Transforming wastewater into energy

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 the wastewater it discharges to the environment, but also to the need for industry to break free from the cost and pollution of fossil fuels - and the financial penalties, such as carbon taxes, they may face.

Crucially for industry using them, the technologies provide reliable and predictable supplies of base load energies.

Besides doing an outstanding job of cleaning the wastewater, by far 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 burned for heating and boilers.

Where a surplus of gas is collected, it can be fed to local electricity generators that provide either on-site energy or direct it back into local grids to earn electricity and carbon credits.

As a result of their efficiency, anaerobic digestion facilities have been recognised by the United Nations Development Programme as one of the most useful decentralised sources of energy supply as they are less capital intensive than large power plants. They can also benefit local communities by providing local energy supplies and eliminate the need for large and often smelly and environmentally-challenging settling lagoons.

With increased focus on climate change mitigation, the re-use of waste as a resource and new technological approaches which have lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, particularly those of emerging regions where infrastructure investment is high, such as Asia, South America and Eastern Europe.

Anaerobic digestion is a biological process whereby 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, such as lakes and rivers. This natural process, with a helping hand from modern technology, is much more efficient as a waste consumer and converter than aerobic and physicochemical processes.

Modern anaerobic processes concentrate the process in closed reactors, operated under ideal temperature and process control, to optimise waste consumption and, in the process, generate large quantities of methane (CH₄) 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: one tonne of COD (chemical oxygen demand) digested anaerobically generates 350 m³ of methane - equivalent to approximately 312 L of fuel oil or the generation of about 1300 kWh of 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 typically achieve it within two years, says Global Water Engineering CEO Jean Pierre Ombregt.

“Most industries have not realised the potential of this green energy cash cow. 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 agricultural industries, and other primary product processing.

“At the same time they are doing water supplies a big favour because, on average, the removal efficiency of GWE’s anaerobic wastewater treatment installations is as high as 90-95%, 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 re-use.”

GWE is introducing new generation technologies to transform waste water by-products from an industrial disposal expense into green energy profits. GWE’s RAPTOR treatment system for organic residues can convert almost any organic residue or energy crop into biogas, valuable electricity or heat, says GWE, which has successfully built and commissioned more than 75 biogas utilisation plants for clients worldwide.

RAPTOR technologies are particularly applicable to such industries as:

• food waste, such as market surplus, kitchen waste, off-specification fruit and vegetables, and excess crops;
• agro-industry residues, like starch and sugar pulps, vegetable or potato waste;
• industrial residues, such as brewery waste (spent grain), fruit processing waste and paper mill sludge;
• energy crops, eg, corn (silage), various grasses, algae.

The diversity of the material to be processed means a range of different pre-treatments are available, allowing the highest possible conversion efficiency.

The GWE closed anaerobic process systems prevent large quantities of CH₄ being emitted into the atmosphere. With CH₄ being 21 times more harmful than CO₂, such anaerobic wastewater solutions can also qualify for Emission Reduction Certificates for projects in countries listed under the United Nations Kyoto Clean Development Mechanism (CDM) and Joint Implementation (JI) programs.

Besides the economic advantage of anaerobic wastewater treatment, there is clearly also the environmental advantage, significantly reducing carbon footprint. This is achieved not only by supplying renewable energy and thus reducing or even eliminating the use of fossil fuels, but also by replacing more traditional, CH₄-polluting, open lagoons and power-consuming, waste-sludge-producing, traditional aerobic WWTPs.