Investigating the Feasibility of Co-pyrolysis of Biosolids with Alum Sludge and Agricultural Waste
Creators: name: Rathnayake, Nimesha
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 400403
subject: Chemical engineering design

description: Biosolids, the by-product of wastewater treatment plants, contain high levels of carbon, nutrients, and other beneficial organic matter, making them suitable for many agricultural and non-agricultural applications. However, sustainable management of biosolids is challenging due to the presence of odours and pathogens, as well as the existence of various emerging contaminants, such as per- and poly-fluoroalkyl substances (PFAS), micro-plastics, and pharmaceutical waste. Thermal treatment methods, specifically pyrolysis, and gasification, have gained significant traction as effective techniques for managing biosolids. Despite the many benefits of these methods, the high heavy metal content, low surface area, carbon content, and calorific value of biochar derived from biosolids limit its applications. To address these limitations, the present thesis explores the viability of co-pyrolysis, a modified pyrolysis process. The main objective of this study is to assess the feasibility of co-pyrolyzing biosolids with agricultural wastes and alum sludge to enhance the properties of the resulting products, biochar, oil, and gas. In particular, this thesis investigates the synergistic effects of feedstock in the co-pyrolysis of biosolids in conjunction with understanding the effect of critical process parameters, such as co-pyrolysis temperature, feedstock composition, and mixing ratio. Additionally, the effectiveness of co-pyrolysis in reducing the heavy metal content present in biochar and destructing PFAS components present in biosolids also was investigated in the current thesis.
Co-pyrolysis experiments of biosolids with alum sludge, wheat straw, and canola straw were performed in a fluidized bed reactor at temperatures ranging from 500-900°C with varying co-feedstock to biosolids mixing ratios. The study found that the co-pyrolysis biochar, oil, and gas yields were heavily dependent on the ash content and volatile matter content in the parent feedstocks. Co-pyrolysis with alum sludge and lignocellulosic biomass produced biochar with significantly fewer heavy metals due to the dilution effect. All three feedstocks studied exhibited synergistic effects, which reduced the biochar and oil yields. However, the mechanisms of synergistic effects varied with the co-feedstock type used in the experiment. In co-pyrolysis oil product analysis, the addition of alum sludge slightly reduced nitrogenated compounds, while the addition of wheat straw and canola straw significantly reduced nitrogenated compounds and polyaromatic compounds (PAC) but increased fuel acidity. Moreover, it was evident that co-pyrolysis temperature and mixing ratio were critical process parameters, and 700°C and a 3:1 mixing ratio were identified as the optimum temperature and mixing ratio respectively in the co-pyrolysis of biosolids.
The study also evaluated the effect of co-pyrolysis on the destruction of PFAS present in biosolids, using alum sludge and wheat straw as co-feedstocks at 600°C. The experimental study found that over 98% of total PFAS were destructed during co-pyrolysis experiments. However, the study also observed the formation of new short-chain PFAS components and a few long-chain PFAS components during co-pyrolysis. 
This thesis also determined the heat requirement of pyrolysis and co-pyrolysis using a TGA-DSC analyser and investigated the correlations between heat requirement and feedstock properties. Finally, the thesis conducted a techno-economic and energy analysis of a novel closed coupled pyrolysis and gasification reactor system to perform pyrolysis and co-pyrolysis of biosolids. ASPEN modelling suggested that this system can operate profitably with biosolids and biosolids blended with wheat straw and alum sludge and generate biochar and surplus energy. The proposed process operated profitably even with biosolids with a calorific value as low as 11 MJ/kg. Based on the results of the studies discussed, it is evident that co-pyrolysis is a promising approach to convert biosolids into value-added products with numerous potential applications. However, further research is needed to refine and evaluate this process for commercial-scale implementation.