Ammonium removal from drinking water - comparison of the breakpoint chlorination and the biological technology

Transcription

1 Conference of Junior Researchers in Civil Engineering 248 Ammonium removal from drinking water - comparison of the breakpoint chlorination and the biological technology Szabolcs Takó BME Department of Sanitary and Environmental Engineering, tako.szabolcs@vkkt.bme.hu Abstract In 2004, Hungary joined to the European Union, which resulted in, harmonization of the law as well. As a result, the limits for drinking water quality parameters became more stringent. In Hungary, currently the most significant problems are the ammonium and arsenic content of the drinking water. This study deals with the effects of ammonium in drinking water and details the possible removal technologies. In Hungary, currently two methods are in use for ammonium removal: the breakpoint chlorination, and biological ammonium removal (nitrification). The two different methods are compared based on my own experiences and also by literary sources. Beside the basic principles of these technologies, the study also deals with the by-product formation and reaction time. Introduction In Hungary, 94.1% of the supplied drinking water originates from subsurface water resources (including the bank filtration wells) (CEEBI, 2009) % of the subsurface water sources originating from deep confined aquifers do not require treatment technology at all, only disinfection is enough before the water is pumped to the distribution system ([9]). Generally, the waters originating from deep confined aquifers do not contain anthropogenic contaminants. However, due to the geological environment, the concentration of certain components (e.g., iron, manganese, ammonium, arsenic, natural organic matter) exceeds the maximum allowable concentration values, therefore the application of water treatment technology is needed. Components Previous limit according to MSZ Current limit according to Government Decree 201/2001 Arsenic 50 µg/l 10 µgl/l Ammonium (surface water) 0.5 mg/l 0.5 mg/l Ammonium (ground water) 0.2 mg/l 0.5 mg/l Ammonium (subsurface water) 2.0 mg/l 0.5 mg/l Iron 0.3 mg/l 0.2 mg/l Manganese 0.1 mg/l 0.05 mg/l Iron and manganese together 0.3 mg/l - Table 1: The maximum allowable concentration values of selected components according to the previous and the current regulation Since 2001, new Hungarian drinking water requirements are in force, which significantly decreased the maximum allowable arsenic and ammonium concentrations (Government Decree No. 201/2001, [3]). In Hungary currently around 1.6 million people at around 600 settlements are supplied with drinking water which contains ammonium concentration higher than the 0.5 mg/l standard value, and around 1.5 million people at 400 settlements consume drinking water above 10 µg/l arsenic concentration. Figure 1. shows the ratio of the Hungarian settlements non-complying the EU Directive for arsenic and ammonium. Around 14.5 % of the total population is affected with elevated arsenic concentrations, 15.5 % for ammonium respectively. 8 % of the total Hungarian population is affected by the arsenic and ammonium problem as well (Government Decree No. 201/2001, [3]). The ammonium does not directly harm the human body in typical ph values (6.5 to 9.5) applied in drinking water treatment. However, it may form nitrite ions under oxidative conditions. Nitrite is a toxic component, because it disables the enzyme lactase in the blood cells, causing hydrogen-peroxide realize. As a result, the hemoglobin is oxidized to methemoglobin, which means that the divalent iron in hemoglobin is oxidized to trivalent iron. Iron(II) is suitable for the oxygen transport, however iron(iii) is unable to do so, because the iron (III) containing hemoglobin connects to oxygen with stable ionic bond, which makes oxygen transport

