CC BY
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18. Eljamal, O., Jinno, K., Hosokawa, T. (2008). A mathematical model of biological clogging of soil-sawdust media. Journal of Environmental Hydrology, 16 (4), 1-12. Available at: https://www.researchgate.net/publication/281260984_A_mathematical_ model_of_biological_clogging_of_soil-sawdust_media
19. Sergienko, I. V., Skopetskiy, V. V., Deyneka, V. S. (1991). Matematicheskoe modelirovanie i issledovanie protsessov v neodnorodnyh sredah. Kyiv: Naukova dumka, 431.
20. Vlasyuk, A. P., Martynyuk, P. M., Fursovych, O. R. (2009). Numerical solution of a one-dimensional problem of filtration consolidation of saline soils in a nonisothermal regime. Journal of Mathematical Sciences, 160 (4), 525-535. doi: https://doi.org/ 10.1007/s10958-009-9518-8
21. Bölter, M., Bloem, J., Meiners, K., Möller, R. (2005). Enumeration and biovolume determination of microbial cells. Microbiological Methods for Assessing Soil Quality, 93-113. doi: https://doi.org/10.1079/9780851990989.0093
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This paper reports a study on the application of aluminum sulfate solution, modified by the magnetic field and electrocoagulation, in the processes of drinking water preparation. The modification of the coagulant solution makes it possible to intensify water purification processes, to reduce reagent consumption by 25-30 %. It has been found that a dose of the modified aluminum sulfate solution of28-30 mg/dm3 improves the efficiency of removal of suspended substances and coloration by 35-40 %. The dosage of the conventional reagent solution was 40 mg/dm3 while reaching the same purification parameters.
Modifying a solution of aluminum sulfate with the magnetic field and electrocoagulation increases the hydraulic size of the coagulated suspension. A change in the hydraulic size in the suspension has been studied at different periods of the year. In winter, when treating water with the modified aluminum sulfate solution, there a decrease in the suspension content whose hydraulic size is 0.1 mm/s and less, from 89 % to 22 %. In this case, the content of suspended substances at settling decreases from 8.5-12.5 mg/dm3 to 5.6-8.3 mg/dm3. In spring, when using the modified coagulant solution, the content of suspension whose size is 0.1 mm/s and less decreased from 55 % to 15 %. In summer, there is an increase in the content of suspension whose size is 0.3-0.5 mm, from 58 % (a conventional reagent solution) to 66 % (the modified reagent solution). This indicates an intensification of the coagulation of impurities and the clarification of water.
The experimental data testify to an increase in the effectiveness of discoloration of natural low-turbid colored water to 63.3-63.9 % for the modified reagent solution at 45.5 % for a conventional reagent solution.
A change in the bacteriological parameters has been determined: the effectiveness of the decrease in a microbial number grows from 11.6-18.7 % to 18.6-25.1 %. In terms of a coli-index, the efficiency of purification grows from 16.6-23.1 % to 23.0-29.5 %
Keywords: drinking water quality, modification of reagent solution, coagulation, aluminum sulfate, hydraulic size, coloration, suspended substances, water clarification, magnetic field,
anode-dissolved iron -□ □-
UDC 628.16
¡DOI: 10.15587/1729-4061.2020.210096|
APPLYING A MODIFIED ALUMINUM SULFATE SOLUTION IN THE PROCESSES OF DRINKING WATER PREPARATION
S . D u s h ki n
PhD, Associate Professor Department of Applied Mechanics and Environmental Protection Technologies National University of Civil Defence of Ukraine Chernyshevska str., 94, Kharkiv, Ukraine, 61023 E-mail: d.akass@ukr.net T. Shevchenko PhD, Associate Professor Department of Water and Wastewater Engineering O. M. Beketov National University of Urban Economy in Kharkiv Marshala Bazhanova str., 17, Kharkiv, Ukraine, 61002 E-mail: Tamara.Shevchenko@kname.edu.ua
Received date 23.07.2020 Copyright © 2020, S. Dushkin, T. Shevchenko
Accepted date 14.08.2020 This is an open access article under the CC BY license
Published date 25.08.2020 (http://creativecommons.org/licenses/by/4.0)
1. Introduction
The purification of water from the surface sources of water supply, used to provide drinking needs, from coarsely-dispersed, colloidal, and other contaminants, mostly employs the physical-chemical technology. This technology includes
the processes of settling and filtration, which are important elements of water supply systems in the preparation of drinking water [1, 2].
One of the most common methods of water purification from coarsely-dispersed and colloidal contaminants is the method of water treatment by coagulants. However, under
the adverse conditions of coagulation, such as insufficient alkalinity, high coloration, and low water temperature, the reagent consumption is quite significant. Therefore, this method needs to be improved, namely: to increase the rate of the formation of coagulated suspension and its deposition.
An analysis of existing methods to improve the operational efficiency of treatment plants in the preparation of drinking water reveals that there are new methods being developed to intensify the operation of these facilities. The most promising are those methods that are related to the modernization of existing, and the development of new, methods and designs of water treatment machines [3, 4].
The preparation of drinking water from surface sources in winter and during a flood period typically involves the larger dosage of reagents because the low temperature of water leads to the deterioration of a coagulation process. This is also contributed to by the insufficient water alkalinity during a flood. However, the use of larger doses of reagents is not always reasonable and leads to a higher load on treatment facilities. In particular, the larger reagent dosage causes the formation of small-size suspension, which is very slowly deposited in the sediment yards and subsequently leads to the rapid clogging of the interporous space of quick filters. Therefore, objective difficulties arise while improving the efficiency of a coagulation process. These difficulties are due to the complication of the process of coagulation of impurities under conditions of low temperature, the high coloration, and insufficient alkalinity of natural water in winter and during the spring flood.
