HORIZONTAL FILTER – CONCEPTS AND APPLICATIONS
A new application introduced in Maharashtra Jeevan Pradhikaran for the upgradation of the existing facilities. The first such experiment was done at Amaravati at Wadali treatment plant. This was created in 1955 but it was not in use since 1960. During scarcity period of 1984-87 Amaravati city required water from all possible sources. A complete renovation was taken up for Wadali plant. For reducing turbidity of raw water from Wadali tank this type of filter was externally introduced. It was found to be successful. This was appreciated by the then Chief Engineer. After this attempt lot of modification were done in the field application based on the actual experience. Opportunity was available to work in the drought prone areas of Amaravati ,Yavatmal and Satara districts. In 1997, while on training in IRC, The Netherlands these modifications could be discussed with the team of experts working on the same subject. All applications were much upheld by the team. To cover all these concepts and also sharing the practical experiences for the Horizontal Filters a paper was presented in a National seminar on New Technology in Water Supply convened by CPHEEO.
1. GENERAL
When Water flows through a filter media, its turbidity is reduced. Filer media naturally available is surface of earth. Rain Water percolates into the soil and then flows from higher elevation to lower elevation. At some point it may come out in the form of spring. This water is very clear in terms of turbidity. During the flow it has a straining action. Filtering area is enormous. Hence, rate of filtration is very low. The action of filtration is also very slow. But the flow is established naturally due to potential difference.
2. MAIN TYPES OF FILTERS
In case of artificial filters there are mainly two types. When water flows from top to bottom through a filter bed, it is a vertical filter. Straining action is through the filter media provided. Rate of filtration, which may be predetermined, goes on diminishing as the filter run is increased and re-stabilization of the filter bed is necessary. Rate of filtration is inversely proportional to the loss of head through filters.
The second type of filter is naturally available or horizontal filter. It can also be created artificially. Water is allowed to enter horizontally through the artificially filled media. In this case also straining takes place. During the flow loss of head is also created. When the loss of head exists straining is effective and filtrate water quality is improved. In this case Velocity of water is very less. It is proportional to loss of head. The rate of filtration is also very less. It is inversely proportional to loss of head.
3. BASIC CONCEPTS OF HORIZONTAL FILTERS
AB and CD are perforated partitions and the media is held in ABCD Boundaries, then water will find its way through the media by adopting any circuitous path. Naturally, the resistance is created by the media. The resistance depends upon the fineness of media; its travel length, and voids in the media. Due to resistance in the media, there is some loss of head. Flow exists through the media. Turbidity existing in the inflow water has to pass through media. It has to cross the barriers in the random way. In the process water being more generous to get through the media, discrete turbidity particles lag behind and in the process gets settled on the surface of the media. This process attracts more turbidity particles and water moves faster rubbing the surfaces and a sort of turbid layer is formed on the media surface. This builds up more resistance of the same media. Process of adsorption is also effective as the time passes on. If the water level of the incoming water is fairly constant, the drawl of water is continuous, unless on the surface CD full resistance has not been developed, flow, is varying according to the effective resistance of the media. Process can be made effective, if the rate of filtration is kept minimum. Recommended rate of filtration is 2.5 to 3 m3/m2/hr. When the filter gets choked, the flow stops, if there is a space above media, parallel to ‘AC’ the water does not flow through filter, but short circuiting is observed and it runs over the media and in that case unfiltered water is observed at the outlet.
4. ARRANGEMENT OF HORIZONTAL FILTERS
Use of Artificial horizontal filters can be done effectively, with the basic concepts of horizontal filters are known. Keeping rate of filtration to the limits 2-5 to 3m3/m2/hr. media boxes can be constructed. Size of media, at least in the first box, shall be more than 25mm for obvious reasons. The flow should be established through the media. The important consideration is, what should be the of optimum length. In case of artificial filters, there are limitations of depth of maximum 1.5 m. In that case, the size of box will have limitations of 1.5x1.5x1.5 m. It gives sectional area of maximum 2.25 sq. m. From this sectional area maximum possible flow desirable is 2.25 m2 3m3/m2/hr i.e. 6.75 m/hr. or 6750 mm/hr. For having minimum flow time through filters, the length of the filter may be equivalent to 15 min. Thus, it will be 6750 mm/4=1687 mm limited to 1.5m. This will ensure slow flow through the media. In the first chamber, however, the flow will be faster initially as only coarser media is placed in the chamber. If total travel time is prolonged, still better results are expected. Further, if the adjoining chamber is filled with media of size 25 to 12 mm, then flow rate is restricted and more head loss is expected. Size of this chamber is also kept as 1.5x1.5x1.5 m. If both these chambers are followed by another box of 1.5m x 1.5m x 1.5 m. size and filled with media of size 12 mm to 6 mm, then flow rates are further restricted. Actually ultimately the Governing rate of filtration is the result of flow through period in the finest media. In the initial chambers flow is obstructed and it gives more settling time to U/S water as compared to that in the finest media. Total length of filters is about 4.5 m and water has to flow from coarser media to finer media.
