PCDD/F levels and phase distributions in a full-scale municipal solid waste incinerator with co-incinerating sewage sludge

Highlights

• PCDD/F emission levels in a MSWI with co-incinerating 5% sludge were investigated.

• PCDD/Fs enriched in condensed water could take maximumly 63% of the total concentrations.

• Memory effect of soot particles in SCR might elevate PCDD/F levels in flue gas.

• De novo synthesis should not be considered as the dominant PCDD/F formation pathway.

• Ideally, 63% sewage sludge yielded in a megacity can be disposed in MSWIs.

Abstract

Co-incinerating sewage sludge in municipal solid waste incinerators (MSWIs) is an up-to-date disposal way with great prospects to market. To verify the environmental safety of this disposal method, a field study was conducted in a MSWI which has achieved PCDD/Fs ultra-low emission. PCDD/F phase partitioning characteristics, congener profiles, and the influence of selective catalytic reduction (SCR) were also investigated. PCDD/F emission levels ranged from 0.0031 to 0.0053 ng I-TEQ/Nm³, distinctly lower than the national standard. For tests co-incinerating 5% sludge, PCDD/F emission levels were averagely 32% lower than tests mono-combusting municipal solid waste. The phase partitioning study found that PCDD/Fs enriched in condensed water took a non-negligible proportion of the total concentration in flue gas. The removal efficiency of SCR in tests co-incinerating sludge was averaged at 41.9%. However, in tests without adding sewage sludge, PCDD/F concentrations in flus gas after SCR were increased. It was found that the elevations were mainly attributed to the increase of low-chlorinated PCDF congeners in gas-phase. By inference, memory effect existing in SCR might be responsible for the increase of PCDD/F levels. PCDD to PCDF ratios in most of the sampling points were >1, suggesting that de novo synthesis is not the dominant formation pathway in the studied incinerator. This study verified that co-incinerating sewage sludge in the MSWI would not elevate the emission levels of PCDD/Fs. If all of the yielded municipal waste is incinerated with adding 5% sewage sludge, more than half of sewage sludge can be disposed safely in Shenzhen.

Introduction

With the rapid population growth and urbanization in China, yields of domestic wastewater and sewage sludge have been increasing substantially in megacities. According to the estimation that every 1000 tons wastewater produces approximately 1 ton of sewage sludge, the actual production of sewage sludge (85% moisture content) in 2017 was approximately 70 billion tons in China (National Bureau of Statistics of China, 2018). There are three major ways to dispose municipal sewage sludge, including composting, landfill, and incineration, respectively. Previous studies have reported that sewage sludge contains high levels of microorganisms, heavy metals and undigested organic pollutants (such as polycyclic aromatic hydrocarbons, pesticides, surfactants, by-products of pharmaceutical and personal care products) (Chanaka Udayanga et al., 2018, Liu et al., 2018). Thus, the existence of hazardous constituents in sewage sludge might pose potential risks to the environment and human health under improper disposal. For instance, sea dumping of sewage sludge has been banned by Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (International Marine Organization, 1972), for protecting the biodiversity and health of marine resources; composting could not be applied on fertilizing agricultural land for sludge with high levels of heavy metals; landfill is not suitable for land shortage regions, and improper management of the leachate could pose environmental risks such as groundwater contamination.

Compared to composting and landfill, incineration is the most complete method for sewage sludge disposal, not only reducing the volume and mass, but also eliminating microorganisms such as pathogenic bacteria and recovering the caloricity of sludge (Cartmell et al., 2006, Fytili and Zabaniotou, 2008, Kelessidis and Stasinakis, 2012). However, due to the high moisture content in sludge, this method normally demands large amounts of energy, resulting in high operating cost (Chen et al., 2012, Hospido et al., 2005). Besides, previous studies have reported that incineration can achieve stabilization and resource recovery of sludge (Lin and Ma, 2012). Currently, co-combusting sludge in fossil-fuel power plants and co-processing sludge in cement kilns are two common applications of thermal-disposal (Wang et al., 2018, Yang et al., 2016, Zhang et al., 2013, Zhang and Zeng, 2019, Zhao, S. et al., 2017). However, in most of regions in southern China, hydroelectric generation of power is the major way rather than coal-fired power. This means that the number of coal power plants is very limited in these regions, and thus, co-combusting sewage sludge in coal power plants is relatively difficult to be applied. Besides, cement kilns normally locate in mountainous areas. For megacities with large population density, there might not be cement kilns nearby. Under these limitations, the application of adding sewage sludge in waste incineration industry might be preferred as a relatively to-be-promoted disposal way yet with great prospect on market. However, the feasibility assessment study on co-combusting sewage sludge in full-scale MSWIs is limited, especially on the emission levels of unintentional by-products.

