Transformation Kinetics of Phosphorus and Nitrogen in Iron-Rich Sewage Sludges during Hydrothermal Treatment and Recovery of Nutrients from Process Water

Abstract

Hydrothermal treatment (HT) is an emerging technique for sustainable sewage sludge management and resource recovery. Many sludges are rich in iron (Fe) due to the common addition of Fe salts in water resource recovery facilities. To develop guidance for reaction conditions targeting nutrient recovery, this study systematically investigated the influence of HT temperature, treatment time, and sludge source on the dynamic speciation evolution of phosphorus (P) and nitrogen (N) during HT of Fe-rich sewage sludge. Complementary chemical extraction and X-ray spectroscopy analyses were conducted to characterize the treatment products. For the sludge mixture (a blend of primary and waste activated sludges), P speciation did not change significantly within 4.5 h at 125 °C HT, while soluble and labile P was converted into insoluble P over time at 175 and 225 °C HT. Strengite (FePO4·2H2O) preferentially formed in the hydrochars with increasing treatment temperature and/or time, whereas 125 °C HT within 1.5 h favored the formation of vivianite (Fe3(PO4)2·8H2O). Organic P was completely decomposed into orthophosphate when the HT temperature reached up to 175 °C. Pyrrole-N was enriched in the hydrochars. Similar reaction pathways were observed during HT of anaerobically digested sludge, though some minor differences in Fe-associated P and organic P were observed. Meanwhile, HT of the two sludges released orthophosphate and ammonia into the process waters at 175 and 225 °C, which can be recovered by a sequential process involving struvite (MgNH4PO4·6H2O) precipitation and air stripping. This study provides new insights into the transformation of P and N during HT of Fe-rich sludges as well as a modular design for maximum P and N recovery from the treatment products.

Experimental-based mechanistic study and optimization of hydrothermal liquefaction of anaerobic digestates

“…The HTL temperatures between 310 and 350 C and the reaction times between 15 and 60 min have been reported to maximize the biocrude yield and minimize the biocrude heteroatom content from a variety of feedstocks, [11][12][13][14] while homogeneous and heterogeneous catalysts have been reported to increase the carbon content in biocrude by catalyzing deoxygenation and denitrogenation reactions. [15][16][17][18] Another branch of HTL research has instead focused on recovering nutrients from HTL products in the form of inorganic fertilizers, for example, struvite (MgNH 4 PO 4 $6H 2 O) crystallization from the HTL-AP under alkaline conditions to produce a slow-release inorganic fertilizer providing plantavailable P, N, and Mg. 19,26,27 To maximize struvite production from the HTL-AP, nutrients in the feedstock should preferentially partition in the HTL-AP, where they tend to assume their bioavailable forms (e.g., NH 4 + , orthophosphates, K + , Ca 2+ , and Mg 2+ ) rather than accumulate in the hydrochar phase in their non-bioavailable forms (e.g., apatite phosphate, 19 calcite, 20 and vivianite 21 ). Since struvite formation is inhibited by the presence of Ca ions, the Ca concentration in the HTL-AP should be minimized, as it would cause the formation of a morethermodynamically stable hydroxyapatite (a less effective Pfertilizer).…”

“…, NH 4 + , orthophosphates, K + , Ca 2+ , and Mg 2+ ) rather than accumulate in the hydrochar phase in their non-bioavailable forms ( e.g. , apatite phosphate, 19 calcite, 20 and vivianite 21 ). Since struvite formation is inhibited by the presence of Ca ions, the Ca concentration in the HTL-AP should be minimized, as it would cause the formation of a more-thermodynamically stable hydroxyapatite (a less effective P-fertilizer).…”

Thermodynamics and Kinetics of Struvite Crystallization from Hydrothermal Liquefaction Aqueous-Phase Considering Hydroxyapatite and Organics Coprecipitation

“…Previous studies on struvite crystallization kinetics from highly controlled synthetic solutions reported that crystallization at temperatures between 10 and 50 °C and alkaline pH between 8 and 10 combined with stirring at 100 to 300 rpm for 30 to 60 min recovers about 80–98% of P in the form of fine-sized struvite crystals. − These studies assumed the absence of foreign ions, e.g., Ca 2+ , Fe 3+ , and Al 3+ , that can form other phosphate minerals, − used low ionic strength (IS) solutions (IS < 0.1 M), and adopted controlled Mg/NH 3 –N/P ratios, e.g., regulated at equimolar or specific molar ratios, that assures the irreversibility and high selectivity of struvite formation . Unfortunately, these conditions are not found in actual HTL-AP product effluents, which contain significant concentrations of foreign ions and polar organic compounds, ,, higher ionic strength (0.2–0.4 M), , and nonequimolar Mg/NH 3 –N/P ratios. − …”

Probing elemental speciation in hydrochar produced from hydrothermal liquefaction of anaerobic digestates using quantitative X-ray diffraction

“…Similarly, previous studies of the speciation of inorganics in hydrochar have only focused on the transformation of P during HTL of wet biomass wastes and have always assumed that P precipitates in hydrochar through the formation of apatite 20,43 (e.g., hydroxyapatite) and non-apatite inorganic phosphate minerals, e.g., vivianite, varulite, and strunzite. 44,45 These studies did not comprehensively assess the prole of P speciation in the aqueous phase (e.g., via functional group identication with 31 P NMR), ignoring the possibility of P precipitation by chemically bonding aqueous aromatics that are susceptible to repolymerization into hydrochar during HTL.…”