![Lecture 30: Slow Sand Filters for Small Canal or Surface Water 8
SPM NIWAS
Slow Sand Filter (SSF)
Slow sand filtration is a simple and reliable process in water treatment in which untreated water is percolated slowly
through a porous sand bed and then collected at the bottom drain. They are relatively inexpensive to build, but do
require highly skilled operators.
➢ diarrheal disease are projected to increase in east, south, and Southeast Asia due to projected changes in
the hydrological cycle associated with climate change [EPA ,US (Feb'12)].](https://image.slidesharecdn.com/pptsession30-250730175120-07a64822/85/Management-of-Water-Treatment-Systems-pdf-8-320.jpg)
![Lecture 30: Slow Sand Filters for Small Canal or Surface Water 15
SPM NIWAS
Typical Slow Sand Filter
[Ref. SSWM University Course]](https://image.slidesharecdn.com/pptsession30-250730175120-07a64822/85/Management-of-Water-Treatment-Systems-pdf-15-320.jpg)
![Lecture 30: Slow Sand Filters for Small Canal or Surface Water 16
SPM NIWAS
Slow Sand Filter
Section
[Ref. SSWM University Course]](https://image.slidesharecdn.com/pptsession30-250730175120-07a64822/85/Management-of-Water-Treatment-Systems-pdf-16-320.jpg)
Management of Water Treatment Systems.pdf
- 1. Dr. Syama Prasad Mookerjee - National Institute of Water & Sanitation, Ministry of Jal Shakti, Govt. of India Module 3 : Management of Water Treatment Systems 30: Slow Sand Filters for Small Canal or Surface Water Certificate Course on Operation & Management of Rural Water Supply Schemes
- 2. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 2 SPM NIWAS Details of Resource Person Dr. Asit Aich, Scientist-C at Dr. Shyama Prasad Mookerjee National Institute of Water & Sanitation, has 37 years of expertise in Environmental Engineering and Waste Management, with a research focus on Bio-methanation. He holds a Ph.D. in Engineering, an MBA in Finance, and degrees in Civil and Environmental Engineering. His contributions include 4 international journal papers, 3 book chapters, and 16 conference papers, showcasing his dedication to sustainable water and sanitation solutions. Scientist C, SPM NIWAS Dr. Asit Aich
- 3. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 3 SPM NIWAS Classification of Filter ❑ ❑Water filtration is the process of removing or reducing the concentration of particulate matter, including suspended particles, parasites, bacteria, algae, viruses, and fungi, as well as other undesirable chemical and biological contaminants from contaminated water to produce safe and clean water. Generally Sand and gravel filters are used for water purification in rural areas. Filtration is an important step in water treatment process for purification ❑ Filters that are used in the water treatment may be classified as: 1. Gravity filters 2. Pressure filters Filter Horizontal Pressure Filter Gravity Filter Pressure Filter Vertical Pressure Filter Slow Sand ( Biological Filter) Rapid Sand (Mechanical Filter)
- 4. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 4 SPM NIWAS Types of Sand Water Filters ❑Types of Sand Filtration Plants generally used extensively in rural water treatment are: ⬥Slow Sand Filter Plant ⬥Rapid Sand Filter Plant ⬥Other types of Filter Plants, which are not used commonly, are: • Pressure filters-used as small treatment plant in Industries. • Roughing filters-may be used to reduce load on the treatment plants. Small streams of water in the catchment areas may carry large particles and floating matter which can entrap such undesirable material prior to the storage structures of the treatment units. Sand filters should not be confused with Roughing Filters. Horizontal-flow gravel filters used as a roughing treatment for turbid raw waters prior to sand filtration.
- 5. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 5 SPM NIWAS Types of Sand Water Filters ❑Types of Sand Filtration Plants generally used extensively in rural water treatment are: ⬥Slow Sand Filter Plant ⬥Rapid Sand Filter Plant ⬥Other types of Filter Plants, which are not used commonly, are: • Pressure filters-used as small treatment plant in Industries. • Roughing filters-may be used to reduce load on the treatment plants. Small streams of water in the catchment areas may carry large particles and floating matter which can entrap such undesirable material prior to the storage structures of the treatment units. Sand filters should not be confused with Roughing Filters. Horizontal-flow gravel filters used as a roughing treatment for turbid raw waters prior to sand filtration.
- 6. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 6 SPM NIWAS Roughing Water Filters Roughing filter is primarily used as a pre-treatment step in water treatment to remove large suspended solids from raw water before further filtration. Roughening filters may be horizontal or vertical Upward flow sand filters Upflow roughening filter is a rapid gravity filter and, as it relies on the force of gravity to move water upwards through a coarse filter media, allowing solids to settle and be captured within the filter media allowing for a relatively fast filtration rate; the filtered water then exits at the top, without the need for additional pressure from a pump and the filtrate is removed from the bottom. Not all rapid filters are necessarily upward flow designs, as some operate with a downward flow. Horizontal flow sand filters Horizontal flow filter is not a rapid sand filter. A horizontal flow filter is a roughing filter that is used before sand filtration, while a rapid sand filter is a type of sand filter used for water treatment.
