Updated: July 2002

1. Equipment overview

Wastewater generated by food processing plants generally contains organic content such as starch, proteins etc. The wastewater with a high BOD, which is a by-product of the soybean steaming process from the fermented soybean production plant described here (Table 1) is no exception.
Stricter enforcement of environmental preservation measures is increasingly being required. Satisfying emissions regulations by using a general-purpose biological process and continued stable processing of waste water requires intensive control by skilled operators, but this is an increasingly tough challenge due to the low allowance limit on removal rates offered by conventional equipment.
By employing submerged membranes with 0.4 µm multiple micro-pores in its processing system, our company has achieved high-load, high removal rate processing of the organic wastewater generated by the plant. In addition, we have designed easily maintained, compact equipment.

Table 1 Raw water quality

2. Construction

The overall process flow of the membrane type wastewater processing system used for the fermented soybean production plant is shown in Fig. 1.

3. Features

(1) High contaminant removal rate of (Refer to Table 2)
1) By employing submerged membranes with micro-holes, wastewater treatment by high concentration active sludge can be applied, enabling simultaneous removal of organic substances with high BOD, as well as of nitrogen compounds which are difficult to break down.
2) Micro filtering by membranes with holes of 0.4 m average diameter eliminates most suspended solids from the treated water.
3) The wastewater from food processing plants is organic in nature, and the treated water has buffering properties
which makes consistent neutralization difficult. The wastewater from a fermented soybean manufacturing plant has a low pH value due to the presence of acetic acid but our system neutralizes the water without requiring addition of chemicals or dedicated pH adjustment equipment.

(2) Reduction of equipment installation area
1) Because the volumetric load in terms of MLSS and BOD can be set higher than for conventional equipment, the capacity of the aeration tank can be reduced without requiring the inclusion of a sludge condensation tank.
(MLSS in the aeration tank = 20,000 mg/l)
(BOD volumetric load = 2.5 kg/m³/day)
2) Liquid-solid separation carried out by submerged membranes eliminates the need for a sedimentation tank.

(3) Simple maintenance
1) Membrane separation does not show any degradation of treated water quality, normally a problem caused by microbial phase change. (Treatment is not affected by the sedimentation characteristics of the active sludge.)
2) The upward flow parallel to the membrane provides the membrane with a self-cleaning function, extending its service life.

Table 2 Treated water quality

Fig.1 Membrane processing flow in the fermented soybean production plant