Nitrification and de nitrification

nitrification and denitrification

1. Briefly explain nitrification and de-nitrification process in the waste water treatment. Explain the techniques of removing phosphates from the waste water 1RR18CV042

2. NITRIFICATION Nitrification is the biological process by which ammonia is first converted to nitrite and then to nitrate. Nitrification can be achieved in any aerobic-biological process at low organic loadings and where suitable environmental conditions are provided. Nitrifying bacteria are slower growing than the heterotrophic bacteria, which comprises the greater proportion of the biomass in both fixed film and suspended growth systems. The key requirement for nitrification to occur, therefore, is that the process should be so controlled that the net rate of accumulation of biomass, and hence, the net rate of withdrawal of biomass from the system, is less than the growth rate of the nitrifying bacteria . The processes currently used in the treatment of wastewater for nitrification are presented as follows. 1RR18CV042

3. Trickling filters The extent of nitrification in trickling filters depended on a variety of factors; including temperature, dissolved oxygen, pH, presence of inhibitors, filter depth and media type, loading rate, and wastewater BOD   Rotating biological contractor RBC biofilm has an initial adsorption of microorganisms to the disk surface to form 1-4 mm thick biofilm that is responsible for BOD removal in rotating biological contractors.  Fixed bed reactor . 1RR18CV042

4. Conventional activated sludge processes at low loadings  the nitrification in a conventional activated sludge system and found that it was relatively low for carbon removal and nitrification of sewage because carbon removal and nitrification occurred in the same reactor with an activated sludge system. Two-stage activated sludge systems with separate carbonaceous oxidation and nitrification systems The nitrification process requires a slow-growing nitrifying bacteria with sludge that has been aged for a long time and high dissolved oxygen concentration. In addition, they were susceptible to inhibition by a wide range of compounds at concentrations so low as not to affect the heterotrophic bacteria.  1RR18CV042

5. DENITRIFICATION Denitrification is the biological process by which nitrate is converted to nitrogen and other gaseous end products. The requirements for the denitrification process are: a) nitrogen present in the form of nitrates b) an organic carbon source c) an anaerobic environment The processes currently used for biological denitrification are presented as follows. 1RR18CV042

6. The first step of oxidation of ammonia is brought about by microbes in the soil which includes bacteria of the genus  Nitrosomonas  and  Nitroso coccus,  and  arcae like Nitrosopumilus maritimus  and  Nitrososphaera Viennese.  The conversion of ammonia into nitrite is the rate-limiting step of nitrification. The second reaction is performed by bacteria of the genus  Nitrobacter  and  Nitrospira. All these microorganisms are chemoautotrophs that utilize the energy from the reaction to produce organic carbon compounds. Nitrification is important in many organisms as it is the only process of obtaining nitrogen source for some microorganisms present in the soil. These organisms convert ammonia into nitrates which is more soluble than ammonia and thus can be taken into the system more conveniently. Besides, it is also important in agricultural systems where ammonia is used as a fertilizer. 1RR18CV042

7. The ammonia is then converted into nitrate which facilitates nitrogen leaching into the plants. Nitrifying bacteria also play an important role in wastewater treatment where different nitrogen compounds are converted into nitrates and then nitrogen before removing the gas out of the water. The nitrification process is controlled by several factors like the availability of oxygen, soil moisture, and the availability of ammonia. The activity of the nitrifying bacteria also decreases in acidic conditions and at a temperature above 35°C. 1RR18CV042

8. DENITRIFICATION Denitrification is a biological process of reduction of nitrate into nitrite, which is then followed by the reduction of nitrate into nitrogen gas that usually results in the removal of nitrogen gas into the air. Denitrification, like nitrification, is a microbial process that is performed by various groups of microorganisms. It is also an important step in the nitrogen cycle where nitrogen is released back into the atmosphere from the ground. In this case, the oxidized products of nitrogen are reduced to its gaseous forms, mainly nitrous oxide (NO 2 ) and nitrogen gas (N 2 ). Denitrification, unlike nitrification, is performed by facultative anaerobes that perform denitrification as anaerobic respiration to reduce oxidized forms into gases. Denitrification takes place at about 10% or less concentration of oxygen and organic carbon compounds. 1RR18CV042

9. The process of denitrification takes place through a set of half-reactions, which are: NO 3 –  + 2H +  + 2e –     →   NO 2 –  + H 2 O NO 2 −  + 2 H +  + e −    →   NO + H 2 O 2NO + 2 H +  + 2 e −   →   N 2 O + H 2 O N 2 O + 2 H +  + 2 e −   →   N 2  + H 2 O The overall reaction can be represented as: 2 NO 3 −  + 10 e −  + 12 H +     →    N 2  + 6 H 2 O 1RR18CV042

