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.