Connected horizontal boxes –
In this case maximum resistance is in the third box. It governs the detention time in the boxes as a whole. First & second chamber losses are less as compared with the third. As retention time in the first & second chamber is more due to higher resistance in the third chamber, a slow settling action also starts. Progressively the flow of water in these boxes is stabilized and comparatively clear water comes out through the third box.
As regards the percentage volume of voids in the three equal boxes, there is no major change. But due to use of finer media the surface area of media is increased. As the depth of the box is 1.5 m, up to which media is filled. The maximum expected head loss is limited to 1.5 m. Hence, if the driving head is insufficient for water to come out of these boxes, naturally there is heading up of water. If heading up of water is more, the driving force is more and hence flow is more. For this heading up of water in the upstream water containers, channels, boxes should have space above, otherwise the water will prefer to overflow than to flow through the strained path. This can be controlled by providing larger area or adding no. of boxes in parallel.
5. MODIFIED ARRANGEMENT OF BOXES
Sequential straining through finer medias in series are definitely giving good results but the frequency of choking the boxes and cleaning thereafter is much more. It is required to watch the process closely, otherwise there are chances that water tring to overflow or flow on top of filters, rather than through the filters. To avoid this, following modifications were thought of and carried out in field.
Coarser and finer media, instead of grading sequentially in the series, in boxes, to follow, they are arranged in one box and such boxes are kept in series with a hydraulic separator in between in the consecutive boxes. Total length of media (effective) is kept constant i.e. 4.5 m.
If the discharge is up to 5 m3/m2/hr. the results in above mentioned horizontal filters are very encouraging. The step by step action in this filter is as following,
a) Incoming flow has lesser resistance through coarser strip. It increases in the medium graded media strip and has the maximum straining action through the finer media. As the resistance is built up upstream box level rises; and being initial box, it is at its maximum level.
b) The stained flow eases out while coming out of the box. It reaches to the blank box and due to drop in gradient, levels at the bottom. It tries to find thorough access in the media box. Due to relative resistance, it tries to rise up in the blank box, till it matches with head loss in the initial box below the original level of water before entering the initial box. After reaching this level, water has no option to find access through the next media box.
c) The process continues in the media box and the blank box in series.
d) It is normally found that in the fourth blank box the resulting water level is substantially below the original level.
6. OBSERVATIONS
The following observations are seen in the field.
a) When flow is stabilized, total incoming flow is equal to total outgoing flow though whole filter.
b) The quality of water is sequentially improved in the blank boxes.
c) There is continuous drop in the water levels in the blank boxes in sequence.
d) Improved quality is proportional to the drop observed.
e) Choking of filter starts in the initial box. It steadily drops the level in blank box.
f) Choking is governed by the incoming turbidity of water. For normal surface water turbidity is up to 50 ppm. The filter box gets choked in a specific time. It is required to take out media, clean with water and refill in the same fashion.
g) Frequency of choking of filters down stream is delayed sequentially.
h) Manual washing of filters can be taken up after specific time for the initial box and one box downstream in rotation.
7. WELL IN WELL TYPE ARRANGEMENT
For rural Water Supply, if the above type of filters are followed after due pre-settling, the results are much better as the incoming turbidity is much within the limits. Typical arrangement of box type of filters, used in rural water supply schemes is shown as below,
According to requirement the sizes of sump can be chosen. However, the above mentioned sizes are more workable. No. of boxes should be minimum 5 No. filled with media. More No. may be suitable, but they do not add to performance. Media boxes are separated by providing openings in the separator walls. Opening area should be minimum 35% of the total cross section surface. Openings should be evenly distributed.
There should be no openings in the top 20 cm depth. From the last box, the water is allowed to enter into sump.
As there is storage in the sump, the water level rises & this in turn checks the relative head loss through the filter boxes. Pumping should be simultaneous. Then the hydraulics is easily be balanced.
In the case of gravity systems, the upstream flow can be controlled but in case of pumped systems the upstream flow should match with the pumping from sump.
8. ARRANGEMENT FOR RIVER INTAKES HAVING LESSER SURFACE FLOW IN SUMMER
When the supply wells are situated on the banks of rivers, normally, the yield in fair seasons is sufficient to cope up with the demands of village. But in the summer, the yield drops down suddenly and the same pump can not work continuously. It results in the shortage of water in comparison with the grown up water demands in the summer period. Hence, the reliability of water supply scheme comes down and users are dissatisfied.
In such case, appropriate size horizontal filters can come to help. These filters can tap surface/subsurface flow from the river and connect it to the supply well. During its horizontal travel the water is filtered through the media. As these filters are hydraulically connected to the supply well; the rate of filtration is equal to the maximum of rate of pumping. However, in the normal working the quantity of water through filters is only to the extent of difference between rate of pumping and the yield of well.
In this slow filtration process, choking of media in the first box is possible. However, cleaning period can be much longer. This can be equivalent time of the peak summer, when the use of filter is really necessary.
With the commencement of rainy season these filters can be isolated by sluice valve. This also avoids turbid water getting into the supply well. During summer, however, the turbidity is well within the range of requirements of operation of these filters.
