WWTP Tour - Secondary Treatment
Oxygen Trains

The activated-sludge technique is a biological treatment process where aerobic (i.e., requiring free dissolved oxygen for their respiration) organisms are contacted with the wastewater in an environment suitable for their proliferation. The function of the activated-sludge unit process is to convert organic substances in finely divided, colloidal, and dissolved form into oxidized products and a settleable floc. This floc, known as "activated sludge", is later removed from the wastewater in the clarifiers leaving a high quality effluent.

Our pure-oxygen system uses covered and staged oxygen trains for treatment. The oxygen trains are the heart of the activated-sludge process. Here the incoming wastewater is mixed with the activated sludge being returned from the secondary clarifiers. The resulting mixture of sludge and wastewater is called "mixed liquor". Pure oxygen is also supplied to the tanks in sufficient quantity to maintain the process. Activated sludge is a living biomass that utilizes the organic substances in the wastewater as a food thereby removing these wastes from the wastewater stream. The biomass which make up the activated sludge are the workers in the treatment process. They also forms a lacy mass that entraps material not used as food. This is a complex and interrelated system of biological processes. Large mixers are used to keep the solids in suspension and oxygen distributed.

The Oxygen Trains discharge to the secondary clarifiers for separation of the "mixed liquor" into it's two main components, activated sludge and treated water. The sludge settles by gravity while, the water flows out the top. A portion of the settled activated sludge is recirculated back to the oxygen trains to be mixed with the incoming wastewater and provide a continuous process. There the biomass begins feeding all over again on the organic material in the wastewater, stabilizing it and creating new organisms. The remaining sludge is sent to the sludge handling facilities for dewatering and stabilization. This is referred to as sludge wasting.

Since oxygen gas is fed to the system, it is devoid of nearly all nitrogen. This decreases, the total gas venting from the system to about 1 percent of the gas vented from an air activated-sludge system. In addition, the covered reactors permit venting of this gas at one point, thus achieving effective odor control and eliminating the biological aerosol problem typical of air systems. Other advantages include increased bacterial activity, decreased sludge volume, reduced aeration tank volume, and improved sludge settleability.

Now settle down, as we're now off to the the Secondary Clarifiers.