Partial nitritation/anammox (PN/A) has been one of the most innovative developments in biological wastewater treatment in recent years. With its discovery in the 1990s a completely new way of ammonium removal from wastewater became available. Over the past decade many technologies have been developed and studied for their applicability to the PN/A concept and several have made it into full-scale. With the perspective of reaching 100 full-scale installations in operation worldwide by 2014 this work presents a summary of PN/A technologies that have been successfully developed, implemented and optimized for high-strength ammonium wastewaters with low C:N ratios and elevated temperatures. The data revealed that more than 50% of all PN/A installations are sequencing batch reactors, 88% of all plants being operated as single-stage systems, and 75% for sidestream treatment of municipal wastewater. Additionally an in-depth survey of 14 full-scale installations was conducted to evaluate practical experiences and report on operational control and troubleshooting. Incoming solids, aeration control and nitrate built up were revealed as the main operational difficulties. The information provided gives a unique/new perspective throughout all the major technologies and discusses the remaining obstacles.

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 ][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 ][2], [ 3 ][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 ][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 ][5]). The bacteria use CO2 as their carbon source for growth and hence do not require organic carbon ([ 1 ][1]). The nitrite required for their growth may be provided by aerobic ammonium-oxidizing bacteria or archaea ([ 2 ][2]). The anammox (I) and nitrification (II) reactions ![][6] 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. ![Figure][7] 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 16 S 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 ([ 6 ][8]–[ 9 ][9]). 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 ][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 ][11], [ 12 ][12]). Granular-sludge reactors have now been developed for removal of organic matter and nutrients under aerobic conditions ([ 13 ][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 ][14]). Because of their high volumetric conversion...

Biogrouting is a biological ground improvement method, in which microorganisms are used to induce carbonate precipitation in the subsurface in order to increase the strength and stiffness of granular soils. In this paper the results of a large-scale experiment ( 100 m3 ) are presented, in which the feasibility of biogrouting as a ground improvement method is investigated using techniques and equipment similar to those used in potential applications. In situ geophysical measurements were used to monitor the biogrouting process during treatment and indicated that the stiffness had increased significantly after one day of treatment. The results of unconfined compressive strength tests on samples which were excavated after treatment were used to assess the distribution of mechanical properties throughout the cemented sand body, which correlated quite well with the results of the in situ geophysical measurements. The stiffness increase could be quantified as a function of the injected volume of grouting agent...

Antibiotic resistance genes (ARGs) are widespread among bacteria. However, not all ARGs pose serious threats to public health, highlighting the importance of identifying those that are high-risk. Here, we developed an ‘omics-based’ framework to evaluate ARG risk considering human-associated-enrichment, gene mobility, and host pathogenicity. Our framework classifies human-associated, mobile ARGs (3.6% of all ARGs) as the highest risk, which we further differentiate as ‘current threats’ (Rank I; 3%) - already present among pathogens - and ‘future threats’ (Rank II; 0.6%) - novel resistance emerging from non-pathogens. Our framework identified 73 ‘current threat’ ARG families. Of these, 35 were among the 37 high-risk ARGs proposed by the World Health Organization and other literature; the remaining 38 were significantly enriched in hospital plasmids. By evaluating all pathogen genomes released since framework construction, we confirmed that ARGs that recently transferred into pathogens were significantly enriched in Rank II (‘future threats’). Lastly, we applied the framework to gut microbiome genomes from fecal microbiota transplantation donors. We found that although ARGs were widespread (73% of genomes), only 8.9% of genomes contained high-risk ARGs. Our framework provides an easy-to-implement approach to identify current and future antimicrobial resistance threats, with potential clinical applications including reducing risk of microbiome-based interventions.