Many supply wells are rejuvenated with the introduction of these filters and round the year reliability of water supply is assured.
Though, theoretically only 3 media boxes are necessary to give the required output, practical considerations have shown that 5/6 media boxes give better results. (Total 12 to 14 boxes, including hydraulic separators), However, the media size can be increased. to 40 to 50 mm metal can be used as coarser media and 12 to 20 mm can be used as finer media. These sizes can work up to 15 m3/m2/hr. rate of pumping.
Disinfection by use of chlorine or bleaching powder is however, must be associated with this system; to ensure quality of water.
These filters can be cleaned manually or by using water jets. As the boxes are closed from top by pre-cast strips of slabs, they are well protected in rainy season and in floods. As the boxes are more or less flush to the natural bed of the river, they are not disturbed during floods.
Approximate construction cost of this arrangement of filter is Rs. 70,000/- excluding the cost of connecting pipe as its length is variable as per the site condition.
The construction can be done in a period of 30 days when the river flows are very less in the summer.
9. COMBINATION OF DWARF BARRIERS ACROS RIVERS AND USE OF HORIZONTAL ROUGHENING FILTERS FOR SURFACE INTAKES-
As shown above, the method used for Rural Water Supply wells connected to surface/subsurface waters of Rivers by horizontal filters, it can be applied to small Urban Water Supply schemes. Main difference in application is, in case of Rural Water Supply, horizontal filter is required to act as a on line treatment unit of filtration, where as the Urban Water Supplies are having a separate water treatment unit. Hence, the contribution by horizontal filters, in case of Urban Water Supply is to act as a pre-treatment unit.
In the summer, the flows in rivers are very less. It becomes difficult to get sufficient water through intakes. This is still more difficult, if it is intended to construct a horizontal filter unit connecting river and jack well. For this purpose and to create a small balancing storage to cope up with summer demands, a dwarf barrier of about 1.2 to 1.5 m height can be easily created across the river. This will not only create the storage but increase the intake water level. It will further facilitate assured intake through Horizontal filter. In the equilibrium stage, level in the jack well and the storage in dwarf barrier will be the same. When the pumping is started, it will create flow through filters at the equivalent rate of pumping. In the process there will be straining action in the filters and head loss will occur along the flow path through filters. It will arrest the turbidity in the incoming water, and comparatively clearer water will reach jack well. In the process initial boxes of horizontal filter will choke. They are required to be cleaned periodically. Water jets can be used for cleaning by connecting pumping systems from jack well. If required, a small drain from blank box can be provided.
Advantages claimed in this process are as following.
1. Jack well silting time is prolonged. It remains comparatively cleaner.
2. Pumping machinery, impellers ensure longer life, as wear & tear with sand, silt and turbidity is comparatively less.
3. Turbidity load is not transferred to Treatment plant and hence the Treatment process can be simpler. Filter runs are increased.
4. The turbidity particles which are carried up to Treatment plant are finer and due to rubbing action the nature of turbidity is changed and settling is easier.
5. On line horizontal filter replaces connecting pipe. Hence, additional cost involved is much less.
6. Periodical cleaning by water jets or manually is possible and chocked turbidity/silt can be washed out and excluded from entering into the main system.
7. When turbidity loads are reduced, the economy is achieved by less use of chemicals in the Treatment Plant.
Pumping rates in semi urban or small municipal council water supply schemes are comparatively larger than those in Rural Water Supply Schemes. Hence, the rate of filtration through horizontal filters is larger. The filters with finer media will not cope up with these flows, even if battery of filters is used. Hence, only coarse media of 50-40 mm is used in such type of filters. No. of boxes can suitably be increased. Filter rates up to, 100 m3/m2/hr. can be used with this media size. The effect of pre-filtration unit is, however, very encouraging. For better control in cleaning operations, sluice valves can be fixed at the starting and at the end of filters. As the filter boxes are closed from Top, by RCC Strips of slab, the media does not get disturbed even in the floods. However, care is required to be taken, when the flood recedes. Hence, the appropriate pitching of the area is required to be done.
10. HORIZONTAL FILTERS AHEAD OF SEDIMENTION PROCESS
In some of the Rural water supply schemes regular filtration process is avoided and only settled water is supplied. As the process of filtration requires maintenance of a skilled person and the same is not available in the remote rural areas, the process is not provided. In such cases the horizontal filters can supplement further treatment to sedimentation. Instead of constructing separate filter unit, use was made to upgrade the system by constructing horizontal filters along the existing masonry benches. The hydraulics from settling tank to P.W. sump is not disturbed, only the water travel was changed from closed conduit to through Horizontal filters to give the desired results.
11. WORKING EFFICIENCIES
In all the applications shown above it is necessary to emphasis that Horizontal filters can work efficiently, but they are required to be maintained properly. The maintenance is not nullified, nor it is pleaded that the system is totally maintenance – free. However, it can be said that the maintenance required is tangible and understandable to the supervisory staff. Highly skilled operation or any specific trained operators are not required. The process of maintenance is simpler and found to be accepted by the villagers.
