About the author: Marc Eeckhaut is executive vice president, marketing and technology, for Global Water Eng. Eeckhaut can be reached at [email protected].
While solar, wind and tidal power grab the daily headlines, a vastly different green energy technology is quietly making real progress across the globe.
Anaerobic wastewater digestion technologies respond not only to industry’s need to thoroughly clean up wastewater discharged into the environment, but also to the need for industry to break free from the cost and pollution of fossil fuels.
Crucial to industry using them, these new technologies provide reliable and predictable supplies of base load energies. Besides effectively cleaning the wastewater, the greatest advantage of anaerobic wastewater treatment is the controlled, continuous production of valuable biogas (methane) that occurs during the wastewater treatment. Rather than polluting the atmosphere, this CH4 is fed back into industrial processes to be used for heating and boilers.
When a surplus of gas is collected, it can be fed to localized electricity generators, providing onsite energy, or directed back into local grids to earn electricity and carbon credits. Ideally, technologies can convert almost any organic residue into biogas.
As a result of their efficiency, anaerobic digestion facilities have been recognized by the United Nations Development Program as one of the most useful decentralized sources of energy supply, as they are less capital-intensive than large power plants. They also can benefit local communities by providing local energy supplies and can eliminate the need for large—and often odorous and environmentally challenging—settling lagoons.
Considering the increasing focus on climate change mitigation, along with the reuse of waste as a resource and new technological approaches that have lowered capital costs, anaerobic digestion has attracted attention among governments in a number of countries, particularly those of emerging regions where infrastructure investment is high. Asia, South America and Eastern Europe are prime examples.
Modern Take On An Ancient Process
Anaerobic digestion is a biological process in which bacteria break down organic material into more basic compounds without requiring oxygen as a component of the process. As plant life arose billions of years ago, anaerobic digestion occurred in natural environments where oxygen was absent—such as swamps, water-logged soils, and in ground continuously covered by water bodies such as lakes and rivers. This natural process, with a helping hand from modern technology, is more efficient as a waste consumer and converter than aerobic and physicochemical processes.
Modern anaerobic processes in environmentally harmonious closed reactors—operated under ideal temperature and process control to optimize waste consumption—generate large quantities of methane from the organic materials in the wastewater.
The quantities of methane produced can diminish or even completely replace the use of fossil fuels in the production process: 1 ton of chemical oxygen demand (COD) digested anaerobically generates 350 cu Nm of methane, equivalent to approximately 312 liters of fuel oil, or about 1,300 kWh of green electricity.
Any factory with a biological waste stream or wastewater with high COD can easily use this technology to generate energy. Some companies making the investment have achieved payback within a year. Most companies typically achieve it within two years, according to Global Water Eng. (GWE) CEO Jean Pierre Ombregt. GWE has been involved in more than 300 water and wastewater projects in Asia, Africa, North and South America, Australia, China and Europe.
“Most industries have not realized the potential of this green energy cash cow,” Ombregt said. “They have mainly been focusing on treating their effluent to meet local discharge standards at the lowest possible investment costs. By doing so, wastewater treatment installations have only generated additional operating costs and have never been seen as revenue generators.
“However, applying anaerobic wastewater treatment sheds a whole different light on the cost structure of wastewater treatment infrastructure. It can now actually become a substantial additional source of income for many factories and processing plants throughout the world, including the food, beverage and agro industry, and other primary product processing.”
At the same time, Ombregt said, this new technology is doing water supplies a big favor because, on average, the removal efficiency of GWE’s anaerobic wastewater treatment installations is as high as 90% to 95%, bringing the organic load down to regulatory discharge standards for some types of wastewater. For more heavily loaded wastewater, relatively small extra post-treatment steps can further purify the effluent, meeting even the most stringent discharge regulations for water reuse.
Breaking Down the Process
The RAPTOR (Rapid Transformation of Organic Residues) treatment system can convert almost any organic residue or energy crop into biogas, valuable electricity or heat. GWE has successfully built and commissioned more than 75 biogas utilization plants for clients including Bonduelle, Budweiser, Chang, Carlsberg, Coca Cola, Corn Products Intl., Danone, Nestlé and others. Projects have been completed involving a wide spectrum of technologies, including anaerobic and aerobic wastewater treatment, biogas reuse, renewable energy, green power, carbon credits, bioethanol, biomass to energy, biowaste to energy, sludge digestion and water recycling
The system is a powerful liquid-state anaerobic digestion process that consists of enhanced pretreatment followed by multi-step biological fermentation.
“A RAPTOR plant is a total solution, starting with logistics for handling the energy crop and ending with the production of biogas, green electricity or steam,” Ombregt said. “A wide range of organic residue types can be processed, resulting in an efficient and rapid conversion of the material to agricultural fertiliser and biogas.”
In the process, the pretreated and blended substrate slurry is transferred into GWE’s Anamix digester, which uses energy-efficient and low-maintenance mechanical mixing. The digester tank comes in sizes up to 12,000 cu meters. Optional extras include a foam breaker fan, a scum buster system and a bottom grit trap. The digester tank is fully insulated, heated by recycling the digester content through a special heat exchanger.
Loading rates of up to 10 to 15 kg COD per cubic meter per day and biogas production rates of up to 6.3 cu Nm per digester per day can be obtained in RAPTOR plants, depending on the nature of the substrate.
The digestate from the digester is usually treated in a centrifuge for removal and disposal of non-digestible solids in the form of wet sludge cake, ideal for use as an agricultural fertiliser. Sludge cake drying and pelletising systems are available.
The liquid concentrate from the digester is added to the fresh solid waste in the slurry making stage or recycled to a TAR treatment—or ultimately disposed of in a conventional wastewater treatment plant.
Biogas generated in the process is desulfurized and partially dried, using GWE’s Sulphurix and Gasodrix systems, and consequently used for green power generation. Alternatively, it can be used in a steam boiler for steam production, in which case desulfurization and drying typically are not needed.
For a Greener Footprint
A notable end result of closed anaerobic process systems is that they prevent large quantities of CH4 from being emitted into the atmosphere. With CH4 being 21 times more harmful than CO2, anaerobic wastewater solutions also can qualify for Emission Reduction Certificates for projects in countries listed under the United Nations Kyoto Clean Development Mechanism and Joint Implementation programs.
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