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http://soundbook.soundkeeper.org/chapter.asp?ContentID=216&SectionID=7

Introduction

Brief History
Federal law requires all treatment plants to operate with a secondary treatment system. This requirement was put in place through the Clean Water Act of 1972[1].
Brief Explination
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 two separate processes, which are biological and settling.There are a variety of secondary treatment process. The following are two common process used by treatment plants:

  • Activated sludge process
  • Trickling filter process

Activated Sludge 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, aslo reffered 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 ttwo 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 throught the final treatment stage before it is released into a natural water system.

Trickling Filter Process

When Primary effluent is transported to a trickling filter system the watsewater 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.

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