CTB3365x Introduction to Water Treatment

Transcription

1 CTB3365x Introduction to Water Treatment W5a2 Nitrogen removal Merle de Kreuk If you already watched the movie about the nitrogen cycle, you understand that with introduction of the Haber-Bosch process, the natural Nitrogen cycle has been disturbed. This has caused nitrate accumulation in drinking water resources, eutrophication and acidification. Therefore, nitrogen removal has been introduced to sewage treatment processes. Today you will learn several configurations for nitrogen removal. Furthermore, you will be able to understand how the sludge load is coupled to the nitrogen removal efficiency and finally, a little bit more is explained about Anammox, the shortcut of the Nitrogen cycle. So, we will focus on the anoxic and aerobic tank of the sewage treatment plant. The first mechanism by which nitrogen is removed in wastewater treatment is bacterial growth. Nitrogen is incorporated in cell material, as it is part of proteins, aminoacids, DNA, certain lipids etc. You might recognize that nitrogen is part of the overall molecular formula of biomass generally expressed by this equation: C5 H7 O2 N. The bacteria can take this nitrogen needed for growth from ammonium in the wastewater. Calculating with the mole mass shows that 14 grams of nitrogen is needed for the growth of 113 grams of biomass. If you know this value, you can calculate whether or not biomass growth alone is enough to remove the nitrogen from your influent. 1

2 Let s consider the average BOD and ammonium concentration in Dutch influent and a sludge growth Yield of 0.6 kg VSS/kg BOD converted. It can be calculated that per liter influent 108 mg VSS is produced, containing about 13 mg N. Would this be enough to get to the effluent demands of 10 mg total N/L, coming from 40 mg/l in the influent? Exactly, no it wouldn t, and that is why other processes are needed: namely nitrification followed by denitrification. We will start with nitrification, the aerobic conversion from ammonia to nitrate. Nitrification is a two-step oxidation process performed by two different species: the ammonium oxidation to nitrite is done by Nitrosomonas species, while the oxidation of nitrite into nitrateis performed by Nitrobacter species. These organisms are Chemo-Litho-autotrophic organisms, so can you tell what their carbon source is? And their electron donor? Indeed, CO2 is the Carbon source, ammonium or nitrite the electron donor and since the organisms are fully aerobic, oxygen serves as electron acceptor. From the overall equation, you can see that for every ammonium oxidized 2 oxygen are needed. This means that each gram of ammonium-n converted, uses 4.57 g of oxygen. Since the catabolism generates energy for the anabolism, part of the ammonium is not oxidized but incorporated in cell mass. This leads to the net value of 4.2 grams of oxygen needed per gram ammonium N removed. In this overview you see the differences between the heterotrophic and autotrophic organisms used in sewage plants. In particular, the different growth rates of the two organisms and their oxygen demands determine their competition in treatment plants. A high sludge loading rate (Bx), means that the food to mass ratio of the system is high and thus a lot of biomass can be produced. This will lead to a low sludge retention time as more sludge must be wasted to keep the concentration in the aeration basin constant. This is a good situation for heterotrophic organisms, since with high maximum growth rates and high affinity for oxygen, heterotrophic organisms can survive at relatively short sludge ages, high loading rates (Bx), and low oxygen concentrations. On the other hand, the autotrophic nitrifiers are slow growing organisms. This means that to achieve nitrification, the solid retention time, or sludge age should be long enough to keep them in the system. A general rule is that the sludge age needs to be at least longer than 2.5 days to guarantee nitrification. This is obtained at sludge loading rates below 0.15 kgbod/kg 2

3 biomass/day. Also the oxygen concentration needs to be relatively high during nitrification as maximum nitrification rates have been observed at DO concentration of 3 to 4 mg/l. With nitrate formation, we are only half way through the nitrogen removal process. In the past some treatment plants were limited only on ammonia concentration in the effluent so nitrification was the only goal. New limits focus on limiting the total nitrogen in the plant effluent, thus the nitrate must also be removed through denitrification where nitrate is converted to di-nitrogen gas. Denitrification is performed by heterotrophic organisms, which are able to use nitrite or nitrate as electron acceptor in the absence of oxygen. It is important to remember that these heterotrophic organisms need an organic carbon source, which can be the BOD from the sewage, but also other organic compounds, such as methanol. The equations show the conversion of these organics with nitrate as electron acceptor. You should notice that this reaction produces hydroxides, which is useful since it compensates the proton production during nitrification, limiting the ph change in the buffered sewage. There are two main configurations in which nitrification and denitrification can take place: pre- and post denitrification. First I will show you the post-denitrification. In this configuration the sewage is first fed to an aeration tank, in which the aerobic biological processes occur. We know that if the sludge retention time is long enough nitrification will occur and ammonium will be oxidized to nitrate. Next, the sludge water mixture will flow to an unaerated tank. The water contains nitrate, so this is an anoxic tank and denitrification could take place. Only, one ingredient for the denitrification is missing. Since the BOD already has been (partly) oxidized in the first step, denitrification will not happen anymore. An additional C-source and electron donor is needed in this reactor for the denitrifying bacteria. This could be solved by diverting some influent to the anoxic tank. However, as the influent also contains ammonium, bypassing the aerobic tank will lead to ammonium being discharged with the effluent. 3

