After filter beds have been cleaned the rate of filtration, for a while, is much faster than at other times, and is often too great for efficient results. Under working conditions an excessive rate of filtration might disturb the jelly layer and permit raw water to pass through with but little nitration. To regulate the rate of filtration, automatic controllers, Fig. 111, are now generally used. In this type of controller the outlet is fitted with removable bushings that regulate the discharge to the desired velocity; if the rate of filtration then becomes greater than can be discharged, water will fill the controller box and raise the float, thus throttling the balance valve and reducing the flow to the required rate.

Fig. 1ll

Fig-. 112