Cleaning period is required to be watched and decided in each case separately. It can range from monthly cleaning of boxes for the horizontal filters used for Rural Water Supplies where no other treatment is included in the process. This cleaning time can be bimonthly in case of Roughening filters for river intakes. However, in any case when the incoming turbidity is more, in rainy season, the cleaning interval is closer. In some cases the horizontal filters are not used in rainy season, as the supply wells yield sufficient water in rainy season.
Regarding the bacteriological efficiencies, proper evaluation is not yet done, but experiments in other countries reveal that with filtration rates up to 4 m3/m2/hr,. the bacteriological efficiency is up to 75 to 80%. However, for all practical purposes, if disinfection is followed by the horizontal filter units, it will take care of removal of remaining bacteriological loads.
For small urban communities and Rural areas maintenance can safely be relied upon with the use of horizontal filters. Even if, there are no set standards of maintenance, the choking of filters is immediately reflected in Overflowing of the units as well as carrying of turbid particles in system, and restoring action becomes obligatory.
12. REVIEW OF WORKS IN THE COUNTRY AND ABROAD
All India Institute of Hygiene and Public Health, Calcutta, has carried out extensive studies on horizontal filters and further finer Treatment by slow sand filters. Turbidity removal is very effective in horizontal filters. Rated treatment capacities are reported up to 2m3/hr/m2 with 50 to 75 ppm Raw Water turbidity. The recorded cleaning period is 140 to 160 days. No chemicals were used and the turbidity removal was to the extent 80 to 85%. Coliform removal is also reported up to 90% minimum. Though HRF is not complete treatment, it is definitely a most appropriate pre-treatment in semi-urban areas and appropriates treatment in Rural areas.
International Reference centre reports that Mr. M.Wegelin has prepared Horizontal flow roughing filtration (HRF), manual for Design, construction and operation. (IRCWD Report No. 06/86). Lot of research in Laboratory and field has been done. Evaluation results have been reported from the works done in Colombia, and Research centre in CINARA. Horizontal roughing filters, is technically known as multistage filtration. They are slightly modified to Dy. RF, known as Dynamic Roughening filters. These are specially taken up for research by International Institute for Infrastructures, Hydraulic and Environmental Engineering in Delft and the University of Valle Cali, Columbia. Dy. RF is especially useful in dealing with variant turbidities and has proved to be very successful pre-treatment. They have reported that about 2.20 person –hrs are required for Manual cleaning, washing, backfilling per Sq.m of Dy. RF. Feasible filtration rates may be up to 7 m3/m2/hr but efficiency of filtration is satisfactory up to 4 m3/m2/hr. Turbidity removals up to 66%, suspended solids removal upto 93% and faecal coliform removal up to 95% have been reported.
Recently research is being done on URFS i.e. roughing filters in series. Manual cleaning is a sort of thing which is not so appropriate in the Treatment process. If not avoided, it can be minimized by the introduction of URFS. The silt storage capacity of URFS is limited and therefore the units are required to be cleaned regularly by draining the units. In URF the most of the sludge remains at the bottom & can be easily drained out. The wash Water required is also comparatively less. It is found that the URFS is more efficient system to handle the treatment process in the Rural Areas. During the initial stages of Research alum was being used in very low dosages, say up to 1 ppm, but with horizontal flow roughing filters or Upward flow roughing filters in series alum dosing ins not insisted.
13. CONCLUSIONS
Horizontal filters as applied in Yavatmal and Amravati Districts of Maharashtra are required to be extensively used wherever suitable. It has variety of applications. The construction is simple and no specific requirement of special material is necessary. The operation & maintenance can be done by unskilled labour and the whole process is economical. Particularly, in river as a source the combination of dwarf storage barriers and Horizontal filters works very satisfactory.
Horizontal filters used as a pre-filter unit is also useful in semi-urban areas. The design for sizes, use of construction materials, choice of filter media, cleaning time backfilling procedure and everything regarding horizontal filter is very simple to understand, and anyone who has once used these filters, have a tendency to apply the same in one form or the other, in the next opportunity he will get for similar execution of works. He can easily decide all the parameters of application.
It is true, that the thorough evaluation of the working of horizontal filters, already constructed have not been done systematically up to now. All technical parameters are not checked and the performance is certified. This work is required to be taken separately by the organization or any educational institute. However, as it has helped to solve the problems of source augmentation, surface water treatment, pretreatment, the process available for application has been put forth. Constitution of HRFS and its arrangement has been evolved by actual field experience, by working out better alternatives. Providing a blank box in the consecutive media boxes works out to be more useful and has been tried successfully. This has not been referred to in the Literature and reported practices in this area.
14. ACKNOWLEDGEMENTS
I am grateful to Shri. R.M. Sagane, Member Secretary (Tech), Maharashtra Jeevan Pradhikaran for having personally inspired for taking up such works in Yavatmal District. The above said compilation of field experience is also due to him. I am also thankful to my co-workers in Yavatmal and Amravati District, for having sincerely done their job in the various applications of Horizontal filters.