2 Conference of Junior Researchers in Civil Engineering 249 impossible, and therefore cause hypoxia (blue baby syndrome) ([19]). Due to the harmful effect of nitrite, the maximum allowable concentration is 0.5 mg/l. 20 Percentage of inhabitants and settlements affected by thearsenic and ammonium problem 15 % Ammonium above 0.5 mg/l Arsenic above 10 µg/l Ammonium above 0.5 mg/l and arsenic above 10 µg/l Percentage of people affected of the total Hungarian population Percentage of settlements affected of the total number of settlements in Hungary Fig. 1: Percentage of inhabitants and settlements affected by the elevated arsenic and ammonium content in drinking water (data source: 201/2001 Government Decree about the quality and monitoring of drinking water, ([5]) This process can be also present in the water supply network if the distributed water contains ammonium, and the circumstances in the supply system favour nitrite formation. This process may result in the presence of nitrite at the consumers. Beside the possible nitrite formation, the other issue related to the presence of ammonium in drinking water is the decrease of the chlorination disinfection efficiency. The ammonium reacts with chlorine forming chloramines, and thus reducing the amount of the disinfectant available for microorganism inactivation. The less efficient disinfection may cause secondary water pollution in the distribution system. Moreover, the resulting chloramines cause the unpleasant smell, which may lead to customer complaints. In Hungary currently two technologies are applied to remove ammonium from drinking water: the breakpoint chlorination, and the biological method based on nitrification. Ammonium removal technologies Breakpoint chlorination The breakpoint chlorination is a commonly applied and well-known method to remove the ammonium. The method has been used for decades. The ammonium reacts with the chlorine based on the following reactions ([6]): In the first step, monochlor- amine (NH 2 Cl) and then dichloro- amine is formed, while the ammonium concentration decreases (Fig. 2), free chlorine appears in the water. When the breaking point is reached, all the ammonium is converted to trichloro- amine (NCl 3 ), and no ammonium is present in the water. The trichloro-amine compounds are instable, and decompose to nitrogen gas (N 2 ). The formed chloramine gives an unpleasant odour to the treated water. The ratio of the mass of the chlorine and ammonium-nitrogen at the breakpoint is 7.6 ([1]). However, this is only a theoretical value, and in practice usually higher chlorine dose is needed due to the presence of the

3 Conference of Junior Researchers in Civil Engineering 250 components being in reduced form (e.g., Fe(II), Mn(II), natural organic matter). Experiments carried out by Laky et al. ([4]) resulted in Cl 2 :NH 4 -N ratio. Figure 2. shows the results of experiments, where in the beginning the ammonium concentration was 1.1 mg/l. Chlorine was needed 9.5 mg/l to the breakpoint. In this case the needed ration was 8.6. Fig. 2: NH4-N [mg/l], Free active chlorine [mg Cl2/L], X Total chlorine [mg Cl 2 /L], Combined chlorine [mg Cl 2 /L] depending on the dose sodium hypochlorite ([4]) The chlorine in the water reacts with organic material, and as a result trihalo methane (THM) and adsorbable organic halides (AOX) are formed. These components may cause cancer and these also were mutagenic ([11]). In Hungary, the recommended value for the AOX compounds is 50 µg/l, and the Government Decree No. 201/2001 sets 50 µg/l as a maximum allowable total THM concentration in drinking water. Therefore, if the breakpoint chlorination is used for ammonium removal, activated carbon adsorber must be installed into the technology in order to remove the THM and AOX compounds. Furthermore, activated carbon catalyzes the decomposition of trichloro-amine to nitrogen gas. This treatment steps makes this technique relatively expensive. Before designing the technology, laboratory tests should be performed in order to study the amount of byproducts formed. The results of such experiment carried out with two types of raw water originating from deep confined are shown in the Figure 3 and 4. It can be concluded that the amount of THM and AOX byproducts increase with increasing reaction time. Moreover, the authors also concluded increased by-product formation with increased chlorine dose (data not shown). In case of both experiments, the THM remained below 20 µg/l, however AOX compounds were above 50 µg/l in all cases. The aim of the laboratory experiments are to minimize the formation of these products, and optimize the breakpoint chlorination in terms of reaction time and chlorine dose. Study by Laky et al. ([4]) concluded that 5-10 minutes reaction time was adequate for breakpoint chlorination, however the reaction time depends on the applied chlorine dose. According to another source ([7]) 10 to 15 minutes contact time is sufficient for the reaction. Biological ammonium removal The nitrifying microorganisms belong to the chemoautotroph bacteria. They use the inorganic carbondioxide as carbon source and ammonium is used as energy source. The nitrification process consists of two steps. As a first step the ammonia oxidizing bacteria (AOB, mainly: Nitrosomonas, in addition Nitrosospira, Nitrosococcus, Nitrosolobus and Nitrosovibrio) oxidizes the ammonium content of the water to nitrite: In the second step the nitrite-oxidizing bacteria (NOB, in particular: Nitrobacter, in addition Nitrospira, Nitrospina and Nitrococcus) oxidizes the nitrite to nitrate:

4 Conference of Junior Researchers in Civil Engineering 251 Fig. 3: NH4-N [mg/l] AOX [µg/l] X THM [µg/l] Breakpoint chlorination in time AOX and THM formation Cl2: NH4-N rate at 9.7 (Laky et al., 2011) Fig. 4: : NH4-N [mg/l] AOX [µg/l] X THM [µg/l] Breakpoint chlorination in time AOX and THM formation Cl2: NH4-N rate at 9.7 (Laky et al., 2011) As a result of the previous reactions the released energy is used for the life processes of the nitrifying microorganisms. The reaction requires dissolved oxygen and appropriate temperature. A minimum 2 mg/l of dissolved oxygen concentration is needed to work nitrifying biofilters properly ([16]). Zhu and Chen ([14]) shown that in oxygen limitation, 14 and 27 C temperature range has no significant effect on the nitrification. The optimum ph range varies between 7.0 and 9.0 ([20]). Due to the nitrification, hydrogen is released, which reduces the ph of the water ([8]). The complete nitrification process is following ([8]): For the oxidation of 1g NH 4 -N to nitrate approximately 4,18g oxygen and 7,07g alkalinity (carbonate and bicarbonate equivalence point during acid neutralization) is needed, and as a result 0.17 g bacteria cell mass is produced ([13]) Because of the risks associated to the biological ammonium removal technology, field tests have to be carried out prior the full-scale application to test if that certain type of water is suitable for the nitrification technology, whether spontaneous nitrification starts, and ammonium is fully converted to nitrate. In general, the nitrification occurs because of the action of the nitrifying bacteria on the filters or on the activated carbon adsorber if aeration is applied and sufficient oxygen concentration is reached before the biological filtration step. In addition to the oxygen there are several factors, which affect the nitrification process such as ph, temperature, dissolved oxygen, turbulence, organic matter content ([20]). The other disadvantage is that microorganisms can grow on the filter. Therefore, great caution is required during operation.

5 Conference of Junior Researchers in Civil Engineering 252 In the following the methodology and the results of such field experiments are presented. I carried out these experiments with raw water originating from a deep confined aquifer containing arsenic, ammonium, iron, manganese, methane, and aggressive carbon-dioxide above the maximum allowable concentration values. iron (mg/l) manganese (mg/l) ammonium (mg/l) arsenic (µg/l) aggressive carbon dioxide (mg/l) methane (NL/m 3 ) typical concentration ~3.7 Table 2: Some typical components of the raw water Fig. 5: The experimental setup used for field tests: 1- raw water of the waterwork, 2- buffer tank (200 L), 3- peristaltic pump, 4- overflow, 5- aeration tank (25L), 6- air installation, 7- overflow, 8- sand filtration, 9- overflow, 10- flush water installation,11- chemical mixing tank, 12- membrane pump, 13- potassium permanganate tank, 14- sand filtration, 15- stand pipe, 16- overflow, 17- flush water installation (sampling valve) ([4]) Figure 5.shows the experimental setup used for the field tests. The applied flow rate was 10 L/h. Due to the non-continous operation of the water treatment plant, first the water was directed to a buffer tank of 200 litre volume. From the buffer tank, the water was introduced to the aeration reactor, where near saturated dissolved oxygen concentration was achieved. Due to aeration, the iron and the part of manganese are oxidized (Fe 2+ Fe 3+, Mn 2+ Mn 4+ ) and thus they are converted solid state, which could be removed by rapid sand filtration.in addition to metal removal, nitrifying bacteria were able to growth on the filter, and therefore nitrification could take place. As a next treatment step, KMnO 4 it was added to the water, which converted the remaining dissolved manganese to solid form. These compounds were removed on the second rapid sand filter (unit No. 14 at Figure 5.) ([15]). Figure 6.shows the nitrogen forms after the first rapid sand filter of the second experimental setup. Nitrification started after treating 400 bed volume of water. Nitrite started to decrease significantly after 800 bed volume and achieved acceptable level after filtering 880 bed volume of water. No nitrogen loss or accumulation was observed in the rapid sand filter (the sum of Ammonium-nitrogen, Nitrite-nitrogen, Nitrate-nitrogen was around the same in the influent and effluent water. The iron concentration was successfully removed after the first filtration stage, while the manganese concentration decreased to acceptable level after KMnO 4 dosing and sand filtration by the second rapid sand filter (data not shown). For arsenic removal the in-situ coagulant (the natural iron content of the water) was sufficient, no extra coagulant was needed in order to convert the dissolved arsenic to particulate from (data not shown). Therefore by the application of the studied technology (methane, aggressive carbon dioxide, iron, manganese and arsenic were successfully removed from the water).