For waste incineration industry, one of the major challenges is how to control the secondary pollution effectively. Air pollution control techniques on regular pollutants such as SO₂, NO_x, and acidic gases are relatively well-developed. However, for heavy metals and unintentionally produced persistent organic pollutants, such as PCDD/Fs and polychlorinated biphenyls (PCBs), excessive emissions are still existing especially under weak supervision. The high toxicities, potential carcinogenic and mutagenic effects of these pollutants have aroused wide concerns both in public and academic (Pagano et al., 2018, Pearson et al., 1997, Rawn et al., 2017; Wuhu Ecology Center, 2016). Previous studies have reported the PCDD/F emission levels of co-incinerating sewage sludge in coal-firing power plants and cement kilns (Conesa et al., 2008, Zhang et al., 2013). However, for the co-combustion of sludge with municipal solid waste (MSW), most studies were conducted in laboratory-scale reactors or through pilot tests (Chen et al., 2018a, Chen et al., 2018b, Mininni et al., 2004, Zhan et al., 2019).

The formation mechanisms of PCDD/Fs in thermal processes have been extensively studied. Two temperature zones have been recognized with different reaction mechanisms: (1) homogenous reaction zone (high temperature, 500–800 °C); and (2) heterogeneous reaction zone (low temperature, 200–500 °C). In low temperature zone, both of surface-mediated precursor reaction and de novo synthesis can lead to the formation of PCDD/Fs (Cormier et al., 2006). Generally, de novo synthesis is recognized as the dominant formation pathway in waste incinerators with the observation of PCDF/PCDD > 1 in flue gas (Chen et al., 2018b, Huang and Buekens, 1995, Moo Been and Chung Han, 1998). However, several recent studies found that PCDF to PCDD ratio is related to the contents of chlorophenols and chlorobenzenes in raw materials. Although study results of these laboratory-based experiments suggested that the ratio might not be an appropriate indicator for PCDD/F formation mechanisms, their experimental conditions were largely different from full-scale waste incinerators. So far, PCDF/PCDD > 1 is still recognized as a decisive indicator for de novo synthesis of PCDD/Fs. With optimizing combustion conditions and the technical progress on air pollution control systems, ultra-low emission of PCDD/Fs has been reported achieved in several newly-built municipal solid waste incinerators (Ma et al., 2019, Song et al., 2019). Since the reaction rate of de novo synthesis is relatively slow, especially under precise control of chlorine content and transition metals in raw feeding materials, PCDD/Fs might be mainly formed through homogeneous reaction in high temperature zone (500–800 °C). Thus, de novo synthesis might not be the absolute dominant formation pathway in MSWIs which have achieved ultra-low PCDD/F emission levels. However, the relevant experimental reports are insufficient, especially in full-scale incinerators.

Apart from the formation of PCDD/Fs in low temperature zone, the chlorinated organics can be destructed as well. Previous studies have reported that in the SCR deacidification tower, PCDD/Fs can be co-decomposed with NO_x through catalysts such as V₂O₅–WO₃–TiO₂ (Finocchio et al., 2006). However, the PCDD/Fs removal efficiencies in practical operation conditions might be different for incinerators with applying various catalysts in SCR system. In addition, soot particles accumulating over time on the inner surface of air pollution control devices (APCDs) could lead to the adsorptive memory effect, and then cause the elevation of PCDD/F levels in flue gas by increasing the temperature in SCR. Thus, for individual MSWIs, it is necessary to investigate the actual PCDD/Fs removal rate and whether the memory effect existing in each APCD sector.

In addition, since the PCDD/Fs inhibition effects of elements such as sulfur, nitrogen, and phosphorus have been widely recognized, adding sewage sludge might reduce the formation of PCDD/Fs in a MSWI. It has been reported that with adding S-/N-inhibitors, the metal catalysts could be poisoned, and the chlorination process would be hindered subsequently (Addink and Altwicker (Addink and Altwicker, 1998, Ke et al., 2010, Ruokojârvi et al., 2001, Ryan et al., 2006). For P-containing inhibitors, the main mechanism is passivating metals, which in detail, NH₄H₂PO₄ could convert CuCl₂ to CuPO₃, blocking the formation of C-Cl bonds (Cao et al., 2004, Fu et al., 2015, Wang et al., 2016). However, it is unsure for a MSWI whether the inhibition effect of sewage sludge is distinct, or the PCDD/F levels would be elevated with co-incinerating sludge, before conducting a field study.

This field study is based on investigations of flue gas PCDD/Fs levels of a MSWI with and without co-combusting sewage sludge. The objectives are (1) to investigate the PCDD/F emission levels of co-incinerating sewage sludge in the MSWI; (2) to track the PCDD/F concentration change in flue gas prior to and after SCR and to understand the possible reasons; (3) to elucidate the dominant PCDD/F formation mechanism through the congener profiles. The ultimate purpose of this study is to verify the feasibility of applying sewage sludge co-incinerating in full-scale MSWIs.