The technology reduces energy and land requirements for wastewater treatment.
The conventional method of treating sewage, and some forms of industrial wastewater, uses an activated sludge process to treat organic material after screening to remove other items. The activated sludge process uses microbial action to transform harmful pollutants into waste products that are safe to either reintroduce to the water cycle or use in some other way, such as spreading on fields as fertilizer.
Microbial action is encouraged and accelerated through the introduction of oxygen. The harmful substances in organic matter are primarily either carbonaceous or nitrogenous. In both cases, microbes can oxidize these substances to produce safe waste products. After screening, sedimentation tanks are used to carry out primary settlement. The actual activated sludge process can then begin, with the growth of aerobic microorganisms encouraged through aeration. This involves both stirring the sludge and blowing air through it. The microbes then consume the organic material, digesting it to produce less harmful oxidized material.
The microbes also group together into suspended clouds of cells, known as biological flocs. These flocs require no surface to start forming and allow the organic material to settle out of water more easily. The final settlement takes place in a clarifier-settler tank, where the biological flocs settle as a “sludge blanket” to produce biofertilizer, leaving only clean water that is suitable for reintroduction into the water cycle. However, the rate of settling is still quite slow—typically about one meter an hour. Large settling tanks are therefore required, and they occupy most of the land in a conventional wastewater treatment plant.
What has been described so far is a relatively simple activated sludge process that is able to deal with primarily carbonaceous matter. This is sometimes acceptable for sewage, although high-quality treatment must also process the nitrogenous matter in sewage. Other forms of wastewater may contain significantly more nitrogenous matter, meaning that treating it becomes essential. Processing nitrogenous matter involves multistep activation tanks with pumps and mixers creating differing levels of mixing and aeration at each stage.
The Nereda Process
The Nereda process uses principles that are similar to the activated sludge process, with microbial action breaking down sewage or industrial wastewater. However, instead of the microbes growing as a floc suspended in the wastewater, they grow on granules within the wastewater. These granules have layers where both aerobic and anaerobic activity takes place, so that both carbonaceous and nitrogenous matter can be broken down in a single-stage process. This greatly reduces the size of tanks required and also saves energy by reducing the need for air circulation.
Greenfield sites can, of course, be designed to use the Nereda process from the outset. However, what gives the process such potential is that existing tanks can be adapted in hybrid capacity extensions and retrofitted plants to convert them to Nereda technology. When retrofitting a facility to the Nereda technology, the capacity of existing plants can be easily doubled or tripled while simultaneously improving effluent quality. This can be achieved at just half of the energy costs of conventional treatment methods and with no chemical usage as the process is entirely biological. The end result is a treatment that works with significantly lower operating costs and has a lower carbon footprint and cleaner discharge.
The award-winning technology to use granular biomass was invented by the Delft University of Technology in the Netherlands and was 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. The first operational plant was built at Epe in the Netherlands in 2011.
“We initially trialed Nereda technology on smaller-scale pilot plants, to prove its effectiveness and demonstrate the ease with which it can be installed,” said René Noppeney, global director of Water Technology Products and Innovation at Royal HaskoningDHV. “Once these were successful and the data backed up our assertions, we were able to take the Nereda 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.”
The Nereda process is much simpler and more compact when compared to conventional biological treatment processes. It can also operate over a wide range of temperatures and climatic conditions. There are now over 70 Nereda plants that are either completed or in development. Sites are located in the United States, the United Kingdom, South Africa, Brazil, India, China, the Philippines and Australia.
United Utilities installed the first UK Nereda pilot plant at its wastewater treatment works at Davyhulme, Manchester, in 2013. Since then, Royal HaskoningDHV has worked with many 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 October 2018 and achieved regulatory compliance in April 2020. Morecambe Wastewater Treatment Works will treat additional flows 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 scheduled to enter service in 2021. Between them, the three UK sites will treat wastewater for a population equivalent (PE) of 147,000.
Accelerated Settling
A key advantage of granular sludge is that it settles much more rapidly, typically about 15 meters per hour, which is 15 times faster than a conventional activated sludge. This allows the Nereda process to operate at higher concentrations, greatly improving the throughput of a tank. The biological treatment and settling can also take place in a single tank, completely eliminating the need for final settling tanks. The combination of rapid settling, higher concentrations and a single tank process make for a much more compact and economical operation. Existing sites can operate at higher capacity and new sites can be constructed at low cost with much smaller tanks and a reduced land requirement.
“In comparison to conventional activated sludge treatment processes, Nereda technology offers lower capex, lower opex, reduced hazard risk and improved effluent quality,” said Noppeney. “The Nereda process is both sustainable and cost-effective, and can reduce energy consumption by up to 50 percent. We think it is the biggest step-change in wastewater treatment since Ardern and Lockett developed the activated sludge process over 100 years ago.”
Improved Treatment Quality
In a conventional activated sludge process, different tanks are required to cultivate aerobic and anaerobic microbes. Complete treatment requires both of these processes. In a granulated sludge process, oxygen does not fully penetrate the granules. This means that there are oxygen-rich regions on the surface and oxygen-poor regions deeper within each granule. Therefore, multiple biological processes can be performed simultaneously, enabling the treatment of both carbonaceous and nitrogenous matters in a single tank. This is much simpler and more economical than the multiple stages, with expensive pumps and mixers normally required for high-quality treatment. Significant energy is also saved by this simplification.
“Nereda technology can provide full nutrient removal so [it] 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,” said Noppeney. “The space-saving allows these works to be installed retrospectively on existing sites, which minimizes the need for new land to be developed and also decreases interruptions to customers’ daily lives.”
Conclusion
The Nereda process has many advantages for wastewater treatment and has the capability to improve water quality while simultaneously reducing costs, land use and energy consumption. It makes business sense because it reduces both capital and operational expense while also demonstrating environmental benefits. Another major factor in the applicability of this technology is that it can be deployed in a wide range of environments, making it suitable for global implementation.