A new application introduced in Maharashtra Jeevan Pradhikaran for the upgradation of the existing facilities. The first such experiment was done at Amaravati at Wadali treatment plant. This was created in 1955 but it was not in use since 1960. During scarcity period of 1984-87 Amaravati city required water from all possible sources. A complete renovation was taken up for Wadali plant. For reducing turbidity of raw water from Wadali tank this type of filter was externally introduced. It was found to be successful. This was appreciated by the then Chief Engineer. After this attempt lot of modification were done in the field application based on the actual experience. Opportunity was available to work in the drought prone areas of Amaravati ,Yavatmal and Satara districts. In 1997, while on training in IRC, The Netherlands these modifications could be discussed with the team of experts working on the same subject. All applications were much upheld by the team. To cover all these concepts and also sharing the practical experiences for the Horizontal Filters a paper was presented in a National seminar on New Technology in Water Supply convened by CPHEEO.
1. GENERAL
When Water flows through a filter media, its turbidity is reduced. Filer media naturally available is surface of earth. Rain Water percolates into the soil and then flows from higher elevation to lower elevation. At some point it may come out in the form of spring. This water is very clear in terms of turbidity. During the flow it has a straining action. Filtering area is enormous. Hence, rate of filtration is very low. The action of filtration is also very slow. But the flow is established naturally due to potential difference.
2. MAIN TYPES OF FILTERS
In case of artificial filters there are mainly two types. When water flows from top to bottom through a filter bed, it is a vertical filter. Straining action is through the filter media provided. Rate of filtration, which may be predetermined, goes on diminishing as the filter run is increased and re-stabilization of the filter bed is necessary. Rate of filtration is inversely proportional to the loss of head through filters.
The second type of filter is naturally available or horizontal filter. It can also be created artificially. Water is allowed to enter horizontally through the artificially filled media. In this case also straining takes place. During the flow loss of head is also created. When the loss of head exists straining is effective and filtrate water quality is improved. In this case Velocity of water is very less. It is proportional to loss of head. The rate of filtration is also very less. It is inversely proportional to loss of head.
3. BASIC CONCEPTS OF HORIZONTAL FILTERS
AB and CD are perforated partitions and the media is held in ABCD Boundaries, then water will find its way through the media by adopting any circuitous path. Naturally, the resistance is created by the media. The resistance depends upon the fineness of media; its travel length, and voids in the media. Due to resistance in the media, there is some loss of head. Flow exists through the media. Turbidity existing in the inflow water has to pass through media. It has to cross the barriers in the random way. In the process water being more generous to get through the media, discrete turbidity particles lag behind and in the process gets settled on the surface of the media. This process attracts more turbidity particles and water moves faster rubbing the surfaces and a sort of turbid layer is formed on the media surface. This builds up more resistance of the same media. Process of adsorption is also effective as the time passes on. If the water level of the incoming water is fairly constant, the drawl of water is continuous, unless on the surface CD full resistance has not been developed, flow, is varying according to the effective resistance of the media. Process can be made effective, if the rate of filtration is kept minimum. Recommended rate of filtration is 2.5 to 3 m3/m2/hr. When the filter gets choked, the flow stops, if there is a space above media, parallel to ‘AC’ the water does not flow through filter, but short circuiting is observed and it runs over the media and in that case unfiltered water is observed at the outlet.
4. ARRANGEMENT OF HORIZONTAL FILTERS
Use of Artificial horizontal filters can be done effectively, with the basic concepts of horizontal filters are known. Keeping rate of filtration to the limits 2-5 to 3m3/m2/hr. media boxes can be constructed. Size of media, at least in the first box, shall be more than 25mm for obvious reasons. The flow should be established through the media. The important consideration is, what should be the of optimum length. In case of artificial filters, there are limitations of depth of maximum 1.5 m. In that case, the size of box will have limitations of 1.5x1.5x1.5 m. It gives sectional area of maximum 2.25 sq. m. From this sectional area maximum possible flow desirable is 2.25 m2 3m3/m2/hr i.e. 6.75 m/hr. or 6750 mm/hr. For having minimum flow time through filters, the length of the filter may be equivalent to 15 min. Thus, it will be 6750 mm/4=1687 mm limited to 1.5m. This will ensure slow flow through the media. In the first chamber, however, the flow will be faster initially as only coarser media is placed in the chamber. If total travel time is prolonged, still better results are expected. Further, if the adjoining chamber is filled with media of size 25 to 12 mm, then flow rate is restricted and more head loss is expected. Size of this chamber is also kept as 1.5x1.5x1.5 m. If both these chambers are followed by another box of 1.5m x 1.5m x 1.5 m. size and filled with media of size 12 mm to 6 mm, then flow rates are further restricted. Actually ultimately the Governing rate of filtration is the result of flow through period in the finest media. In the initial chambers flow is obstructed and it gives more settling time to U/S water as compared to that in the finest media. Total length of filters is about 4.5 m and water has to flow from coarser media to finer media.
