The Effects And Mechanism Of Free Ammonia Pre-treatment On Waste Activated Sludge During Anaerobic Digestion

The global energy crisis caused by the depletion of traditional energy sources and the environmental pollution are major problems now.In recent years,anaerobic digestion technology has been widely u tilized to treat municipal sludge.However,the long start-up and processing time of anaerobic digestion is a serious issue that limits this technology.Previously it has been reported that FA?free ammonia?pretreatment could improve methane production,however,they did not explain the reason and details about this technology was not described.In view of the present situation and shortcomings of the research,the following research work has been carried out.First,the effect of pretreatment of different concentrations of FA under different pH conditions?8.5,9.5,and 10?on anaerobic digestion and methane production from sludge was explored.Different pretreatments were inferred by measuring various indicators of pretreated sludge supernatants,and the effects of the method on the properties of the sludge were explored.Then,qualitative and quantitative analysis of the causes of different methane yields was performed by three-dimensional fluorescence spectro scopy.Finally,the anaerobic digestion products were used as substrates to conduct water distribution experiments,and mathematical models were used to analyze the effects of different concentrations o f FA on anaerobic digestion.After different pretreatments of the sludge,it was found that the FA concentration was greater at the same pH,and the soluble chemical oxygen demand?SCOD?,soluble protein and the higher the concentration of polysaccharides.It showed that the greater the concentration of pretr eatment FA,the stronger the destructive effect on the sludge floc.Experimental results showed that at pH 8.5 level,with an increase of FA level from 18.5 to 92.5 mg/L?i.e.,NH₄⁺-N:100-500 mg/L;pH 8.5?the maximum methane yield varied between 194.0±3.9 and 196.9±7.7 mL/g of VSS.At pH 9.5 or 10 level,however,with an increase of initial FA level from 103.2 to 516.2 mg/L,the maximal methane yield increased linearly.The mecha nism studies revealed that FA pretreatment at high levels?e.g.,>100 mg/L?not only accelerated the disintegration of WAS but also enhanced the biodegradability of WAS,which provided more substrates,especially the biodegradable substrates,available fo r the subsequent methane production.Although pH in the digesters was adjusted to 7.0±0.1,the high levels of NH₄⁺-N added or released led to substantial levels of residual FA ranging from 4.4 to 11.6 mg/L.It was found that this level of FA inhibited h omoacetogenesis and methanogenesis significantly,though hydrolysis,acidogenesis,and acetogenesis processes were una ffected largely by this range of FA.Although the pH of the sludge in each reactor was adjusted to 7.0±0.1 before the anaerobic digestion stage,the ammonium nitrogen released from the initial addition and release during the anaerobic digestion stage can cause some FA remaining in the reactor.In this test,concentrations ranged from 4.4 mg/L to 11.6mg/L.A series of batch test s were conducted to simulate various stages of anaerobic digestion.It was found that FA in this concentration range can inhibit homoacetogenesis and methanogenesis significantly,but the hydrolysis,acidification and acetification processes are not affected by FA in this range o f concentrations.Finally,through the actual economic analysis of the actual WWTP?wastewater treatment plant?,it can be seen that the FA pretreatment can save more costs compared with the alkaline pretreatment method,indicating that the FA pretreatment technology has a good future in practice.