Recirculating nutrients in aquaponics – worm filtration for treating sludge

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By Milthon Lujan

As production of farmed animals continues to grow, treating their effluents correctly becomes increasingly important. In many countries the nitrate load of soil and groundwater is already at a critical level or exceeds the official guideline values. The random distribution of manure as fertilizer accounts for large parts of this load.

Aquaponic producers try to counteract this overload by using fish-based excretions as a nutrient source for plants. Hence, the nutrients are recycled and stay in the system. However, only the dissolved nutrients are currently taken up by the plants, resulting in a lack of some nutrients. The solids usually are disposed within the municipal sewage system or being used as agricultural fertilizer, even though they are rich in nutrients. Nutrients in effluent sludge are often carbon-bound and have to be processed in order to make the nutrients available for crop production. A very promising on-site low-energy treatment option for aquaculture sludge is using worms to digest the effluents from fish production, also known as vermifiltration.

To analyze the potential of this treatment, scientists from Rostock University have built a pilot-scale setup with worms, that are fed sludge from African catfish (Clarias gariepinus) production. They want to investigate whether worm digestion supports the mineralization of organic bound nutrients. Previous studies have shown that “worm filtration” effectively reduces biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solid (TSS) loads in sewage effluents. Thus, the application of worms can be seen as an additional filtration device, which transforms aquaculture sludge into high quality biological fertilizer – a valuable by-product.

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In addition, worm filtration can be used to remove contaminants from organic effluents due to bioaccumulation. Last but not least, the worm biomass, which contains (up to 65% of) high-quality lysine-rich protein, may act as a suitable source for substituting fishmeal in future fish diets.

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