Turning food waste into food: Measuring carbon, nitrogen, and phosphorus efficiency in coupled vermicomposting-aquaponics systems

Background/Question/Methods

In his classic paper “The Strategy of Ecosystem Development”, Eugene Odum dedicated an entire section to “Prospects for a detritus agriculture.”  Forty-four years later, more ecologists are recognizing that our current reliance upon high-input agriculture is not sustainable.  We explored the potential to turn food waste (coffee grounds) into new food (tilapia and basil) through coupling vermicomposting and aquaponics systems.  Across six independent systems, we fed tilapia various ratios of commercial fish food and dried worms, and we measured the biomass production and nutrient sequestration in fish and basil, as well as water chemistry throughout the eight-week experiment.   We also measured the effects of various rates of worm harvests on the average size of worms, worm bin respiration rates, and carbon:nitrogen (C:N) ratios of the final vermicompost.  

Results/Conclusions

Fish biomass production efficiency was maximized using a diet of 100% commercial fish food, converting 12% of food C into new biomass, in contrast to a 4.4% efficiency with a diet of 50% dried worms.  However, the amount of fish production (as C or N) produced per unit of supplemental commercial food C or N was optimized at a diet of 40% worms.  Basil biomass production sequestered between 18-55% of total N inputs.  Mean biomass of individual worms did not vary with harvest rate, although worm population at our highest harvest rate (150 worms per week) appeared to be on the verge of crashing.  Harvesting any worms led to decreased worm-bin respiration rates and higher vermicompost C:N.  Our results highlight the trade-offs between compost quality and worm production, but they suggest that coupling vermicomposting and aquaponics has potential to redirect significant amounts of nutrients from food waste into new food.