The primary sludge pumped from the bottom of the primary clarifiers in Step Three, along with the continuous flow of waste activated sludge from the aeration / activated sludge process in Step Four,
must be treated to reduce volume and produce a usable end product. The
sludge treatment process involves four steps as described below.
Step One: Air Floatation Thickening (AFT)
In this step, the activated sludge removed from the aeration process is
thickened by attaching the biological solids to minute bubbles of air.
The floating mass of solids is then removed using surface skimmers.
The water removed from the solids is returned to the head of the plant
for treatment. In this step, the solids concentration is increased from
less than 1% to 3.5%.
Step Two: Anaerobic Digestion The thickened activated sludge from the AFT is combined with the primary sludge in this step and pumped into the primary digester. In this tank, the combined sludge is heated to 98 degrees Fahrenheit and allowed to mix for an average of 35 days. This process is also biological, but uses anaerobic bacteria for treatment. The pollutants are digested and converted to cell mass, water, methane gas, and carbon dioxide gas. The process is heated entirely by the methane gas produced by the biological activity in the digester.
Step Three: Gravity Belt Thickening After digestion, sludge is pumped from the digester system to the gravity belt thickener to be thickened once again. To allow the water to drain away from the solids in the sludge, polymer is added to the sludge as it is pumped from the digester. The polymer treated sludge is then directed to a porous, traveling belt where the water drains through the belt and into a collection basin.
This water, called filtrate, is very high in ammonia and is pumped
to a holding tank where it is slowly metered back to the head of the
plant for treatment over an extended time period. The thickened sludge
is taken off the porous belt and pumped to storage where it is held for
up to eight months before it is pumped out to tank trucks which take it
to agricultural spreading sites. The thickening process is a batch
process operated approximately eight hours every week.
Step Four: Biosolids Hauling and Injection
The ultimate disposal of the remaining sludge from the treatment
process is an important recycling measure exercised for the mutual
benefit of the city and area farmers. The sludge held in storage after
thickening in step three is hauled by tank truck in the spring and fall
by a contracted biosolids handling company.
The sludge, now
termed biosolids, is transported to farm fields where it is pumped by
injection equipment. This equipment injects the biosolids into the soil
where it provides the farmer with valuable soil amendments including
ammonia, nitrogen, phosphorus, and potassium. The city’s treatment
facility produces an average of 1.7 million gallons of biosolids