The removal of uranium (VI) from aqueous solution by the anaerobically digested sewage sludge with hydrothermal pretreatment

Anaerobic digestion (AD) with hydrothermal (HT) pretreatment (sequential HT-AD treatment) is a novel technology for sludge management. HT-AD sludge is rich in functional groups and its applications as pollutant sorbents might be a win-win strategy. This study investigated the removal of uranium (VI) from water using HT-AD sludge as affected by solution pH, temperature, and ion strength. The reusability and heavy metal risk of HT-AD sludge were also assessed. The batch sorption experiments demonstrated that even at an acidic initial pH of 3.2, the maximum adsorption of HT-AD sludge for uranium (VI) reached 117.13 mg/g, higher than that of most carbon-based materials. The inner-sphere and out-sphere complexation between uranium (VI) and the HT-AD sludge dominated the adsorption when pH was in the range of 2-6 and 6-11, respectively. The FTIR and XPS analysis indicated that the primary mechanisms of uranium (VI) adsorption by the HT-AD sludge were the surface complexation and the electric attraction between uranium (VI) and the functional groups (e.g. -COO-) on HT-AD sludge. The removal rate of uranium (VI) by HT-AD sludge only decreased by ∼7% after 3 consecutive adsorption cycles. Leaching experiment showed that less than 5% of the total heavy metal were released from HT-AD sludge. Our research proved that HT-AD sludge can be used as an efficient uranium (VI) adsorbent with good reusability and environmental safety.

Anaerobic co-digestion of the aqueous phase from hydrothermally treated waste activated sludge with primary sewage sludge. A kinetic study

The mesophilic anaerobic co-digestion of the liquid fraction from hydrothermal carbonization (LFHTC) of dewatered waste activated sludge with primary sewage sludge (PSS) has been studied. Mixtures of different composition (25, 50 and 75% of LFHTC on a chemical oxygen demand (COD) basis), as well as the individual substrates, have been tested using two inocula (flocculent (FS) and granular (GS) sludges). Methane production decreased as the LFHTC/PSS ratio increased, which can be related to the presence of recalcitrant compounds in the LFHTC, such as alkenes, phenolics, and other oxygen- and nitrogen-bearing aromatics hard-to-degrade through anaerobic digestion. Methane yield reached 248 ± 11 mL CH₄ STP/g COD_added with the GS inoculum and 25% LFHTC. A 74 and a 30% increase of methane production was achieved in the 25% LFHTC runs respect to the obtained in the similar experiments with 100% LFHTC, using the FS and GS inocula, respectively. In those late runs, the COD was reduced more than 86%, with a negligible concentration of total volatile fatty acids. With both inocula, total Kjeldahl nitrogen hydrolysis increased as the LFHTC to PSS mixture ratio decreased, reaching values higher than 79% at the end of the experiments. Methane yield values fitted well the first-order, Cone and Weibull kinetic models for both inocula. Significant differences in the kinetic constant values, ranging from 0.100 to 0.168 d^-1 and 0.059-0.068 d^-1, were found with the FS and GS inocula, respectively. The results obtained support the potential integration of HTC of dewatered waste activated sludge in wastewater treatment plants.

Characterization of hydrothermal carbonization products (hydrochars and spent liquor) and their biomethane production performance

To optimize the energy yield (EY) of food waste (FW) via hydrothermal carbonization (HTC), a response surface method was applied. Hydrochars and spent liquor were further conducted to evaluate their characterization and anaerobic digestion potential. Results found that optimal parameters for HTC of FW were suggested as temperature of 260 °C, reaction time of 4 h and moisture of 80%, with higher EY of 66.1%. Higher heating value, good combustion quality, lower H/C and O/C ratios indicated that hydrochar could be utilized as a safe solid fuel. Biochemical methane potential (BMP) experiment showed that spent liquor and hydrochars could be used as feedstocks for anaerobic digestion. Interestingly, hydrochars added in the spent liquor could promote the specific methane yield, which was 2.53 times higher than no addition of hydrochars. The finding of this study could provide useful information for HTC of FW and the utilization of hydrochars and spent liquor.

Evolution of phosphorus complexation and mineralogy during (hydro)thermal treatments of activated and anaerobically digested sludge: Insights from sequential extraction and P K-edge XANES

(Hydro)thermal treatments of sewage sludge is a promising option that can simultaneously target safe waste disposal, energy recovery, and nutrient recovery/recycling. The speciation of phosphorus (P) in sludge is of great relevance to P reclamation/recycling and soil application of sludge-derived products, thus it is critical to understand the effects of different treatment techniques and conditions on P speciation. This study systematically characterized P speciation (i.e. complexation and mineral forms) in chars derived from pyrolysis and hydrothermal carbonization (HTC) of municipal sewage sludges. Combined sequential extraction and P K-edge X-ray absorption near edge structure (XANES) spectroscopy analysis revealed the dependence of P transformation on treatment conditions and metal composition in the feedstocks. Pyrolysis of sludges decreased the relative abundance of phytic acid while increased the abundance of Al-associated P. HTC thoroughly homogenized and exposed P for interaction with various metals/minerals, with the final P speciation closely related to the composition/speciation of metals and their affinities to P. Results from this study revealed the mechanisms of P transformation during (hydro)thermal treatments of sewage sludges, and might be applicable to other biosolids. It also provided fundamental knowledge basis for the design and selection of waste management strategies for better P (re)cycling and reclamation.