Connected horizontal boxes –
In this case maximum resistance is in the third box. It governs the detention time in the boxes as a whole. First & second chamber losses are less as compared with the third. As retention time in the first & second chamber is more due to higher resistance in the third chamber, a slow settling action also starts. Progressively the flow of water in these boxes is stabilized and comparatively clear water comes out through the third box.
As regards the percentage volume of voids in the three equal boxes, there is no major change. But due to use of finer media the surface area of media is increased. As the depth of the box is 1.5 m, up to which media is filled. The maximum expected head loss is limited to 1.5 m. Hence, if the driving head is insufficient for water to come out of these boxes, naturally there is heading up of water. If heading up of water is more, the driving force is more and hence flow is more. For this heading up of water in the upstream water containers, channels, boxes should have space above, otherwise the water will prefer to overflow than to flow through the strained path. This can be controlled by providing larger area or adding no. of boxes in parallel.
5. MODIFIED ARRANGEMENT OF BOXES
Sequential straining through finer medias in series are definitely giving good results but the frequency of choking the boxes and cleaning thereafter is much more. It is required to watch the process closely, otherwise there are chances that water tring to overflow or flow on top of filters, rather than through the filters. To avoid this, following modifications were thought of and carried out in field.
Coarser and finer media, instead of grading sequentially in the series, in boxes, to follow, they are arranged in one box and such boxes are kept in series with a hydraulic separator in between in the consecutive boxes. Total length of media (effective) is kept constant i.e. 4.5 m.
If the discharge is up to 5 m3/m2/hr. the results in above mentioned horizontal filters are very encouraging. The step by step action in this filter is as following,
a) Incoming flow has lesser resistance through coarser strip. It increases in the medium graded media strip and has the maximum straining action through the finer media. As the resistance is built up upstream box level rises; and being initial box, it is at its maximum level.
b) The stained flow eases out while coming out of the box. It reaches to the blank box and due to drop in gradient, levels at the bottom. It tries to find thorough access in the media box. Due to relative resistance, it tries to rise up in the blank box, till it matches with head loss in the initial box below the original level of water before entering the initial box. After reaching this level, water has no option to find access through the next media box.
c) The process continues in the media box and the blank box in series.
d) It is normally found that in the fourth blank box the resulting water level is substantially below the original level.
6. OBSERVATIONS
The following observations are seen in the field.
a) When flow is stabilized, total incoming flow is equal to total outgoing flow though whole filter.
b) The quality of water is sequentially improved in the blank boxes.
c) There is continuous drop in the water levels in the blank boxes in sequence.
d) Improved quality is proportional to the drop observed.
e) Choking of filter starts in the initial box. It steadily drops the level in blank box.
f) Choking is governed by the incoming turbidity of water. For normal surface water turbidity is up to 50 ppm. The filter box gets choked in a specific time. It is required to take out media, clean with water and refill in the same fashion.
g) Frequency of choking of filters down stream is delayed sequentially.
h) Manual washing of filters can be taken up after specific time for the initial box and one box downstream in rotation.
7. WELL IN WELL TYPE ARRANGEMENT
For rural Water Supply, if the above type of filters are followed after due pre-settling, the results are much better as the incoming turbidity is much within the limits. Typical arrangement of box type of filters, used in rural water supply schemes is shown as below,
According to requirement the sizes of sump can be chosen. However, the above mentioned sizes are more workable. No. of boxes should be minimum 5 No. filled with media. More No. may be suitable, but they do not add to performance. Media boxes are separated by providing openings in the separator walls. Opening area should be minimum 35% of the total cross section surface. Openings should be evenly distributed.
There should be no openings in the top 20 cm depth. From the last box, the water is allowed to enter into sump.
As there is storage in the sump, the water level rises & this in turn checks the relative head loss through the filter boxes. Pumping should be simultaneous. Then the hydraulics is easily be balanced.
In the case of gravity systems, the upstream flow can be controlled but in case of pumped systems the upstream flow should match with the pumping from sump.
8. ARRANGEMENT FOR RIVER INTAKES HAVING LESSER SURFACE FLOW IN SUMMER
When the supply wells are situated on the banks of rivers, normally, the yield in fair seasons is sufficient to cope up with the demands of village. But in the summer, the yield drops down suddenly and the same pump can not work continuously. It results in the shortage of water in comparison with the grown up water demands in the summer period. Hence, the reliability of water supply scheme comes down and users are dissatisfied.
In such case, appropriate size horizontal filters can come to help. These filters can tap surface/subsurface flow from the river and connect it to the supply well. During its horizontal travel the water is filtered through the media. As these filters are hydraulically connected to the supply well; the rate of filtration is equal to the maximum of rate of pumping. However, in the normal working the quantity of water through filters is only to the extent of difference between rate of pumping and the yield of well.
In this slow filtration process, choking of media in the first box is possible. However, cleaning period can be much longer. This can be equivalent time of the peak summer, when the use of filter is really necessary.
With the commencement of rainy season these filters can be isolated by sluice valve. This also avoids turbid water getting into the supply well. During summer, however, the turbidity is well within the range of requirements of operation of these filters.
