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Research Paper & Presentation

Advancement & simplification in operation &
control of rapid gravity filter beds

Rapid gravity filtration process:

During the process of filtration, impurities in water are removed by passing it through sand media. They precipitate in the voids between grains of the filter bed, reducing the effective pore space and increasing the resistance against downward water movement. During the progress of a filter run (service cycle) this resistance (headloss or pressure drop) becomes so high, that the filter bed requires cleaning.

The rate of filtration is expressed in lit/sqm/hr or in terms of velocity as meter/hr. sudden fluctuations in the rate of filtration are likely to produce undesirable filtrate quality. It is also likely to disturb the delicate physio-chemical mechanisms which influence the separation in the granular media. Therefore, the filters need to be operated at a uniform loading rate & any increase or decrease in flow rate has to be gradual.

Gravity filter operation is essentially controlled by the following two modes.

Constant rate filtration:

A]	With control on effluent side or downstream

B]	With control on influent side or upstream.

Declining rate filtration

1. A] CONSTANT RATE FILTRATION WITH EFFLUENT CONTROL (BY MECHANICAL RATE CONTROLLERS):

Principle:

This system is one of the most popular and widely practiced in the operation of gravity filters in India. It ensures a constant rate of filtration by compensating for increasing headloss during a filter run by incorporating mechanical flow controller on the effluent path. The mechanical rate controller provides a progressive deceasing resistance to the flow & maintains a uniform flow.

Description and working of the control mechanism : (fig. 1)

This is generally a control valve, operated by a lever mechanism connected to a float placed in the filter control chamber. The float and control valve are isolated by a partition wall.

The effluent pipe from filter is connected to this flow control valve through the bed isolation valve. Filtered water is discharged through the control valve and flows over a measuring weir.

The rate setter device consists of a telescopic tube with a circular weir at the top. This telescopic tube is located on the upstream side of the measuring weir. The height of circular weir is so adjusted that when the effluent flows over the measuring weir, a portion of water spills over the circular weir and falls into the telescopic tube.

The telescopic tube is connected to the float chamber by means of a bend. From the telescopic tube the water trickles down to the float chamber. In the float chamber a submerged orifice is provided (orifice gear) which continuously discharges the water to the drain. When there is a balance of water coming in and going out of the float chamber, the level of the float gets set.

The slightest difference in the arte of inflow and outflow upsets this balance. This causes changes in the float level and automatically adjusts opening of control valve through the connecting lever until the balance is restored. The telescopic pipe can be raised or lowered to give the desired discharge.

The flow control valve is many a time also controlled by a second float located in the filter bed again through a lever arrangement. This float closes down the control valve when the level of water in the filter drops below a preset desired level. This helps to avoid the dewatering of filter bed.

Thus with this mechanism the operation of filter bed incorporates the characteristics of constant rate as well as constant level filtration.

The depth of water in these types of filters is normally 2.6m to 3.0m. The filter bed is due for cleaning or for a backwash when either of the two conditions mentioned below occurs earlier.

1.	Development of maximum or terminal headloss.
2.	Breakthrough in the filtered water turbidity.

Disadvantages :

In most of these plants the designed depth of water column over sand media is 1.30m to 1.60m. The maximum allowable head loss is normally designed as 2.0m to 2.5m. As a result the level of outlet weir in the rate control chamber is below the top of the sand media. The failure of rate control mechanism invariably leads to the dewatering of bed. At many plants one can see the incoming water directly pouring on the sand media. Alternate dewatering of sand deteriorates the health of filter bed, rapidly resulting in poor filtrate quality. The changes of build up of negative headloss are higher due to the said location of the outlet weir.

a.	Incompatible and faulty design of equipments with respect to the filter hydraulics.

b.	Inherent complexity and nonflexibility of the mechanical equipments and parts used in the assembly.

c.	Lack of literature and operating instructions by the manufacturers. Non availability of skilled personnel for erection of the equipment, repairs and maintenance.

