8937-17 Authorize Design-Build Contract for WWTP

Home Browse Search

The primary digestion system consists of two (2) 75' diameter x 225 side water depth (SWD) circular anaerobic digestion tanks. Primary and waste activated sludge along with skimming from the primary and final clarifiers is pumped to the primary digesters. The primary digestion system is designed to operate in the mesophilic range (95 °- 105 °). The thickened primary and waste activated sludge mixture is pumped continuously throughout a 24 -hour period to the primary digesters. Alternating the feeding of sludge between the primary digester tanks on a 24 -hour cycle allows for the best methane gas production. Each primary digester tank contains three (3) mechanical mixers used to insure a homogenous sludge mixture. The level in the primary digestion tanks is controlled by gravity feeding sludge from the primary digester tank to the corresponding secondary digestion tank. Sludge Recirculation /Heating Primary digested sludge is recirculated from approximately the mid -point of the primary digestion tank by the means of a recirculating pump /grinder combination. The fresh sludge being pumped into the primary digestion tank mixes with the warmer recirculated sludge. If the incoming /recirculated sludge mixture is warm enough it bypasses the heat exchangers otherwise it passes through the heat exchanger for further heating. This is accomplished by the use of temperature probes and automated valves. Boilers /Heat Exchangers The boilers /heat exchangers serve a dual purpose. The heat exchanger portion of the system is used to maintain the temperature of the .recirculated /incoming sludge in a mesophilic range. The second function is to heat water that is recirculated throughout the facility for heating the buildings during cold weather. The boilers are dual fuel capable; this allows them to burn not only methane gas produced by the primary digestion process but also heating oil. Heating oil is only used when there is insufficient methane gas or the BTU value of the methane gas is too low to maintain the desired water temperature in the boilers. In addition, there is an electric boiler used to supplement the heating of the water used in heating the buildings. Note: No heating oil has been used in the last four (4) yews. Secondary Digestion Tanks There are two (2) secondary digestion tanks 60' diameter x 22.5' SWD. Sludge from the primary digester tanks flows by gravity into the corresponding secondary digestion tank. The secondary digestion tanks are not heated. The secondary digestion tanks are primarily being used as holding tanks for digested sludge until it can be dewatered for disposal. There is a jet nozzle mixing system located in each of the secondary digestion tanks that keeps the sludge in a homogenous state. Presently the secondary digestion tanks are kept at a low level in order to keep supernatant solids from decanting back into the facility. Methane Gas Collection /Use Methane gas is collected under the fixed covers of the primary digestion tanks. As the pressure of the methane gas increases it is routed through a series of gas collection lines 3 1 P a g e