Many supply wells are rejuvenated with the introduction of these filters and round the year reliability of water supply is assured.
Though, theoretically only 3 media boxes are necessary to give the required output, practical considerations have shown that 5/6 media boxes give better results. (Total 12 to 14 boxes, including hydraulic separators), However, the media size can be increased. to 40 to 50 mm metal can be used as coarser media and 12 to 20 mm can be used as finer media. These sizes can work up to 15 m3/m2/hr. rate of pumping.
Disinfection by use of chlorine or bleaching powder is however, must be associated with this system; to ensure quality of water.
These filters can be cleaned manually or by using water jets. As the boxes are closed from top by pre-cast strips of slabs, they are well protected in rainy season and in floods. As the boxes are more or less flush to the natural bed of the river, they are not disturbed during floods.
Approximate construction cost of this arrangement of filter is Rs. 70,000/- excluding the cost of connecting pipe as its length is variable as per the site condition.
The construction can be done in a period of 30 days when the river flows are very less in the summer.
9. COMBINATION OF DWARF BARRIERS ACROS RIVERS AND USE OF HORIZONTAL ROUGHENING FILTERS FOR SURFACE INTAKES-
As shown above, the method used for Rural Water Supply wells connected to surface/subsurface waters of Rivers by horizontal filters, it can be applied to small Urban Water Supply schemes. Main difference in application is, in case of Rural Water Supply, horizontal filter is required to act as a on line treatment unit of filtration, where as the Urban Water Supplies are having a separate water treatment unit. Hence, the contribution by horizontal filters, in case of Urban Water Supply is to act as a pre-treatment unit.
In the summer, the flows in rivers are very less. It becomes difficult to get sufficient water through intakes. This is still more difficult, if it is intended to construct a horizontal filter unit connecting river and jack well. For this purpose and to create a small balancing storage to cope up with summer demands, a dwarf barrier of about 1.2 to 1.5 m height can be easily created across the river. This will not only create the storage but increase the intake water level. It will further facilitate assured intake through Horizontal filter. In the equilibrium stage, level in the jack well and the storage in dwarf barrier will be the same. When the pumping is started, it will create flow through filters at the equivalent rate of pumping. In the process there will be straining action in the filters and head loss will occur along the flow path through filters. It will arrest the turbidity in the incoming water, and comparatively clearer water will reach jack well. In the process initial boxes of horizontal filter will choke. They are required to be cleaned periodically. Water jets can be used for cleaning by connecting pumping systems from jack well. If required, a small drain from blank box can be provided.
Advantages claimed in this process are as following.
1. Jack well silting time is prolonged. It remains comparatively cleaner.
2. Pumping machinery, impellers ensure longer life, as wear & tear with sand, silt and turbidity is comparatively less.
3. Turbidity load is not transferred to Treatment plant and hence the Treatment process can be simpler. Filter runs are increased.
4. The turbidity particles which are carried up to Treatment plant are finer and due to rubbing action the nature of turbidity is changed and settling is easier.
5. On line horizontal filter replaces connecting pipe. Hence, additional cost involved is much less.
6. Periodical cleaning by water jets or manually is possible and chocked turbidity/silt can be washed out and excluded from entering into the main system.
7. When turbidity loads are reduced, the economy is achieved by less use of chemicals in the Treatment Plant.
Pumping rates in semi urban or small municipal council water supply schemes are comparatively larger than those in Rural Water Supply Schemes. Hence, the rate of filtration through horizontal filters is larger. The filters with finer media will not cope up with these flows, even if battery of filters is used. Hence, only coarse media of 50-40 mm is used in such type of filters. No. of boxes can suitably be increased. Filter rates up to, 100 m3/m2/hr. can be used with this media size. The effect of pre-filtration unit is, however, very encouraging. For better control in cleaning operations, sluice valves can be fixed at the starting and at the end of filters. As the filter boxes are closed from Top, by RCC Strips of slab, the media does not get disturbed even in the floods. However, care is required to be taken, when the flood recedes. Hence, the appropriate pitching of the area is required to be done.
10. HORIZONTAL FILTERS AHEAD OF SEDIMENTION PROCESS
In some of the Rural water supply schemes regular filtration process is avoided and only settled water is supplied. As the process of filtration requires maintenance of a skilled person and the same is not available in the remote rural areas, the process is not provided. In such cases the horizontal filters can supplement further treatment to sedimentation. Instead of constructing separate filter unit, use was made to upgrade the system by constructing horizontal filters along the existing masonry benches. The hydraulics from settling tank to P.W. sump is not disturbed, only the water travel was changed from closed conduit to through Horizontal filters to give the desired results.
11. WORKING EFFICIENCIES
In all the applications shown above it is necessary to emphasis that Horizontal filters can work efficiently, but they are required to be maintained properly. The maintenance is not nullified, nor it is pleaded that the system is totally maintenance – free. However, it can be said that the maintenance required is tangible and understandable to the supervisory staff. Highly skilled operation or any specific trained operators are not required. The process of maintenance is simpler and found to be accepted by the villagers.
