Effects of chemical additives on emissions of ammonia and greenhouse gas during sewage sludge composting

https://doi.org/10.1016/j.psep.2020.05.056Get rights and content

Highlights

  • Magnesium chloride and ferrous sulfate addition for co-composting were studied.

  • Addition of magnesium chloride and ferrous sulfate decreased pH.

  • Addition of magnesium chloride and ferrous sulfate increased ammonium content.

  • Magnesium chloride addition reduced CH4 and NH3 emissions by 22.9 % and 58.3 %.

  • NH3 and CH4 emissions decreased by 82.9 % and 24.9 % with ferrous sulfate addition.

Abstract

Chemical additives of magnesium chloride (MaC) and ferrous sulfate (FS) were investigated in sewage sludge composting. A treatment without chemical additive was used as the control group (CK). The effects of MaC and FS on ammonia (NH3) and greenhouse gases (CO2, N2O and CH4) emissions were evaluated. The results showed that the addition of MaC and FS promoted the degradation of organic matter and increased the ammonium concentration of the compost product. The maximum pH values decreased from 8.98 (CK) to 8.49 and 8.0 in MaC and FS treatments, respectively. In addition, the NH3 emission for the MaC and FS treatments reduced by 58.3 % and 82.9 %, respectively, whereas the methane (CH4) emission decreased by 22.9 % and 24.9 %, respectively. However, in FS treatment, the carbon dioxide (CO2) and nitrous oxide (N2O) emissions were higher than those in CK. Furthermore, the FS treatment exhibited lower germination index values (71.4 %) and higher electrical conductivity (6.62 mS/cm). Therefore, MaC is suggested as an additive for reducing greenhouse gas emission and conserving nitrogen during sewage sludge composting.

Introduction

In recent years, the number of wastewater treatment plants (WWTPs) has gradually increased, and sewage sludge (SS), which is the inevitable byproduct of wastewater bio-treatment process, has become a sensitive environmental issue (Raheem et al., 2017). According to Cai et al. (2016), in 2014, approximately 129 million tons of municipal wastewater was treated daily in WWTPs, and during this process, approximately 30 million tons of SS was produced annually in China. SS contains a high concentration of pollutants, such as heavy metals, pathogenic microbes, and organic micropollutants, which harm human health by polluting soil, water resources, and ambient air (Han et al., 2018). Additionally, SS includes massive organic matters and plant nutrients. Therefore, a reasonable disposal method cannot only resolve the derivative environmental problem of SS, but also provide approaches for resource reutilization.

Numerous chemical and biological methods are available for SS treatment, including aerobic composting, incineration, anaerobic digestion, and landfill. Among these methods, composting is an effective option for reducing the volume of SS discharge and simultaneously converting SS into value-added fertilizers (Muktadirul Bari Chowdhury et al., 2013). It may be defined as the biological decomposition and stabilization of organic substrates to produce a final product that is stable, free of pathogens and plant seeds, and can be beneficially applied to land (Chen, 2012). However, during composting process, NH3 and greenhouse gas are emitted because of the decomposition of organic waste (Wang and Zeng, 2017). The emissions of undesired gases such as CO2, CH4, N2O, and NH3 can cause secondary pollution and reduce the agricultural value of the compost product (Piippo et al., 2018). NH3 and N2O emission is the main reason for nitrogen loss during the composting process. It was reported that approximately 20 %–60 % and 0.1 %–9.9 % of the total nitrogen has been lost by NH3 and N2O emissions, respectively (Chen et al., 2018; Jiang et al., 2018a, 2018b).

Many approaches have been applied to mitigate nitrogen loss and promote composting efficiency, including using different bulking agents, using chemical or mineral additives, changing aeration frequency and temperature, and adding absorbent materials (Awasthi et al., 2016a; Chen et al., 2018; Han et al., 2018; Wang et al., 2018). Among these methods, using chemical or mineral additives is considered effective owing to engineering practicability and the associated effective composting process (Chen et al., 2015; McCrory and Hobbs, 2001; Pan et al., 2018). The use of composting additives primarily focuses on struvite formation and pH adjustment to conserve nitrogen. For example, Wang et al. (2013) discovered that the addition of Mg and P salts effectively reduced nitrogen loss from 40.8% to 23.3% by the formation of struvite during food waste composting. Wang et al. (2016) investigated the effect of lime addition on N conservation through struvite formation during food waste composting. The author indicated that although the addition of lime did not affect struvite formation, > 2.25 % lime resulted in more nitrogen loss due to the high pH.

