3.2　Temperature

Temperature is closely related to biogas digestion. From a certain point of view, temperature plays a key role in gas production. Within a certain range, the higher the temperature, the higher the gas yields (Table 3.1).

Table 3.1　Gas yield of rural small digesters at different temperatures

Biogas bacteria have their own temperature ranges like other microbes, so the fermentation temperature varies too. Generally, 45～60 ℃ is taken in thermophilic fermentation; 30～45℃ is viewed in mesophilic fermentation. Rural small digesters usually work at ambient temperature and the fermentation temperature is much influenced by atmospheric (or air) temperature and seasons, so we call it fermentation at ambient temperature. According to the determination by Zhejiang University in Hangzhou, the temperature of the digester slurry changes greatly under the influence of ambient temperature (Table 3.2)

Table 3.2　Change of digester temperature (Hangzhou, Zhejiang)

Note: Digester 3 with plastic cover.

In January, as the air temperature was the lowest (−1.3～6.3 ℃), and the temperature of the digestion slurry was also the lowest (only 11～12 ℃). Whereas in July which got the highest temperature in the year (average 27.4～33.2 ℃), the slurry was at 21～24 ℃. Because of the difference of the temperature between January and July, the gas yields were also quite different. Generally, the gas yield of rural small digesters is the lowest in Jan. (0.08～0.1 m3/m3. digester); while in July the gas yield is the highest [0.2～0.3m3/(m3·digester)] due to the high digestion temperature. China has vast area and the temperature varies in different places, thus the gas yield differs obviously. The data stated above can only represents one region.

Moreover, there is a close relationship among the air temperature, earth temperature and digester temperature (Table 3.3). The digester (or fermentation slurry) temperature is affected directly by earth temperature, which alters with air temperature. The deeper the digester under the earth is, the less the slurry temperature changes and the difference between air temperature and digester temperature becomes greater. On the contrary, the nearer the digester to the ground is, the greater the slurry temperature changes and closer to the air temperature. So in summer the nearer the digester to the ground, the higher the slurry temperature will be. Whereas in winter, the nearer the digester to the ground, the lower the slurry temperature will be. If a digester is 1m under the ground, the earth temperature changes slowly with little influence by daily air temperature. When a digester is built 1.9m under the ground, the earth temperature is almost equal to the digester temperature.

Table 3.3　Relationships among air temperature, earth temperature and digester temperature (Hangzhou, Zhejiang)

The relationship between gas production and temperature is only a phenomenon in appearance. The essence is the digestion rate of feedstock. The higher the temperature, the faster the decomposition. It would take 50 days to digest cattle dung completely at an average temperature of 75°F (23.9 ℃), 70 days for crop waste and 50～60 days for cattle dung-crop waste mixture. If the temperature is controlled to 90～100°F (32.2～37.8 ℃), the fermentation period of cattle dung will not exceed 28 days and less than 45 days for crop waste.

Tests have shown that within 15～35 ℃, the total gas yield of feedstock per ton is almost equal in a fermentation period. The fermentation period is 12 months at 15℃, whereas one month at 35℃, i. e. the total gas yield of one month under 35℃is equal to that of 12 months under 15℃.

Table 3.4 shows the relationship between manure fermentation period and the temperature. The lower the temperature, the longer the fermentation period and vice versa.

Table 3.4　Relationship between manure fermentation period and temperature

A sudden change of fermentation temperature can affect gas production obviously. In general, the change of ±5 ℃ will make gas yield drop sharply and even stop. But when the temperature recovers, the fermentation can continue. In case the digester is charged fully, the effects of temperature drop to the activity of methanogenic bacteria will be greater to the acid-forming bacteria, which will lead to a severe unbalance between acid-forming bacteria and methane-forming bacteria and cause disorder of the normal fermentation similarly, if we increase the temperature suddenly to 50℃ in a digester which is in normal fermentation at 35℃, the gas production will drop abruptly.