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 can exit the vermifilter and block irrigation lines. Instead of settling tanks, inline filters could be used but these would require regular maintenance. Settling tanks not only settle out any remaining solids, 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 drums and tank fittings. The drums are joined together, one tank fitting joins two drums.

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

Water exits from inside one drum through here...

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

Note that the final settling tank that discharges to the pumpout drum should have an outlet slightly higher than the preceding settling tanks. This ensures the outlets from each settling tank are below the water level in the tank so that scum on the surface does not drain into the next tank.

The final outlet (red arrow) sets the water level in the preceding drums. The outlets for the settling drums (purple arrows) are below the water level set by the final settling drum.  This is the equilibrium water level, whereby when more water is added from the primary digester, the same volume exits the settling drums into the pump-out drum. 

Make sure this level is not too high, because the first drum requires surge capacity. If a surge of water is added (e.g.  emptying a bath), capacity is required until that volume exits the system.

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