Editing Activated sludge (section)

==Plant types==
There are a variety of types of activated sludge plants.<ref name=Beychok/> These include:

===Package plants===
There are a wide range of types of package plants, often serving small communities or industrial plants that may use hybrid treatment processes often involving the use of aerobic sludge to treat the incoming sewage. In such plants the primary settlement stage of treatment may be omitted. In these plants, a biotic floc is created which provides the required substrate.  Package plants are designed and fabricated by specialty engineering firms in dimensions that allow for their transportation to the job site in public highways, typically width and height of {{convert|12|x|12|ft|m|order=flip}}.  Length varies with capacity with larger plants being fabricated in pieces and welded on site. Steel is preferred over synthetic materials (e.g., plastic) for its durability. Package plants are commonly variants of [[extended aeration]], to promote the "fit and forget" approach required for small communities without dedicated operational staff. There are various standards to assist with their design.<ref>{{Cite web |url=http://www.britishwater.co.uk/Document/Download.aspx?uid=3d63842c-eb86-48b1-be17-12eebf7487a5 |title=Code of Practice, ''Flows and Loads-2'', British Water |access-date=2007-09-08 |archive-url=https://web.archive.org/web/20090326193950/http://www.britishwater.co.uk/Document/Download.aspx?uid=3d63842c-eb86-48b1-be17-12eebf7487a5 |archive-date=2009-03-26 |url-status=dead }}</ref><ref>[http://products.ihs.com/cis/Doc.aspx?AuthCode=&DocNum=252510 Review of UK and international standards] {{webarchive |url=https://web.archive.org/web/20070928105619/http://products.ihs.com/cis/Doc.aspx?AuthCode=&DocNum=252510 |date=September 28, 2007 }}</ref><ref>[http://www.standardsdirect.org/standards/standards4/StandardsCatalogue24_view_5158.html British Standard BS 6297:1983]</ref>

To use less space, treat difficult waste, and intermittent flows, a number of designs of hybrid treatment plants have been produced. Such plants often combine at least two stages of the three main treatment stages into one combined stage. In the UK, where a large number of wastewater treatment plants serve small populations, package plants are a viable alternative to building a large structure for each process stage. In the US, package plants are typically used in rural areas, highway rest stops and trailer parks.<ref name="EPA Package">EPA. Washington, DC (2000). [http://www3.epa.gov/npdes/pubs/package_plant.pdf "Package Plants."] Wastewater Technology Fact Sheet. Document no. EPA 832-F-00-016.</ref>

Package plants may be referred to as ''high charged'' or ''low charged''. This refers to the way the biological load is processed. In high charged systems, the biological stage is presented with a high organic load and the combined floc and organic material is then oxygenated for a few hours before being charged again with a new load. In the low charged system the biological stage contains a low organic load and is combined with [[Flocculation|flocculate]] for longer times.

===Oxidation ditch===
In some areas, where more land is available, sewage is treated in large round or oval ditches with one or more horizontal aerators typically called brush or disc aerators which drive the mixed liquor around the ditch and provide aeration.<ref name=Beychok/> These are oxidation ditches, often referred to by manufacturer's trade names such as Pasveer, Orbal, or Carrousel. They have the advantage that they are relatively easy to maintain and are resilient to shock loads that often occur in smaller communities (i.e. at breakfast time and in the evening).

Oxidation ditches are installed commonly as 'fit & forget' technology, with typical design parameters of a [[hydraulic retention time]] of 24 – 48 hours, and a sludge age of 12 – 20 days. This compares with nitrifying activated sludge plants having a retention time of 8 hours, and a sludge age of 8 – 12 days.

===Deep shaft / Vertical treatment===
Where land is in short supply sewage may be treated by injection of oxygen into a pressured return sludge stream which is injected into the base of a deep columnar tank buried in the ground. Such shafts may be up to {{convert|100|m|ft}} deep and are filled with sewage liquor. As the sewage rises the oxygen forced into solution by the pressure at the base of the shaft breaks out as molecular oxygen providing a highly efficient source of oxygen for the activated sludge biota. The rising oxygen and injected return sludge provide the physical mechanism for mixing of the sewage and sludge. Mixed sludge and sewage is decanted at the surface and separated into supernatant and sludge components. The efficiency of deep shaft treatment can be high.

