Nutrient recovery from digestion waste: from field experimentation to model-based optimisation
The increasing awareness of natural resource depletion, the increasing demand of nutrients and energy for food production, and the more and more stringent nutrient discharge and fertilisation levels, have resulted in an increased attention for nutrient recovery from municipal and agricultural wastes.
The PhD dissertation of Céline Vaneeckhaute aims at stimulating the transition to a bio-based economy by providing (tools to develop) sustainable strategies for nutrient recovery from organic wastes following bio-energy production through anaerobic digestion (= digestion waste). Particular attention is paid to the valorisation of the recovered products as renewable substitutes for chemical fertilisers and/or as sustainable organo-mineral fertilisers in agriculture. Three complementary research phases were conducted: 1) technology inventory and product classification, 2) product value evaluation, 3) process modelling and optimisation.
An important outcome is the development and implementation of advanced dynamic models for anaerobic digestion and nutrient recovery technologies, such as ammonia stripping, acidic air scrubbing, and phosphorus precipitation/crystallisation. The models are based on a detailed chemical solution speciation and combined with biological and physicochemical reaction kinetics. This allows to accurately predict the reactor pH, all relevant precipitation reactions (e.g. struvite), gas-liquid exchange reactions, biogas production, etc. The models were successfully used to increase process understanding, for operational and economic treatment train configuration and optimisation, and will be further upgraded to allow for sizing and process control of anaerobic digestion and nutrient recovery processes. An optimisation experiment for a large-scale anaerobic digestion and nutrient recovery project resulted in potential financial benefits of €2.5-6 per m³ of manure based on net variable cost calculations and €1.8 per m³ of manure, equivalent with €35 per ton solids, on average over 20 years when also taking into account capital costs.
Overall, the PhD dissertation will serve as an important reference for future anaerobic digestion and nutrient recovery projects (e.g. the biogas plant at Quebec City, Canada), particularly to optimise the process operational settings, costs, and quality of the recovered biogas and fertilisers.
Author: Céline Vaneeckhaute