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The aims of this study were to investigate the optimum conditions to produce acetic acid from various excess sludge as carbon sources for biological phosphorus removal processes. Five substrates were used to produce acetic acid which were waste from conventional activated sludge (CAS), waste from Enhanced biological phosphorus removal (EBPR), swine manure (SM), waste from biogas systems (raw sludge, RS) and waste from fermented biogas production system after 10 days of fermentation (fermentation sludge, FS). The fermentation was carried in batch fermenter with ten substrates series; series 1-2 using WAS from CAS, series (3-6) using WAS from EBPR, series 7 using pig manure and the rest of the series using excess sludge from biological gas production. RS and FS series were combined with acid fermentation (AF) in three different ratios; FS : AF (1 : 1), RS : AF (4 : 1) and RS : AF (1 : 4) in series 8, 9 and 10, respectively. Further investigation regarding the influence of carbon sources, temperature, pretreatment and the proportion of excess sludge (FS and AF) and acid fermentation (AF) was also conducted. The results showed that the highest concentration of acetic acid was 1,406 mg COD/L in the reactor containing FS : AF 1 : 1 (series 8) after 96 hours and control conditions at 35 °C whereas similar combinations in reactor 9 and 10 produced the acetic acid of 1,018 and 792 mg/L, respectively. The paired t-test of series 8-10 showed significant difference (p < 0.05) for FS in series 8 and RS in series 9-10. The composition in experimental series 8 can be used to produce acetic acid as a carbon source for the biological phosphorus in the removal system since it has a potential to reduce the cost of additional acetic acid.
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