The integrated utilization of wastes in a cement kiln has become an international megatrend. Various hazardous compounds containing in wastes might be enriched in the particulate matter (PM) emitted during the waste disposal and cause adverse environmental consequences. In order to evaluate the potential environmental issues caused by the PM emission from the cement kiln for co-processing wastes, the chemical characteristics and formation mechanism of the PM was investigated for the first time by using a laboratory-scale tube furnace to simulate this process. PM was collected to analyze the major components (heavy metals, inorganic and organic components) and morphologies. The results reveal that volatile heavy metals in municipal sewage sludge (MSS) can be enriched in PM. The process can elevate the levels of sulfate and the organic contents in PM, and the existence of organic moieties such as CNþ and CHNþ suggest that the co-processing of MSS can affect the PM organic compositions. Single particle imaging analyses indicate that PM can be classified into nine types based on their elemental compositions. Further analysis reveals that most particles usually mix with other types of particles and those mixed particles may cause a more serious impact on the environment. Based on the TEM observation, a two-step formation mechanism of PM is proposed. Our findings point out the potential environmental adverse effects and call attentions to a full environment assessment of this new type of cement production process, and also indicates stringent measures should be taken to reduce PM emissions during this process.

Introduction

Municipal sewage sludge (MSS) is a common waste worldwide, and its production has reached 30 Mt/y with an annual increasing rate of 8e10% (Xiao et al., 2017). The accumulation of MSS has become a serious environmental concern. Landfill and incineration are the two most common routes for MSS disposal in China (Zhou et al., 2014). Such methods have their own bottlenecks. For example, the reduction of land resource and undesired emissions (e.g., leachate and landfill gas) to ambient water, soil and air restricts the implementation of landfills (Jin et al., 2017). Incineration can also aggravate emissions of hazardous substances such as heavy metals and some organic species into the environment. The methods of gasification, pyrolysis and digestion have been proposed for the disposal of MSS, but the associated intensive energy consumption, complex operation, etc., may also limit further development of those methods. The integrated utilization of wastes such as MSS in cement kiln has become a promising method and attracts public interests due to its multiple advantages. For example, the MSS contains Ca, Si and Al, which are similar to the raw materials for cement production and the co-processing of MSS in a cement kiln can save natural resources. Dried MSS also contains significant calorific values, which could save energy at the same time. The silicate in the cement clinker such as tricalcium silicate and dicalcium silicate can help stabilize the heavy metals during the high temperature calcination process. Hazardous organic substances might also be decomposed due to the high temperature and long residence time in a cement kiln. The co-processing of wastes in cement kiln can also receive subsidies from the government. Despite aforementioned advantages, this disposal method may pose some environmental problems, such as the emission of particulate matter (PM) into the air.