Combined filtration and oxalic acid leaching for recovering phosphorus from hydrothermally carbonized sewage sludge. Issue 1 (February 2021)

• Record Type: Journal Article
• Authors: Waldmüller, Wolfgang; Herdzik, Szymon; Gaderer, Matthias
• Is Part Of: Journal of environmental chemical engineering. Volume 9:Issue 1(2021)
• Publication Date: 2021-02
• Subjects: AC ash content -- DM dry matter content -- EDX energy-dispersive X-ray analysis -- HC hydrochar -- HHV higher heating value -- HTC hydrothermal carbonization -- PZC point of zero charge -- SEM scanning electron microscopy -- SS sewage sludge -- XRD X-ray diffractometry -- db dry basis -- wb wet basis; Hydrothermal carbonization -- Calorific value -- Phosphate -- Precipitation -- Hydrochar; Chemical engineering -- Environmental aspects -- Periodicals; Environmental engineering -- Periodicals; Chemical engineering -- Environmental aspects; Environmental engineering; Periodicals; 660.0286
• DOI: 10.1016/j.jece.2020.104800
• Languages: English
• ISSNs: 2213-2929

Abstract

Graphical abstract: Highlights: An HTC slurry of sewage sludge was filtered, washed and leached with oxalic acid. The permeability of the hydrochar cake is poor, but increases during leaching. Acidic leaching solubilizes mainly inorganics, in particular phosphate. Leached iron(II), calcium and magnesium precipitate as their oxalate salts. Abstract: Hydrothermal carbonization is a promising pretreatment for the energetic utilization of municipal sewage sludge. However, the carbonized slurry must be dewatered and the valuable compound phosphate recovered. This article reveals a new approach of combined slurry filtration and subsequent phosphate leaching using oxalic acid (18–72 g l −1 ) in a single apparatus for the first time. A slurry of hydrothermally carbonized sludge was filtered, washed, leached twice with oxalic acid and washed again. A thorough characterization of the filtration performance and the process products was conducted. The suspension is difficult to filter, but the permeability of the filter cake increases during acidic leaching. The specific filter cake resistance of (4.5 ± 2.2) · 10 15 m −2 indicates poor filterability of the hydrochar slurry. 41–83% of phosphate and sodium are leached from the hydrochar, while all other investigated inorganics are leached to a lower extent. 12–31% of iron, calcium and manganese are removed from the hydrochar. The relative contents of carbon, hydrogen and oxygen increase in the residual hydrochar after leaching. After leaching, oxalic acid accounts 10.5–16.9% of the dry hydrochar due to adsorption and poor cake washing. Co-leached calcium, bivalent iron and magnesium precipitate from the leachate as oxalate salts. In conclusion, combined filtration and phosphate leaching enables efficient phosphate removal and generates an ash-reduced hydrochar for energetic utilization. However, to enable its technical application, the filtration performance and system integration need to undergo further development.