Design of Dual Media Filters to Suit Existing Water Treatment Plants in India

1. INTRODUCTION

Most of the water treatment plants in India are being overloaded in an attempt to cope up with the ever in­creasing water demand. The existing rapid sand filters form a bottleneck for such overloading with the result that part of the water has to be bypassed without filtration, or it finds its way through cracked filter beds, thereby escaping treatment. Poor pre-treatment and non-availability of skilled person­nel for operating the plant make the situation worse, resulting in large and unpredictable changes in treated water quality. A fact-finding survey by NEERI revealed deteriorated condi­tions of water treatment plants in India during 1972 and the conditions have remained practically the same till now. It is felt that conversion of existing rapid sand filters to high rate dual media filters may form the most practical and economical interim mea­sure that could be adopted to improve the conditions.

No doubt, the solution to the prob­lem lies in improvement in pre-treat­ment conditions and augmentation of plant capacities. But it must be ad­mitted that skilled personnel and equipment necessary for proper control of coagulation and availability of finan­cial resources for installation and maintenance of improved pre-treat­ment facilities, or construction of sup­plementary units are beyond the reach of most of the water works in India.

Does it mean that the present state of affairs should continue till these obsta­cles are overcome?

Fortunately, dual media filters have a capacity of functioning even under con­ditions of poor pre-treatment and in­creased flow. Experience in the West as well as in India confirm this. Another advantage in adoption of these filters is that existing filters can be converted to dual media filters with minor changes at a very low cost. The design of such filters is really a difficult task as no established norms can.be adopted due to limitations imposed by existing condi­tions. The present paper tries to evolve a rational approach for design of such converted filters.

Design of dual media filters for new installations has not been considered here as it would be based on entirely different aspects.

2. DESIGN CONSTRAINTS

Design of any filter is based on sus­pension characteristics, filter structure, operating parameters and expected filtrate quality. Considering these, the constraints imposed by prevailing con­ditions in the water treatment plants can be described as follows:

(i) Suspension Characteristics:

Due to uncontrolled alum dose, in­effective flocculation and short circuit­ing in settling tanks, the water ap­plied to the filters has large concentra­tion of turbidity, with significant per­centage of large settleable flocs along­with partially stabilized but unfloccu­- lated colloids. The observations at Kanpur Water Works indicated that the turbidity range is from 15-30 FTU, with about 40 per cent of large floc turbidity which invariably gets strained in the top filter layers.

This can be taken as a representative characteristic of applied water, as in most of the cases the situation is more or less identical. Moreover, along with rise in turbidity levels, there is a change in nature of turbidity also. Thus in rainy season the turbidity levels may shoot upto 100 FTU and during late winter and summer "difficult to floccu­late" algal turbidity may pose prob­lems.

(ii) Filter Structure:

Most of the filtration plants are pro­vided with rate controllers and have submerged inlet flow conditions, thus limiting the standing water depth over the filter: medium. The structure of the filter does not provide any allow­ance for increasing the depth of filter medium, which is restricted to 0.6 to 0.75 m. The limiting head loss avail­able for filtration seldom exceeds 2.0 m, thereby preventing longer filter runs

(iii) Operating Parameters:

The filters are operated by semi­skilled personnel and in some plants, even by coolies; as such, the control of filter operation is very poor. The head loss gauges, rate controllers and dis­charge measuring devices are out of order in most of the cases due to negli­gence. The backwashing is done on a routine time cycle, decided on the con­venience of shift duties. As a result the filter run is terminated only on the basis of time interval without consider­ing limiting head loss, turbidity of filtrate, etc. Any new design of the filter must take into consideration this lacuna in operation, which may not be altered unless skilled personnel are employed. Therefore the filter should be designed to give good filtrate quality and acceptable filtration rate during the entire filter run which should be a pre-determined time interval.

(iv) Expected filtrate quality:

The current design practice in deve­loped countries aims at filtrate turbi­dity to be consistently below 0.2 FTU. However, the filtrate turbidity at such a low level, though desirable, is neither economically feasible under present conditions, nor essential from the point of view of acceptable drinking water standards which permit turbidity up to 5 FTU. Considering the techno-econo­mical feasibility, the filtrate turbidity of 1 FTU may be taken for design.

3. SELECTION OF HIGH RATE FILTER

Of all the high rate filters available, dual, multimedia and mixed media filters can be adopted easily for con­verting old filters. The multimedia filters, though more suitable, may not be practically feasible due to difficulty in the availability and cost of procur­ing and placing different filter media. Mixed media filters advocated by Culp and Culp suffer from the same draw­backs. Dual media filters, though not truly coarse to fine graded, still pro­vide an effective and feasible alterna­tive.

