Monday, July 1, 2013

Nitrate Accumulation in Denitrification Systems – The Role of Dissolved Oxygen and Substrate Limitation.

This is an excerpt from an article written by JoAnn Silverstein from the University of Colorado on what kind of bacteria is needed to make dissimilative denitrification. It is a long article so I though I would only give the pertinent information that relates to the Anoxic Filtration System and is one way of removing nitrates from a closed recirculating system like our ponds. So if there was any uncertainty in your minds about what I have been saying, I hope this article will reinforce what you have been reading.
Also I have added some test-tubes that show bacteria and the placement of that bacterium in relationship to oxygen levels. Look at test-tube number 3 with the Facultative bacteria and how evenly it is displaced in the test-tube. This is why when test were done to see if oxygen levels in the Anoxic Filter made an impact on the overall oxygen levels of the pond; it showed negative. The water that came out of the filter had the same oxygen levels as that of the ponds bulk water.

 Nitrate Accumulation in Denitrification Systems –

The Role of Dissolved Oxygen and Substrate Limitation
JoAnn Silverstein
Department of Civil
University of Colorado
CB 428
Boulder, CO 80309-0428
Abstract not


       “Recirculating aquaculture systems (RASs) rear high densities of fish while the culture water is continuously recycled, thereby employing water conservation techniques. Since fresh water addition is minimized, the quality of the culture water can deteriorate quite rapidly from the accumulation of ammonia and particulate waste generated from the metabolism of feed. Aquaculturists employ common wastewater treatment techniques in RASs to yield an environment that is conducive to rearing aquatic organisms. Solids removal is typically achieved through clarification or filtration, while nitrification is employed to convert ammonia to nitrate, via nitrite, in order to prevent free ammonia toxicity (8). The combined implementation of the nitrification process and decreased water exchanges leads to the accumulation of nitrates over time in recirculating aquaculture systems (3). Chronic toxicity to certain fish species (6), as well as tightening water regulations with regard to nutrient discharge, have led to concern over the accumulation of nitrates in recirculating systems.

Biological denitrification can be used to remove nitrates from RAS waters. Denitrification is the dissimilative reduction of nitrate (NO3-) to nitrogen gas (N2), through the production of nitrite (NO2-) and gaseous nitric oxide (NO) and nitrous oxide (N2O) intermediates.

This process is performed by heterotrophic bacteria under anoxic conditions and uses nitrate as a terminal electron acceptor in the presence of a carbon and energy source.
An electron donor is required as a carbon and energy source to fuel the denitrification process. Dissolved organic carbon (DOC) compounds accumulate in RASs as a result of the introduction of feed, and the extent of accumulation is greatly affected by fish stocking densities and feeding rates (5). However, these systems typically possess relatively low concentrations of DOC (3). Wastewater treatment plants often add an exogenous carbon source, such as methanol or acetate, when a carbon deficiency exists (2, 11), though the associated cost does not make this an attractive option for aquaculturists. Growing NO3 - NO2 – NO (g) N2O (g) N2 (g) 172 interest has been expressed for using biosolids as a carbon supplement in the denitrification process. Fermented municipal sludge and swine waste have been shown to be good electron donors, effecting enhanced denitrification rates over methanol and acetate alone (7). Fish waste and uneaten feed constitute a source of organic matter produced within the fish culture unit that can be used to generate a suitable carbon source for the denitrification process (1, 10). Since this organic matter is in the particulate form and not readily available for microbial use, hydrolysis and fermentation can be applied to convert these substances into volatile fatty acids (VFAs), which can be more easily consumed by denitrifying microorganisms (4, 7). The use of an organic substrate that is prevalent in the system is aimed towards the development of a self-sustaining treatment process. In addition, the amount of particulate waste requiring disposal is reduced by converting a fraction of the particulate matter into a soluble form that is consumed by the denitrification process.

Biofilters are an attached growth process in which a biofilm is generated from the propagation of microorganisms on an inert surface. Biofilters maintain a higher active fraction of biomass, as compared to suspended growth environments, which enables the use of a smaller reactor (9). The efficient operation and compact size makes biofilters an attractive treatment device for the aquaculture industry, as is illustrated by their wide scale use in the performance of nitrification. Complete nitrogen removal can be achieved in recirculating aquaculture systems through the implementation of a coupled biofiltration treatment scheme employing nitrification and denitrification.

This study was designed to investigate the removal of nitrates from recirculating aquaculture system waters using a denitrifying biofilter to reduce nitrate to nitrogen gas and a supplemental carbon source provided through the fermentation of fish food. Implications for full-scale operation are discussed.”

Aerobic and anaerobic bacteria can be identified by growing them in liquid culture:
3. Facultative anaerobic organism (continuum with "facultative aerobic organism")

Photo above of test tubes from:
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Anoxic Filtration Book... Still free on Apple's iBook store


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