Editing Biofilter (section)

==Use in aquaculture==
The use of biofilters is common in closed [[aquaculture]] systems, such as [[recirculating aquaculture systems]] (RAS). The biofiltration techniques used in aquaculture can be separated into three categories: biological, physical, and chemical. The primary biological method is nitrification; physical methods include mechanical techniques and sedimentation, and chemical methods are usually used in tandem with one of the other methods.<ref>{{cite journal |last1=Crab |first1=Roselien |last2=Avnimelech |first2=Yoram |last3=Defoirdt |first3=Tom |last4=Bossier |first4=Peter |last5=Verstraete |first5=Willy |title=Nitrogen removal techniques in aquaculture for a sustainable production |journal=Aquaculture |date=September 2007 |volume=270 |issue=1–4 |pages=1–14 |doi=10.1016/j.aquaculture.2007.05.006 |bibcode=2007Aquac.270....1C |url=http://www.vliz.be/imisdocs/publications/261082.pdf }}</ref> Some farms use seaweed, such as those from the genera Ulva, to take excess nutrients out of the water and release oxygen into the ecosystem in a “recirculation system” while also serving as a source of income when they sell the seaweed for safe human consumption.<ref>{{cite journal |last1=Neori |first1=Amir |last2=Chopin |first2=Thierry |last3=Troell |first3=Max |last4=Buschmann |first4=Alejandro H. |last5=Kraemer |first5=George P. |last6=Halling |first6=Christina |last7=Shpigel |first7=Muki |last8=Yarish |first8=Charles |title=Integrated aquaculture: rationale, evolution and state of the art emphasizing seaweed biofiltration in modern mariculture |journal=Aquaculture |date=March 2004 |volume=231 |issue=1–4 |pages=361–391 |doi=10.1016/j.aquaculture.2003.11.015 |bibcode=2004Aquac.231..361N }}</ref>

Many designs are used, with different benefits and drawbacks, however the function is the same: reducing water exchanges by converting [[ammonia]] to [[nitrate]].  Ammonia (NH<sub>4</sub><sup>+</sup> and NH<sub>3</sub>) originates from the brachial [[excretion]] from the [[gill]]s of [[aquatic animal]]s and from the [[decomposition]] of organic matter.  As ammonia-N is highly toxic, this is converted to a less toxic form of nitrite (by ''[[Nitrosomonas]]'' sp.) and then to an even less toxic form of nitrate (by ''[[Nitrobacter]]'' sp.).  This "nitrification" process requires oxygen (aerobic conditions), without which the biofilter can crash.  Furthermore, as this nitrification cycle produces H<sup>+</sup>, the pH can decrease, which necessitates the use of buffers such as [[Lime (material)|lime]].