Editing Activated sludge (section)

==Process description==
The process takes advantage of [[aerobic organism|aerobic]] micro-organisms that can digest organic matter in sewage, and clump together by [[flocculation]] entrapping fine particulate matter as they do so. It thereby produces a liquid that is relatively free from suspended solids and organic material, and flocculated particles that will readily settle out and can be removed.<ref>{{cite web |url=http://www.nesc.wvu.edu/pdf/WW/publications/pipline/PL_SP03.pdf |title=Explaining the Activated Sludge Process |access-date=6 February 2022 |publisher= University of Virginia - National small flows clearing house |date= 2003 |volume=14 |number=2 |archive-url=https://web.archive.org/web/20120817144056/http://www.nesc.wvu.edu/pdf/WW/publications/pipline/PL_SP03.pdf |archive-date=17 August 2012 |url-status=dead}}</ref>

The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items:
* Aeration tank where air (or oxygen) is injected in the mixed liquor.
* Settling tank (usually referred to as "final clarifier" or "secondary settling tank") to allow the biological flocs (the sludge blanket) to settle, thus separating the biological sludge from the clear treated water.
  
Treatment of nitrogenous or phosphorous matter comprises the addition of an anoxic compartment inside the aeration tank in order to perform the nitrification-denitrification process more efficiently. First, ammonia is oxidized to nitrite, which is then converted into nitrate in aerobic conditions (aeration compartment). Facultative bacteria then reduce the nitrate to nitrogen gas in anoxic conditions (anoxic compartment). Moreover, the organisms used for the phosphorus uptake (Polyphosphate Accumulating Organisms) are more efficient under anoxic conditions. These microorganisms accumulate large amounts of phosphates in their cells and are settled in the secondary clarifier. The settled sludge is either disposed of as waste activated sludge or reused in the aeration tank as return activated sludge. Some sludge must always be returned to the aeration tanks to maintain an adequate population of organisms.

The yield of PAOs (Polyphosphate Accumulating Organisms) is reduced between 70 and 80% under aerobic conditions. Even though the phosphorus can be removed upstream of the aeration tank by chemical precipitation (adding metal ions such as: calcium, aluminum or iron), the biological phosphorus removal is more economic due to the saving of chemicals.

===Bioreactor and final clarifier===
The process involves air or [[oxygen]] being introduced into a mixture of screened, and primary treated sewage or industrial wastewater ([[wastewater]]) combined with organisms to develop a biological [[flocculation|floc]] which reduces the [[organic matter|organic]] content of the [[sewage]]. This material, which in healthy sludge is a brown floc, is largely composed of [[Saprotrophic bacteria]] but also has an important [[protozoa]]n flora component mainly composed of [[amoeba]]e, [[Spirotrich]]s, [[Peritrich]]s including [[Vorticella|Vorticellids]] and a range of other filter-feeding species. Other important constituents include motile and sedentary [[Rotifer]]s. In poorly managed activated sludge, a range of [[mucilage|mucilaginous]] filamentous bacteria can develop - including ''[[Sphaerotilus natans]]'', ''[[Gordonia (bacterium)|Gordonia]]'',<ref>{{cite journal | vauthors = Oerther DB, de los Reyes FL, Hernandez M, Raskin L | title = Simultaneous oligonucleotide probe hybridization and immunostaining for in situ detection of Gordona species in activated sludge | journal = FEMS Microbiology Ecology | volume = 29 | issue = 2 | pages = 129–136 | date = 1999 | doi = 10.1111/j.1574-6941.1999.tb00604.x | doi-access = free }}</ref> and other microorganisms -  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.

The combination of wastewater and biological mass is commonly known as ''mixed liquor''. In all activated sludge plants, once the 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 wastewater entering the tank. This fraction of the floc is called ''return activated sludge'' (R.A.S.).

The space required for a sewage treatment plant can be reduced by using a [[membrane bioreactor]] to remove some wastewater from the mixed liquor prior to treatment. This results in a more concentrated waste product that can then be treated using the activated sludge process.

Many sewage treatment plants use [[axial flow pump]]s to transfer nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification.  These pumps are often referred to as internal mixed liquor recycle pumps (IMLR pumps). The raw sewage, the RAS, and the nitrified mixed liquor are mixed by [[submersible mixer]]s in the anoxic zones in order to achieve denitrification.

