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Introduction

Wastewater treatment is critical in maintaining the quality of water. Wastewater consists of water exiting residential, business, and industry buildings via drains; this includes water from toilets, washer machines, dish washers, showers, and baths. The water is transported to local wastewater treatment facilities and undergoes a series of treatments in order to remove pollutants that

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 systems are 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 a 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
  • Trickling filter
  • Oxidation ponds

There are pros and cons to each of these three processes. Cost along with space are two factors that will often determine which technique is appropriate. additionally wastewater treatment plants need to consider maintenance, reliability, and effectiveness of the system.

Activated Sludge

Process: During the activated sludge process primary effluent flows into an aeration tank, where it is mixed with microorganisms . The aeration tank injects a steady supply of oxygen or air into the wastewater, ensuring that the organisms have an adequate supply of oxygen needed to breakdown the organic matter that remains in the effluent. The effluent then flows into secondary settling tanks. At this point the sludge goes in one of two directions; 1. back to the aeration tank or 2. to the sludge digester[2]. 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 operating cost (air pumps)
  2. High energy expenses for oxygen demand

Trickling Filter

Process: When primary effluent is transported to a trickling filter system the wastewater is dispensed upon a bed of media, such as rocks, stones, plastics, or salts. The effluent flows through the material at a slow enough rate to allow microbial growth on the surface of the media. Once microbial growth takes place additional wastewater flow has contact with microorganisms; this contact ensures that the organic matter in the primary effluent is 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 a key figure in the oxidation pond system. Algae are much like the aeration tank in the activated sludge system; they deliver a steady flow of oxygen.
Pros:

  1. Small energy input
  2. Degrades Nitrogen and Phosphorus

Cons:

  1. Occupies a large area
  2. Possible odors
  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 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 is transported to a tertiary treatment[3] system and disinfection 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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