Updated: July 2002

1. Purpose

The Mitsubishi Methano-Pack Process (UASB Process) is a biological water treatment system to efficiently treat organic wastewater including food processing wastewater by a compact energy-saving reactor. Energy can be efficiently recovered as methane gas generated during the course of the treatment. Conventional aerobic treatment processes such as the activated sludge process to treat organic wastewater involve many inconveniences. That is, they require much power for aeration, produce excess sludge in large quantities and suffer sludge bulking. The UASB process, which overcomes these difficulties, is a newly-developed anaerobic treatment method.

2. Outline of the UASB Process

1) Principle of anaerobic treatment
Organic matter in the wastewater is degraded and treated through two steps of biochemical reaction by the facultative anaerobic bacteria such as acid-formation bacteria and the strictly anaerobic bacteria. In the first acid fermentation step, the organic matter is degraded into low-grade fatty acid, carbon dioxide, hydrogen and so on. In the next step for methane fermentation, the low-grade fatty acid is degraded and methane gas is produced. Through the UASB process, the acid fermentation and methane fermentation proceed correlatively both in the equalization tank and UASB reactor to degrade and remove organic substances.
2) Granular sludge
Organic matter is degraded by utilizing granular sludge self-granulated to1-3mm diameter by anaerobic bacteria whose dominant is Methanothrix. This granular sludge is superior to others in having greater sedimentation velocity (20-40m/h) and very high organic degrading activity. These enable the equipment to be compact and economical.
3) UASB reactor
The following figure shows the structure of the UASB reactor and the general view of the installation. The reactor consists basically of influent water distribution device, reaction section (sludge zone) and three-phase separator. Wastewater entered from the influent water distribution device is distributed uniformly in the reactor, moves up the reaction section slowly and comes in contact with granular sludge where organic matter is degraded. The treated water is discharged from the reactor after the gas and fine granule are removed by the three-phase separator.

3. Distinctive Features

1) Comparison with conventional anaerobic treatment
The space for UASB treatment is much smaller than that needed for conventional sludge suspended type anaerobic treatment. There is no need to use expensive carrier, as required for the fixed bed type.
2) High-loading CODcr can be obtained
As the concentration of a very active granular sludge in the reactor can be kept at high concentration, it is possible to treat efficiently in a very short time.
(Removal efficiency: BOD; 90-97%, CODcr; 88 -95%, at CODcr load; 9-15kg CODcr/d)
3) Energy saving
It requires no power cost needed for aeration which accounts for 50 - 70 % of power required for all the equipment in aerobic treatment. Energy can be recovered from methane gas generated during the anaerobic treatment (0.3-0.35m³N/removed CODcr).
4) Excess sludge
The amount of excess sludge produced is reduced to 1/3- 1/7 of the produced sludge during aerobic treatment, drastically reducing the sludge treatment cost.

Principle of the reactor

Mitsubishi Methano-Pack Process