Effect Of Free Ammonia Pretreatment On Hydrogen Production From Sludge By Dark Fermentation And Its Mechanisms

Due to the energy crisis and environmental pollution,To develop the green new energy and efficient control of environmental pollution have become the focus of worldwide attention.Hydrogen energy,because of its high calorific value,no pollution,stable combustion,and renewable,has been attracting people's attention.Hydrogen is recognized as one of the few high-quality alternative energy sources that can replace primary fossil energy.Ammonium and/or free ammonia?FA,a non-ionized form of ammonium?is generally considered to inhibit the activity of anaerobic digestion microorganisms in the activated sludge?WAS?.However,the research in this paper shows that the presence of free ammonia greatly improves the hydrogen production efficiency of alkaline pretreatment?pH=9.5?sludge dark fermentation.The specific experimental process is divided into the following steps:Firstly,in the presence of different concentrations of ammonium?NH₄⁺-N?,the batch experiments of hydrogen production by dark fermentation of alkaline pretreatment activated sludge showed that the existence of NH₄⁺-N or free ammonia greatly improved the hydrogen production efficiency of dark fermentation.With the increase of initial NH₄⁺-N level from 36to 266 mg/L,the maximal hydrogen production from alkaline?pH 9.5?pretreated-sludge increased from 7.3 to 15.6 mL per gram volatile suspended solids?VSS?.Further increase of NH₄⁺-N to 308 mg/L caused a slight decrease of hydrogen yield?15.0 mL/g VSS?.Then,in the presence of the same concentration of NH₄⁺-N as above,by acid pretreatment sludge dark fermentation batch experiments,we determined that FA,not NH₄⁺-N,is the real factor to improve the efficiency of hydrogen production.Secondly,by exploring the effect of FA on sludge dissolution,It was found that the presence of free ammonia facilitated the releases of both extracellular and intracellular constituents,which thereby provided more substrates for subsequent hydrogen production.Thirdly,by simulating the batching experiments of various stages of fermentation,this study explored the impacts of FA as a pretreatment method on the dark fermentation hydrolysis,acidification,acetogenesis,homoacetogenesis,methanogenesis and sulfate-reducing process.The results demonstrate that free ammonia is indeed a biological inhibitor,free ammonia at the tested levels?i.e.,0⁴44 mg/L?inhibited all other bio-processes except for the acetogenesis.Nonetheless,its inhibition to the hydrogen consumption process?ie,homoacetogenic,methanogenic,and sulfate reduction processes?is much more severe than that to hydrogen production processes?such as hydrolysis and acidogenesis processes?.Finally,the enzymatic activity in a long-term semi-continuous reactor was further studied.The results show that free ammonia posed slight impacts on protease,butyrate kinase,acetate kinase,CoA-transferase,and[FeFe]hydrogenase activities but largely suppressed the activities of coenzyme F420,carbon monoxide dehydrogenase,and adenylyl sulfate reductase,which were consistent with the chemical analyses performed above.It is proved that FA can improve the hydrogen production by sludge dark fermentation,and the mechanism of FA improving the hydrogen production by sludge dark fermentation is clarified for the first time.This study provides a theoretical basis for engineers and technicians to use sludge dark fermentation technology to produce hydrogen,and provides a reference and technical support for formulating more economical strategies for the treatment and disposal of excess activated sludge in sewage treatment plants in the future.