Ammonia Inhibition And Its Mechanism On Thermophilic Microaerobic Digestion For Sewage Sludge Tretment

Thermophilic Micro-Aerobic Digestion?TMAD?process overcomes the deficiency of traditional sludge digestion process,accelerates the degradation rate of organic matter,and shortens the sludge residence time.However,the higher concentration of ammonia nitrogen in the process of TMAD digestion will adversely affect the process of sludge stabilization.How ammonia nitrogen affects the process of aerobic digestion of sludge,as well as the mode and mechanism of its inhibitory effect,has not been systematically studied and is worth further investigation.In this case,the effect of ammonia nitrogen concentration on sludge stabilization was studied by adding ammonium bicarbonate of different concentrations in the digestive system in batch test.Total ammonia nitrogen?TAN?is considered as an appropriate index to measure ammonia nitrogen inhibition.When the TAN concentration reached 1000 mg L^-1,short-chain fatty acids?SCFAs?were accumulated mainly in the form of acetic acid at a concentration of 13,250 mg L^-1.The ratio of acetic acid to total SCFAs was 75.5%,the removal rate of sludge digester VS was as low as 31.5%,and ammonia nitrogen had an adverse effect on sludge stability.There are three pathways of action are closely related to the inhibition of ammonia nitrogen.Free ammonia is actively diffused into the cell body,resulting in the loss of intracellular K+ and Mg²⁺,thus affecting the normal physiological metabolism of microorganisms.Ionic ammonium can exchange with Mg²⁺ ions on the cell membrane and affect the enzyme activity.Reactive oxygen species?ROS?are the inevitable by-products of micro-aerobic metabolism,and their rapid accumulation leads to excessive oxidation of intracellular substances.With the continuous advance of digestion process,the content of O2-in sludge digester with obvious inhibitory effect was not less than 0.229 mol g^-1 after sludge digestion for 16 days.SOD and CAT enzyme activity began to be higher than that in the early stage of digestion,which partially alleviated the oxidative stress of thermophiles.Although the post-digestion TAN was still higher than 1000 mg L^-1,the inhibitory digestion reactor maintained a good VS reduction effect.Since the PH value is closely related to the production of short-chain fatty acids?SCFAs?and the release of ammonia,this paper investigated the effects of different PH values on the biodegradation characteristics of TMAD process.Alkaline digestion condition?p H 9.0?has better sludge stabilization performance than acid and neutral digestion system.When the digestion time reached 20 days,the removal rate of VS under acidic condition was only 25.4%,while that under alkaline condition was as high as 48.3%.However,the accumulation of SCFAs under this condition is relatively low,which reflects that the mild alkaline p H value can promote the oxidative degradation of substrates.Under alkaline digestion conditions,microbial by-products,humic acids and fulvic acids could be detected at the same time on day 8.But it decreases rapidly in the later period of digestion,especially the humic acid and fulvic acid substances are too low to be detected.Different proton transfer pathways indicate that the acidic digestion environment is prone to potassium deficiency,enzyme activity reduction and oxidative stress.The alkaline environment is conducive to promote substrate oxidation,accelerate the process of humus,prevent the accumulation of acid and ammonia nitrogen and cause inhibition.The effect of activated oxygen scavenger?fulminic acid,tea polyphenols?on sludge digestion system was studied in this paper.When the digestion time reached 16 days,the removal rate of the blank group VS was 32.2%,and the removal rate of the sludge system added fullyric acid VS increased to 40.3%,and the sludge stabilization effect was more outstanding.At the 16 th day,SCFAs in the sludge digestion system with furic acid was reduced to 3000 mg L^-1,and the degradation rate of organic matters was faster than other systems.In addition,organic matter with molecular weight greater than 1000 KDa increased from 14.06% to 26.2%.The formation of macromolecules is evident.Compared with other sludge digestion systems,the fluorescence intensity of humic acid substances in sludge digestion system added fulvic acid was as high as 600.Meanwhile,the oxidase activity was relatively high.It can effectively reduce the adverse effect of reactive oxygen species on microorganisms and promote the stabilization process of sludge.High abundance of functional microorganisms such as Paenalcaligene and Pseudogracilibacillus in alkaline sludge digestion system maintains high oxidase activity and alleviates potential adverse effects of oxidative stress on microorganisms.