| The issues of low sludge hydrolysis rate and methane production rate still exist in traditional anaerobic fermentation process. Research has shown that microbial electrolysis can effectively improve the anaerobic methane production rate and enhance the stability of anaerobic fermentation, but carbon conversion rate of sludge is still low and hydrolysis is still the rate-limiting step. In order to further improve the carbon conversion rate and methanogenic rate of sludge, this study proposes pretreated sludge by chemicals to jointly improve the sludge hydrolysis and methane production rate. Chemical pretreatments were established as an effective way to promote dissolution of intracellular and extracellular polymeric substance. The transformation of substrates was studied in the sludge fermentation and microbial electrolysis process, an optimum chemical pretreatment was chosen to maximize cascade conversion rate of sludge and its influence on sludge conversion efficiency was analyzed and evaluated.Five pretreatment chemicals were applied into sludge fermentation for preparation of sludge fermentation liquid, followed with the utilization of fermentation liquid by microbial electrolysis which was regarded as a new technology model. Its influence on the subsequent accumulation of volatile fatty acid(VFAs) and hydrogen production by electrolysis was studied. In mesophilic fermentation conditions for three days, the amounts of VFAs accumulation were cyclodextrin > rhamnolipid > peracetic acid> SDS> Na OH> blank, respectively, wherein the cyclodextrin group received the maximum VFAs accumulation that reached 4713 mg COD/L and was 3.95 times of the blank, the rhamnolipid reached 3503 mg COD/L which was 2.94 times of the blank. Mechanisms of chemical pretreatments for promoting acid production were different which lead to the different proportion of the components VFAs and then affected the subsequent microbial electrolysis capacity. The performance of microbial electrolysis for hydrogen production using the sludge fermentation liquid was tested. The results showed that the greater proportion of small molecule volatile fatty acid(acetic acid and propionic acid) was in favor of higher energy recovery efficiency of microbial electrolysis process for hydrogen production. The highest hydrogen conversion rate was 8.5 ± 0.5 mg H2/g VSS through maximizing use of organic by the coupling system, the conversion efficiency of different carbon sources in descending order are: acetic acid, propionic acid, n-butyric acid, n-valeric acid, isovaleric acid and isobutyric acid with the conversion coefficient of 341.32 mg COD/d(R2 = 0.97), 147.99 mg COD/d(R2 = 0.99), 68.94 mg COD /d(R2 = 0.91), 53.44 mg COD/ d(R2 = 0.99), 47.86 mg COD/d(R2 = 0.96), 36.09 mg COD/d(R2 = 0.97), respectively.At room temperature, the effects of chemical reagents for sludge fermentation processes and coupling processes are studied to find the right chemical pretreatment for the subsequent coupling process. Investigation of chemical reagents for methane production is done to discovery their effects on methanogenic bacteria. The results show that SDS has a strong inhibitory effect on methanogenic bacteria which leads to none detection of methane, cyclodextrin group has a certain inhibitory effect for methanogens, rhamnolipid has no inhibition on methanogens substantially which is conducive to be applied to the coupling process for sludge methane production. Select rhamnolipid as pretreatment agent to enhance the performance of microbial electrolysis combined anaerobic fermentation coupling process. After rhamnolipid pretreatment, both the methane yield and production rate of the coupled system have increased, the total output of methane reached 78.4 mg CH4/g VSS, the maximum methane production rate reached 156 m L/d which is 1.5 and 1.7 times of the group without rhamnolipid, respectively.The results showed that suitable pretreatment means can effectively promote sludge hydrolysis for acid production and microbial electrolysis process for hydrogen production. Rhamnolipid as a kind of biosurfactants, shows its environmental friendliness and is respected to promote sludge fermentation for acid accumulation and enhance performance of the coupling processes. The application of rhamnolipid on sludge pretreatment has a great value to further shorten the sludge fermentation time and improve methane production rate in microbial electrolysis combined sludge fermentation system. |
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