d. The floc (also referred to as slurry or sludge) appears as small, grayish-rust
colored particles floating about in suspension in the lower portion of the erdlator. These
particles are slightly heavier than water, so that in the upper portion of the erdlator tank,
or separator zone elements, water is recirculated to the wet well tank and the flow
controller valve (see figure 5-11, circle D) is set on a rate of flow which wiII just hold the
coating on the elements (10 gpm).
e. Clear water at the top of the erdlator tank is collected in the effluent launder
(see figure 5-10, circle I) that acts as a double-edged weir. From the effluent launder,
the clear water passes through a discharge tube (see figure 5-10, circle J) into the wet
well tank (see figure 5-10, circle K) that provides limited storage for coagulated water
and acts as a sump for the filter pump. An overflow pipe in the wet well tank permits
operation of the erdlator at rated capacity when the filter is stopped. A float valve
connected to the raw water line permits raw water to enter the erdlator assembly when
the water in the wet well falls below a preset level. When this occurs, a warning light
and warning buzzer are actuated to alert the erdlator operator of the condition.
f. The slurry level is controlled by a continuous withdrawal of a small amount of
slurry from the top of the slurry blanket by means of a draw- off port through the slurry
weir box (see figure 5-10, circle L) into the sludge concentrator tank (see figure 5-10,
circle M). The sludge concentrator tank functions as a small auxiliary clarifier. It
provides a longer holding period for slurry concentration and permits settling of slurry in
the bottom of the tank for continuous or intermittent drainage to waste.
The construction and operation of a diatomite filter was discussed in paragraph
1-14. The following discussion will amplify the details of filtration in the WPU during the
three phases of operation: precoat, filtration, and backwashing.
a. Precoat. Before water can be filtered, it is necessary to precoat the filter
elements with diatomaceous earth. The erdlator operator prepares slurry consisting of
1/2-pound diatomaceous earth in one gallon of coagulated water and pours it into the
precoat funnel (see figure 5-10, circle A). The filter pump is then started and coagulated
water from the erdlator wet well tank (see figure 5-6) is introduced into the filter. Filtered
water is recirculated back to the wet well tank until precoating is completed in order to
ensure that no unfiltered or partially filtered water is pumped into the storage tanks.
This may be determined by observing the filter elements through the filter window (see
figure 5-11, circle B). Initially, the water inside the filter appears cloudy as the diatomite
particles move about in suspension. As precoating continues, the water gradually
becomes clearer. When the precoating process is completed, the filter elements can be
viewed through the window. Each filter will be covered by a white film of diatomite.