Groundwater treatment using polystyrene foam filters with upflow filtration

The groundwater is less susceptible to anthropogenic influence and the water quality of these sources is more stable. At the same time, the groundwater often has an increasing iron concentration. Despite the sufficient availability of proven methods for iron removal from water, it becomes necessary to search for more resource and energy saving technologies which is due to the constant rise in the cost of resources for the construction and operation of water treatment plants. The aeration methods for the iron removal from groundwater are often used, as cheaper ones. The polystyrene foam filters with upflow filtration are of particular interest, as the most reliable in operation among the developed types of filters with floating medium. A further reduction the cost of water is possible by using granules of foam polystyrene obtained by a production method, which ensures an increased grain size in comparison with the traditional method of their obtaining. The positive experience in such filters implementation allowed to recommend the placement of iron removal equipment in the existing water towers.


Introduction
The availability of sufficient water amount of adequate quality is one of the main factors of safe living conditions and sustainable development of the state [1,2].However, human activity negatively affects the environment, including water resources, which leads to their significant pollution and exhaustion [3][4][5].
Thus, according to prediction, a half of the world's population will experience water shortages in the near future [6].The preference is given to groundwater (artesian, springs) underflow when choosing a source of water supply [7,8].
Groundwater is the main source of drinking water in many countries [9][10][11].As a rule, a significant proportion of protected groundwater sources have elevated concentrations of iron, less often hydrogen sulphide, ammonium, manganese, hardness salts, mineralization and other contaminants [12,13].
Iron content is associated with regional, climatic, landscape and hydrological features.It is known that an increased concentration of iron in water gives it a brownish colour, an unpleasant metallic taste, and causes overgrowth of water supply networks and fixtures [14,15].
Iron in natural waters can be in the form of ferrous and ferric ions [16,17], colloids of inorganic and organic origin, which has a significant impact on the choice of method of iron removal.According to [6] methods of iron removal from water are divided into reagentless, reagent, ionexchange, membrane and biochemical.At the same time in [18] it is proposed to divide them into four groups: traditional strategies, biological strategies, strategies based on membrane technology and strategies based on nanotechnology.
The preference is given to reagentless (aeration) methods for the iron removal from water because they are relatively simple to operate and easily automated [14,19].The most widely used aeration methods include contact iron removal, in which water is fed directly into the granular medium after aeration [6].In this case the medium grains can have a larger size than in deep aeration.In addition to quartz sand, the other natural and artificial materials are used as a filter granular medium.The type of filtration medium, parameters of its grains and layer height have a significant impact on the efficiency of water treatment, size of filtration units and their productivity.The polystyrene foam is one of the economically suitable medium [6,20].
The work is devoted to the development of resource and energy-saving schemes of contact iron removal from groundwater at polystyrene foam filters with upflow filtration.

Materials and methods
Foam polystyrene filters with upflow filtration have been investigated for a long time in the laboratory of the department of water supply, drainage and drilling at the National University of Water and Environmental Engineering [6,20].
The plant for studying the iron removal from water consisted of a filtering column, a filtration rate regulator, a piezometer shield and a unit for dosing the iron solution.The filtering column was made of a transparent pipe 150 mm in diameter and 2000 mm in height.A cone was fixed below to it with a branch pipe for connection of aerated water and a washout pipe.A tank of 500 mm in diameter was connected above.It was possible to visually observe the processes occurring in the sublattice space thanks to such construction of the plant.Four branch pipes for the piezometers connection and five samplers were arrange along the height of the filtration column d.The amount of water sampled by the samplers did not exceed 5% of the flow rate passing through the filtration column.The head losses in the filtration column was monitored with the help of the piezometer shield.
The construction features of filters, water treatment efficiency, hydraulic laws of processes in the medium for different operating conditions were studied.The impact of granulometric composition of the medium, layer height, filtration rate, initial water quality parameters (iron concentration, pH, alkalinity, etc.) on these processes was studied.Special attention was paid to investigation of the medium regeneration by different methods.Extensive pilot and industrial production studies were conducted in the water supply systems of a number of settlements of the north-western regions of Ukraine at the introduced plants for iron removal from water with foam polystyrene filters of productivity from 2 to 900 m 3 /day.
The polystyrene foam medium can be obtained in the production conditions (plants of producing the polystyrene pellets) and directly at the place of use.As a rule, the polystyrene is foamed by steam in the first case and by water in the second case.The medium amount in the first case is larger and its density and cost is lower.It is advisable to use gravity polystyrene foam filters with increased granule size at the iron removal from water for medium and large consumers (figure 1).The use of industrially manufactured polystyrene foam pellets allows reducing the cost of medium purchase by 4.5 times.

