The following article will guide you about how to remove nitrogen from sewage effluents.
Nitrogen in sewage can exist in four forms: organic nitrogen, ammonia nitrogen, nitrite nitrogen and nitrate nitrogen. Organic nitrogen and ammonia nitrogen are the principal forms in untreated sewage. As shown in Table 8.1, the total nitrogen concentration in untreated sewage varies from 20 to 85 mg/l.
Of this total, about 40 per cent is organic nitrogen and 60 per cent is ammonia nitrogen. The organic nitrogen plus ammonia nitrogen is termed as Total Kjeldahl Nitrogen (TKN). In nature, in the nitrogen cycle (see Fig. 8.2), organic nitrogen and ammonia nitrogen are converted first to nitrite and then to nitrate. The overall reaction starting with ammonia is-
For this reaction to go to completion, 4.57 g of oxygen are required per g of ammonia nitrogen. This oxygen demand is known as nitrogenous oxygen demand (NOD). It may also be noted that 7.1 g of alkalinity (as CaCO3) is required per g of ammonia nitrogen.
The problem with nitrogen in sewage is related primarily to the oxygen demand that can be exerted if sewage having large amount of ammonia nitrogen is discharged to a body of water. Further nitrogen in the form of ammonia is toxic to fish and excess quantities of nitrogen in any form may lead to eutrophication of surface waters.
Hence discharge of sewage having large amount of nitrogen to a body of water may result in public health hazard. The problem can be mitigated either by converting the ammonia and organic nitrogen to nitrate or by eliminating the nitrogen from the sewage before it is discharged to a body of water.
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The methods commonly adopted for the removal of nitrogen from sewage are as indicated below:
1. Biological nitrification—denitrification
2. Physical and chemical processes
1. Biological Nitrification—Denitrification:
The removal of nitrogen by this method is carried out in two steps viz. as discussed below:
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(i) Nitrification, and
(ii) Denitrification
In nitrification process the nitrogen present in the sewage in the form of ammonia is aerobically converted to nitrate in two steps by nitrifying autotrophic bacteria. In the first step the ammonium ion is converted to nitrite by Nitrosomonas a genera of aerobic autotrophic bacteria that utilize ammonia as their sole source of energy.
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This energy reaction is indicated by the following equation:
In the second step nitrite is converted to nitrate by Nitrobacter which is also a genera of aerobic autotrophic bacteria that utilize nitrite as their sole source of energy.
This energy reaction is indicated by the following equation:
The biological processes used for nitrification are identified as aerobic suspended growth processes and aerobic attached growth processes. Nitrification can be accomplished either in the same reactor used for the removal of carbonaceous BOD or in a separate reactor.
When nitrification is accomplished along with carbonaceous BOD removal or carbon oxidation in the same reactor the process is known as combined carbon oxidation nitrification process or single-stage nitrification process. On the other hand when carbonaceous BOD removal or carbon oxidation and nitrification are accomplished in separate reactors the process is known as separate-stage nitrification or two-stage nitrification process.
Fig. 18.5 shows a flow diagram for a separate-stage nitrification or two-stage nitrification process. In the first-stage reactor carbonaceous material is oxidized. The second-stage reactor, enriched with nitrifying autotrophic bacteria, receives the first-stage effluent with residual carbon and almost entire amount of ammonia for nitrification. The separate-stage nitrification process provides flexibility in operation to achieve optimum performance.
Combined Carbon Oxidation Nitrification Process:
Combined carbon oxidation nitrification process can be accomplished in any of the suspended-growth activated sludge processes by maintaining conditions suitable for the growth of nitrifying organisms. For example, in most warm climates, increased nitrification can be brought about simply by increasing the mean cell residence time and the supply of air.
The two attached-growth processes that can be used for combined carbon oxidation nitrification processes are the trickling filter and rotating biological contactor. As with the suspended-growth processes, nitrification in the attached-growth process can also be brought about or encouraged by suitable adjustment of the operating parameters. This can usually be accomplished by reducing the applied loading rate.
