Phosphorus recovery by chemical precipitation from the spent liquor of hydrothermal carbonization of biogenic municipal waste

Abstract

Phosphorus (P) is an essential macronutrient that helps plant growth by forming energy releasing compounds which is usually supplied by the addition of synthetic fertilizer. P recovery from renewable resources have recently garnered interest due to the projection of eventual depletion of phosphate rock (PR). In this work, organic fractions of the municipal solid waste (MSW) was subjected to P recovery by combined application of hydrothermal carbonization (HTC) and chemical precipitation. The biogenic municipal waste (BMW) was found to be highly wet (~80% moisture) and volatile (containing >70% VM of the dried weight). Considerable amounts of multivalent cations in the waste were quantified too and it was found particularly Ca-enriched (2.70 % of the DW). Nutrient incorporation within the hydrochar and solubilisation into the hydrothermal carbonization liquor (HTCL) were studied following the HTC process conducted at two different conditions – at 190℃ for 30 min and at 250℃ for 70 min. It was revealed that despite a loss in yield, P was almost entirely transferred to the hydrochar during reaction at the more severe condition. Respectively, 79% and 95% of the total phosphorus were retained in the hydrochars obtained at 190℃ and 250℃. Among other nutrients, both nitrate (>70%) and total Kjeldahl (>51%) nitrogen, and calcium (>63%) were found to be distributed predominantly in the solid fraction whereas Na (>67%) and K (>63%) were majorly dissolved in the HTC. As retained P content in the HTCL was too low, extraction experiments were carried out to release P from the hydrochars using both 2.5 M HCl and 2.5 M H_2 SO_4. HCl displayed comparatively better performance in the liquefaction of most of the nutrients. The kinetics study showed that most of the P was released almost instantaneously and a 2nd order kinetics was mostly followed in the leaching. Later, precipitation experiments using simulated solutions were carried out and optimized for three variables i.e., pH, Mg^(2+): 〖PO_4〗^(3-) molar ratio, Ca^(2+): 〖PO_4〗^(3-) molar ratio. Models were developed for the responses whose significance based on p-values and fitting quality based on R^2-values was satisfactory. 51.58% of P was recovered in the solids and 98.05% solid purity was achieved at the distinct optimums. Similar level of performances had not translated for the runs with a 1:1 (v/v) HTCL – leachate mixtures, as the purity dropped to 26.05% at the optimum, largely attributed to the Ca-enriched nature of the feedstock.