III Middle East Regional Conference on Civil Engineering Technology and III International Symposium on Environmental Hydrology, American Society of Civil Engineers - Egypt Section
(ASCE-EGS), Cairo, Egypt, 8-10 April 2002
Efficiency of Iron and Manganese Removal From Groundwater Using Aeration Tower in Nile Valley, Egypt Ahmed Khalaf Abdel-Lah 1, Mohamed Shamrukh Mahmoud 2, and Shehata Atia Abadai 3
ABSTRACT: Iron and manganese are found with high concentrations in the ground water of Nile Valley aquifer. They probably originate from the dissolution of iron and manganese-bearing minerals exist in the aquifer. Iron and manganese are common water contaminants that are not considered health hazards. Their presence in water results in staining as well as, offensive tastes and appearances. In an effort to provide adequate and safe drinking water to the customers and to comply with the standards, a treatment process has been introduced. In the last five years, a few numbers of treatment units were constructed utilizing the processes of aeration tower and filtration to remove iron and manganese from the wells in the Nile Valley. A case study has been conducted, autumn of 2001, to assess the efficiency of an aerationtower treatment process located at the city of Farshout (Qena governorate), Nile Valley. Water samples were collected and concentrations of iron and manganese have been measured for the wellfield, filtered and distributed water. Quality of raw water during the investigation period was fairly consistent. This oxidation process was successful in reducing iron level to below standard of 0.3 mg/L. nevertheless, manganese concentration was still higher than allowable level of 0.1 mg/L.
INTRODUCTION Elevated levels of iron and manganese in drinking water lead to health and aesthetic concerns. Such as problems associated with staining of cloths and plumping fixtures, and boiled vegetables, and incrustation of water mains. The later process of water mains leads to customer complaints of high turbid and brownish/black water at tap (Odell, 2001). In addition, water containing iron and manganese promotes the growth of iron and manganesetolerant bacteria in pipelines with accompanying increases in friction loss and power consumption. Therefore, the Egyptian drinking water guidelines specify a maximum concentration of 0.3 mg/L and 0.1 mg/L for iron and manganese, respectively. 1
Department of Civil Engineering, Faculty of Engineering, Assiut University, EGYPT e-mail:
[email protected] 2 Department of Civil Engineering, Faculty of Engineering, ElMinia University, EGYPT e-mail:
[email protected] 3 Research Scientist, Egyptalum Company, EGYPT 1
The majority of ground water contains iron and manganese in solution that are derived from the location and past movement of subsurface water. The type and concentration of these salts depend on the environment of the aquifer and the surrounding rocks. Such as: amphiboles, ferromanganesion mica, ferrous sulfate, iron pyrite, magnetite, sandstone rocks, and iron clay mineral are the source of dissolved iron. The dissolved manganese comes most often from metamorphic rocks, mica biotite, and amphibole hornblende minerals (AWWA, 1990). Iron and manganese that found in ground water system are predominately found in their reduced forms: ferrous iron ion (Fe2+) and manganous manganese ion (Mn2+). Iron and manganese removal can be accomplished with chemical oxidation of soluble form to its insoluble oxide form. The insoluble form is then subsequently removed during the downstream processes (e.g., filtration). The oxidation of those reduced forms of iron and manganese results in formation of ferric iron (Fe3+) and manganic manganese (Mn4+), sometimes (Mn3+) is formed as well. These oxidized forms of these compounds generally precipitate as iron hydroxide (Fe(OH)3) and manganese dioxide (MnO2). Oxidized iron and manganese can result in particles smaller than 5 micron in size. Then, the precipitates that are formed can be removed by filtration processes, although the precipitates are oven very small (Odell, 2001). In addition, oxidation reaction kinetics can vary greatly depending on type of oxidant used, pH, water temperature, and the presence of organic complexes. The most commonly used oxidants are chlorine, chlorine dioxide, and potassium permanganate (Odell, 2001)Often multiple oxidants are used and contact chambers are included in the design. The following equations represent the oxidation of iron and manganese by oxygen, respectively (Coulter and Gagnon, 2001):
2Fe 2 O2 2H 2 O 2FeO 2 4H
……..………………(1)
2Mn 2 O2 2H 2 O 2MnO2 4H
………………..….(2)
This paper will present case study of actual full-scale aeration tower system. This system was designed specially to treat iron and manganese from ground water in the Nile Valley aquifer. Process design information and water samples analysis will be presented to assess the efficiency of removal process of the treatment plant.
IRON AND MANGANESE REMOVAL PLANT The study site, Farshout City, is one of Qena governorate cities which is located in the southern part of the Nile Valley. Its population about 70 thousands and it depends on ground water as a source of drinking water demands. In late eighties, high concentrations of iron and manganese were observed in the ground water of the extraction wells. Therefore, a treatment plant was designed and constructed to remove that elevated concentrations of iron and manganese from the ground water in year 1998. In fact, a few number of this treatment model were constructed in scattered sites within the Arab Republic of Egypt. Most of those plants were designed and constructed by Qaha Company for Chemical Industrials, Egypt.
