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Data Entry: June 2011
Name of Technology Anaerobic Digestion Process with Ozonation for Economically Feasible Energy Production from Municipal Sludge
Technology Owner Hitachi Plant Technologies, Ltd.
Kurita Water Industries Ltd.
General Description In the conventional anaerobic sludge digestion, sewage sludge contains much inert organic matter which is not easily biodegraded, its digestibility (degradation ratio of organic matter in sludge) is low at around 50%, and biogas production rate is low. Consequently, digestion gas is not generally used for gas power generation.
This technology aims to reduce the amount of sludge to be disposed of by combining digestive promotion device using ozonation with conventional anaerobic digestion. The whole system will also be configured to reduce power generation costs, by generating power from increased digestion gas.
Components of Technology The major components of this technology are: (1) Anaerobic digestion tank, (2) Solid-liquid separator, (3) Ozonation equipment, (4) Digestion gas refiner, (5) Digestion gas power generator and (6) Dewatering equipment. These components have the following functions.

(1)Anaerobic digestion tank
Organic matter in fed sewage sludge is digested and converted to biogas. This technology can use existing equipment for this purpose.

(2)Solid-liquid separator
There are two functions, the one is to separate solids and liquids for the mixture of part of digested sludge and fed sewage sludge to keep sludge concentration high in the anaerobic digestion tank. The other is to withdraw excess digested sludge, in which inorganic solids are more concentrated. The equipment consists of a centrifuge, coagulant addition device, etc.

(3)Ozonation equipment
Part of the digested sludge is ozonated to convert inert organic matter in the sludge to a biodegradable form. The equipment consists of a pre-aeration tank, ozonaton reactor, gas-liquid separator, ozone generator, deodorization equipment, etc.

(4)Digestion gas refiner
Hydrogen sulfide and siloxane in the digestion gas are removed with desulfurizing agent and siloxane adsorber (activated carbon) so that the power generator can use the digestion gas. This technology produces more digestion gas than the conventional system. However, the peak gas production rate is the same or less than that of the conventional system, by feeding uniformly sewage sludge to the digestion tank. Therefore, this technology uses existing desulfurization equipment. Regarding siloxane removal, equipment is newly installed according as the gas volume used for power generation.

(5)Digestion gas power generator
Power is generated using the digestion gas refined by the equipment described in (4) and a gas engine. The outputs of this equipment are electricity and heat (hot water or steam).

(6)Dewatering equipment
Excess digested sludge withdrawn with the centrifuge described in (2) is dewatered. This technology uses existing equipment.



Schematic diagram of the technology


Anaerobic digestion tank setup
Case 1: Mesophilic digestion
Case 2: Thermophilic digestion + Mesophilic digestion

Features of Technology This technology enhances solid degradation ratio of sewage sludge by combining ozonation and solid-liquid separation of digested sludge with conventional anaerobic digestion. In addition, sludge for disposal is reduced with lower water content of dewatered sludge, in which inorganic solids are more concentrated by centrifugal separation. This technology also increases the digestion gas production without receiving garbage or other non-sewage sludge biomass.
Meanwhile, the increased digestion gas production raises the economy of power generation from digestion gas, which is stabilized by incorporating siloxane removal and other digestion gas refininer.
In this technology, the whole system is configured so as to reduce power generation costs. Each component and equipment has the following features.

(1)Ozonation equipment
The equipment ozonates part of digested sludge, converts inert organic matter in the sludge to a biodegradable form, and then returns it to the digestion tank. Through these processes, higher digestibility and more digestion gas production than the conventional system are achieved.
Meanwhile, in the gas-liquid separator, waste ozone is separated from treated sludge, so the ozonation equipment prevents oxidant (waste ozone) from entering the digestion tank and detoxifies waste ozone through decomposition and deodorization processes for release to the atmosphere, thereby controlling safety.



Flowchart of ozonation equipment

(2) Solid-liquid separator
A centrifuge performs solid-liquid separation of part of the digested sludge, and the concentrated sludge is returned to the digestion tank. These processes ensure high-concentration digestion operation. Meanwhile, the solids retention time (SRT) in the digestion tank is extended to accelerate the digestion.
When withdrawing excess digested sludge, centrifugal separation is performed at a low solids recovery rate without adding coagulant. This is intended to concentrate more inorganic solids. Lower water content of dewatered sludge leads to reduction of the amount of sludge to be disposed of.

(3)Digestion gas refiner and power generator
Compared with the conventional system, a larger generator can be applied since more digestion gas is used as fuel, benefiting from economy of scale. Furthermore, gas power generation using digestion gas after removal of impurities (siloxane [organosilicon compounds], hydrogen sulfide, etc.) improves the operating conditions of the generator, enabling power to be generated stably.

(4)Digestion tank
This technology assumes the use of two types of digestion tank.
In Case 1, the mesophilic digestion process is designed for mesophilic digestion, which is used widely in Japan.
In Case 2, thermophilic and mesophilic processes are combined to achieve, where multiple digestion tanks are incorporated, a higher digestibility than in the mesophilic process alone, thereby improving the digestibility sufficiently even in higher sludge loading on digestion tanks and benefiting from the resulting increase in biogas production.


Case 1: mesophilic digestion process




Case 2: combined thermophilic and mesophilic digestion processes



Digestion tank setup

Evaluation of Technology

This technology was developed in the LOTUS Project (Lead to Outstanding technology for Utilization of Sludge Project) and evaluated and reviewed by the Sewerage Technology Development Project Committee under the Ministry of Land, Infrastructure and Transport in Japan in 2008.

[Link to the technology evaluation report]

Contact Information Water Treatment Business Division, Environment Preservation & Water Treatment Group, Hitachi Plant Technologies, Ltd.
URL: http://www.hitachi-pt.com/products/es/water/wastewater2/index.html
For inquiry: http://www.hitachi-pt.com/contact/index.html

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