Citation
Ong, Keat Khim
(2007)
Influence Of Ph And Initial Sludge Concentration On Anaerobic Digestion Of Waste Activated Sludge.
PhD thesis, Universiti Putra Malaysia.
Abstract
Although anaerobic digestion is a common process for treatment of sludge, the
digestion rate remains low. Hydrolysis is the first step of anaerobic digestion process
and generally is considered to be the rate-limiting step for the overall digestion process.
Hence, improvement in the hydrolysis rate is needed to enhance anaerobic digestion of
sludge. Chemical hydrolysis can be used as an alternative as it could improve the
digestion performance of the sludge. However, hydrolysis process relies on enzymes
excreted by the fermentative acidogens, and is strongly pH dependent. Consequently,
determination of the optimum pH of sludge digestion is essential and required. In spite
of pH, initial concentration of sludge is also an important factor that influences the
anaerobic digestion performance of sludge. However, concentrations of total suspended
solids (TSS) at a secondary clarifier and a thickener are 1.0% and 4.0% TSS,
respectively.This study focuses on a laboratory-scale research on improvement of anaerobic
digestion of waste activated sludge (WAS) which was performed by chemical
hydrolysis using 0.5 M H2SO4 and 4.0 M NaOH at temperature of 29 ± 1 °C and
agitation of 150 rpm. The research was conducted in two phases: the first phase
investigated the effect of pH at pH 6.00, 7.00, 8.00, 9.00 and uncontrolled pH on the
digestion performance at initial concentration of 4.0% TSS; the second phase
investigated and compared the performance of anaerobic digestion at initial
concentrations of 1.0% and 4.0% TSS at the optimum pH which was determined from
the first phase. Subsequently, determination of kinetic parameters and followed by
developments of mathematical models and computer programs were performed at both
phases.
Significantly higher average removals, removal rates and rate constants of TSS, volatile
suspended solids (VSS), total chemical oxygen demand (TCOD) and particulate
chemical oxygen demand (PCOD) at controlled pH compared to that at uncontrolled
pH. The average removals and removal rates at uncontrolled pH could be improved by
1 to 4 times by controlled pH. Meanwhile rate constants could be improved by 1 to 7.
The highest average removal, removal rate and rate constant were found at pH 6.00
followed by pH 8.00, 7.00, 9.00 and uncontrolled pH. The results also indicated that the
optimum pH was at pH 6.00. The improvement of sludge digestion was attributed to the
enhancement of sludge solubilisation by the chemical hydrolysis as higher soluble
chemical oxygen demand (SCOD) concentration was observed at the controlled pH.There was no significant difference in rate constant and removal of the sludge.
However, significantly higher average removal rates of TSS, VSS, TCOD and PCOD
were achieved at initial concentration of 4.0% TSS. The improvement of the average
removal rates of TSS, VSS, TCOD and PCOD was 2.5 to 3.4 times over that at initial
concentration of 1.0% TSS giving rise to a higher digestion performance at initial
concentration of 4.0% TSS.
The experimental results fitted well in first-order models in both phases. High
relationships between simulated and experimental results were obtained from both
phases experiments with correlation coefficients (R2) ranging from 0.80 to 0.99. Hence,
the developed mathematical models can be considered as a useful tool for predicting the
concentrations remaining and removals of TSS, VSS, TCOD and PCOD as well as
digestion time.
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