=== Sludge production===
Activated sludge is also the name given to the active biological material produced by activated sludge plants. Excess sludge is called "surplus activated sludge" or "waste activated sludge" and is removed from the treatment process to keep "food to biomass" (F/M) ratio in balance (where biomass refers to the activated sludge). This [[sewage sludge]] is usually mixed with primary sludge from the primary clarifiers and undergoes further [[sewage sludge treatment|sludge treatment]] for example by [[anaerobic digestion]], followed by thickening, dewatering, [[composting]] and land application.

The amount of sewage sludge produced from the activated sludge process is directly proportional to the amount of wastewater treated. The total sludge production consists of the sum of primary sludge from the primary sedimentation tanks as well as waste activated sludge from the bioreactors. The activated sludge process produces about {{convert|70|–|100|g/m3|oz/cuyd}} of waste activated sludge (that is grams of dry solids produced per cubic metre of wastewater treated). {{convert|80|g/m3|oz/cuyd}} is regarded as being typical.<ref name=":1">{{cite book|title = Wastewater engineering : treatment and reuse|url = https://archive.org/details/wastewaterengine00tcho|url-access = limited|publisher = Metcalf & Eddy, Inc., McGraw Hill, USA|isbn = 0-07-112250-8|page = [https://archive.org/details/wastewaterengine00tcho/page/n1482 1456]|edition = 4th|year = 2003}}</ref> In addition, about {{convert|110|–|170|g/m3|oz/cuyd}} of primary sludge is produced in the primary sedimentation tanks which most - but not all - of the activated sludge process configurations use.<ref name=":1" />

===Process control===

The general process control method is to monitor sludge blanket level, SVI (Sludge Volume Index), MCRT (Mean Cell Residence Time), F/M (Food to Microorganism), as well as the biota of the activated sludge and the major nutrients DO ([[Dissolved oxygen]]), [[nitrogen]], [[phosphate]], BOD ([[Biochemical oxygen demand]]), and COD ([[Chemical oxygen demand]]). In the reactor/aerator and clarifier system, the sludge blanket is measured from the bottom of the clarifier to the level of settled solids in the clarifier's water column; this, in large plants, can be done up to three times a day.

The SVI is the volume of settled sludge occupied by a given mass of dry sludge solids. It is calculated by dividing the volume of settled sludge in a mixed liquor sample, measured in milliliters per liter of sample (after 30 minutes of settling), by the MLSS (Mixed Liquor Suspended Solids), measured in grams per liter.<ref name="MECC">{{cite web|url=https://water.mecc.edu/courses/ENV149/lesson7.htm|title=Lesson 7: Activated Sludge|website=Water/Wastewater Distance Learning|publisher=[[Mountain Empire Community College]]|date=2013-03-19 <!--Based on browser-derived page modification date as of access date-->|access-date=2022-02-19}}</ref><ref name="WastewaterMath">{{cite web|url=http://www.ragsdaleandassociates.com/WastewaterSystemOperatorsManual/Chapter%2017%20-%20Mathematics.pdf|title=Mathematics For Wastewater Operators|archive-url=https://web.archive.org/web/20120907112903/http://www.ragsdaleandassociates.com/WastewaterSystemOperatorsManual/Chapter%2017%20-%20Mathematics.pdf|archive-date=2012-09-07|url-status=usurped}}</ref> The MCRT is the total mass (in kilograms or pounds) of mixed liquor suspended solids in the aerator and clarifier divided by the mass flow rate (in kilograms/pounds per day) of mixed liquor suspended solids leaving as WAS and final effluent.<ref name="MECC" /><ref name="WastewaterMath" /> The F/M is the ratio of food fed to the microorganisms each day to the mass of microorganisms held under aeration. Specifically, it is the amount of BOD fed to the aerator (in kilograms/pounds per day) divided by the amount (in kilograms or pounds) of [[Mixed Liquor Volatile Suspended Solids]] (MLVSS) under aeration. Note:  Some references use MLSS (Mixed Liquor Suspended Solids) for expedience, but MLVSS is considered more accurate for the measure of microorganisms.<ref name="MECC" /><ref name="WastewaterMath" /> Again, due to expedience, COD is generally used, in lieu of BOD, as BOD takes five days for results.

