Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 150 (2016) 2399 – 2404
International Conference on Industrial Engineering, ICIE 2016
Improving the Efficiency of Biological Treatment of Domestic Wastewater by Using Acoustic and Hydrodynamic Effect S. Denisov, S. Maksimov*, E. Gordeef South Ural State University, 76, Lenin Avenue, Chelyabinsk, 454080, The Russian Federation
Abstract Effective domestic wastewater treatment is a significant problem of the construction industry. To solve this problem, one has to design and operate a chain of successive constructions. Biological treatment is the final link in this chain. The method is based on a degradation process that affects dissolved organic waste in the water. As urban population is rapidly growing, and households use more and more appliances that utilize water, engineers face a number of issues in relation to making biological treatment more efficient. It is due to the fact that such issues are the most long-lasting and energy-intensive problems in the abovementioned chain. The solution we propose is arotary machine with flow modulation. This approach allows to activate protozoa and effectively saturate the treated medium with oxygen. The device presented herein allows to optimize the hydrodynamic and acoustic pulse impact on the milieu of these microorganisms. Thanks to the extended interface area, the device enables effective control over the regeneration of such microorganisms as well as oxygen saturation of the medium. © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license ©2016 2016The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of ICIE 2016. Peer-review under responsibility of the organizing committee of ICIE 2016
Keywords: domestic wastewater; biological method; regeneration microorganisms; hydrodynamic impact; acoustic; oxygen saturation; intensification of process.
1. Introduction Modern construction industry is characterized by the development and introduction of new and innovative materials and technologies to increase the efficiency of processes and reduce economic costs. Water disposal and wastewater treatment are not an exception. The problem of modern treatment facilities is compounded by increasing
* Corresponding author. Tel.: +7-351-366-5844; fax: +7-351-366-6403. E-mail address:
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1877-7058 © 2016 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 ICIE 2016
doi:10.1016/j.proeng.2016.07.308
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population and cities since the load on water treatment enterprises using traditional equipment and techniques is increased. The increase in volumes of waste regulations reduces the quality of cleaning. As a rule, new additional treatment facilities are not built, therefore improving the efficiency of the treatment of domestic wastewater is an actual problem of water disposal [1-3]. As a rule, treatment facilities are successively built chain of different methods of extraction pollutions. Domestic water goes through the following steps. 1. Mechanical purification, at which suspended particles are separated. Grit chambers and grease trap installed in series are used for these purposes. 2. The second step is connected with purification of dissolved impurities. In this case, generally used aeration tank which is an open container in the form of a long serpentine passage. Feeding wastewater accompanied by constant mixing, in certain proportions, microbial biomass as an aqueous suspension. Water in its structure consists of the weight of organic compounds which are human waste and at the same time they are a kind of food for certain microorganisms. These microorganisms metabolize organics therefore they grow and reproduce, and the concentration of pollutants in the water is dropped. Biological method is ecologically clean therefore to replace it using more effective technologies of complex chemical processes is not possible. After cleaning water must not break the balance and harm the environment [4, 5]. 2. Actuality Saturated with oxygen water is a prerequisite for the processing of dissolved organic substances, growth and reproduction of microorganisms that form the activated sludge. Saturation of the aqueous solution with oxygen in the aero tanks is carried out by bubbling – passing through powerful air compressors through the treated media. Air rising from the bottom of the aeration tank partially activates sludge, breaks it and oxygenates the environment. It is important to note that elementary microorganisms consume only dissolved oxygen which determines the most important characteristic of the aeration tank. The amount of dissolved oxygen is a measure of technological and structural perfection of devices that provide oxygen saturation of the processed environment. Analysis of mechanism treatment of biological wastewater shows that a major disadvantage of the existing technology is its low efficiency because it is difficult to control the "work" process of microorganisms and the required concentration of oxygen aquatic environment is quickly reduced [6, 7]. When bubbling air bubbles continuously collide with each other, merge and split up; increasing their size and dispersion. It decreases area of the interface and the number of dissolved oxygen. Therefore, in the last stages of the biological treatment it is observed "anoxia" microorganisms that directly determine the quality of processing of organic compounds. Floccula of the activated sludge is a friable flocculent accumulation of coagulated protozoa. As a rule a size of floccula is small and varies from 50 microns to 4...6 mm. Floccula consists of ten and a hundred microorganisms and it has disproportionately smaller size relative to the air bubbles. It cannot be effectively broken by bubbling or mixers. Most of the return activated sludge does not operate because of protozoa being inside a large number of floccules isolated from the external environment [8, 9]. 3. Formulation of the problem Protozoa pass all phases throughout their life. In the active growth period the intensive work for the wastewater treatment is observed. At this stage high resistance of microorganisms to physical and chemical effects of the environment provides them high viability. Thus, the development of methods and devices of activated sludge and transfer them to the active growth phase, as well as effective oxygen saturation of processed medium is economically viable and promising task of water treatment. To achieve objectives it is possible to use modern devices of efficient oxygen dissolution (by increasing the area of the interface) and purposeful dimensional regeneration of microorganisms increasing their activity and ability to prolong clean medium from the dissolved impurities. Keeping the same sequence of the cleaning process, it is possible to improve its efficiency by conversion of important address characteristics and process variables [10, 11, 12].
