Solar-assisted thermal pre-treatment of waste activated sludge in a tubular photoreactor: Operational and economic insights

Abstract

Anaerobic digestion (AD) is crucial for sludge stabilisation and biogas generation, yet its efficiency is often limited by the slow hydrolysis of waste-activated sludge (WAS). This study explores a novel pre-treatment approach that integrates low-temperature heating with solar radiation in a tubular photoreactorcomprising a borosilicate-glass tube over a reflective surfaceto enhance urban WAS disintegration and boost methane production in AD. Lab-scale experiments (1L) evaluated the effects of temperature (5070°C), radiant power (0.450.83WUV), and reflector geometries. Under simulated sunlight (0.66 WUV), a 3-hour pre-treatment increased WAS disintegration by up to 17%, enhanced EPS release, and boosted the methane production by up to 74%, with a peak of 212±6L CH4 kgVS1 at 70°C (7.6 kJUV L1). These outcomes confirm a synergistic effect between thermal and solar energy input. Pilot-scale experiments (15L) under natural sunlight, without external heating, confirmed an 85% increase in methane production. Solar radiation alone raised WAS temperature from 20 to 63°C in 4h (16.6 kJUV L1) and sustained it for 3 more hours (12.1 kJUV L1), achieving a WAS disintegration degree of 12%. An economic assessment compared two scenarios: 1) one relying solely on solar radiation (28.7 kJUV L1) captured by compound parabolic collectors (CPCs); and 2) another adding external heating (12.1 kJUV L1; 1672 MWh). Although external heating reduces the required CPC area, its high electricity demand diminishes economic feasibility. Consequently, scenario 1 offers a more favourable balance between capital and operating expenses, reducing total yearly costs by 23%.