Updated: July 2002

1. Purpose

The yeast cycle system is a wastewater treatment system employing yeast which is combined with subsequent utilization of excess yeast. This system, as depicted by its name, can be readily adopted by a recycling society. The yeast cycle system has many features which are not seen in the conventional activated sludge method, and is the most suitable for industrial wastewater treatment requiring high efficiency and stability."Efficiency"here implies a smaller site area, less aeration and lower sludge production."Stability"implies high stability to load fluctuation and good solid-liquid separation. In addition, the system also has the merit that excess yeast can be reused for fertilizer and livestock feed.

2. System Overview

3. Features

1) Since the system can operate under super high loading with as much as 10-15kg BOD/(m3d), the equipment is compact and the construction cost will be also going down.
2) The yeast treatment part is the most suitable for the pre-treatment device because only it can remove 90% or more of influent BOD.
3) The system can decompose lopids by as much as 10,000 mg/l(measuring n-hexane as the solvent) without a dissolved air floatation apparatus.
4) The system is not affected by sharp loading fluctuations and is not subject to bulking phenomenon.
5) The running cost is low at about 2/3 times that of the activated sludge method.
6) Excess sludge production is lower, decreasing to 1/2 to 1/3 times that of the activated sludge method.
7) Excess yeast, containing a lot of protein, lipid and vitamins, can be used for livestock feed and mushroom production as additives.

Wastewater treatment facility of Yeast cycle system

4. Principle

In nature, yeast usually occurs where organic substances are abundant, such as sap and nectar. In industry, it has been used for the fermentation of high organic-content materials such as miso, soy sauce and sake. Therefore, treatment system employing yeast is the most suitable for industrial wastewater in which the concentration of organic matter (BOD value) is 1,000 mg/l or more. The yeast treatment is able to operate at 1.0 to 1.5 kgBOD/kg yeast/day of high load. Therefore, the treatment capacity of the system is 3 to 5 times higher than the general activated sludge method. While activated sludge consists of bacteria, the system uses yeast belonging to the eumycetes subdivision. Since the system makes functional and efficient use of the physiological differences between the two kinds of microorganisms, it can perform a high-load treatment efficiently, even if the wastewater containing oil and fat, high-content organics, antibiotics, sterilizers and high-content salt. The yeast floc formed by intertwining yeast mycelia is so air-permeable that the quantity of air required is capable of being reduced to 2/3 times that in the general activated sludge method. In addition, the sedimentation of the yeast floc satisfactorily maintains an SVI value between 50 and 60 ml/g, resulting in good stability of the effluent water quality which is not affected by load fluctuations. Yeast is subject to convert energy from the oxidation of organics into heat which maintains a higher temperature of the reaction tank and thus decreases yeast growth.

Photograph of a Yeast Floc