Therefore, there is a need to increase the efficiency of the process of coagulating impurities of natural water during different seasons when applying new technological purification techniques.
An analysis of the design and operational materials for the water supply treatment facilities has allowed us to establish that the following technological schemes of water clarification and discoloration [4, 5] are the most widespread:
- a scheme implying the two-stage treatment of water with coagulation of impurities in the free volume (reaction chambers, sedimentation yards, filters);
- a scheme involving the two-stage treatment of water with coagulation of impurities in the limited volume of the suspended sediment (a clarifier with suspended sediment, filters);
- a scheme with the one-stage water treatment and the use of contact coagulation.
The technological schemes for the purification of natural water for drinking water supply do not always yield the necessary quality of water purification and require significant reagent consumption. Therefore, the development and substantiation of technological schemes for drinking water preparation are one of the important tasks in the intensification of operation of water supply facilities.
2. Literature review and problem statement
The application of a new natural mineral coagulant, instead of the generally accepted aluminum sulfate, is considered in work [6]. The natural material contains the following components: SiO2 - 75-80 %, Al2O3 - 18-22 %, Fe2O3 -0.5-1 %, H20 - 0.2-0.5 %, CaSO4 - 0.3-0.5 %, CaC03 -0.12-0.8 %. The use of a mineral coagulant of the natural origin makes it possible to abandon the application of chemical coagulants and flocculants and reduces chemical pollution of drinking water. The authors determined the optimum tech-
nological parameters of treatment facilities operation in case of using a natural mineral coagulant. However, the application of the specified method at water treatment plants is limited because of the presence or absence of a natural material's deposit, which could be applied instead of chemical reagents.
Paper [7] analyzed the application of new processes to intensify the operation of water treatment facilities. The expediency of using a non-thermal plasma, the stirring reactors with partitions, sonication, twisting a flux, the application of rotating plates, ultraviolet irradiation, microwaves processing, ozone flotation, inkjet oscillatory devices was considered taking into consideration several aspects. These aspects include capital expenditure, full operational cost, performance, sustainability, logistical costs, a possibility for adaptation by other industries, novelty, and so on. Each of the above methods requires further study, experimental research, and the consideration of individual components of an object at which any method is introduced.
Study [8] investigated, as an additive, monodispersed silica, synthesized by the Stober method, in the ultra-low concentrations for the synthesis of organo-inorganic membranes based on polyester sulfone for the potential use in water treatment. The study results showed that the polyester sulfone (PES) membrane, alloyed with monodispersed silica, has improved characteristics. The hydrophilicity of the modified membranes was also increased due to the high affinity of SiO2 nanoparticles to water, which resulted in higher permeability. However, in the interval of high concentrations of the SiO2 nano-particles the permeability of the modified membranes decreases due to the clogging of pores and a change in the macro voids in the membrane sublayer. At the same time, the selectivity of the modified membranes was improved, which is an indicator of a narrower size of pores. The optimum permeability was attained with the addition of 0.30 % of nano-silica. The application of the modified membranes in water treatment is a promising method but high capital expenditures for the construction of membrane units and the need for preliminary fine mechanical purification lead to an increase in the cost of water purification.
The impact of a magnetic field on the process of reduction in scale formation in hydraulic systems is considered in paper [9]. The use of harmful chemicals to reduce scale, while being effective, brings more harm than good as it makes water unsuitable for human consumption and causes an environmental imbalance. Magnetic processing devices against scale offer a cleaner solution to solve this problem. However, data on the magnetic treatment of water are very different and do not yield a consensus on the operational mode of magnetic devices.
One of the basic requirements for water purification plants is the reliability and continuity of their work. The latter to a large extent can be achieved by using in water purification processes of the modified reagent solutions, and, in particular, for the preparation of drinking water - the modified coagulant solution - aluminum sulfate. The specified method makes it possible to improve the quality of water purification, in terms of the suspended substances and coloration, by, on average, 25-30 %, to reduce the dosage of a coagulant by an average of 20-25 %. It is also possible to improve the productivity of treatment facilities by an average of 15-20 % and reduce the cost of water purification. The latter, in turn, would reduce capital expenditures for the construction of treatment facilities in water supply systems and improve the anthropogenic-environmental safety in drinking water purification processes [10, 11].
The mechanism of influence of the modified reagent solutions on water purification processes can be explained by a definite factor: the overlay of a magnetic field on aqueous solutions. An outer magnetic field overlap changes the structure of aqueous solutions and creates conditions for the formation of ion associations of the submicroscopic and colloidal degree of dispersiveness [11, 12]. In turn, the ionic associates, which emerged under the influence of a magnetic field, are the embryos of a new phase of the submicroscopic and colloidal stages of dispersiveness and, upon their stabilization, perform the role of additional coagulation centers. The stabilization of ion associates is carried out with the help of anode-dissolved iron [11].
It was established that the use of the modified coagulant solution for water purification promotes the formation of denser shielded films that slow the diffusion of counter water flows and compounds, formed during coagulation, through the shielding hydrated films.
When using the modified coagulant solution, the formation of the shielding films at the surface of the coagulated impurities occurs slower, while the dehydration of the hydrated structured complexes, formed during water purification, accelerates.
