Transforming waste into green energy

While solar power, wind power and tidal power grab the daily headlines, a vastly different green energy technology is quietly making progress.

Anaerobic waste water digestion technologies respond not only to industry's need to clean up the waste water 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.

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

Apart from 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 methane 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 localised electricity generators that provide either on-site energy or direct it back into local grids to earn electricity and carbon credits.

Technologies such as Global Water Engineering's Raptor technologies can convert almost any organic residue into biogas. 

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.

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 environmentally harmonious 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 ton of COD (chemical oxygen demand) digested anaerobically generates 350 Nm3 of methane, equivalent to approximately 312 litres of fuel oil, or generates about 1,300 kWh of green electricity.

Any factory with a biological waste stream or wastewater with high COD (Chemical Oxygen Demand) can 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, whose company has been involved in more than 300 water and waste water projects around the world.

One of the most recent in Australasia is the new Bluetongue Brewery in NSW, where a water recovery/green energy plant designed to target world's best-practice water reuse standards in the food and beverage industry has exceeded its designers' expectations in its first year of service.

"Most industries have not realised the potential of this green energy cash cow," says Ombregt.

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 facilities.

"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 to 95 percent, bringing the organic load down to regulatory discharge standards for some types of wastewater.

"For more heavily loaded wastewaters, relatively small extra post-treatment steps can further purify the effluent, meeting even the most stringent discharge regulations for water re-use," adds Ombregt.

A wide range of organic residue types can be processed resulting in a conversion of the material to agricultural fertiliser and biogas.GWE is introducing new generation technologies to transform waste water byproducts from an industrial disposal expense into green energy profits.

The 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 built and commissioned more than 75 biogas utilisation plants for clients worldwide.

GWE's Raptor technology stands for Rapid Transformation of Organic Residues. It's a liquid-state anaerobic digestion process that consists of enhanced pre-treatment 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. A wide range of organic residue types can be processed, resulting in a conversion of the material to agricultural fertiliser and biogas.

Raptor technologies are 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, for example corn (silage), various grasses, algae

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

In the Raptor process, the pre-treated and blended substrate slurry is transferred into GWE's Anamix digester that uses energy efficient and low maintenance mechanical mixing. The digester tank comes in sizes up to 12,000m3.

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/m3 per day, and biogas production rates of up to 6.3 Nm3 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 Raptor process is desulphurised 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 desulphurisation and drying are typically not needed. 

Raptor technologies is distributed in Australasia by CST Wastewater Solutions.

[Mike Bambridge is Managing Director, CST Wastewater Solutions.] 

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