Activated Sludge

Activated sludge is a process dealing with the treatment of sewage and industrial wastewaters. Atmospheric air or pure oxygen is bubbled through primary treated sewage (or industrial wastewater) combined with organisms to develop a biological floc which reduces the organic content of the sewage. The combination of raw sewage (or industrial wastewater) and biological mass is commonly known as Mixed Liquor. In all activated sludge plants, once the sewage (or industrial wastewater) has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new sewage (or industrial wastewater) entering the tank. This fraction of the floc is called Return Activated Sludge (R.A.S.). Excess sludge which eventually accumulates beyond what is returned is called Waste Activated Sludge (W.A.S.). W.A.S is removed from the treatment process to keep the ratio of biomass to food supplied (sewage or wastewater) in balance. This is called the F:M ratio. W.A.S is stored away from the main treatment process in storage tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.

Activated sludge is also the name given to the active biological material produced by activated sludge plants and which affects all the purification processes. This material, which in healthy sludge is a brown floc, is largely composed of saprotrophic bacteria but also has an important protozoan flora mainly composed of amoebae, Spirotrichs, Peritrichs including Vorticellids and a range of other filter feeding species. Other important constituents include motile and sedentary Rotifers. In poorly managed activated sludge, a range of mucilaginous filamentous bacteria can develop including Sphaerotilus natans which produces a sludge that is difficult to settle and can result in the sludge blanket decanting over the weirs in the settlement tank to severely contaminate the final effluent quality. This material is often described as sewage fungus but true fungal communities are relatively uncommon.

Denitrification

Denitrification is a microbially facilitated process of dissimilatory nitrate reduction that may ultimately produce molecular nitrogen (N₂) through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favourable include: nitrate (NO₃^-), nitrite (NO₂^-), nitric oxide (NO), and nitrous oxide (N₂O). In terms of the general nitrogen cycle, denitrification completes the cycle by returning N₂ to the atmosphere. The process is performed primarily by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), although autotrophic denitrifiers have also been identified (e.g.,Thiobacillus denitrificans). Denitrifiers are represented in all main phylogenetic groups. Generally several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.

Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen, a more energetically favourable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in environments where oxygen consumption exceeds the rate of oxygen supply, such as in some soils and groundwater, wetlands, poorly ventilated corners of the ocean, and in seafloor sediments.

Denitrification generally proceeds through some combination of the following intermediate forms:

NO₃- → NO₂-→ NO → N₂O → N₂ gas

The complete denitrification process can be expressed as a redox reaction:

2NO₃^- + 10e^- + 12H⁺ → N₂ + 6H₂O

This reaction shows a fractionation in isotope composition. Lighter isotopes of nitrogen are preferred in the reaction, leaving the heavier nitrogen isotopes in the residual matter. The process can cause delta-values of up to -40, where delta is a representation of the difference in isotopic composition. This can be used to identify denitrification processes in nature.

Denitrification is commonly used to remove nitrogen from sewage and municipal wastewater. It is also an instrumental process in wetlands and riparian zones for the removal of excess nitrate from groundwater resulting from excessive agricultural or residential fertiliser usage.

Direct reduction from nitrate to ammonium, a process known as dissimilatory nitrate reduction to ammonium or DNRA, is also possible for organisms that have the nrf-gene. This is less common than denitrification in most ecosystems as a means of nitrate reduction.

Reduction under anoxic conditions can also occur through process called anaerobic ammonia oxidation (Anammox), this reaction is expressed as the following:

NH₄⁺ + NO₂^- → N₂ + 2H₂O

In some wastewater treatment plants, small amounts of methanol are added to the wastewater to provide a carbon source for the denitrification bacteria.

Nitrification

The oxidation of ammonia into nitrite is performed by two groups of organisms, ammonia oxidizing bacteria and ammonia oxidizing archaea.^[1] Ammonia oxidizing bacteria can be found among the β- and γ-proteobacteria.^[2] In soils the most studied ammonia oxidizing bacteria belong to the genera Nitrosomonas and Nitrosococcus. Although in soils ammonia oxidation occurs by both bacteria and archaea, archaeal ammonia oxidizers dominate in both soils and marine environments,^[3][4] suggesting that Crenarchaeota may be greater contributors to ammonia oxidation in these environments. The second step (oxidation of nitrite into nitrate) is (mainly) done by bacteria of the genus Nitrobacter. Both steps are producing energy to be coupled to ATP synthesis. Nitrifying organisms are chemoautotrophs, and use carbon dioxide as their carbon source for growth.

Nitrification also plays an important role in the removal of nitrogen from municipal wastewater. The conventional removal is nitrification, followed by denitrification. The cost of this process resides mainly in aeration (bringing oxygen in the reactor) and the addition of an external carbon source (e.g. methanol) for the denitrification.

In most environments both organisms are found together, yielding nitrate as the final product. It is possible however to design systems in which selectively nitrite is formed (the Sharon process).

Together with ammonification, nitrification forms a mineralization process which refers to the complete decomposition of organic material, with the release of available nitrogen compounds. This replenishes the nitrogen cycle.

1. NH₃ + CO₂ + 1.5 O₂ → NO₂^- + H₂O + H^{+ (Nitrosomonas)}
   
2. NO₂^- + CO₂ + 0.5 O₂ → NO₃^{- (Nitrobacter)}