The latter is confirmed by the results of the relaxation timing measurement T by a method of nuclear magnetic resonance (NMR) in the coagulation process of impurities with the regular and modified solutions of aluminum sulfate (Fig. 1) [13]. When a magnetic field acts on a coagulant solution, there is a disruption of the thermodynamic equilibrium in the solution. When the magnetic field acts no longer, the hydrodynamic system must again reach the equilibrium. The process of establishing an equilibrium in the hydrody-namic system is called the relaxation. The time over which the equilibrium sets in the system is called the relaxation time. The relaxation measurements based on the NMR method makes it possible to analyze the dynamics of molecules movement, determine the coefficient of self-diffusion, to investigate the processes of hydration, as well as other phenomena that occur in water-dispersed systems [14].
t, hours
Fig. 1. Results of the relaxation time T1 measurement in the coagulation process of impurities with the conventional and modified aluminum sulfate solutions: ♦ — the modified coagulant solution; ■ — a conventional coagulant solution
The character of the nuclear relaxation dependence Tf(t) is due to the mechanism of impurities coagulation. The first section of the curves in Fig. 1 is characterized by the time of relaxation, caused probably by the release of water molecules from the associated state as a result of the increase in the specific surface of the coagulation structure. In the second section Tf(t), the relaxation time slows down sharply, which corresponds to the intensive formation of coagulation structures. Subsequently, the coagulation structure changes slightly - the time of relaxation does not significantly change.
Table 1 [13] gives the results of relaxation time measurements of the modified and activated aluminum sulfate coagulant solution that show the time interval during which the nonequilibrium closed macroscopic system enters equilibrium.
Table 1
A change in the relaxation time of the conventional and modified solutions of aluminum sulfate reagents
Type of an aluminum sulfate reagent solution Relaxation time 71, m/s Change T1, %
Conventional 1,033-1,035 -
Modified 1,108-1,179 7.2-14.1
The analysis of experimental data indicates that the modification of the coagulant solution leads to a more substantial violation of the dynamic equilibrium of the water-dispersed system. Therefore, it creates more favorable conditions for the formation of ion associates - the embryos of a new phase of the submicroscopic and colloidal stage of dispersiveness, which perform the role of additional coagulation centers in water purification [7, 12].
The use of modified coagulant solutions makes it possible to prolong the relaxation time T1 in the process of impurities coagulation. The latter is probably caused by the presence of additional coagulation centers and, consequently, a more significant violation of the dynamic equilibrium in the water-dispersed system in the process of water purification.
Experimental data acquired when using an NMR method indicate that the active centers of the colloidal stage of dis-persiveness intensify the processes of water purification. These active centers, formed in a solution of aluminum sulfate under the action of a magnetic field, after stabilizing them by anode-dissolved iron, act as the additional centers of coagulation.
The theory of the formation of ion associates during the magnetic treatment of a solution is substantiated in works [15-17].
A coagulant solution is activated in a special device, which involves the formation of ion associates resulting from magnetic treatment and binding them with the anode-dissolved iron [18].
The problematic issue is the study of the effect of the modified aluminum sulfate solution on the processes of clarification and discoloration of natural waters in surface sources. Constant changes in the qualitative characteristics of natural water, such as temperature, coloration, the content of the suspended substance in different seasons, require constant changes in the technological process of purification. Therefore, it is important to investigate the influence of the aluminum sulfate solution, modified with the magnetic field and electrocoagu-lation, on the qualitative indexes of purified water. It is also necessary to compare the amount of the formed coagulated suspended substance in terms of the hydraulic size when using the modified and conventional coagulant solutions.
One of the important parameters of drinking water purification is its bacteriological safety. The effect of a modified aluminum sulfate solution on indicators such as the general microbial number and a coli-index has not been investigated up to now. Therefore, these issues require further research.
3. The aim and objectives of the study
The aim of this work is to establish the feasibility of applying a modified coagulant aluminum sulfate solution in
the preparation of drinking water. To accomplish the aim, the following tasks have been set:
- to experimentally investigate the effect of the modified solution of aluminum sulfate coagulant on the hydraulic size of the coagulated suspended substance;
- to examine the effect of the content of suspended substances in starting water on the effectiveness of clarification when using the modified solution of aluminum sulfate coagulant;
- to establish the possibility of reducing the estimation dosage of the reagent when using the modified solution of aluminum sulfate coagulant;
- to investigate the impact of the modified solutions of aluminum sulfate coagulant on a change in the bacterial indicators of water.
4. Materials and methods to study the application of the modified solution of aluminum sulfate coagulant in the preparation of drinking water
The study on the intensification of water supply facilities' operation using the modified solution of aluminum sulfate coagulant in the preparation of drinking water was performed under laboratory conditions. In addition, a series of experiments were carried out at a pilot installation, located at the water treatment facilities.
A scheme of the chain of machines for the activation of a coagulant solution is shown in Fig. 2.
A modifier consists of two consistently connected devices: a magnetic activator and an electric coagulator. A reagent solution, when passing a working gap between a magnetic circuit casing and a pole nozzle, is exposed to a magnetic field, generated by an electromagnetic coil. Next, the solution enters the electric coagulator where it is saturated with anode-dissolved iron. A modifier to activate a coagulant solution is shown in Fig. 3.
The reagent modifier is installed on a reagent pipe before feeding a coagulant solution to a mixer.
This paper considers the method of improving the operational efficiency of the treatment facilities for a water supply system in the preparation of drinking water by using the modified solution of aluminum sulfate coagulant.