4. DESIGN OF DUAL MEDIA FILTER

Filter Media:

Due to non,.availability of anthracite in India, various other materials like high grade bituminous coal (Param­sh:vam1 et al 1973, Ranade et al'" 1975), crushed coconut shell (Kardile8 1972), berry seeds (Bhole and Nashik­kar1 1974) and kernels of stone fruits like apricots (Ranade and Agrawal 1974) have been tried and were found to be suitable as coarse filter media. Considering the cost, availability and filtration properties, high grade bitu­minous coal seems to be a better choice. However when local conditions favour, use of indigenous filter media may be made as the coarse media. Recent attempts at using plastic granules as filter media may be explored further in. this respect. The lower layer would be of sand for all such filters(ii)

Size Gradation:

High percentage of large flocs in the influent to filter suggests the use of much coarser medium to prevent sur­face mat formation, whereas improper flocculation of some colloids necessiatates greater depth of coarse medium followed by fine medium, with a sharp interface. However, it is reported that use of fine sand produces intermixing. The use of high density fine medium like garnet immediately below the coarse medium may be tried, in order to achieve optimum design. Such com­bination, though it seems to be strange, may give better performance. How­

ever, from availability and cost points of view, use of garnet sand may not be feasible.

Opinions differ regarding the desir­able amount of intermixing at the junction of coarse and fine media. Culp and Culp recommend significant intermixing to achieve ideal "coarse to fine" gradation and claim better filtrate quality and longer filter runs. How­ever there are sufficient reasons to adopt a sharp interface for the filters designed to suit Indian conditions. The turbidity of applied water is of two distinct types and requires two-stage treatment consisting of fine media filterfollowing a coarse media filter. There is less control over the grading of filter media and if the gradation is selected to have significant intermixing there may remain large portion of fine mate­rial which may cause dense packing of intermixing layer, thus defeating the purpose of coarse to fine gradation. This suggests that some degree of in­ termixing is inevitable under present conditions but the design should aim at minimising it.

As regards selection of size grading of filter media, the norm of equal hydraulic settling may be followed as a guide line. But to ensure minimum intermixing, the 10% (wt basis) coarsest grains of top medium and 10% (wt basis) and  finest grains of fine medium (Effective size) should be considered as representative sizes, as they would determine the degree of intermixing. According to Brossman and Malina if the size ratio is in the range of 2 : 1 for coarse and fine media (in case of coal and sand), separation of layers would occur and the same norm can be adhered to in the design. If the densities of coarse and fine media are assumed to be 1.2 and 2.65 respectively as in case of coal and sand, the speci­fications may be stipulated as follows:

Coarse media:

Size range 0.85 to 1.6 mm

E.S. = 1.00 mm U.C. = 1.3 to 1.5

Fine media:

Size range 0.55 to 0.9 mm

E.S. = 0.60 mm U.C. = 1.3 to 1.5

Depth of Media:

Experiments conducted with various combinations of relative depths of indi­vidual media, under high rates of filtra­tion have conclusively proved that coarse media with a depth of 40 to 50 cm can effectively remove large floc turbidity without excessive head loss build-up and can flocculate unflocculat­ed particles. Turbidity reaching the lower sand layers is always less than 5 FTU, even under varying influent turbidity concentrations. The greater depth also prevents clogging of beds when algal suspensions are encounter­ed. Considering the permissible standards for filtrate turbidity, sand layer

of 15 cm thickness is more than enough to ensure filtrate turbidity of less than 1 FTU during the entire filter run.

Use of greater depth of coarse medium also helps in preventing surface mat formation at the interface. It may be argued that a sharp interface would favour such surface straining, but it should be pointed out that the turbidity reaching interface being low, the surface mat formation would take a much longer time as compared to that in a conventional rapid sand filter. The backwash interval for Dual media filters would be much less than for conventional filters, depending on turbidity storage capacity of coarse layer. An increase of head loss due to surface mat formation would not limit the filter run.

normal rates of filtration, becomes a controlling factor at higher rates as the head loss increase with the square of velocity. Fortunately a change in the number of openings on the existing laterals can solve the problem. How­ever, this must be done judiciously after studying the hydraulics of the system, otherwise backwashing may become non-uniform and ineffective after such modifications.

