Sludge-to-gas: aquaculture fish sludge shows varying hydrogen production potential over the lifetime of a fish.

Aims: The fish industry is growing, with millions of tons of fish cultured in off-land and on-land aquaculture systems. A major challenge is the continuous production of sludge containing fish secretions and uneaten fish feed. Anaerobic digestion (AD) can efficiently use this sludge, involving biological degradation of organic matter by microbes under anoxic conditions, forming valuable biogases like methane. Recently, there is an increasing need for green hydrogen (H2), but the potential of using fish sludge for H2 production has not been fully explored. A key complicating factor is the changing sludge composition during a fish's lifetime.

Methods and results: Salmon fish sludge was sampled from a land-based recirculating aquaculture system (RAS) over 6 months, covering fresh, brackish, and seawater stages. Sludge was used for AD batch incubation to investigate H2 and liquid compound productivity. Chemical- and DNA-based microbial community analysis of the sludge showed significant changes over time. This was also reflected in changing ad productivity. Highest H2 production (24.5 ± 17.5 mL H2 gas per 1-g dry sludge) was within the freshwater phase, followed by brackish. Concurrently in all enrichments we observed high production of CO2 (75.3 ± 30.1 mL CO2 per 1 g of dry sludge) and volatile fatty acids, especially acetic acid, propionic acid, and lactic acid.

Conclusions: The microbial community in fish sludge can naturally produce H2 and other valuable products, with productivity varying over the fish's lifetime. The key governing factor for H2 production is the type of fish sludge and availability of electron acceptors.