Sulfur dioxide and o-xylene co-treatment in biofilter: Performance, bacterial populations and bioaerosols emissions
Graphical abstract
Introduction
With increased treatment rates and treatment capacity of municipal sewage treatment plants, the production of sewage sludge with 80% moisture content from wastewater treatment plants reached up to 27.86 million tons in 2016. Besides moisture, organic matter, and inorganic salts, the sludge contains hazardous substances such as pathogens and heavy metals, which pose threats to human health and the environment. Sanitary landfills, composting, and incineration are three main technologies that are frequently used in sludge treatment. Among these methods, sludge incineration is the most effective and reliable technology because it minimizes the sludge volume and recovers dry sludge. The pathogens and other toxic compounds can be destroyed during the incineration process. Co-processing of sewage sludge in cement kilns is an emerging technology, characterized by feeding the sewage sludge into the cement kiln for incineration (Lv et al., 2016). The moisture content of the sludge should be reduced to below 30% before incineration in the cement kiln. Thermal drying by waste heat of the cement kiln exhaust gases is an economic way of sludge desiccation. In Europe, particularly in Germany, the sludge drying method has been frequently applied in cement kilns and coal-fired power stations (Kelessidis and Stasinakis, 2012). This environmentally friendly disposal method has attracted increasing attention in China and has been employed in Beijing, Shanghai, and Guangzhou in recent years.
In the sludge drying process, the temperature reaches up to 290°C to sterilize the sludge and destroy its gel structure. Large amounts of off-gases containing odors and volatile organic compounds (VOC) are released into the atmosphere during the process, which cause odorous air pollution that is harmful to human health and the environment. SO2, NH3, and VOC are frequently present in the emissions from the sewage sludge desiccation process. Effective treatment technologies are needed to reduce these emissions. Biofiltration is a method with low operating costs and small amounts of hazardous residual waste, and is efficient in the purification of off-gases with large flow rates and low concentrations. Biological technologies were used to remove hydrogen sulfide, mercaptans, ammonia, and amines from air. Approximately 10 years later, this technology has been extended to the removal of VOC, volatile organic sulfur, and SO2. Several studies have reported effective elimination of aromatic VOC, including BTEX by these technologies (Jeong et al., 2009, Rahul et al., 2013, Wang et al., 2013). Philip and Deshusses (2003) developed a combined bioreactor system containing a biotrickling filter followed by a biological post-treatment unit to treat SO2 from flue gases. Their results showed that nearly 100% SO2 removal efficiency was achieved by using this treatment system. Although biofilters have many advantages, there are still some problems with those methods and the performance of biofilters in the treatment of gases containing complex pollutants needs further improvements. In addition, biofilters inhabited by bacteria and fungi are sources of microbial bioaerosol emissions (Martens et al., 2001). Because of arising public concern about such emissions and possible health risks to the exposed people, further research should be conducted to understand the degree of emissions from airborne microorganisms in biofilters during the odor or VOC treatment processes.
In the present study, a laboratory-scale biofilter was established to treat the waste gas containing SO2 and o-xylene. SO2 and o-xylene were selected as representatives of sulfur-containing compounds and monoaromatic hydrocarbon compounds that are usually present in the sludge-drying tail gases. The performance of the biofilter was evaluated for over 6 months by varying the inlet load of o-xylene and SO2. The microbial population in the biofilter was analyzed using PCR-DGGE (denaturing gradient gel electrophoresis) methods to investigate the relationship between microbial characteristics and the removal of waste gas mixture with o-xylene and SO2. Bioaerosol emissions from the biofilter were also studied.
Section snippets
Experiment and procedure
The co-treatment of SO2 and o-xylene was carried out in a biofilter with a height of 2.5 m and a diameter of 0.2 m (Fig. 1). The effective volume of the biofilter was 0.056 m3. Polyurethane foam cubes with 97% of porosity were packed in the biofilter column. The volume of each cube was approximately 1 cm3. The packing density was 12–15 kg/m3. Bacteria or other microorganisms were grown on polyurethane foam cubes. SO2 and o-xylene were selected as representative compounds to be treated by the
Performance of the biofilter for SO2 and o-xylene removal
The evaluation of the biofilter performance was conducted for over 6 months by step-varying the inlet loads of SO2 and o-xylene. The entire operation period was divided into 5 stages according to the SO2 and o-xylene loads. Stages I, II, III, IV, and V lasted for 32, 34, 32, 38, and 50 days, respectively. The operation time, average concentration, and removal efficiency of SO2 and o-xylene in each operational stage are summarized in Table 1. At Stage I and Stage II, SO2 was the sole pollutant for
Conclusions
The results of this study indicated that SO2 and o-xylene could be co-treated effectively in a single biofilter. The removal of SO2 and o-xylene in the biofilter was dependent on their concentration ratio in the inlet stream. The removal of o-xylene was influenced by the amount of SO2. At same time, the presence of o-xylene at low concentrations enhanced the SO2 removal efficiency.
SOB and XB thrived and proliferated vigorously in the biofilter when SO2 and o-xylene were the main components to
Acknowledgements
The authors express their sincere acknowledgements to Editage for the English revision of the manuscript. The work was financially supported by the National Natural Science Foundation of China (Nos. 51478456 and 51308527).
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