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McDonald Group International, Inc

Water, Wastewater, Environmental Engineering 



Overview of Treatment Processes Available

Wastewater that is primarily domestic in origen can be treated with the following processes:

  • Biological Aerobic
  • Biological Anaerobic
  • Physical
  • Physical-Chemical

Generally speaking, there is an inverse relationship between the land area needed for the treatment plant and the level of complexity in the plant. Aerobic and physical chemical plants will require far less space than anaerobic processes such as facultative lagoons. In addition, the more complex plants provide a far higher level of treatment.

Aerobic, Biological Treatment of Domestic Wastewater

In this process, the raw wastewater is first aerated in an aeration tank. This allows a certain kind of microorganism already present in the water, called aerobic bacteria, to grow. As the bacteria grow, they consume the other oxygen depleting material in the water and grow into visible "chunks" of material called floc or sludge. After aeration, the sludge filled water flows to a settling tank. In this tank, the sludge settles out of the water leaving a clear zone of treated water at the top and settled sludge at the bottom. The settled sludge is recycled out of the settling tank and mixed with the raw wastewater. The clear water at the top is disinfected with chlorine, and can then be discharged.

There are several variations on this process, such as:
  • Extended Aeration
  • Contact Stabilization
  • StepFeed Aeration
  • Nutrient Removal

The variations are based on:

  • How long the wastewater is aerated;
  • Where the sludge recycle comes back to;
  • Where the raw wastewater is introduced;
  • Control of dissolved oxygen in the plant
In an extended aeration plant, the buildup of sludge in the plant is slow, but does have to be periodically removed. When it is time to be removed, sludge can be diverted to a holding tank called a digester, where it can be thickened and stabilized. The waste sludge can be used as a fertilizer in certain circumstances. Other processes can be added to the plant to treat the water enough so that it can be used for irrigation or to remove nutrients such as phosphorus and nitrogen. For treating domestic wastewater and some kinds of industrial wastewater, removal of nitrogen and phosphorus can be accomplished biologically. as is done in the plant in the picture below. The addition of filters and providing higher doses of chlorine removes most of the residual suspended solid matter left over from secondary treatment processes. This allows the water to be recycled for irrigation in parks, golf courses, and so forth. A advanced treatment wastewater plant appears in the photo below: