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Research Paper & Presentation
 
Evaluation of performance of new technologies:
A high-rate unconventional simplified plant at Trimbakeshar
 
  The main criterion to qualify as an appropriate technology is that the end product, plants should be cheap enough for mass application. Secondly their operation and maintenance should be simple so that the plants could be managed within the rural infrastructure. The third criterion is of course the output, treated water quality should not be sacrificed to achieve said criteria, but the standards for the same should be realistic.

Many alternative technologies are emerging on the world scenario to quality for the above criteria. To name a few like non-mechanical flocculation systems, shallow depth sedimentation, multi-media filters, declining rate filter control systems etc. Judicious combinations of these unit processes and if required their amalgamation with conventional simplified water treatment plants.

At Trimbakeshwar, Near Nasik, a high rate, unconventional plant was constructed by the Maharashtra Water Supply and Sewerage Board in the year 1990-1991. This paper basically deals with the evaluation of the performance of this during the monsoon of the year 1991.
   
2. General description and purpose of the scheme:
   
  Trimbakeshwar is a small picturesque town, located at about 30 kms from Nasik in Maharashtra state. It is surrounded by the hills of Sanhyadries from three sides. The river Godavari originates from these hills. It is a holy place for devotees of the Hindu religion.

During the later half of the year 1991, a Hindu religious festival �Kumbh-Mela� was organized at this place. As this particular festival repeats only after twelve years, millions of devotees from all over India took the opportunity to visit this place.

To cater to the need of this population and also to the population of the town of drinking water, water supply scheme for this town was executed well before the onset of this event.

The source of raw water is a Minor irrigation tank at village Amboli, located at about 10 kms from Trimbakeshwar. The designed capacity of the scheme is 2.4 million liters per day (MLD). The raw water is pumped through the rising main up to the treatment plant, which is located on the slopes of a hill. After purification the filtered water is stored in a Ground Service reservoir (G.S.R.) of capacity 0.6 million liters. The distribution to the town further is by gravity.
   
3. The treatment plant:
   
  A high-rate, unconventional simplified water treatment plant is provided to supply purified water to the town.

Various units of the plant are as follows:
   
 
  (a) Receiving chamber or stilling chamber
(b) Mixing cum measuring weir
(c) Mixing channel
(d) Gravel bed flocculation tanks
(e) Tube settling tanks
(f) Dual media filter beds
(g) Filter control room
(h) Chemical house
(i) Wash water tank
(j) Sludge disposal system
   
   
4. Brief description of the units:
   
 
a] Stilling chamber :
   
 
  Simplified W.T.P. at Trimbakeshwar, cap. 2.4 MLD.
(Note: Details of filter control room are not seen in the section)
   
  Legend:
 
1. Raw water inlet
2. Stilling chamber
3. Alum solution feeder pipe
4. Mixing channel
5. Inlet channel
6. Gravel bed flocculator
7. Tube settling tanks
8. Settled water collection channel
9. Filter inlet channel
10. Duel media filter bed
11. Chemical house (first floor)
12. Alum solution tank		 13. Walkway
14. Staircase
15. Porch
16. Washing arrangement for Gravel bed
      flocculator
17. Sludge drain chamber
18. Floc collector pipes
19. Floc distributor pipes
20. Collection troughs
21. Inlet to filter pipe
22. Overflow pipe
23. Cross troughs
   
 
   
  Legend:
 
1. Stilling chamber
2. Gravel bed flocculator
3. Tube settling tank
4. Duel media filter bed
5. Control room building		 6. Pure water sump
7. Filter control room
8. Chemical house
9. Wash water tank
   
 
  Simplified W.T.P. at Trimbkeshwar, Cap. 2.4 M.L.D.
Section-AA
  Legend:
 
1. Inlet channel
2. Inlet openings
3. Gravel bed flocculator
4. Tube settling tank
5. Collection channel
6. Corbel of M.S. Grill
7. M.S. Grill		 8. Floc collector pipe
9. Hopper pit
10. Floc distributeor pipe
11. Tube modules
12. Collection troughs
13. Corbel
   
 
  Fig. 4 Perspective view of the simplified W.T.P. at Trimbakeshwar
Capacity 2.4 M.L.D.
  Legend:
 
1. Porch
2. Filter Control Room
3. Staircase Room 		4. Chemical House
5. Washwater Tank
6. Hydraulic Units
   
  Function: This is the first unit of the plant. It receives raw water from the rising main. The turbulence created at the inlet of pipe is dampened in the chamber. Thus non-turbulent flow is created on the upstream side of the measuring weir to facilitate measurement of flow over weir. Heavy particles like sand are also trapped in the chamber.
   
 
No. of units : One
Size: 0.8m x 1.5m x 3.5m water depth (w.d)
Design features: Detention time – 2.5 min.
Design features: Upwards
   
b] Mixing cum measuring weir :
   
  Function: To measure the raw water flow. To create turbulence on the downstream side
for instantaneous mixing of raw water and alum dose.
   
 
No. of units : One
Size: Length – 0.8m, Height above mixing channel – 0.5m
Type of weir : Sharp crested
   
c] Mixing channel :
   
  Function: To provide further uniform mixing of alum dose for thorough coagulation of water.
   
 
No. of units : One
Size: Length – above 0.8m, Width- 0.8m, Depth of water-0.2m.
Design features: Velocity – 0.6m/sec, Detention time- 30 sec.
Mixing device: 100 mm dia. A.C. pipes of 250mm ht. embedded in staggered fashion in the
bottom slab.
   
d] Gravel bed flocculator :
   
  Function:To provide multiple recontacts to suspended solids to agglomerate into settleable flocs, while the coagulated water passes through the voids of gravel.
   
 
No. of units : Two in parallel.
Size: 2.5m x 3.4m x 3.0m w.d. over hopper, each.
Detention time: 30 min.
Volumetric loading: 2000 lph/cum
Surface loading: 6000 lph/sqm
Size of gravel: 30mm to 70mm (top to bottom)
Depth of gravel: 2.2m
Direction of flow: Downwards
   
e] Tube settling tanks :
   
  Function:Solid-liquid separation. Effective collection of settled water and disposal of sludge.
   
 
No. of units : Two in parallel.
Size: 3.4m x 3.4m x 3.0m w.d. above hopper, each.
Surface loading: 4500 lph/sqm
Detention time: 40 min.
Size of tube: 50mm x 50mm
Length: 600mm
Materia: Rigid PVC
Angle of inclination: 60 degrees
Module height: 500mm
Floc distribution pipes: 200mm dia RPVC, 2 No per unit
Settled water collectors: F.R.P. trough 150mm x 200mm, each 4 Nos. per unit.
Direction of flow: Upwards.
   
f] Dual media filter beds :
   
  Function:This is a coarse to fine filter bed (from top to bottom), provided to produce filtered water of turbidity less than 2 ppm.
   
 
No. of units : Two
Size: 3.1m x 2.1m x 3.0m w.d. each
Filtration rate: 6600 lph/sqm
Filter media: Supporting gravel 0.6m,
Size: 2mm to 60mm
Quartzite sand: 05.mm 0.4m, E.S.
Crushed coconut shell: 0.4m, Av. Size: 1 to 2mm
Filter backwash:
Hard wash only, Rate:		 700 lpm/sqm
Filter under drain: M.S. Manifold pipe and RPVC laterals.
Filter control: “Controlled head”, variable declining rate.
Washwater disposal: 2 No. cross-troughs per unit. Central gullet located in between two units.
   
g] Filter control room: (Ground floor) :
   
  This room is constructed adjacent to the filter beds with a common wall. The two outlet pipes from the filter units are connected to a common conduit, which conveys the filtered water to the rate control chamber via a Master rate control valve. The filters work on the principle of �Controlled head� variable decling rate control system. The master rate control valve governs the total output going to the rate control chamber. In the rate control chamber a measuring weir is provided to govern the output. The elevation of weir also controls the maximum allowable headloss which is 2.0m. Arrangements for post chlorination are also housed in this room.
   
h] Chemical house: (First floor) :
   
  This room is located directly above the filter control room. Two No of alum tanks of 1500 litres capacity each are provided in this room. Outlet pipe from the constant head dosing tank conveys the solution to the slum feeder pipe which is located on the downstream side of the mixing weir. The feeder pipe is perforated at the bottom to distribute alum solution uniformly across the breadth of the channel for instantaneous mixing with raw water.
   
i] Washwater tank :
   
  A washwater tank of 80,000 litres capacity is provided directly above the chemical house. Thus the filter control room building is a two storied structure with control room on ground floor, chemical house on first floor and washwater tank on the roof slab. The washwater tank is provided with 8.5m head above the filter bottom.
   
5. Salient features of the plant :
   
 
a] The entire hydraulic process is non-mechanical. All the unit processes make effective use of the hydrostatic pressure only. The external energy input is only in the form of pumping head at the stilling chamber. The total headloss in the system is 3.75m.
   
b] For filter control, mechanical equipments are not provided. The filters work on the principle of variable declining rate control. The headloss is measured by simple manometric arrangement.
   
c] The entire plant is designed from the operator�s point of view. The operator has easy access to all units. Stress is laid to provide his access paths in such a way that he has to travel minimum distances with minimum changes in the elevation.
   
d] The execution of the plant was completed in twelve months period. The cost of the plant is Rs. 8.5 lakhs which is about 75% of the cost of a CONVENTIONAL PLANT OF THE SAME CAPACITY.
   
6. Evaluation of performance of the plant during monsoon::
   
 
a] For performance evaluation of the plant, the period of six months starting from June 11th upto November 30th is selected. Reservoir being a source, during this period there was a gradual and uniform change in raw water turbidity from high to low. Therefore the period from June to November is important from observation point of view. This can be observed from the tabulated data. In the data only average trend in turbidities is considered.
   
b] The plant was operated and maintained by the Maharashtra water supply and sewerage board up to the month of December 1991. Thereafter the plant was handed over to the local Municipality.
   
c] The staff at the plant during the observation period of six months consisted of one plant operated daily anywhere from 8 to 24 hours depending upon the demand during Kumbh-Mela. Further the unreliability of electric power supply caused stoppages due to which the plant was many a times operated intermittently. The plant operating staff was thus required to perform under taxing conditions. In this case telephone facility was provided at the plant to co-ordinate with headworks and distribution. The following performance analysis of the plant is examined in the light of the abovesaid factors.
   
7. Observations on the performance of gravel bed flocculator and tube settling tank: (Pretreatment unit)
   
 
a] The performance of both the units is evaluated simultaneously as it is found out that the settled water quality to a great extent is directly related to the performance of gravel bed flocculator.
   
b] From the tabulated data it can e ceen that from June 11th to July 31st (beginning of monsoon), the raw water is having the highest turbidity of the season. With proper alum dose, the settled water quality is found to be satisfactory. The plant was commissioned in the month of April. Up to June it was receiving raw water of fairly low turbidity. The voids in the gravel bed flocculator were comparatively clean during this period.
   
c] From the tabulated data it can e ceen that from June 11th to July 31st (beginning of monsoon), the raw water is having the highest turbidity of the season. With proper alum dose, the settled water quality is found to be satisfactory. The plant was commissioned in the month of April. Up to June it was receiving raw water of fairly low turbidity. The voids in the gravel bed flocculator were comparatively clean during this period.

For benefit of the readers typical calculations for a single row data are worked out as follows :
   
  Column 1.
  Period under consideration: June 11th to June 31th
No. of days: 20, twenty
   
  Column 1.
  Period under consideration: June 11th to June 31th
No. of days: 20, twenty
   
  Column 2.
  Total cumulative pumping: 184 hr.
Total quality of water supplied: 184 x 90 = 16560 cum
   
  Column 3.
  Period under consideration: June 11th to June 31th
No. of days: 20, twenty
   
  Column 3, 4, 5.
 
Average turbidity : Raw-raw water
: Set-settled water
: Fil-filtered water
   
  Column 6.
  Actual requirement of alum: 881 Kg
   
  Column 7.
  Average dose of alum: (881 x 10x 100)/16560 = 53 ppm
   
  Column 8.
  Total quantity of washwater for filter: 300 cum
Required (60cum/wash/bed)
No. of beds=2
   
  Column 9.
  Percentage of washwater (with re-: (300 x 100)/1650=1.8% spect to total water filtered)
   
  Column 10.
  Column 8.
  Total quantity of washwater for filter: 300 cum
Required (60cum/wash/bed)
No. of beds=2
   
  Column 11.
  No. of backwashes for gravel bed flocculator
   
  The cause of deterioration in the settled water turbidity was identified and the operator
was instructed to backwash the gravel bed periodically at a regular interval of 7 to 10
days.
   
d] From Sept. 1st to Sept. 30th it can be seen that the raw water turbidity is moderate to low (100 ppm and less). The settled water quality is improved and is well within the limits. This is attributed to proper and regular cleaning of gravel bed flocculator.
   
e] The performance of the gravel bed flocculator and tube settling tank is summarized in the graph (fig. 5) showing raw water turbidity against removal efficiency of tube settling unit. The curves A and B show that for raw water turbidity of 100 to 400 ppm the removal efficiency is lower than the expected behavior of the curve. This is attributed to the inadequate cleaning of gravel bed flocculator as explained in point No. 3.
   
f] It was observed that the headloss development in the gravel bed flocculator was minimal even when the bed was not washed over a period of month. But the effect of the clogging of bed was directly reflected in the quality of settled water.
   
g] It can be controlled from the above observations that the best performance from the tube settling tank can be achieved when the gravel bed flocculator is clean and the voids are free of silt. The turbidity removal efficiency of the tube settling tank was upto 85% for raw water turbidity upto 100 ppm. The efficiency is more than 500 ppm upto 2000 ppm the removal efficiency is more or less constant and is between 98% to 99%.
   
h] The alum dose during the abovesaid period was monitored from time to time on te jar test equipment. The percentage of A1203 (alumina) in the alum was about 12%. The strength of the alum solution was

Kept at 1 to 2%. The raw water pH from June to October varied from 7.4 to 7.6 The filtered water pH was found to be 6.8 to 7.0.
   
i] The draining of sludge from the hoppers of the tube settling tank was done once or twice in twelve hours for raw water turbidity up to 500 ppm. The draining interval was required to be reduced upto 4 hours for raw water turbidity from 500 to 2000 ppm..
   
j] The quantity of wash water provided for backwashing of one gravel bed flocculator was 40 cum. The rate of backwashing was 500 to 600 lit/sqm/min. The total quantity of wash water required during the period from June to November was 0.5% of te total water treated through the plant. The frequency of gravel bed backwash is reduced during the period of low turbidity for period onwards November. Only one or two washes were recommended in a month during this period. Therefore the consumption of washwater will significantly reduce if calculated over a period of a year.
   
8. Observations on performance of dual media filter beds :
   
 
a] The dual media filters were able to tackle settled water turbidity even upto 30 ppm comfortably. During the operation, stress was laid to produce filtered water of turbidity less than 1 ppm. The average length of filter run was observed from 42 hours to 102 hours (Calculated for every 15 days for the period under consideration). This is as per expectation as the designed rate of filtration is moderate (6600 lph/sqm) for a dual media bed.
   
b] For backwashing of filters the operator was instructed to use either of the following criteria which ever occurred earlier.
   
 
1. Maximum development of headloss upto 1.8m.
   
2. Breakthrough of filtered water turbidity above 1.0 ppm.

It was observed that breakthrough of turbidity occurred earlier than the maximum development of headloss for majority of the time. This can be attributed to many factors but the author is of the opinion that quartzite sand which was used as a fine media was used as a fine media was little coarser than the recommended value.
   
c] The two filter beds are located in a single bax with washwater disposal gutter at the centre. The capacity of washwater tank is enough for washing of one bed only. During backwashing the plant has to be stopped as the design of pipeline does permit independent washing of one of the beds while other is in operation. (This is possible in small capacity intermittently operated plants as the operational hours are limited from 8 to 16). Further in case of two beds, when one of the beds is taken out of operation for washing, the other bed faces difficulties in tackling 100% overloading.)

It was observed from the logbook that sometimes the operator has backwashed both the beds simultaneously. As enough washwater was not available at the designed rate, this resulted in inadequate cleaning. During the successive filter run there was rapid development of headloss. This is attributed to insufficient training of the operator. This was detected early and the washing of the beds was subsequently corrected.
   
d] The need for effective backwashing of filtered beds was repeatedly stressed to the plant operator. He was told to observe two basic things.
   