The qualitative characteristics of water from the examined water supply sources are given in Table 2. We examined the samples of water from the river Seversky Donets (Kharkiv oblast) and model samples (tap water introduced with a clay extraction).
The study was conducted during the winter period of 2017 and the spring and summer periods of 2018. A 10 % aluminum sulfate solution was used as a reagent. Based on the initial characteristics of the treated water, the following dosage of the reagent was established: 38-40 mg/dm3 for the conventional aluminum sulfate solution, and 28-30 mg/dm3 for the modified coagulant solution.
The main accepted technological criterion of the efficiency of using the modified solution of aluminum sulfate coagulant is the residual content of suspended substances in the clarified water. It is this indicator that is one of the main parameters that characterize both the operation of treatment facilities within a water supply system in general and of separate structures such as settling ponds, filters, etc.
The coefficient of an increase in water purification efficiency (K) was determined from ratio (1):
K = Dl, D
(1)
Fig. 2. Scheme of the chain of machines for the activation of coagulant solutions
where D0 is the residual content of suspended substances in clarified water (the optical density of suspended substance in a sample) when treated with a conventional reagent solution, mg/dm3; Dm is the residual content of suspended substances in clarified water (the optical density of suspended substance in a sample) when treating water with a modified coagulant solution, mg/dm3.
Fig. 3. Modifier for the activation of reagent solutions: 1 — electromagnetic system casing; 2 — coil; 3 — core; 4 — diamagnetic plate; 5 — waterproof pads; 6 — pole nozzle; 7 — magnetic circuit body; 8 — coupler; 9 — electric coagulator body; 10 — inlet and outlet branch pipe; 11, 12 — partitions for discharge and passing; 13 — plates; 14 — fitting for hydrogen discharge; 15 — connector terminal; 16 — drain nozzle
Qualitative characteristic of the examined water supply sources
Study period Temperature, °C Suspended substances, mg/dm3 Coloration, degrees PCS* pH General hardness, mol/cm3 Alkalinity, mol/dm3
Water samples from the river Seversky Donets
Winter
January 0.5-1.1 8.5-10.4 34-35 7.6-7.9 3.35-3.65 3.1-3.3
February 0.5-2.6 8.7-12.5 35-42 7.5-7.7 3.65-3.80 3.3-3.5
Spring
March 2.5-6.5 10.5-14.8 35-42 7.2-7.5 2.95-3.25 2.3-2.8
April 2.5-9.5 11.3-16.5 41-53 7.4-7.6 2.90-3.15 2.7-3.1
Summer
June 17.5-23.6 11.2-22.3 45-65 7.3-7.7 3.95-4.10 2.6-3.1
July 22.4-23.8 17.4-24.3 55-70 7.5-7.7 3.75-4.05 2.4-2.7
August 22.3-25.3 16.5-38.5 55-85 7.2-7.6 3.95-4.20 2.5-2.8
Autumn
September 17.3-19.5 17.2-41.3 47-65 7.7-7.9 2.95-3.15 2.5-2.7
October 10.3-17.6 14.6-41.6 50-65 7.5-7.9 2.95-3.15 2.6-3.15
Model water
2017-2018 1.1-25.4 25-500 20-100 7.2-7.5 4.35-4.65 2.55-2.85
Note: PCS* - platinum-cobalt scale
The influence of the modified coagulant solution on the hydraulic size of the coagulated suspended substance, which settles at a speed of 1.2 mm/s and 0.2 mm/s and faster, was determined from ratio (2):
E = Em 100%,
(2)
where E is the effectiveness of the impact of the modified coagulant solution, %; EM is the percentage of the suspended substance, settled in the water, treated with the modified coagulant solution, %; E0 is the percent of the suspended substance, settled in the water, at regular coagulation, %.
During the study of drinking water purification involving the modified aluminum sulfate solution, the following aspects were examined:
1) the influence of the modified solution of aluminum sulfate coagulant on the hydraulic size of the coagulated suspended substance;
2) the influence of the content of suspended substances in starting water on the efficiency of clarification when applying the modified solution of aluminum sulfate coagulant;
3) the reduction in the estimated doses of a reagent when using the modified solution of aluminum sulfate coagulant;
4 ) the influence of the modified solutions of aluminum sulfate coagulant on a change in the bacterial indicators of water.
5. Studying the application of the modified solution of aluminum sulfate coagulant in the preparation of drinking water
5. 1. The influence of the modified solution of aluminum sulfate coagulant on the hydraulic size of the coagulated suspended substance
The magnitude of the hydraulic size of the coagulated suspended substance is one of the indicators of the operation of sedimentation ponds, clarifiers with suspended sediment, and other water cleaning facilities. The rapid and complete separa-
tion of a heterogeneous system, which natural water is, depends to a certain extent on the hydraulic size of the suspended substance formed during water treatment with coagulants.
Using a modified coagulant solution for water purification makes it possible to increase the hydraulic size of the coagulated suspended substance and thereby to intensify the processes of water clarification. Treating water with a modified coagulant solution increases the hydraulic size of the coagulated suspended substance, which leads to the faster clarification of water in a sediment pond. When treating water with a conventional coagulant solution (Fig. 4) in winter, a sediment pond contains mainly small suspended substances that are difficult to remove (about 85 %; 0.1 mm/s and less). These small suspended substances are removed along with the clarified water. A suspended substance whose hydraulic size is 0.2 mm/s and larger accounts for about 15 %.