It is fairly complicated to balance the sum total of effluent passing through the individual beds to the total inflow.

Problems with the mechanical rate control arrangement : As explained in the ‘Introduction’, the non-functioning of the mechanical rate of flow controllers could be due to one or more of the following reasons:

An adjustment to rate setter device induces sudden fluctuations in the flow rate. During operation water level on the bed is held almost constant and is allowed to vary only slightly. When the total plant input increases or if one of the beds is taken out for a backwash, the momentary surge due to abrupt increase in the rate of flow is detrimental to the filtered water quality.

To measure the headloss through the filter bed and to measure the quantity of filtrate, equipments are required to be installed and maintained. They have their own problems.
In order to avoid these difficulties and to simplify and regulate the filter operation, alternative options are available now. These alternative systems are based on simple hydraulic principles rather than having a dependence on mechanical controls. This single factor makes them an attractive and appropriate option. They also have some additional inbuilt safety factors.

1. B] CONSTANT RATE FILTRATION WITH INFLUENT CONTROL OR INFLUENT FLOW SPLITTING:

Principle:

As against control over the filter effluent described in the earlier paras, this system exercises control over the filter influent.

This system ensures a constant rate of flow of effluent during the service cycle by allowing the water level to rise in the bed. The rising of water level overcomes the increasing headloss caused due to the resistance of the media. The influent flow to filters is governed by means of weirs and effluent side is left uncontrolled.

Process description and details : (fig. 2)

The entire flow of influent (clarified water conveyed to the filters) is split equally at the inlet of filter beds by means of simple weirs. Rectangular weirs with equal dimensions are located at the same elevation in the concrete weir boxes in each filter. The influent channel is connected to the inlet of weir boxes by means of isolation gates.

The influent enters into weir boxes, flows over the weirs and then is conveyed into the bed by a vertical pipe reaching the top of the gullet. In the process the influent is split equally over all the beds and each bed receives roughly equal and desired amount of water.

The effluent from filters flows over a measuring weir located in the outlet chamber. The filter outlet pipe is connected to the chamber by means of an isolation valve. The elevation of the weir is provided above the top of sand media to avoid dewatering of the bed.

During the service cycle the filter inlet and filter outlets are opened completely. As the filter is backwashed prior to the service cycle, media will offer the least resistance and there will be minimum water level over the bed. As the service run progresses the media will offer greater resistance, till it reaches the level of influent weir which is also the terminal headloss for the service cycle.

Thus the entire headloss is accommodated in terms of water head over the top of the media. These beds require standing water depth of 2.25m to 3.50m over the filter media and hence are sometime known as deep box filters.

In practice the elevation of outlet weir is provided about 0.10m to 0.25m above top of the sand media. The initial headloss is about 0.20m to 0.25m. The depth of water over outlet chamber is 0.10m to 0.70m above top of the sand. Assuming that the maximum allowable headloss in the bed is 2.0m to 2.50m, the high water level (HWL) at the end of filter run is 2.40m to 3.20m above top of the media. Assuming that the depth of filter media and supporting gravel to be about 1.20m to 1.30m, the minimum water depth in the bed is required to be 3.60m to 4.50m.

The split flow weirs are located in the concrete box which is located directly above the central or the side gullet of the filter. The height of the weir above the bottom of the box is 0.40m to 0.60m. The sizing of the box and that of the inlet port should be larger enough so that they do not cause excessive turbulence over the weir.

The weir plates (A.S.S. flat of about 50 x 6 sizes) should be firmly embedded in the concrete. All weirs should be at same level so they split or distribute the water at +5% accuracy over all the beds. This system is incorporated and is working satisfactorily in many plants in Maharashtra and Gujarat.

Advantages :

1.	From operation and control point of view these are very simple filters as there is no induced control by any mechanical means. The filter bed is taken out of the service cycle for backwash once the water level in that bed reaches up to the influent weir level.

