In Taiwan, the dewatered sludge of sewage treatment plant is mostly contracted out for transportation. Not only its cost is expensive, but also its exact destination is hard to efficiently control. Although sewage sludge is processed by thickening and dewatering in the treatment plant, the water content of the dewatered sludge is still high, ranging from 75% to 85%. Therefore, it will directly cause the increase of the transportation cost of the sludge, and is liable to pollute the environment.
This study is based on a real case to dry the dewatered sludge of a secondary treatment plant in the northern Taiwan. To prevent from the high-temperature deterioration of the reusable material, the highest drying temperature was set at 105℃ and the water content of the dry sludge was reduced to about 30%. The relevant operational parameters were recorded on energy consumption and cost, followed by the benefit assessment of sludge-drying deduction.
In addition, the examinations of constituents and heavy metals of the dried sludge were outsourced to the qualified laboratories. According to the examination results, the adaptable reuse method was recommended. Furthermore, to evaluate the adaptability of the localization model of the combined treatment of the sludge in Taiwan, the approach of Yokohama City in Japan was referred.
Based on the prevailing regulations and limitations on the sludge reuse in Taiwan, it is found that the heavy metal contents, such as Cd, Hg, Cr, Ni, and Zn, are higher than those of the hazardous constituents limited in the ancillary compost fertilizer or the ancillary organic plantation media of the Fertilizer Management Law.
In the case study of drying deduction of 53%, the sludge drying cost per metric ton ranges from NTD2,005 to NTD3,600. Compared with the directly outsourcing hauling and transportation cost of NTD4,500 per metric ton, it has potential economic benefit from drying deduction, in addition to reducing the sludge weight on a large scale, facilitating to clearance in plant, and promoting subsequent multipurpose reuse.
Recovery energy from sludge is the application of biomass energy. Because there is no sludge-exclusive incinerator in Taiwan, the incineration of the dry sludge is usually mixed with the municipal waste. This study was based on the practical cases in Taipei City and New Taipei City to use the existing municipal waste incinerators in the regions to burn the sewage sludge in the regions concerned. Through the reuse of the recovery energy from sludge in the sewage treatment plant, and the power generation of the co-generation and the reuse of the materialized ash of the incinerators, the re-usage rate of the sludge will be increased and the complete zero waste will be reached.
It is expected to associate with the sludge deduction, treatment, and reuse technologies in Taiwan to initiate the market of the multipurpose reuse, to well use the existing treatment facilities to fill up the rated garbage delivery amount guaranteed by the government, to increase the incineration rate of the garbage incinerator, to reduce the loading of the landfill, to achieve a triple benefit of pollution reduction, energy saving and carbon reduction, and resource recycle, and to comply with the international trend toward the energy recycle for exact solving the problem with the final treatment and disposal of sludge.