100 90 80 70 60 50 40 30 20 10
0.1 0.2 0.3 0.4 0.5
Hydraulic size of coagulated suspended substance, mm/s
. conventional coagulant solution
. modified coagulant solution
Fig. 4. Effect of the modified solution Al2(SO4)3 on the hydraulic size of the coagulated suspended substance in the winter period
Treating water with a modified solution of aluminum sulfate coagulant decreases the content of the smallest sus-
0
pended substance (up to 22 %), thus increasing the content of suspended substances, detained by a sediment pond (up to 34 %) and improving the quality of the clarified water.
The effect of the modified solution Al2(SO4)3 on the hydraulic size of the coagulated suspended substance in the spring period is shown in Fig. 5. During the spring flood, the coagulation process improves due to the rise of water temperature in a source.
The study that had been conducted in summer (Fig. 6) showed that the amount of small suspended substances (0.1 mm/s and less) is incomparably lower compared to the winter period and a flood. At the same time, there is a predomination of the suspended substances with a hydraulic size of 0.2 mm/s. The content of the suspended substance whose size is 0.3-0.5 mm/s is 58 % (a conventional reagent solution) and 66 % (the modified reagent solution).
100
90 ! 80 J 70
!Si
•o 60
& 3
50 40 30 20 10
0.1 0.2 0.3 0.4 0.5
Hydraulic size of coagulated suspended substance, mm/s
conventional coagulant solution
modified coagulant solution
Fig. 5. Effect of the modified solution Al2(SO4)3 on the hydraulic size of the coagulated suspended substance in the spring period
100 90 80 70 60 50 40 30 20 10 0
0.1
0.2
0.3
0.4
0.5
Hydraulic size of coagulated suspended substance, mm/s
conventional coagulant solution
modified coagulant solution
Fig. 6. Effect of the modified solution Al2(SO4)3 on the hydraulic size of the coagulated suspended substance in the summer period
In the winter period, treating natural water with the modified solution of aluminum sulfate decreases the content of suspension with a hydraulic size of 0.1 mm/s and less, from 85 % to 22 %. The content of suspended substances decreases at settling from 8.5-12.5 mg/dm3 to 5.6-8.3 mg/dm3. In the period of spring flood, using the modified aluminum sulfate
solution decreased the content of suspension of 0.1 mm/s and less, from 55 % to 15 %. In summer, the content of suspension with a size of 0.3-0.5 mm/s increases, from 58 % (a conventional reagent solution) to 66 % (the modified reagent solution). This indicates that the use of the modified aluminum sulfate solution could intensify the process of coagulation and natural water clarification.
5. 2. Effect of the content of the starting suspended substance on water clarification when using a modified coagulant solution
According to the recommendations given in [19], the surface source water used to supply water can be divided into the following groups:
1) highly turbid waters (mountain rivers and rivers in the periods of floods and rains): the amount of suspended substances is 500-1,000 mg/dm3;
2) turbid colored waters: the amount of suspended substances is 100-500 mg/dm3; coloration, 20-60 degrees; the amount of suspended substances is up to 100 mg/dm3; coloration, 30-100 degrees;
3) low turbid colored waters: the amount of suspended substances is up to 10 mg/dm3; coloration, exceeds 50 degrees.
The surface sources of water supply in Eastern Europe mainly belong to the second and third groups, that is, those are the low turbid colored waters (for example, the river Dnipro water) and turbid colored waters (for example water from the channel Seversky Donets - Donbas in separate time of year, a series of sources of water supply in the western regions of Ukraine, etc.). Therefore, in order to determine the feasibility of using a modified solution of aluminum sulfate coagulant in water purification processes, our study investigated the following:
1. The efficiency of clarification of low turbid colored waters using the modified solution of aluminum sulfate coagulant depending on the coloration and temperature of the starting water.
2. The dependence of changes in the coloration and residual content of suspended substances in clarified water on the content of suspended substances in the starting water.
The study involved the model water and water from the river Seversky Donets with the following indicators in terms of the suspended substances and coloration:
1) low turbid colored waters:
- the amount of suspended substances is 5-10 mg/dm3;
- coloration - 20-100 degrees;
2) turbid colored waters:
- the amount of suspended substances is up to 500 mg/dm3;
- coloration - 20-100 degrees.
The results of our study involving the low turbid colored waters at a temperature of the starting water of 1.4-1.9 °C and 12.1-12.6 °C are given in Table 3.
The experimental data indicate a high enough efficiency of the clarification of low turbid colored waters using the modified solution of aluminum sulfate coagulant. The effect of the modified coagulant solution is significantly larger at the water coloration over 40 degrees, which is of certain importance in the purification of highly turbid waters. The experimental findings show that the coloration of the clarified water is almost independent of the starting water temperature.
The effectiveness of the discoloration of turbid colored waters when treating water with the modified solutions of aluminum sulfate coagulants depending on the content of suspended substances in the clarified water and at a coloration of 100 degrees is shown in Fig. 7.