10. The process is primarily performed by heterotrophic bacteria like  Para coccus dentifrices  and some species of Pseudomonas, but some autotrophic denitrifies like  Thiobacillus dentifrices  are also present. Denitrification is an important microbiological process that is performed naturally in both terrestrial and marine environments. Besides, denitrification follows nitrification in wastewater treatments to convert nitrogen-rich compounds into nitrogen gas before being released into the atmosphere. However, sometimes denitrification can be disadvantageous by removing the NO 3 –  present in the soil, thus reducing the extent of leaching. Denitrification is controlled by various factors like the concentration of oxygen and carbons, even though some aerobic bacteria of the genus Proteobacteria, might facilitate denitrification even in the presence of oxygen. 1RR18CV042

11. TECHNIQUES OF REMOVING PHOSPHATES FROM THE WASTE WATER 1RR18CV042

12. The main phosphate removal processes are : Treatment of raw/primary wastewater Treatment of final effluent of  biological plants (post precipitation) Treatment contemporary to the secondary biologic reaction (co-precipitation). The first process is included in the general category of chemical precipitation processes. Phosphorous is removed with 90% efficiency and the final P concentration is lower than 0.5 mg/l. The chemical dosage for P removal is the same as the dosage needed for BOD and SS removal, which uses the main part of these chemicals. As mentioned above lime consumption is dependent on the alkalinity of the wastewater: only 10% of the lime fed is used in the phosphorous removal reaction. The remaining amount reacts with water alkalinity, with softening. 1RR18CV042

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14. The post precipitation is a standard treatment of a secondary effluent, usually using only metallic reagents. It is the process that gives the highest efficiency in phosphorous removal. Efficiency can reach 95%, and P concentration in the effluent can be lower than 0.5 mg/l. Post precipitation gives also a good removal of the SS that escape the final sedimentation of the secondary process. Advantage is also to guarantee purification efficiency at a certain extent even if the biological process is not efficient for some reason. The chemical action is stronger, since the previous biologic treatment transforms part of the organic phosphates in orthophosphates. Disadvantages are high costs for the treatment plant (big ponds and  mixing  devices) and sometimes a too dilute effluent. Using ferric salts there is also the risk of having some iron in the effluent, with residual coloration. The metallic ions dosage is about 1.5-2.5 ions for every phosphorus ion (on average about 10-30 g/mc of water). 1RR18CV042

15. The coprecipitation process is particularly suitable for active sludge plants, where the chemicals are fed directly in the aeration tank or before it. The continuous sludge recirculation, together with the coagulation-flocculation and adsorption process due to active sludge, allows a reduction in chemical consumption. Moreover the costs for the plant are lower, since there is no need for big post precipitation ponds. In this process the chemical added are only iron and aluminum, lime is added only for pH correction. Lower costs and more simplicity are contrasted by a phosphorous removal efficiency lower than with post precipitation (below 85%). The phosphorous concentration in the final effluent is about 1 mg/l. Another disadvantage is that biological and chemical sludge are mixed, so they cannot be used separately in next stages. Mixed  sludge  need bigger sedimentation tanks than activated sludge. 1RR18CV042

16. In the anaerobic zone : Under anaerobic conditions, PAO assimilate fermentation products (i.e. volatile fatty acids) into storage products within the cells with the concomitant release of phosphorous from stored polyphosphates. Acetate is produced by fermentation of bs COD, which is dissolved degradable organic material that can be easily assimilated by the biomass. Using energy available from stored polyphosphates, the PAO assimilate acetate and produce intracellular polyhydroxy butyrate (PHB) storage products. Concurrent with the acetate uptake is the release of orthophosphates, as well as magnesium , potassiu m , calciumcations . The PHB content in the PAO increases as the polyphosphate decreases. Biological processes 1RR18CV042

17. In the aerobic zone : energy is produced by the oxidation of storage products and polyphosphate storage within the cell increases. Stored PHB is metabolized, providing energy from oxidation and carbon for new cell growth. Some glycogen is produced from PHB metabolism. The energy released from PHB oxidation is used to form polyphosphate bonds in cell storage. The soluble orthophosphate is removed from solution and incorporated into polyphosphates within the bacterial cell. PHB utilization also enhances cell growth and this new biomass with high polyphosphate storage accounts for phosphorous removal. As a portion of the biomass is wasted, the stored phosphorous is removed from the biotreatment reactor for ultimate disposal with the waste sludge. The amount of phosphorous removed by biological storage can be estimated from the amount of bs COD that is available in the wastewater influent. Better performance for BPR systems is achieved when bs COD acetate is available at a steady rate 1RR18CV042

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