Now, let's backtrack a little and see what happens to the
solids which have been settled out of the liquid sewage. The
settled solids, from the primary clarifiers, are pumped to the
digesters where the solids are stabilized.
Activated sludge solids from the secondary clarifiers which
are not returned to the aerators are wasted. The DAF (Dissolved
Air Flotation) thickener tanks receive the wasted solids. Solids
enter the DAF tank where they are mixed with water and compressed
air. As the air and water mix, solid particles are lifted to
the surface by rising air bubbles in the tank.
The floating solids are then collected by a series of tank
skimmers while the water is recycled back to the raw sewer to
be processed through the plant. The solids from the DAF are pumped
to the anaerobic digesters.
The City of Elmhurst is modifying its anaerobic digester design
to separate the two phases of anaerobic digestion. New acid faced
tanks have been installed on the east side of the existing north
digester. These tanks will hold the raw and waste activated sludge
for a short period of time, oftentimes less than 24 hours. The
separation of these two processes will allow the different groups
of bacteria that carry out the anaerobic digestion to perform
in an optimum environment for each group. The acid producing
bacteria will have their individual stage and the gas producing
bacteria will have theirs. Experience has shown at other wastewater
plants around the country that when these two processes are separated,
the quality of the digestion is improved and the production methane
gas is increased. The anaerobic digestion process will dissolve
the solids to their basic components.
In the anaerobic digesters another group of bacteria begin
to digest and dissolve the solids to their basic components.
This process uses bacteria which do not need atmospheric oxygen
to survive, so therefore, no air is bubbled into the tanks. In
fact, air mixed with the gasses may be explosive, so we strive
to keep all air out. The anaerobic digesters produce a stable
sludge which is readily dewatered. The process is also a source
of methane gas, which is used as a fuel source for heating the
digesters, heating several buildings, and fueling the engine
generator to produce electricity. The digester is kept at an
optimum temperature of between 90-95 degrees F. About 40,000
cubic feet of methane gas is produced per day.