Because the granules that comprise the heart of the Nereda process settle much faster than the suspended material normally generated during a conventional activated-sludge process, biological treatment can occur in a single reactor, obviating the need for such steps as secondary clarification. © DHV
A Dutch wastewater treatment plant now includes the first full-scale municipal application of a biological treatment process that uses a single reactor—saving space, energy, and costs.
June 19, 2012—The recent upgrade of a Dutch wastewater treatment plant (WWTP) features the first full-scale municipal application of the biological treatment process known as Nereda. Developed by a consortium of public and private institutions in the Netherlands, the Nereda process facilitates the growth of microorganisms that form compact granules. Because the granules settle much more quickly than the suspended material normally generated during the conventional activated sludge process, biological treatment can occur in a single reactor, obviating the need for such steps as secondary clarification. The dense granules also enable higher biomass concentrations in the reactor. As a result, the Nereda process offers the prospect of a smaller facility footprint, lower capital and operation costs, and decreased energy and chemical demands.
First discovered by researchers at the Delft University of Technology during the 1990s, the Nereda process was developed into a commercially available technology by a public-private partnership that includes the university; the engineering consulting firm DHV, of Amersfoort, the Netherlands; five local water agencies, or district water boards; and the Foundation for Applied Water Research, also of Amersfoort. Known more commonly by its Dutch acronym STOWA, the foundation coordinates applied research for multiple district water boards within the Netherlands.
As part of the Nereda process, microorganisms grow in the shape of compact granules rather than in the form of the fluffy suspended material—commonly known as “floc”—that typically results during conventional activated sludge treatment. “We force [the microorganisms] to grow into granules by changing the process conditions,” says Helle van der Roest, DHV’s leading professional for water treatment. To survive, the microorganisms create biopolymers that protect them against challenging process conditions, including wide ranges in pH. “We have seen that this granular system is so much more reliable and especially robust against all kinds of difficult circumstances,” van der Roest notes.
The Nereda process operates on a batch treatment basis; influent entering the reactor displaces wastewater that has been treated in the previous batch. Aeration then occurs within the reactor, promoting growth of the granular biomass. Because of their unique structure, the granules are able to simultaneously remove organic compounds and nutrients from the wastewater. The outer layer of the granule consists of microorganisms that thrive under aerobic conditions. These microorganisms remove organic matter and convert ammonium to nitrate. The inner layer of the granule consists of microorganisms that operate under either anoxic or anaerobic conditions. These microorganisms reduce the nitrate to nitrogen gas and remove phosphate, completing the process of nutrient removal without chemical dosing.
The recently completed upgrade of the 8,000 m3/d wastewater
treatment plant in the Dutch town of Epe incorporated three
4,500 m3 reactors dedicated to the Nereda biological treatment
process. © DHV
The final step of the Nereda process involves the rapid settling of the compact granules within the same reactor in which the biological treatment occurs. Because the granules settle between 10 to 100 times more quickly than flocs from conventional activated sludge, “we don’t need separate tanks for separation of sludge and the treated wastewater,” van der Roest says. And by eliminating the requirement for secondary clarification, the Nereda process obviates the need to use pumps and related equipment to recirculate returned activated sludge from secondary clarifiers to aeration tanks. “That makes a big difference,” he says, by greatly reducing space requirements and energy demand.
In early May, the Dutch town of Epe formally commissioned its WWTP following an upgrade of the facility to include the Nereda process, marking the first time that the technology has been employed on a full-scale basis at a municipal facility. More than 40 years old, the WWTP is owned by Waterschap Veluwe, the local district water board. The Epe WWTP has a design capacity of 8,000 m3
/d, though during periods of wet weather it can operate up to a maximum hydraulic capacity of 36,000 m3
/d. Because its treated effluent comprises most of the flow within the small, sensitive water body to which it discharges, the upgraded Epe WWTP has discharge limits of 5 mg/L of total nitrogen and 0.2 mg/L of total phosphorus. These nutrient limits are among the strictest for wastewater facilities throughout the Netherlands, van der Roest says.
DHV served as the designer and main contractor for the upgrade of the Epe WWTP and is responsible for its operation and maintenance through 2012. Meanwhile, the engineering firm is teaching operators from the local district water board how to operate the upgraded WWTP, which includes a pretreatment screening facility. Because slaughterhouse waste is part of the influent, the WWTP also employs a system for removing fat in advance of the three 4,500 m3
Nereda tanks. To ensure compliance with the stringent limit on total phosphorus, the wastewater undergoes sand filtration following biological treatment. However, such tertiary treatment would not be required in most other applications, van der Roest says.
The total cost to design and construct the Epe plant upgrade was €15 million (U.S.$18.8 million). For the installation at the Epe plant, the Nereda process is estimated to cost approximately 25 to 30 percent less than a conventional activated-sludge treatment system, while offering a similar reduction in terms of operating costs, van der Roest says. Additionally, the facility uses about 30 percent less energy compared to an activated-sludge system.
For municipalities needing to expand treatment capacity in constrained settings, the Nereda process offers certain advantages, says Ronald Niermans, the intellectual property and licensing manager for DHV. In particular, the process can enable a facility to increase its treatment capacity without expanding its footprint. As a result, Nereda has the potential to be “very advantageous in retrofit situations,” he says. The process has also been used successfully in several industrial applications.
Four other Nereda facilities are under construction in the Netherlands, as are two plants in South Africa. In May, the municipality of Ryki, Poland, awarded a contract to DHV to design a Nereda facility there.