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Using BOD/COD for Aeration Control : Week 9/5/2011

Saving Electrical Energy in Waste Water Treatment

In last fortnight, we mentioned that Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are two important parameters in any Waste Water Treatment process. The consumption patterns of Oxygen are important in the process of Waste Water Treatment, in order to preserve the receiving water bodies and their aquatic life.

Efficient Aeration Control to optimize the process and reduce operating costs.

One of the major use of energy in a waste water treatment facility is for the operation of mechanical systems designed to introduce oxygen into the wastewater. This oxygen is necessary for biological treatment of carbonaceous matter (secondary treatment) and for oxidation of ammonia into nitrite and nitrate (nitrification).

Improper aeration can also have negative effects on sludge characteristics and on anaerobic and facultative processes such as denitrification and phosphorous removal.

If improved treatment is required or if peak treatment demand at times exceeds the original design capacity, these plants may experience poor process performance during certain periods, poor energy performance during other periods and composite performance approaching or exceeding permit requirements.

Aeration control can be divided into two broad subjects:

  • Measurement of process variables to provide information for control adjustments and
  • Implementation of the aeration control adjustments.
  • Let's see what are the available methods for measurement of process variables & their usefulness to the user:

    I) MEASUREMENT OF PROCESS VARIABLES

    (A) LAB Measurement:

    • Measurement after few hours
    • Manual error while measuring in the Lab
    • No control over aeration process
    • Results in excess / insufficient DO level due to which bacteria can die / septic condition
    • Continuous aeration leading to wastage of electrical energy

    (B) On line Measurement:

    • Continuous measurement and control of water parameters in Aeration Basin.
    • Uniform and universally standard measurement techniques give repetitive results.
    • Consistent measurement results in smooth operation of aeration system
    • Possible to control precise DO level with the help of control over aeration speed
    • May also help to keep BOD / COD values in control

    The option (B) can result in substantial energy conservation

    II) IMPLEMENTATION OF THE AERATION CONTROL ADJUSTMENTS:

    Transfer of oxygen can be accomplished by several methods. One method is the delivery of air through a network of pipes and diffusers located at the bottom of treatment basins (diffused aeration). Another method is the high speed operation of brushes, paddles and/or other mechanical arms at or near the surface of the wastewater (mechanical mixing).

    Some treatment facilities do not have the capability of adjusting the aeration rate in response to changes in demand. At these locations, the amount of aeration supplied to the process is always designed to be more than the expected demand. This results in energy costs that are well in excess of the minimum required for the treatment.

    Is there any way by which we can ensure proper measurement as well as control of aeration? Read more about it in our next article…