Tackling ammonia in effluents
In recent times, due to the limited availability of water resources and rapid industrialization, it is mandatory to preserve water quality. Ammonia is one of the most common pollutant observed in industrial and agricultural effluents. It is used as a bleaching agent in chemical industries and also in production of fertilizers, plastics and explosives. In effluents ammonia exists in equilibrium with ammonium ions. The concentration of ammonia gas increases with increase in pH and temperature. At neutral pH most of the ammonia exists as dissolved ammonium ions, but as the pH increases to 12, both ammonium ions and gaseous ammonia coexist in the effluent. The estimation of Ammonia in effluents is generally carried out by Nessler’s method and is determined as Ammonical Nitrogen (NH3-N) in ppm.
Although ammonia is a naturally occurring compound and an essential macronutrient for plant growth, it is also toxic to aquatic life at higher concentrations. The presence of gaseous Ammonia in effluent can lead to corrosion of utility lines and may release foul pungent odour in its vicinity. The regulatory bodies across the world have therefore raised their concerns regarding the hazards of aberrant discharge of effluents with high levels of ammonia. Thus it is imperative to address this issue before the arrival of an ecological crisis. The common methods adopted for are physical (ammonia stripping), chemical (oxidizing agents), Ion Exchange and Biological (Nitrosomonas/Nitrobacter).
The stripping of ammonia in effluents is carried out in a modified cooling tower. The pH of the effluent is kept above 9 and it is passed through a packed column provided with a sparger. Air at high pressure is constantly sparged through the column and the temperature is maintained at 120°F. The gaseous Ammonia is carried out of the effluent system through the generated air current. The process efficiency is highly dependent on the temperature of the system and requires high economic expenditure for maintenance of the utilities.
The removal of ammonia by treatment with oxidizing agents is also incorporated in settling tanks post secondary treatments. Sodium Hypochlorite is one of the most common reagent used to oxidize ammonia in effluents. But stringent monitoring of application is required due to the inherent toxicity of these reagents.
Ion exchange Chromatography:
Ion Exchange chromatography for ammonia removal involves passing the effluent through zeolite packed columns. The column containing Clinoptilolite (a form of Zeolite) has high affinity towards NH4+ ions and is able to scavenge all traces of the pollutant in the effluent. The recovery product obtained after treatment is ammonium sulphate, which can be used as a chemical fertilizer, while the treated water can be recycled for irrigation in agricultural fields. The only drawback of this process is the high cost involved in regeneration of column and disposal of residual wastes generated.
Biological ammonia removal in effluents is either aerobic ( Nitrifying and Denitrifying bacteria) or anaerobic (Anammox organisms). In the aerobic processes two group of bacteria sequentially convert Ammonia/ Ammonium ions(NH4+) to atmospheric Nitrogen (N2). The first group of bacteria are called Nitrifying Bacteria and generally involve bacterial species of Nitrosomonas spp, Nitrococcus spp. etc.These bacteria using their advanced enzyme machinery convert the dissolved ammonium ions to nitrites (NO2) and later nitrites (NO2) are converted to Nitrates (NO3). The second group of Bacteria are called Denitrifying Bacteria and involve bacterial species of Micrococcus spp., Pseudomonas spp. etc. These bacteria convert Nitrates (NO3) to atmospheric nitrogen (N2) under low oxygen conditions. Thus the coupled action of both nitrifying and denitrifying bacteria results in complete removal of ammonia from the effluent.
Anaerobic ammonia oxidation (Anammox) is a process where ammonical nitrogen is converted into atmospheric nitrogen under anaerobic condition. These bacteria are slow growers with a generation time of 10 days and hence they are generally cultured in sequential batch reactor . Some of the common examples of Anammox bacteria are Kuenenia stuttgartiensis, Anammoxoglobus spp.Therefore, microbial augmentation favors better efficiency of ammonia removal in effluents. Microbial growth utilizes the indigenous organic matter for development of biomass, thus providing a dual advantage of lowering the COD and Ammonical nitrogen content in the effluent.
To conclude one can find many methods to resolve the problem of ammonia in effluent, but to obtain a results that can sustain the need of the hour the best bet will rather be microbial augmentation.