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The search and development of devices that allow to intensify the process is necessary due to: oxygen dissolved in the treated environment, increasing the area of the interface without increasing the volume of air supplied; effective influence and control in the process of dimensional regeneration of microorganisms that significantly increase their ability to long cleaning activity of water environment from dissolved organic compounds. 4. Theoretical part Biological treatment is ecologically pure and less expensive compared to other methods. However, to find the optimal solutions of design and technological problems we have to clearly imagine mechanisms and stages of a complex biological process. Biological treatment process can be represented as three stages. The first stage is characterized by a considerable withdrawal of pollutants during the time filling in wastewater. It is explained by the huge surface area of the sludge – 105 m2/kg .At this period cleaning is carried out by mass transfer processes in the system "activated sludge – biological pollution" and the emerging bio-oxidation process readily degradable organic matter. At this stage, for a period of from 0,5 to 2,0 hours, the content of organic pollutants that are determined by BOD5 index lowered by 50 – 60% [13-16]. Second stage continues the process of adsorption of organic pollution and starts the mechanism of active oxidation of exoenzymes which gives off the activated sludge in the processing of biomass. This stage is characterized by a decrease in oxygen consumption and the water gradually accumulates dissolved oxygen. As a result, the flow of benthic processes by exoenzymes, organic pollutants are oxidized about 75% that characterized by BOD5 index. Composition wastewater determines the duration of the second stage which may be equal from 2 to 4 hours. The third stage of endogenous power the oxidation is carried by and enzymes inside the cells. In biomass the compounds that difficult to oxidize are oxidized, the conversion of ammonium nitrogen to nitrite salts and nitrate as well as regeneration of the activated sludge. The speed of oxygen consumption increases again. The process is carried out in aeration tanks for domestic wastewater 6...8 hours and 10...20 hours at a joint cleaning domestic and industrial wastewater. The process of endogenous power depends on the amount of organic matter, the age of the activated sludge and its residence time in the aeration tanks [17, 18]. Isolation of polysaccharide gel on the surface of bacterial cells during the third stage form clusters of microorganisms (flocculation).Efficiency and quality of biological treatment is determined by the structure and biological properties of the sludge flakes. Sludge flakes have a density of 1,1...1,37 g/cm3 and size from 53 to 212 micron are normally flowing cleaning process. Effectiveness of the biological purification can be achieved only in case of activated sludge is flocculated state. Thus, the quality of treated water is determined by the ability of the sludge to flocculation process [19-21]. Depending on the specific load on the sewage treatment facilities activated sludge is divided into the following groups. Treatment facilities with high specific loads (400...600 mg BOD/gram of activated sludge) form the first group of the activated sludge, working part-time oxidation. At the same time biocenosis is formed that has a poor species composition. The second group provides a complete oxidation of dissolved organic matter (sludge load of 250...300 mg/g). This type of operation is typical for wastewater treatment of mixed composition – domestic and industrial. Multicomponent water pollution provides a high level of fitness of biocenosis in a significant range of everchanging conditions. The composition of the activated sludge in this group is varied in types and dynamically reacts to changing external conditions. As a rule, we do not exhibit the numerical superiority one of protozoa species or such domination is minimum. The third group is typical for treatment facilities which work at specific loads 80...150 mg/g, that ensures complete oxidation and nitrification of nitrogen-containing contaminants. At low loads on the activated sludge and the developed process of nitrification we observe complete oxidation of entering to cleaning of dissolved organic substances which creates conditions for the formation the ecologically perfect biocenosis – nitrifying activated
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sludge. Activated sludge of this group is characterized by considerable sizes, filled with gas bubbles. The spontaneous sludge flotation caused by processes of denitrification is observed. Low loaded sludge defined by rich species of microorganisms, dynamically reacts to changing external conditions for sustainable and efficient water treatment. In this case, even under the action of highly concentrated industrial wastewater, biocenosis retains its structure, population integrity and high level of enzymatic oxidation. In determining the growth of dependency of microorganisms in a heterogeneous culture we estimate the biomass of dry substance. Height is determined by a curve which is characterized by the presence of several development phases, each of them is characterized by specific conditions of existence of bioculture. Phase I – initial microbial growth phase - is called the lag phase. In the lag phase can be distinguished into two periods. The first period determines the cell adaptation to the external environment, actively synthesizing the adaptive enzymes. This part of the curve is characterized by a slight increase of biomass due to the increase in cell size. The second period of the lag phase has a significant increase, reaching to the end of the first phase of the maximum value. The phase is characterized by an unlimited number of nutrients. Young and biologically active cells have strong specific characters. Phase III – the phase of slowdown. In this part the rate of cell division is reduced due to the decrease of nutrients. At the same time toxic metabolic products are accumulate in the environment. Phase IV – stationary phase - characterized by the depletion of the stock nutrients. If the cells are in the early stages of a stationary growth if they move in an environment rich in nutrients, can begin the exponential growth of the culture without the lag phase. However, cell metabolism is disrupted when there are no food sources for a long time. Phase V - exponential phase of cell death. In the initial period of this phase a significant population of cells still alive and uses as a food source endogenous substrates. This process is called endogenous respiration. In the initial period the cells oxidize the stocked substances, and the final one- cellular lipids, carbohydrates and proteins. The phenomenon of autoxidation cellular material reduces the biomass. In the V-phase the cells are characterized by a smaller size, but a high resistance to chemical and physical disturbances from the environment. Quality of biological purification methods depends primarily on the rate of oxidation of the surface of microorganisms belonging to the activated sludge. The oxidation rate is determined by the delivery rate of organic substances to bacterial cell surfaces. Delivery is carried out by means of molecular diffusion. Diffusion processes occur in the layer II (Fig. 1). The thinner the layer II, the faster the diffusion and the higher saturation speed of floccules with organic matter from wastewater. To reduce the boundary layer it requires efficient mixing [22-25]. When supersaturation of floccule its size reaches a significant critical values and its ability to clean falls. Thus, a swollen sludge, which must be disposed of and support optimum size of floccules using modes of mechanical action.
