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Biogas slurry can be used for fish farming
At present, the implementation of biogas projects across different regions plays a vital role in building a new socialist countryside and promoting a resource-efficient and environmentally friendly society. The production of biogas using agricultural waste such as crop straw and livestock manure, and its use for lighting and cooking, not only helps in recycling resources but also contributes to energy conservation. However, the by-products generated from biogas production, such as biogas residue and slurry, have become a new challenge that needs to be addressed. In fact, biogas slurry is rich in amino acids and trace elements, making it an excellent organic fertilizer for aquaculture. Below is a brief introduction of this technique for reference.
Biogas residue can be directly used as fish feed or mixed with commercial feed to enhance fish nutrition. Biogas slurry does not require solid-liquid separation before application and can be directly poured into fish ponds to promote plankton growth. Both residue and slurry can be used alternately or simultaneously. However, biogas slurry has a certain level of reducing properties, so it should be left for more than three hours after removal from the digester before being applied. It's important to note that unfermented slurry can deplete oxygen in the pond water, especially during hot summer months, so it's better to use it sparingly or avoid it altogether.
When raising fish with biogas residue and slurry, there are no strict requirements for the pond itself, but it's preferable to maintain a water depth of over 1 meter and ensure the water is clean and free of debris. A polyculture system that combines filter-feeding fish, omnivorous fish, and herbivorous species is recommended. Typically, about 30% of the stock can be filter feeders like silver carp, 40–50% omnivores such as common carp, and 20–30% herbivores like grass carp. This approach helps optimize nutrient utilization. Fish of various sizes can be introduced, and regular harvesting and rotation practices can improve overall productivity.
In terms of management, the application of biogas residue and slurry should be adjusted based on the specific conditions of the pond. Normally, no more than 300 kg of surface material per mu should be applied at one time, with biogas residue not exceeding 150 kg per application. It’s advisable to apply it no more than three times a week. Applications should be done on sunny days, ideally every other day using the "Quanchiposa" method. Biogas residue can also be distributed at fixed points throughout the pond. The exact timing 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 least 25 cm. If the transparency is lower, it’s best to avoid applying the residue and slurry. During hot summers, care must be taken to monitor for signs of fish floating heads, and applications should be avoided during thunderstorms. Overuse of biogas slurry and residue should be avoided, following the principle of small, frequent applications. If oxygen depletion occurs, immediate aeration or water replacement is necessary. If diseases are observed, the cause should be identified promptly and appropriate treatments applied.