Surface aerators are commonly quoted as having an aeration efficiency of 0.5–1.5&nbsp;kg O<sub>2</sub>/kWh (1.1–3.3&nbsp;lb O<sub>2</sub>/kWh), diffused aeration as 1.5–2.5&nbsp;kg O<sub>2</sub>/kWh (3.3–5.5&nbsp;lb O<sub>2</sub>/kWh). Deep Shaft claims 5–8&nbsp;kg O<sub>2</sub>/kWh (11–18&nbsp;lb O<sub>2</sub>/kWh).

However, the costs of construction are high. Deep Shaft has seen the greatest uptake in Japan,<ref>[http://wholewater.com/H2OTREATMENT/verticalshaftprojects.htm Vertical Shaft Projects]</ref>{{deadlink|date=February 2025}} because of the land area issues. Deep Shaft was developed by [[Imperial Chemical Industries|ICI]], as a spin-off from their [[Pruteen]] process. In the UK it is found at three sites: Tilbury, Anglian water, treating a wastewater with a high industrial contribution;<ref>[http://cat.inist.fr/?aModele=afficheN&cpsidt=8054833 Tilbury construction]</ref> Southport, United Utilities, because of land space issues; and Billingham, ICI, again treating industrial effluent, and built (after the Tilbury shafts) by ICI to help the agent sell more.

DeepShaft is a patented, licensed, process. The licensee has changed several times and currently (2015) Noram Engineering<ref>{{Cite web |url=http://www.noram-eng.com/groups/biosystems-group-overview.html |title=NORAM VERTREAT (VERtical TREATment) |access-date=2015-08-13 |archive-url=https://web.archive.org/web/20150703162448/http://www.noram-eng.com/groups/biosystems-group-overview.html |archive-date=2015-07-03 |url-status=dead }}</ref> sells it.

===Surface-aerated basins===
{{further|Aerated lagoon}}
[[Image:Surface-Aerated Basin.png|thumb|upright=1.25|A Typical Surface-Aerated Basing (using motor-driven floating aerators)]]
Most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or air) and microbial action. Surface-aerated basins achieve 80 to 90% removal of [[Biochemical oxygen demand|BOD]] with retention times of 1 to 10 days.<ref name=Basin>{{cite journal|author=Beychok, M.R.|year=1971|title=Performance of surface-aerated basins|journal=Chemical Engineering Progress Symposium Series|volume=67|issue=107|pages=322–339}} [http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=7112203&q=&uid=788301038&setcookie=yes Available at CSA Illumina website] {{webarchive|url=https://web.archive.org/web/20071114140052/http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=7112203&q=&uid=788301038&setcookie=yes |date=2007-11-14 }}</ref> The basins may range in depth from {{convert|1.5|to|5.0|m|ft}} and utilize motor-driven aerators floating on the surface of the wastewater.<ref name=Basin/>

In an aerated basin system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kilograms [[Oxygen|O<sub>2</sub>]]/[[Kilowatt-hour|kWh]] (4.0 to 6.0&nbsp;lb O<sub>2</sub>/kWh). However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units.<ref name=Basin/>

Biological oxidation processes are sensitive to temperature and, between {{convert|0|and|40|°C|°F}}, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between {{convert|4|and|32|°C|°F}}.<ref name=Basin/>

=== Sequencing batch reactors (SBRs) ===
{{Main|Sequencing batch reactor}}
[[Sequencing batch reactor]]s (SBRs) treat wastewater in batches within the same vessel. This means that the bioreactor and final clarifier are not separated in space but in a timed sequence. The installation consists of at least two identically equipped tanks with a common inlet, which can be alternated between them. While one tank is in settle/decant mode the other is [[aeration|aerating]] and filling.