Technical details for Continuous Sand Filter Plant for Providing Safe Drinking Water Supply Scheme

Transcription

1 Technical details for Continuous Sand Filter Plant for Providing Safe Drinking Water Supply Scheme TREATMENT PHILOSOPHY Source ( Canal ) Raw Water Pump Continuous Sand Filter Filtered Water Tank and Pump Activated Carbon Filter Disinfection by Chlorination -> Over Head Tank The Continuous sand filter plant is a continuous up flow deep bed granular media filter for raw water filtration for potable use. The backwash should also be continuous to facilitate filter media to be moving in down flow direction and continuously cleaned by recycling the sand internally. Continuous sand filter plant are made of mild steel plate, in the filter, flow comes in radically and washing is done continuously. Feed is introduced into the bottom of the filter, then flows upward through a series of tubes and is evenly distributed into the sand bed through the open bottom of an inlet distribution hood, the influent moves upward through downward moving saving sand bed with the solids being trapped in the sand. The filter medium is sand, contained in the annular space of pipes of a closed cylindrical shell. Initial treated water enters into a central hollow column and permeates radially through sand and is collected through peripheral ducts and flows out of the shell. The dirty filter media is continuously drawn from the bottom and airlifted to a compartment at the top of the filters where it is washed, the filter media sinking to the cylinder while the washed water carrying dirt escapes out through an over flow device. The continuous operation results in a constant and low pressure drop. Maximum clean filtrate is assured since the filter had always contains a certain amount of solids which improve filtration. This mode of operation also enables the filter to accept a highly polluted flow with high efficiency. Working Principal The feed is introduced into the lower part of the filter bed through riser tubes which discharge under a distribution hood. Filtration takes place upwards through the sand bed which should moving downwards. Most of the suspended solids in the feed should be separated near the feed level, which means that the dirtiest sand will be found in the lower part of the filter. The sand bed should be kept in a slow downward motion by an airlift pump taking out dirty sand from close to the bottom of the filter tank. In the air-lift pump the sand is subjected to a thorough mechanical agitation by the action of air bubbles and the dirt is separated from the

2 grains of sand. The filter bed is continuously cleaned while the filter is in operation. The dirtiest sand is continuously taken out of the filter bed, washed and returned to the clean part of the sand bed. This means that the filter does not have to be taken out of operation for backwashing. The dirt is finally rinsed away from the sand in the sand washer, which is placed concentrically around the upper part of the air-lift pump. The clean sand return to the top of the filter bed and Reject water is consistently taken out from the sand washer. The filtrate leaves the filter as an overflow. Plant runs on the principal of Continuous backwash operation and handles suspended solids load upto 150 ppm and it operates at atmospheric pressure. Raw water analysis Total Suspended Solids ppm : < 150 (Considered) Turbidity NTU : < 150 Temperature Deg. Celcius : 45 Treated Water Quality Total Suspended Solids ppm : < 5 Turbidity NTU : < 5 The clean filtrate exists from the sand bed, overflows a weir and is discharged from the filter. Simultaneously the sand bed, along with the accumulated solids is drawn downward into the suction of an airlift pipe, which is positioned in the center of the filter. The impurities are scoured loose from the sand during this upward flow. The sand then falls downward into the sand washer where it is rinsed in countercurrent flow with a small amount of filtrate. The cleaned sand falls back to the surface of the filter bed and takes part in the filtration process again. The sand is redistributed by means of a sand distribution cone. The dirty liquid is discharged through the wastewater outlet. The treated water after filtration is then taken back in to clear water tank. Working of Air-lift pump The placed airlift pump in sand filter should be placed vertically in the protecting tube in its correct position, ensuring it is fully submerged in water. The suction end of the pump will be located close to the bottom of the cone of the filter and the upper part will be connected with the sand washer in the upper part of the filter unit, above the sand level. Compressed air is fed in to the pump through the pipe connection and the static pressure from the surrounding water will therefore force water up through the pump tube and the suction created at the lower end of the air-lift should be strong enough to lift a mixture of sand and water to the level Controlled vessel. In this process when sand leaves the discharge opening of the airlift and enters the larger diameter level controlled vessel the flow velocity should be decreased so that the sand immediately will start to fall down through the sand washer. The suspended solids, which are much smaller and with lower density than the sand grains, will follow the wash water out through the wash water outlet pipe and the air bubbles will rise upwards to the free atmosphere. The rising air bubbles quite often have such a velocity that they will cause splashing.

