CONVERSION OF SOLID WASTE TO FUEL-GRADE BIOGAS FROM A LAND-BASED AQUACULTURE SYSTEM FOR ATLANTIC SALMON
As land-based production of Atlantic salmon rapidly expands, efficient life-support systems must be developed and optimized to increase water recirculation and prevent environmental pollution. Specifically , land-based salmon operations will generate 20-30 tons of sludge for every 1000 tons of fish produced. Treating these amounts of sludge in a way that is environmentally responsible is of paramount importance. Our approach has been to use anaerobic digestion bioreactors, in which methanogens convert the solid waste into fuel-grade methane. We have successfully developed this approach to convert over 90% of solid waste produced by warm-water marine fish (seabream, seabass) into combustible biogas that contains 65-70% methane. Building upon our experience, we are now studying and implementing the technology in cold water, both fresh and saline, focusing on Atlantic salmon. Sludge obtained from a large commercial Atlantic salmon smolt operation in Norway was shipped to our lab and was used to develop and enrich a methanogenic consortium that has been custom-tailored to this sludge. Experimenting with different temperatures, d ry matter concentrations and C/N ratios, conditions for optimal methane production were determined. The consortium was then scaled up in a 250 liter fermenter and shipped to Norway where it was use to inoculate a 100 m3 biogas bioreactor, with excellent results. The biogas generated is currently used on-site to operate methane-driven boilers to heat farm water, thus offsetting some of the energy cost of the operation.
Currently, we are developing and testing efficiencies of sludge-to-biogas conversion for post-smolt Atlantic salmon produced at 20 PPT salinity and 13o C. Fish were stocked in two 6m3 tanks equipped with life- support systems allowing water recirculation. Sludge collected post- drum filter was used as the initial inoculum source to develop, enrich and optimize the methanogenic consortium converting salmon waste to biogas. The optimized consortium was scaled-up in a 20 liter fermenter and inoculated into the 1000 liter anaerobic digester. As fish grew toward market size at densities of up to 65 kg/m3, we collected increasingly larger quantities of biogas, up to 380 liters /day, with 70% methane content, which is directly combustible ( right; photo of burning biogas collected from the anaerobic digester ).
These studies demonstrated that Atlantic salmon solid waste produced in RAS, whether fresh-water or saline, can be efficiently converted to fuel-grade biogas to be used on- site to reduce energy costs. Complete operational parameters and results will be discussed.
