Hydrothermal carbonization (HTC) of dairy waste: effect of temperature and initial acidity on the composition and quality of solid and liquid products

Abstract

Background: Hydrothermal carbonization (HTC) of dairy processing waste was performed to investigate the effect of temperature and initial pH on the yield and composition of the solid (hydrochar) and liquor produced. All hydrochars met the EU requirements of organo-mineral solid fertilizers defined in the Fertilizing Products Regulation in terms of phosphorus (P) and mineral content. Methods: Laboratory scale HTC was performed using pressurized reactors, and the products (solid and liquid) were collected, stored and analyzed for elemental composition and nutrient content using Inductively coupled plasma optical emission spectroscopy (ICP-OES), ultraviolet-visible spectrophotometry (UV-Vis) and other analytic techniques. Results: Maximum hydrochar yield (60.67%) was observed at T=180℃ and pH=2.25, whereas the maximum P-recovery was 80.38% at T=220℃ and pH=4.6. The heavy metal content of the hydrochars was mostly compliant with EU limitations, except for Ni at T=220℃ and pH=8.32. Meanwhile, further study of Chromium (Cr) species is essential to assess the fertilizer quality of the hydrochars. For the liquid product, the increase in temperature beyond 200℃, coupled with an increase in initial acidity (pH=2.25) drove P into the liquor. Simultaneously, increasing HTC temperature and acidity increased the concentration of NO3- and NH4+ in the liquid products to a maximum of 278 and 148 mg/L, respectively, at T=180℃ and pH=4.6. Furthermore, no direct relation between final pH of liquor and NH4+ concentration was observed. Conclusions: HTC improved the nutrient content of dairy waste, allowing for the production of potential solid organo-mineral fertilizers requiring additional treatment to ensure safe fertilizer application.

Phosphorus fertiliser equivalent value of dairy processing sludge‐derived STRUBIAS products using ryegrass (Lolium perenne L.) and spring wheat (Triticum aestivum)

“…All ash and HC samples contained a significant amount of nutrients and metals, except NH 4 ‐N, because P and metals are most likely to remain and concentrate in solid residues during thermochemical process (Shackley et al., 2010). Three HCs in this study were produced from an Fe‐DPS and different initial acidity was used, which can affect HC yield (Khalaf et al., 2022) but did not affect the HC characteristics.…”

“…Three HCs in this study were pro-TA B L E 3 Effects of treatment and P rate on the grass and spring wheat dry matter yield and P uptake over the course of the phosphorus mineral fertiliser equivalency (P-MFE) experiment. duced from an Fe-DPS and different initial acidity was used, which can affect HC yield (Khalaf et al, 2022) but did not affect the HC characteristics.…”

Fertiliser equivalent value of dairy processing sludge-derived STRUBIAS products using ryegrass (Lolium perenne L.) and spring wheat (Triticum aestivum)

“…All ash and HC samples contained a signi cant amount of nutrients and heavy metals, except NH 4 -H, because P and metals are most likely to remain and concentrate in solid residues during thermo-chemical process (Shackley et al, 2010). Three HCs in this study were produced from a Fe-DPS and different initial acidity was used, which can affect hydrochar yield (Khalaf et al, 2022) but did not affect the HC characteristics.…”

Soil microorganisms increase Olsen phosphorus from poorly soluble organic phosphate: A soil incubation study

“…In recycled fertilizers, a large proportion of the P can be strongly chelated by iron (Fe), aluminium (Al) or calcium (Ca) cations or organic molecules, thus reducing P availability to plants. Large efforts are made during the production phase to reduce the formation of insoluble P molecules in chars (Han et al, 2022; Khalaf et al, 2022; Mercl et al, 2020), ashes (Ahmed et al, 2021; Kumpiene et al, 2016) or during the formation of P‐salt precipitates such as struvite (Numviyimana et al, 2020) or vivianite (Prot et al, 2021). Unfortunately, it remains poorly understood how the solubilities of poorly soluble Fe‐P, Al‐P, Ca‐P or organic‐P compare to one another.…”