Study On Recycling Digestive Effluent In The System Of Anaerobic Fermentation

Anaerobic fermentation has become the first way of dealing with animal manure with the growing scale of livestock production. It will produce a large number of anaerobic digestive emissions, if the anaerobic digestion can not address outward emissions, it will inevitably cause secondary pollution, and contrary to the original purpose of cleaning up the environment of anaerobic fermentation; if the treatment meet emissions standards, it will result in biogas construction costs and operating costs. Therefore, the treatment and disposal problems of the anaerobic digestion have restricted the development of anaerobic fermentation of wet and lead to the bottlenecks. The most economical and effective method of dealing with anaerobic digestion is to recycle fermentation system. However, recycling will have a certain impact on the volatile acids, ammonia nitrogen content, COD, etc. in the anaerobic fermentation system. Effects may be advantages and disadvantages. The test studied on some problems of recycling digestive effluent.Cow manure is raw material for the two studies in this experiment: in the two-phase anaerobic digestion, the emissions of digestive fluid in the methanogenic phase used to dilute the fresh cow manure and then add to the acid tank after solid-liquid separation. The test would contrast the different amount of recycled digestive effluent, the test period is 25 days; the digestion of the methanogenic phase is not solid-liquid separation and it directed to the acid tank as a certain proportion, 25 days of the test period, changed the reflux ratio of the digestive effluent. The results show that:1. The recycled digestive effluent after solid-liquid separation in the two-phase anaerobic fermentation: pH value had great affect, pH value was affected acidification effect with a high value when the recycled amount of 800 mL. It made a greater impact to the volatile acid content, and it could lead to propionic acid accumulation, the higher amount recycled, the higher propionic acid accumulation is more obvious. The recycled digestive effluent affected the ammonia nitrogen in the digestive system, the higher amount is recycled, the ammonia nitrogen concentration was much higher. In this test, the total COD removal rate in each group were 62.61%, 70.05%, 77.49%, it shows that COD removal increased accordingly with increasing the amount of recycled digestive effluent; the gas production had great influence on increasing the amount of recycled digestive effluent, the total gas production of the three groups were 139. 5 L, 146.82 L, 163.27 L, and the average methane content of the three groups were 67.49%, 69.30%, 66.16%, it shows that the higher amount is recycled, the total gas production is much higher, however, it does not affect the methane content.2. Digestive effluent without solid-liquid separation of the different rate on the two-phase anaerobic digestion: the pH value has influenced to a certain extent, but pH were all at appropriate values. Digestion of different rate impacted on the volatile acid content, but it did not effect on the propionic acid content significantly. There was no significant that it affected the ammonia nitrogen concentration when the digestive effluent rate were 0, 20%, and 25%, the ammonia nitrogen concentration of the group with the rate of 30% was significantly different from other groups, the ammonia nitrogen concentration in the methanogenic phase was affected by different rate of recycled effluent. With the improvement of digestive effluent rate , COD removal rate was much better, four groups of COD removal rate was 56.12%, 72.58%, 75.02%, 77.89%, total gas production were 155.35 L, 156.82 L, 165.48 L, 175.27 L, the average percentage of methane content was 67.52%, 68.94%, 71.3%, 67.3%. The rate of 25% has higher methane content.3. It shows that all above of the two methods of recycled could be reduced the efflux of part of digestion: in the test of recycled digestive effluent after solid-liquid separation, it might be suitable to choose a lower amount of digestive effluent, because the high recycled amount would lead to the accumulation of propionic acid and ammonia nitrogen. In the test of different rate of recycled effluent without solid-liquid separation, 20~30% of the rate were reasonable, the best rate was 25% when it considered the methane content.