| Pursuing the goal of carbon neutral results in reconsideration of energy recovery fromexcess sludge in wastewater treatment plants (WWTPs). However, traditional mesophilicanaerobic digestion (AD) is slow in organic conversion and impure in biogas, especially thelow ratio of methane (CH4), which reduces the calorific value of biogas and limits theachievement of carbon neutral of WWTPs. Under the circumstance, some trials wereconducted to reduce CO2content and to increase methane (CH4) production in biogas withexogenous hydrogen (H2) stimulation in AD. However, the mechanism of enhancing theCH4yield by exogenous H2and the influence of exogenous H2on whole AD process has notbeen fully understood.A model calculating the energy consumption and production of WWTPs was firstconfigured to evaluate the possibility of achieving “carbon neutral” in WWTPs. Then theinfluences of exogenous H2on the patywasys of H2consumption and the whole anaerobicdigestion process, especially hydrolysis and acetogenesis, were studied and analyzed inbatch test.Based on mass balance, influent quality and energy index are coupled, and energybalance models and analyzing functions are configured, which are applied to evaluate thebalance of energy consumption and energy recovery of WWTPs. Validated and analyzed onthe basis of a practical set of data from a practical WWTP, the configured models aredemonstrated to be accurate and practical. It is revealed by the model analyses that WWTPsin China are not so easy to approach to “carbon neutral” under the circumstance of energyrecovery (CH4) due to low influent organic load. But energy recovery could compensate atleast half of consumed energy and thus reduce CO2emission by50%indirectly.This present study also focuses on intervention of exogenous H2, and thus thepathways of H2consumption was especially concerned due to increased H2partial pressure(pH2). At a high partial H2pressure (1.2atm), hydrogenotrophic methanogens (HMs) andhomoacetogens (HAs) could contribute to H2consumption for60%and40%respectively.This is because the population of HAs was two orders of magnitude less than that of HMs.Moreover, Ksand Vmaxof HAs were respectively10times higher and4times lower thanthose of HMs, revealing that a low H2partial pressure (in normal AD) makes HAscontribute much less to H2consumption. There should be an upper limited capacity of HMsfor H2consumption. Even under the condition where HAs are out-competed for H2consumption with HMs, HMs would not consume anymore H2. For this reason, any attempt to inhibit HAs would be pointless in technology.Moreover, the influence of exogenous H2on upstream process of whole mesophilic AD,especially the inhibiting effect on the degradation of VFAs, was experimentally studied.And the optimal H2injection and best injection method were also investigated andevaluated on the basis of sequencing AD system (SAD). And the mechanism of echancingCH4by exogenous H2was studied by experiments. The experimental results revealed thatthe optimal initial injection amount of H2was0.33atm which increased the ratio of CH4inbiogas to71%. The study investigating the mechanism suggested that the injection ofexogenous H2not only reduced the endogenous CO2but also promoted the degradation rateof VSS which increased10%. In addition, the present study also evaluated the enhancementeffect of different injection method of exogenous H2by adjusting the injection time point tothe middle of reaction. The results showed that this injection method also enchanced theCH4production but has insignificant improvement to VSS degradation. |
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