0
Effect of the modified solution of aluminum sulfate coagulant on the discoloration of low turbid colored waters
Experiment No. Coloration of water to be clarified, degrees Water temperature, °C Coloration of clarified water, degrees Change in the clarified water coloration, %
Modified coagulant solution Conventional coagulant solution Modified coagulant solution Conventional coagulant solution
1 20 1.4-1.9 15 17 33.3 17.6
40 26 32 53.8 25.0
60 37 43 62.2 39.5
80 49 59 63.3 35.6
100 62 76 61.3 31.6
2 20 12.1-12.6 15 16 33.3 25.0
40 26 33 53.8 21.2
60 37 44 62.2 36.4
80 49 55 63.3 45.5
100 61 72 63.9 38.9
8.5 8 7.5 7 6.5 6 5.5 5
40
35
30
25
20 % -o
15
10
5
50 100 150 200 250 300 350 400 450 500 Content of suspended substances, mg/dm3
^ - coloration of clarified water when using a conventional coagulant solution;
- coloration of clarified water when using a modified coagulant solution; ♦ - residual content of suspended substance when using a conventional coagulant solution;
0 - residual content of suspended substance when using a modified coagulant solution
Fig. 7. Change in the coloration and residual content of suspended substances in clarified water when treating it with the coagulant solutions depending on the content of suspended substances (/■=17.6-17.9 °C, coloration - 60 degrees)
When applying a modified reagent solution, the maximum values of the effectiveness of clarification of 97.3-98.1 % were obtained at the initial concentrations of suspended sub-
stance of 200-350 mg/dm3. When using a regular solution of aluminum sulfate at the same concentrations of suspended substance, the effectiveness of clarification amounted to 96.8-97.8 %. Afterward, there is a tendency to decrease the efficiency although the value of the coloration of clarified water is low .1 m enough. The coloration of the clarified water when treating it with a modified solution of aluminum sulfate coagulant is approximately at the same level. It was found that the purified water coloration does not depend on the content of suspended substances in the starting water, which is consistent with the research findings reported in [20, 21].
5. 3. Reduction of the estimation dosage of a reagent when using a modified solution of aluminum sulfate coagulant
Detecting the intensified drinking water purification processes when using a modified solution of the reagent has allowed us to assume the possibility of reducing the estimated reagent dosage while increasing a load on a sediment pond without compromising the quality of clarification. The estimated dosage of a regular solution of aluminum sulfate was 38-40 mg/dm3.
The effect of lowering the dosage of a coagulant on water transparency is shown in Fig. 8. Reducing the coagulant dosage at regular coagulation is inappropriate as it reduces the transparency of clarified water.
140 130 § 120 £ 110 § 100
| 90 §
a 80 1 70 ^ 60 50 40
Winter
Spring flood
Summer
100 90 80 70 100 90 80 70 100 90 80 70 Coagulant dosage, % of estimated -Modified coagulant solution —à Conventional coagulant solution
Fig. 8. Effect of coagulant dosage reduction on the transparency of clarified water
The dosage of a conventional aluminum sulfate solution ranged between 38-40 mg/dm3. In this case, the estimated dosage of the modified reagent solution was 28-30 mg/dm3 while achieving the same purification parameters. However, the use of the modified aluminum sulfate solution is advisable for water whose starting concentration is 150-300 mg/dm3 with a coloration of up to 20-100 degrees platinum-cobalt scale.
When using a modified coagulant solution, the dosage can be reduced by an average of 25-30 % without compromising the quality of water clarified.
5. 4. Effect of the modified solution of aluminum sulfate coagulant on a change in the water bacterial indicators
The study of the impact of the modified solution of aluminum sulfate coagulant on a change in the bacterial indexes of water was performed during different periods of 2018, namely: winter period - January, February; spring period -April, May. The qualitative characteristics of starting water are given in Table 4.
Table 4
The qualitative characteristics of clarified water
Parameter Study period
winter spring
Temperature, °C 2.7-3.5 10.2-10.7
General microbial number, CFU per 1 cm3 265 440
Coli-index per 1 dm3 145 160
The selection and preparation of samples for sanitary-bacteriological analysis were carried out by a membrane filtration method; determining the general microbial number -according to [22, 23].
The results of our study are given in Table 5.
Data in Table 5 show that applying a modified solution of aluminum sulfate coagulant makes it possible to improve the bacteriological indexes in the clarified water. Using an aluminum sulfate solution modified by the magnetic field and electrocoagulation has established that the quality of water purification, in terms of the bacteriological characteristics, improves compared to water treatment involving a conventional coagulant solution.
The following improvement of drinking water quality has been detected in terms of the bacteriological indicators: the efficiency of a decrease in the microbial number grows from 11.6-18.7 % to 18.6-25.1 %. The coli-index-based efficiency of purification grows from 16.6-23.1 % to 23.0-29.5 %.
It should be emphasized that the increase in bacteriological contamination in the spring period does not decrease the effectiveness of clarification when using a modified coagulant solution. The rise in temperature in the spring period makes it possible to intensify the process of coagulation of impurities, which promotes the improved efficiency of bacteriological pollution removal in particular.
6. Discussion of results of studying the application of a modified solution of aluminum sulfate coagulant in the preparation of drinking water
The mechanism of the influence of the modified solutions of reagents on water purification processes can be explained by the following factors: exposing aqueous solutions to an external magnetic field changes their structure and creates conditions for the formation of ion associates. These ionic associates, which were generated under the influence of a magnetic field, are the embryos of a new phase of the sub-microscopic and colloidal stages of dispersiveness. After they are stabilized by anode-dissolved iron, they act as additional centers of coagulation.
It is established that using the modified coagulant solution for water purification promotes the formation of denser shielded films that slow the diffusion of counter water flows and compounds, formed during coagulation, through the shielding hydrate films. These factors make it possible to intensify the process of impurities coagulation, increasing the efficiency of clarification and discoloration of natural water.
