As meat consumption grows worldwide, intensive livestock production is predicted to keep increasing, and consequently the amounts of manure. According to the Food and Agricultural Organization of the United Nations (FAO) pig farming is currently the main source of animal protein for human nutrition.
Pig slurry, manure plus liquid waste of farms, is a rich material in nitrogen, phosphate and other plants nutrients, and therefore is often applied to corps as a fertiliser. Nevertheless, as nitrogen can easily leach, this practise endangers the quality of ground waters and in return superficial and marine water bodies. Europe has promoted a strict legislation (EC Nitrate Directive, 1991) designing Nitrate Vulnerable Zones (NVZs) where nitrate pollution risk is high. This points out the necessity of implementing cost-effective solutions for further treatment of pig slurry.
In this sense, duckweed ponds for treating pig slurry arise as an efficient and low-cost option. Duckweed is a small floating macrophyte that has high capacity for removing dissolved nutrients from water (mainly nitrogen in ammonia form and phosphorus compounds) as well as for reducing organic matter and suspended solids. The advantage of this plant group is that the uptake of nitrogen and phosphorus from swine wastewater bioremediates the pig slurry, and at the same time, produces large amounts of biomass. This has two main benefits (i) the biomass generated has high protein content thus having great nutritional value for producing animal feed, and (ii) high levels of starch can be found in duckweed biomass which can be used for production of bioethanol. Taking this into account, novel technological solutions can be implemented for adapting the existing open ponds and converting them into smart automated reactors with monitoring of nutrient matrix condition and automatic harvesting of biomass.
This approach for pig slurry bioremediation points at a system that fits perfectly in a desirable circular economy, where the waste management produces profitable resources.