Co-composting of sewage sludge and Phragmites australis using different insulating strategies

Highlights

• Insulating strategy notably influenced the laboratory-scale composting behaviors.

• A centre oriented real-time temperature compensation was applied in the composting.

• Five composting reactors with different insulating strategies were compared.

• Co-composting of sludge and wetland plants is meaningful for the running of WWTP.

Abstract

Insulating strategies are indispensable for laboratory-scale composting reactors, however, current insulation methods interfere with the aerobic fermentation behaviors related to composting. To address this issue, a centre-oriented real-time temperature compensation strategy was designed in this study. Five 9 L reactors (R1–R5) with different insulation strategies were used for the co-composting of dewatered sludge and Phragmites australis and compared. The process performance was assessed by monitoring the temperature, O₂ and CO₂ emissions, the physical–chemical properties of the composting materials were evaluated by measuring the organic matter (OM), carbon nitrogen ratio (C/N), pH, electrical conductivity (EC), and fluorescence excitation-emission matrix (EEM) spectra. And a 16S rDNA analysis was used to quantify the evolution of bacterial community. The main findings are as follows. Compared with R1 as a control, the insulating strategies can increase the maximum temperature and prolong the thermophilic phase of composting. Comparing R1 and R3 showed that real-time temperature compensation can better restore the real fermentation of the compost. The results showed that R5 had the best composting effect, reaching 69.8 °C, which was 25.1%, 29.7%, 19.3%, and 17.3% higher than R1, R2, R3, and R4, respectively, and remaining in the thermophilic phase for 4.24 d, which is 1.4, 1.5, 1.3, and 0.2 times longer than R1, R2, R3, and R4, respectively. Furthermore, it can significantly reduce the temperature difference between the centre and edge of the reactor, which improved the composting material allocation efficiency and composting process control accuracy, further providing a basis for the actual full-scale composting operation.

Introduction

Composting is widely used for converting organic solid wastes into a stable and safe compost that can be reused in agriculture (Meng et al., 2018). The aerobic composting process involves physical, chemical, and biochemical reactions, which are affected by factors such as temperature, particle size, and moisture content (MC). Composting experiments are necessary for determining optimal composting conditions. However, a full-scale composting experiment is expensive, labor intensive, and difficult to control. Thus, laboratory-scale composting reactors, which are easier to operate and can track the composting process, are widely applied.

Laboratory-scale composting results often vary widely due to different equipment and environmental conditions. This is mainly due to the limited amounts of organic substrates and easier heat losses; hence, a rapid drop in temperature is typically observed in small reactors (Petiot and de Guardia, 2004). Therefore, insulating strategies are widely used in laboratory-scale composting reactors.

As shown in Table S1, many laboratory-scale composting reactors (≤100 L) that are commonly used to study the aerobic fermentation behavior of various organic waste materials have been described in the literature. The larger the volume of the composting reactor, the simpler the insulation method needed, such as using polyurethane foam, or mineral wool. With a decrease in composting reactor volume, it becomes more difficult for the reactor to achieve and maintain high-temperature composting conditions through self-heating. In this case, covering it with insulation material is no longer applicable; most solutions applied to reduce the heat loss from the wall involved placing the reactor in a constant temperature incubator or a water bath for heat losses. However, applying a fixed temperature throughout the entire composting process typically causes artificial conditions, which greatly influence the aerobic fermentation behavior of the organic waste, and even becomes a prerequisite for the start-up and operation of the system, a point which few studies pay attention to. Furthermore, the strong contrast between the temperature inside and versus outside of the compost pile is one of the most challenging aspects of composting. The results from Lau et al. (1992) showed that the vertical gradient between the centre and surface probes (a distance of approximately 30 cm) of the 55 L reactor is greater than 5 °C, (Elwell et al., 1996), indicating that there is a temperature gradient of up to 25 °C between the top and bottom of the 208 L reactor. The temperature difference between the inside and outside of a laboratory-scale composting reactor results in the uneven fermentation of materials and unrepresentative sampling. Hence, it is necessary to improve the consistency between the centre and the edge temperatures in laboratory-scale composting reactors.

In this study, dewatered sludge, an inevitable by-product of the wastewater treatment process (Bai et al., 2018), was selected as the raw material for composting. The dewatered sludge has the characteristics of a high MC (approximately 80%), viscosity, and the low carbon nitrogen ratio (C/N) (Huang et al., 2015), making it unsuitable for composting alone. Therefore, the dewatered sludge should be mixed with dry materials which are rich in carbon, such as sawdust, biochar, wheat straw and green wastes, to adjust the MC and C/N ratio to an appropriate range (Arias et al., 2017). In this study, Phragmites australis was used as a bulking agent and carbon source that was mixed with the dewatered sludge for composting, to adjust the MC, C/N ratio, and maintain the air spaces. These raw materials were obtained from the Yanghu wastewater treatment plant located in Changsha, China. The plant has the capacity to treat 120,000 m³ of domestic wastewater per day, and it adopts the combined treatment technologies of a modified sequencing batch reactor, micro-flocculation filtration, constructed wetland and disinfection. The constructed wetland, spread over 114,136 m², is mainly vegetated with Phragmites australis, Typha orientalis, and Iris pseudacorus. These wetland macrophytes will be harvested twice a year, and the annual output of the biomass can reach up to 1500 t. Therefore, the co-composting has provided a potential approach for the sustainable management of the by-products from this plant.

Five kinds of composting reactors (R1–R5) with different insulating strategies were utilized in this study, including one set to room temperature without artificial insulation (R1), one with a thermostat incubator at 30 °C (R2), and three with real-time temperature compensation with maximum heating thresholds of 30 °C, 50 °C, and 70 °C, respectively (R3–R5). The effects of different temperature compensation methods on the reactors were then compared. In summary, the purpose of this study is to design a small compost reactor system with a centre oriented real-time temperature compensation strategy that allows the wall temperature of the reactor to follow the centre temperature of the substrate to achieve more accurate and realistic temperature compensations. The study aims to restore the heat and mass transfer process in the core region of the aerobic fermentation with fewer organic solid wastes, simulate the thermal effects often observed in full-scale composting, and explore the impacts on composting through chemical and biochemical indicators.