Presence of iron and manganese in water not only affects the organoleptic properties of water, but also can cause a number of problems in drinking water treatments. Their removal in drinking water preparation processes becomes more complicated in the presence of hydrogen sulfide and ammonia in water. There are certain commercialized products at the market that are used for removal of manganese, iron and ammonia, but it is of crucial importance to establish an appropriate order of removal in the technological process during drinking water treatment. Through the various combinations of commercialized filtration media, the removal of iron, manganese, hydrogen sulfide and ammonia, was being examined and on the basis of obtained results their effectiveness was estimated. Research results have shown that hydrogen sulfide is pollutant that causes problems during the adsorption in removing manganes. Ammonia, which is bonded to hydrogen sulphide influences the volume of treated water when it comes to removing the iron and manganese. Decrease in the concentration of hydrogen sulfide at the entrance to Filtersorb FMH for four times, has led to an increase in the volume of treated water in the amount of two times, followed by the breakthrough point of concentration of manganese. For complete usage capacity of commercialized products for the removal of these pollutants, finding their mutual bond in compounds which are present in the water, is of the importance.
The presence of iron, manganese, hydrogen sulfide and ammonia in ground and surface waters, can cause a number of problems that are related to the health safety of water. Iron and manganese are colourless in the dissolved form, but in contact with air turn into an insoluble form, and their deposition causes reddish or brown- black colour of the water, metallic taste and unpleasant odour, which impairs the organoleptic properties of water and may promote the growth of certain types of chlorine tolerant micro-organisms [
Iron and manganese dissolved in water are usually in the form of bicarbonate, sulfate, or hydroxide, and may also be affiliated to a specific organic substance [
Recommended concentrations by European Economic Community (EEC) for drinking water are: for iron 0.2 mg/L, for manganese 0.05 mg/L and for ammonia 0.5 mg/L [
Pollutants such as ammonia, iron and manganese can be removed from the water by chemical or biological means. Physico-chemical oxidation of ammonia is carried out by ion exchange processes, microfiltration, reverse osmosis, “air stripping” processes or by using a strong oxidizing agents. Oxidation of the ammonia is achieved by the simple aeration. At neutral pH values, the oxidation of Mn2+ to Mn4+ represents a slow process [
Commercial products that can be found on the market for removal of iron, manganese and hydrogen sulfide are: Greensand, Greensand Plus and Filtersorb FMH. The basis of Greensand makes glauconite, whereas the basis of Greensand plus is siliceous sand, while the basis of Filtersorb FMH is dolomite, which are covered with manganese dioxide as a catalyst in oxidation-reduction reactions of iron, manganese and hydrogen sulfide. The optimum pH value for the operation of the filter medium is within the range from 6.2 to 8.8. Regeneration or renewal of such media is carried out by continuous or discontinuous dosing of potassium permanganate. The removing capacity of the Filtersorb FMH separately are: for iron 3000 mg/L, for manganese 1500 mg/L, and for hydrogen sulfide 500 mg/L [
For the removal of ammonia the following media are available on the market: ion exchange resins, natural zeolite―clinoptilolite (alumino-silicate mineral comprising a replaceable cations based on the alkali and alkaline earth metal: Na, K, Ca and Mg), and a synthetic zeolite―Crystal Right™ (CWG GmbH, Mannheim, Germany). Benefits of zeolite compared to ion exchange resins are: efficient removal of ammonia at lower temperatures, compact size (which facilitate maintainance) and selectivity to ammonia. Only at pH < 8.0 in ammonium in ionic form may be removed, and the optimal ion exchange is carried out at a pH ≤ 7.0. During the reaction of ion exchange, Na+ ion is replaced with
Simultaneous biological removal of these three pollutants is a very complex process, mainly due to the different values of the redox potential, which are required for their oxidation. Researchers drew the conclusion [
However, this scientific area requires more thorough research of pollutants removal in the presence of hydrogen sulfide. Sequence of pollutants removal can be significantly influenced by the presence of ions HS−, which has a lower oxidation potential compared to manganese, and higher comparing to iron and ammonia.
For these reasons, there were examined the possibilities of removing these pollutants from the water and followed by their removal efficiency. In particular, the efficiency of hydrogen sulfide and ammonia removal were examined as well as their individual impact on the removing of iron and manganese.
