Recycling of MSWI Bottom Ash: A Review of Chemical Barriers, Engineering Applications and Treatment Technologies

Bottom ash from municipal solid waste incineration is an underutilized secondary resource, which currently gains large attention due to increased landfill costs and the push towards a circular economy. Due to the high concentrations and mobility of pollutants, bottom ash cannot readily replace virgin construction materials. Over the last decade, many research efforts have addressed these issues in view of newly developed engineering applications. However, the required quality of bottom ash varies for each application. In this review we focus on the ternary relationship between engineering applications, chemical barriers/limitations and treatment technologies for municipal solid waste incinerator bottom ash. For each intended engineering application [loose(bulk) construction aggregates; sand, aggregate or cement replacement in concrete; raw material for cement or ceramics] the appropriate treatment technologies are selected to overcome identified chemical barriers. This allows future top-down design decisions, starting from the most promising engineering application of bottom ash. The main chemical barrier for bottom ash recycling as loose construction aggregates is the leaching of heavy metals and/or metalloids. This can be overcome by size separation, carbonation, mild heat treatment or by using mineral additives. In structured concrete, the presence of metallic aluminum or zinc causes early cracking and a high chloride concentration causes corrosion of reinforcement steel. Therefore, recent developments in wet/semi-dry separations facilitated enhanced eddy current separation to remove non-ferrous metals. The washing of bottom ash to remove chloride, is to date the sole technology to prepare bottom ash as raw material for cement kilns. Finally, when bottom ash is used as feedstock for ceramics production, recent knowledge was generated to allow for selecting thermal process parameters in such a way that leaching of both heavy metals and metalloids is minimized.