Advances in sewage sludge application and treatment: Process integration of plasma pyrolysis and anaerobic digestion with the resource recovery

Multi-criteria optimisation of subcritical wet oxidation for sludge treatment

Subcritical wet oxidation (SWO) is an environmentally-friendly solution for sewage sludge volume reduction. However, little study has comprehensively optimised SWO conditions across various aspects. This study developed a multi-objective model using genetic algorithms (GAs) to optimise SWO conditions, considering sludge deconstruction, emissions, energy balance, and resource recovery. The multi-criteria optimisation approach highlights the significant environmental benefits of SWO, including substantial sludge volume reduction and effective pollutant removal. An in-depth analysis of temperature, reaction time, and severity factor revealed their critical roles in enhancing sludge deconstruction and resource recovery efficiency. GAs predicted optimal conditions at 271 ± 2 °C and 51 ± 1 min, with confirmation experiments showing only 12% discrepancy between predicted and actual outcomes. This study provides practical insights for efficient sewage sludge treatment and sustainable wastewater management.

Recycling of sludge residue as a coagulant for phosphorus removal from aqueous solutions

Phosphorus pollution poses a significant challenge in addressing water contamination. The coagulant is one of the effective methods to remove phosphorus from wastewater. Abundant Al and Fe oxides in sludge residue make it have great potential to synthesize water treatment coagulants. However, the utilization of sludge residue for preparation of coagulant was seldom investigated. In this study, we fabricated a novel coagulant, polyaluminum ferric chloride (SM-PAC), using sludge residue as a raw material through acid leaching and polymerization processes. Characterization results confirm that the parameters of SM-PAC meet the specifications outlined in the national standard (GB/T 22627-2022). We investigated the effects of pH, dosage, initial phosphorus concentration, and contact time on the removal efficiency of SM-PAC. As anticipated, the prepared SM-PAC exhibited a significant efficacy in removing phosphorus, meeting the discharge standards set for municipal sewage. Furthermore, the adsorption kinetics analysis suggests that the predominant mode of phosphorus adsorption on SM-PAC is chemical adsorption. Furthermore, the SM-PAC was employed in the actual wastewater treatment plant and exhibited excellent efficiency in phosphorus removal. The utilization of SM-PAC can not only effectively address the issue of sludge disposal but also achieve the goal of "treating waste with waste." It is expected that the proposed method of reusing sludge residue as a resource can provide a sustainable way to synthesize a coagulant for phosphorus removal.

Pyrolysed sewage sludge for metal(loid) removal and immobilisation in contrasting soils: Exploring variety of risk elements across contamination levels

Efficient treatment of sewage sludge may transform waste into stable materials with minimised hazardous properties ready for secondary use. Pyrolysed sewage sludge, sludgechar, has multiple environmental benefits including contaminant sorption capacity and nutrient recycling. The properties of five sludgechars were tested firstly for adsorption efficiency in laboratory solutions before prospective application to soils. A wide variety of metal(loid)s (As, Cd, Co, Cr, Cu, Ni, Pb, Sb, and Zn) was involved. Secondly, the sludgechars (3 % v/v) were incubated in five soils differing in (multi)-metal(loid) presence and the level of contamination. The main aim was to evaluate the metal(loid) immobilisation potential of the sludgechars for soil remediation. Moreover, nutrient supply was investigated to comprehensively assess the material's benefits for soils. All sludgechars were efficient (up to 100 %) for the removal of metal cations while their efficiency for metal(loid) anions was limited in aqueous solutions. Phosphates and sulphates were identified crucial for metal(loid) capture, based on SEM/EDS, XRD and MINTEQ findings. In soils, important fluctuations were observed for Zn, being partially immobilised by the sludgechars in high-Zntot soils, while partially solubilised in moderate to low-Zntot soils. Moreover, pH showed to be crucial for material stability, metal(loid) adsorption ability and their immobilisation in soils. Although metal(loid) retention was generally low in soils, nutrient enrichment was significant after sludgechar application. Long-term evaluation of the material sorption efficiency, nutrient supply, and ageing in soil environments will be necessary in future studies.

A review on waste biomass-to-energy: integrated thermochemical and biochemical conversion for resource recovery

Integrating thermochemical–biochemical methods overcomes the single-path limits for bioenergy production. This synergy lowers costs and enhances energy sustainability, highlighting waste-to-energy's vital role in the circular economy transition.