By William Atkinson
In conventional sewage plants, microorganisms digest solid waste in activated sludge, converting the organic matter into methane but leaving liquid waste that contains ammonium and phosphates. These must be removed before the water can be discharged into rivers; however, it demands a great deal of energy.
The process requires bacteria that convert ammonium into nitrate, and these bacteria require oxygen that must be constantly supplied to the treatment tanks via pumps. The nitrate is then converted into nitrogen gas by more bacteria, known as denitrifying bacteria, that also require methanol to be added to the mix. Overall, the whole process requires a significant amount of energy.
According to Kung-Hui Chu, Ph.D., P.E., one of the current challenges in the industry is the nitrogen cycle and emission of greenhouse gases. Chu is an assistant professor of environmental and water resources at Texas A&M University's Zechry Department of Civil Engineering, Interdisciplinary Faculty of Toxicology & Water Program. "Currently, the process of removing nitrogen from wastewater is not that efficient," she said. "In addition, when you try to remove nitrogen, it requires oxygen, which is very energy-intensive."
Is there a better way? Chu thinks there might be. "Anammox seems to have potential, in that it reduces oxygen demand and can reduce emissions during denitrification," she said.
Anammox is an abbreviation for anaerobic ammonium oxidation, a microbial process of the nitrogen cycle. The bacteria involved in this process were identified in the late 1990s. "Anammox is a class of bacteria, a group of organisms, that eat ammonia and breathe nitrites," explained Charles B. Bott, Ph.D., P.E., BCEE, chief of special projects, for Hampton Roads Sanitation District (Virginia Beach, Va.).
Normally, ammonium is removed by oxidizing it to nitrate and then to nitrogen gas. With the anammox process, ammonium is directly converted to nitrogen gas, requiring no organic carbon and minimal use of energy. The anammox bacteria do not require oxygen to oxidize the ammonium.
Benefits and Drawbacks
As noted, anammox can eat ammonium without the need for oxygen. Likewise, the anammox bacteria reduce the nitrogen cycle by converting ammonium directly into nitrogen gas. Compared to the traditional nitrification/denitrification process, the anammox process has several advantages. One is that the need for oxygen is reduced significantly, which also reduces the energy required to introduce the oxygen. Another benefit, unlike the traditional nitrification/denitrification process, entails the anammox bacteria do not require an organic carbon source. Thirdly, the anammox bacteria produce very little biomass, thus reducing the amount of sludge to manage. "As such, the incentive to use anammox is cost reduction because it offers a lot of advantages in terms of removing nitrogen with much less energy and chemicals," said Dr. Bott.
"Anammox may be the next generation of activated sludge," said Amit Pramanik, Ph.D., BCEEM, service program director for the Water Environment Research Foundation (WERF; Alexandria, Va.). "Anammox is a form of microbial process that provides a pathway to reduce energy and chemical costs. As a result, utilities are somewhat excited about anammox, especially since it provides an opportunity to save energy and chemical costs."
While anammox holds significant potential, it isn't without its drawbacks. "There isn't a lot of practical use of it in wastewater treatment plants yet," said Dr. Chu. One reason is that the bacteria grow very slowly. "It can take two weeks or more for just one cell to grow to two cells," she explained, "and they also require specific environmental conditions." Since the bacteria grow so slowly and require a much longer residence time, it may require more time than a traditional treatment process allows.
Anammox Process Availability
The anammox bacteria were discovered in the late 1990s by researchers at the Delft University of Technology in the Netherlands. "The researchers named the organism anammox, which stands for anaerobic ammonium oxidation," said Dr. Bott.
These researchers began working with Paques in the Netherlands to develop the anammox process. "This is one of three or four processes that are now available and have been built full-scale around the world," he said. "Paques markets their process, which is a granular sludge process, under the name Anammox." As such, besides being a class of bacteria going under the name anammox (lowercase "a"), Anammox® (capital "A") is also the registered name for the anammox-based ammonium removal technology that was developed by the Delft University of Technology and Paques.
However, according to Dr. Bott, it turns out that the anammox bacteria is not the only organism involved in the process. "There are actually two organisms," he said. "One is anammox; the other is the traditional ammonia oxide bacteria." This has led to a new term being coined: deammonification. It refers to the combination of aerobic and anaerobic ammonia oxidation, "which is what all of these processes use, including the Paques process," he said.
As a result, according to Dr. Bott, the more comprehensive term used to describe the process is deammonification, since anammox only describes half of what is going on in the process.
"There are several vendors that grow, harvest and sell the anammox bacteria," said WERF's Dr. Pramanik. "Most of them also sell the treatment process configurations, concentrators and control mechanisms since the bacteria are valuable and can be recycled instead of wasted." According to Dr. Pramanik, these vendors include: Paques in the Netherlands, which offers the Anammox® process; World Water Works in the U.S., which offers the DEMON® process; AnoxKaldnes, which offers the ANITA™ Mox process; and Degremont's Cleargreen™ process.
