Keywords biomass boiler, biomass resource, Small scale sewage treatment plant, twin drum type dryer,

Practical Research on a Sewage Sludge Fuelization System for Small-scale Sewage Treatment Plants Eiji Tochioka 1,Hidekazu Nagasawa 1, Shuichi Ochi 1 Tsuneo Nishida 2, Kazuya Hibino 2 1 Japan Institute of Wastewater Engineering Technology, Shinjuku-ku,Tokyo, Japan 2 Water Supply and Sewage Sect., Industrial Construction Dept.,Minokamo City, Gifu Pref, Japan Short Abstract This research developed sewage sludge fuelization technology for small-scale sewage treatment plants, which, by effectively using the biomass resource, sewage sludge, can reduce sludge disposal cost at the same time as it can cut emissions of greenhouse effect gases. This technology is a fuelization system that uses solid fuel made by mixing and molding dried sludge produced from sewage sludge with wood chips, and recovers heat for the drying process from steam produced in a biomass boiler (below called the boiler ). Because it is a system that uses the energy contained in sewage sludge instead of fossil fuel, it can be counted on to save energy, cut sludge disposal costs, and reduce emissions of greenhouse effect gases. This paper reports on the properties of the twin drum type dryer (below called the drum type dryer ) and the boiler, which form the heart of this technology, based on the results of proving tests. Keywords biomass boiler, biomass resource, Small scale sewage treatment plant, twin drum type dryer, Introduction Japan is conducting extensive development of technologies intended to effectively use sewage sludge in order to prevent global warming and to conserve energy, a type of technology which is positioned as a very important policy for Japan, which intends to become a recycling society. But in Japan, these technologies are being developed for large-scale treatment plants, while none have been established for smallscale sewage treatment plants such as a plant with a design dry weather daily maximum treatment quantity of less than 5,000m 3 /day. And at the end of 2008, there were 1,162 small-scale treatment plants equal to about half of all treatment plants. At small-scale sewage treatment plants, the building costs and maintenance costs per unit of population tend to be high, posing a challenge for regional governments with small financial resources. In response to such circumstances, the authors have developed a sewage sludge fuelization technology intended to effectively use sewage sludge, which is a carbon neutral biomass resource, as one countermeasure to overcome this challenge. To introduce this fuelization technology as a practical system, they have conducted proving tests to clarify the properties of the dryer and the properties of the boiler, which are the system s major components. This development and proving tests are described below. Methods 1. Overview of the technology Using this technology, steam is supplied to a drum type dryer with two rotating drums (see Figure 1), dewatered sludge is inserted when the surface temperature of the drums is between 110 and 120ºC, and the steam pressure is 0.4MPa, forming dried sludge Page 1 of 8

with the decline of its calorific value inhibited by low temperature drying. After this dried sludge has been mixed with wood chips so it can be used as boiler fuel with high energy efficiency, it is pelletized in a pellet molder, and then used as boiler fuel (see Fig. 2, below called, mixed pellets ). Figure 3 shows the overall flow of the system. Dewatered sludge with water content of about 85% is continually fed to the drum type dryer, where it is wound between and applied as a thin membrane to the two rotating drums heated by the boiler steam, drying to a water content of about 20%, then it is mixed with wood chips. Next it is pelletized to form pellets with diameter of 7mm and length of 20mm in a pellet molder. The mixed pellets are used as boiler fuel. The boiler, which has a furnace bed area of about 0.16m 2 providing combustion capacity equal to incineration capacity of about 30kg/h, burns at a combustion temperature of about 850ºC, producing steam which is then supplied to the drum type dryer. Odors produced in the drum type dryer are extracted by a suction fan, deodorized in a chemical cleaning tower, then exhausted into the atmosphere through an activated carbon adsorption tower. After the chemical used in the chemical cleaning tower is neutralized to reduce its reverse flow load, it is discharged into the existing water treatment facility. Figure 1 Drum type dryer Dewatered sludge Dried (Water content 85%) (Water content 20%) Dewatering machine To Water treatment Dry exhaust gas Drainage water Twin-drum type dryer Chemical cleaning tower Figure 2 Mixed Pellets Wood chips hopper Automatic hopper scale Mist separator Drainage water Wood chips Hot water Steam Deodorizing fan Pellet molder Air supply Water supply Mixed pellets Blower Activated carbon absorption tower water softener Water supply tank Pellet steam boiler Exhaust air Figure 3. Fuelization System Flow Incinerator ash Steam 2. Description of the tests The tests included a test to clarify the drying properties of the drum type dryer, a test to clarify the combustion properties of the dryer, and an evaluation of the environmental performance of the boiler exhaust gas. 2.1. Drying test of the drum type dryer The drum type dryer, which operates on the indirect steam heating principle, winds the dewatered sludge between the two drums which have been heated by the steam as it spreads it in a thin layer on the drum surfaces, achieving drying in a short period of time. For this test, the three types of tests shown below were performed to study the drying properties of the drum type dryer. Based on the results of a separately performed pelletizing test, it was known that conditions for pelletizing good quality pellets are dried sludge water content from 15 to 25% with no Exhaust air Cyclone Ash Page 2 of 8

