THERMAL DRYING IS APPROPRIATE FOR MEDIUM SIZED PLANTS
Abstract:In 1996, the City of Garland embarked on a process to select a private venture to use its landfill gas to thermally dry the solids from the City's two wastewater treatment centers (WWTC), Duck Creek and Rowlett Creek. Solids generated at the 30-million-gallon-per-day (mgd) Duck Creek WWTC are pumped to and combined with the solids produced at the 24-mgd Rowlett Creek WWTC. All solids are then anaerobically digested. Biosolids are dewatered then hauled by the City to its landfill.
The City realized that there are opportunities to reuse biosolids for land application. The City also recognized that by diverting these biosolids from its landfill, it could realize a significant benefit, namely increasing the life of its landfill. Therefore, the City sought to develop a plan to eliminate landfilling dewatered biosolids. City staff were very interested in thermal drying technologies because of the potential use of landfill gas from its landfill, which is located three miles from the Rowlett Creek WWTC. Consequently, the City requested proposals from vendors to construct a landfill gas recovery system and a thermal drying system at the Rowlett Creek WWTC using landfill gas and possibly digester gas.
By using tax credits under the IRS Section 29 Federal tax credits, Zapco Development Corporation (Zapco) became the low bidder. Although startup was planned for October 1999, the system did not achieve full operation until May 2001.
Due to these tax credits, Zapco was able to design, build, and operate for 10 years a thermal drying facility to handle an average of 22,000 wet tons per year (about 15 dry tons per day) and initially charge the City only 11/wet ton of dewatered biosolids, or about 44/dry ton, escalated annually by the Consumer Price Index. Zapco is required to reuse or dispose of all dewatered biosolids from Garland regardless of the operation of the thermal dryer. In addition, the City shares equally in any revenues received from the sale of the dried product.
This is clearly a most attractive situation. This project reduces Garland's overall landfill disposal costs and will save about 6.5 million over 20 years. About 7 percent of the landfill material is diverted to beneficial use as Exceptional Quality biosolids. Other benefits are also very significant. For instance, the project eliminates almost 60 tons per year of volatile organic compounds (VOCs) emitted from the landfill, and virtually no additional fossil fuels will be required in the thermal drying system. In addition, since the dryer only uses about 20 percent of the landfill gas, the other 80 percent will be used to generate electricity.
Although the IRS tax credit program was stopped in 1998, the current energy shortage, especially in California, has prompted Congress to look at renewing these credits.
Document Type: Research Article
Publication date: January 1, 2001
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