To ensure good bacterial settlement and to avoid sedimentation problems caused by filamentous bacteria, plants using atmospheric air as an oxygen source should maintain a dissolved oxygen (DO) level of about 2 mg/L in the aeration tank. In pure oxygen systems, DO levels are usually in the range of 4 to 10 mg/L. Operators should monitor the tank for low DO bacteria, such as S. natans, type 1701 and H. hydrossis, which indicate low DO conditions by elevated effluent turbidity and dark activated sludge with foul odours. Many plants have on-line monitoring equipment that continuously measures and records DO levels at specific points within the aeration tank. These on-line analysers send data to the SCADA system and allow automatic control of the aeration system to maintain a predetermined DO level. Whether generated automatically or taken manually, regular monitoring is necessary to favour organisms that settle well rather than filaments. However, operating the aeration system involves finding a balance between sufficient oxygen for proper treatment and the energy cost, which represents approximately 90% of the total treatment cost.<ref>{{cite web|title=Lesson 8: The Activated Sludge Process|website=water.mecc.edu|url=https://water.mecc.edu/courses/Env149/lesson8.htm|access-date=19 August 2024}}</ref>

Based on these control methods, the amount of settled solids in the mixed liquor can be varied by wasting activated sludge (WAS) or returning activated sludge (RAS).{{citation needed|date=December 2019}}
The returning activated sludge is designed to recycle a portion of the activated sludge from the secondary clarifier back to the aeration tank. It usually includes a pump that draws the portion back. 
The RAS line is designed considering the potential for clogging, settling, and other relatable issues that manage to impact the flow of the activated sludge back to the aeration tank. This line must handle the required flow of the plant and has to be designed to minimize the risk of solids settling or accumulating.

=== Nitrification and Denitrification ===
Ammonium can have a toxic effect on aquatic organism. Nitrification also takes place in bodies of water, which leads to oxygen depletion. Furthermore, nitrate and ammonium are eutrophying (fertilizing) nutrients that can impair water bodies. For these reasons, nitrification and, in many cases, nitrogen removal is necessary.

Two special steps are required for nitrogen removal:

a) Nitrification: Oxidation of ammonium nitrogen and organically bound nitrogen to nitrate. Nitrification is very sensitive to inhibitors and can lead to a pH value in poorly buffered water.<ref>{{Cite book |last=Mohren |first=Andreas |title=Nitrifikation-Ammoniakoxidation}}</ref>

Nitrification takes places in following steps:

# <math>\mathrm {\ NH_4^+ + 1.5 \ O_2 \longrightarrow \ NO_2^- + 2 H^+ + H_2O + Energy}</math>
# <math>\mathrm {\ NO_2^- + 0.5 \ O_2 \longrightarrow \ NO_3^- + Energy}

</math>

this results in:

<math>\mathrm {\ NH_4^+ + 2 \ O_2 \longrightarrow \ NO_3^- + 2H^+ + H_2O + Energy}
</math>

Nitrification is associated with the production of acid (H+). This puts a strain on the buffering capacity of the water or a pH value shift may occur, which impairs the process.

b) Denitrification: Reduction of nitrate nitrogen to molecular nitrogen, which escapes from the wastewater into the atmosphere. This step can be carried out by microorganisms commonly found in sewage treatment plants. However, these only use the nitrate as an electron acceptor if no dissolved oxygen is present.

<math>\mathrm {\ 2 \ NO_3^- + 2 \ H^+ + 10 \ H \longrightarrow \ N_2 + 6 \ H_2O}
</math>

In order for denitrification to take place in the activated sludge process, an electron source, a reductant, must therefore also be present that can reduce sufficient nitrate to N2. If there is too little substrate in the raw wastewater, this can be added artificially. In addition, denitrification corrects the change in H+ concentration (pH value shift) that occurs during nitrification. This is particularly important for poorly buffered water.

Nitrification and denitrification are in considerable contradiction with regard to the required environmental conditions. Nitrification requires oxygen and CO2. Denitrification only takes place in the absence of dissolved oxygen and with a sufficient supply of oxidizable substances.