TM 5-814-3/AFM 88-11, Volume III
(5) pH control. The pH level of the sludge inside the digester is a critical factor in anaerobic digestion
and will be kept as near to 7.0 as possible, with a range of 6.6 to 7.4 considered acceptable. Also, monitoring
of the volatile acids-to -alkalinity ratio is important. The pH is maintained with bicarbonate buffering and,
when natural buffering fails and the pH becomes less than 6.6, hydrated lime (calcium hydroxide) should be
added to the digester. Design provisions must be made that will provide a simple means for adding lime to
the digester if and when needed. One of the more practical means is to provide for convenient manual
addition of lime to the raw sludge pit before the raw sludge is pumped to the digester.
c. Tank element design.
(1) Tank dimensions. No particular shape possesses advantages over all others but circular tanks are
more popular. Circular tanks will not be less than 20 feet or more than 100 feet in diameter. Side-wall water
depths will be a minimum of 20 feet and a maximum of 30 feet. A 2.5 feet freeboard will be provided between
the top of the wall and the working liquid level. With mechanisms for removing sludge, the bottoms of the
tanks will be flat; overwise, hopper bottoms with steep slopes of 3 feet horizontal to 1 foot vertical will be
provided. All tanks designed for treatment plants rated at or above 1.0 million gallons per day will be multiple
units.
(2) Covers. Two types of covers are used on sludge digestion tanks, fixed and floating. If a combination
of covers is used, fixed covers will be used for the primary stage of a two-stage digestion process, and
floating covers will be used for the secondary stage. In lieu of floating covers on separate digesters and in
cold regions where freezing ice and snow are problems, fixed covers may be used provided a gas collection
dome is installed in the top of the cover. At least two access manholes will be provided in the tank roofs. In
addition, the tank covers will be provided with sampling wells, pressure and vacuum relief valves, and flame
traps.
(3) Control chamber. Entrance to the control chamber must be designed with the safety of the operator
and the equipment foremost. The chamber will be well-lighted, ventilated, and equipped with a water service
and drain. All sludge-heating equipment, gas piping, gas meters, controls and appurtenances will be located
in a separate structure. All the above-mentioned structures will be of explosion-proof construction.
(4) Piping. The particular piping requirements for sludge digesters will include provisions for adding
sludge, withdrawing sludge, multi-level supernatant removal points, heating, recirculating sludge or super-
natant, flushing, sampling gas collection, and gas recirculating. All supernatant will be returned to process
for further treatment. Supernatant draw-off facilities will be designed to provide variable-rate return to
prevent plant upset.
(5) Heating. The method to be used for heating sludge digestion tanks is the circulation of the contents
of the tank through a heat exchanger. Heated tanks will be insulated and the heating equipment sized to main-
tain a temperature of 95 degrees Fahrenheit during the coldest weather conditions.
(6) Chemical feeding. Practical means for feeding lime or other chemicals that are commonly used to
correct digester operation problems must be included as part of the digester design.
(7) Gas collection. Sludge gas will be collected from the digesters either for utilization or for burning
it to waste. Two-stage units will provide interconnecting lines, permitting transfer and storage from one unit
to the other. Gas withdrawal will be from a common point.
(8) Gas utilization. Gas storage facilities will have to be provided if the gas is to be utilized and not
wasted by burning. Sludge gas has a heat value of between 500 and 700 British thermal units per cubic foot.
An average gas yield is 15 cubic feet per pound of volatile solid destroyed.
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