|
|
| Fig. 1: Sludge from a wastewater treatment plant. (Source: Wikimedia Commons) |
Urbanization and industrialization have increased the amount of sludge produced. Sludge is the byproduct of wastewater and sewage treatment (see Fig. 1). Today, large amounts of sludge are generated on a daily basis by cities around the world. China, in particular, has produced approximately 40 million metric tons of sludge through 2015. [1]
Sludge can cause environmental damage when it is dumped under landfills or is discharged into rivers or coastal waters. In 2009, sludge buried in Shenzhen, China, came bursting up from the ground. The polluted matter traveled to a river nearby, going all the way to Hong Kong. [2]
Thus, technology that converts sludge to energy kills two birds with one stone.
Two cities on opposite sides of Earth are leading the way for recycling sludge to generate energy: Davyhulme, UK, and Xiangyang, China. Sludge recycling facility in Davyhulme is the largest in the world, handling the waste created by 1.2 million residents of Manchester. Inside the plant, sludge is fed into a huge metal container that exerts high pressure and temperature to the matter; after a complex process that resembles pressure cooking, toxic organisms die, and biogas and clean sludge is produced. The natural gas produced is sold back to fuel the UK's power grid. [3]
Xiangyang is one of the first cities in the developing world to invest in anaerobic digestion, and methane capture and utilization. A Chinese firm called Toven decided to build a sludge treatment plant near Xiangyang, and now it uses sludge produced by the city's 2 million residents to produce compressed natural gas. The natural gas is sold to gas stations, used to fuel homes and cars. [1] While creating enormous revenue, the plant has reduced air pollution within the city by efficiently degenerating sludge and organic waste. By implementing the technical processes of high temperature thermal hydrolysis, anaerobic digestion, and methane capture and utilization, and by producing and using biochar soil, Xiangyang project actually recovered and reutilized the nutrients in sludge and avoided pollution of water bodies. Thus, according to the World Resources Institute, this project grasped the opportunity to recover bio-energy (biogas) from sludge and kitchen waste, helping cities move toward clean energy, and reducing greenhouse gas emissions associated with sludge and kitchen waste treatment. [1]
The amount of sludge generated per person per day is same around the world - about 0.07 kg/day of dried solids of sludge per day. The energy extraction from this mass is maximized if you simply burn it. Since the energy content of sludge is about that of wood, sludge energy content per person per day is
Since 1 kWh is 3.6 × 106 joules, this amounts to about 0.5 kWh/day, the value of which in rich countries is about $0.05. Because the steam generators that make the electricity are about 50% efficient, this is about $0.03. Taking off the energy lost due to the heat dissipation during energy transfer from various reducing matters to methane, the energy consumption for maintaining the microbial activity and, the residual reducing matters in the wastewater after treatment, 80% of the chemical energy contained in the original reducing matters can be transferred into methane.[6] Considering that only around 35% of the chemical energy of methane can be converted into electricity through combustion process, the overall energy recovery efficiency is around 28%. According to Fu et al., the Xiangyang project will produce 45.4 million m3 of natural gas during the 21 years of operation. [1] This natural gas will be compressed to replace about 60,000 m3 of gasoline, reducing emissions of CO2e by an extra 140,000 metric tons. [4]
As human life becomes more comfortable, people waste huge amounts of food and also use more detergent, which causes serious water pollution. Also, as the economy grows, the waste water from the factories leads to water pollution. Sewage treatment can eliminate sand, sticks and debris in sewage and also microorganisms. It can decompose, eliminate and purify wasted materials. Therefore, sewage treatment is critical for good sanitation and the maintenance of water quality. High temperature thermal hydrolysis technology, which converts sludge into natural gas, will help solve the environmental and energy crises in the future. It can benefit all parties, including municipal governments, firms, households, and the environment. Sludge can be black gold. However, we have to also consider that extracting energy from wastewater cannot affect the overall energy consumption structure in the world, since it only contributes a very small portion of the total energy consumption, even with 100% energy recovery efficiency. [5]
© Dahee Chung. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] X. Fu et al., "Sludge to Energy: An Environment-Energy-Economic Assessment of Methane Capture from Sludge in Xiangyang City, Hubei Province," World Resources Institute, March 2017.
[2] M. Griffiths et al., "Water Management," EC-Link, March 2017.
[3] R. Sutton et al., "Davyhulme WwTW Modernisation Project," UK Water Projects, 2016.
[4] P. L. McCarty, J. Bae and J. Kim, "Domestic Wastewater Treatment as a Net Energy Producer - Can This be Achieved?" Environ. Sci. Technol. 45, 7100 (2011).
[5] X. Xie, "Energy From Wastewater," Physics 240 Stanford University, Fall 2011.
[6] A. Karagiannidis et al., "Evaluation of Sewage Sludge Production and Utilization in Greece in the Frame of Integrated Energy Recovery.," Desalin. Water Treat. 33, 185 (2011).