2.	As the outlet weir of the filter bed is located above the top of sand media there is no chance of even accidental dewatering of bed as well that of development of negative head loss.

3.	As the headloss in the filter bed is directly visible to the operator in terms of water level over the bed, essentially one can dispense off with the headloss indicators.

4.	The same is the case of rate of flow indicator, as total inflow gets equally divided in each bed and the same is coming out of the filter beds; there is no need of measurement of flow from the individual beds.

5.	Perhaps the greatest advantage of this system is its ability to tackle the shock loading. During increase in the inflow or during backwash of one of the beds, the additional water gets uniformly distributed over rest of the beds. The water level in the rest of the beds increases gradually and so does the flow until sufficient head is achieved to handle the flow. Thus the increase or decrease in the rate of flow is gradual and has least harmful effects on the quality of filtrate.

Disadvantages:

The only disadvantage this system has is that the water depth being 3.50 4.50m, filter boxes are deep and the cost of civil works will be more than that for filters having mechanical rate controllers. But, taking into consideration the enormous advantages it offers over filters with rate controllers, this should be a non issue.

2 ] Variable declining rate filtration (VDRF):

Principle:

In the VDRF system, essentially the entire filter battery media resistance is kept under control by periodic cleaning of individual beds in a predetermined sequence. The rate of filtration through an individual bed is highest after the backwash and then is on a gradual decline till the end of its filter run. Normally there is no control over the outlet valves and the water level in the entire battery falls or rises gradually to compensate for the average media resistance.

Process description & details: (figs. 3 & 4)

The general arrangement of beds is similar to the filter beds with influent splitting weirs with the following changes:

1.	The influent splitting weirs are not provided.

2.	Influent header channel is deep and large enough so that all the beds in the battery have same water level all the time.

The filter influent enters the bed below the low water level of filter. The influent channel or header and filter inlet ports are large enough so as not to cause any appreciable headloss. As a result water level is same in all the filter beds. The effluent flow from filters is not controlled.

All the filters in the battery (bank) served by the common influent channel tend to get clogged and the flow through the dirtiest bed decreases most rapidly. This causes the flow to redistribute itself automatically through the common influent channel over other filters. The water level rises slightly in redistribution of flow to provide the additional head needed by the cleaner filters to pick up the decreased flow of the dirtier filters.

As the effluent from each filter is uncontrolled, the cleanest bed accepts the greatest flow increase in the redistribution and operates at the highest rate. The dirtiest filter operates at the lowest rate. Thus at any given instance the cleaner filters are above the average plant rate and the dirtier filters are below average rate.

As the effluent from each filter is uncontrolled, the cleanest bed accepts the greatest flow increase in the redistribution and operates at the highest rate. The dirtiest filter operates at the lowest rate. Thus at any given instance the cleaner filters are above the average plant rate and the dirtier filters are below average rate.

The lowering down of water level over entire battery will cause proportionate decline of flow rates over the other beds. The changes in the water level in the battery are very gradual. As the bed that is the cleanest will draw the maximum water, the effluent quality from the battery is least affected.

To determine which bed is in the immediate need of backwashing, only the water lever over the outlet measuring weir has to be checked. The filter giving the least output is then backwashed. Otherwise the backwash sequence is decided at a fixed time interval. The bed which has served the longest service cycle is taken up for backwash.

In practice, the filter beds are sized for the average rate of filtration. The maximum and minimum filtration rates normally preferred are 150% and 50% of the average rate.

Advantages :

1.	The main advantage of declining rate filtration claimed is that the filtrate quality from the battery is always better than obtained with constant rate filtration as the flow decreases progressively through the dirtier filters.

2.	The available headloss that has to be provided can be decreased because the headloss through the under drains and effluent pipe system decreases as the flow rate through filter decreases towards the end of the filter run. This head then becomes available to sustain the filter run for a longer period. The recovery of available head varies approximately as a function of the square of the flow rate.