Cleaning period is required to be watched and decided in each case separately. It can range from monthly cleaning of boxes for the horizontal filters used for Rural Water Supplies where no other treatment is included in the process. This cleaning time can be bimonthly in case of Roughening filters for river intakes. However, in any case when the incoming turbidity is more, in rainy season, the cleaning interval is closer. In some cases the horizontal filters are not used in rainy season, as the supply wells yield sufficient water in rainy season.
Regarding the bacteriological efficiencies, proper evaluation is not yet done, but experiments in other countries reveal that with filtration rates up to 4 m3/m2/hr,. the bacteriological efficiency is up to 75 to 80%. However, for all practical purposes, if disinfection is followed by the horizontal filter units, it will take care of removal of remaining bacteriological loads.
For small urban communities and Rural areas maintenance can safely be relied upon with the use of horizontal filters. Even if, there are no set standards of maintenance, the choking of filters is immediately reflected in Overflowing of the units as well as carrying of turbid particles in system, and restoring action becomes obligatory.
12. REVIEW OF WORKS IN THE COUNTRY AND ABROAD
All India Institute of Hygiene and Public Health, Calcutta, has carried out extensive studies on horizontal filters and further finer Treatment by slow sand filters. Turbidity removal is very effective in horizontal filters. Rated treatment capacities are reported up to 2m3/hr/m2 with 50 to 75 ppm Raw Water turbidity. The recorded cleaning period is 140 to 160 days. No chemicals were used and the turbidity removal was to the extent 80 to 85%. Coliform removal is also reported up to 90% minimum. Though HRF is not complete treatment, it is definitely a most appropriate pre-treatment in semi-urban areas and appropriates treatment in Rural areas.
International Reference centre reports that Mr. M.Wegelin has prepared Horizontal flow roughing filtration (HRF), manual for Design, construction and operation. (IRCWD Report No. 06/86). Lot of research in Laboratory and field has been done. Evaluation results have been reported from the works done in Colombia, and Research centre in CINARA. Horizontal roughing filters, is technically known as multistage filtration. They are slightly modified to Dy. RF, known as Dynamic Roughening filters. These are specially taken up for research by International Institute for Infrastructures, Hydraulic and Environmental Engineering in Delft and the University of Valle Cali, Columbia. Dy. RF is especially useful in dealing with variant turbidities and has proved to be very successful pre-treatment. They have reported that about 2.20 person –hrs are required for Manual cleaning, washing, backfilling per Sq.m of Dy. RF. Feasible filtration rates may be up to 7 m3/m2/hr but efficiency of filtration is satisfactory up to 4 m3/m2/hr. Turbidity removals up to 66%, suspended solids removal upto 93% and faecal coliform removal up to 95% have been reported.
Recently research is being done on URFS i.e. roughing filters in series. Manual cleaning is a sort of thing which is not so appropriate in the Treatment process. If not avoided, it can be minimized by the introduction of URFS. The silt storage capacity of URFS is limited and therefore the units are required to be cleaned regularly by draining the units. In URF the most of the sludge remains at the bottom & can be easily drained out. The wash Water required is also comparatively less. It is found that the URFS is more efficient system to handle the treatment process in the Rural Areas. During the initial stages of Research alum was being used in very low dosages, say up to 1 ppm, but with horizontal flow roughing filters or Upward flow roughing filters in series alum dosing ins not insisted.
13. CONCLUSIONS
Horizontal filters as applied in Yavatmal and Amravati Districts of Maharashtra are required to be extensively used wherever suitable. It has variety of applications. The construction is simple and no specific requirement of special material is necessary. The operation & maintenance can be done by unskilled labour and the whole process is economical. Particularly, in river as a source the combination of dwarf storage barriers and Horizontal filters works very satisfactory.
Horizontal filters used as a pre-filter unit is also useful in semi-urban areas. The design for sizes, use of construction materials, choice of filter media, cleaning time backfilling procedure and everything regarding horizontal filter is very simple to understand, and anyone who has once used these filters, have a tendency to apply the same in one form or the other, in the next opportunity he will get for similar execution of works. He can easily decide all the parameters of application.
It is true, that the thorough evaluation of the working of horizontal filters, already constructed have not been done systematically up to now. All technical parameters are not checked and the performance is certified. This work is required to be taken separately by the organization or any educational institute. However, as it has helped to solve the problems of source augmentation, surface water treatment, pretreatment, the process available for application has been put forth. Constitution of HRFS and its arrangement has been evolved by actual field experience, by working out better alternatives. Providing a blank box in the consecutive media boxes works out to be more useful and has been tried successfully. This has not been referred to in the Literature and reported practices in this area.
14. ACKNOWLEDGEMENTS
I am grateful to Shri. R.M. Sagane, Member Secretary (Tech), Maharashtra Jeevan Pradhikaran for having personally inspired for taking up such works in Yavatmal District. The above said compilation of field experience is also due to him. I am also thankful to my co-workers in Yavatmal and Amravati District, for having sincerely done their job in the various applications of Horizontal filters.
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