Results and discussion
Foam polystyrene filters with upflow filtration can be used in the reagent schemes of water clarification and decolorization, in the schemes of iron removal from water, for water pretreatment.It is recommended to use aeration followed by filtration aimed at removing water treatment from iron ions.It is known that a catalytic film of iron oxides should form on the surface of the medium in order to obtain the necessary effect of iron removal from water, which is not washed off after the filters washout.The catalytic film of iron oxides on the surface of the medium intensifies the adsorption of ferrous iron and its oxidation.Thus, there is no direct contact of the treated water with the surface of the floating medium (polystyrene foam).The ordinary foam polystyrene filters are used at low concentrations of iron ions in the source water and at significant concentrations of iron ions with a growing layer of suspended sediment and foam polystyrene medium.The construction of such filters provides for a sediment layer in the sub-filter space, where the main iron extraction takes place and the final one is directly in the foam polystyrene medium.The sediment layer increases over time, increasing the efficiency of water treatment in it.A part of the sediment is discharged with the washout water.The use of expanded polystyrene filters with a growing sediment layer with each washout of the medium makes it possible to significantly reduce the building area of the wastewater treatment plant.At the same time, the total height of the filtration room will be slightly higher.
We propose the concentration of iron in water after passing the growing layer of suspended sediment [F e l ] (mg/L) to determine by the equation where [F e 0 ] (mg/L) is the concentration of iron in source water; V (m/h) is the filtration rate; H 1 (m) is the height of the suspended sediment layer; K 1 (1/h) is the coefficient, taking into account the physicochemical parameters of water and contact medium.The value of the coefficient K 1 was obtained experimentally and is 0.7-2.5 1/h.The studies were conduct at a water temperature of 5-15 • C for the purpose of determining H 1 , the concentration of iron in the source water 15-30 mg/L and the rate of upflow of 2.5-4.0 m/h.The following empirical dependence H 1 (cm) was obtained based on these studies where H 0 (cm) is the initial height of sediment layer; t (h) is the duration of water treatment in the growing layer of suspended sediment.
To check the adequacy of the last equation in figure 2 the dependences of the growth of the suspended layer of deposit obtained experimentally were built with points and by straight lines were based on equation ( 2).It can be seen from figure 2, that the growth rate of the sediment layer calculated by the equation ( 2) agrees well enough with the data obtained in the process of research, the error does not exceed 15%.The peculiarity of the foam polystyrene filters operation with the growing sediment layer is a gradual reduction of iron concentration in the water entering the polystyrene foam medium.We propose the concentration of iron in the filtrate [F e f ] (mg/L) to determine by the equation where L (m) is the thickness of filtration medium layer; K 2 (1/h) is a coefficient considering the impact of catalytic properties of the filtration medium to the efficiency of impurities detention from water.Coefficient value K 2 is obtained experimentally and is 15-20 1/h.The concentration of iron in the filtrate during the iron removal from water at polystyrene foam filters with the growing layer of suspended sediment is determined by substituting equations ( 1) and (2) into equation (3).
The increased sub-filter space is the feature of the foam polystyrene filter with a growing layer of deposit.The height of the sub-filter space H u (m) to ensure the required initial height of the growing layer of suspended sediment after washout, is The height of sub-filter space increases with increasing duration and the washout intensity.The washout intensity and duration of polystyrene foam depends on its height and granulometric composition.At the same time, a longer duration and intensity of washout is necessary with an increase in the height and diameter of the grains of the medium.Therefore, it is necessary to accept fine-grained medium to reduce the height of the sub-filter space.
Table 1 shows the results of four gravity filters with polystyrene foam medium obtained in production conditions.The total capacity of the filters was up to 1000 m 3 /day.The filtration was conducted by upflow through the polystyrene foam medium with the equivalent grain diameter of 2.8 mm.The filtration rate was accepted as 7 m/h and the medium layer height was 1.0 m based on the results of laboratory tests.The washout of polystyrene medium was carried out by treated water from the combined above-filter space of filters by opening the gate valve on the washout pipeline.Filters are taken for washout alternately once a day with a calculated washout intensity of 18 L/(s•m 2 ) and a duration of 3 min.The semi-annual studies showed satisfactory quality of the filtrate by the concentration of total iron, which did not exceed 0.1 mg/L.
The results of long-term laboratory and industrial studies on the implemented water treatment plants with gravity polystyrene foam filters showed their efficiency, reliability and ease of service [6,20].This experience allowed us to recommend the placement of the iron removal equipment in the existing steel water towers and the tower of water iron removal plants for small consumers (rural water supply systems) (figure 3).It allows to avoid the construction of filtering plants and clean water reservoirs, the construction of network pumps and filters body, which significantly reduces building costs in comparison with the previous scheme.We have proposed and implemented a combined medium consisting of small granules of polystyrene foamed with water and large granules foamed with steam to ensure the required efficiency of the iron removal from water and to reduce the cost of the medium.Tower iron removal plants operate in the discrete mode depending on the volume of the tower tank and the mode of water consumption.
Granular filters, including polystyrene foam filters, need periodic regeneration of the medium layer (washout).Reducing the volume of washout reduces the harm to the environment, reduces the size of plants for the water treatment, energy costs for its pumping and the cost of water.The water from the above-filter space is used for the polystyrene foam filters washout.Therefore, the reduction of washout volume will also reduce the total height of the filter.We have tested the impulse washout of polystyrene foam medium, which allows to save 15-40% of washout water in comparison with the continuous water washout.

