Akinori NISHII*
Naoaki KATAOKA*
Shinya HIGUCHI*
*
Swing Corporation
Sewage sludge has high energy potential as continuously usable biomass energy that is collected at treatment plants through the sewerage system. However, in Japan, because the facility costs for the utilization of sewage sludge are high, its effective utilization ratio has been low. Aiming to use the unused biomass energy, we developed a semi-dry anaerobic digester, which is low-cost and high-efficiency sludge digestion equipment. The semi-dry anaerobic digester can not only much more highly concentrate feed sludge than before, but also shorten the digestion period for efficient gas collection. Based on these improvements, the capacity of the digestion tank can be minimized to one-eighth of that of conventional ones, and the amount of effectively usable gas can be increased. This paper introduces the semi-dry system along with the results of demonstration tests.
Keywords: Methane fermentation, Anaerobic digestion, Sewage sludge, Biosolids, Energy recovery
Sewage sludge is a biomass with following characteristics.
High energy potential
Collection through the sewerage system (No need of transport)
Continuous usability
In Japan, however, its utilization ratio as energy has remained at only a little more than 10 %1). Anaerobic digestion (methane fermentation) is an excellent process that can convert the energy of sewage sludge into a usable form; however, because conventional anaerobic digestion facilities require a large capacity digestion tank, and because the amount of heat consumed by the anaerobic digestion facility itself is large, the problem arises that the proportion of effectively usable heat is low. In addition, because the introduction of the facility is highly costly, it is not very popular.
With this background, we at Swing Corporation, have focused on energy recovery efficiency and developed a semi-dry anaerobic digester, as inexpensive anaerobic digestion equipment.
For the development of this Semi-Dry Anaerobic Digester, the following points were the priority issues.
（1）
Low cost
（2）
Less energy consumption by the equipment itself, and a greater amount of effectively usable heat
（3）
Easy maintenance and management
In order to tackle these challenges, we first considered decreasing the size of the digestion tank.
Since the capacity of the digestion tank i determined by the [volume of feed sludge] × [digestion period], decreasing the size of the digestion tank can be attained by reducing these values. The method for this is shown in Figure 1 and the next section.
Fig. 1 Concept of Semi-Dry Anaerobic Digester
The feed sludge volume is reduced by thickening it; however, thickening feed sludge can raise the following concerns.
ammonia inhibition in the digestion process
inadequate mixture due to high viscosity in the digestion tank
Therefore, laboratory experiments, etc., were conducted to verify and confirm that the optimum feed sludge concentration is approximately 8%. In the conventional standard sewage sludge digestion facility, the feed sludge concentration is 2 to 4 %2). By setting the sludge concentration to 8 %, the feed sludge volume decreases to 1/4 to 1/2 of a conventional facility. As a result, the digestion tank capacity and the amount of heat required for heating the feed sludge can be reduced almost proportionally.
From the relation between the digestion period and gas production amount obtained from the actual data (right in Fig. 1), it can be understood that most of the gas production occurs within 15 days of digestion. In order to improve the gas recovery rate per digestion tank capacity, the digestion period, which is conventionally 20 to 30 days3), was set to 15 days for this equipment. As a result, the digestion tank capacity can be reduced to 1/2 to 3/4 of a conventional tank.
As described in Sections 2-1 and 2-2, it is expected that this equipment can decrease the digestion tank capacity up to 1/8 as compared with that of conventional digestion equipment.
Furthermore, because the amount of heat required for heating can be reduced by decreasing the feed sludge volume and decreasing the size of the digestion tank, it is possible to increase the amount of effectively usable gas (Figure 2).
Fig. 2 Benefits of the equipment
As shown in Figure 3, the Semi-Dry Anaerobic Digester consists of the thickening part and the digestion part. Each will be described in detail below.
Fig. 3 Schematic diagram of Semi-Dry Anaerobic Digester
The mixed raw sludge, which was coagulated by adding a polymer coagulant, is fed into the horizontal screen inlet of the thickening part and transported on the screen in the exit direction by a special mechanism. The coagulated feed sludge is thickened while being transported. The sludge on the screen is further thickened by the back pressure mechanism installed at the screen exit.
The sludge thickened to about 8 % in the thickening part is injected into the digestion tank directly or by a pressure pump. The temperature inside the digestion tank is maintained at 35 ℃ for mesophilic digestion by heating equipment. The feed sludge inside the tank is agitated and mixed by a mechanical agitator, and anaerobically digested. The capacity of the digestion tank is designed for a digestion period of 15 days for efficient gas recovery. The digestion method is one step digestion and the digestion tank is generally of the cylindrical steel plate type; however, existing RC structure tanks of various shapes are also applicable.