A higher pH (> 8) can accelerate NH3 emission (Ekinci et al., 2000; Jiang et al., 2014; Matsumura et al., 2010). Kirchmann and Witter (1989) reported that the pH value in equilibrium between ammonium and NH3 gas is a key factor for regulating NH3. The authors indicated that NH3 loss rapidly decreased below pH = 7 and increased for an initial pH above 8. Several studies recommended avoiding or decreasing the use of alkali additives during composting to reduce NH3 emission (Fang and Wong, 1999; Pan et al., 2018).

Therefore, acidic substances, including phosphogypsum, apple pomace, calcium superphosphate, citric acid, elemental sulfur, phosphoric acid and calcium superphosphate, have been used as pH regulators to control nitrogen loss (Jiang et al., 2014; Lim et al., 2017; Luo et al., 2013; Pan et al., 2018). The type and composition of acidic additives are crucial for their practical application. Some acid additives have been proven to be questionable; for instance, although metal sulfur and phosphoric acid can reduce NH3 emission, the quality of composting product is low (Pan et al., 2018). Yuan et al. (2015) discovered that the presence of FeCl3 decreased the amounts of NH3 and H2S emitted during municipal kitchen waste composting, whereas the NH4+N concentration of final composts increased significantly and hence decreased the compost maturity. Pan et al. (2018) indicated that the addition of elemental sulfur limited organic matter degradation because of the low associated pH (< 6.0). Although these acidic additives are effective in mitigating N loss, microbial activity can be inhibited and the ion concentration of composts can be increased (Husted et al., 1991; Yuan et al., 2015). Using inappropriate composting additives may yield adverse effects on the composting process and compost safety. Hence, despite the additive or composting substance, the application of acidic additives will be affected.

In the present study, the chemical additives of magnesium chloride (MaC) and ferrous sulfate (FS) were used as pH regulators during SS composting; they have been widely used in agriculture, food, decontamination, and addressing polluted water. To the best of our knowledge, a systematic comparative evaluation of the use of MaC and FS on SS composting quality improvement, N conservation, and greenhouse gas emission is unavailable. Therefore, the aim of this study is to evaluate and compare the effects of MaC and FS on NH3 and greenhouse gas emissions as well as analyze the correlation between physiochemical properties (organic matter, pH, total kjeldahl nitrogen, temperature, electrical conductivity, ammonium and nitrate) and greenhouse gas and ammonia emissions during the composting.

Section snippets

Composting materials

Dewatered SS was collected from a WWTP (Henan Province, in China), and sawdust was collected from a local wood processing plant and was crushed with a disintegrator to less than 1 cm before being mixed with SS as a bulking agent. The SS and sawdust (dry weight) were mixed at a 1:1 ratio to maintain the C/N ratio at approximately 20 and moisture content 60 %. The MaC and FS added to the mixture were of chemically pure grade. The basic properties of initial materials are presented in Table 1.

Experimental design and lab–scale composting methods

The

Effect of additives on temperature, OM, O2, and pH

Temperature can reflect the material properties and microbial activity during composting; it is also an important indicator to assess whether aerobic composting has been completed successfully. As shown in Fig. 1a, the temperature changing trends of treatments MaC and FS indicated no significant differences; however, both of them were higher than that of CK. All composting treatments went through the three typical degradation phases: the mesophilic, thermophilic, and curing phases. The

Conclusions

This study indicated that the addition of acidic additives promoted the degradation of OM and increased the temperature and O2 consumption during SS composting. The pH of MaC and FS addition treatments decreased significantly, whereas the NH4+-N content of the compost increased by 60.4 % and 66.3 % in MaC and FS treatments, respectively, compared with that of CK. The addition of MaC and FS decreased NH3 emission by 58.3 % and 82.9 %, respectively; and decreased CH4 emission by 22.9 % and 24.9

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51508167 and 41805123), the Key Scientific Research Project of Universities in Henan Province (16A560022 and 17B610006) and PhD research startup foundation of Henan Normal University (5101219170108 and 5101219170113).

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