(v) Operation of the Filter :

variable conditions of turbidity and filter components, it is recommended that the filter should be designed to operate with declining rate range of 300 Ipm/sqm to 200 lpm/sqm with 12 hours of filter run. The design is on a much conservative side and would guarantee good filtrate quality. There may be objection to adoption of 12 hours' filter run as the present prac­tice is to have 24 hours filter run. But it should be remembered that the filtration capacity of the converted filter is more than double that of the original filter and a little increase in operation cost is fully justified as the percentage of backwash water require­ment is constant. Moreover this can be adopted without any special train­ing to operators. The backwash pro­cedure would remain unchanged for the converted filters(vi) Performance of the Filter:

Ample data are available on the per­formance of dual media filters, both on pilot scale and field scale, it has been established that these filters can operate at high rates of filtration with consistent high removal efficiency of turbidity and bacteria. Use of such filters in existing water treatment plants would improve filtrate quality and augment the capacity at a rela­tively low cost.

It must be pointed out here that con­version of filter is only an interim mea­sure and improvement in the pre-treat­ment conditions cannot be overlooked.

As the funds become available pre­treatment can be improved by:

1. Proper coagulant dose and effi­cient mixing arrangement.

2. Use of polyelectrolytes to streng­ then the floc.

3. Effective flocculation of all the colloids.

4. Improvement in settling tanks and augmentation of their capa­

cities by use of tube settlers.

5-. Employment of skilled personnel and special training facilities.

Once these changes come into effect the same dual media filters would give excellent filtrate turbidities with longer filter runs of 24 to 36 hours, as the

inlet water would then have turbidity

 of less than 5 F. T.U. and would be of

filterable nature.

(vii) Cost of Conversion :

The cost of the conversion includes the cost of modifications in influent and underdrainage system and cost of plac­ing new media. From the experience gained at Kanpur and Ichhalkaranji Water Works it can be said that this cost is around Rs. 6,000 per mld of plant capacity or Rs. 3,000 per mId of increased capacity If a conversion of a fair sized filtration plant is done the

cost can be further reduced5. CONCLUSION

Dual media filters offer the most appropriate and economical solution -to the present problem of overloaded and inefficient water treatment plants in India. The novelty of these filters 1s that they can be incorporated in the existing system without much change in plant structure or method of opera­tion. The technical know-how for such conversion is fully developed in India.

and the process of conversion of filters can start on a large scale, awaiting only the decision of the concerned authori­ties to solve such -urgent problems.

REFERENCES

1. "Survey of Water Treatment Plants", Technical Digest No.3, CPHERI, Nagpur, (January 1971).

2. Culp G. L. and Culp R. L., "New Con­cepts in Water Purification", Van Nos­trand Reinhold Company, New York, (1974).

3. Conley, W. R., "High Rate Filtration",

      J.A.W.W.A. 64, 3, p. 203, (1972).

4. Ranade S. V., "Engineering and Theoreti­cal Investigations on Dual Media Filters using Indian Bituminous Coals", Ph.D. Thesis, J.I.T., K;anpur, (July 1976).

5. Ranade S. V., Agrawal G. D. and Misra Y. D., "Conversion of Rapid Sand Filter into Dual Media Filter", J. Inst. Public Health Engrs., J., 2, p. 12, (1975).

6. "Manual on Water Supply and Treat­ment", Central Public Health and En­vironmental Engineering Organization, Government of India, (1977).

7. Paramsivan et al., "Bituminous Coal ­A substitute for Anthracite in two Layer Filtration of Water", Indian J. Env. Health, 15, p. 178, (1973).

8. Kardile, J. N. (1972) "Crushed Coconut Shell as a New Filter Media for Dual and Multilayer Filters", J.I.W.W.A. 1, 1, p. 28, (1972).

9. Bhole A. G. and Nashikkar J. T., "Berry Seed Shell as Filter Media", J. Inst. o! Engrs 54, PH 2, p. 45, (1974).

10. Ranade S. V. and Agrawal G. D., "Use of Vegetable Wastes as a Filter Media" presented at the Conference on Engg. Materials and Equipment, The Associa­tion of Engineers, Cal lcutta (1974).

11. Brossman, D. R. and Malina J. F. Jr., "Intermixing of Dual Media Filters and Effects on Performanl;e", Technical Re­port EHE 72-4 CRWR 86, Env. Engg. Lab., University of Texas, Austin, Texas, U.S.A., (1972).

13. Gadgil J. M., "Effect of Media Depths on Performance of Dual Media Filters", M.E. Thesis, Walchand College of Engg., Sangli, (1979).

14. Ranade S. V., Agrawal G. D. and Misra Y. D., "FulJ Scale Trials on Converted Dual Media Filter", J. I.W.W.A., (Octo­ber 1976).