Acid-Induced Phosphorus Release from Hydrothermally Carbonized Sewage Sludge

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 Abstract

Phosphorus recovery from hydrothermal carbonization of organic waste: a review

“…Acids enhance the dissolution of the complexes that bind P to metal ions, but the efficiency of extraction changes with varying acid strengths and with the extent of complexity of the material. Generally, strong acids (such as H 2 SO 4 and HCl) are used to achieve higher P‐recovery; however, organic acids (such as oxalic and citric acids) are currently being used to overcome the potential environmental drawbacks induced by using inorganic acids 102 . The difference in composition also plays a role in defining the degree of extraction, since different metal ions have different solubilities with acids.…”

Environmental Life Cycle Assessment of Hydrothermal Carbonization of Sewage Sludge and Its Products Valorization Pathways

“…In Table 5 the main assumptions are reported. The separated P leachate is assumed to be used directly in agricultural land, even though [38] post treatments and/or fertilizers production processes are generally recommended [43].…”

Influence of Post- and Pre-Acid Treatment during Hydrothermal Carbonization of Sewage Sludge on P-Transformation and the Characteristics of Hydrochar

“…Phosphorus (P) is an indispensable element for all life forms, and its continuous supply is vital for sustainable food production globally. Further, P is also used in various industrial applications, such as production of detergents, paints, food and beverages [1]. As a result of the versatility P possesses, the natural source of P is being unsustainably exploited.…”

“…Generally, after HTC of sewage sludge, a significantly higher portion of P is transferred and accumulated onto hydrochar. The building-up of P in hydrochar produced after HTC of sewage sludge is mainly influenced by the utilization of coagulant in the WWTP [1]. Metal salts such as Al 2 (SO 4 ) 3 and FeCl 3 are generally utilized in the WWTP to precipitate P from wastewater [12].…”

“…During HTC, the presence of metal salts in sewage sludge, in turn, fosters the P transformation into hydrochar, along with the majority of heavy metals [13]. An effective method for leaching the trapped P in hydrochar is by treating hydrochar in inorganic acid solutions at an extremely low pH [1,14].…”