Conclusions
Foam polystyrene filters can be used in schemes for iron removal from water.At a low concentration of iron ions the polystyrene foam filter is used in the source water without a contact mass at the bottom of the filter, and at a significant concentration of iron ions is used with a growing layer of deposit and the polystyrene foam medium.The implementation of contact water iron removal schemes with foam polystyrene filters with upflow filtration allows to reduce building and operation costs during reconstruction of existing and building new water iron removal plants.In addition, the use of tower water iron removal plants in existing rural water supply systems significantly reduces the assembling time.Ease of maintenance of polystyrene filters with upflow filtration makes them competitive in the absence of highly qualified service personnel.It is reasonable to apply mathematical modelling as one of the directions of future studies for substantiation of rational constructive and technological parameters of foam polystyrene filters operation and service life of the medium.

Figure 1 .
Figure1.Iron removal from water at the gravity polystyrene foam filters with upward filtration: 1 -the groundwater supply pipeline; 2 -the aerator; 3 -the air separator; 4 -the filter body; 5the polystyrene foam medium of industrial production; 6 -the bottom drainage and distribution system; 7 -the holding grid; 8 -the above-filter volume of water; 9 -the pipeline for filtrate removal; 10 -the washout pipeline.

Figure 3 .
Figure 3. Reconstruction of metal water tower using the polystyrene foam filter: a) the general view of the tower-column; b) the holding grid, pipelines: the inlet water supply (left), the aerated water (centre), the filtrate intake (right); c) the air separator (left) and the inlet water supply pipeline (right).
.06 is the transfer coefficient; i (L/(s•m 2 )) is the washout intensity; t b (min) is the washout time; h 0 (m) is the initial height of the suspended sediment layer which depends on the quality of initial water and the upflow rate.

Table 1 .
Results of the iron removal from water.