| Summary: | In Malaysia, Sewage Treatment Plant Sludge (STP sludge)
is the largest contributor of organic pollution to water
resources and soil. Its contribution is top listed (64.4%),
followed by animal husbandry wastes (32.2%), agro based
pollutants (1.7%) and industrial effluents (1.3%) in terms
of BOD load. In this study, STP sludge was used as
substrate because it is an inexpensive and easily available
raw material and a good source for growth of
microorganisms due to enough nutrients and trace
elements present for citric acid production with optimum
process conditions. Citric acid is one of the important
chemicals used in various industrial processes. It is
estimated that about 500,000 tons of citric acid is
produced annually by fermentation of expensive raw
materials like glucose and sucrose. The optimum process
factors determined by central composite design (CCD)
using the statistical software MINITAB from previous
study were: substrate concentration of 1%; both wheat
flour and glucose concentrations of 4%; fermentation
period of 6 days with absence of ammonium nitrate. The
strain Aspergillus niger (A-SS106), isolated and screened
by another work, was used throughout the study.
Evaluation of the treatment performance was done in
terms of maximum citric acid production, biosolids
accumulation, chemical oxygen demand (COD) removal
and reduction of reducing sugar. The maximum citric acid
production was 1.296 g/l on 6th day at pH 1.57 with the
reduction of COD concentration from 82.5 g/l to 22.6 g/l.
Reducing sugar showed a decreasing trend from the
beginning but maximum decrease was observed on the last
two days of the fermentation process.
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