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The complete water supply system consists from three parts. A schematic diagram showing the plant system is given in Figure 1. Those three parts are the extraction deep wells, the removal plant, and the distribution pipeline network. The ground water is extracted throughout a four deep wells each 85m length and 140m3/h production capacity. All four wells extract ground water from the sand-gravel layer of the Nile Valley aquifer. The extracted ground water is pumped to the waterfall aerator that is followed by the filtration process.
Chlorine
Calcium hydroxide
Potassium Permanganat e
Distribution system
Aeration Tower
Pressure Filter
Well field
Sand&gravel aquifer
Figure 1. Schematic diagram of the iron and manganese removal plant from the well water, Nile Valley aquifer.
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Then, the finished water is pumped to the distribution pipelines that serve the city. Chlorine gas is applied after the aeration process and prior to the filtration process as a disinfectant. The maximum total capacity of the treatment plant is 8000 m3/day when the four wells are pumped. The current operation schedule of the wells is two or three wells are on from 7:00AM to 2:00PM and one well is on from 2:00PM to 7:00AM. The oxidation of iron and manganese in this site is achieved using the waterfall aerator tower. The tower is circular in shape and constructed from reinforced concrete. It is consists of three trays (six compartments) with total high of 20m, Figure 1. Every tray is equipped with perforated bottoms over which water is distributed and allowed to fall into a collection basin at the base. The holes diameter of those perforated trays is about 12.5cm. Thus, the falling water is broken into thin films, thereby increasing the area of water exposed to air per unit volume. In this removal unit, Fe and Mn oxidation was designed depending on the ideal theoretical stoichiometric ratio of oxidant to metal for each reaction, see two previous equations (Casale et al., 2001). According to the plant manual, the estimated volume of air necessary to oxide iron and manganese is 0.5 and 1.0 liter for 1.0 gm of iron and manganese, respectively (Qaha, 1995). It should be noted that estimating of water aeration demand is an inexact science due to the large number of items that influence oxygen uptake. Lack of sufficient aeration capacity is perhaps the number one cause of aeration system that fails to improve water quality. The falling water is then collected downstream from the lower tray orifices into a collection basin. To remove oxidized iron and manganese precipitated from solution, the flow is delivered to a pressure filter. Then the filtered effluent from the pressure filter is delivered to service. In this treatment plant, there are two feeding points, one for potassium permanganate and the other for alkali. Potassium permanganate is used as oxidant in case of observed elevated concentrations of iron and manganese in the well water. Alkali, calcium hydroxide, is fed to raise the pH, if required. Because of some complaints, neither potassium permanganate nor calcium hydroxide is fed during the current operation of the plant. At the end, chlorine gas is added to the filtered water as a disinfectant.
SAMPLING PROGRAM Few complaints have been raised regarding the quality of the treated water from the Farshout plant. Therefore, an investigation study was carried out for this plant. Many grab samples were collected from different points. The locations of samples are: four production wells, aerator inlet, filter exit, and downtown of distribution pipelines (network). These samples were analyzed chemically and biologically in the field using Hach DR 2000 spectrophotometer, pH meter, and TDS meter. Many other quality parameters were measured as shown in Table 1. In addition to current measurements, more water samples have been collected and analyzed in the laboratory of The Aluminum Company of Egypt located near Farshout City. Many water quality parameters were measured for each sample which has been collected two months before. Consequently, the concentrations of iron and manganese were taken from that reported analysis. Those previous results were added to the current investigation results.
