Study Of Anaerobic Digestion And Strengthening Treatment Effect Of Flocculated Sludge

In sewage treatment, flocculation process could do well in removing suspended solids and colloidal materials, reducing the burden on the subsequent biochemical process, but it would generate a lot of flocculated sludge. There are few study on flocculated sludge treatment reported, which is hindering the promotion and application of flocculation processes. Anaerobic digestion is one of the common methods of sludge treatment. The generation, anaerobic digestion(batch,semi-continuous) and strengthening treatment(alkaline pretreatment, mixing digestion) of flocculated sludge were systematically studied to provide technical and theoretical basis for practical application.In order to produce flocculated sludge, domestic sewage was flocculated with polyferric chloride(PFC) in combinational dosing way(coagulant aid:polyacrylamide PAM). Based on the results of flocculation test, the optimum flocculation conditions were chosen as follows: pH value of 7, water temperature of20 ℃, PFC dosage of 180mg/L, PAM dosage of 0.5mg/L. The batchs anaerobic digestion test of flocculated sludge were taken for the operating conditions such as10%sludge ratio, pH 7, 35℃, of semi-continuous anaerobic digestion test.The result of semi-continuous test showed that: 1) the digestion effect of flocculated sludge was worse than the waste activated sludge under the same concentration and organic load.The removal rates of total chemical oxygen demand(TCOD), the degradation effect of volatile solid(VS) and the average daily gas production of flocculated sludge were5.8%, 6.6% and 3.8% respectively lower than waste activated sludge; 2) flocculant did have an effect on waste sludge digestion with the effect of digestion decreased.The TCOD, VS removal rates were reduced by 28.3%, 31.7%, and the gas production fell by 51.0%. The pH environment stability of the system deteriorated with the obvious pH fluctuations; 3) microscopic analysis of sludge flocs in each system demonstrated that, the changes of floc structure were consistent with the degree of digestion. The changes of floc structure could reflect the sludge cracking process well in anaerobic digestion, and could explain the microscopic process of sludge digestion.The study of flocculated sludge alkaline hydrolysis pretreatment show that: 1)after alkaline hydrolysis for 24 hours under the pH of 11, the solubility and chemical oxygen demand(SCOD) value of sludge reached 3641.0 mg/L, increased by 203.4%comparing with no alkaline hydrolysis sludge, and the removal rate of volatile suspended solid(VSS) reached 38.6%. Alkaline pretreatment can effectively promote the transfer in organic matter from the solid phase into the liquid phase, which could reduce the difficulty of subsequent anaerobic digestion. The changes of floc structure in morphology could confirm that the effect of alkaline pretreatment would be better at higher pH and longer action time; 2) the pH of flocculated sludge pretreated with alkaline was adjusted to 7.5 for semi-continuous anaerobic digestion. Comparing to sludge without alkaline-pretreatment, the TCOD removal rate, VS degradation rate and daily gas production of alkaline- pretreatment sludge were increased by 52.4%,48.5% and 73.8% respectively, which indicated that the effect of anaerobic digestion was strengthened, and the time of gas production peaking shortened 8d; 3) because ofthe too high pH value, the activity of anaerobic microorganisms was inhibited. It made the digestion effect of alkaline sludge decreased when the pH did not be adjusted. The TCOD and VS removal rates decreased by 28.8% and 33.5% than the original flocculated sludge.The effect of anaerobic co-digestion of flocculated sludge and waste activated sludge at different mixing ratios and dosing rates has been studied. The results showed that the best effect of anaerobic digestion of flocculated sludge / waste activated sludge(VS/VS) at 1:2 was achieved under the 5% rate of dosing: the TCOD and VS degradation rates increased by 33.9% and 39.0 % comparing with flocculatied sludge digestion alone. The effect of reduction was obvious while the VS/TS value decreased by 15.6% than the original mixed sludge. The gas production rate also increased by 132ml/g VS than the flocculated sludge digestion alone. Therefore,co-digestion with waste activated sludge could significantly improve the efficiency of anaerobic digestion of flocculated sludge.In summary, flocculation with PFC could effectively remove the COD of sewage under certain conditions, and produce large flocculated sludge. But the anaerobic digestion effect of flocculated sludge was poorer than waste sludge. Alkaline pretreatment and mixing with waste sludge were two ways to enhance the digestion effect of flocculated sludge, which could improve the digestion effect efficiency.