Wastewater Treatment Plant Case Study
A large size electrical energy producing company has wastewater treatment plants for treatment of (a) industrial wastewater and (b) domestic (bathroom) wastewater. The domestic wastewater treatment plant (WWTP) had several problems in meeting National Pollution Elimination Discharge System (NPDES) limits, including E.Coli limits.
Sources of receiving any amount of industrial wastewater to the domestic WWTP were eliminated in order to avoid toxicity to the biomass of the domestic WWTP. One of the main reasons that the WWTP had problems is due to oversize of the WWTP. The WWTP was designed for treating 20,000.00 gallons of wastewater per day. The actual wastewater flow that was coming to the WWTP for treatment was only 4000.00 to 50000.00 gallons per day.
The conventional activated WWTP was designed for 24 hours residence time. Due to oversize of the treatment plant, the residence time was from 96 hours to 120 hours. To save energy and to prolong the lives of the blowers, the aeration system for the activated sludge aeration chamber was "ON" for one hour and "OFF" for one hour. The said conditions caused septic conditions contributing the violation of E.Coli limits.
Measurement of dissolve oxygen (D.O) at different depth of the aeration chamber of the WWTP was taken. The D.O in the activated sludge aeration chamber was sometimes below 1 mg / l.
Briefly in order to resolve the problem, (a) short term corrective measure and (b) long term corrective measures were taken.
Short Term Measure
- Septic sludge from the aerating chamber was pumped out and fresh activated sludge was added.
- Aeration pattern was changed first from one hour "ON" to one hour "OFF" to aeration at all times until the D.O. reached at least to a level of 4.0 mg / l, for a week and then aeration was tapered off to 45 minutes "ON" and 15 minutes "OFF" mode.
- Chlorination dosage for disinfections was adjusted.
- Special technique was developed in collecting E. Coil samples.
Long Term Measure
Different Options were considered. The WWTP plant has been operating for without much mechanical maintenance for more than 13 years. Normally, such a package plant will have a life of 20 to 25 years. The package plant is corroded.
- First option was considered to replace the WWTP with a smaller size WWTP plant like sequential batch reactor, bio reactor, membrane bio reactor, SVM, conventional package plant etc. The company was not willing to install and operate a new plant due to budgetary constrain. Cost on installation of a new WWTP would have been any where from $250,000.00 to $600,000.00.
- In order to meet the budgetary constrain, it was decided to divide the existing 20,000.00 gallons per day aeration chamber of the WWTP to two (2) 10,000.00 gallons aeration chambers by mechanical means. One of the two chambers will be used for aeration purpose. The second one will be at stand-by. This change will reduce the chance of septic conditions due to reduced residence time. During the change, the other needed mechanical maintenance will be made which will prolong the life of the WWTP to another 10 to 15 years.
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