Solid fuel production through hydrothermal carbonization of sewage sludge and microalgae Chlorella sp. from wastewater treatment plant
Introduction
Currently, wastewater treatment plant (WWTP) has been widely facilitated to lower contaminant discharge and to prevent pollution of natural water system through wastewater collection and treatment, respectively. Many of WWTPs were well-designed to satisfy the certain requirements for water quality of effluent. However, in recent years, the problems of huge amount of energy consuming in the WWTPs while they protect human health and the environment were issued with respect to the energy crisis over the world (Scott et al., 2011). The general energy consumption in conventional WWTP forms about 25–40% of total plant operating costs (Venkatesh and Brattebø, 2011, Panepinto et al., 2016), and the energy consumption of WWTPs in Republic of Korea, a typical semi developed country, possess 0.5% of the national annual electricity consumption (Chae and Kang, 2013). Also noteworthy, the present energy consumption in WWTPs may continue to increase with strengthening regulations according to improvement of quality of life and economic development (Mo and Zhang, 2013).
To overcome encountered high energy consuming problem and to improve sustainability of WWTPs, a concept of energy self-sufficient WWTP has been introduced through energy audit and refinement, aging system and facility replacement, and renewable energy production (Howarth and Monasterolo, 2016). The signification of the concept is deeply related to the interrelationships and dependencies between energy and essential function of WWTPs (Yang and Chen, 2016). Nowadays, optimizing energy efficiency and reducing net energy consumption through energy harvesting from WWTPs receive a great scientific attention (McCarty et al., 2011, Chen and Chen, 2013, Gu et al., 2017). The researches throughout the recent decade have shown that wastewater and biosolids from WWTPs can be used as an alternative source for renewable energy production. They also provided renewable energy production strategies, mostly in the form of gas and heat, to achieve carbon neutralization and to design energy self-sufficient WWTPs.
The waste sludge (i.e., biosolid) has high moisture content so that converting to solid fuel through hydrothermal carbonization (HTC) may be an attractive way to produce renewable energy from WWTP. The HTC is based on a thermochemical reaction, which occurs in the presence of moisture under moderate temperature range (180–350 °C), to convert biomass into multifunctional carbonaceous material (i.e., hydrochar) (Lee et al., 2018). The produced hydrochar can be widely used for the industrial, environmental, and agricultural fields (Lang et al., 2018, Melo et al., 2018, Wang et al., 2018). Among the various applications, the using of hydrochar as alternative solid fuel has benefit due to the improvement of hydrochar properties obtained by HTC compared to those of the untreated biomass in terms of heating value, fuel ratio, and aromatic structure (Kim et al., 2016). However, the hydrochar which made of waste sludge has an obstacle to use for solid fuel because waste sludge commonly has high ash content (Kim et al., 2014). When the ash content in hydrochar is higher than maximum permissible level of regulation for renewable solid fuel, the hydrochar will lose its own value for solid fuel using or need huge investment for facilities to reduce air pollutant emission. In this sense, it is inevitable co-hydrothermal treatment of waste sludge from WWTP with low ash containing material for solid fuel.
The high levels of nutrients (i.e., N and P) are contained in WWTP effluents, and the nutrients can be applied in microalgae cultivation (Chen et al., 2015). It has been demonstrated that many species of microalgae are able to uptake and remove nutrients from wastewater effectively (Wang et al., 2010, Cho et al., 2011). Currently, the integrating algal based wastewater treatment and microalgae cultivation toward energy production received the most attention. In the our previous study (not published), we also confirmed that microalgae (Chlorella sp.) could remove about 90% (w/w) of nutrients (92.2% of TN and 87.8% of TP) in municipal WWTP effluent, and the biomass productivity ratio of microalgae to the sludge was 1:9 (w/w). With regard to the productivity ratio and ash content in biomass, the mixture of microalgae and waste sludge with original productive percentage seemed feasible for potential using as solid fuel through HTC treatment.
This study mainly dealt with the renewable energy production approaches in municipal WWTP, which combine solid fuel production with enabling waste management and nutrient recycling. To this end, the mixture of microalgae cultivated by using municipal wastewater and waste sludge from WWTP was converted into hydrochar under various temperatures (180–300 °C) of HTC treatment. Next, the properties of produced hydrochars were investigated in terms of feasible using for solid fuel, product yield, and energy recovery efficiency. The improvement of hydrochar properties according to blending waste sludge with microalgae was focused. Finally, the properties of hydrochar as solid fuel was compared to that of widely used low ranked coals.
Section snippets
Materials
Primary sludge (PS) was collected from a municipal WWTP (Gwangju, Republic of Korea). The lower ash content and higher calorific value of PS than those of secondary sludge were confirmed in our previous study, thus, the PS was considered as feedstock for the HTC process to produce hydrochar in this study. The collected PS was stored at 4 °C until it was used. Microalgae (Chlorella sp.) was originally obtained from an agricultural research institute in Korea and cultivated using a
Effects of hydrothermal carbonization on biomass properties
PS, Chlorella sp., and mixture samples were converted into hydrochar during the HTC treatment, and the physicochemical properties of hydrochars are shown in Table 1.
The ash content of raw PS was 17.05% and gradually increased up to 33.51% with the increase in HTC treatment temperature (Table 1). In general, ash content of sludge from WWTPs can vary according to sludge types (18.5–34.9% w/w, dry) and the observed ash content of raw PS was in accordance with previous studies (Peterson et al., 2008
Conclusion
PS from the municipal WWTP was blended with Chlorella sp., cultivated using municipal WWTP effluent, and the mixture was converted into hydrochar through HTC. This study focused on the feasibility of using the produced hydrochar as a solid fuel, in terms of suggesting a way of renewable energy production in WWTP. The physicochemical properties and energy-related properties of PS were improved via blending with Chlorella sp.. The temperature about 210 °C for HTC treatment was deemed suitable for
Acknowledgements
This paper was supported by Konkuk University in 2018.
References (38)
Biomass-coal co-combustion: opportunity for affordable renewable energy
Fuel
(2005)- et al.
Estimating the energy independence of a municipal wastewater treatment plant incorporating green energy resources
Energy Convers. Manag.
(2013) - et al.
Enhancing the productivity of microalgae cultivated in wastewater toward biofuel production: a critical review
Appl. Energy
(2015) - et al.
Net energy production and emissions mitigation of domestic wastewater treatment system: a comparison of different biogas–sludge use alternatives
Bioresour. Technol.
(2013) - et al.
Reuse of effluent water from a municipal wastewater treatment plant in microalgae cultivation for biofuel production
Bioresour. Technol.
(2011) - et al.
Combustion properties of some power station biomass fuels
Fuel
(2010) - et al.
Heat of reaction measurements for hydrothermal carbonization of biomass
Bioresour. Technol.
(2011) - et al.
The feasibility and challenges of energy self-sufficient wastewater treatment plants
Appl. Energy
(2017) - et al.
Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior
Appl. Energy
(2013) - et al.
Understanding barriers to decision making in the UK energy-food-water nexus: the added value of interdisciplinary approaches
Environ. Sci. Policy
(2016)