| There are mainly two kinds of sludge in the municipal wastewater treatment plant, i.e., primary and secondary sludge. They account for 0.3-0.5% of the treatment wastewater quantity in the case of sludge containing water ration of 97%. The studies on the anaerobic fermentation of sludge in the literature mainly focus on the treatment of the primary or its mixture with excess activated sludge. However, there are very few studies regarding the anaerobic fermentation of single excess activated sludge. The aim of hydrolysis and acidfication of excess activated sludge is to improve its biological degradability in the terms of higher soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) productions. SCOD and VFAs, especially acetate and propionate in VFAs, are recognized as the most suitable carbon substrates for enhanced biological phosphorus removal (EBPR). In order to achieve this aim, some effecting factors, such as environmental parameters and operational conditions of fermentation of excess activated sludge were investigated in the test.Firstly, through the comparison of aerated and anaerobic fermentation production of excess activated sludge, it was obvious that the SCOD and VFAs production under anaerobic conditions were higher than those under aerated conditions. Therefore it was important to avoid coming into oxygen during the fermentation process.Secondly, the production of SCOD and VFAs was conducted at different mixing methods and stir rate. Experimental results showed that mechanical mixing method was more advantageous than magnetic and shaking mixing methods for effective contact of sludge granules. At the same time, it was found that the quicker or slower stir rate for mechanical mixing was adverse to the production of SCOD and VFAs. The 60~80rpm mechanical sitr speed was selected in the test.Thirdly, the anaerobic fermentation from excess activated sludge was conducted at different pH values ranging from 4.0 to 11.0 at 21 ± 1℃. Experimental resultsshowed that the SCOD production at pH 8.0-11.0 was higher than pH 4.0-7.0 during the whole fermentation time of 20 days. Especially at pH 10.0, the VFAs production during 8-day fermentation ( 2708.02mgCOD/L ) was closely to its largest value (2770.40mgCOD/L, arrived on the 12th day) and higher than that at other pHs. For example, it was respectively over 3 and 4 times of that at pH 5.0 and pH uncontrolled. Among six kinds of individual VFAs, acetate and propionate were main prevalent products and acetate production was higher than propionate. And the percentage of the sum of acetate and propionate production accounting for the total VFAs was 60-70% at pHlO.O. Iso-valeriate and iso-butyrate production were second main composition of total VFAs. However, n-butyrate and n-valeriate production was low, especially the percentage of n-valeriate production accounting for total VFAs was lower than 10%. The soluble protein and carbohydrate production were higher at alkaline condition than closely neutral situation. However, the released orthophosphate and ammonia-nitrogen by acid adjusted were higher than those by alkaline adjusted.In addition, different methods were applied to change the pH controlled strategies. One was the comparison of controlled time of pH5.0 or pHlO.O (1-day vs 20-day). These results showed that the production of SCOD, total VFAs, individual VFA, soluble protein and carbohydrate etc. at pHlO.O were higher than those at pH 5.0 no matter than pH was controlled for lday or 20 day. Besides these, during the shorter fermentation time (2~4day) the production of SCOD and total VFAs at pHlO.O of 1- day controlled were close to those at pHlO.O of 20-day controlled. Then the production values at pHlO.O of 20-day controlled were prevalent. So if the aim of test is to get more SCOD and VFAs in shorter fermentation time, it was feasible that the pH value was pre-controlled alkaline. Another change strategy of pH was that the pH was pre-controlled 10.0 or 5.0 and the excess sludge was quick stir (410~430rpm)for 2-hour. Then stir rate was cut down to 60~80rpm and at the same time pH value was adjusted 6.0, 7.0 and 8.0 again. Experimental results showed that the production of SCOD, total VFAs, individual VFA, soluble protein and carbohydrate etc. under the previous condition were higher than those under the latter condition. So under the pH pre-controlled to 10.0 and quick stir for short timecondition, the production of SCOD and VFAs were higher than those at close to neutral pH value. And the corrupt action of higher alkaline and acidic to the reactors was might avoided.At 10~35°C, when pH value was controlled 10.0, the rate of acidification was limited by the rate of hydrolysis. And the hydrolysis of the overall paniculate COD of excess activated sludge was described first-order kinetics. The first-order hydrolysis rate constant kh was revealed strong temperature dependency. Which follows Van't Hoff guide line and Arrhenius experiential equation.The mechanism of VFAs production increased significantly under alkaline conditions was investigated. Results showed that as more soluble COD, definitely more soluble protein was provided as the substrate for producing more VFAs, whereas less or even no VFAs were consumed by methanogens at alkaline pH, the VFAs production was therefore remarkably improved. Further investigation revealed that the formation of VFA at pH 10.0 was dominated by the biological effects.
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