THE OVERALL PROCESS:
An overview of worldwide processes
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Sewage goes through several stages of treatment. There is a stage called
Preliminary
treatment that deals with large solids through the use of screens
or by the use of ore maceration. These solids are then returned to the
sewage flow. Also in this stage grit is removed in special tanks. The sewage
then receives primary treatment consisting sedimentation tanks where solids
settle out and form sludge. The next stage, secondary treatment
occurs when microorganisms are used to take organic matter out of solution.
The result is a sludge which can be settled out in a final settling tank.
Tertiary treatment further reduces the organic matter content if
necessary. Nutrient removal reduces the phosphorus and nitrogen in the
sewage which hinders plant growth in the receiving waters.
BIOLOGICAL FILTERS (The start of secondary treatment):
Sedimentation treatment even when aided by chemical treatment can not reach the high standards set today. Thus a second stage or secondary treatment was required. At first secondary treatment consisted of treating the sewage on percolating filters. These consisted of circular beds of stone about 1m deep over which the settled sewage was distributed by rotating pipes that sprinkled the sewage over stones. Gradually a slime built up on the surface of the stones and absorbed dissolved material out of the settled sewage through microbial action thus reducing the BOD of the affluent. As the slime layer built up on the stones it increased the tendency to shear off. It was found necessary to trap this slime by passing the effluent from the filters through a second set of settling tanks. The effluent after this secondary treatment and settlement would meet the required standards.
ACTIVATED SLUDGE
In 1914 a new type of secondary treatment was developed. Studies foutnd that continually aerating effluent from settlement tank flocs would form sludge that could absorb additional dissolved organic matter in the same way as in the percolating filter.The retention period in these tanks was generally 8 hours and this became the conventional period for activated sludge systems Over the years different forms of aeration developed. The sludge from conventional activated sludge was unstable and required further treatment by digestion..
EXTENDED AERATION
In 1960 a system activated sludge based on a long period of aeration
24-36 hours was developed in Holland. Aeration was carried out in an Oxidation
Ditch. This method of aeration uses a horizontal shaft which is attached
to a series of short paddles which dip into the sewage in the ditch. On
rotating the shaft the sewage is aerated and propelled around the ditch.
The aerated sewage is formed into a brown floc like sludge which settles
out in a separate settling tank. Thus clear treated effluent is drawn off
the top of the settling tank and sludge is drawn off the bottom of the
tank. The advantage of this system is that the sludge is stable and needs
no further treatment except dewatering. The disadvantage is that the power
absorbed is high because of the long period of aeration, thus the system
is suitable for small plants of less than say 20,000 population.
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| 1.Catoma Wastewater Plant |
| 2. Towassa Wastewater Plant |
| 3. Econchate Wastewater Plant |
| 4. Pike Road Wastewater Plant |
| 5. Stewart McLean Lagoon |
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| 1.Eclecttic Town Lagoon and Sprayfield |
| 2. Millbrook Wastewater Treatment Plant |
| 3.City of Tallassee Lagoon |
| 4. Wetumpka Wastewater Treatment Plant |
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| 1. Autauga creek Wastewater Treatment Plant |
| 2. AutaugavilleWastewater Treatment Plant |
| 3. Pine Creek Wastewater Treatment Plant |
Information regarding the above plants, including permit information, can be obtained at: http://www.epa.gov/enviro/html/pcs/pcs_query_java.html