with an upper (flowing-through) compartment for primary settling and a lower one for
sludge digestion or secondary settling (see Figures 1-8 and 2-11). It may be
rectangular or circular (see Figure 2-12). Solids settling from the wastewater detained
in the tank pass through a slot in the hopper bottom of the flowing-through
compartment. The slot is overlapped so that gas produced by and rising from the
digesting sludge in the bottom compartment is diverted and does not enter the flowing-
through compartment. This permits a settling of the solids that is unhampered by rising
gas. The inside walls and a hopper should be scraped frequently with a squeegee to
prevent build-up of septic solids. The flowing-through slot is cleaned by dropping one
end of a heavy chain through the slot and dragging the chain along the length of the
slot. The operation and cleaning of the digestion compartment is similar to that for
separate sludge digester (para 2-14).
d. Maintenance. Good housekeeping at the settling tank is essential to prevent
odors, flies, and unsightly appearance. Floating solids passing out with the effluent can
clog filtering equipment and grease can cause ponding of filter media. Normal or
regular inspecting, cleaning, adjusting, and lubricating of all mechanical equipment
should be scheduled. Daily inspections for unusual noises, vibrations, and uneven
action of mechanical equipment should be done. If these indications are noted, the unit
should be shut off and the cause found and corrected.
(1) Skimming devices. Floating material must be removed once each shift
or more often if present in large quantities. Some mechanical skimmers automatically
remove materials to a sump for disposal; other tanks have a manually operated
skimming tool. However, a hand-skimming tool (see Figure 2-8) should always be used
to facilitate entrance of skimmings into the pipe or trough. When skimmings are
pumped to the digester, a minimum of wastewater and wash water should go with it to
prevent upsetting the digester operation. If large quantities of fairly dry skimmings tend
to upset digestion, they should be collected in a covered can with openings for draining
excess water and hauled to a sanitary fill or incinerator. The can must not be placed
where drainage becomes a nuisance. If a fill is not available, skimmings are removed to
a trench and covered with at least two feet of earth. A spray of water under pressure
directed against floating material frequently settles it.
Perimeter and bottoms.
(a) Sidewalls of channels, baffles, weirs, and tanks are kept clean of
grease and other solids by hosing, scraping, or brushing once each day or more often, if
(b) Dead ends and corners are brushed at least once each shift. Fine
sand and gravel are removed for burial or used as fill.
(c) Decks and walks are hosed at least once each day. Where
pressure is not available for hosing, secondary effluent may be used with a portable
pump, pressure system, or other pumping equipment.