Water is one of nature's most important gifts to mankind. It is a vital natural resource which forms the basis of all life. Water is a vital resource in all economic activities ranging from agriculture to industry. Due to the increasing human population, there is severe stress on water resources. In the twentieth century, vast amounts of gas, liquid, and solid waste were introduced into the environment due to the rapid industrialization thus leading to pollution.
Wastewater is formed when concentration of certain constituents of water gets increased beyond the background limit and thus cannot be useful for productive use. Waste-water quality may be defined by its physical, chemical, and biological characteristics. The physical constituents include the total solids, turbidity, colour, odour, temperature, density and conductivity. The chemical constituents are inorganic chemical characteristics (free ammonia, organic nitrogen, total kjeldehl nitrogen, nitrites, nitrates, phosphorous, pH, alkalinity, chloride, sulphate, etc) and organic chemical characteristics (biochemical oxygen demand, chemical oxygen demand, total organic carbon, etc). The biological characteristics of wastewater are coliform organisms, toxicity, etc. Both constituents and concentrations vary with time and local conditions.
All living organisms require the nutrient nitrogen for the production of amino acids, the primary constituents of compounds such as proteins and nucleotides, which are vital to cellular function. The availability of biologically useable forms of nitrogen plays a major role in controlling global productivity. Over 95% of total nitrogen is not available to most organisms, leaving the bioavailable, or fixed nitrogen in short supply. Fixed nitrogen is nitrogen that most organisms can assimilate into new biomass and is composed of the inorganic nitrogen species ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), and certain organic nitrogen species. These species are restricted to nitrogen that is not bonded to another nitrogen atom. The vast majority of the fixed nitrogen in the water bodies is in the form of NO3-. Non-bioavailable nitrogen is composed of dissolved dinitrogen (N2) and nitrous oxide (N2O) gases, which cannot be assimilated by most biota, except for a small group of organisms that can convert N2 into fixed forms.
In nature, nitrogen occurs in several forms and oxidation states, with organic and inorganic compounds that exhibit a wide range of reaction, transformation or transport pathways in the biosphere. The most common and important forms of nitrogen in wastewater and their corresponding oxidation state in water environment are ammonia (NH3, -III), ammonium (NH4+, -III), nitrogen gas (N2, 0), nitrite (NO2-, +III) and nitrate (NO3-, +V). The various forms of nitrogen can deplete dissolved oxygen levels in receiving waters, stimulate aquatic growth, exhibit toxicity toward aquatic life, affect chlorine disinfection efficiency, present public health hazards, and affect the suitability of wastewater reuse. Nitrogenous materials enter the aquatic environment from natural or man-caused sources. Natural sources include precipitation, dustfall, non-urban runoff, and biological fixation. Fertilization of agricultural land and combustion of fossil fuels lead to increase quantities of nitrogen in the aquatic environment. Other man-related sources include urban and livestock feedlot run-off, municipal wastewater effluents, and subsurface drainage wastes.
Depending on the kind of nitrogen compounds present in wastewater, nitrogen removal requires up to three processes in sequence: ammonification, nitrification, and denitrification.
The main organic nitrogen compounds in wastewater are heterocyclic compounds (e.g., nucleic acids) and proteins. Proteolysis and degradation of amino acids leads to liberation of ammonia by the various mechanisms of ammonification including hydrolytic, oxidative, reductive, and desaturative deamination. A significant amount of ammonia from ammonification of amino acids is assimilated in aerobic treatment processes for growth of bacteria (surplus sludge formation). It can be estimated that bacteria consist of roughly 50% protein and that the nitrogen content of protein is about 16%. Thus, for synthesis of 1 g of bacterial biomass, about 0.08 g of ammonia-N is required. To eliminate ammonia that is not used for cell growth during wastewater treatment, it must first be nitrified and then denitrified to molecular nitrogen or anaerobically oxidized with nitrite.
Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. Degradation of ammonia to nitrite is usually the rate limiting step of nitrification. Biological nitrification is the work of two bacterial genera: Nitrosomonas, which oxidize ammonia to the intermediate product nitrite, and Nitrobacter, which convert nitrite to nitrate as shown in equation 1 and 2.
Denitrification is the removal of oxidized nitrogen from wastewater by converting it to N2 gas which escapes to the atmosphere. Under anoxic conditions nitrite and nitrate serve as electron acceptors instead of O2 and organic substrates as electron donors for ATP production at very low oxygen concentration. Besides heterotrophic denitrification, denitrification can also be performed by chemolitho-autotrophic bacteria with H2 or with reduced sulfate compounds as electron acceptors.se reactions are slow and require long