Editing Biofilter (section)

==Water treatment==
[[Image:Trickle Filter.svg|thumb|right|A typical complete trickling filter system for treating wastewaters.<ref name=Beychok>{{cite book|author=Beychok, Milton R. |title=Aqueous Wastes from Petroleum and Petrochemical Plants| edition=1st| publisher=John Wiley & Sons Ltd|year=1967|lccn=67019834 |title-link=Aqueous Wastes from Petroleum and Petrochemical Plants}}</ref>]]
[[Image:Trickle Filter Cross-section.png|thumb|right|Image 1: A schematic cross-section of the contact face of the bed media in a trickling filter.]]

Biofiltration was first introduced in England in 1893 as a trickling filter for wastewater treatment and has since been successfully used for the treatment of different types of water.<ref name="Chaudhary, 2003">{{cite journal |last1=Chaudhary |first1=Durgananda Singh |last2=Vigneswaran |first2=Saravanamuthu |last3=Ngo |first3=Huu-Hao |last4=Shim |first4=Wang Geun |last5=Moon |first5=Hee |title=Biofilter in water and wastewater treatment |journal=Korean Journal of Chemical Engineering |date=November 2003 |volume=20 |issue=6 |pages=1054–1065 |doi=10.1007/BF02706936 |s2cid=10028364 }}</ref> Biological treatment has been used in Europe to filter surface water for drinking purposes since the early 1900s and is now receiving more interest worldwide. Biofiltration is also common in [[wastewater treatment]], [[aquaculture]] and [[greywater]] recycling, as a way to minimize water replacement while increasing [[water quality]].

===Biofiltration process===
<!-- Deleted image removed: [[Image:biofilter.jpg|thumb|right|Biofilter installation at a commercial composting facility.]] -->
A biofilter is a bed of media on which microorganisms attach and grow to form a biological layer called [[biofilm]]. Biofiltration is thus usually referred to as a fixed–film process. Generally, the biofilm is formed by a community of different microorganisms ([[bacteria]], [[fungi]], [[yeast]], etc.), macro-organisms ([[protozoa]], worms, insect's larvae, etc.) and extracellular polymeric substances (EPS) (Flemming and Wingender, 2010). Air or water flows through a media bed and any suspended compounds are transferred into a surface biofilm where microorganisms are held to degrade pollutants'''.''' The aspect of the [[biofilm]]<ref>{{cite journal|author1=H.C. Flemming  |author2=J. Wingender |name-list-style=amp |title=The biofilm matrix|journal=Nature Reviews Microbiology |volume=8 |issue=9 |pages=623–633 |year=2010|doi=10.1038/nrmicro2415 |pmid=20676145 |s2cid=28850938 }}</ref> is usually slimy and muddy.

Water to be treated can be applied intermittently or continuously over the media, via upflow or downflow. Typically, a biofilter has two or three phases, depending on the feeding strategy (percolating or submerged biofilter):

* a solid phase (media)

* a liquid phase (water);
* a gaseous phase (air).

Organic matter and other water components diffuse into the biofilm where the treatment occurs, mostly by [[biodegradation]]. Biofiltration processes are usually [[Cellular respiration#Aerobic respiration|aerobic]], which means that microorganisms require oxygen for their metabolism. Oxygen can be supplied to the biofilm, either concurrently or countercurrently with water flow. Aeration occurs passively by the natural flow of air through the process (three phase biofilter) or by forced air supplied by blowers.

Microorganisms' activity is a key-factor of the process performance. The main influencing factors are the water composition, the biofilter hydraulic loading, the type of media, the feeding strategy (percolation or submerged media), the age of the biofilm, temperature, aeration, etc.

