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

obtain high treatment efficiencies of 76% TKN, 60% COD, 84% phosprorus removal at
HRT of 7 hours and with a F/M ratio of 1.73 g COD/g MLSS.d. The maximum specific
growth rate of C. utilis was found to be 0.19 h -1
Hu (1989) used ten different yeast strains in cultures to treat vermicelli wastewater
which contained BOD ranging from 24,000 to 44,000 mg/L and high concentration of
starch, lactic acid and protein. Based on the ability of starch degradation, protein
hydrolysis and lactic acid tolerance, these yeast strains were screened from 391 colonies
isolated from soil samples. Most of them could grow well with pH range of 3.0-5.0, 4.0
being the optimum. The results shows that the two strains could reduce soluble COD by
92% at HRT of 7 days, F/M ratio of 0.48 g COD/g MLSS.d and VLR of 1.03 kg
COD/m 3 .d. Due to the poor settling ability of yeasts, they could not be flocculated or
settled as in a conventional activated sludge process and were easily washed out with the
effluent. Therefore, the HRT in this process have to keep long and same as the SRT. The
author postulated that the fungi contamination prevented the formation of yeast flocs.
Chigusa, et al. (1996) used nine different strains of yeasts capable of decomposing
the oil to treat wastewater from oil manufacturing plants. A pilot scale yeast treatment
system had been run for one year. According to the results, 10,000 mg/L of hexane extracts
in the raw wastewater were reduced by the yeast mixture to about 100 mg/L.
Elmaleh et al.(1996) investigated the yeast treatment of highly concentrated acidic
wastewater from the food processing industry. The strain Candida utilis was cultured in
continuously completed mixed reactors. This system did not have a separate settling tank;
the SRT and HRT of the system were identical. A mixture of acetic acid, propionic and
butyric acid was the carbon source of feed wastewater. The pH was maintained at 3.5 to
prevent any bacterial contamination. The TOC removal obtained was 97% at high loading
rates (30 kg TOC/m 3 .d). The growth yield and maximum specific growth rate of yeasts
were similar to those for conventional activated sludge (µmax = 0.5 h - ; Y = 0.85-1.05 kg
SS/kg TOC for acetic acid).
Olive mill wastewater normally contains high concentration of fats, sugars, phenols,
volatile fatty acids which contribute to a very high COD concentration (100,000-200,000
mg/L). Scioli and Vollaro (1997) reported that Yarrowia lipolytica cultured in aerated
fermenter was capable of reducing the COD level of olive oil processing wastewater by
80% in 24 h. Fats and sugars were completely assimilated while methanol and ethanol
were present. The effluent had a pleasant smell due to the presence of these compounds.
The authors asserted that a possible approach for pollution reduction in olive-oil-producing
countries is to use membrane to filter effluent before discharging into the sewage system.
Useful biomass (40% protein) and valuable lipase enzyme could also be obtained in this
process.
Arnold, et al. (2000) investigated the ability of selected yeast strains (C. utilis and
Galactomyces geotrichum) to purify silage wastewater containing high COD concentration
of 30,000 to 80,000 mg/L by using the shaker-flask. High removal efficiencies of COD
(74-95%), VFA (85-99%) and phosphate (82-99%) were obtained after 24 hrs and some
ammonia was also removed. During treatment, pH rose from initial values of 3.7-5.8 to
8.5-9.0. This was presumably due to removal of lactic acid and VFAs. An efficient
removal of P from the system could lead to the shortage of phosphorus.
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