Using Electrical Methods to Increase Sedimentation Rate of Sludge in Drinking Water of Purification Stations during the Rainy Season

Abstract


Introduction
Water is the lifeblood of man, so interest in the quality of water began more than five thousand years ago, and this concern was limited to the color, taste, and smell of water. Water is an essential element for all living things (there is no life without water) with water. In the past, civilizations were established on the banks of rivers and near their estuaries, and the depletion of water is one of the most important reasons for the demise of those civilizations. Water has a basic and actual connection with human development and daily needs water in all of its vital, domestic, religious, industrial, commercial, and other activities. Human has made a good choice of appropriate water sources and devoted all possibilities to preserving them from pollution. Since a person relies on his senses when evaluating drinking water, he relies on the water's external properties, smell, or taste. Therefore, turbid water is considered a danger to health and refuses to be used for drinking, in addition to the fact that some salts and others are dissolved in drinking water cannot be distinguished with his senses (WHO, 2017a).
For this reason, some treatment processes have been used in a limited way and during separate historical periods such as (boiling, filtration, sedimentation, and the use of salts such as alum and other traditional methods) (Cescon and Jiang, 2020). Most non-developed countries do not have advanced processing units, so they depend mainly on processing units' conventional access to water within international health standards (Fssai, 2015;Hassinger and Watson, 2016;WHO, 2017a). Globally, several tests are conducted on the water to determine its suitability for different uses (WHO, 2017b). As is well known, water contaminated with suspended matter is one of the most important causes of diseases and even threatens human life (Ramal, 2010). Removing suspended materials from drinking water is necessary to protect public health on the one hand and to prevent operational problems in the distribution network on the other hand. Suspended materials due to the size of the micro-clay, which takes a long time to settle, leading to the stopping water supply in the stations during the rainy season due to the presence of large quantities of mud. Recently, radical changes in treatment techniques have been carried out, in many cases due to the severe shortage that many countries of the world suffer from drinking water, or as a result of pollution of water sources, like the most industrialized countries (Bukhary et al., 2020 andSmith, 2002). These factors have led to the search for new sources other than traditional sources, which naturally require advanced processing techniques in addition to traditional treatment such as the use of activated carbon, ozone, and environmentally friendly methods (Campinas et al., 2016;Malik et al., 2001;Reimanis et al., 2013).
The mechanism of action of the electric charges applied to the sedimentation basins containing the clays is the electrostatic attraction between the suspended particles of the suspended clays carrying the negative charge and the positive charges emanating from the electric current between the electrodes in the sedimentation basin (Alaydi, 2016). As a result of the accumulation of positive charges around each granule, the clay granules gather together, which leads to an increase in their mass and thus flocculate and precipitate to the bottom of the pot with a sufficient period for the growth of the formed colloidal mass (Elsaim et al., 2019).
The areas around the Tigris River in Iraq are exposed to erosion due to rain. The products of erosion, including soil, are transferred to the Tigris River through torrents and streams, carrying large quantities of mud, as well as the products of washing roads and streets due to rain, and all of this reaches the Tigris River, which will be a repository for all waste, especially mud. Therefore, water purification stations will suffer from the problem of getting rid of the suspended sludge load before pumping water to consumers. The rapid development in the field of water treatment techniques to reach a high degree of purity using effective and practical methods that must be taken into consideration, including the proposal to use electrodes inside basins (where mud is deposited) as advanced stages of the water purification process. Note that this method has never been used in water purification stations in Iraq.
The study aims to use the electric current to accelerate the sedimentation of plankton in the river water loaded with large quantities of mud in the rainy season. This saves the time required to filter the water in the sedimentation basins in the drinking water purification station.

Sampling
Fifty letter turbidate water sample was taken from the Tigris River during winter season in January, 2017. The sample transported to the lap in department of physics science in plastic containers.

Preparation of the electrolysis cell
Three types of electrical cells were prepared in the electrical laboratory of the Department of the Physics Sciences / University of Mosul. In the first type, copper electrodes of dimensions (10 x 5 cm)  The electrodes were connected (the distance between them was 3.1 cm) with two devices and placed inside the sedimentation basin containing river water with a high percentage of suspended mud (1000 NTU).

Physical Tests
• An electric current passed to separately monitor the effect of time, voltage, and current, as well as their combined effect on the sludge deposition velocity, as follows: a. The river water (250 ml) was placed in a glass beaker and the poles were immersed in it to a limit of (6 cm), the voltage was fixed at 20 volts and the current at 0.085 amperes. 20 ml water was taken at different times (10, 20, and 30) for minutes. b. The current was changed (0.065, 0.085, 0.125, and 0.15 amperes) with a fixed time of 20 minutes and a constant voltage of 28 volts . c. The voltages were changed (20, 22, 25, and 28 volts), with a fixed time of 20 minutes, and a constant current of 0.085 amps . d. Measuring the degree of turbidity by adding (1000 ml) and (3000 ml) respectively of river water containing clays. Using a voltage of 20 volts and a current of 0.085 amperes with variable times. • Measuring the oxidation-reduction potential (Eh), and the acidity function (pH) of a river water sample containing clays. The oxidation-reduction potential of samples was measured in the laboratory of physics/ Department of Chemistry Sciences at the University of Mosul. • Checking the turbidity by using turbidity meters, (model LAB-IR, manufactured by Lovibond/ Germany in 2000) in lab of University of Mosul Environmental researches center to measure the turbidity of samples before and after treatment by taking samples from the pot that contains the water.