Separate-Stage Nitrification:
Both suspended-growth and attached-growth processes are used to achieve separate-stage nitrification. In most details, separate-stage suspended-growth nitrification processes are similar in design to the activated-sludge process. Both continuous-flow stirred-tank staged-flow reactors and plug-flow reactors may be used. When very low ammonia concentrations are desired, the staged-flow or plug-flow reactors are preferred.
Three different types of attached-growth processes are used for separate-stage nitrification. They are trickling filter process, rotating biological contactor and packed-bed reactor. As shown in Fig. 18.6 a packed-bed reactor consists of a container (reactor) that is packed with a medium to which nitrifying microorganisms can become attached. Sewage is introduced from the bottom of the reactor through an appropriate underdrain inlet chamber. Air or pure oxygen necessary for the process is also introduced with the sewage.
The suspended-growth nitrification process is significantly affected by the following factors: ammonia nitrite concentration, BOD5/TKN (Total Kjeldahl Nitrogen) ratio, Dissolved Oxygen (DO) concentration, temperature, and pH.
The effect of these factors on the nitrification process is discussed below:
It has been observed that the concentration of ammonia and nitrate affects the maximum growth rate of nitrosomonas and nitrobacter. Further the growth of nitrobacter is considerably greater than that of nitrosomonas and hence ammonia-nitrite concentration is significant in the nitrification process.
Nitrifying organisms are present in almost all aerobic biological treatment processes, but usually their numbers are limited. The fraction of nitrifying organisms present in the mixed liquor of activated sludge process has been correlated to the BOD5/TKN ratio. For BOD5/TKN ratios between 1 and 3, which roughly correspond to the values encountered in separate-stage nitrification systems, the fraction of nitrifying organisms is estimated to vary from 0.21 at a ratio of 1, to 0.083 at a ratio of 3.
In most activated sludge processes, the fraction of nitrifying organisms would therefore be considerably less than the 0.083 value. It has been found that when the BOD5/TKN ratio is greater than about 5, the process can be classified as a combined carbon oxidation nitrification process, and when the ratio is less than 3, it can be classified as a separate-stage nitrification process.
The Dissolved Oxygen (DO) level affects the maximum growth rate of the nitrifying organisms.
Temperature has a significant effect on nitrification rate. The overall nitrification rate decreases with decreasing temperature.
Nitrifying bacteria are quite sensitive to pH. It has been observed that the maximum rate of nitrification occurs between pH values of about 7.2 and 9.0.
The performance of the attached-growth nitrification processes is significantly affected by the loading rate. In general with the reduction in the loading rate the nitrification increases. For example in a trickling filter with rock medium, as the loading rate varies from 0.16 to 0.10 kg BOD5 per m3 per day the nitrification varies from 75 to 85 per cent, and when the loading rate varies from 0.10 to 0.05 kg BOD5 per m3 per day the nitrification varies from 85 to 95 per cent.
(ii) Biological Denitrification:
In denitrification process the nitrogen present in the sewage in the form of nitrate is converted to nitrogen gas that escapes from the sewage. Biological denitrification is accomplished under anaerobic conditions by nitrate-reducing bacteria which utilize nitrate as a hydrogen acceptor.
Most of the nitrate- reducing bacteria are facultative anaerobic heterotrophs the principal genera of which are Pseudomonas, Micrococcus, Achromobacter and Bacillus. These bacteria require organic carbon for energy while carrying out denitrification and hence in the denitrification process it is necessary to have a source of organic carbon.
The sources of organic carbon used are the endogenous decay of the organisms or the carbon present in the sewage, or the supplemental sources of organic carbon such as methanol or some other suitable material.
Using methanol (CH3OH) as the organic carbon source, the energy reaction may be represented by the following equations:
The biological processes used for denitrification are also identified as suspended growth processes and attached growth processes. Further denitrification can also be accomplished either in combination with carbon oxidation nitrification process in a single unit process without any intermediate steps or in a separate reactor.
In the combined carbon oxidation nitrification-denitrification processes, either the endogenous decay of the organisms or the carbon in the sewage is used to achieve denitrification. In separate reactors used for denitrification, methanol or some other suitable material is used to achieve denitrification.
