Dual drain tank
Today, circular tanks with a dual drain outlet system are used on some farms (Fig. 5.10). Here the tank is used as the first purification step. In a dual drain system gravitational forces, and the fact that the waste particles are denser than water, are uti-lized to separate the particles and collect them at one point in the tank where a particle outlet is placed and a small amount of water can be with-drawn to flush the particles out, while the main water flow can be withdrawn elsewhere, normally higher up in the water mass. Several methods can be used to do this, and a number of different dual drain systems exist.
By separating the water flow in this way, purification is achieved inside the tank. The particle concentration in the particle outlet is considerably higher than in the outlet withdrawn higher up in the water mass. This can be the first purification step, and may be the only one. Since the amount of water coming through the particle outlet is much lower than the total flow, only a small part of the water flow has to be treated (Fig. 5.11), so a much smaller filter can be used. As the water flow through the particle outlet can be stable, a swirl separator is very suitable. The amount of water sent through the particle outlet in the dual drain system varies from 0.5 to 20% depending on the system used.