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ScienceDirect Procedia Engineering 117 (2015) 107 – 113
International Scientific Conference Urban Civil Engineering and Municipal Facilities, SPbUCEMF-2015
Simultaneous Denitrification and Nitrification in the Lab-scale Oxidation Ditch with Low C/N Ratio Gogina Elena, Gulshin Igor* Moscow State University of Civil Engineering, Yaroslavskoye shosse, 26, Moscow, 129337, Russia
Abstract In this study optimal operational parameters for carbon and nitrogen removal (specifically dissolved oxygen concentrations in different zones, hydraulic retention time and flow velocity) for wastewater typical for the Moscow region (low C/N and C/M ratio) in the Oxidation Ditch have been determined. The processes of nitrification and carbon oxidation in the studied system were stable. The possibility of simultaneous denitri-nitrification in the Lab-scale Oxidation Ditch with Low C/N ratio has been proved. The results of this study will be used in further researches. © 2015 2015The The Authors. Published by Elsevier Ltd. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SPbUCEMF-2015. Peer-review under responsibility of the organizing committee of SPbUCEMF-2015
Keywords: biological wastewater treatment, oxidation ditch, denitrification, lab-scale modelling, nutrient optimization.
1. Introduction Environmental protection is one of the priority directions of science nowadays. It is because almost any technical or social development of society affects environmental issues, including the protection of water resources. Wastewater treatment plays a cruel role in this protection so there is a huge amount of science researches in this field in the world. Although the main purposes of water specialists all over the world are the same, there are some differences in various countries. Some phenomena strongly affect the development of wastewater treatment systems in Russia. Two of them are the process of suburbanization and serious tightening of wastewater discharge standards in the nineties years of the
* Corresponding author. Tel.: +7 (495) 781-80-07; +7 (499) 183-44-38 E-mail address:
[email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SPbUCEMF-2015
doi:10.1016/j.proeng.2015.08.130
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previous century. The suburbanization process leads to increased demand for small wastewater treatment plants especially for the Moscow region (as the most dynamically developing region in Russia). Small-sized WWTP in Russia have to be ease in maintenance due to the deficit of qualified staff, so there are some constraints on technologies and constructions that can be used. At the same time, new discharge standards require modern technologies for deep nitrogen and phosphorus removal with minimal monetary costs [1, 2]. One of the most effective methods for the treatment of small quantities of wastewater at low capital and operational costs is the Oxidation Ditch (OD) [3, 4, 5]. Dr. A. Passveer developed it in 1953 in The Research Institute for Public Health Engineering. The main features of the original OD are the simplicity of design and equipment. Moreover, operating in the extended aeration mode, it can obtain high treatment efficiency together with a high degree of sludge stabilization [6, 7, 8]. The extended aeration mode leads to an increasing volume of bioreactor, and therefore, the appearance of a buffer zone. It allows neutralizing a significant excess of average flow and concentration of pollutants, which is important for small wastewater treatment systems [9, 10]. In Russia, due to the introduction of stricter effluent standards, there is a strong necessary in methods for deep removal of nutrients. As far as concerns nitrogen removal, there is no alternative to the biological nitrificationdenitrification process [11, 12, 13]. In oxidation ditch systems, denitrification and nitrification can be carried out due optimal aeration management, with or without specific anoxic and aerobic zones. In the second case, it is a simultaneous denitri-nitrification (SND). OD is a situation where SND often has a significant impact [14, 15, 16]. The physical explanation is that SND occurs within activated-sludge flocs because of dissolved oxygen concentration gradients arising from diffusional limitations [17, 18, 19, 20]. Simultaneous denitri-nitrification is of interest because it allows saving energy and space. As is it known, one of the conditions for the denitrification is a sufficient amount of organics in influent. Wastewaters in the Moscow region have a lower concentration of organics due to infiltration and increased water consumption by the population. Therefore, the experiment took into account low C/N ratio. The investigation has carried out in the research laboratory of biological wastewater treatment in the Moscow State University of Civil Engineering for four months. The main objective of the present study was to determine optimal operational parameters for carbon and nitrogen removal (specifically dissolved oxygen concentrations in different zones, hydraulic retention time and flow velocity) for wastewater typical for the Moscow region (low C/N and C/M ratio) in the Oxidation Ditch. It is important to note, that it was the preliminary work, the results will be used in further researches. 2. Materials and methods Lab-scale model of the Oxidation Ditch consisted of bioreactor (A) and secondary clarifier (B) (Fig. 1). Lab-OD bioreactor was a complete mixed reactor with mixed liquor flow circuit. The experiment consisted of two stages. In the first stage, OD has worked in the full-aeration mode for two months. In the second stage were established both aerobic and anoxic zones. The principle of operation of the Oxidation Ditch was as follows: metering pump supplied synthetically created wastewater (5) into the feed hopper (3), which distributes the liquid waste to the top of zone 1 of the Oxidation Ditch (1). Low-powered mechanical stirrer (4) created a directed liquid stream witch passed in succession through zone 1 and zone 2 (2). Then the main part of the liquid flow circulated to zone 1 and another part (7) directed to the secondary clarifier. The precipitated activated sludge returned to the Oxidation Ditch as a returned activated sludge (RAS) (6). The clarified liquid was removed from the system as the effluent (8). There were two aeration points: high-intensity aeration (9) and low-intensity (10).
