Doctor of Philosophy / Agronomy / Nathan O. Nelson / Agricultural fields contribute phosphorus (P) to water bodies, which can degrade water quality. The P index (PI) is a tool to assess the risk of P-loss from agricultural fields. However, due to limited measured data, P indices have not been rigorously evaluated. The Agricultural Policy/Environmental Extender (APEX) model could be used to generate P-loss datasets for P index evaluation and revision. The objectives of the study were to i) determine effects of APEX calibration practices on P-loss estimates from diverse management systems, ii) determine fertilizer and poultry litter management effects on P-loss, iii) evaluate and update the Kansas PI using P-loss simulated by APEX and iv) determine appropriate adsorption isotherms with advection-dispersion equation with column leaching experiment. Runoff data from field studies in Franklin and Crawford counties were used to calibrate and validate APEX. Poultry litter and inorganic fertilizer application timing, rate, method, and soil test P concentration effects on P loss were analyzed using location-specific models. A column leaching laboratory study was also conducted to test the adsorption isotherms. Location-specific model satisfactorily simulated runoff, total P (TP) and dissolved P (DP) loss meeting minimum model performance criteria for 2/3 of the tests whereas management-specific models only met the criteria in 1/3 of the tests. Applying manure or fertilizer during late fall resulted in relatively lower TP loss compared to spring applications before planting. The Kansas-PI rating and the APEX simulated P-loss were correlated with r² of 0.40 (p<0.001). Adjusting the weighting factors for Prate, soil test P, and erosion improved the correlation (r² = 0.46; p<0.001. Using a component PI structure and determining the weighting factors by multiple linear regression substantially improved the correlation between the PI and TP loss (r² = 0.69; p<0.001). In the P-leaching experiment, both the linear and nonlinear adsorption isotherms did not fit the experimental data. A multi-reactional advection-dispersion model that better describes all the P processes and complexities in soils should be included in the future. These procedures can provide a roadmap for others interested P transport in soils and using computer models in evaluation, and modifying their PI.

With a growing population but scarce primary phosphorus sources, recycling of the vital element has become an important research area throughout the last decades. Several streams in society are potential resources for recirculation but municipal sewage is considered one of the most available materials. With current technologies in wastewater treatment, over 95 % of the influent phosphorus is captured in the sludge along with a variety of other nutrients. However, due to increasing fractions of pharmaceutical residues and heavy metals also following the sludge, direct use as fertiliser is being phased out in most European countries in favour of extraction methods. Extraction of nutrients from the sludge is problematic mainly because of dewaterability difficulties. Thus, pretreatment of the material is required to access the desired components at a reasonable cost and energy consumption. Hydrothermal carbonisation (HTC) is a technology showing high potential for treatment of wet carbonaceous material without necessity of prior drying. The resulting product is hygenised, essentially free from pharmaceuticals and easily dewatered. In this Master’s thesis principal conditions for release of phosphorus from HTC converted digested sludge under acid leaching have been experimentally investigated. Dependence of time, temperature, dry solids (DS) content of HTC sludge and pH have been studied. Also, differences arising from acid type have been considered by comparing acidulation with sulphuric acid and hydrochloric acid. A short investigation of the recovery of the dissolved phosphorus from leachate by precipitation was also performed where calcium ions were added to both sulphuric and hydrochloric acid leachates. Extraction of phosphorus from HTC converted sludge has shown to be easier than from pure metal phosphates under comparable leaching conditions and pH values. Also, the dissolved phosphorus concentrations obtained in the presence of HTC converted sludge was higher than for theoretical equilibrium concentrations where all phosphorus is in the form of iron(III) or aluminium(III) phosphate. A maximum leachate phosphorus concentration was around 2500 mg/L, recorded in leaching experiments performed at a dry HTC product concentration of 10 % (w/w) in an extraction solution of water acidified with sulphuric acid. Leaching procedures performed at pH values between 2 and 1 with 1 and 5 % DS HTC product resulted in dissolution of 90 % of ingoing phosphorus at an acid charge of 0.5 kg H2SO4/kg DS HTC product. At this chemical charge, release of phosphorus from converted sludge is fast. Similar amounts of dissolved phosphorus were recorded after 15 min as after 16 h retention time. Possibly, time dependence becomes relevant at lower charges. The dissolution of phosphorus is negatively affected by temperature increases at moderate acid loads, and by possibly by hydrochloric acid at pH values below 2. Addition of calcium gave a dissolved phosphorus reduction of 99.9 % in both the sulphuric acid and hydrochloric acid leachates. Gypsum, CaSO4, also precipitates from the sulphuric acid leachate resulting in 67 % more dry mass. Due to high release of metals during acidulation, the precipitate was also contaminated with large fractions of metals in addition to calcium. In summary, this investigation has demonstrated that up to 90 % of the phosphorus content of the HTC converted sludge can be released by acid leaching, and almost 100 % of the phosphorus can be recovered from the leachate by precipitation with calcium ions.