A test to demonstrate the performance of this equipment was carried out at a pilot plant installed in a sewage treatment plant. The flow sheet of the pilot plant is shown in Figure 4, and the photos of the demonstration test equipment are shown in Figure 5 and Figure 6. Mixed raw sludge (primary sludge: excess solid mixing ratio = 2.5 : 1), which is a mixture of gravity thickened primary sludge at the treatment plant and excess sludge was used as raw sludge. Cationic polymer coagulant was added to this raw sludge, then the sludge was thickened at the thickening part and fed into the digestion part.
Test conditions and results are shown in the Table, Figure 7, Figure 8, and Figure 9. The thickened sludge with a concentration of 8.4 % was obtained in the thickening part under the condition of a chemical feeding rate of 0.5 % (vs. TS) or less, for feed sludge with an average concentration of 1.4 %. The solid recovery rate was 95 % or above on average. At the digestion part, this thickened sludge was used as feed sludge into the digestion tank, and a continuous mesophilic digestion test with an average digestion period of 14.5 days was carried out.
As a result, the pH values of the digested sludge were stabilizing (average 7.3) throughout the test period, and the organic acid concentration was 88 mg/ℓ at maximum, which was a satisfactory level. In addition, the digested sludge concentration and VS decomposition rate against the feed sludge with an average concentration of 8.4 % were 4.2 % and 56.3 % on average, respectively, confirming satisfactory organic decomposition performance.
It has been indicated for the digestion of sewage sludge, the VS decomposition rate of which is generally 40 to 60%3), that the same level of decomposition rate can be obtained even with this equipment with a shortened digestion period.
The digestion gas production amount per digestion tank capacity during the test period was 2.1 m3 /(m3 ･d) (NTP) on average. The value is approximately 1.0 m3 / (m3 ･d) (NTP) at maximum2), 3) for the conventional mesophilic digestion of sewage sludge; it was confirmed that the present equipment has high gas recovery efficiency.
Fig. 4 Demonstration test equipment flow sheet
Fig. 5 Demonstration test equipment (thickening part)
Fig. 6 Demonstration test equipment (digestion part)
| Test conditions | |||
| Thickening Part | Coagulant type | Polymer coagulant (single-coagulant sludge conditioning) | |
| Chemical feeding rate % (vs TS) | Average: 0.41 Range: 0.30 to 0.52 |
||
| Digestion Part | Digestion temperature °C | 35 to 37 | |
| Average digestion period | Average: 14.5 Range: 12.0 to 16.0 |
||
| Test results | |||
|
Thickening Part |
Thickening part feed sludge concentration % | Average: 1.4 Range: 0.9 to 1.9 |
|
| Thickened sludge concentration % (digestion tank feed sludge concentration) | Average: 8.4 Range: 7.1 to 9.8 |
||
| Solid recovery rate % | Average: 96.0 Range: 94.2 to 99.1 |
||
|
Digestion Part |
Digested sludge properties |
pH | Average: 7.3 Range: 6.9 to 7.6 |
| TS concentration＊1 % | Average: 4.2 Range: 3.7 to 4.7 |
||
| VS concentration＊2 % | Average: 3.3 Range: 2.9 to 3.6 |
||
| VS decomposition rate % | Average: 56.3 Range: 46.6 to 64.6 |
||
| Digestion gas production rate per digestion tank capacity [m3 /(m3 ･d) (NTP)] | Average: 2.1 Range: 1.7 to 2.5 |
||
Fig. 7 Thickening part test result
Fig. 8 Changes in digested sludge pH and organic acid concentration
Fig. 9 Changes in feed sludge and digested sludge concentrations
Sewage sludge has high energy potential. Although it is continuously usable biomass energy that is collected at sewage treatment plants, its utilization ratio as a source of energy still remains at a low level. The Semi-Dry Anaerobic Digester developed by our company is lower in cost and higher in energy recovery efficiency compared to the conventional sewage sludge digestion facilities. Therefore, we believe that it can contribute to the promotion of sewage sludge utilization as an energy source.
1) Web page of Ministry of Land, Infrastructure and Transport Systematization of Resource/Energy Recycling Condition of resource/energy use in sewer system
2) Guidelines for Sewage Works Technical Management Practical Edition (2014), Japan Sewage Works Association: p. 822, p. 833.
3) Guidelines and Commentary for Planning/Design Sewerage Facility vol. 2 (2009), Japan Sewage Works Association: p. 341, p. 343, p. 359.
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