Deammonification involves partial nitration by ammonia oxidizing bacteria (AOB), combined with anaerobic ammonium oxidation (anammox). It provides between 70 to 90 percent nitrogen removal, with a 65 percent reduction in aeration energy, a 100 percent reduction in supplemental carbon and an approximate 50 percent reduction in alkalinity requirements compared to traditional nitrification/denitrification.
"Deammonification can save about 60 percent of the energy needed for conventional nitrogen processes," said WERF's Dr. Pramanik. Deammonification consumes carbon dioxide rather than releasing it from the carbon oxidation process. "There is a huge reduction in the alkalinity demand for nitrogen removal," explained Dr. Pramanik, "and there is also a significant reduction in sludge production."
Currently, as discussed below, the deammonification process is gaining a lot of traction in wastewater sidestream treatment. In fact, WERF's Nutrient Removal Challenge Team has created a sidestream deammonification compendium, which is available on its website (www.werf.org). "It provides information on what deammonification is, what its benefits are and why it is so exciting for the industry in terms of energy reduction and chemical savings," said Dr. Pramanik. "The compendium also has a list of installations worldwide which utilize sidestream deammonification technology."
According to Jeff C. Moeller, P.E., WERF's director of water technologies, the organization is very interested in working with members on pilots, testing and demonstrations in order to help them reduce the costs and risks associated with this new technology deployment.
Right now, WERF believes that the deammmonification process is viable for utilities in the treatment of sidestream - the concentrated nitrogen stream that comes from digesters. "This has been working quite well in Europe and is beginning to work in the U.S.," said Dr. Pramanik. The key is getting the conditions just right: having the right process configuration, concentrating the nitrogen in the digester and getting the anammox bugs to grow, recover and concentrate. "This requires the right environment and temperature," said Dr. Pramanik. "But in plants where this is being done, they are reporting significant energy and chemical savings."
In fact, a public-private partnership has made possible the first operating implementation of the DEMON® sidestream deammonification process in North America. Hampton Roads Sanitation District (HRSD), a regional wastewater treatment utility that serves 17 localities in Virginia, and World Water Works, which specializes in developing and providing efficient wastewater treatment solutions, joined forces to install a centrifuge centrate treatment process previously used only in Europe.
The DEMON process is one of several available forms of single-step (in the same tank) partial nitration-anammox designed for centrate treatment of anaerobically-digested biosolids and operated as a sequencing batch reactor (SBR).
HRSD's York River Treatment Plant currently has a fully aerobic nitrifying activated sludge system with post-denitrification filters. "We have a full-scale sidestream anammox process running right now and have since last October, the first one in North America," said Dr. Bott.
The DEMON process was seeded with 5,000 gallons of concentrated anammox biomass by mixing the imported anammox bacteria with plant effluent in October 2012. After some minor modifications to the equalization tank were made in November and December, the process achieved its operational objectives for ammonia loading and total nitrogen removal in January 2013.
"It is effective in a sidestream process because streams that are produced internal to the treatment plant are warm, high in ammonia and low in organics," said Dr. Bott. "We have been very pleased with the results."
According to Dr. Bott, sidestream represents 15 to 25 percent of a plant's nitrogen load. "The real goal is to get the deammonification process on a whole plant flow, mainstream," he said. "This is the Holy Grail of nitrogen removal, and there are lots of people around the world trying to figure out how to do this," he said. "We're currently working on a pilot study on this in collaboration with others."
One of these groups is WERF. "We are currently working on a new mainstream deammonification pilot project here in the U.S., financed by WERF, with EPA funding and with significant leveraged funding from Hampton Roads Sanitation District as well as DC Water," said Dr. Pramanik. "This game-changing technology leap requires a lot of work to be done in this space. However, with the help of the international team of experts on this research, we should be able to succeed."WW
About the Author: William Atkinson is a correspondent for WaterWorld Magazine. He has been a full-time freelance business magazine writer since 1976, specializing in infrastructure, sustainability, supply chain, risk management, and safety/health.
The American Association of Environmental Engineers and Scientists (AAEES) recently recognized World Water Works and Hampton Roads Sanitation District (HRSD) with an Honor Award for Environmental Sustainability for implementation of the first DEMON® sidestream deammonification system in North America.
HRSD's York River Treatment Plant's existing sequential batch reactors (SBR) and equalization tanks made it an ideal candidate for the DEMON system because it could be easily retrofitted for the process. The installation took less than four months, and successful operation has continued since early January 2013.
The project demonstrated that sidestream nitrogen removal using partial nitritation and anammox not only improves nitrogen removal performance, but it also results in savings in aeration energy, chemicals (alkalinity and supplemental carbon) and sludge production.
The total operational savings associated with the DEMON process is estimated to be $192,000 per year. This savings is attributed to reductions in the usage of methanol, sodium hydroxide (caustic) and electrical power for blower operation. Total nitrogen and ammonia removal efficiencies of more than 80 and 90 percent, respectively, have been achieved.