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Wastewater
Treatment Articles Index
Alternative means of sewage treatment are of great interest when the natural soils are non-porous and/or the water table is high. These conditions preclude the use of septic tanks with tile fields. Essentially, the treated effluent needs to be discharged at a rate faster than it can percolate into the ground. Therefore, the attention is directed to discharging the treated effluent water to a drain, stream, or river.
Treatment Plant
One option is to treat the sewage in a treatment plant and continuously discharge the effluent. The effluent from the plant often needs to be so clean that the effluent often needs to pass through a sand filter similar to that used to treat drinking water. You may hear it said, "The river is polluting the effluent".
The effluent from a treatment plant needs to be sampled and tested quite frequently. The pumping and treatment equipment need to be maintained in very good working order so there is never a discharge of untreated or partially treated wastewater. This assures that the effluent continually meets the standards. If the effluent doesn't meet the requirements, the community will be cited with a violation of its permit.
The capital, operation, and maintenance costs of this type of treatment can be high.
Lagoons
Another option is to treat the sewage in lagoons where the sun, wind, bacteria, and algae naturally remove the pollutants over a six-month period. Effluent from these lagoons can be discharged to a drain, stream, or river during the spring and the fall. Lagoons typically have relatively low operation and maintenance costs. The capital cost can be high because the lagoons need to be lined with a plastic liner placed over an impermeable clay base.
Lagoon treatment can be very effective. However, lagoons are often viewed as providing a low level of treatment because of high bacteria counts and high solids content. The bacteria counts can sometimes be attributed to wild life. The solids often consist of the algae grown during the treatment of the wastewater. In addition, the treatment is dependent on "Mother Nature". The licensed operator may need to postpone the effluent discharge until the lagoon water quality improves.
Wetlands
A third option is to construct wetlands to treat the sewage. Many successful constructed wetlands treatment installations are located in the warmer climates. There have also been some successes in the northern climates. However, the inefficiency of wetlands treatment in cold weather will probably necessitate that the wastewater be stored in lagoons during the winter in the northern states.
The State of Michigan has regulations on wetlands treatment that apply to groundwater discharges of less than 20,000 gallons per day. These regulations probably do not apply to situations where the treated effluent needs to be discharged at a rate faster than it can percolate into the ground. In these cases, the limitation is the hydraulic capacity of the ground to accept the water not the treatment efficiency.
Lagoons + Wetlands
Lagoon treatment and wetlands treatment have advantages that can be combined into an economically feasible hybrid alternative. The system will better protect the environment.
Lagoon treatment has the following advantages and disadvantages:
Advantages of Lagoons:
- Low operation and maintenance costs
- Close operator attention, sampling, and testing are required only during the seasonal discharges in the spring and fall.
- Lagoons are highly efficient in the summer.
- Treatment may be protecting the environment more than is apparent due to the limited discharge periods.
- Treatment may be protecting the environment more than is apparent because many measurements of high bacteria counts and solids concentrations are not related to contamination from human sewage.
Disadvantages of Lagoons:
- Six months of storage is required.
- The allowable discharge periods (March 1 to May 31 and October 1 to December 31) are not fully utilized. Discharges are often delayed because of low quality test results on the effluent.
- Effluent quality is adversely affected by waterfowl.
- Effluent quality is adversely affected by algae growth.
Wetlands treatment can be combined with lagoons to overcome some of the disadvantages of lagoon treatment while economically providing excellent protection for the environment. The wetlands portion of the treatment can occur before or after the lagoon treatment. It can even occur in parallel with the lagoons during the fall discharge period. Flow management is needed.
The wastewater can flow through the wetlands prior to flowing into the lagoons. This will reduce the biological loading on the lagoons. The lagoon effluent quality can thereby be improved. As a result, effluent discharges from the lagoons can more readily occur throughout the allowable discharge period.
The lagoons can also act as a storage basin for the wetlands treatment. During colder winter weather, the wetlands are far less efficient than in the summer. During the winter, the flow to the wetlands treatment can be proportionately reduced with the excess flow going to the lagoons.
The wetlands can also be operated in parallel with the lagoons during the fall discharge period. That is, part of the wastewater can flow to the wetlands and a portion can flow to the lagoons in October and November. The quality of the effluent from the wetlands treatment may very well meet the lagoon effluent quality requirements.
With this hybrid "Lagoons + Wetlands" treatment, there is great opportunity to more effectively protect the environment. However, the size of the lagoon storage needs to be reduced from the 6-month storage capacity to offset the cost of the wetlands. This approach will definitely bring wastewater treatment a step closer to "Mother Nature".
Wastewater Treatment Articles Index
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