Effect Of Calcium Peroxide On The Production Of Hydrogen From Anaerobic Fermentation Of Waste Activated Sludge And Its Relevant Mechanisms

Activated sludge process is a universal aerobic biological treatment method in the current wastewater treatment plants?WWTPs?.The method has the advantages of low operating cost,easy management,effective removal of organic matter and suspended solids contained in sewage,and good removal effect on pollutants such as nitrogen and phosphorus.However,the process could lead to the production of large amounts of waste activated sludge?WAS?Calcium peroxide?CaO₂?,one of versatile and harmless inorganic peroxy compounds,has been recently used as a highly effective oxidant to degrade pollutants in sludge and a method to achieve sludge resource utilization.In recent years,it has been reported that CaO₂ can effectively improve the dewatering of WAS,and can promote the production of short-chain fatty acids in anaerobic fermentation systems.Therefore,CaO₂ is increasingly used as an additive for residual activated sludge pretreatment.Although many efforts have been dedicated to this topic,the effect of oxidizing agent in the anaerobic fermentation system on hydrogen production has seldom documented in the literature,and its specific mechanisms are also still unknown.Therefore,the main purpose of this study is to investigate the effect of CaO₂ on hydrogen production from sludge anaerobic fermentation.Firstly,the effects of different doses of CaO₂ on the accumulative production of hydrogen in sludge anaerobic fermentation were investigated.Firstly,the cumulative production of H₂ in fermentators with different concentrations of CaO₂ was studied by H₂ production batch test and COD mass balance analysis.The cumulative H₂ volume in the CaO₂-involved system was significantly higher than that in the natural conditio n.Therefore,it is found that the concentration of CaO₂ was closely related to the process of H₂production.When CaO₂ increased from 0.05 to 0.25 g/gVSS,the maximum H₂cumulative production increased from 0.77 to 10.55 mL/g VSS.Secondly,this study explored the effects of CaO₂ on the sludge dissolution by EEM fluorescence spectrometry and t he determination of the ratio of bacterial living cells.The experimental results shown that the presence of CaO₂ acceleratesd the breakage and lysis of sludge cells,which leads to the release of cellular organic matter and increases the accumulation of fermentation substrates,thus laying a good material foundation for subsequent biochemical reactions.In addition to this,during the research,it was found that CaO₂ also significantly improved the biodegradability of organic substance released from sludge cells,converting it into substances that can be utilized by microorganism.Therefore,CaO₂ increase the possibility of increasing hydrogen production by fermentation.Thirdly,by simulating the batching experiments of various stages of fermentation,this study explored the impacts of CaO₂ as a pretreatment method on the dark fermentation hydrolysis,acidification,acetogenesis,homoacetogenesis,methanogenesis and sulfate-reducing process.The results demonstrate that CaO₂ at the tested levels inhibited all other bio-processes except for the acetogenesis.Nonetheless,its inhibition to the H₂ consumption process is much more severe than that to hydrogen production processes.The enzyme activity test in the long-term semi-continuous reactor also verified the above experimental resu lts.Under the condition of 0.2g CaO₂/g VSS,CaO₂ posed slight impacts on[FeFe]hydrogenase activities?only 7%?but largely suppressed the activities of coenzyme F420?reached to 12%?.Finally,the experiment analyzed the contribution of various hydrolysates of CaO ₂to H₂ production.Results shown that the mainly decomposition products of CaO₂ are Ca²⁺,alkali,·OH,and·O^2-,and further studies found that among the four main decomposition products,the presence of Ca²⁺has no significant effect on H₂accumulation.Alkali,·OH,and·O^2-play a major role in this series of processes andcontributions of alkali,·OH,and·O₂^-to the increased hydrogen production were in the order of alkali>·O₂^->·OH,which was in accordance with the observed order of hydrogen production.This paper demonstrated the effect of CaO ₂ on the hydrogen production and the specific mechanism of this process,which effectively fills this knowledge gap.This is the first work demonstrating that CaO₂ can enhance hydrogen production from WAS.The findings reported in our paper not only expand the application field of CaO ₂ but also deepen the understanding of the CaO₂-involved sludge fermentation process.