Our secondary, or biological treatment, simply duplicates nature’s own water purification process. Except that it’s done under contained and controlled conditions and usually at faster rates.
In the presence of oxygen in the water, microorganisms (mostly bacteria and fungi) consume the oxygen. In the process they eat the dissolved organic matter in the
effluent, and convert it into carbon dioxide and water.
Normally this is done in large aerated stabilization basins covering 20 to 50 acres with aerators mixing the effluent with the air to make the process work. In our case a large area was not available at the mill site. Instead, we installed a “high rate oxygen activated
sludge” secondary treatment system.
This system is a totally enclosed concrete basin divided into two sides, each with four chambers. Again, microorganisms do the work by feeding on nutrients. To keep the microorganisms healthy, we add ammonia and phosphoric acid to the primary treated effluent. Sludge from the secondary
clarifiers (Return Activated Sludge or RAS) is also added to maintain a healthy population of microorganisms.
Aerators in each of the four chambers splash the effluent mixture up and into an atmosphere of nearly pure oxygen. The high concentration of oxygen allows a small space to hold a very large population of microorganisms, which consume the dissolved organic material (
BOD) in the effluent. After this treatment, the effluent released into the Kaminsitiquia River will only consume a small amount of dissolved oxygen leaving enough oxygen for fish and other biological organisms to survive.
But before the effluent is released into the river, the solid material must be removed. The effluent is sent to
secondary clarifiers, which work in the same way as the primary clarifiers. The solid material to be removed consists mainly of microorganisms (bio-mass). The material is composed of some living and some dead organisms. Because this is a living system, organisms are aging and dying off as new ones are being produced.
As the sludge is collected from the bottom of the secondary clarifier, part of the effluent stream is sent to the sludge treatment system. Here it’s
de-watered and mixed with primary sludge and sawdust before being burned in #6 Power Boiler (as Waste Activated Sludge or WAS). The remaining secondary sludge goes back to the front end of the secondary treatment system (as Return Activated Sludge or RAS) to provide additional living organisms to keep the system going.