Raw sludge is pumped to a primary digester and heated to body temperature (97°F to 99°F). At this temperature bacteria breaks down the organic matter to water and gases. Not all of the organic waste can be broken down, what is left is processed more at the Secondary Digester and Belt Filter Press.
The primary digesters are circular with fixed covers and measure 60 feet in diameter and 20 feet in depth. The volume of each primary digester is 422,770 gallons and a total volume for all three primary digesters is 1.27 million gallons. The design detention time for the primary digesters is 18.75 days. The primary digesters are mixed using three individual draft tube mixers. Two of the mixers run in forward and the third mixer runs in reverse. The order is changed every 8 hours. This helps to keep any scum blanket from forming on top of the digester. The sludge is displaced by raw sludge addition and is transferred to the secondary digester through a series of overflows. The primary clarifier sludge gets pumped into digester #1 from 8:00 a.m. to 4:00 p.m., digester #2 from 4:00 p.m. to 12:00 a.m. and digester #3 from 12:00 a.m. to 8:00 a.m. The design loading is 0.21 lbs/ft3/day of dry solids and loading for January 2007 to October 2008 was 0.17 lbs/ft3/day.
Each digester has one heat exchanger rated at 500,000 BTU/hr and one centrifugal sludge-circulating pump pumping at 300 gpm.
Two gas-fired boilers with an output of 2,000,000 BTU/hr provide heat through a series of three-way valves. The boilers are dual fuel units; they normally run on sludge gas but can be changed over to natural gas if the volume of sludge gas drops too low. Sludge gas is produced as part of the sludge digestion. The three primary digesters provide the production of the gas supply. Digester #4 can produce some, but is mainly for storage, and therefore digester #4 is not heated or mixed. The sludge gas is mainly made up of methane and carbon dioxide. The digesters make an average 126,317 cu/ft/day of gas and use 97% of it to run the boilers. The gas system is made up of six-inch pipe connecting all of the four digesters together. The secondary digester has a floating gasholder that provides a means for maintaining even gas pressure as production and consumption of the sludge gas varies. Each primary digester has a pressure/vacuum relief and flame arrester assembly with gate valve at the center of the fixed cover. The pressure and the vacuum relief covers rest on a seat, and have removable weights to adjust settings. The normal range of pressure for the pressure relief is 11 to 13 inches of water column. The total heat exchange capacity for the sludge heating system is 36 million BTU/day with the energy required to raise the digester contents one degree F/day being 10.8 million BTU/day. The digesters are kept at a temperature of 98 degrees Fahrenheit. This is done by continuously pumping the sludge though the heat exchanger with the 3-way valve set to let enough hot boiler water to the heat exchanger to keep the sludge-out temperature at 100 degrees. The three primary digesters are monitored for percent solids, volatile solids, VA/alkalinities and pH on a weekly basis.
The raw sludge volatile solids are an average of 75%, this is reduced to 51% in the primary digesters. The percent of sludge volume reduction from raw to land applied was 67.5% in 2007.