Anaerobic technologies cleanly transform wastewater into green energy

Anaerobic wastewater digestion technologies respond not only to industry’s need to thoroughly clean up the wastewater it discharges to the environment, but also the need to break free from the cost and pollution of fossil fuels. The technologies provide reliable and predictable supplies of base load energies.

Besides doing a good job of cleaning the wastewater, the advantage of anaerobic wastewater treatment is the controlled, continuous production of valuable biogas (methane) that occurs during the treatment. Rather than polluting the atmosphere, the methane (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 localised electricity generators that provide 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.

Ancient process with a modern twist

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 ground continuously covered by water, such as lakes and rivers. This natural process, with the help of modern technology, is much more efficient as a waste consumer and converter than aerobic and physico-chemical processes.

Modern anaerobic processes vastly concentrate the process in environmentally harmonious closed reactors, operated under temperature and process control to optimise waste consumption and, in the process, generate large quantities of 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 Nm³ of methane, equivalent to approximately 312 L of fuel oil, or generates about 1300 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 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 wastewater projects around the world. One of the most recent in Australasia is the Bluetongue Brewery in NSW, where a water recovery/green energy plant designed to target best-practice water re-use 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. 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,” Ombregt said.

“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, 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 wastewaters, 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 technologies to transform wastewater by-products from an industrial disposal expense into green energy profits. The company says its Raptor treatment system for organic residues can convert almost any organic residue or energy crop into biogas, valuable electricity or heat. Raptor stands for Rapid Transformation of Organic Residues. It’s a powerful liquid-state anaerobic digestion process that consists of enhanced pre-treatment followed by multistep biological fermentation.

CST Wastewater Solutions Managing Director Michael Bambridge says 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 an efficient and rapid 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, 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 Raptor pre-treatments is available to allow the highest possible conversion efficiency.

Rapid anaerobic digestion

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 12,000 m³. 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 10-15 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, suitable for use as an agricultural fertiliser. The liquid concentrate from the digester is either added to the fresh solid waste in the slurry-making stage, recycled to a TAR treatment or disposed of in a wastewater treatment plant.

Biogas generated in the Raptor process is desulfurised 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 desulfurisation and drying are typically not needed.

For a greener footprint

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, GWE’s anaerobic wastewater solutions can also qualify for Emission Reduction Certificates for projects in countries listed under the United Nations Kyoto Clean Development Mechanism and Joint Implementation programs.

Besides the economic advantage of GWE’s anaerobic wastewater treatment, there is also the environmental advantage, significantly reducing factories’ 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 by replacing power-consuming and waste sludge producing traditional aerobic WWTPs.