The first step of wastewater treatment consists of primary clarification
utilizing four 60-foot diameter basins. Designed for a two-hour detention time,
these clarifiers serve to settle out the larger solids. The settled solids are called
primary sludge (bio-solids) which is pumped from the bottom of the clarifiers for
The primary clarifier effluent flows to four 64-foot square aeration basins.
This stage is the complete mix activated sludge process that mixes
and provides oxygen to the wastewater to optimize the growth of bacteria and other
organisms for the biological treatment of the waste. The basins are designed to
provide four to six hours detention time.
The activated sludge then flows to four 65-foot diameter secondary clarifiers.
These clarifiers are designed for a two- to three-hour detention time and provide
the final settling of activated sludge solids in order to achieve liquids - solids
separation. A portion of these settled solids (activated sludge) is returned to
the aeration basins to maintain optimum biological growth and conditions. Excess
activated sludge is pumped to additional solids processes.
Secondary clarifier effluent has now achieved over 90% removal of suspended solids
and biochemical oxygen demand, and is then discharged to Rock Creek, a tributary
to the Missouri River.
Solids Processing and Disposal
The sludges (bio-solids) removed from the wastewater must now be de-watered and
conditioned for final disposal. Sludges from the primary clarifiers are sent through
cyclone degritting and a gravity thickener. The primary sludge is
delivered at .5% solid slurry through cyclone degritting and thickener which converts
the solids to 8 to 12% settled thickened sludge. To secure the thickening process,
a mild dose of sodium hypochlorite is applied.
The secondary sludges (waste activated sludge) are sent to dissolved air flotation
units of process. This waste activated sludge of .8 to 1.5% solids is now injected
with air and polymer which thickens the sludge to 4%.
The gravity thickened (primary sludge) and flotation thickened waste activated sludge
are delivered to a mixing/blending tank. Mixing and blending are accomplished
with air diffusers. The blended sludges are now pumped through grinders and conditioned
further in a low pressure wet air oxidation unit (thermal conditioning).
Wet Air Oxidation
Wet Air Oxidation is designed to receive a 5% blended primary and secondary sludge
mixture. The system is also designed to condition 3,200 gallons per hour. The process
utilizes a high pressure bag pump which operates at approximately 450 psig. The
pump sends sludge through two sludge-to-sludge heat exchangers and a 30-inch diameter
by 30-foot high reactor.
A high pressure air compressor injects air into the sludge flow at the entrance
of the first heat exchanger inflow pipe. A steam generator operating at 400 psig
sends hot steam into the reactor. The reactor operates at 385° F for
30 minutes at a pressure range of 310 psig.
The heated sludge is then delivered to a decant basin which allows the 5%
oxidized sludge to settle to a 12 to 15% solids content. The water overflow at the
decant basin, along with gravity thickener overflows are sent back to the aeration
basin for further wastewater treatment.
Gravity Belt Filter Presses
Thickened, oxidized sludge is pumped to
gravity belt presses where the slurried
sludge is spread between woven cloth belts that travel through a series of large
rollers that successively squeeze the remaining water from the sludge. The resulting
sludge "cake" rolls off the press in a sheet of solid sludge that is dry enough
to easily incinerate.
Fluidized Bed Incinerator
The sludge cake is screw augered into a fluidized bed incinerator that contains
a bed of fine sand suspended by constant air injection and incinerated at a temperature
of 1100°F. The sludge typically contains enough volatile organic matter that it
self sustains the burning process without the addition of fuel oil. Incineration
reduces the sludge volume as much as 90% and results in a fine ash that flows through
the exhaust of the incinerator where it is converted to an ash slurry when washed
down in a three tray scrubber. The scrubbed, hot air is continuously monitored for
air quality prior to the stack exhaust and the washed down ash is pumped to an ash
press similar to the sludge presses. The ash is processed into a dry product
that is conveyed to dump trucks for final disposal at the landfill.