poly ferric sulphate added. So although in the past, dewatering machines supplied poly ferric sulphate to lower the water content of dewatered sludge, considering pelletizing, a single high polymer flocculent is believed to be appropriate. During the (1) drum clearance and the (2) drying steam pressure described below, to confirm the impact on the drum type dryer of adding or not adding poly ferric sulphate, the percentage of poly ferric sulphate added was varied. And to clarify the range of application of the drum type dryer, the percentages of surplus sludge and high polymer flocculent added were varied to adjust the properties and water content etc. of the dewatered sludge. (1) Drum clearance The clearance between the two drums was varied to study the relationship of the drum clearance with the spreading width of the dewatered sludge on the drum surface and the dewatered sludge water content. Table 1 shows an overview of the test. Table 1. Overview of the Drum Clearance Adjustment Test Dewatering Drying Test items Drum clearance Surplus sludge (m 3 /h) 6 9 Poly ferric sulfate addition rate (%/TS) High polymer flocculent addition rate (%/TS) 0~9.4 0 1.2~2.7 1.7 Drum clearance (mm) 0.2~1.0 0.25~0.40 Drum rotation speed (min-l) 2.45 2.45 Drying steam pressure (MPa) 0.4 0.4 (2) Drying steam pressure The drying steam pressure was varied in steps from 0.2 to 0.4MPa at the same time as the drum clearance was adjusted at each drying steam pressure, to study dried sludge water content. Table 2 shows an overview of the test. Table 2. Overview of the Steam Pressure Adjustment Test Dewatering Drying Test items Steam pressure Surplus sludge (m 3 /h) 6 Poly ferric sulfate addition rate (%/TS) High polymer flocculent addition rate (%/TS) 6.8 0 0.98 1.7 Drum clearance (mm) 0.2~0.5 0.1~0.3 Drying steam pressure (MPa) 0.2 0.3 0.4 0.4 (3) Dewatered sludge treatment quantity The relationship of the dewatered sludge treatment quantity in the drum type dryer with the quantity of evaporative water and quantity of drying steam used was studied. Table 3 shows an overview of this test. Table 3. Overview of Dewatered Sludge Treatment Quantity Adjustment Test Dewatering Drying Test items Dewatered sludge treatment quantity Surplus sludge (m 3 /h) 6~9 Poly ferric sulfate addition rate (%/TS) 0 High polymer flocculent addition rate (%/TS) 1.7~2.1 Dewatered sludge water content (%) 85±1 Dewatered sludge treatment quantity (kg-wet/h) 95~143 Drying steam pressure (MPa) 0.4 2.2. Combustion test of the boiler The boiler was supplied with mixed pellets as fuel and the quantity of combustion was adjusted to maintain the specified steam pressure of 0.6MPa. The target ignition loss for the combustion residue was 15% or less, a value permitting its disposal as landfill. The following two kinds of test were performed to study the combustion properties of the boiler. (1)Air ratio The relationship of the air ratio with the quantity of steam produced was studied in order to confirm the appropriate air ratio Page 3 of 8