The performed research is carried out on a pilot plant with capacity of 0.01 m3/h, in order to define the process of purification of ground water from wells in the Fruska Gora region (Pannonian Plain, North Serbia), which is characterized by a specific chemical composition. The chemical composition of this water is characterized by increased levels of hydrogen sulfide, ammonia, iron and manganese. The processes of removal of ammonia, iron and manganese and hydrogen sulfide were carried out on a pilot plant, which was consisted of a column of plexiglas, with a continuous flow of raw water (
Water samples were taken at the entrance of each column and behind each of the columns, for the purpose of determining the concentration of the individual parameters.
In the first case (A on the
In the second case (B on the
After each of the columns the outlets have been installed separately (with a Teflon valve), through which sampling of water was carried out after a certain volume of flown water (I, II, III on the
For the purpose of this research raw groundwater was taken from wells with depths of 220 m, which is characterized by increased levels of hydrogen sulfide in the range of 0.100 to 0.657 mg/L, ammonia from 0.670 to 1.400 mg/L, iron of 0.820 to 1.380 mg/L, manganese, from 0.048 to 0.159 mg/L, while the pH value was in the range from 7.2 to 7.6. The chemical composition of this water is characterized by poor organoleptic properties and unpleasant odour as a consequence of the above-mentioned chemical composition of the water.
Bearing in mind the quoted specific composition of raw water, it was difficult to determine the sequence of the removal of certain pollutants in drinking water treatment. Since the usage of chemicals in the procedures for the drinking water treatment causes the formation of undesirable side products, the process of adsorption are becoming more widely represented nowadays. In the process of adsorption one of the key factors is the sequence of the removal of specific contaminants. Hydrogen sulfide, ammonia, iron and manganese are pollutants which cause interferences in the process of adsorption, influencing the adsorption capacity, in case the order of removal has not been chosen adequately. Their mutual influence was examined through the relation of the inlet and outlet concentrations of hydrogen sulfide, ammonia, iron and manganese, as a function of the volume of treated water per unit volume of the adsorption medium (
In order to explain in detail the process of removing these substances from the examined water, it is necessary to consider the adsorption processes from the point of view of adsorption equilibrium. At equilibrium a relationship exists between the concentration of the species in solution,
where:
where:
and approximately presented as a straight line:
where
from which a relationship between a value of
Note that
Equation (7) represents an operating line (from a mass balance) for the system. If the time elapsed is long enough for equilibrium to be established then this equation becomes:
In this case the long enough time is the time elapsed from the beginning of the contact of the raw water (starting concentration of the adsorbate,
Using Equation (1) and Equation (8), one can write:
where
On the presented diagram (
Based on this criteria, in the first case it was found: treated water 239 dm3 water/dm3 adsorption medium was treated, up to saturation point on FMH, related to the point of saturation which is defined with concentration of 0.3 mg/L of iron in the effluent.
However, looking across the line of mean values, in this case, the concentration of manganese in the breakthrough point (239 dm3) was about 0.137 mg/L, which indicates that it is not only the manganese from the raw water (0.100 mg/L), but eluation of manganese, which is coated with Filtersorb FMH. In this case, the concentration of hydrogen sulfide observed across the line of the average value was 0.208 mg/L, which represents
equalizing inlets and outlets and the very end of the process of removing hydrogen sulfide from the raw water. On this basis, it can be concluded that the first breakthrough occurred manganese, and then it came to the breakthrough of hydrogen sulfide before it came to the point of saturation defined over iron concentration of 0.3 mg/L. This is a very important indicator that the adsorption capacity in this medium can not be fully utilized when it comes to removing iron since it occurs well before the eluation of manganese and it undermines the quality of purified water and prevents further work.
Based on the observed line of the average values, it can be concluded that, at the outset of the column, the effect of the iron reduction was 87.3%, and in the breakthrough point (0.3 mg/L) was 63.4%. However, the percentage removal of hydrogen sulfide in the initial period was 89.6% and on the 236 BV removal of hydrogen sulfide terminated. The percentage of manganese removal in the initial period amounted to 81.7% and at 163.4 BV removing of manganese was stopped. After that, the eluation of manganese occurred and manganese concentration was increased in relation to the inlet. As a result, no matter on still removing of hydrogen sulfide and iron, the process must be stopped.