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ANALYSIS OF RESULTS As general, the most of quality parameters of well water, as shown in Table 1, are under the standards of drinking water. From the comparison of water quality parameters pre- and posttreatment processes, the following results can be drawn. The influence of aeration and filtration processes on quality parameters such as pH, TDS, nitrate, phosphate, and sulfate is negligible, Table 1. This effect was anticipated because of the oxidation state of those species. As shown in Table 1, it is observed that there are higher concentrations of nitrite in filtrate water than feed water before treatment. From the literature, the transformation of nitrogen among nitrite, nitrate, ammonium, and other forms is a complex process. Therefore, those transformations may be the important source of this nitrite increasing in the filtrate water. To explain this increasing, more measurements of nitrogen are required. Table 1. Water quality analysis sampled in 2001 location
pH
TDS
Fe
Mn
NO3
NO2
PO4
SO4
Cl2
Well 1
7.7
610
2.63
0.52
3.52
0.013
0.38
-
-
Well 2
7.5
450
0.62
0.45
7.10
0.026
0.50
120
-
Well 3
7.6
460
0.35
0.38
5.30
0.013
0.46
195
-
Well 4
7.5
560
2.67
-
4.40
0.063
0.30
-
-
Treated
7.9
480
0.10
0.35
6.15
0.50
0.33
125
1.75
Service
7.6
510
0.19
0.31
5.90
0.58
-
150
0.14
Removing of iron and manganese is the main objective of this treatment plant. The concentrations of Fe and Mn in feed and filtered water for three measurements at different days are plotted in Figure 2. The concentration of feed raw water is estimated according to the number of pumping wells and the concentrations in each one. It is observed that there is no consistency in Fe and Mn concentrations with time. Furthermore, concentrations of iron in wellfield water are varied greatly from one well to another. Nevertheless, there is small variation of manganese concentrations from well to another. Figure 2 illustrates the mean concentrations of iron and manganese pre-and post-treatment plant for the three sampling days. It is clear that aeration followed by filtration can be effective technique for iron removal in Nile Valley. Removal efficiency for iron is typically above 85%. Furthermore, the filtrate iron concentrations are less than Egyptian standards (0.3 mg/L). Nevertheless, the aeration is not effective technique for manganese removal, as shown in Figure 2. The efficiency of manganese removal is typically under 35%. In addition, the manganese concentrations in filtrate water are still higher than standards (0.1 mg/L). Thus, aeration oxidizes manganese less than iron. These observations of Fe and Mn removal by aeration are in agreement with many other studies in the literature (Casale et al., 2001; Coluter and Gagnon, 2001; Odell, 2001). Therefore, oxidation of manganese with aeration is generally not practiced because it is very slow and dependent on pH. It is reported that, below pH value of 8.6, the oxidation of Mn is very slow. Therefore, the feeding points of Potassium permanganate and alkali must be re-considered in the plant operation.
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Feed water
2.0
Fe Concentration (mg/L)
Filtrate
1.6
City
1.2 0.8 0.4
0 #1
#2
#3
Sampling dates
(a) Iron concentrations
Feed water
Mn Concentration (mg/L)
1.0
Filtrate
0.8
City
0.6 0.4 0.2
0
#1
#2
#3
Sampling dates
(b) Manganese concentrations Figure 2. Concentrations of iron and manganese pre- and post-treatment for the three sampling days
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CONCLUSION Efficiency of iron and manganese removal using aeration tower designed by Qaha Company and located at Farshout City was investigated. Wellfield water quality during the study was fairly consistent. The Farshout treatment removal plant for wells was found to be highly efficient in iron removal. The technology of this unit seems to be more acceptable and adaptable at low levels of iron concentrations. However, this technology is not effective for manganese removal. This observed limitation of the treatment plant force the designer and operator to do further development. It seems that the feed point of potassium permanganate included within the plant must be applied. In addition, raising pH of feed water may be an another solution to help in manganese oxidation with aeration. In addition, the old ages of water pipelines network seems are working to decrease water quality at consumer tap.
REFERENCES APHA (American Public Health Association), AWWA (American Water Works Association) and WEF (Water Environment Federation) (1995) Standard Methods for Examination of Water and Wastewater. 20th ed. Washington, D.C.: American Public Health Association. AWWA (American Water Works Association) and ASCE (American Society of Civil Engineers) (1990) Water Treatment Plant Design. 2nd ed., New York: McGraw Hill, Inc. Casale, R. J., LeChevallier, M. W., and Pontius, F. W. (2001) Review of Manganese Control and Related Manganese Issues. AWWA Annual Conference Proceedings, Washington D.C. Coulter, S., and Gagnon, G. A. (2001) Optimization of Manganese Removal by Filtration. AWWA Annual Conference Proceedings, Washington D.C. Deshpande, Shard S. and Brigano, Frank A. (1990) Iron and Manganese Removal. AWWA Annual Conference proceedings. Jankauskas, J., Valentukevičienė, M., and Karosas, T. (2000). Efficiency Investigation of Domestic Equipment for Drinking-Water Treatment. Environmental Engineering, Vol. VIII, No 2. P. 86-91. Laughton, Richard V. (1987). Iron and Manganese Removal, Prospect Park Halton Hills (Acton), Ontario Regional Municipality of Halton. 1987 Joint Annual Conference, Ontario Section, American Water Works Association, Ottawa, Ontario, Canada. Odell, L. (2001) High Rate Iron and Manganese Removal for Small Systems. AWWA Annual Conference Proceedings, Washington D.C., USA. Qaha (1995) Manual of Specification, Operation, and Maintenance of Well Water Removing Iron and Manganese, Qaha Company for Chemical Industrials, Qaha, Qlubiah, Egypt. Schneider, C.; Johns, P.; and Huehmer, Robert P. (2001). Removal of Manganese by Microfiltration in a Water Treatment Plant. AWWA, Membrane Technology Conference Proceedings.
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