Nutrient recovery from anaerobically digested organic wastes

Abstract

Effluents (digestates) resulting from anaerobic digestion of wet organic wastes can be
recycled as fertilizers. Separation of such digestates into a wet and a dry fraction is common.
The solid fraction is often sold as a soil enhancement product while the liquid fraction is
usually discarded as wastewater. A large portion of the digestate nutrients is dissolved as ions
in the liquid. The aim of this study is to establish an efficient way to convert this into a liquid
"organic fertilizer". Enhancement of the nutrient concentration is necessary in order to make
the final product commercially acceptable. Direct evaporative concentration is not suitable as
it would lead to a significant loss of ammonia due to the fact that most of the available
nitrogen in these digestates is present in the ammonium/ammonia form. Thus stabilizing the
product by partial nitrification prior to evaporation is proposed based on a series of
experiments conducted to evaluate the appropriateness of this approach.
The ammonium-N concentration of the digestate used was 1.7 g/L. The nitrification does not
appear to be inhibited by ammonia, nitrate or nitrite accumulation, except when pH > 7.5, a
situation that can be avoided by regulating the feeding rate based on pH measurements.
Significant nitrite accumulation was not observed either. Nitrification is however consistently
limited by alkalinity so that only about 75% of the ammonia can be converted to nitrate
without alkalinity supplementation. The nitrification brings the pH down below 5.0 where the
remaining ammonia is present as> 99% NH/. At this condition the nitrified digestate can be
evaporated without a significant nitrogen loss and the product is recognized to be a high
quality liquid fertilizer. It is suggested that the production of concentrated partially nitrified
organic fertilizers can become a sustainable way of nutrient recovery from anaerobic
digestates originating from municipal organic wastes.