  (i) After backwashing of filters the initial headloss should register in-between 0.10 to 0.15m.

(ii) Periodically check should be kept on media expansion during the backwashing operation with the help of simple expansion stick. The desired expansion of the media should be 30 to 40%.
   
e] 1. Maximum development of headloss upto 1.8m. 2. Breakthrough of filtered water turbidity above 1.0 ppm. It was observed that breakthrough of turbidity occurred earlier than the maximum development of headloss for majority of the time. This can be attributed to many factors but the author is of the opinion that quartzite sand which was used as a fine media was used as a fine media was little coarser than the recommended value.
   
f] The bacteriological test was carried out once during the observation period for the settled water and non-chlorinated filtered water. The reduction in coliform count was found to be 98%.
   
g] The filter beds were operated on the principle of controlled head variable declining rate control system. This was achieved by washing the filter beds in staggered fashion so that the state of clogging of both the beds was different. The output from both the beds was controlled by a single master control valve. This resulted in rate of flow from each filter bed proportionate to its state of clogging. From operation point of view this system was found to be suitable as mechanical filter control gear was not required.
   
9. Conclusion :
   
 
a] When gravel bed flocculator is adopted preceding tube settling tank, the clogging of gravel bed directly affects the settled water quality. The gravel bed floculator should be backwashed periodically during the period of high turbidity atleast once in 7 to 10 days.
   
b] The gravel bed flocculator should be adopted for low turbidity and low silt raw waters. The upper limit of turbidity should not cross 3000 ppm over a long duration.
   
c] The deteriorated settled water quality in the month of August (For raw water turbidity of 400 to 70 ppm) may be tempted to be attributed to the colloidal suspension. But the jar test and observations at other plants have confirmed the reasons discussed above in details.
   
d] Overall the tube settling process was found to be extremely stable. The settled water turbidity was consistently satisfactory inspite of some operational in-adequacies.
   
e] The dual media filter beds consistently produced filtered water of very high quality. The filters were able to tackle turturbidity shockloadings comfortably. The filter runs averaged 67.5 hrs over the period of six months. Declining rate control system was found to be simple from operation point of view.
   
f] Some basic training especially on the maintenance aspects was required to be imparted to the operators. The plant operators had no previous experience of operating water purification plants. With a little guidance and experience they were able to grasp the various techniques easily.
   
g] Under the given circumstances and environment, the performance of the plant was found to be satisfactory. There is definitely some scope for refinement in the training of the staff and maintenance of the plant. For mass application of the technology grade examination for the operators or training programs on the basic aspects of treatment have become most vital.
   
h] In the absence of training and welfare of the operating staff of the local bodies (especially in the rural area) it is possible that the end product of all this excerise i.e. potable water quality may get affected. However inherently strong or appropriate may be a technology, the plant operator is the single most crucial link for its successful implementation.
   
i] New development are taking place in this field basically from these type of plant scale observations. Plastic flocculation media, floc modules have shown a great promise by eliminating the inherent problem of clogging encountered in the gravel bed. Similarly in urban environment mechanical flocculators have been provided with good results. Tube settling system is the heart of such plants. It may be possible to improvise on the flocculation and filtration aspect upto a certain extent in the near future, but tube settling system is found to be the most efficient and robust mechanism.
   
10. Acknowledgements :
   
  The authors would like to thank the Member Secretary of M.W.S.S>B. and the chief Engineer (urban) for encouraging such type of studies. Thanks are also due to the Superintending Engineer (Jalgaon), the Executive Engineer (Nasik and staff of ENE Devision and Sub- Division as the detailed observations were possible with their kind co-operation.
   
  References:
   
 
1. Simple methods in water purification: Dr. J.N. Kardile
2. New concepts in water purification: Culp & Culp
3. Surface water treatment for communities in Developing Countries: Shultz & Okun
4. Design of water treatment plants: A.W.W.A. Publication
5. C.P.I.I.E.E.O. Manual on water supply and Treatment (1991)
6. Various papers published in the Journals of IWWA, AWWA.
 
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