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Fig. 1. The Lab-scale model of the Oxidation Ditch (the scheme and natural view).
The total working volume of the Oxidation Ditch was 18 L (the volume of zona 1 was 8.15 L and of zona 2 – 9.85 L). The influent rate was 2.4 L/h, which gave a hydraulic retention time (HRT) of 7.5, 3.4 and 4.1 h respectively. The working volume of secondary clarifier was 4 L; time of clarification was 1.7 h. RAS ratio was 200 %. Dissolved Oxygen concentrations, pH and temperature were measured with WTW ProfiLine Oxi 3310. The analyses of chemical oxygen demand (COD), ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), PO 4 3-, and mixed liquor suspended solids (MLSS) and Sludge Volume Index (SVI) were made according to the standard methods. Biological Oxygen Demand (BOD5) was measured with respiration method with WTW OxiTop Control 12. 3. Results and discussion The experiment consisted of two stages. The first stage was to achieve the operating mode and stable results for the BOD and ammonia removal. The Oxidation Ditch in this stage worked in the full-aeration mode (oxygen was supplied to both zones of the bioreactor). During this stage has been received optimal oxygen mode for nitrification. On the day 49 of the experiment, the system has reached a stable operational mode. According to the chemical analyses, efficiency of organic removal was 95 – 96 %; efficiency of ammonia removal was 80 – 90 %. It was decided to run denitri-nitrification mode. The intensity of aeration has been significantly reduced. Circulating fluid flow created mainly by mechanical stirrer. Horizontal velocity of a fluid stream has reduced to the minimal values (one full turn in two minutes). Concentrations of dissolved oxygen in different parts of the reactor were about 2 – 3 mg/L. Despite the high concentration of dissolved oxygen, the process of denitrification started. It is noteworthy that it was achieved in a small single reactor with a relatively strong mixing of the activated sludge across different oxygen zones. Therefore, denitrification in this case was a result of processes, which occurred into activated sludge flocs, specifically SND. From the obtained data, basic characteristics of the system`s biochemistry processes have been constructed. Characteristics are in the form of graphs. Graphs of the specific oxidation rate, depending on the influent and effluent BOD5 (Fig. 2 and Fig. 3 respectively), show the stable operation of the system. Deviating points correspond
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to the results of chemical analyzes attributable to the period of commissioning. Upon the stable operation of the model has been reached, BOD removal was effectively both the first and the second phase of the experiment.
Fig. 2. Specific oxidation rate in relation of BOD5 (in the influent).
Fig. 3. Specific oxidation rate in relation of BOD5 (in the effluent).
Michaelis–Menten constant and the maximum reaction velocity were calculated with the corresponding Lineweaver–Burk plot of Specific oxidation rate; Km = 2.94, Vmax = 16.03 mgBOD/(g·h). The reaction velocity is:
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V=
Vmax S Km S
16.3 Lex . 2.941 Lex
(1)
Plot of nitrification rate in relation of ammonia nitrogen concentration in the effluent is on the Fig. 4. As it seen, the nitrification process is stable and corresponds to the biological nitrification law involving autotrophic nitrifying bacteria. This suggests that all necessary conditions for effective nitrification process were achieved.
