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Biogas development and comprehensive utilization technologies are production models that rely on land resources as a foundation, solar energy as a power source, and biogas as a linking element; they integrate energy production, livestock breeding, and crop cultivation, and combine biogas, biology, fertilizers, and feed in an organic way. These technologies allow for the full utilization of farmyards for pig raising, vegetable cultivation, and environmental purification. Biogas can be used for cooking, lighting, and storing grain ； Biogas slurry and residue can be used for foliar spraying on crops to improve soil fertility, produce green food, and meet people’s living needs. I. Technical key points for constructing biogas digesters: Biogas production and comprehensive utilization technologies have gained recognition among farmers today. However, the quality of biogas digester construction is the crucial factor determining whether these technologies can be utilized comprehensively. Technical expertise is required on-site during digester construction to ensure that all technical aspects are met properly. It mainly includes: (1) Selecting the appropriate location for building the digester: ① Building the biogas digester under a sunny pigpen not only saves space but also helps to maintain the temperature of the digester during winter ； ②For farm households with the necessary resources, a \"four-in-one\" integrated approach is adopted, with part of the material handling area located in a solar greenhouse; the carbon dioxide produced there is used to support the growth of vegetables. (2) Selection of digester capacity: For a family of 5, it is appropriate to build a biogas digester with a capacity of 6 m3. (3) Meeting all the conditions of the feces fermentation process: ① A strict anaerobic environment. That is, the pool must be sealed to ensure no air leakage or water seepage ； ②Sufficient fermentation raw materials. Generally, 3 pigs or 2 cows, along with the human excreta from household members, are sufficient to provide the raw materials needed for fermentation in a 6 m3 biogas digester ； ③Appropriate pH level. The optimal pH value for biogas fermentation is 6.8–7.4 ； ④The concentration of the raw material should be 8–10%; it is slightly lower in summer and slightly higher in winter ； ⑤The fermentation temperature of biogas. Biogas fermentation is closely related to temperature; fermentation stops when the temperature drops below 8°C. Below 65°C, the higher the temperature, the higher the gas production rate. Above 65°C, it still has an impact on gas production. (4) Increase the actual residence time of the raw materials in the tank, and properly address the issue of residue sedimentation within the tank. (5) Prevent the material liquid from forming a crust in the tank. (6) Select an appropriate inoculum to facilitate rapid gas production. (7) Strengthen daily management to prevent substances such as pesticides and tobacco leaves, which can affect the normal activity of bacteria and methanogens, from entering the pond, and use them in the correct manner. II. Key points of biogas comprehensive utilization technology: Biogas comprehensive utilization technology refers to the integrated use of biogas, biogas sludge, and biogas liquid – products resulting from the fermentation of human and animal manure. Biogas sludge and biogas liquid contain a large number of microbial metabolites, which include various vitamins, proteins, enzymes, trace elements, etc., and have a positive regulatory effect on the growth of crops. In addition to being used for lighting and cooking, biogas can also be used for storing grain and preserving fruits. Therefore, biogas sludge, biogas liquid, and biogas have wide-ranging applications, mainly including the following uses: (1) Soaking seeds in biogas liquid can increase the seed germination rate by 15–20% or more, result in seeds emerging 1–2 days earlier, lead to stronger and healthier seedlings, and improve their resistance to diseases. Method of operation: Use plastic woven bags with good water permeability to hold 15–20 kilograms of the selected seeds (make sure to leave some space). Seal the bags tightly, then place them in the middle or lower part of the soaking area. Seeds with shells, such as rice, should be soaked for 48–60 hours, while seedless seeds like corn should be soaked for 12–18 hours. After that, take the seeds out, rinse them thoroughly, let them dry, and promote germination before sowing. (2) Foliar application of biogas slurry: It can increase the yield and quality of crops, and it helps to control various pests and diseases such as aphids, red and yellow spiders, sheath blight, and grape powdery mildew; the effects are particularly noticeable in vegetables and fruits. Procedure: Use biogas slurry that has been producing gas normally for over 3 months; filter the slurry and then spray it on the underside of the leaves, avoiding application during the hot hours at noon. (3) Feeding pigs with biogas slurry: Mixing biogas slurry from biogas digesters that produce ample gas with pig feed in a 1:1 ratio can help pigs gain weight more quickly and become more resistant to diseases. This approach allows for a reduction of about 100–150 kilograms of feed per pig, thereby lowering the costs of pig farming and increasing farmers’ incomes. (4) Using biogas slurry and residue for fish farming: It can improve the water environment, enabling fish to grow faster and achieve better quality. Generally, 150–200 kilograms of biogas slurry and biogas residue are applied per mu of water surface, with no more than 100 kilograms applied at each time. (5) Biogas slurry fertilization: It can improve soil fertility and help produce organic food. (6) In addition to lighting and cooking, biogas can also be used for storing grain and preserving fruits. It not only extends the storage period, eliminates pests and diseases, and ensures the quality of grain and fruits, but it can also be used for sericulture and hatching, primarily to provide energy for heating purposes. III. Investment and Benefits A 6 m3 high-efficiency anaerobic digester with gravity-fed feeding and discharging requires an investment of 600–700 yuan for its construction (including cement, accessories, and labor costs); together with the construction of pigsties and toilets, the total investment amounts to 1500–1800 yuan. Biogas digesters have a service life of 20 years, and building one once allows for long-term benefits. The gas production rate per cubic meter of tank volume is generally 0.2–0.3 m3/m3 per day; thus, more than 300 m3 of biogas can be produced in a year, which is equivalent to 100–120 kilograms of liquefied petroleum gas. At the current price of liquefied gas, this can result in savings of over 400 yuan in fuel costs per year. One cubic meter of biogas can be used to cook three meals for a family of 4–5 people, which is equivalent to lighting a 60W bulb for 6 hours. If biogas lamps and stoves are used in greenhouses, it helps to raise the temperature inside the greenhouse as well as the CO2 concentration, thereby promoting vegetable growth and increasing yields. Furthermore, biogas slurry and residue are high-quality fertilizers that are free from bacteria and offer both rapid and gradual nutrient release. A 6 cubic meter biogas digester can produce 15 tons of biogas slurry and 2.5 tons of residue per year, which is equivalent to the fertilizer effect of 480 kilograms of ammonium carbonate; this helps to save 200 yuan in fertilizer costs each year. By establishing integrated facilities that combine biogas digesters, pigsties, toilets, and solar greenhouses, not only is the courtyard environment improved, but fuel costs are also reduced thanks to the use of biogas, and fertilizer expenses are lowered by utilizing biogas fertilizer. Each farmer can raise 15 pigs per year, resulting in an annual net income of 2,000–3,000 yuan. By constructing a plastic greenhouse covering 0.5 mu, the net income from growing vegetables in such a greenhouse can exceed 4,000 yuan per year. In summary, by developing household biogas digesters, each farming family can generate an annual profit of over 7,000 yuan.