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Wastewater generated at Citizens Gas from coking operations is treated in a biological nitrification system. The treatment system consists of a one million-gallon equalization tank, a one million-gallon aeration basin, and a 65ft clarifier. The nitrification reactions generate acid, which is neutralized using 4-8 tons/day of soda ash. The annual soda ash cost at Citizens Gas can range from 240,000 to 480,000, depending on usage rate. In an attempt to reduce chemical cost, a treatability test was conducted to evaluate the potential to reduce soda ash cost by converting the existing nitrification system to a Nite/Denite system. Nite/Denite was developed in 1985 by ENSR Corporation to completely nitrify and denitrify high ammonia content coke plant wastewater in a single reaction tank system. Denitrification would generate alkalinity neutralizing a portion of the acid generated during nitrification. The lower demand for the external source of alkalinity would reduce the annual operational cost.

A continuous stirred tank activated sludge reactor was set up in the laboratory to determine the effectiveness of Nite/Denite in reducing the soda ash consumption at Citizens Gas WWTP. Actual wastewater from the facility was used in the treatability tests. Nite/Denite mode of operation was achieved by turning the airflow to the reactor ‘on’ and ‘off’ during a 60-minute cycle to create aerobic and anaerobic conditions in the same reactor. During the aerobic cycle, ammonia was nitrified to nitrite and nitrate, generating acid. During the anaerobic cycle, nitrite and nitrate were reduced to nitrogen gas, generating alkalinity. This alkalinity neutralized a portion of the acid generated in the aerobic cycle.

Results of the treatability studies indicated that soda ash consumption dropped by approximately 41% in the reactor under the Nite/Denite operating mode, while maintaining 10-ppm ammonia in the reactor.

Citizens Gas could convert the existing nitrification system to a Nite/Denite system with minor design and operational modifications. The only design modification would be the installation of mixing equipment (floating or side entry), which would provide mixing in the aeration basin during the anaerobic cycle. Minor computer programming change would also be required to turn the blowers on and off to create aerobic and anaerobic conditions in the aeration basin. The total cost for the above modifications including engineering is estimated to be less than 50,000.

A preliminary operational cost estimate was also conducted to determine the potential savings that would be realized if the above modifications were made. Converting the existing system to a Nite/Denite system would reduce the total annual soda ash and power costs by approximately 179,225.

Treatability studies also indicated that the extent of denitrification achieved under the current operating conditions is limited by the amount of BOD in the influent wastewater. To increase the extent of denitrification would require that an external source of BOD (e.g. methanol) be added to the wastewater. Although this would increase the design and operational cost, it could potentially further reduce the soda ash cost.
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Document Type: Research Article

Publication date: 2002-01-01

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