3Denitrification is the process of removing nitrate(NO3) or nitrites(NO2) from ground and/or surface waters by reducing them, in presence of a electron donor, to nitrogen gas(N2).The electron donor, usually carbon, is referred to as the energy source.
4But isn’t nitrate important for life? Nitrate is a form of nitrogen usually found in the soil. Most crops need nitrogen as it forms part of DNA and proteins in plant and animal cells. Hence in the right amounts, nitrogen is essential to all forms of life.Crops usually obtain it by absorbing it in form of nitrates from the soil while animals get it from feeding on plants.
5Major Sources include: Nitrogen-fixation from atmosphere by lightning, leguminous plants & N-fixing bacteria.FertilizersManureMunicipal wastewater
6Why then denitrify?As a result of leaching, nitrates usually find their way to groundwater.When found in concentrated amounts in ground water and surface water, such water can have harmful effects both to humans and animals if used for drinking.
7Drinking water high in nitrates concentration results to an interference with one’s red blood cells ability to transport oxygen in the body.The result to both humans and animals is an illness known as methemoglobinemia, mainly found in infants.Hence there is need to monitor groundwater used for drinking and carry out de-nitrification if found to have excess nitrates.
8The maximum limit, as set by the World Health Organization is: For humans: 10 mg/ℓ NO3For animals: 100 mg/ℓNO3
9Apart from biological de-nitrification, are there other methods that can be used?
10Other processes can be used such as: 1. Physical processes – Reverse osmosis, ion exchange and electrodialysis2. Chemical de-nitrificationPROS’Highly EfficientCONS’High operating costsPROS’Highly EfficientCONS’Ammonia is formed
11Advantages of Biological Denitrification: High nitrate removal efficiency,Moderate operating costHigh process stability and reliabilityThe process can be controlled with ease
12Biological Denitrification Process The raw water to be denitrified is fed into a bioreactor (with the presence of denitrifying bacteria-innoculum)Most wastewater to be denitrified contains enough carbon(energy) source for the process and thus with the right conditions, nitrates are converted to N2 gas through a number of reduction steps:
13Nitrates are first converted into nitrites, then to nitric oxide, then to nitrous oxide and finally to nitrogen.NO3- →NO2- → NO → N2O → N2Nitric oxide, nitrous oxide and nitrogen are gaseous products and they can be safely released into the atmosphere.
14Commonly used denitrifying bacteria include: LactobacillusPseudomonasAchromobacterBrevibacteriumAerobacterBacillusProteusFlavobacteriumAlcaligenesMicrococcus
15If water to be treated is deficient in dissolved carbon, then additional carbon(energy) source/ electron donor must be supplied.
16Commonly used energy sources include: MethanolEthanolAcetateGlucoseMolassesLactate
18Sawdust as an Energy Source Research has shown that biodegradable solid waste can be used effectively as a carbon(energy) source.Research carried out on the effectiveness of sawdust for raw water containing mgN/L, which is a probable value for groundwater, has shown nitrates removal of up to 100%The usability of sawdust as a carbon source presents a great advantage since it is readily available as a waste product of the forest industry.
19Sawdust is mainly composed of cellulose, hemicellulose and lignin. Of the three, Cellulose forms the greatest percentage and is the major source of carbon.For sawdust to be used as an energy source, a special set of bacteria capable of degrading cellulose, needs to be availed.
20Examples of bacteria that can degrade cellulose: i) Fermentative anaerobes, e.g. Clostridium, Ruminococcus & Butyrivibrio,spp.ii) Aerobic bacteria, e.g. Cytophaga, Sporocytophaga, spp. Cellulomonas & Thermobifida, spp.
21These bacteria degrade cellulose to form products such as methane, acetate, ethanol which avail the carbon necessary for de-nitrification.
22What are the optimum conditions for this bio-reaction?
23pHDifferent microorganisms require different pH levels to thrive. The optimum pH for de-nitrification has been found to be between 6-8.
24Temperature:Temperatures of between 20-30˚C have been found to be optimum.