4 An alternative option to avoid this is the dosage of an external C-source in the anoxic tank, such as methanol, but this adds operational complexity and is expensive. A solution to avoid these drawbacks is to put the anoxic tank before the aeration tank in a setup known as pre denitrification. In this configuration the BOD containing sewage goes directly to the anoxic tank to act as the C source and electron donor for the denitrifying bacteria. However, as you might have noticed, the difficulty with this setup is that the influent sewage contains ammonium that needs to first be aerated for nitrification before, it can be denitrified. Part of the nitrate from the aeration tank will therefore be recycled to the anoxic tank with the return sludge from the final clarifier. However, this flow is typically far too small to significantly reduce the nitrate concentration in the effluent. As a solution, we introduce an additional recycle from the aeration tank to the anoxic tank. Now it is possible to denitrify in the anoxic tank, since both BOD from the influent and nitrate from the recycle are present. In the aerobic tank, remaining BOD will be further oxidized by oxygen and influent ammonium that passed the anoxic tank will be nitrified to nitrate. Because not all the water can be recycled, some nitrate will always make its way to the effluent in a pre-denitrification setup which limits the ultimate nitrogen removal capacity of this configuration. Can you make a quick and dirty estimation of the ratio influent/return flow if the influent contains 40 mg ammonium/l and effluent nitrate demands are below 10 mg/l? Indeed, the recycle needs to be around 3 times the influent flow. Of course, we are ignoring some parameters in this estimation: Some nitrate is also returned with the return sludge flow, so this can be subtracted from the internal nitrate return flow. There will also be some nitrogen removal by growth that is not incorporated in this balance. An internal recycle ratio of 3 to 4 is typical for activated sludge systems. Lower ratios can be used if influent total nitrogen concentrations are low. Higher recycle ratios are generally avoided, since the additional effect on the nitrate concentration in the effluent is minimal and too much oxygen from the aeration tank will be introduced in the anoxic tank. 4

5 A special case of nitrification/denitrification can be found in the ultra-low loaded Carrousel system developed in the Netherlands. This system is based on general oxidation ditch technology and is designed as a race track, with quite high water flows. Influent can pass a separate anoxic zone first, but can also be introduced in the aerated zone of a circuit system. By measuring the actual oxygen concentration at different points in the reactor, the aeration can be precisely controlled. In that way aerated and anoxic zones can be alternated, leading to a good nitrogen removal by nitrification and denitrification in the different zones of the track. The recycle rate in such system is very high, since only a small part leaves the reactor every lap and thus hydraulic residence times are relatively long. Effluent total nitrogen concentrations well below 10 mg/l are reached with these systems. So, conventional N-removal needs lots of oxygen in the first phase and an electron donor in its second phase. In the late 90 s, a shortcut in the nitrogen cycle was discovered: Anammox (ANaerobic AMMonium OXidation). The Anammox bacteria can use ammonium and nitrite to produce di-nitrogen gas directly. Main advantages of this process are that only half of the ammonium has to be converted to nitrite. Therefore, it only consumes 38% of the oxygen compared to full nitrification. This leads to large energy savings. Furthermore, no carbon source is needed as in conventional denitrification, because these organisms are autotrophs. Since they have their optimum growth rate at elevated temperatures, the Anammox process is very suitable for the nitrogen removal from rejection water of sludge digestion. Anammox bacteria have a very low growth rate, with a doubling time ranging from 9 days to two weeks and therefore sludge retention in Anammox systems is very important. This can be done in a two-step or one step Anammox reactor. In a two-step reactor the water is first partially nitrified in a Sharon reactor into a mixture of nitrite and ammonium, after which it is fed to the anoxic Anammox reactor. One step Anammox reactors come in many configurations with names, as Demon, Oland and Anammox. All make use of the growth of Nitrosomonas species in combination with Anammox. Preferably they both grow in a granule or biofilm, in which the outer layer of oxygen consuming nitrosomonas shield the Anammox that are inhibited by oxygen. 5

6 The first application of Anammox was built in 2002 at a sludge treatment plant in Rotterdam, The Netherlands. This plant was already equipped with partial nitritation, to treat the rejection water from the sludge digester. A one to one mixture of ammonium and nitrite is fed to the reactor and converted by the Anammox bacteria. With an influent concentration of 1 g nitrogen/l, a conversion rate of 5 kg N per cubic meter reactor per day is reached. A new development that is being studied is the application of Anammox in the main stream of a wastewater treatment plant at colder temperatures, but more about that in the Master-track Watermanagment, which can be followed on campus as well as online. 6