The advantages of combined processes are:
(1) Reduction in the volume of air needed to achieve nitrification and BOD5 removal;
(2) Potential elimination of the supplemental organic carbon sources (e.g., methanol) required for complete denitrification; and
(3) Elimination of intermediate clarifiers required in a staged nitrification-denitrification system.
However, in combined processes denitrification rates are low, large structures are required and there is lower nitrogen removal than in methanol-based systems. As such denitrification is usually carried out in a separate reactor.
Fig. 18.7 shows a flow diagram for a separate suspended-growth denitrification process. The reactor used in this case is similar to that used in a normal activated sludge process with recycle of denitrifying organisms and with mixing but no aeration. Both continuous-flow stirred-tank and plug-flow reactors may be used, but plug-flow reactors are usually preferred.
Mixing in the reactor is accomplished using submerged paddles which mix the contents sufficiently to keep the bacterial mass in suspension without producing undue aeration. The nitrogen gas released during the denitrification process often becomes attached to the biological solids, and hence a nitrogen release step is included between the reactor and the settling facilities that are used to separate the biological solids.
The removal of the attached nitrogen gas bubbles can be accomplished either in aerated channels that can be used to connect the biological reactor and the settling facilities or in a separate tank in which the solids are aerated for a short period of time (30 to 60 minutes).
The suspended-growth denitrification process is significantly affected by the following factors – nitrate concentration, carbon concentration, temperature and pH.
The effect of these factors on the denitrification process is discussed below:
The concentration of nitrate affects the maximum growth rate of the organisms responsible for denitrification.
The carbon concentration also affects the maximum growth rate of the organisms responsible for denitrification.
The temperature affects the effluent quality (measured in terms of the amount of nitrate in the effluent) which will deteriorate at low temperatures.
The optimum pH range is between about 6.5 and 7.5 and the optimum condition is around 7.0.
A flow diagram for a three-stage suspended-growth biological treatment process for the removal of nitrogen from domestic sewage is shown schematically in Fig. 18.8 which incorporates organic carbon conversion (or carbonaceous BOD removal or carbon oxidation), nitrification and denitrification.
Typical design parameters for each of the processes shown in Fig. 18.8 are given in Table 18.2. As shown in Fig. 18.8 and reported in Table 18.2, a continuous-flow stirred-tank reactor is used for the activated sludge process for organic carbon conversion, and plug-flow mixed reactors are used for nitrification and denitrification.
A number of separate attached-growth denitrification processes have been developed out of which packed-bed reactor is commonly used. The reactor is completely submerged so that no air can enter. Alternatively the reactor is covered and filled with nitrogen gas, which eliminates the necessity of having to submerge the medium to maintain anaerobic conditions. The performance of the attached-growth denitrification processes is significantly affected by the loading rate. In general with the reduction in the loading rate the denitrification increases.
2. Nitrogen Removal by Physical and Chemical Processes:
The principal physical and chemical processes used for nitrogen removal are:
(i) Air stripping of ammonia,
(ii) Breakpoint chlorination; and
(iii) Ion exchange.
(i) Air Stripping of Ammonia:
The air stripping of ammonia from sewage is a modification of the aeration process used for the removal of gases dissolved in water. At neutral pH (i.e., pH = 7) nitrogen is present in sewage in soluble ionic ammonium (NH4+) form.
The ammonium ions in sewage exist in equilibrium with dissolved ammonia gas (NH3) as shown in the following equation:
The equilibrium of ammonium ions and dissolved ammonia gas in sewage is controlled by both pH and temperature as shown in Fig. 18.9. As the pH of the sewage is increased above 7 (i.e., OH– concentration increases), the equilibrium is shifted to the left in equation 18.13, and the ammonium ion is converted to ammonia.
For example, at 20°C and pH 10 the percentage distribution of NH3 is about 82%, and at 20°C and pH 11 the percentage distribution of NH3 is about 95%. Thus at pH 11 most of the ammonium ions are converted to dissolved ammonia gas, which may be removed as a free gas by agitating the sewage in the presence of air. This is usually accomplished in a packed tray tower equipped with an air blower.
The pH of sewage may be raised by adding lime [Ca(OH)2] to the sewage. The amount of lime required to raise the pH of sewage to 11 depends on the alkalinity (i.e., mg/l of CaCO3) of the untreated sewage.