Fig. 1. Structure of floccules in the diffusion model I – scrubbing medium; II – the boundary layer between the processing layer and the organic medium; III – the volume inside floccula with variable concentrations of organic substrate; IV – the volume inside floccula with zero concentration of organic substrate
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Thus, the efficiency of the biological cleaning process depends on the size of floccules, phase of development of bioculture and value of dissolved oxygen throughout the biological treatment. 5. Practical significance To increase the performance of biological treatment we can use modern devices of effectively dissolving oxygen in a liquid medium at the expense of increasing the area of the interface without increasing the volume of supplied air. However, it is necessary to carry out targeted dimensional regeneration of microorganisms to certain values, thereby increasing their activity and the ability to prolonged purification of medium from the dissolved impurities. Keeping the same cleaning process sequence we can improve the efficiency of its targeted transformation of significant characteristics and process variables [26, 27]. To activate sludge and effective dissolution of oxygen in the processed environment we propose to use a rotary device with flow modulation (RDFM). Common to all devices of similar design is the modulation of the flow passage area and, thus the modulation flow rate, volume, fraction of liquid medium in the gap between the rotor and stator. The device operation can be described as follows. The treated fluid is fed under pressure or by gravity through the inlet into the cavity of the spinning rotor. It passes through the channels of the rotor and the stator and leaves the machine through the outlet. The liquid flow rate in the channel of the stator is variable. When distributing the excess pressure of pulse, in the stator channel there is a reduced momentary (negative) pressure. The volume of fluid which entered the channel of the stator tends to leave the channel, and inertial forces create tensile stress in the liquid that causes a cavitation. In the view of the fact that the fluid flow rate in the channel of the stator is great and has fluctuations, the flow has a developed turbulence. The above-listed features of the operation of rotary devices suggest the possibility of their use for activating sludge and efficient dissolution of oxygen. The use of these devices in the schemes of biological treatment has been considered [28]. However, the mediums that hydroacoustic processed have constant physical characteristics such as density, viscosity and others. Therefore, the basic geometric parameters of the device, on which depends the effectiveness of media processing, such as the size of evenly spaced holes in the rotor and the stator are not optimal for the specific conditions of the apparatus. Moreover, the frequency of acoustic oscillations creating is limited by speed rotation widespread motors that are used as a direct drive rotor which limits use of the device. Therefore, during of the work we propose to increase the efficiency of RDFM by changing its structure using optimizing the pulse of hydrodynamic and acoustic impact on the flow treated environment by adjusting the geometric parameters of the stator holes and (or) of the rotor, as well as expand the range of acoustic oscillation frequencies 6. Conclusion The scheme and the technology can significantly improve the quality of biological treatment due to optimal selection of parameters and automatic operation of the rotary machine. The new design allows us to adjust during operation the parameters such as the duty cycle pulse (period pulse repetition to their duration), as well as the time of the pulse. The proposed activities will ensure the effective management of the regeneration of protozoa organisms, oxygen saturation of the medium by increasing the area of the interface without increasing the volume of supply air, thereby greatly enhancing the quality and speed of the process of biological treatment of domestic wastewater. References [1] L.L. Paal', Ja.Ja. Karu, H.A. Mel'der, B.N. Repin, Reference of cleaning of natural and waste water, Moscow, 2004. [2] M.V. Bljashina, L.A. Sabre, Using anaerobic- aerobic bioreactor for wastewater treatment, Water Treatment. 4 (2013) 19–23. [3] Z.R. Hasanova, G.Ju. Fjodorov, Basic facilities for biological treatment of sewage at housing and communal enterprises, Vestnik magistratury, Master's Herald, Publisher: Colloquium Ltd. 44 (2015) 7375.
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