Wastewater treatment of sewage – and some forms of industrial wastewater – introduces oxygen in order to promote microbial action. This transforms harmful pollutants into waste products that are safe to reuse or reintroduce to the water cycle. The activated sludge process is commonly used for the microbial breakdown of sewage. A new form of this process, called the Nereda process, claims to reduce energy bills and require a smaller area than traditional methods. It has been installed in at least four sites in the UK: Kendal, Failsworth and Morecambe, as well as Davyhulme (Manchester).
The Nereda process is an anaerobic wastewater treatment technology that purifies water using granular biomass. This technology was invented by the Delft University of Technology in the Netherlands and developed in a public-private partnership between the University, the Dutch Foundation for Applied Water Research (STOWA), the Dutch Water Authorities and Royal HaskoningDHV, an international engineering consultancy with headquarters in Amersfoort, Netherlands.
It can be used in a range of applications, including Greenfield sites, hybrid capacity extensions or new-build and retrofitted plants (to convert existing conventional CAS or SBR plants to Nereda technology). As a result of the retrofitting of a facility to Nereda technology, the capacity of existing plants can be easily doubled or tripled, while improving effluent quality at the same time. This can be achieved at only half of the energy costs of conventional treatment methods and no chemical usage as the process is entirely biological. The end result is a treatment works with significantly lower operating costs, a lower carbon footprint and cleaner discharge.
CONVENTIONAL PROCESS
The activated sludge process is used to oxidise carbonaceous and nitrogenous matters in wastewater. For sewage, this is carried out after screening and primary settlement in sedimentation tanks. Aerobic microorganisms digest the organic material, producing biological ‘flocs’ that can be settled out of the water. A floc is a type of biofilm that forms as a suspended cloud of cells, which do not require a surface on which to grow.
Most commonly, the process is used for carbonaceous matter. In this case, the process has two main stages: An aeration tank where the wastewater is mixed with air to promote the microbial activity and a settling tank (‘final clarifier’ or ‘secondary settling tank’) where the biological flocs settle as a ‘sludge blanket’. The breakdown of nitrogenous matter requires additional steps. High quality treatment of sewage therefore requires different process conditions, with pumps and mixers used to ensure the correct circulation at each stage.
An issue with the activated sludge process is that the biological flocs do not readily settle. The typical settling rate is about one metre per hour. This means that large settling tanks are required, which uses a significant amount of land.
THE NEREDA PROCESS
The Nereda process is a greatly simplified and more compact biological treatment process. Instead of the microbes growing as a floc, suspended in the wastewater, they grow on granules within the wastewater. One of its most useful characteristics is that the process can operate in a variety of temperatures and climatic conditions. The first operational plant was built at Epe in the Netherlands in 2011. Such is the technology’s emergence, there are now over 70 Nereda plants either completed or in development as far afield as the United States, South Africa, Brazil, India, China, the Philippines and Australia.
René Noppeney, global director of Water Technology Products & Innovation at Royal HaskoningDHV, is a central figure in the Nereda technology success story: “We initially trialled Nereda technology on smaller-scale pilot plants, to prove its effectiveness and demonstrate the ease with which it can be installed. Once these were successful and the data backed up our assertions, we were able to take the process worldwide. We have seen the Nereda technology implemented in over 70 wastewater treatment facilities across Europe and throughout the world. To install Nereda technology, we sometimes work with trusted local partners and licensees or deliver the technology directly to the water authorities.”
Granular sludge has greatly improved settling characteristics. The settling rate is approximately 15 metres per hour – 15 times faster than a conventional activated sludge. The Nereda process uses these improved settling characteristics to operate at higher concentrations. The need for final settling tanks is completely removed, with the entire process taking place in a single tank. This can enable an existing site to operate at higher capacity. Alternatively, new sites can be constructed more economically, with much smaller tanks and using smaller areas of land.
Another advantage of the Nereda process is improved treatment quality. Because oxygen does not fully penetrate the granules, there are oxygen-rich and oxygen-poor regions within each granule. Multiple biological processes, such as treatment of both carbonaceous and nitrogenous matters, can therefore be performed simultaneously. The multiple stages normally required for high-quality treatment are combined into a single tank. This means fewer pumps and mixers, less cost and less energy use.
NEREDA TECH IN THE UK
Nereda technology was originally presented to the UK water industry in 2013 when United Utilities installed the first UK Nereda pilot plant at its wastewater treatment works at Davyhulme, in Manchester. Since then Royal HaskoningDHV has worked with many of the UK water companies to test the technology and implement full-scale Nereda plants in the UK.
The Kendal Wastewater Treatment Plant, on the bank of the River Kent, initially commissioned its Nereda plant in in October 2018 and just achieved regulatory compliance in April of this year. Morecambe wastewater treatment works will treat additional flows in order to reduce the number of storm water spills, increasing the quality of Morecambe’s bathing waters. Another Nereda plant at Failsworth is under construction and due to enter service in 2020. Between them, the three sites will treat waste for a population equivalent of 147,000.
As Noppeney explains: “Nereda technology can provide full nutrient removal so will allow cost-effective solutions to be implemented on sites that will be subjected to increased load or need to meet more stringent effluent requirements in the future. The space-saving allows these works to be installed retrospectively on existing sites, which minimises the need for new land to be developed and also decreases interruptions to customers’ daily lives.
“In comparison to conventional activated sludge treatment processes, Nereda technology offers lower capex, lower opex, reduced hazard risk and improved effluent quality. The Nereda process is both sustainable and cost-effective, and can reduce energy consumption by up to 50%. We think it is the biggest step-change in wastewater treatment since Ardern and Lockett developed the activated sludge process over 100 years ago.”
The Nereda process looks set to be a key technology in improving water quality within the constraints of increasing pressure on land use and the need to reduce energy consumption. This is a cost-effective approach that reduces both capital and operational expense while also giving environmental benefits. Critically, it is suited to use in a wide range of environments, making it suitable for world-wide deployment.