3.4　Feedstock for Biogas Fermentation

Biogas microbes need to consume the nutrients continuously from the environment during the fermentation in order to keep growing and generating biogas. Therefore various kinds of nutrient should be provided. Sufficient feedstock is the material base for biogas production, and feedstock is the most important prerequisite for digestion process. So it is necessary to know the properties of raw materials for digestion.

3.4.1　Properties and Classification of Feedstock

Almost all the organic matter can be used as the feedstock. The main feedstock in Chinese rural areas is made of two parts, namely crop waste (straw and stalks) and human and animal wastes. According to the difference of carbon/nitrogen ratio (C/N) of feedstock, they can be roughly divided into two kinds, i.e. nitrogen-rich raw material and carbon-rich raw material.

3.4.1.1　Nitrogen-rich Raw Material

It mainly indicates human and animal wastes. They are characterized by fine particles, some low molecular compounds, higher nitrogen content, lower C/N ratio of less than 25/1 and higher speed of biodegradation and gas generation during the fermentation.

3.4.1.2　Carbon-rich Raw Material

Agricultural wastes such as various straw and stems belong to this kind of raw materials, which are generally composed of cellulose, semi-cellulose, lignin, wax and other complex compound in structure. They contain higher carbon constituent, with C/N ratio above 30/1 and slower gas generation in the digestion process.

3.4.2　Properties of Common Feedstock in Gas Production

There are many kinds of feedstock for biogas fermentation, and each has its own property in gas production (including gas production rate of feedstock and gas production speed etc.) According to the property of feedstock in gas production, it is possible to estimate the gas yield of the raw material and the feasibility of the fermentation technology.

However, the gas production rate of feedstock is influenced by the quality of the feedstock and the conditions for the fermentation. For the same kind of raw material in different areas will result in different gas production rate of feedstock. And fermentation temperature will also give great influence to the gas production rate for the same feedstock (See Table 3.5 and Table 3.6). To learn the properties of common feedstock in China, we adopted a test method to determine the potentiality of feedstock in gas production. The testing conditions are as follows:

Fermentation temperature: 35℃

Duration of fermentation: 60 d (excreta)

90 d (straw and stalks)

Table 3.5 shows the rate of gas generation with different materials in different regions.

3.4.2.1　Rate of Gas Production of Feedstock

Different materials have different gas production rate. Generally, the rate of carbon-rich raw materials is higher than that of nitrogen-rich raw materials, i. e. the gas production rate of various crop wastes is higher than that of excreta as shown in Table 3.5 and Table 3.6. It can be seen from Table 3.5 that corn stalks stand in the highest position up to 0.55m3gas/kg TS, while cow dung retains in the lowest only 0.11m3gas/kg TS.

3.4.2.2　Speed of Gas Production

Because of the difference of various feedstock in composition, the more the constituent in the feedstock which can easily be degraded and utilized by microbes to form methane, the faster the speed of gas production and vice versa. The gas production rate of nitrogen-rich raw material is faster than that of carbon-rich raw material as shown in Table 3.7. In the first 10d of fermentation, gas yield of nitrogen-rich raw materials (excreta) accounts for 34.4％～46％ of the total gas yield;while that of carbon-rich raw materials (straw and stalks) only 8.8％. Within 30 d, gas yield of excreta accounts for 86.2％～94.1％; whereas that of rice straw only 53.7％.

According to the different rate and speed of gas production with different raw materials, it is necessary to use the feedstock in proper proportion of high-rate material to low-rate material and high-speed to low-speed materials. Therefore, it can be assure both higher gas yield and constant gas generation during the fermentation.

3.4.3　Proportion of Feedstock

When making the proportion of raw materials, the following points should be put into consideration.

3.4.3.1　Ratio of Carbon and Nitrogen (C/N)

The growth and catabolism of microbes need various kinds of nutrients, especially elements of carbon, nitrogen and phosphorus are necessary. Hence, besides the property of gas production of feedstock, it is also necessary to take the proper ratio of carbon and nitrogen (C/N) into account. Carbon is utilized for energy and nitrogen for the building of the cell structure. A specific group of microbes always consume these two elements in proportion. So far there are still various views existing on the proper C/N ratio of feedstock in rural areas. Some people hold that the ratio may not be so strict in biogas fermentation, but it is commonly recognized that a C/N ratio of 20/1～30/1 is acceptable.

Various kinds of raw materials vary in the content of carbon and nitrogen as shown in Table 3.8. In general, crop stalk and straw contain much carbon with higher C/N ratio, in which the C/N ratio of dry wheat straw can reach 87/1, while excreta contains less carbon with lower C/N ratio. Thereby, it is necessary to add nitrogen-rich raw material to high C/N raw material in order to lower the C/N ratio. Adding human and animal waste is usually done when straw and stalk are used as feedstock. Supplement of ammonium hydrogen carbonate or other nitrogen fertilizer is needed if necessary.

It is useful to prepare feedstock at a certain proportion of carbon and nitrogen in biogas fermentation. In fact, the raw materials for biogas fermentation are very complex, and carbon and nitrogen in organic materials exist in different ways. Both lignin and glucose are carbon-compounds. Glucose is easy to be degraded by microbes to produce methane whereas lignin is very difficult. Different nitrogencompounds have different capabilities in degradation and utilization by microbes. Thus, for C/N ratio of feedstock it is necessary to draw a line of demarcation between those raw materials in which carbon or nitrogen is easy to be utilized and those in which carbon or nitrogen is difficult to be utilized.

Based on the content of carbons and nitrogen in the feedstock the C/N ratio can be calculated using the following formula,

where　C——percentage of carbon in the raw material;

N——percentage of nitrogen in the raw material;

X——weight of the raw material;

K——C/N ratio of mixed raw material.

Human excreta:　　C1=2.5％,N1=0.85％,X1=100;

Pig dung:　　C2=7.8％,N2=0.6％,X2=100;

Rice straw:　　C3=4.2％,N3=0.63％,X3is unknown;

C/N ratio K = 25:1

Then put the data stated above into the formula, X3=98.9 (kg)

3.4.3.2　Manure/Straw Ratio

Human and animal excreta and crop waste are the chief fermentative raw materials in Chinese rural areas (Table 3.9), and also the two kinds of distinctive feedstock in property of gas production. As C/N ratio is not required strictly in biogas fermentation and it's difficult to prepare feedstock according to a certain C/N ratio in the countryside, the proportion of manure and straw and stalks in the total solid should be clear when TS concentration is taken into account. The reason is that when the raw materials with the same TS concentration differ in proportion of excreta and crop waste, the fermentation effects will be different undoubtedly. Tests have shown that manure is an important factor which will influence gas production when feedstock is prepared for start-up of the digester with semi-continuous or batch fermentation process. Generally, the proper ratio should be 2:1, at least over 1:1. If possible, the more the excreta are, the faster the start up of the digester will be.