The amount of air required to remove the ammonia from the sewage in a stripping tower is determined by the following steady-state materials balance equation for the stripping tower:
The sewage with raised pH is introduced from the top of the ammonia-stripping tower and after passing through the tower leaves at the bottom of the tower. The air is introduced from the bottom of the tower and as it passes through the tower agitates the sewage so that dissolved ammonia gas is released and mixture of air and ammonia gas leaves the tower at the top.
The ammonia-stripping tower can be of either cross flow type or countercurrent flow type with induced air circulation. The efficient performance of the tower depends on a prolonged and intimate contact between the air and the sewage. This is accomplished by tower packings. The ideal tower packing is the one which has a large surface area and low resistance to air flow.
The advantages of air stripping for nitrogen removal are:
i. Simplicity of Operation,
ii. Ease of control, and
iii. Low cost relative to other nitrogen removal processes.
The disadvantages of air stripping for nitrogen removal are:
i. Calcium carbonate scaling within the tower and feed lines, and
ii. Poor performance during cold weather operation.
(ii) Breakpoint Chlorination:
In breakpoint chlorination process sufficient amount of chlorine is added to oxidize the ammonia nitrogen in solution to nitrogen gas and other stable compounds.
A representative equation that can be used to describe the overall reaction that takes place in the breakpoint chlorination process is as indicated below:
The stoichiometric mass ratio of chlorine as Cl2 to ammonia as N as computed from equation 18.15 is 7.6:1. In practice the ratio has been found to vary from about 8:1 to 10:1.
From both laboratory and full-scale testing programs, it has been found that the optimum pH operating range for breakpoint chlorination is between 6 and 7. If breakpoint chlorination is accomplished outside this range, it has been observed that the chlorine dosage required to reach the breakpoint increases significantly and that the rate of reaction is slower. The temperature, however, does not have a major effect on the process in the range normally encountered in sewage treatment.
The breakpoint chlorination process can be used for the removal of ammonia nitrogen from treatment plant effluents either alone or in combination with other processes. Usually to avoid the large chlorine dosage required when used alone, it is used in conjunction with other nitrogen removal processes. Further to optimize the performance of this process and to minimize equipment and facility costs, flow equalization is usually required.
The advantage of breakpoint chlorination process is that with proper control and flow equalization, all the ammonia nitrogen in the sewage can be reduced to zero. An added advantage of this process is that disinfection of the sewage is achieved at the same time.
(iii) Ion Exchange:
Ion exchange is a unit process by which ions of a given species are displaced from an. insoluble exchange material by ions of a different species in solution. It may be operated in either a batch or a continuous mode. In a batch process the ion-exchange resin is simply stirred with the sewage to be treated in a reactor until the reaction is complete. The spent resin is removed by settling and subsequently it is regenerated and reused. In a continuous process, the exchange material is placed in a bed or a packed column, and the sewage to be treated is passed through it.
Both natural and synthetic ion-exchange resins are available, but synthetic resins are used more widely because of their durability. Nevertheless, some natural resins (zeolites) have found application in the removal of ammonia from sewage. Out of the various natural zeolites that have been investigated, Hector Clinoptilolite has proved to be one of the most effective.
One of the novel features of this zeolite is the regeneration system employed. Upon exhaustion the zeolite is regenerated with lime Ca(OH)2. The ammonium ion removed from the zeolite is converted to ammonia because of the high pH. At this point the regenerating solution is passed through a stripping tower for removal of the ammonia. The stripped liquid is collected in a storage tank for subsequent reuse. The advantage of this system is that there is no process waste containing ammonia for which ultimate disposal must be provided.
A major problem in this case is the formation of calcium carbonate precipitates within the zeolite exchanged bed and in the stripping tower and piping appurtenances. The zeolite bed is equipped with backwash facilities to remove the carbonate deposits that form within the zeolite bed.
A serious problem associated with application of ion exchange to the treatment of sewage effluents is resin binding caused by the residual organic matter found in effluent from biological treatment. This problem is solved partially by pre-filtering the sewage or by using scavenger exchange resins before application to the exchange column.