Nutrient recycling using biowastes from diverse sources

Abstract

Phosphorus is a limiting nutrient in crop growth and thus widely used as an agricultural fertiliser. It cannot be substituted by another element, nor newly synthesized. Hence, global food production is ultimately dependent on it and the demand will increase with global population growth. Most phosphorus is applied to soil as mineral P fertiliser, a non-renewable resource derived from mined phosphate rock and consequently there has been increasing interest in the use of treated biowastes to return P into agricultural soils. This research aimed to establish the physical and chemical properties of hydrochar obtained from biowastes from diverse sources with a view to their eventual use as a P fertiliser in organic farming. After collecting biowaste samples including biogas slurry, liquid pig manure and struvite the samples were subjected to hydrothermal carbonisation (HTC) treatment. The treated samples were analysed visually with a microscope to estimate grain size, and homogeneity. In the second phase of the research, the chemical properties of the samples were determined. Elemental concentration including phosphorus, nitrogen, potassium, magnesium as well as heavy metals were measured. After this different amendments, including various minerals containing carbonate and phosphate, sea grass, struvite and wood ash, were added to the liquid pig manure to establish whether they influence nutrient concentration as well as P solubility and availability in carbonised pig manure. The visual analysis indicated that increasing HTC temperature, pressure and processing time resulted generally in more homogeneous particle sizes as well as an increased abundance of smaller pores compared to the unchanged feedstock. The chemical analyses showed that sufficient P is available in all three carbonized biowastes used, thus making them suitable as fertiliser. However, only small differences were found between hydrochars made from the pure liquid pig slurry and those from amended liquid pig manure. The PAH and heavy metal concentrations in the amended and treated biowastes were generally low. P availability was unknown in carbonised pig manure before this research. This researched has shown the potential of biowastes in conventional and organic farming.