Energy recovery from industrial wastewater sludge: Multi-sectoral evaluation of calorific value and heavy metal content

Industrial wastewater sludge represents a significant environmental burden due to the increasing volume generated and the limitations of conventional disposal methods such as landfilling and incineration. As industries seek sustainable and cost-effective waste management alternatives, energy recovery from sludge has emerged as a promising solution. This study investigates the potential of industrial wastewater sludge as an alternative solid fuel by evaluating the physicochemical characteristics of six sludge types collected from different industrial sectors: Beverage Production, Beverage Recycling, Dairy Processing, Flavour Industry, Oleochemical, and Paper Processing. Comprehensive analyses were conducted, including MC determination, proximate analysis, calorific value measurement using bomb calorimetry, thermogravimetric analysis, and heavy metal quantification via Flame Atomic Absorption Spectroscopy. The calorific values ranged from 11.85 to 31.31 kJ/g, with Dairy Processing wastewater sludge recorded the highest average HHV at 31.308 kJ/g, placing it within the typical coal range (26.50–34.63 kJ/g). Meanwhile, Beverage Production, and Oleochemical sludges showed lower energy content than coal but surpassed both charcoal benchmarks (19.09 and 20.09 kJ/g). Proximate analysis revealed high volatile matter and fixed carbon content, further supporting their suitability for thermal energy recovery. However, sludges with high MC, particularly from beverage industries, require pre-treatment for efficient combustion. The study underscores the viability of industrial sludge as a renewable energy source and recommends optimization of treatment processes.