Thesis - faculty.ait.ac.th - Asian Institute of Technology

2.12 Yeasts
2.12.1 Introduction
The yeast degrade organics either anaerobically (fermentation) or aerobically
(oxidation). The most typical yeast process applied in food or beverage industries is
anaerobic, also known as alcoholic fermentation. The end products of fermentation can be
alcohols, acids, esters, glycerol and aldehydes. A typical reaction of sugar fermentation by
yeasts is shown in the following reaction:
C6H12O6 + nutrients C2H5OH + CO2 + new biomass
Under aerobic process, complete oxidation of organics yields carbon dioxide and
water. Abundant supply of oxygen enhances considerable yeast growth; whereas
incomplete oxidation is accompanied by the accumulation of acids and other intermediary
products. There are differences in the compounds which can be assimilated by various
species of yeasts. Some can degrade pentoses, polysaccharides (starch), sugars, alcohols,
organic acids (lactic, acetic, citric) and other organic substrates.
COHNS + O2 + nutrients CO2 + H2O + new biomass + end products
(Organic matter)
Yeasts may utilize the nitrogen required in their metabolism for the synthesis of
protein from organic (amino acids, urea, vitamins, peptone, aliphatic amines, etc.) and
inorganic sources (ammonia, nitrite and nitrate). Most species can utilize the ammonium
ionmaking it appropriate for leachate treatment. Other nutrients required for yeast growth
include phosphorous, sulfur (organic sulfur and sulphate), minerals (potassium, magnesium,
sodium and calcium). The C:N:P ratio of Candida utilis biomass was found to be 100:20:5.
Therefore, nutrient demands of yeasts are higher than that of bacteria whose BOD5:N:P
ratio is 100:5:1 (Defrance, 1993).
Yeasts can grow in a wide pH range (from 2.2 to 8.0). In general, yeasts grow well
on media with acid reactions (from 3.8 to 4.0), whereas optimum pH values for bacteria
growth range from 7.5 to 8.5. Yeasts have been used in the fermentation industry which
requiring operation at a high substrate concentrations and under high loads. It is noted that
yeast can be utilized to treat the wastewater containing solids, high concentrations of
organic matter and salt, and other substances, which are difficult to treat using activated
sludge process (Nishihara ESRC Ltd., 2001). Furthermore, yeasts can grow in
temperatures ranging from 0 to 47 o C, the optimum temperature being from 20 to 30 o C.
2.12.2 Applications of Yeasts for Wastewater Treatment
Miskiewicz, et al. (1982) developed yeast based treatment for fresh piggery wastes
by adding carbon source (beet molasses or sucrose). Candida tropicalis, Candida
tropicalis, Candida robusta and Candida utilis were the yeast strains that were cultured in
the aerated batch reactor. According to the study, molasses are the most appropriate carbon
source of yeast. The use of raw piggery waste without carbon supplement leads to low
biomass yield and low treatment efficiency, inspite of nutrients (N, P) content being high.
The culture of C. utilis on molasses-enriched piggery waste (5,570 mg COD/L) could
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