Fig. 4. Specific nitrification rate in relation of ammonia nitrogen (in the effluent).
Michaelis–Menten constant for nitrification were calculated with the corresponding Lineweaver–Burk plot of nitrification rate in relation of ammonia nitrogen concentration in the effluent; K MN = 25, VmaxN = 67.11 mgNH4/(g·h). The reaction velocity is: VN =
VmaxN S K mN S
67.11 C NH 4 25 C NH 4
.
(2)
Plot of specific denitrification rate in relation of BOD5 in the effluent is on the Fig. 5. This graph allows us to estimate the stability of denitrification process, produced by heterotrophic bacteria. This suggests the possibility of an efficient denitrification in the system (if to create the necessary conditions).
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Fig. 5. Specific denitrification rate in relation of BOD5 (in the effluent).
4. Conclusions In general, the experimental results can be regarded as positive. Stable operation of a lab-scale model of the Oxidation Ditch was achieved. Processes of biological wastewater treatment in the system correspond to the basic principles of enzyme kinetics. Nitrification was almost complete with the laboratory conditions when dissolved oxygen was about 2-3 mg/l. In the same time macro-anoxic zones in the Oxidation Ditch and micro-anoxic zones in the activated sludge flocs provided the SND process. The poor denitrification process associated with low C/N ratio and high DO level. The optimal HRT was 7.5 h, the reactor worked in extended aeration mode. The results of this study will be used in further researches. References [1] Gogina, E.S., Gulshin, I.A. The single-sludge denitri-nitrification system in reconstruction of wastewater treatment plants in the Russian Federation (2014) Applied Mechanics and Materials, 580-583, pp. 2367-2369. [2] Andrianova, M.J., Molodkina, L.M., Chusov, A.N. Changing of contaminants content and disperse state during treatment and transportation of drinking water (2014) Applied Mechanics and Materials, 587-589, pp. 573-577. [3] Pang, H., Shi, H., Shi, H. Flow characteristic and wastewater treatment performance of a pilot-scale airlift oxidation ditch (2009) Frontiers of Environmental Science & Engineering in China, 3 (4), pp. 470-476. [4] Vatin, N.I., Chechevichkin, V.N. Chechevichkin, A.V., Shilova, Y., Yakunin, L.A. Application of natural zeolites for aquatic and air medium purification (2014) Applied Mechanics and Materials, 587-589, pp. 565-572. [5] Andrianova, M.J., Vorobjev, K.V., Lednova, J.A., Chusov, A.N. A short-term model experiment of organic pollutants treatment with aquatic macrophytes in industrial and municipal waste waters (2014) Applied Mechanics and Materials, 587-589, pp. 653-656. [6] Stamou, A., Katsiri, A., Mantziaras, I., Boshnakov, K., Koumanova, B., Stoyanov, S. Modeling of an Alternating Oxidation Ditch System (1999) Water Science and Technology, 39 (4), pp. 169-176. [7] Gogina, E.S., Makisha, N.A. Reconstruction of waste water treatment plants in Russia, approaches and solutions (2013) Applied Mechanics and Materials, 361-363, pp. 628-631. [8] Makisha, N.A., Yantsen, O.V. Laboratory modeling and research of waste water treatment processes in biofilters with polymer feed (2014) Applied Mechanics and Materials, 587-589, pp. 640-643 [9] Gogina, E.S., Gulshin, I.A. Issledovanie raboty modeli tsirkulyatsionnogo okislitel`nogo kanala [Research of the Oxidation Ditch model] (2014) Vestnik MGSU, 12, pp. 162-171. [10] O.A. Ruzhitskaya, E.S. Gogina. Removal of phosphates from wastewater and intensify the biological wastewater treatment process from organic pollution (2014) Advanced Materials Research, 919-921, pp. 2153-2156. [11] Abusam, A., Keesman, K.J., Meinema, K., Van Straten, G. Oxygen transfer rate estimation in oxidation ditches from clean water measurments (2001) Water Research, 35 (8), pp. 2058-2064
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