(quantity of air) in the boiler. Table 4 shows an overview of the test. Table 4. Overview of the Air Ratio Adjustment Test Test items Air ratio Quantity of mixed pellets supplied (kg/h) 40 Mixing ratio (dried sludge: wood) 6:4 Air quantity (m 3 /h) 2~330 Air ratio (-) 1.6~2.0 Boiler (2) Pellet type A variety of pellet conditions were prepared by varying the percentages of sludge and wood chips in the mixture, and these were studied to clarify the types of pellets effective in the boiler and to study the pellet combustion rate and quantity of combustion of each type of pellet during total combustion. Table 5 shows an overview of the test. Table 5. Overview of the Pellet Type Adjustment Test Boiler Mixing ratio (dried sludge: wood) Quantity of pellets supplied (kg/h) Test items Pellet type 10:0~0:10 5~30 Air ratio (-) 1.7 Explanatory note) The following five types of pellet fuels were used. i. Only sludge pellets ii. Only wooden pellets iii. Sludge pellets + wooden pellets (below referred to as No. 1 ) iv. Mixed pellets combining dried sludge and No. 1 wood chips * (below referred to as No. 2-1 ) v. Mixed pellets combining dried sludge and No. 2 wood chips * (below referred to as No. 2-2 ) *The differences between No. 1 wood chips and No. 2 wood chips are differences between manufacturers; but little differences between their compositions is observed. 2.3. Evaluation of environmental performance of boiler exhaust gas The dust, sulfur oxides and nitrogen oxides in the boiler exhaust gas were studied, confirming the need for a bag filter or other dust collector. Results 1. Drying test results of the drum type dryer (1)Results of drum clearance adjustment test Figures 4, 5, and 6 show the results of tests performed adjusting the drum clearance. The test results in Figures 4 and 5 show that as the drum clearance narrows, the spreading width of dewatered sludge on the drum surface tends to increase and the dewatered sludge water content tends to decrease. Figure 6 confirms that the rise of the dewatered sludge water content increases the spreading width of the dewatered sludge. According to the above, regardless of whether or not poly ferric sulphate is added, and even when the dewatered sludge water content is relatively high at about 85%, adjusting the drum clearance can spread the dewatered sludge in a thinner layer, permitting the water to be evaporated efficiently, and this in turn, permits the adjustment to the water quantity suitable for pelletizing. Page 4 of 8

Drum surface spreading width of dewatered sludge (cm) Dried sludge water content (%) Dewatered sludge water content (%) Cake spreading width (surplus 6m3/h) Cake spreading width (surplus 9m3/h) 150 140 130 120 110 60 50 40 30 20 10 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 Drum clearance (mm) Figure 4. Relationship of Drum Clearance with Drum Surface Spreading Width of Dewatered Sludge Dried sludge water content (surplus 6m3/h) Dried sludge water content (surplus 9m3/h) 60 50 40 30 20 10 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 Drum clearance (mm) Figure 5. Relationship of Drum Clearance with Dried Sludge Water Content 95 85 75 Clearance 0.3mm Clearance 0.4mm Clearance 0.35mm 0 10 20 30 40 50 60 110 120 130 140 150 Drum spreading width of dewatered sludge (cm) Figure 6. Relationship of Spreading Width of Dewatered Sludge with Cake Water Content (2)Results of the drying steam pressure Figure 7 shows the results of the test performed adjusting the drying steam pressure. Figure 7 has confirmed that as the drying steam pressure rises, the dried sludge water content tends to decrease. It is also confirmed that adjusting the drum clearance under a constant steam pressure can adjust the drying steam water content, and that at steam pressure of 0.4MPa, it was possible to obtain a water content suitable for pelletizing. And the drum clearance was adjusted without poly ferric sulphate under steam pressure of 0.4MPa, but it was possible to adjust the water content in the same way as poly ferric sulphate. Dried sludge water content (%) 60 50 40 30 With poly ferric Clearance 0.5mm Clearance 0.2mm Clearance 0.5mm Clearance 0.3mm 20 Clearance from 0.1 to 0.3, Clearance 10 without poly ferric sulfate 0.2mm 0 0.0 0.1 0.2 0.3 0.4 0.5 Drying steam pressure (MPa) Without poly ferric Figure 7. Relationship of Drying Steam Pressure with Dried Sludge Water Content Clearance 0.5mm (3) Dewatered sludge treatment quantity Figure 8 shows the results of a test performed by varying the dewatered sludge treatment quantity in order to study the relationship of the quantity of evaporative water with the quantity of drying steam. According to the results of the test, the quantity of drying steam needed to evaporate 1 kilogram of water was about 1.31kg. And the results of the study of thermal efficiency (drying efficiency) obtained a rate of about %. The drying efficiency was trial calculated using equation (a)*. Page 5 of 8

