| |
| Services |
 |
|
| 1. Greenfield or new water treatment plants based on Tube Settler technology / clarifiers read more... |
| 2. Single or multimedia gravity filter beds. read more... |
| 3. Complete water treatment systems and Water supply schemes read more... |
| 4. Augmentation or retrofit of existing plants for capacity or quality up-gradation read more... |
5. Mass application of small capacity standardized plants
read more... |
| 6. Sewage Treatment Plant. |
| 7. Project Management Consultancy. (PMC).. |
| 8. Civil, Structural, Electrical & Automation Engineering design & drawings. |
|
| Research Paper & Presentation |
|
| |
|
|
| Nashik Thermal Power Station, Maharashtra, INDIA |
| |
| Augmentation of existing water treatment plants |
|
|
|
| Introduction : |
|
The existing plant has a receiving chamber and a mixing channel. From the common mixing channel
the flow is divided into three streams. Each stream consists of a secondary distribution channel
and two numbers of settling tanks. Each settling tank has a size of 10m x 10m. The vertical water
depth of tank is 2.0 m, and overall depth of the tank up to bottom of the hopper is 10m. Entry to
each tank is in the form of a pipe, which is extended up to 1.0 m above bottom of the hopper. The
settled water collection is by means of collection troughs. The construction is 30 years old and is
of concrete and masonry. The surface loading on the tanks works out to be 2000 lit/sqm/hr (0.80
gpm/sqft). After filtration, 70% of treated water goes to softening plant (make up for cooling
towers), 20% to demineralization (DM) plant (Boiler water make up) and 10% to domestic colony.
In spite of using different chemicals and coagulants the plant was not able to reduce the organic
load appreciably. As a result sometime back a new stream was constructed with a different raw
water source (after spending a considerable amount of money) for domestic and drinking water
consumption. However the organics in treated water continued to affect the softening and DM
operations. The regeneration chemical consumption went up, the service cycle of the units got
reduced and backwashing frequency increased with long periods of backwash.
Jar tests confirmed that effective coagulation (rapid mix) and flocculation (slow mix) processes
were essential for removal of organics prior to settling. Accordingly, one stream (central) of
400 cum/hr consisting of two settling tanks was taken up for up-gradation in the month of June
2002.
The modifications consisted of incorporating one number of mechanical rapid mix unit
and two numbers of flocculators (slow-mixers) in one tank and clarifying the flocculated water
by retrofitting the other tank with tube settlers. This required a basic change in flow distribution
arrangement, as the two tanks were to be connected in series instead of parallel as in the existing
plant. There was no vertical partition wall between the two adjoining settling tanks and this feature
helped in the conversion. Thus the resulting size of the settling unit was 10.0m x 20.0m. Inlet to this
stream was provided by creating an opening in the common feed channel. Gates were provided
at this inlet and in the flow path of two remaining streams to regulate the flow. In short the flow
pattern was changed from vertical to predominantly horizontal, to suit the coagulation-flocculation-
tube settler system.
The location of rapid mix unit and slow mixers in the first settling tank was a complicated choice
due to the unusual tank geometry (Side water depth of 1.6m and tapering of tank into hopper
shape below that). Conceptually the tank was divided into two parts, 1/3 towards inlet side and 2/3
towards outlet side. The outlet side portion was treated as flocculation zone.
The rapid mix unit consisted of a pitch-blade turbine (upward pumping). To define the flow path,
a CS shell of 1.75m diam. and 3.5m height was suspended from the trussed bridge walkway. The
bridge with a span of 10m was supported at the two ends over the side walls. The shell (and the
mixer) was located at the center. The mixer was thus located at 1/3 the distance (at 3.3m) of total
tank length (10m) from the inlet of the tank. Inlet to the shell was provided at the top and outlet
at the bottom to make it a down-flow unit. Raw water from the newly created gate-opening was
conveyed to this chamber by a CS open channel. The outlet of the shell chamber was provided with
a baffle and wing walls to direct the coagulated water flow towards initial 1/3 section. This was
required to be done to avoid possible short-circuiting.
The flocculation zone thus had plan dimensions of 6.7m length and 10m width. Two numbers
of vertical paddle type flocculator were suspended over another trussed bridge in the path of
coagulated water flowing towards the tube settling section. The bridge was positioned at the center
of this compartment with a provision to shift it by +1.0m along the length. The distance between
two flocculator agitators was provided less than � (4.0m) of tank width (10m). Again the provision
was made to shift their location by + 0.50m. Defining effective volume (for that matter the effective
tank depth) for flocculation was virtually impossible due to the tank geometry described earlier.
However as the flashmixer outlet was 3.5m below the top water level, it was considered for
calculating the paddle area and power requirement. Variable speed drive was incorporated to
impart G value from 30 to 80 m/sec/m. The drives for rapid mixer and slow mixer were located on
the trussed bridge platform.
The adjoining settling tank was converted to tube settling tank by covering 9.0m x 10.0m of plan
area by tube modules (Surface loading rate: 5000 lit/sqm/hr, roughly 2 gpm/sqft). Tube modules
fabricated out of 50mm x 50 mm square tubes, each having a length of 0.6 m and inclination of
60 degrees were placed 1.0m below the top water level. The modules were spanned into three
equal portions over total length of 10m. They were supported from bottom over trusses with a
span of 10m. A CS baffle was provided at the inlet side, extending up to 1.50m below the bottom
of the modules. The existing collection troughs were corroded and hence were replaced by new CS
troughs. The existing sludge drainage system was retained as it was.
The modifications and retrofitting work took 30 days during which the central stream was isolated.
Proper dosing arrangements were made using a rapid mix. The unit was provided with a flow
measuring weir in the settled water channel, as there was no other place where this could be
achieved. The modified plant has shown promising results. The unit was commissioned during
monsoon but due to dilution effect the organic load was comparatively less. For raw water turbidity
up to 40 NTU, the clarified water turbidity was consistently less than 5 NTU. For raw water COD
of 10 to 20 mg/l, clarified water has reported COD reduction of 40 to 60%. A low dose of 2 mg/l of
prechlorination is practiced as the post treatment units having resin beds discourage presence of
chlorine. Tube settling portion has reported good quality of sludge, including dead lumps of algae.
During commissioning the total plant flow was 900 cum/hr. The tube settling stream has reduced
the load over two remaining streams, thus improving the overall plant efficiency. An indirect
indication of improved water quality includes absence of fishy smell from the filter house and
longer filter runs (18-20 hrs against 4-6 hours before modification).
The location of mixers in the first tank was a complicated choice due to non-regular plant geometry.
Lack of three dimensional flow analysis facility compelled use of common sense, experience and a
bit of guesswork. However enough flexibility was provided to change location of the bridge and the
drives if the situation so demanded. The total cost of the job was INR 1.50million (USD 0.03million).
Conversion of the remaining two streams is envisaged in future once the performance of the
modified unit is established.
 Settling Tank Diagram |
|
| Acknowledgements : |
|
|
Authors will like to thank all organizations ( govt., private and semi-govt.) who have participated in these case studies. Thanks are also due to their managers, engineers, chemists and operators who have given invaluable help to compile this data. |
| |
|
|
|
| Top |
| |
| |
|
