The Optimization Of Sludge Fermentation For Volatile Fatty Acids Production

As we all known, in many cities of our China, the influent water contains insufficient carbon source in some wastewater treatment plant, particularly in urban wastewater treatment plant. Accordingly, it becomes necessary to add extra carbon source to such wastewater to achieve satisfying BNR performance. Recently, production of SCOD, especially short-chain fatty acids (SCFAs) by fermentation of sludge generated in municipal wastewater treatment plants (MWTP) has attracted much attention, because it can not only take advantage of organic wastes for reclamation but also improve the SCOD concentration of influent.Primary sludge (PS) and waste activated sludge (WAS) are the two different wastes generated in MWTP. This thesis focused on the anaerobic fermentation of the two types of sludge, discussed the effect of pH and fermentation time on the products such as SCOD, carbohydrate, protein, etc as well as by-products like ammonia and phosphate, finally proposed optimal parameters for different sludge fermentation. On that basis, sludge fermentation reactor was designed and we investigated the dissolving process of organic matter, the separation of mixed fermentation liquid, transfer efficiency of organic substrate and so on. Finally results provide reference for actual project which aimed to provide maximum, continuous, stable and efficient carbon source for BNR system with insufficient carbon source.Firstly, sequencing batch method used in this experiment was to examine the different processes of fermentation of primary sludge and activated sludge, under different pH conditions and fermentation time. The results show: alkaline conditions are obviously benefit to the dissolution of SCOD and VFAs. SCOD dissolved rapidly in the initial stage, for primary sludge the maximum rate achieved at 2d, while for activated sludge was at 4d; Relatively long fermentation time was benefit to the production of VFAs. Primary sludge achieved maximum output at 7.5to 9.5d, while activated was at 7.5d; Combined with SCOD and VFAs two factors the optimum fermentation conditions of primary sludge is pH at 9 and fermentation time in 9.5 days, while for activated sludge is pH at 10 and the fermentation time in 7.5 days.Secondly, comparation of the two sludge fermentation shows: primary sludge can produce a large number of VFAs in a relatively short time with low ammonia and phosphate load, therefore, fermentation of sole primary sludge is the most effective and efficient manner as additional carbon source for BNR system; Though the amount of SCOD dissolved by activated sludge is much higher than primary sludge (about 25.4%) after stabilization under pH at 10, the phosphorus and ammonia release are also higher than primary sludge (about 58.7%, 73.1%, respectively). As we all know great deal of ammonia and phosphorus release undoubtedly aggravated the waste water processing load of the follow-up system, in order to ease this negative impact, fermentation of mixed sludge as the carbon sources is proposed in this paper, meanwhile, suggested that the optimum conditions is pH at 10 and fermentation time in 8 days, under the combination of the fermentation characteristics of the two types of sludge.Thirdly, the research of continuous flow of alkaline fermentation reactor for activated sludge shows: the biomass of the sludge fermentation area was stable (SS 6890mg/L, VSS 4079 mg/L) and the production of SCOD and VFAs was also steady (3063 mg COD/L, 1252 mg COD/L, respectively); The main VFAs was acetic acid (41.12%), followed by propionic acid (23.57%) and isovaleric acid (15.38%); ammonia content was relatively low (66.15mg/L) and approach to sewage waste water, therefore, the effect to follow-up system can be ignored. However, the amount of phosphorus was relatively high (201.4mg/L), necessary pretreatment was needed to lower phosphorus load before utilization, such as chemical precipitation.Finally, the element-balance analysis of which C,N and P from the solid phase of sludge before fermentation to liquid phase after fermentation, shows that: the transfer efficiency was 3.2%,16.8% and 17.3%, respectively. Lower C transfer efficiency manifests larger sludge development space for ever exploit. The difficult problem of separation the fermentation mixture can be solved efficiently by elutriation technology, and the efficiency was 68%, which can be improved by continuous elutriation. The content of C,N,P keep equilibrium in the process of fermentation. C can effectively transfer from fermentation mixture before elutriation to the liquid phase of Static sink tank, where the elutriation efficiency was 86%.