By studying data in Fig. 4-6, one can point to the following: the application of the modified solution of aluminum sulfate coagulant makes it possible to increase the efficiency of removal of the smallest suspended substance. Thus, in winter, the content of suspended substances with a hydraulic size of 0.1 mm/s in the clarified water decreases from 87 % to 22 %.
Table 5
Study period January 2018 February 2018 April 2018 May 2018
Bacteriological indicators of water to be clarified
General microbial number, CFU per 1 cm3 394 365 581 629
Coli-index per 1 dm3 165 171 591 611
Bacteriological indicators of clarified water
General microbial number, CFU per 1 cm3 conventional coagulant solution 353 322 496 530
modified coagulant solution 332 304 442 451
Coli index per 1 dm3 conventional coagulant solution 142 144 491 495
modified coagulant solution 137 134 133 132
Improved indicators of clarified water, %
General microbial number, CFU per 1 cm3 conventional coagulant solution 11.6 13.4 17.1 18.7
modified coagulant solution 18.6 20.1 21.3 25.1
Coli index per 1 dm3 conventional coagulant solution 16.6 18.7 20.0 23.1
modified coagulant solution 23.0 27.6 28.5 29.5
tî
Effect of the modified solution of aluminum sulfate coagulant on the improvement of water bacteriological indexes
In the period of spring flood, the coagulation process improves due to the rise of water temperature in a source (Fig. 5). The content of small suspended substance decreases (to 55 %), the amount of suspended substances with a hydraulic size of 0.2 mm/s decreases, by 27 %; 0.3 mm/s and larger - by 18 %.
The content of small suspended substance (less than 0.1 mm/s) is 15 %, and, at regular coagulation, is 55 %, the content of suspended substance with a hydraulic size of 0.2 mm/s is approximately the same - 31 % and 27 %. However, the amount of the suspended substance with a hydraulic size of 0.3 mm/s and larger increases by about 2 times, 33 % and 15 %, respectively.
The suspended substances with a hydraulic size of 0.4 mm/s and larger, when using a modified coagulant, account for 21 % of the total content, and, at coagulation with the regular reagent - only 3 %.
During our study in summer (Fig. 6) it was found that the amount of small suspended substance with a hydraulic size of 0.1 mm/s and less is 1.5-2 times lower than that in the winter period and flood when using the modified coagulant solution. Using the modified coagulant solution increases the percentage of suspended substance detention, whose hydraulic size is 0.3-0.5 mm/s. Thus, when using a regular coagulant solution, the suspended substance of 0.3-0.5 mm/s accounts for 58 %, and when using the modified solution of aluminum sulfate coagulant - 66 %.
An analysis of Fig. 4-6 has revealed that when reducing the content of the starting suspended substance to 25-50 mg/dm3, the effectiveness of coagulant solution application decreases: at a concentration of suspended substance of 25 mg/dm3, the substances with a hydraulic size of 0.2 mm/s are removed by 23.1 % and 12.5 %; at a concentration of suspended substance of 50 mg/dm3, respectively, 26.2 % and 12.5 %; 100 mg/dm3 - 41.6 % and 24.1 %.
Thus, the effectiveness of the impact of the modified coagulant solution on hydraulic size increases with a decrease in the latter, as evidenced by data in Fig. 4-6. The amount of suspended substances that deposited, of any hydraulic size, in the period of spring flood is higher compared to the winter period. This refers to the deposition of suspended substance in water treated with a modified solution and a conventional coagulant.
The effect of the modified solution of aluminum sulfate coagulant on a change in the coloration of starting natural water is shown in Table 3, which demonstrates that the effectiveness of lowering the coloration is higher when using a modified coagulant solution. Thus, when applying a conventional coagulant solution, the maximum efficiency value was 45.5 %; when using a modified solution - 63.9 %. However, it should be emphasized that the temperature of the starting water did not have a significant impact on the effectiveness of lowering the water coloration. Thus, when using the modified coagulant solution, both in winter, at low temperature (t = 1.4-1.9 °C), and in the spring period (t = 12.1-12.6 °C) the coloration decreased in about the same range of 61.3-63.9 % and amounted to about 31.5 degrees platinum-cobalt scale.
Fig. 7 shows that the content of suspended substances in clarified water when using the modified solution of aluminum sulfate coagulant is affected by the concentration of suspended substance in the starting water. Thus, at a starting concentration of suspended substance of 200-350 mg/dm3, the efficiency of using the modified coagulant solution reaches
a maximum value of 97.3-98.1 %. However, it should be noted that the coloration of clarified water is not in any way affected by the concentration of suspended substance in the starting water. Nevertheless, the modification of a coagulant solution increases the efficiency of lowering the coloration of water. Thus, when using a conventional solution of aluminum sulfate coagulant, the effectiveness of reducing the coloration of water is about 64 %, and, when using a modified coagulant solution, 69 %.
When studying data in Fig. 8, one can note that when applying a modified coagulant solution, it is possible to achieve a reduction in the estimated dosage of the reagent. It is established that the dosage of a regular solution of aluminum sulfate of 38-40 mg/dm3 improves the quality of clarified water, in terms of the suspended substances and coloration, by 35-40 %. In this case, the estimated dosage of the modified reagent solution was 28-30 mg/dm3 while achieving the same purification parameters. However, the use of the modified aluminum sulfate solution is advisable for water with a starting suspended substance concentration of 50-300 mg/dm3 and a coloration of 20-100 degrees platinum-cobalt scale.
