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Biogas slurry can be used for fish farming
At present, the implementation of biogas projects across different regions has become a crucial strategy for building a new socialist countryside and promoting a resource-efficient and environmentally friendly society. The production of biogas from organic materials such as crop straw and livestock manure not only helps in waste management but also provides a renewable energy source for lighting and cooking. However, the by-products generated during biogas production—such as biogas residue and slurry—pose new challenges that require effective solutions. Fortunately, biogas slurry is rich in amino acids and trace elements, making it an excellent natural fertilizer for aquaculture. This technique is briefly explained here for reference.
Biogas residue can be directly used as fish feed or mixed with commercial feed to enhance fish nutrition. Unlike solid-liquid separation, biogas slurry can be applied directly into fish ponds to promote plankton growth, which serves as a natural food source for fish. Both biogas residue and slurry can be used alternately or simultaneously. It’s important to note that biogas slurry should not be applied immediately after removal from the digester; it should sit for at least three hours to reduce its reducing properties. Unfermented slurry can deplete oxygen levels in the pond, especially during hot summer months, so it's advisable to use it sparingly or avoid it altogether in such conditions.
When raising fish using biogas residue and slurry, there are no strict requirements for pond design, but the water depth should ideally be over 1 meter, and the water should be clean and free from debris. A polyculture system that combines filter-feeding, omnivorous, and herbivorous fish species is recommended. Typically, around 30% filter-feeding fish, 40–50% omnivorous fish, and 20–30% herbivorous fish can be stocked. Species like silver carp, grass carp, eel, and mackerel are commonly used. The amount of biogas residue and slurry applied depends on availability, and in high-yield ponds, commercial feed can also be added to boost productivity. Introducing fish of different sizes and practicing regular harvesting and rotation can improve overall efficiency.
In terms of management, the application of biogas residue and slurry should be adjusted based on pond conditions. Normally, surface water per mu should not exceed 300 kg per application, with biogas residue limited to 150 kg per session. No more than three applications per week are recommended. Applications should be done on sunny days, ideally every other day using the "Quanchiposa" method. Biogas residue can also be distributed at fixed points. The frequency and amount should be adjusted according to the season and water quality. During April to May and October to November, the water transparency should be maintained at a minimum of 25 cm. If the transparency drops below this level, application should be suspended. In summer, monitoring for signs of fish floating head is essential, and applications should be avoided during hot weather or thunderstorms. Overuse of biogas slurry and residue should be avoided, following the principle of small and frequent applications. In case of oxygen deficiency or fish disease, immediate action such as aeration or adding fresh water is necessary, and proper diagnosis and treatment should be carried out if any health issues arise.