Improving Sludge Dewatering Performance With The Platinum System

Introduction
The acceleration of urbanization which increases overall sludge volume, and expanding environmental regulations combined with dramatically rising disposal and operation costs for sewage treatment plants, as well as 'Greenhouse Gas Emissions' is a serious problem. As a solution, the anaerobic digestion process has been adopted by sewage treatment plants as the technology which can achieve and recover bioenergy. However, after anaerobic digestion, the dewatering property of the sludge is significantly reduced due to decomposition of the fibrous material and formation of fine particles; Therefore, improvements were strongly required.
For this situation, we have focused on the decomposition of fibrous materials in the sludge during the anaerobic digestion process and have developed the Platinum System. This system will recover the fibrous materials from the primary sludge (which contains an abundant amount) and the recovered fibers serve as a dewatering aid efficiently removing water from the digested sludge (Figure 1). The fibrous material regarded as toilet paper origin is plentiful in the primary sludge (Ruiken et al. 2013; Tochioka et al. 2019). The recovery unit of this System recovers the fibrous material effectively from the sludge. The recovery unit consists of a sludge mill, recovery device and pump. After grinding the primary sludge up with the sludge mill as shown in Figure 2, the fibrous materials are washed through the recovery device, and then added to the digested sludge as a dewatering aid. The Platinum System has been brought into more than one Japanese sewage-treatment plant so far and has proven that it is possible to increase the cake solids by up to 33 %. The study of the effect on the overall sludge treatment showed that the amount of digestion gas generated was reduced by recovering fibrous materials that were conventionally fed into the digester, while the amount of dewatered sludge could be significantly reduced by increasing the cake solids through reintroducing the recovered fiber to the dewatering process (Tochioka et al. 2021). The first System was introduced into the Nanbu sewage treatment plant in Kumamoto-shi in Japan; actual operation started in April 2021. Another System (#2) will be introduced into the Marugame-shi sewage treatment plant which is being constructed in Marugame-shi, Kagawa in Japan. Estimated completion in March 2023. Six months of data (April 2021 to September 2021) from the System introduced at Kumamoto-shi was evaluated. The impact to the dewatering sludge emission rate and the digestion gas emission rate were confirmed. The reduction of the maintenance costs and greenhouse gas emissions were also gathered and will be reported.
Method
1. Equipment Outline
The outline of the Nanbu treatment plant and the equipment outline of the System are indicated in Table1. Resulting cake from the System is reused as cement material. The upper value for cake solids is contracted at 25% as the acceptable standard for cake to be used by a cement company.
2. Operation Data
To evaluate the performance of the System for six months, from April to September 2021, the operational data of the influent sewage volume, cake solid content, polymer coagulant addition rate, dewatered sludge, and digestion gas volume before and after the introduction of this System were arranged so that the changes over time could be seen on a monthly basis. The data for three years from 2018 to 2020 were used for the data before the introduction of this system.
3. Comparison of Data
The average values of the operation results for the three years from 2018 to 2020 were used as comparison data; the reduction in maintenance costs and greenhouse gas emissions due to the introduction of the System were calculated (Table 2).
4. Data Results and Consideration
Comparison of the average influent from April to September shows the volume of influent was higher in 2021 than in other years. Although there is some variation from year to year, the increase in influent volume from July to August can be attributed to unusually high rainwater in August 2021 (Figure 3). - Cake solids increased from a 3-year average of 19.5% to 24.5% (Figure 4). - Cake volume decreased from a 3 year average of 19.24 t/day to 14.56 t/day (Figure 5). - Digestion gas decreased from a 3-year average of 3,437m3/day to 3,321m3/day; the digester gas was reduced by only about 3% (Figure 6).
Results
1. Maintenance and Operation Costs It was confirmed that $154K (during 6 months) can be reduced by introduction of the System (Figure 7). The maintenance cost is expected to be reduced by $268K per year. The cost reduction is greater than expectations (Figure 8). 2. Greenhouse Gas Emissions It was confirmed that the installation of this System reduced greenhouse gas emissions by 528 t-CO2. As a result of estimating the reduction effect for one year, the greenhouse gas emissions are expected to be reduced by 1,024 t-CO2 per year (Figure 9).　In this case, the reduction is equivalent to about 40%, demonstrating that the introduction of this system will make a significant contribution to the reduction of greenhouse gas emissions.
Conclusions/Significance
We have investigated the operation of the System for six months and found that it can be used as a technology to improve the dewaterability of digested sludge, steadily increase the cake solids content, and significantly reduce the amount of dewatered sludge. In addition, it was proven that the technology can reduce the cost of sludge treatment and disposal, and reduce greenhouse gas emissions by about 40%. The fact that the System can contribute to the reduction of maintenance costs and greenhouse gas emissions is expected to lead to widespread use of the System.