- 7. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 7 SPM NIWAS Slow Sand Water Filter The most widely used sand filtration in rural water supply schemes which is effective, low cost system of water treatment if operated and managed correctly. Slow Sand Water Filter
- 8. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 8 SPM NIWAS Slow Sand Filter (SSF) Slow sand filtration is a simple and reliable process in water treatment in which untreated water is percolated slowly through a porous sand bed and then collected at the bottom drain. They are relatively inexpensive to build, but do require highly skilled operators. ➢ diarrheal disease are projected to increase in east, south, and Southeast Asia due to projected changes in the hydrological cycle associated with climate change [EPA ,US (Feb'12)].
- 9. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 9 SPM NIWAS Slow Sand Water Treatment Process Screen Sediment ation Pre Chlorination (Optional) Distribution Filtration Source Raw Water Storage ( Clear Water Reservoir) Post Chlorination
- 10. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 10 SPM NIWAS Working Principles of Slow Sand Filters ❖ In slow sand filter contaminants from water are removed through a combination of physical straining and microbiological processes in a shallow, low-surface-loading bed of fine sand. Used primarily for surface water treatment. ❖ Slow sand filters differ from all other filters in that they work by using a complex biofilm that grows naturally on the surface of the sand. ❖ Slow sand filters work through the formation of a gelatinous layer or biofilm called the hypogel layer or Scmutzdecke in the top few mm. of the fine sand layer. ❖ ■ The Schmutzdecke is formed over the filter bed in the first 10–20 days of operation and consists of bacteria, fungi, protoxoa, rotifer and a range of aquatic insect larvae that actually removes the organic matters and a large variety of microorganism. ■ This biological layer (also called dirty skin) of microorganisms, is a dense biofilm that traps and removes foreign particles from water as it passes through the filter.
- 11. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 11 SPM NIWAS Working Principles of Slow Sand Filters ❖ As an epigeal biofilm ages, more algae tend to develop and larger aquatic organisms may be present including some bryozoa snails and Annelied worms. ❖ The surface biofilm is the layer that provides the effective purification in potable water treatment, the underlying sand providing the support medium for this biological treatment layer. As water passes through the hypogeal layer, particles of foreign matter are trapped in the mucilaginous matrix and soluble organic material is adsorbed. The contaminants are metabolised by the bacteria, fungi and protozoa.
- 12. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 12 SPM NIWAS Working of Slow Sand Filters (SSF) ❖ The Schmutzdecke layer in slow sand filters (SSFs) is essential for pathogen removal, however, this layer is also responsible for increased head loss. Slow sand filters generally can : ■ Reduce water turbidity to less than 1 NTU ■ Remove total organic content by 10% ■ Remove 95% of coliform bacteria ■ Improve water's color, taste, and odor, good for ammonia and manganese removal. SSF is not suitable for color removal. SSF do not produce significant sludge.
- 13. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 13 SPM NIWAS Slow Sand Filters ⬥ Shape : Rectangular or cylinder in cross section ( Concrete or masonry) ⬥ Depth: 1–3 m ▪ Bed depth of 0.3 to 0.6. comprising 0.2 to 0.4 mm sand rest stone. ▪ The length and breadth of the tanks are determined by the flow rate desired for the filters. - Typically have a loading rate of 200–400 litres (0.20–0.40 m3 ) per sq. m./ hour. throughput is 0.25 m/hr. The water produced from a slow sand filter is of excellent quality may be about 90–99% bacterial cell count reduction. ⬥ The length and breadth of the tanks are determined by the flow rate desired for the filters, which typically have a loading rate of 200–400 litres (0.20–0.40 m3 ) per sq. m. per hour. Generally two slow sand filters are provided in parallel, one filter unit is kept in operation and other for maintenance.
- 14. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 14 SPM NIWAS Slow Sand Filter bed ❑ Typical Slow Sand Filter bed is a series of (Top to bottom) : Layer 1 - A thick layer of fine sand (0.15 to 0.25 mm at Top) followed by Layer 2 - Layers of sand are placed on top followed by (0.25 to 0.40 mm) Layer 2 - A cover of coarse gravel (2.0 to 4.0mm) Layer 3 - that are covered with a layer of pebbles (4.0 to 6mm) Layer 4 – Herringbone drains at bottom (10 to 25mm) The whole depth of filter material may be 0.60 to 1.2 m. in depth, the majority of which will be fine sand material. On top of the sand bed sits a supernatant layer of unpurified water.
- 15. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 15 SPM NIWAS Typical Slow Sand Filter [Ref. SSWM University Course]
- 16. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 16 SPM NIWAS Slow Sand Filter Section [Ref. SSWM University Course]
- 17. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 17 SPM NIWAS O&M of Slow Sand Filter ⬥ Slow sand filters slowly lose their performance as the biofilm thickens and thereby reduces the rate of flow through the filter. So, it is necessary to refurbish the filter. Two methods are commonly used to do this 1. The top few mm. of fine sand is scraped off to expose a new layer of clean sand. Water is then decanted back into the filter and re-circulated for a few hours to allow a new biofilm to develop. The filter is then filled to full volume and brought back into service. 2. The second method, sometimes called, involves lowering the water level to just above the hypogeal layer (called wet harrowing), stirring the sand; thus precipitating any solids held in that layer and allowing the remaining water to wash through the sand. The filter column is then filled to full capacity and brought back into service. Wet harrowing can allow the filter to be brought back into service more quickly.