6 Conference of Junior Researchers in Civil Engineering 253 Fig. 6: Change of nitrogen-forms in effluent water of the 1 st filter (the applied technology: aeration + filtration + potassiumpermanganate dosing + filtration) NH 4 -N [mg/l] NO 2 -N [mg/l] NO 3 -N [mg/l] (own experiments [4], [5]) Conclusions In Hungary, the two most widely used technologies are the breakpoint chlorination and the biological ammonium removal based on nitrification. When comparing the two technologies it can be said that the advantages of the biological system are that no by-product is generated (if the nitrite formation can be controlled), while breakpoint chlorination may produce chlorinated organic compounds (such as THM and AOX). Because of the presence of harmful by-products activated carbon adsorber has to be installed in the technology, which makes this technique relatively expensive (Benedek, 1990). Laboratory experiments showed that even in case of relatively low THM concentration, the formed AOX may be above the acceptable concentration (50 µg/l). The needed chlorine doses are usually higher than the theoretical value (Cl2:NH4-N ratio of 7.6), and the reaction time is 5-15 minutes depending on the chlorine dose. Continously operating field experiments were carried out to study the biological ammonium removal technology. In case of the studied raw water the nitrification started spontaneously after treating 400 bed volume of water. At the end of experiments there was no nitrite in the treated water, which means that complete nitrification (conversion of ammonium to nitrate) was achieved. In case of each raw water quality field tests and laboratory experiments have to be carried out for decision support. Beside the technical aspects (applicability of the technology), economic considerations have to be also taken into account before the decision is made, which removal technology to use. In my future research I am planning to test different methods, which can be used to test the sustainability, affordability of the technical solutions for small drinking water treatment plants (below 500 m 3 /day capacity). These methods include e.g., the dynamic cost analysis ([2]), cost-benefit analysis ([18]) and benchmarking ([17]). Acknowledgement This work is connected to the scientific program of the Development of quality-oriented and harmonized R+D+I strategy and functional model at BME. This project is supported by the New Széchenyi Plan (Project ID: TÁMOP-4.2.1/B-09/1/KMR ). References [1] A. E. Griffin, N. S. Chamberlin: Relation of Ammonia-Nitrogen to Break-Point Chlorination, American Journal of Public Health and the Nations Health: August 1941, Vol. 31, No. 8: [2] Dynamic cost analysis, a methodological guide MASZESZ, Budapest, 2011 (in Hungarian: Dinamikus költségelemzés, Módszertani útmutató MASZESZ) [3] Government Decree No. 201/2001. (X.25.) on the quality standards and monitoring of drinking waters (2001). (accessed 28 March 2012) [4] D. Laky, I. Licskó: Ammónium, vas, mangán, metán és arzén eltávolítása mélységi vizekből komplex technológiai megoldások értékelése a hálózatban lejátszódó vízminőség változások figyelembe vételével, javaslat a szolgáltatási pontokon a jogszabályi és fogyasztói igényeket leginkább kielégítő technológiára, kutatási jelentés, 2010

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