The mechanisms by which certain microorganisms can attach and colonize on the surface of filter media of a biofilter can be via transportation, initial adhesion, firm attachment, and colonization [Van Loosdrecht et al., 1990]. The transportation of microorganisms to the surface of the filter media is further controlled by four main processes of diffusion (Brownian motion), convection, sedimentation, and active mobility of the microorganisms. The overall filtration process consists of microorganism attachment, substrate utilization which causes biomass growth, to biomass detachment.<ref name="Chaudhary, 2003"/>

===Types of filtering media===

Most biofilters use media such as sand, crushed rock, river gravel, or some form of plastic or ceramic material shaped as small beads and rings.<ref>{{Cite web|url=https://cals.arizona.edu/azaqua/ista/ISTA7/RecircWorkshop/Workshop%20PP%20%20&%20Misc%20Papers%20Adobe%202006/7%20Biofiltration/Nitrification-Biofiltration/Biofiltration-Nitrification%20Design%20Overview.pdf|title=Biofiltration-Nitrification Design Overview|last=Ebeling|first=James|access-date=November 25, 2018}}</ref>

===Advantages===

Although biological filters have simple superficial structures, their internal hydrodynamics and the microorganisms' biology and ecology are complex and variable.<ref>{{cite book|author1=C.R. Curds  |author2=H.A. Hawkes |name-list-style=amp |title=Ecological Aspects of Used-Water Treatment|publisher=The Processes and their Ecology Vol.3|year=1983|url=https://books.google.com/books?id=D0FRAAAAMAAJ|isbn=9780121995027 }}</ref> These characteristics confer robustness to the process. In other words, the process has the capacity to maintain its performance or rapidly return to initial levels following a period of no flow, of intense use, toxic shocks, media backwash (high rate biofiltration processes), etc.

The structure of the biofilm protects microorganisms from difficult environmental conditions and retains the biomass inside the process, even when conditions are not optimal for its growth. Biofiltration processes offer the following advantages: (Rittmann et al., 1988):

* Since microorganisms are retained within the biofilm, biofiltration allows the development of microorganisms with relatively low specific growth rates;
* Biofilters are less subject to variable or intermittent loading and to [[Water hammer|hydraulic shock]];<ref>{{cite book |author1=P.W. Westerman |author2=J.R. Bicudo |author3=A. Kantardjieff |name-list-style=amp |title=Aerobic fixed-media biofilter treatment of flushed swine manure |publisher=ASAE Annual International Meeting - Florida |year=1998 |url=http://eurekamag.com/research/003/032/aerobic-fixed-media-biofilter-cure-flushed-pig-manure.php |access-date=2013-06-19 |archive-date=2013-10-17 |archive-url=https://web.archive.org/web/20131017174912/http://eurekamag.com/research/003/032/aerobic-fixed-media-biofilter-cure-flushed-pig-manure.php |url-status=dead }}</ref>
* Operational costs are usually lower than for [[activated sludge]]; 
* The final treatment result is less influenced by biomass separation since the biomass concentration at the effluent is much lower than for suspended biomass processes;
* The attached biomass becomes more specialized (higher concentration of relevant organisms) at a given point in the process train because there is no [[biomass return]].<ref>{{cite journal|author=H. Odegaard|title=Innovations in wastewater treatment: the moving bed biofilm process|journal=Water Science and Technology|year=2006|volume=53|issue=9|pages=17–33|doi=10.2166/wst.2006.284|pmid=16841724|url=http://www.iwaponline.com/wst/05309/wst053090017.htm|access-date=2013-06-19|archive-url=https://web.archive.org/web/20131018001647/http://www.iwaponline.com/wst/05309/wst053090017.htm|archive-date=2013-10-18|url-status=dead}}</ref>

===Drawbacks===

Because filtration and growth of biomass leads to an accumulation of matter in the filtering media, this type of fixed-film process is subject to [[bioclogging]] and flow channeling. Depending on the type of application and on the media used for microbial growth, bioclogging can be controlled using physical and/or chemical methods. Backwash steps can be implemented using air and/or water to disrupt the biomat and recover flow whenever possible. Chemicals such as oxidizing ([[peroxide]], [[ozone]]) or biocide agents can also be used.

Biofiltration can require a large area for some treatment techniques (suspended growth and attached growth processes) as well as long hydraulic retention times (anaerobic lagoon and anaerobic baffled reactor).<ref>{{Cite journal |last1=Ali Musa |first1=Mohammed |last2=Idrus |first2=Syazwani |title=Physical and Biological Treatment Technologies of Slaughterhouse Wastewater: A Review |journal=Sustainability |date=2021 |volume=13 |issue=9 |page=4656 |doi=10.3390/su13094656 |doi-access=free }}</ref>