3 Working of Sand Washer The sand washer consists of a number of rings placed to create a labyrinth. As the sand falls down through the labyrinth the sand grains are subjected to a whirling movement and simultaneously meet a flow of clean water. The level difference (pressure difference) must be adjusted so that good washing of the sand is achieved at lowest possible wash water flow. The dirt particles will be flushed away and follow the rising was water flow. The wash water over flows through a number of slots to the collecting vessel and then discharged through the wash water outlet pipe. Cleaning of Media In case of continues sand filter plant for the continuous cleaning at the media, a small supply of air is continuously introduced into the airlift which is centrally located at the bottom of the tank. This airlift, which is encased in the airlift protective housing, extends to the top of the unit. The filtered solids and sand are pulled into the airlift and conveyed to the top of the unit. The agitation and the turbulence within the airlift cleanses the sand of the filtered solids. These solids and sand exit the airlift into the central compartment/washer section. Within this section, the sand is further washed and the solids and sand are separated. The clean sand, having fallen through the washer, is distributed evenly over the sand bed. The washer/central compartment section is isolated from the filtrate except where the sand exits at the bottom of the washer. The blow up of the washer shows that a small amount of filtrate flows up through the sand washer assembly and acts as the reject water. The reject water traps all of the separated solids and this stream flows over the reject weir and out of the filter through the reject nozzle. Activated Carbon Filter Media Filtered Water shall be passed through the bed of Activated Carbon filter media plant to remove the smell, odor from filtered water and also removes organics & disinfection byproducts from the water having facility for manual back washing as and when required or periodically back washing to reactivate the activated filter media. The continuous sand filter should have following operational advantages :- Continuous Operation - Simple wash water handling - No stoppage for backwashing - Constantly high filtrate quality - Low, constant wash water flow. - High Load Capacity - Reduced need for preliminary separation - Contact filtration - Takes care of solids carryover from previous stages

4 - High sludge accumulation capacity - Simplicity - No backwash water storage needed - No backwash water pumps - No expensive automatic valves - No filter floors - Standard filter sand - No moving parts MAJOR ADVANTAGES, FEATUERS, COST SAVING BY INSTALLATION OF CSF WITH ACF PLANT 1. Continuous Sand Filter (CSF) is the only known technology currently commercial available which can generate potable drinking water for raw surface water with turbidity up to 150 NTU. 2. CSF is the only known technology which required the least amount of land for water treatment plant based on surface water. 3. CSF is the only known technology currently commercial available which is not dependent upon the quality /dedication of man power employed for delivery potable water quality. 4. CSF is the only known technology currently commercial available for which the plant is not required to shutdown for backwash since this technology having inbuilt backwash process. 5. In view of state Govt. endeavor to provide O&M of water works in the rural areas to gram Panchayat for their dedicated water works as single village scheme CSF technology is emerging as a solution for creation of mini water works for single village scheme in the rural areas. 6. CSF with ACF is the only known technology which is commercially available which apart from removing the turbidity up to 150NTU can remove the false smell and odor from the water during filtration process. 7. By using CSF with ACF technology following structure will be eliminate :- A. Clear water tank B. Slow sand filter bed, C. High level tank, D. Even Pump chamber can also be construct under OHSR by elimination of all these structure department can save massive land requirement and construction cost.

5 Since CSF is very Robust and easy to operate that PHED Department can also transfer these schemes to gram Panchayat or they can ask the supplier to carryout O&M for 5-10 years. TECHNICAL PARAMETER for Capacity LTR Per Hour 1 RAW WATER PUMP WITH MOTOR a Quantity 1 No. b Capacity 50 m 3 /Hr c Material of construction C.I. d Head 35 mtr e Make KSB/ Kirloskar f Type of Pump Submersible/ Horizontal Centrifugal 2 CONTINUOUS SAND FILTER PLANT a Type of Filter Type of Filter b Design As per Good Engg. Practice with Sand washer, air lift pump with all internal accessories of the plant as per manufacturer design technology. c Height 5850 mm d Diameter 2550 mm e Normal flow 50m3/hr f Material of Construction of Sand Stainless Steel Washer g Design Pressure Atmospheric h Design Temperature Ambient i Material of Construction Mild Steel Minimum Shell thickness=7mm j Piping IS: 1239 k External Protection Red Oxide Primer & Epoxy Paint l Filter media Natural Quartz Sand as per IS: 8419 (Part-I) with latest amendment if any m Chemical Dosing system for disinfection (non electricity base) Non electricity based water powered dosing pump of DOSATRON 3 FILTER WATER TRANSFER PUMP WITH MOTOR a Quantity 1 No. b Capacity 50m3/hr c Material of construction C.I. d Head 35 mtr e Make KSB / Kirloskar 4 BREAK TANK a Capacity Ltrs. Material of construction Stainless steel Grade SS 304L having minimum thickness 2mm (tolerance on thickness -0.25mm)

6 5 ACTIVATED FILTER MEDIA PLANT a Type of Filter Activated Filter Media plant b Design As per Good Engg. Practice c Height 3200mm d Diameter 1600mm e Normal flow 50m3/hr f Material of Construction Mild Steel Minimum Shell thickness=7mm g External Protection Red Oxide Primer & Epoxy Paint h Inlet Distribution System Bell Mouth i Internal Filter water collection system Stainless Steel Grade SS304 Header & Laterals j Under Bed material Activated Carbon filter media, pebbles etc.

