Acid-Induced Phosphorus Release from Hydrothermally Carbonized Sewage Sludge

Carla Pérez,Jerker Fick,Stina Jansson,Jean-François Boily,Tomas Gustafsson

doi:10.1007/s12649-021-01463-5

Abstract

Phosphorus (P) recovery from P-rich residues is crucial to sustain food and industrial demands globally, as phosphate rock reserves are being depleted. The aim of this study is to investigate the speciation and recovery of P from hydrochars (HC) of a metal-bearing sewage sludge (SS) produced by hydrothermal carbonization (HTC). We here focus on extractions by acid leaching as P cannot be directly recovered by HTC due to insoluble metal-P compounds. Acid leaching of SS and HCs was investigated using H2SO4 and HCl over a range of leaching times, and explained in terms of how composition affects P and metal release efficiency. HTC at 180, 215 and 250 °C showed that P remained immobilized (> 75% of total P) in the HCs. More than 95% was present as inorganic P, and was the direct consequence of the double addition of iron salts in the wastewater treatment plant. Leaching experiments in 2.5 M acid solutions showed that a near complete release of P could be achieved in HCs, while it was only incomplete in SS (up to 85%). Lower acid concentrations were ineffective for total P recovery. Treatment temperature exceeding 180 °C however decreased P release rates, such that total removal took at least 2 h of reaction time instead of a few minutes. On the other hand, acid leaching transferred more than 70% of iron, manganese, copper and zinc into the leachate, necessitating a post-treatment purification process. This work therefore reveals that HC produced at low HTC temperatures could offer promising avenues for time- and energy-efficient P recovery from SS.Graphic

Highlights

Phosphorus (P) is an essential element, and its availability is crucial to sustain food and industrial demands globally
The hydrochar yield recovered after hydrothermal carbonization (HTC) reactions (Table 1) decreased from 67.1% at 180 °C to 59.5% at 250 °C, while the ash content increased with reaction severity
The increase in ash content was the result from losses in volatile matter, and from the dissolution of organic matter into the process water [7], which correlated with the enrichment of P and other inorganic matter in the HC (Table 2)

Summary

Introduction

Phosphorus (P) is an essential element, and its availability is crucial to sustain food and industrial demands globally. The current growth in the human population is increasing the demand for food, resulting in a rapid increase in P consumption [3]. While phosphate rock is the only accessible source of P [1] it is a non-renewable resource from only a handful of countries (Morocco, US, Jordan, China and South Africa), and it is critically approaching near-depletion in the near future [4]. Noting that remaining P sources are of lower quality and/or difficult to extract [5], there is an urgent need for efficiently managing and recycling of P-rich residues to address the worlds future demands

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