When applying a conventional solution of aluminum sulfate coagulant, the reduction in its estimated dosage by 30 % led to the worsening of the process of clarification during our study periods. Using the modified solution of aluminum sulfate coagulant can reduce the estimated dosage by 20-30 % without compromising drinking water quality. The better process of clarification in the summer period is due to improving the coagulation process by increasing the temperature of source water.
An analysis of data in Table 5 reveals that the quality of water purification, in terms of the bacteriological characteristics, when using a modified coagulant solution, is much higher than when treating water using a conventional coagulant solution. Thus, the reduction in the microbial number, when using a conventional coagulant solution, is 11.6-18.7 % on average; and when using the modified coagulant solution - 18.6-25.1 %.
A similar phenomenon is observed when analyzing values for the coli-index: a regular coagulant solution -16.6-23.1 %; the modified coagulant solution is 23.0-29.5 %.
During the study, only one type of coagulant was selected - aluminum sulfate. Therefore, in the future, it is necessary to study the effect of modifying other types of coagulants and flocculants, which are used in the preparation of drinking water. It is also necessary to compare the effectiveness of the magnetic field influence on coagulants and flocculants, to test the effectiveness of the modified reagent solutions at actual water treatment plants.
The subject of further study should be the comparison of the efficiency of methods for intensifying the operation of treatment facilities, namely: a modified coagulant solution and, for example, a mixture of the coagulant and flocculant. Attention should also be given to the methods of intermittent and portioned coagulation, to investigate their effectiveness compared to the modified reagent solutions.
7. Conclusions
1. Our experimental study into the influence of the modi-
fied solution of aluminum sulfate coagulant on the hydraulic
size of coagulated suspended substance has confirmed the
expediency of its application. Using the modified solution of aluminum sulfate coagulant makes it possible to remove the smallest suspended substance from water (0.1 mm/s and less) without changing the technological parameters of drinking water clarification. The study data, collected during different periods of the year (winter, spring, summer), have shown the greater efficacy of coagulation when using the modified solution of aluminum sulfate coagulant compared to applying a conventional coagulant solution.
2. It has been determined that the efficiency of water clarification when using a modified coagulant solution influences the content of suspended substances in the starting water. It was found that the efficacy of clarification is lower at a low concentration of suspended substance of 25-50 mg/dm3 and when the starting concentration of suspended substances exceeds 350 mg/dm3. The best result of our study was demonstrated at a content of the suspended substances in the starting water, to be purified, of 150-350 mg/dm3. When the content of suspended substances exceeds 350 mg/dm3, the use of a modified coagulant solution for water treatment becomes impractical.
It should be emphasized that the content of suspended substance in starting water has an effect only on the process of water clarification. The process of discoloration of drinking water during purification in a sediment pond when using the modified solution of aluminum sulfate coagulant is not significantly affected by the content of the starting suspended substance.
3. Using the modified solution of aluminum sulfate coagulant makes it possible to reduce the estimated dosage of coagulant by 20-30 % without compromising the quality of drinking water. It is established that the dosage of a regular solution of aluminum sulfate of 38-40 mg/dm3 improves the quality of clarified water, in terms of the suspended substances and coloration, by 35-40 %. In this case, the estimated dosage of the modified reagent solution was 28-30 mg/dm3 while achieving the same purification parameters. However, the use of the modified aluminum sulfate solution is advisable for water with a starting concentration of 50-300 mg/dm3 and whose coloration is up to 20-100 degrees platinum-cobalt scale.
The mechanism of influence of the modified reagent solution on water purification processes can be explained by a change in the coagulant solution structure. Exposing aqueous solutions to an external magnetic field changes
their structure and creates conditions for the formation of ion associates of the submicroscopic and colloidal degree of dispersiveness. The ion associates that form under the action of a magnetic field are the embryos of a new phase of the sub-microscopic and colloidal levels of dispersiveness; after they are stabilized by anode-dissolved iron they act as additional coagulation centers.
4. It is discovered that the application of the modified solution of aluminum sulfate coagulant makes it possible to improve bacteriological indexes in water that is purified. Thus, we have established the improvement of drinking water quality, in terms of the bacteriological characteristics: the efficiency of reducing a microbial number grows from 11.6-18.7 % to 18.6-25.1 %. In terms of the coli-index, the efficiency of purification increases from 16.6-23.1 % to 23.0-29.5 %.
It should be emphasized that the increase in the total microbial number by 1.6 times and in the coli-index by 3.46 times in spring, when using a modified coagulant solution, makes it possible to preserve the reduction of bacteriological parameters. A decrease in the microbial number was 21.3-25.1 % and 17.1-18.7 % for the modified and conventional aluminum sulfate solutions, respectively. The coli-index also decreased - 28.5-29.5 % and 20.0-23.1 % for the modified and conventional aluminum sulfate solutions, respectively.
The rise in temperature to 10.2-10.7 °C in the spring period makes it possible to intensify the process of impurities coagulation when using the modified coagulant solution. This, in turn, contributes to the increased efficiency in the removal of bacteriological pollutants by 1.39 times in terms of the general microbial number, and by 4.63 times in terms of the coli-index. When using a conventional aluminum sulfate solution, the total microbial number decreased by 1.19 times, and the coli-index - by 1.23 times.
Acknowledgments
The study on the application of a modified solution of aluminum sulfate coagulant in drinking water preparation processes was performed as part of the research work «Substantiation of Technical Solutions to Improve Water Treatment Processes» (Ukraine State Registration No. 0118U000650).
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