3.	In case of rise in the total input or during backwash of one of the beds, the water level gradually rises in the remaining filters until sufficient head is available to handle the higher flow received. The rate increase or decrease is achieved slowly and gradually providing least harmful effect to the filtrate quality.

4.	Headloss is evident to an operator in terms of water level in battery and hence costly and elaborate arrangement to read headloss is not required.

5.	The effluent or outlet weir which is located above the top of sand prevents accidental dewatering of filter bed and also eliminates the possibility of development of negative head.

6.	The depth of water required in a filter box is less than that required for filters with influent flow splitting weirs.

Disadvantages:

Variation to VDRF System: (fig. 5 )

In a ‘controlled or restricted head’ declining rate operation, a resistance in the form of an orifice plate or a valve is incorporated on the effluent (filtrate) piping to control the excess rate that may generate after putting into service a newly back-washed bed.

Another variation of a ‘controlled head’ system is especially suitable to convert the existing filter beds with constant rate application into VDRF system but could also be applied to new filters where standing water depth is less. The effluent from individual filters is connected to a common header. The header is connected to an outlet chamber through a master rate control valve. The control valve can be operated manually or with mechanical flow controller (but only one rate controller for the entire battery) to govern the plant rate or to control the water level in the filter battery.

The individual filter outlet valves are required to be throttled for some period to check the excessive flow after they are put into a new service cycle.

The old rapid sand filter banks at the Pune Municipal Corporation’s Cantonment Water Works were converted into the above described system by the M.W.S.S.B. in the year 1980.

The VDRF system coupled with dual media filters could be a boon to the hundreds of existing plants in India. Most of these plants are with rate controllers and need improvement to the filtrate quality and augmentation of their capacities. These plants could be modified at a fraction of the cost of new plants with these techniques.

Conclusion:

A graphic comparison of headlosses, flow rates and water levels as a function of filtration time for constant rate effluent control, constant rate-influent control and the declining rate control systems is given in Fig. 6

The references and actual field experience conclusively suggest (in the opinion of the author) that the constant rate filtration with split flow weirs and declining rate filtration systems have emerged as an appropriate alternative to the mechanical rate of flow controllers.

These systems are relatively simple to design, execute, install and operate. It is also true that inherent controls by hydraulic means are any time better than induced mechanical controls, from operation point of view.

Due to lack of proper dissemination of information about the developmental work taking place abroad and in India and due to the conservative approach of persons/departments working in this field these systems have taken a long time to get accepted and practiced in our country.

Constant rate filtration with influent control can e adopted for small plants having one or two beds and up to large plants having six/eight beds, while the declining rate filter control option is more attractive for a battery of large number of filter beds having minimum number of three to four beds.

As a number of new filtration plants are expected to be installed in our country in the near future, it is essential that we closely examine these simple and appropriate systems and incorporate them in our future designs.

Acknowledgements :

Thanks are due to engineers and officers of the Maharashtra Jeevan Pradhikaran, MJP (formerly known as MWSSB), Gujarat Water Supply and Sewerage Board (GWSSB), Kerala Water Authority (KWA), Water Treatment DIvn., Thermax Ltd., Pune for allowing the author to design and incorporate these techniques at some of the filtration plants referred to in this paper and study their utility.

Reference:

Manual on Water Supply and Treatment, (C.P.H.E.E.O. 1991)

Upgrading Existing Water Treatment Plants : AWWA Annual Conference, 1974, Boston.

a]	Upgrading Existing filtration plants : John L. Cleasby

b]	Filter rate control without rate controllers : J.L. Cleasby

Surface Water treatment for communities in developing countries : Schulz & Okun.

Simple methods in water purification : J.N. Kardile

Managing Water treatment plants- Performance of dual media declining rate filters :
V. Venkattappaiah, D.M. Mohan, A. Bapu Reddi, JIWWA, JAN-Mar. 1992.

Handbook of public water systems : Culp / Wesner / Culp.

Journals of IWWA, AWWA and IWSA.

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