*Equation (a): drying efficiency (%) = drying steam energy/input steam energy Evaporative water quantity (kg/h) 150 140 130 120 110 60 50 110 120 130 140 150 160 1 1 Drying steam quantity (kg/h) Figure 8. Relationship of Drying Steam Quantity with Evaporative Water Quantity 2. Results of the combustion test of the boiler (1)Air ratio results Figure 9 shows the results of the test performed by adjusting the air ratio supplied to the boiler. According to Figure 9, more steam is produced at an air ratio between 1.6 and 1.7 than between 1.9 and 2.0. And the confirmation of the ignition loss of the incineration residue showed a decline of about 8 points at an air ratio between 1.6 and 1.7, revealing that because the supplied air is not preheated, an air ratio between 1.6 and 1.7 is more appropriate. Quantity of steam produced (kg/h) 200 1 1 1 160 150 140 130 120 110 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 (2)Results of test adjusting the type of pellets Figure 10 shows the results of a test performed by adjusting the percentages of sludge and wood chips in the mixture. Figure 10 shows that the lower the percentage of sludge, the higher the combustion rate. A comparison of various kinds of pellets at a sludge ratio of 60% shows that the combustion rate was higher in the case of the mixture of pellets prepared by mixing the wood chips with the dried sludge before pelletizing (No. 2) than it was with the No. 1 mixture prepared by mixing sludge pellets and wood pellets. Specifically the combustion rate of No. 2-1 pellets was 17% higher than that of No. 1 pellets. This is presumed to be a result of the fact that when single pellets made of dried sludge and wood chips, which have different combustion rates, are burned, the wood chips burn first, forming voids, which accelerate the combustion of the sludge. And the quantity of pellets treated per 1m 2 of furnace bed area during complete combustion (see Fig. 11, ignition loss value of the incineration residue is several percent) was 135kg/h for No. 2-1 and 121kg/h for No. 2-2. It was also confirmed that another property of mixed pellets is that when wood chips are mixed in during pelletization, these pellets are more easily pelletized by the pellet molder and retain their shapes better than pellets consisting only of sludge. Air ratio (-) Figure 9. Relationship of Air Ratio with Quantity of Steam Produced Page 6 of 8

Combustion rate (m/h) 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Wood pellets No. 1 pellets No. 2-2 pellets Sludge pellets No. 2-1 pellets 0 10 20 30 40 50 60 Sludge percentage (%) Figure 10. Relationship of Sludge Percentage with Combustion Rate Figure 11. State of Incineration Residue 3. Evaluating the environmental performance of the boiler exhaust gas Table 6 shows the results of the analysis of the boiler exhaust gas. The results show that on June 21, 2011, all items excluding dust satisfied various standard values. The exhaust gas dust removal equipment on the present system consists of only a cyclone, so in order to stably satisfy all standard values in the future, additional dust removal equipment such as a bag filter will be added to the system. And it was possible to restrict the dinitrogen monoxide, which is a greenhouse effect gas, to 39ppm by maintaining the combustion temperature at about 850 ºC. Table 6. Results of Analysis of Exhaust Gas Items analyzed Sulfur oxides Dust Nitrogen oxides Hydrogen chloride Dioxins Dinitrogen monoxide Analysis results Unit 2011. 2011. 6.21 3.4 Nm 3 /h 0.05 0.10 g/nm 3 (O 2=12%) 0.22 0.096 ppm (O 2=12%) 150 230 mg/nm 3 (O 2=12%) 1 2 Remarks 0.98Nm 3 /h or less 0.15g/Nm 3 or less 250ppm or less 0mg/Nm 3 or less Ng- TEQ/Nm 3 0.0028 5g-TEQ/Nm 3 or less ppm 39 Explanatory note) The standard value for dioxins is the standard value under the Act on Special Measures against Dioxins, and other standard values are standard values under the Air Pollution Control Law. Discussion and Conclusions The results of the proving tests have revealed the following facts regarding the drying properties of the drum type dryer, the combustion properties of the boiler, and the environmental performance of the boiler exhaust gas. (1) Drying properties of the drum type dryer It is possible to adjust the dried sludge water content to a water content suitable for pelletizing by adjusting the drying steam pressure and drum clearance, and stable dried sludge can be obtained by maintaining the water content of the dewatered sludge which is supplied at a relatively high level of 85% by effectively using the drum surfaces. And at a steam pressure of 0.4MPa, the quantity of drying steam required to evaporate 1kg of water was about 1.31kg, and the drying efficiency was about %. (2) Combustion properties of the boiler Wood chips were mixed with sewage sludge to improve the combustion efficiency of the Page 7 of 8

boiler, but pelletizing dried sludge and wood chips and using these pellets as fuel increased the combustion rate by more than about 17% over the combustion rate obtained burning a mixture of sludge pellets and wood pellets. The quantity of pellets treated per 1m 2 of furnace bed obtained using mixed pellets is about 120kg/h. And mixed pellets were more easily pelletized by the pellet molder and retain their shapes better. (3)Evaluation of the environmental performance of the boiler exhaust gas Using the present system, only dust is occasionally excessive, but installing a bag filter or other dust removal equipment can ensure that the standard value is adequately satisfied. And it is possible to hold down dinitrogen monoxide, which is a greenhouse effect gas, by maintaining the combustion temperature at about 850 ºC. This research was a study of the maintenance and economic characteristics of the fuelization system. The actual equipment will be constructed based on these research results. References T. Nishida, K. Hibino, E. Tochioka, M. Minami: Joint Research on a Fuelization System for Small-scale Sewage Treatment Plants, Forty-eighth Conference on Sewage System Research, pp. 839 841. Page 8 of 8