The purification of domestic wastewater produces large quantities of sewage sludge, and the urbanization and industrialization in China has resulted in a dramatic increase in the volume of wastewater and sewage sludge. The moisture content of dewatered sewage sludge is about 80%, which causes a series of problems in terms of sludge treatment and disposal. Therefore, reducing sludge moisture is important to the reduction of sludge volume and quantity.

Sewage sludge bio-drying is an economical and energy-saving method of simplifying thermophilic aerobic fermentation that utilizes the biological energy produced by microbial fermentation to activate bound water and evaporate moisture, resulting in rapid reduction of the moisture in the bio-drying material.

The water mass balance of sewage sludge bio-drying pile indicates that variations in pile moisture are associated with water input and water output. Water input includes: (1) water generation, which is water produced by microbial metabolism during organic matter decomposition; and (2) aeration water input, which is moisture added to the pile during forced aeration. Water output includes the water evaporated from the bio-drying material and the leachate produced during bio-drying.

To illustrate the water variation based on the water mass balance, Prof. CHEN Tongbin, the director of Center for Environmental Remediation, Institute of Geographic Sciences and Resources Research, Chinese Academy of Sciences (IGSNRR, CAS), and his research fellows investigated the variations in water input and evaporation of sewage sludge bio-drying pile in terms of a water mass balance developed via in situ moisture and vapor measurement.

According to the findings of the study, from day 2, the water evaporation began to increase dramatically, reaching the peak value of 47.8 kg/m² on day 4. It then gradually decreased to 29.0 kg/m² on day 9. The mean evaporation decreased to 5.49 kg/m² during the last 3 days (the bio-drying period was 20 days). The metabolic water generation increased dramatically during the first 3 days, reaching its peak value of 12.0 kg/m² on day 3. Once the cooling phase started, the water generation decreased gradually to a mean of 0.60 kg/m². During the thermophilic phase, the mean values of apparent moisture reduction, water evaporation and water generation increased to 28.8±7.90 kg d^-1, 37.7±7.09 kg d^-1 and 7.93±2.39 kg d^-1, respectively.

The findings suggest that the peak values of water generation and evaporation were attained in the thermophilic phase and the water evaporation was much greater than water generation, throughout the sewage sludge bio-drying process, aeration facilitated water evaporation.

The related results have been published in the journal of Bioresource Technology(Lu Cai, Ding Gao, Tong-Bin Chen, Hong-Tao Liu, Guo-Di Zheng, Qi-Wei Yang.Moisture variation associated with water input and evaporation during sewage sludge bio-drying. Bioresource Technology. 2012, 117: 13-19).