Plants can be made energy self-sufficient via “bio-refinery” technologies
Anaerobic digestion of biosolids helps produce methane for fuel
by George N Abrahim & Erik Bundgaard
The conventional wastewater treatment plant (WWTP) is perceived as being a cost centre because of the nature of its function. Driven by sustainable development principles, however, the WWTP of the 21st century will be far different: self-sufficient in terms of energy, producing recoverable and marketable substances, and generating less waste.
This will require a paradigm shift and a change in the technological processes used, transforming tomorrow’s installations into a ”bio-refinery”. To achieve this, researchers and developers now focus on the following:
1 Energy optimisation: Power usage is the highest single expense in a conventional activated sludge plant. Computational fluid dynamics (CFD), a computer-based methodology that simulates the hydrodynamics of processes by evaluating flow patterns and local variables, is now used to reduce energy consumption. Recent advances provide insight into the dynamics of multiphase flows, allowing designers to improve efficiencies.
Veolia offers a wide range of alternative green technologies to reduce the use of chemicals in water treatment, such as non-chemical treatment technologies, reverse osmosis systems for blow-down reduction, condensate polishers for recycling and reuse, and brine recovery systems (photo credit: Veoliawater.com)
2 Advanced process control and instrumentation: Adopting advanced process control and operation technologies helps reduce energy usage. Residual dissolved oxygen is often key to optimising treatment performance and energy reduction. Advanced control strategies and decision support tools now efficiently adjust aeration parameters.
The use of the latest online analysis sensors for both nitrates and ammonia parameters also allows operators to monitor the effluent quality in real time, thereby cutting energy consumption by 15–20%.
3 Energy-neutral biosolids management: Thermal drying is one of the most sustainable methods for balancing economic advantages with environmental stewardship. The BioCon Energy Recovery System (ERS), for instance, dries the biosolids using heat generated from biosolid combustion, halving operating costs. It also lowers hauling costs by reducing material transported out of the plant.
In addition, the BioCon Energy Exchange System (BEES) for treatment plants with anaerobic digestion processes makes it possible to use all biogas from the digester to generate heat for the drying process.
4 Advanced anaerobic digestion of biosolids: Many WWTPs use this technique to reduce biosolid volume. Anaerobic digestion produces biogas containing methane, which is used by many Asian and European plants to fuel generators.
There have been changes, however, to this traditional process. Exelys™ technology increases the concentration of solids to be treated, thus reducing the volume to be heated. It can reduce the quantity of sludge produced by 20–50% and increase the production of biogas by 20–50%, so the whole WWTP is virtually energy-neutral.
Operating the Exelys™ system in a digestion-lysis-digestion configuration makes it possible to recover a maximum of the sludge’s energy potential. This optimises the anaerobic digestion process and maximises the production of renewable biogas energy.
At the same time, using digesters poses challenges. Following anaerobic digestion, most plants dewater the sludge, which generates a filtrate concentrated with ammonia-nitrogen. This is often recycled to the head of the plant and treated in aeration basins, representing a significant energy cost. To address this issue, a recently developed application, called ANITA™ Mox, uses a bacterium that performs the task of nitrification without consuming carbon. This is based on moving bed biofilm technology, and uses 60% less energy than the conventional nitrification process.
5 Wastewater reuse and recycling: Recycling and reusing treated wastewater is now playing a growing role in integrated resource management and sustainable development. In areas where new sources of freshwater cannot be developed, recycling wastewater can help satisfy demand in a sustainable manner.
6 Reducing the environmental footprint: The environmental footprint is now used to measure innovations in the development of clean water technologies. Viewing carbon and water together provides a more comprehensive picture of these challenges, especially in the water and wastewater industry.
One recently developed tool that encompasses the complexity of the water cycle is the Water Impact Index (WII). This is the first indicator to enable comprehensive assessment of the impact of human activity on water resources.
Another recently developed tool in Asia, Green2Biz, reconciles environmental sustainability, system performances and financial profitability.
The wastewater treatment plant as a waste refinery
Source: Veolia Water Solutions & Technologies
George N Abrahim is vice-president of strategic initiatives and Erik Bundgaard is director of technology, carbon manager, for Veolia Water Solutions & Technologies