Free Ammonia Aids Ultrasound Pretreatment To Enhance Short-chain Fatty Acids Production From Sludge Anaerobic Fermentation

The economy of contemporary China is developing faster and faster,people's living standards have been significantly improved,and the discharge of domestic wastewater is also increasing.Therefore,there are new requirements for wastewater treatment plants(WWTPs),the amount of wastewater to be treated must be increased,and the degree of treatment must be strengthened.The widespread use of activated sludge process in China will generate much waste activated excess sludge,which will increase the treatment burden of wastewater treatment plants.The reduction,recycling,and harmlessness of sludge have attracted the attention of many researchers.This subject uses free ammonia(FA)to aid ultrasound pretreatment to improve the production of short-chain fatty acids(SCFA)by sludge anaerobic fermentation.On the one hand,the sludge is scientifically and effectively treated,which relieves the pressure of the wastewater treatment plants,on the other hand,the sludge is also utilized resourcefully.This topic studies the following aspects.First,the effect of free ammonia aids ultrasound pretreatment on the production of short-chain fatty acids by sludge anaerobic fermentation was explored.Taking the maximum SCFA production as the standard,under certain ultrasound pretreatment conditions,the optimal initial free ammonia concentration is determined.Ultrasound pretreatment technology has the advantages of no pollution,high energy density and fast decomposition speed,which can accelerate sludge decomposition and promote the production of short-chain fatty acids by anaerobic fermentation.However,due to the high activity of short-chain fatty acid consumers,the accumulation of short-chain fatty acids is limited.Therefore,the application of ultrasound pretreatment in SCFA production of sludge anaerobic fermentation is limited.Therefore,this study proposes an effective strategy to use free ammonia to aid ultrasound pretreatment in order to increase the SCFA production of sludge anaerobic fermentation.The experimental results showed that the best pretreatment condition is:pretreatment with ultrasound density of 2 w/m L for 15 min,and then pretreatment with initial free ammonia at a concentration of 60 mg/L for 2days.At this time,the accumulation of short-chain fatty acids reached a maximum of 316.7±14.9 mg COD/g VSS,which was 2.2 times and 1.7times that of SCFA production by ultrasound and free ammonia pretreatment respectively.Analysis of the proportion of SCFA found that acetic and propionic are the most abundant,and these two acids are also the preferred carbon sources for wastewater treatment.Second,the mechanism is studied.By investigating the effect of free ammonia aids ultrasound pretreatment on the soluble chemical oxygen demand,VSS reduction,protein and polysaccharides in the sludge fermentation liquid.It was found that free ammonia can aid ultrasound pretreatment to accelerate the solubilization of sludge,so that sludge releases more organic matters compared with the ultrasound and free ammonia pretreatment alone.Excitation emission matrix(EEM)fluorescence spectrometry analysis showed that free ammonian aids ultrasound pretreatment can release more biodegradable organic matters than ultrasound and free ammonia pretreatment alone;The water distribution experiment and the measurement of enzyme activity showed that free ammonia aids ultrasound pretreatment has varying degrees of inhibition on hydrolyzing microorganisms,acidifying microorganisms,and methanogens,but it has a stronger inhibitory effect on methanogens.Thereby effectively inhibiting the consumption of SCFA and increasing the accumulation of SCFA.That is to say,this method can inhibit the processes of hydrolysis,acidogenesis and methanogenesis processes,but the inhibition of methanogenesis is higher than that of hydrolysis and acidogenesis.Finally,the ultrasound density and time are optimized,and then the method is applied to the wastewater treatment plants for economic analysis.Through the comparative analysis of SCFA accumulation,it is determined that the ultrasound density can be optimized to 1.5 w/mL to save energy consumption.The research results of this paper promote the application of ultrasound pretreatment technology in the real WWTPs environment.Compared with the traditional treatment mode of single ultrasound pretreatment sludge,the treatment mode of free ammonia aids ultrasound pretreatment to enhance short-chain fatty acids production from sludge anaerobic fermentation in a wastewater treatment plant with a daily treatment capacity of 10~5m3 is economical.Since free ammonia is a non-ionic form of ammonia,it can be obtained in situ,so using free ammonia assisted ultrasound pretreatment of waste activated sludge to produce short-chain fatty acids has a good application prospect.