The amount of each of the elements under study is presented on the
In the period up to the last 239 dm3 water/dm3 of adsorption medium, Filtersorb FMH adsorbed 119.9 mg Fe, which means 0.076 mgFe/g FMH, and up to the last 163.4 BV adsorbed 89.0 mg Fe or 0.057 mgFe/g FMH. Also, up to the last 236 BV (when removing of hydrogen sulfide was stopped), 16.8 mg of hydrogen sulfide was adsorbed, which means 0.0107 mgH2S/g FMH, and up to the last 163.4 BV (when the breakthrough of manganese was occurred), 13.7 mg of hydrogen sulfide was adsorbed, which is 0.0088 mgH2S/g FMH. Up to the last flown 163.4 BV, 4.55 mg Mn was adsorbed, which is 0.0029 mgMn/g FMH. After this column, the water has been flowing through a column filled with Crystal Right™ for removing of ammonia, in order to quality parameters were within the values recommended by the Regulations for drinking water. The removal of ammonia was terminated up to the past 249.6 BV, and to the past 121.0 BV, ammonia value at the outlet exceeded 0.5 mg/L (which is recommended value by the Regulations for drinking water [
In the second case (case B on the
Up to the past flown 765.2 BV the removal of ammonia was stopped, and up to the past 503.6 BV the value of ammonia at the outlet exceeding the 0.5 mg/L. Based on the criteria when it comes to the point of saturation which is defined with iron concentration of 0.3 mg/L in the effluent, in this case it was found that up to saturation point of FMH, 410 dm3 water/dm3 adsorption medium was processed. Observed via line of the average values, it can be concluded that, at the very beginning of the column, the effect of reduction of iron was 84.7%, while in the breakthrough point of 0.3 mg/L, the effect of reduction was 56%. However, the percentage of hydrogen sulfide removal, in the initial period amounted 99%, and at 342.9 BV removing of hydrogen sulfide was stopped. The percentage of manganese removal in the initial period was 93.7%, and at 302.2 BV removing of manganese was stopped. After the last flown 216.8 BV, values for manganese began to exceed the maximum allowable value of 0.05 mg/L [
The results showed that, in both cases, the volume of treated water directly related to the concentration of input parameters. Based on these results, it can be seen that in both cases after a certain volume of passed water, the intensively increasing of manganese concentrations began, as long as the concentration at the outlet was not equal to the concentration at the inlet, and from that moment the eluation of manganese from Filtersorb FMH began. From the standpoint of mutual competition, it was shown that the affinity of iron towards this medium was highest, while the presence of hydrogen sulfide affected the removal of manganese, and this dependence is shown in
Due to the fact that adsorption is not a selective method, it was necessary to determine the relationship between the parameters, in order to establish the appropriate sequence of removal. On the basis of the inlet and outlet concentration of hydrogen sulfide from the Filtersorb FMH (
Based on the cumulative amount of adsorbed hydrogen sulfide, ammonia, iron and manganese in the second examinated case, it can be concluded that ammonia removal is followed by removing of hydrogen sulfide, which increases the volume of treated water about 2 times, in the case of the removing of manganese. Ammonia removal and simultaneously removing of hydrogen sulfide, affects the increase in the volume of treated water, also in the case of removing of iron. Although there are on the market applied commercialized products for water treatment for the removal of iron, manganese and hydrogen sulfide (Greensand, Greensand Plus and Filtersorb FMH), their use is difficult, because adsorption method is not selective. Thus, it is necessary to determine the appropriate sequence of removal these parameters on the specific medium or perform the multi-stage adsorption. This can be achieved, not only through the values of the electropotential, but the correlation between parameters must be determined, for the appropriate sequence of their removal. In this way, mutual interference in adsorption can be avoid, which greatly simplifies the application of commercialized media for removing of these parameters.
4. Conclusion
Iron, manganese, ammonia and hydrogen sulfide are pollutants that due to their values of electropotential can cause mutual interference during the adsorption, which significantly hinders the application of commercialized media for their removal. Hydrogen sulfide is a pollutant that causes problems in removing, primarily manganese during adsorption and ammonia, which is attached to it affects the volume of treated water when it comes to the removal of iron and manganese. Decrease in the concentration of hydrogen sulfide at the entrance to Filtersorb FMH for four times, has led to an increase in the volume of treated water in the amount of two times, determined by the breakthrough point of manganese. For the reason that adsorption is not selective method, for complete usage capacity of commercialized products for the removal of these pollutants, finding their mutual bond in compounds which are present in the water, is of the importance. Research has shown that the adsorption capacity cannot be sufficiently utilized unless their mutual bond in compounds is found, and appropriate sequence of removal is previously determined.
This research was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, Project Number OI 176018.