A comprehensive analytical study of sludge from water reclamation plants in Singapore for advanced thermochemical conversion processes

Abstract

Rising demand on wastewater treatment caused by population growth, urbanisation and industrialisation was producing increasing amount of sludge residues as byproduct. Innovative management strategy and technology for energy and resource recovery were therefore required to properly handle sludge which consisted significant amount of moisture, ash, nitrogen, sulphur, and heavy metals. Mechanical dewatering, thermal drying, agricultural utilisation, landfilling, and incineration were commonly applied for sludge treatment and final disposal but facing issues on low dewatering effectiveness, high energy consumptions, restrictions on agricultural reuse, limitation on landfill availability, low energy recovery efficiency and generation of flue gas with considerable amount of pollutants. Recently, sludge attracted great interests as a potential feedstock for advanced thermochemical conversion processes such as pyrolysis and gasification which were capable to convert cheap heterogeneous waste mixtures into valuable homogenous products such as synthetic gas, pyrolysis oil and char. However, composition and thermal degradation behaviours of sludge were found to be highly complex and distinctive as compared to other traditional feedstock and therefore created significant challenges in engineering design and system optimisation. To address the need of fundamental research on sludge, a comprehensive analytical study specifically for advanced thermochemical conversion processes was performed. 14 different sludge samples were collected based on type, plant, and batch categorisation from all operating Water Reclamation Plants (WRPs) in Singapore. Existing and proposed characterisation methods were applied and examined by using samples collected. Although no general consensus was achieved on the most efficient conversion method, thermally induced degradations occurred in all thermochemical processes and were considered as primary reactions important to reactor design and process modelling. Therefore comparison study on sludge thermal degradation behaviours and products evolution by using complementary analytical and experimental pyrolysis was carried out. Synthetic sludge study was also performed based on biomass composition analytical procedure and model compounds selection method developed in this study to address issues of viii uncertainty in kinetics modelling caused by heterogeneity of sludge and limited understanding on impacts of sludge composition to thermochemical processes. Qualitative similarities and quantitative variations on thermochemical characteristics and composition of different sludge samples were identified which suggested the presence of waste mixtures with comparable main components but in varying proportions. Oxidation of inorganics (apparent oxidation degree, O D = 47.07 ±19.59%) during ash formation caused deviations on characterisation results of sludge which could be reduced by alternative parameters and comparison bases proposed. Thermal degradation behaviours and products distribution found to be vari...