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Introduction

Brief History

In response to the escalating pollution of natural bodies of water, the Federal Government endorsed the Clean Water Act [1] in 1972. This act required that wastewater treatment plants operate with a secondary treatment system. The system was designed to remove pollutants that generally would be released into a natural body of water, such as, lakes, rivers, and bays.

Brief Explanation

Secondary treatment acts as an additional filtration system in the wastewater treatment process. Once wastewater has gone through the primary treatment stage the effluent will undergo a secondary treatment in order to remove both small suspended and soluble organic materials that pass through the primary treatment stage (on average 60% of suspended solids and 35% of BOD5 is removed by primary treatment [1]). Upon entering secondary treatment effluent will undergo a biological process.There are a variety of secondary treatment process. The following are conventional processes used by treatment plants:

  • Activated sludge process
  • Trickling filter process
  • Oxidation ponds

There are pros and cons to each of these three processes. Cost and the population size are two factors that will often determine which technique will be used. In addition to cost and population size wastewater treatment plants will consider maintenance, reliability, and effectiveness of the system.

Activated Sludge Process

  • Process:

During the activated sludge process primary effluent will flow into an aeration tank, where it will mix with microorganisms . The aeration tank injects a steady supply of oxygen into the wastewater-microorganism mix, referred to as sludge, ensuring that the organisms have an adequate supply of oxygen needed to breakdown the organic matter that remains in the effluent. The effluent will then flow into secondary settling tanks. At this point the sludge will take one of two directions; 1. back to the aeration tank or 2. to the sludge digester. The treated water will enter the tertiary treatment stage, here it will go through the final treatment stage before it is released into a natural water system.

  • Pros:
  1. Low construction cost
  2. Occupies small area
  3. Relatively low odor
  4. Removes a high percent of BOD
  • Cons:
  1. High operation cost
  2. High energy expenses for oxygen demand

Trickling Filter Process

  • Process:

When Primary effluent is transported to a trickling filter system the wastewater will be dispensed upon a bed of material, such as rocks, stones, plastics, or salts. The effluent will flow through the material at a slow enough rate to allow microbial growth. Once microbial growth takes place additional wastewater flow will have contact with microorganisms; this contact will ensure that the organic matter in the primary effluent will be broken down. The effluent that flows through the bed of material will be transported to the secondary settling tank. The materials that settle will either enter a digester or re-enter the trickling system.

  • Pros:
  1. Low Construction Cost
  2. Straightforward system
  • Cons:
  1. Temperature Dependent
  2. Vulnerable to congestion, which can lead to flooding and system failure, low oxygen supply, and restricted water flow
  3. Occupies a large area
  4. High Maintenance

Oxidation Ponds

  • Process:

Oxidation ponds are large and shallow; a typical depth would range from 1-2.5m. The ponds are composed of microorganisms, which feed on the organic matter received from primary effluent. Algae are the byproduct of the decaying organic matter. They are a key figure in the oxidation pond system. Algae are much like the aeration tank in the activated sludge system; they maintain a steady flow of oxygen.

  • Pros:
  1. Small energy input
  2. Degrades Nitrogen and Phosphorus
  • Cons:
  1. Occupies a large area
  2. Possible odor
  3. Slow process
  4. Long retention times
  5. Climate dependent

Conclusion

All secondary treatment systems use a biological process to break down organic matter. The biological process that occurs naturally in nature is accelerated in the treatment system. Secondary treatment is highly effective and will typically remove 85% of BOD and suspended solids [2]). Water exiting secondary treatment will still carry nitrogen, Phosphorus, heavy metals, pathogens, and bacteria. This water will travel to a tertiary treatment system for further removal of pollutants.

Links

References

  1. avis, Mackenzie L., Masten, Susan J. (2004) Principles of Environmental Engineering and Science New York, McGraw-Hill
  2. avis, Mackenzie L., Masten, Susan J. (2004) Principles of Environmental Engineering and Science New York, McGraw-Hill
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