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Sewage Treatment with Anammox

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Science  07 May 2010:
Vol. 328, Issue 5979, pp. 702-703
DOI: 10.1126/science.1185941

Organic matter must be removed from sewage to protect the quality of the water bodies that it is discharged to. Most current sewage treatment plants are aimed at removing organic matter only. They are energy-inefficient, whereas potentially the organic matter could be regarded as a source of energy. However, organic carbon is not the only pollutant in sewage: Fixed nitrogen such as ammonium (NH4+) and nitrate (NO3) must be removed to avoid toxic algal blooms in the environment. Conventional wastewater treatment systems for nitrogen removal require a lot of energy to create aerobic conditions for bacterial nitrification, and also use organic carbon to help remove nitrate by bacterial denitrification (see the figure). An alternative approach is the use of anoxic ammonium-oxidizing (anammox) bacteria, which require less energy (1) but grow relatively slowly. We explore process innovations that can speed up the anammox process and use all organic matter as much as possible for energy generation.

The anammox process is responsible for at least 50% of the nitrogen turnover in marine environments (2, 3) and occurs in nature at both low and high temperatures and salinities. It is a shortcut in the nitrogen cycle (see the figure) that was discovered in the early 1990s (4). The anammox bacteria, which belong to the group Planctomycetes, contain a membrane-bound organelle in which ammonium and nitrite are converted to nitrogen gas via the toxic and extremely energy-rich hydrazine intermediate. Special lipids found in these bacteria, ladderanes, are believed to assist in keeping the hydrazine within this organelle (5). The bacteria use CO2 as their carbon source for growth and hence do not require organic carbon (1). The nitrite required for their growth may be provided by aerobic ammonium-oxidizing bacteria or archaea (2). The anammox (I) and nitrification (II) reactions


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In conventional sewage treatment, organic matter is combusted to carbon dioxide by microorganisms growing in flocs, generally referred to as an “activated sludge.” This process requires a lot of electrical energy input for air pumps to provide oxygen for oxidation of ammonium to nitrate (see table S1 in the Supporting Online Material). Furthermore, denitrification often requires extra organic matter (methanol) for the removal of nitrate as nitrogen gas in the final step. Sometimes, a fraction of the total energy used in existing sewage treatment plants is recovered as biogas (CH4), produced by the excess bacterial biomass (sludge).

A better approach would be to design the treatment process such that aeration is largely circumvented and most organic compounds are converted to biogas in an anaerobic treatment process. This is feasible with the present state of the art. However, in such a design, the organic compounds would no longer be available for denitrification, resulting in a large excess of ammonium and hence a potential nitrogen pollution problem.

The nitrogen cycle including anammox.

Molecular nitrogen (N2) is fixed biologically or industrially to ammonium (NH4+), the main fertilizer for plants. When ammonium is released to the environment, it may be oxidized by aerobic, nitrifying bacteria and archaea to nitrite (NO2) and nitrate (NO3), respectively, which plants can use as an additional nitrogen source. Under anaerobic conditions, nitrate and nitrite may be reduced back to ammonium, or to nitrogen gas through denitrification. Nitrite can also be combined with ammonium to give nitrogen gas in the anammox reaction. (Background) Outer layer of a compact nitrogen-producing granule for possible use in energy-generating wastewater treatment. The anammox bacteria (red) are on the inside of the granule; the nitrite-producing bacteria (blue) reside in a 40-µm-thick layer on the outside, ensuring that oxygen does not reach the anoxic anammox bacteria. The bacteria have been stained with fluorescent 16S rDNA probes.

PHOTO CREDIT: MARI WINKLER/DELFT UNIVERSITY OF TECHNOLOGY

Use of the anammox process can overcome this problem, because it does not require organic compounds for nitrogen removal (69). However, anammox bacteria grow slowly (generation times of 10 to 12 days at 35°C), and the anammox process is therefore currently limited to treating warm wastewater with high ammonium content. In the first designs, reactions I and II were carried out in consecutive reactors, but these were later combined in a single oxygen-limited reactor (10) where an ammox and nitrite-producing bacteria coexist (see the figure). Because of the low specific conversion rates of the one-reactor process, the bottleneck in this combination has been insufficient biomass retention.

Much more biomass could be retained if fast-settling, compact granules of a coculture of nitrification and anammox bacteria (a “granular sludge”) were used. In the 1980s, the use of anaerobic methane-producing granular sludge was the key in the development of efficient anaerobic wastewater treatment in sludge blanket and fluidized bed reactors (11, 12). Granular-sludge reactors have now been developed for removal of organic matter and nutrients under aerobic conditions (13). Granular-sludge reactors achieve a very high volumetric conversion rate due to a large surface area for mass transfer. They also do not trap the inert particles that are inevitably present in wastewater and that would strongly reduce the specific activity of the sludge.

The selective production of granules has also been successfully applied on nitrifying/anammox sludge in sludge blanket reactors that are operated with oxygen-limited aeration. The result is a substantial improvement in the energy management of wastewater facilities, as reported for a case in the Netherlands (14). Because of their high volumetric conversion rate, granular-sludge systems also offer the possibility for application of anammox for sewage treatment at the low temperatures and concentrations that are typical for municipal wastewater.

The availability of a fast anammox process opens real perspectives for a complete redesign of the present energy-consuming into an energy-yielding wastewater treatment (see table S1). The principle of this process would be as follows. Sewage is first led to a very-high-load activated sludge system, where soluble organic matter is converted to biomass with maximal growth yield that can be flocculated and separated together with the nondegraded suspended and colloidal material in a settler. In this way most organic matter is removed and concentrated, enabling its use in the generation of biogas (methane) in a digestion process. The effluent of the first stage is combined with the digester effluent and treated in a granular-sludge anammox reactor. The remaining nitrate will be below the required effluent standards for nitrogen discharge to the environment.

Calculations based on (7) show that the wastewater treatment process with anammox in the main line would yield 24 watt hours per person per day (Wh p−1 d−1), compared to a consumption of 44 Wh p−1 d−1 in conventional treatment. The realization of such a substantial gain presents great challenges in terms of technology and investment. However, we are convinced that it is feasible and expect that these and other innovations may stimulate investigators to take up the challenge of making global wastewater treatment energy-neutral or even energy-generating, as called for by many governmental programs, such as (15).

The discovery and practical application of high-rate nitrogen removal processes for wastewater treatment offer an enormous opportunity to make our wastewater treatment not only sustainable, but also a basis for the production of clean water that can be recycled for a variety of purposes.

Supporting Online Material

www.sciencemag.org/cgi/content/full/328/5979/702/DC1

Table S1

References and Notes

  1. See energy.ca.gov/water-energy/index.html or epa.gov/waterinfrastructure/bettermanagement_energy.html
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