Nitrogen removal techniques
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| Nitrogen removal techniques |
Introduction
While aquaculture industry intensively develops, its environmental impact increases. Discharges from aquaculture deteriorate the receiving environment and the need for fishmeal and fish oil for fish feed production increases.
Rotating biological contactors, trickling filters, bead filters and fluidized sand biofilters are conventionally used in intensive aquaculture systems to remove nitrogen from culture water.
Besides these conventional water treatment systems, there are other possible modi operandi to recycle aquaculture water and simultaneously produce fish feed.
These double-purpose techniques are the periphyton treatment technique, which is applicable to extensive systems, and the proteinaceous bio-flocs technology, which can be used in extensive as well as in intensive systems.
In addition to maintenance of good water quality, both techniques provide an inexpensive feed source and a higher efficiency of nutrient conversion of feed. The bioflocs technology has the advantage over the other techniques that it is relatively inexpensive; this makes it an economically viable approach for sustainable aquaculture.
Overview of problem
Aquaculture is a rapidly growing food producing sector. The sector has grown at an average rate of 8.9% per year since 1970, compared to only 1.2% for capture fisheries and 2.8% for terrestrial farmed meat-production systems over the same period (FAO, 2004).
In contrast to aquaculture, capture fisheries landings as a whole isstagnant. Although catch rates for some species did not decline during the 1990s, most ocean fisheries stocks are now recognized as fully or over fished.
The worldwide decline of ocean fisheries stocks and the further expansion of the human population are an incentive for the further growth of aquaculture.
Despite the growth of the sector,aquaculture production still needs to increase 5-fold in the next 2 decades in order to satisfy the minimum protein requirement for human nutrition (FAO, 2004).
The intensive development of the aquaculture industry has been accompanied by an increase in environmental impacts. The production process generates substantial amounts of polluted effluent, containing uneaten feed and feces (Read and Fernandes, 2003).
Discharges from aquaculture into the aquatic environment contain nutrients, various organic and inorganic compounds such as ammonium, phosphorus, dissolved organic carbon and organic matter (Piedrahita, 2003;Sugiura et al., 2006).
The high levels of nutrients cause environmental deterioration of the receiving water bodies. In addition, the drained water may increase the occurrence of pathogenic microorganisms and introduce invading pathogen species (Thompson et al., 2002).
To produce 1 kg live weight fish one needs 1–3 kg dry weight feed (assuming a food conversion ratio about 1–3)(Naylor et al., 2000).
About 36% of the feed is excreted as a form of organic waste (Brune et al., 2003). Around 75% of the feed N and P are unutilized and remain as waste in the water (Piedrahita, 2003; Gutierrez-Wing and Malone,2006).
An intensive aquaculture system, which contains 3 ton tilapia, can be compared on a biomass basis to a human community with 50 inhabitants (Helfman et al.,1997).
This intensive aquaculture system can also be compared on grounds of waste generation to a community of around 240 inhabitants (Aziz and Tebbutt, 1980;Flemish government, 2005). It can thus be concluded that live fish biomass generates approximately 5 times more waste than live human biomass.
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| Nitrogen removal techniques |
The reason is that the scope of digestion in fish is limited; a relatively large fraction of feed remains undigested and is excreted (Amirkolaie, 2005).
The feeding habit of fish is reflected in the digestive anatomy. The gut length of fish is short and the ratio of gut length to body length is small(Hertrampf and Piedad-Pascual, 2000).
For instance, the intestine of carp is 2.0–2.5 times longer than the body,while that of cattle and sheep is respectively 20 and 30 times longer.
The human intestine is about 3 to 4 times longer than the body. Consequently, in fish, the chymestays in the gut only for a short time. For this reason, fish feed must have a high digestibility.
Typically, fish body contains 65 to 75% protein (Hertrampf and Piedad-Pascual, 2000). In addition, fish use proteins for energy production to a large extent, unlike terrestrial animals that use mostly carbohydrates and lipids (Hepher, 1988).
Fish protein requirement, therefore, is about two to three times higher than that of mammals. Ammonium is one of the end products of protein metabolism (Walsh and Wright,1995).
All these factors contribute to the high nitrogen residues in aquaculture water. In water, NH3 (ammonia) and NH4+(ammonium) are in equilibrium depending on the pH and the temperature (Timmons et al.,2002).
The sum of the two forms is called total ammonium nitrogen (TAN). Although both NH3 and NH4+ may be toxic to fish, unionized ammonia is the more toxic form attributable to the fact that it is uncharged and lipid soluble and consequently traverses biological membranes more readily than the charged and hydrated NH4+ions (Körner et al., 2001).
Ammonia-N is toxic to commercially cultured fish at concentrations above1.5 mg N/l. In most cases, the acceptable level of unionized ammonia in aquaculture systems is only 0.025 mgN/l (Neori et al., 2004; Chen et al., 2006).
However, the toxicity threshold depends strongly on the species, size,fine solids, refractory organics, surface-active com-pounds, metals, and nitrate (Colt, 2006).
In addition to the generation of large amounts of waste,the use of fish meal and fish oil as prime constituents of feed is another non-sustainable practice in aquaculture.
Approximately one-third of the global fishmeal production is converted to aquaculture feeds (Delgado et al.,2003). The proportion of fish meal supplies used for fish production increased from 10% in 1988 to 17% in 1994 and 33% in 1997 (Naylor et al., 2000).
Hence, aquaculture is a possible panacea, but also a promoter of the collapse of fisheries stocks worldwide. The ratio of wild fish:fed farmed fish (both live weight base) is about 1.41:1 for tilapia and 5.16:1 for marine finfish, (Naylor et al., 2000).
Purchase of commercially prepared feed for fish culture comprises 50% or more in the production costs; this is primarily due to the cost of the protein component (Bender et al., 2004).
On average some 25% of the nutrient input of these feed sources is converted into harvestable products (Avnimelech and Lacher, 1979;Boyd, 1985; Muthuwani and Lin, 1996; Avnimelech and Ritvo, 2003).
To make further sustainable increase of aquaculture production possible, the search for inexpensive protein sources and a higher efficiency of nutrient conversion of feed is needed.
By Crab, Roselien & Avnimelech, Yoram & Defoirdt, Tom & Bossier, Peter & Verstraete
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