Results and Discussion
During the passage of electric current, an interaction between some components of the clay with the electrode material, and flocculates to form a colloidal substance and precipitates. Since the reduction potential of river water contains very few muds about 6.5 x 10-3 V, therefore, applying a weak voltage could cause a change to the charges of the surfaces of the suspended muds in the water and thus lead to an increase in its fluctuation and sedimentation. A weak current and voltage were shed and increased, gradually to see the effect on the speed of clay deposition with time.
The degrees or values of turbidity of the water were measured to observe the increase in water purity during the passage of electric current in the models of distilled water, river water containing mud, as well as tap water. Table (1) shows the turbidity values in the water of the river containing suspended mud, which was used in the research. The degree of turbidity was more than (NTU 1000), which is a very high value when compared to the turbidity of tap water (NTU 19.2) and distilled water (NTU 0.35).  (Bejjany et al., 2017).

Samples
Turbidity (NTU) River water containing clays >1000 Tap water 19,2 Distilled water 0.35 Table (2) shows the effect of a variable electric current with constant both voltage and time. It was observed that an increase in the amount of current leads to a decrease in the degree of turbidity, Fig. (4) shows relationship between the degree of turbidity and the current for river water after passing a variable current with a fixed time of 20 minutes (because at time 10 min. settlement of Suspension materials is not complete) and a voltage of 28 volts. It was observed that a green suspended substance of copper carbonate was formed (when using a copper electrode) as a result of the interaction of the electrode material with the carbonate in the water (Fig. 5), and this substance caused turbidity after the deposition of clay, but it quickly deposited at the bottom of the pot and also accumulated towards the top after about ten minutes. These carbonates can be precipitated in less time by adding alum to the water. Table  (2) shows the degree of turbidity after the deposition of copper carbonate.   Fig. 6). It is noted that the change of current has a greater effect on the rate of clay deposition and the decrease in the degree of turbidity. This is because the increase in the current means that greater charges are passed, and thus the charges increase on the surfaces of the suspended particles, because of clay have charges on surface of its sheets so it's effected by any change in current and while the charges increase the clay particles will flocculate together.  The voltage was also fixed at 20 volts and the current at 0.085 amperes (which is the highest that the device used in the laboratory can be measured together) and the turbidity was measured at different times (Table 4 and Fig. 7).  To find out the effect of the amount of water at the same current, and voltage (0.085A and 20V) respectively, and using the same electrodes, the turbidity was estimated at the volumes 250 ml, 1000 ml, and 3000 ml. Fig. (8) shows that the amount of turbidity does not change significantly despite the doubling of the volume from 250 ml to 3000 ml (12 times), this reflects the effectiveness of the electric method in the sedimentation of mud and suspended matter.   (6) and Fig. (10) show the time relationship with the degree of turbidity when using a quantity of water 1000 ml and 3000 ml, respectively. It appears from this that the sedimentation process needs a slightly longer period with an increase in the amount of water used for purification purposes.  9. The relationship between the degree of turbidity and time for (1000 ml) of river water at 20 V. and 0.085 A Fig. 11 shows the difference between the percentage of turbidity before passing the electric current and after passing the current at different times with constant current and voltage difference on a sample containing (3000 ml) of river water with suspended mud.  . 10. The relationship between the degree of turbidity and time for (3000 ml) of river water at 20 V. and 0.085 A. with variable times Fig. 11. Shows a difference between the percentage of turbidity before the electric current and after the current at different times with constant current and voltage on a sample containing 3000 ml of river water with suspended mud.
The copper electrodes were replaced with carbon electrodes as shown in Fig. (2) to avoid contamination with the copper element resulting from its interaction with the carbonates in the water, thus increasing the percentage of copper in the water. It was found that replacing the copper electrodes with carbon electrodes also led to the sedimentation of clay at the same speed Table (7), and it is considered safer in terms of health and pollution (Changjia et al., 2019;Dvorak & Skipton, 2013).

Conclusions
The deposition of portable mud in river waters during the rainy season takes a long time, which calls for the necessity of stopping pumping water into the stations to avoid damage resulting from the presence of large quantities of mud. Therefore, it was necessary to find ways to increase the speed of deposition of these muds to avoid these damages. The use of electrical methods with low voltage and current is considered one of the safest methods from a chemical and physical point of view, as it does not affect the properties of water. The current and voltage used in different ranges led to the sedimentation of suspended mud in a short time, about 20-30 minutes, and they gave good results, as the turbidity in some samples ranged from about 1.3 -6.7, and this is considered good from an economic point of view. Size and number of electrodes and distance between the electrodes depended on the volume of the basin in station, the depletion of the poles over time will be under control by the operator. Both Copper and carbon electrodes will show a high efficiency and similar effectiveness to deposition of clays, but the study prefers to use carbon electrodes because of carbon is an inert and inactive element. It was found from the results of the study that this method was highly effective in purifying drinking water from suspended materials, and it had advantages as it does not cause a change in the chemical or physical composition of water and it is economically feasible.
It should be noted water pumping stations stop the water supply process during the period of high turbidity of the river in heavy rain periods, it is possible from the idea of research that the purification and supply process continue intermittently, to perform the sedimentation process and pulling sludge from the basins, instead of the station ceasing to supply the pure water completely. Remove sludge from the basin is the same technique of removing the sediments from sediment basin that done by the operators in supply station.