7 COMPARISON OF SLOW SAND FILTER TECHNOLOGY WITH CONTINUOUS SAND FILTER WITH ACF TECHNOLOGY 1. Continuous Sand Filter is an ideal technology which performs even at high turbidity level in raw water, that is, up to 150 ppm and above. Whereas performance of Slow sand filter is affected even at low turbidity level and its rate of Filteration declines over a period of time resulting in effecting its performance. Slow Sand Filter can cater to max NTU Turbidity and can not withstand any high level Turbidity especially during the Rainy season. 2. Space requirement for slow sand filter is quite high in comparison to the space required for Continuous Sand filter With Activated Carbon filter plant technology that is, it requires only 400 sqft of space thus land requirement is saved on account of this plant. 3. Operation of Plant is also a very Vital parameter for the success and sustainability of any Technology. It has been often observed that Slow sand Filteration plants now a days are not performing because of lack of good operators from the Department and casual persons deputed by contractors has made the technology virtually defunct. 4. In water logged area where slow sand filters are not workable to construct, department construct COMPACT TREATMENT PLANT because Overall trend of spring level in some areas is on rising side that is why construction of R.C.C. compact treatment plant has been proposed, because if old traditional type masonry structures are proposed those will have to be abounded on submerging of these in the under ground water (spring level) as it is expected that spring level may rise to the Ground Level in near future and the expenditure incurred on these structures will go waste that is why R.C.C. structure on the ground level have been proposed for long durability and safety. Department experienced that CTP is very costlier then the continuous sand filter plant even if land and maintenance cost to be consider then CSF is anytime economical then the any other available filtration technology. 5. Expenditure of maintenance cost to maintain civil structures are very high whereas in case of continuous sand filter plant will be maintenance free because it is epoxy coated thus no corrosion effect frequently.

8 6. Construction is quite time consuming for slow sand filter but much less time is required for the installation of Continuous Sand filter With Activated Carbon filter plant as modular units available are easy to install. 7. Filtration rate in slow sand filter is very slow whereas filtration rate is quite higher in CSF technology. 8. No back washing is possible in slow sand filter and slow sand filter is to be closed for scrapping of silted top layer in this process heavy man power is required, very time consuming and loss of filter media can not be ruled out. Continuous back washing is part of the operative system in continuous sand filter and plant is not to be closed. 9. Initially water quality of output is quite good in slow Sand filter but gradually quality and quantity of output water is reduced. Whereas, quality and quantity of water filtration in Continuous Sand filter With Activated Carbon filter plant is not affected with time as there is no moving part in the plant and washing of media is continuous. From the above, it is noticed that continuous sand filter is certainly better in comparison to slow sand filter in effective performance.

9 Comparison of Filtration Technologies Employed for Potable Water Supply S.No. Description Slow Sand Filter {7.6.2 of CPHEEO Manual-1999} Rapid Sand Filter {7.6.3 of CPHEEO Manual-1999} Pressure Filter {7.6.6 of CPHEEO Manual-1999} Continuous Sand Filter with ACF Land Area Required High Moderate Low Low 2 Capacity to handle Upto 20 NTU Upto 40 NTU Upto 40 NTU Upto 150 NTU maximum Turbidity (NTU) 3 Requirement of water Not Required Required Required Not Required storage for backwashing 4 Requirement of Pump for Not Required Required Required Not Required Backwash 5 Effect of Manpower High High Moderate Low quality on deliverable quality of water 7 Cost of Operation & High High High Low Maintenance 8 Effect of Backwashing on simultaneous continuation of filtration process Not feasible since technology has limitations Not feasible since technology has limitations Not feasible since technology has limitations Feasible since technology supports (Autowash) 9 Quality consistency of Inconsistent Inconsistent Inconsistent Consistent filtrate 10 Pre-treatment of Raw Not required Required Not required Not required water 11 Ease of Not feasible Not feasible Feasible Feasible Construction/Erection in Water logged areas 12 Uniformity of Filter media Not Required Not required Not required Required 13 Filter media statics Static Static Static Dynamic 14 Susceptibility for a part of High Very High Low NIL Filter media notcontributing to operation 15 Susceptibility for Algae High Very High NIL NIL formation during operation 16 Time required for Erection Months Months 3-4 Months 3-4 Months 17 Filtration process w.r.t. Atmospheric pressure Atmospheric pressure Atmospheric pressure Under High Pressure Atmospheric pressure

10 18 Total Power Consumption Low Moderate High Moderate 19 Manpower requirement for Backwashing & Recommissioning 20 Ease of Componentwise Inspection/ Performance Monitoring/ Repair in O&M 21 Capacity to remove foul smell/odour 22 Susceptibility of Stratification of Filter media leading to issues of deliverable quality 23 Susceptibility of reduction in rate-of-filtration on account of clogging of Filter media 24 Cost for maintenance of Civil Structures/Shells 25 Vulnerability of Backwash water to mix with other portions of sand bed 26 Life Span years subject to optimum periodic maintenance of Civil works 27 Feasibility for Automated Operations High Moderate Moderate Low Not possible Not possible Not possible Possible NIL NIL NIL AVAILABLE Moderate High High Nil High Very High Moderate Nil High High Moderate Low NIL Moderate High NIL years subject to optimum periodic maintenance of Civil works 8-12 years subject to optimum periodic maintenance of Mild steel components years even with little maintenance NIL NIL NIL High