceding settling tanks. This raises the water level in the series of settling tanks above the level of their outlets (purple arrows), so that scum on the water surface does not drain from one tank into the next tank.

When more water enters the settling tanks from the primary digester, the same volume exits the last settling tank and into the pump-out tank, creating an equilibrium water level. 

However, the water level will temporarily raise above that equilibrium level when water enters the settling tanks faster than it exits. When a surge of water is added (e.g.  emptying a washing machine or sink), the water level rises until that volume of water exits the system. Therefore the first settling tank requires surge capacity.  

The surge capacity in the diagram above is the volume above the equilibrium water level (red arrows)

Remember, the water level in the LAST settling tank sets the surge capacity in the FIRST settling tank. Don't put the outlets too near the top of the tank, in 200 litre drums allow 20-30 litres capacity above the tank fittings.