Aeration in a Wastewater Treatment Plant

Aeration is the addition of oxygen to wastewater.  Aeration chambers are used in packaged and extended aeration plants to provide an aerobic environment for the bacteria breaking down the organic matter.  RBC's and trickling filters add oxygen by passing the microorganisms or wastewater through the air.  Step aerators are often used at the end of the treatment process to increase the DO levels before releasing the effluent.  

Aeration Chambers

In packaged plants and extended aeration plants, oxygen is forced into the wastewater using an aerator.  In these cases, the size of the bubbles in the pumped air will influence the concentration of oxygen in the water.  Small bubbles can be twice as effective at the transfer of oxygen from the bubble to the surrounding water because of the greater surface area to mass ratio of the bubble.  

The amount of air pumped
into the water will also influence the oxygen concentration in the water in a packaged plant and an extended aeration plant.  The amount of air pumped is proportional to the amount of time - the longer air is pumped through the water, the more oxygen will enter the water.  As a result, extended aeration plants tend to have a greater D.O. concentration than packaged plants due to the extra two hours during which air is pumped through the wastewater.  

The Cost of Adding Oxygen to Water

Introducing oxygen to water can be very expensive.  The cost of pumping oxygen into water for B.O.D. removal varies from a high of $400 per ton of B.O.D removed to $1,200 per ton of B.O.D. removed.  

This cost can also be considered in terms of environmental degradation.  $1 of energy cost puts 10 pounds of carbon dioxide into the air.  This does not even include the oxides of nitrogen which are introduced into the air.  

If the energy is harvested by burning coal, we must also consider the sulphur released into the air.  Coal costs $20 per ton.  Each ton of coal is 5% sulphur, meaning that 100 pounds of sulphur is released into the air for every ton of coal which is burned.  

So let's calculate how much environmental pollution occurs for every ton of B.O.D. removed from wastewater using the lower figure above of $400/ton:

So 4,000 pounds of carbon dioxide are released into the air for every ton of B.O.D. removed from wastewater.  In addition:

And 2,000 pounds of sulphur are also released for every ton of B.O.D. consumed.  

Given both the monetary and environmental costs, it is easy to see why we strive to maximize the efficiency of oxygen addition to wastewater.  The most efficient method of introducing oxygen to water is by mimicking nature and moving water through the air.  Oxidation ditches use this method to some extent, as do trickling filters and RBC's.