The effect of physiochemical properties and moisture content of sewage sludge on its heat value  

Project Title: The effect of physiochemical properties and moisture content of sewage sludge on its heat value

Project Chinese Title: 城 市 污 泥 的 理 化 特 性 和 水 含 量 對 其 熱 值 的 影 響

Applicant: The Hong Kong University of Science and Technology

Total Approved Grant: $150,000

Duration: 01/06/99 to 31/05/2000

Project Status/Remarks: Completed

Scope:

The objectives of this project are to determine (1) the effect of moisture content on heat value of sludge produced in Hong Kong, and (2) the dewaterability of sludge at different levels of salinity and under the addition of chemical coagulants in order to have a better recovery of the thermal energy from a sludge incineration system. The project was comprised of the following three parts: sludge collection and physicochemical analysis, determination of the relationship between heat value and moisture content, and evaluation of the effectiveness of chemical coagulants in enhancing sludge dewaterability. Two typical kinds of sludge were studied: chemical sludge from the wastewater treatment work at Stonecutters and biological activated sludge from the Tai Po wastewater treatment plant.

Summary of the Findings/Outcomes:

The dewatered biological sludge has volatile solids varying from 171,000 to 245,000 ppm, about 72-78% of the total solids. The suspended solids of the sludge are in the range of 207,600 to 317,000 ppm. No clear trend in the variation of moisture content with different times of the day or days of the week was observed. The moisture content of dewatered sludge varies from 65 to 80% with an average value of 72%. A slight increase in the gross heat value was observed from noon to midnight. A linear dependence of gross heat value and moisture content is obtained. For the above ranges of moisture content, the gross heat value varies from 1500 to 2300 BTU/lbm. Significant amounts of ammonia, organic nitrogen, phosphate, potassium and heavy metals were found. The average salinity of the raw biological sludge is about 11,000 ppm. The dewatered chemical sludge shows a much higher gross heat value (3150 Btu/lbm) than the biological sludge. Its organic nitrogen and potassium contents are also significantly higher, although the heavy metal contents are comparable.

Based on theoretical analysis, it is found that the dewatered sludge is not required to be further dried in order to have an autothermic incineration, but an enhancement of the solid content of the dewatered sludge is still desirable. Since the salinity in the raw biological sludge is significantly high, its effect on dewaterability was investigated by determining the specific resistance on filtration with and without the addition of commonly used chemical coagulants. Electro-osmosis dewatering does not show significant advantages probably due to the high conductivity of the sludge. The final solid content of the dewatered sludge using mechanical dewatering processes such as filter press or centrifugation was also determined using a laboratory-scale centrifuge. It was found that the increase of salinity improves the sludge dewaterability and also the final solid content of the dewatered sludge.