Anaerobic digestion (AD) is a key technology for energy recovery in wastewater treatment plants, converting organic matter into methane-rich biogas. However, its efficiency is constrained by slow reaction rates, particularly during hydrolysis and methanogenesis, necessitating large reactor footprints for effective sludge digestion. Alternative AD configurations for process intensification present a promising solution to address these limitations by altering the design and operational setup of the AD process. In this review, key configuration-based AD intensification strategies were systematically analyzed, including recuperative thickening, single-stage thermophilic AD, acid/gas two-stage AD, temperature-phased AD, and multi-stage AD systems. The mechanisms, governing factors, efficiency gains, and scalability of these technologies were critically examined. These configurations demonstrated substantial improvements in methane production rates, process intensification, and the removal of solids and organics. Single-stage thermophilic and cascade AD technologies showed the highest potential for full-scale implementation, supported by successful real-world applications. Conversely, recuperative thickening exhibited promising results at lab and pilot scales but remains limited by its lower technology readiness level. Furthermore, the integration potential of such alternative systems with other intensification technologies was explored, highlighting synergistic opportunities for further optimization. This review provides critical insights into means to intensify AD process through alternative process configurations, offering a comprehensive guide for their application in biogas upgrading. It also identifies key challenges and outlines actionable steps to advance these systems toward widespread adoption in full-scale AD operations.

Phosphorus recovery from wastewater can improve resource circularity while reducing eutrophication risks. This study directly compared calcium- and magnesium-modified commercial biochar for phosphate capture from a controlled synthetic solution and real anaerobic digestion centrate. Biochar was modified with Ca and Mg salts at biochar:salt mass ratios of 1:1–1:20 and characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) before and after adsorption. In synthetic solution, 24-h experimental adsorption capacities reached 55.8 and 24.6 mg P gBC⁻¹ for Ca- and Mg-modified biochar, respectively. In centrate, the selected 1:5 Ca-BC and Mg-BC conditions achieved 83.6 and 54.5 mg P gBC⁻¹, corresponding to calculated phosphate-P removals of 69.6% and 45.4%, respectively. Ca-modified biochar showed stronger phosphate-associated surface changes and higher measured phosphate uptake under the tested conditions, while FTIR, SEM-EDS, pH behavior, and kinetic modeling were interpreted as supporting evidence of adsorption behavior rather than definitive identification of specific mineral phases or molecular-scale mechanisms. The 1:5 biochar:salt ratio was selected because higher modifier loadings did not provide proportional gains in phosphate capture. Phosphate-loaded Ca- and Mg-modified biochars achieved 100% germination for wheat and lettuce, with no evidence of early phytotoxicity. Overall, Ca-modified biochar showed higher phosphate-capture performance, while both phosphate-loaded materials demonstrated preliminary germination compatibility; phosphorus release, leaching, and plant phosphorus uptake were not evaluated.