PhD dissertation “Towards waste valorisation: phosphorous and potassium resource recovery for fertiliser production” by Emma Company Masó

To preserve soil fertility, agriculture needs to return the nutrients removed with harvested crops to the soil.  To date, this has been achieved by means of mineral fertilisers, obtained from non-renewable resources, or by applying livestock manure and/or sewage sludge. However, inadequate management of waste streams causes nutrient losses through volatilization or leaching, thereby contributing to the contamination of surface and groundwater bodies. To address this, a paradigm shift based on circular economy is needed to recover the nutrients contained in waste. Emma Company’s doctoral thesis focuses on the recovery of phosphorus (P), potassium (K) and nitrogen (N) from pig manure and urban wastewater to produce struvite and K-struvite, two slow-release fertilizers. The work investigates precipitation as a nutrient recovery method and explores electrochemical precipitation which, unlike the former, does not require the addition of external chemical reagents.

The experimental work was initially conducted at the laboratories of the “Laboratory of Chemical and Environmental Engineering” (LEQUIA) of the University of Girona. Subsequently, validations were carried out in two pilot plants (one of chemical precipitation and another one of electrochemical precipitation) located in industrial facilities in Catalonia: a pig farm (“Mas Monellots”) and the urban wastewater treatment plant of Terrassa (Barcelona). Altogether, these validations have the ultimate goal to demonstrate the transferability and operativity of precipitation processes under real conditions.

Main results obtained are:

• Demonstration of the feasibility of recovering K and P as K-struvite from pig slurry. From the denitrified effluent, 90% of K was recovered through the addition of external magnesium (Mg) and P sources. From the sludge fraction, nutrient recovery was enhanced up to 75% through an acidification pre-treatment using oxalic acid to mobilise insoluble nutrients while simultaneously blocking Ca²⁺ ions that interfere with struvite recovery.
• Development of electrochemical pH control systems capable of promoting nutrient precipitation without the addition of chemical reagents, achieving phosphorus recovery efficiencies above 90%.
• Optimisation of the electrochemical design by adjusting reactor configuration parameters and integrating electrochemical pH control and crystallisation into a single tubular electrocrystalliser. Nutrient recovery efficiency was improved up to 1.62 grams of dry matter per litre treated while energy consumption was reduced up to 0.12 kWh per kg of phosphorous recovered.
• Development of strategies to avoid anodic chloride oxidation during electrochemical precipitation processes in residual streams.

These results represent an important step towards effective wastewater valorisation and the sustainable production of the so-called “second-generation fertilisers”, that contain valuable microorganisms for soils and crops. To sum up, the thesis demonstrates that nutrient recovery through precipitation technologies can contribute to reducing dependence on mineral resources, decreasing the environmental impact associated with waste management, and advancing towards more sustainable agricultural production and consumption models.

Main publication: Emma Company et al, Exploring the recovery of potassium-rich struvite after a nitrification-denitrification process in pig slurry treatment, Science of The Total Environment, 847, 2022, 157574, https://doi.org/10.1016/j.scitotenv.2022.157574.