Study On The Ammonia Inhibition In Mesophilic And Thermophilic Anaerobic Digestion Of Animal Carcass Wastewater

Animal carcass wastewater contained high levels of organic matter and protein,which produced in the animal carcass cooking process at high temperature and pressure.The temperature of this wastewater could reach 60~70 ℃.For this kind of wastewater,thermophilic anaerobic digestion should be a considerable technology in selection process,because it had a higher organic load rate and biogas yield than mesophilic anaerobic digestion.However,thermophilic anaerobic digestion with high concentration of protein wastewater was highly susceptible to ammonia inhibition,resulting in low COD removal rate,organic acid accumulation and low methane yield.Study on the characteristics of thermophilic anaerobic digestion of animal carcass wastewater and the mechanism of ammonia inhibition were of great significance to the resource treatment of animal carcass wastewater.In this paper,ABR was used to treat the animal carcass wastewater.The start-up and operation characteristics of the ABR from the medium temperature to the high temperature were studied.The effect of ammonia existence form and concentration on mesophilic and thermophilic anaerobic digestion were also investigated by batch experiments.Finally,the difference in efficiency between mesophilic and thermophilic anaerobic digestion was analyzed.The results were as follows:When the temperature of ABR increased from 35 ℃ to 55 ℃ gradually,the COD removal rate decreased by 24%,the methane yields decreased by 33%,the effluent concentration of VFAs increased by 1.9 times.The ratio of VFAs to alkalinity was below 0.38,ABR did not appear rancidity.Free ammonia(FAN)concentration increased from 98 mg/L to 340 mg/L,thermophilic anaerobic digestion of animal carcass wastewater containing high concentration of protein was significantly inhibited by FAN.Thermophilic anaerobic ammonia inhibition batch experiments showed that thermophilic acidogenesis and methanogenesis were all inhibited by FAN,methanogenesis became the rate limiting step of thermophilic anaerobic digestion.The inhibition degree increased with pH and TAN concentration increased.The IC50 of FAN for acidogenesis and methanogenesis were 843 and 453 mg/L at pH value of 8.2.Methanogenesis was more sensitive to FAN than acidogenesis which lead to the accumulation of VFAs in the thernophilic reactor.The accumulation type of VFAs was acetic acid,which indicated that aceticlastic methanogens was inhibited by FAN.Mesophilic anaerobic ammonia inhibition batch experiments showed that the inhibition degree of mesophilic acidogenesis and methanogenesis were less than thermophilic at the same value of pH and TAN concentration.The increase of FAN concentration under high temperature was the main reason for this phenomenon.The accumulation of VFAs in mesophilic anaerobic batch reactor was still predominantly acetic acid,but propionic acid and butyric acid were also accumulated in the process of increasing TAN concentration.Thermophilic methanogens had stronger tolerance to FAN than mesophilic methanogens.The IC50 of FAN for mesophilic methanogens was only 324 mg/L at pH 8.2.When the temperature of ABR increased from 35 ℃ to 55 ℃,the pH of the second and third compartment of ABR were increased from 7.56 and 7.63 to 7.76 and 8.18 respectively because of the proteinrich characteristics of animal carcass wastewater and the flow characteristics of ABR.This led the FAN concentration of the second and third compartment increased from 83 and 98mg/L to 165 and 340mg/L.These two levels of FAN concentration were obviously higher than the tolerance threshold of thermophilic bacteria to FAN,causing severe inhibition of anaerobic microorganisms in the second and third Chambers of ABR and accumulation of VFAs.The synergistic effect of pH,FAN and VFAs led to an ‘inhibited steady state’ of the thermophilic reactor,which was the main reason that thermophilic anaerobic digestion efficiency was much lower than mesophilic anaerobic digestion.