- 18. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 18 SPM NIWAS O&M Activities for Slow Sand Filter ❑ O & M Activities for Slow Sand Filter ⬥ a) Daily activities ⬥ Check the rate of filtration on the flow indicator – adjust the rate of filtration as needed by turning the filtered water valve ⬥ Check the water level in the filter – adjust the inlet vale as needed to maintain a constant water level ⬥ Remove scum and floating material by further opening the inlet valve for short time ⬥ Check the water level in the clear well ⬥ Sample and check water turbidity – if the inflow turbidity is too high close the intake; if the outflow turbidity is too high report to the supervisor ⬥ Testing water quality ⬥ Complete the log book ⬥ Testing Water Quality: Daily monitoring of water quality may be done whether it is slow sand filter or rapid sand filter. If the water supply scheme is having laboratory at the water treatment plant site, water quality testing both the raw water and treated water may be carried out daily.
- 19. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 19 SPM NIWAS O&M Activities for Slow Sand Filter ❑ Monthly Activities ■ Shut down the filter unit – remove scum and floating material; brush the filter walls; close the inlet, filtered water and distribution valves; drain water to 20 cm below the sand level; increase the filtration rate in the other filter to 0.2 m/h. ■ Clean the drained down filter bed – wash boots and equipment before use; scrape upper 2-3 cm in narrow strips and remove scrapings from filter; check, and service, exposed inlet and drain valves; remove cleaning equipment and level sand surface; check and record depth of sand bed; adjust inlet box to the new sand level. ■ Re-start the filter – open the recharge valve; check sand surface and level as needed; when water is 20 cm above the sand, open the inlet valve; open the filtered water valve and stop when filtration rate reaches 0.02 m/h; open waste valve for outflow water to flow to waste; open filtered water valve to increase filtration rate every hour by 0.02 m/h until a rate of 0.1 m/h is reached; adjust and check flow daily until safe to drink; close waste valve and open distribution valve to pass filtered water into the supply; decrease filtration rate of other filter to 0.1 m/h. ■ Wash the filter scrapings and store the clean sand.
- 20. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 20 SPM NIWAS O&M Activities for Slow Sand Filter ⬥ Annual Activities ■ Check if filter is water tight: close all valves and fill filter box from inlet valve until it overflows – close valve; leave for 24 hours and check if water level reduces; if filter box leaks, report for repair; open filtered water valve to fill outlet chamber and when full, close valve; leave for 24 hours and check if water level reduces; if chamber leaks, report for repair; open drain valve to empty filter; clean the clear well in the outlet chamber; restart filter as per the month clean. ⬥ Every two years, activities ■ Re-sand the filter units – clean the filter as in a monthly filter clean; open drain valve to empty water from the sand bed; remove strip of old sand to one side; place new clean sand on top of exposed gravel, and level; place old sand on top of the new sand to the correct depth of 0.8 m in total, and level the surface; continue in strips until filter is resanded; adjust inlet box to new sand level. Re-start the filter as per the monthly clean plan. ⬥ Random checks ■ Checks on the functioning of the plant by supervising staff including turbidity tests through a turbidity meter, and bacteriological tests of the filtered water.
- 21. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 21 SPM NIWAS Record keeping of Slow Sand Filter Operation ❑ Records have to be kept for the following activities. ■ Daily Source water quality ■ Daily Treated water quality ■ Names of chemicals used ■ Rates of feedings of chemicals ■ Daily consumption of chemical and quality of water treated ■ Dates of cleaning of filter feds, sedimentation tank and clear water reservoir ■ The date and hour of return to full service (end of re-ripening period) ■ Raw and filtered water levels (measured each day at the same hour) and daily loss of head. ■ The filtration rate, the hourly variations, if any. ■ The quality of raw water in physical terms (turbidity, colour) and bacteriological terms (total bacterial count, E.Coli.) determined by samples taken each day at the same hour. ■ The same quality factors of the filtered water. ■ Any incidents occurring e.g. plankton development, rising Schmutzdecke, and unusual weather conditions.
- 22. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 22 SPM NIWAS Slow sand filter Standard Slow sand filter design Depth of water over sand bed Shape: 1–2 m. Rectangular or cylindrical in cross section Depth of Filter media Sand size 0.6 to 1.2 m. 0.15–0.40 mm Gravel layer thickness 300 mm of 10–25 mm gravel Filtration rate 0.1–0.3 m/h Filter run time 50 days
- 23. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 23 SPM NIWAS Advantages of Slow Sand Filter ❑ Advantage of Slow Sand Filter ⬥ SSFs improve water quality by removing suspended particles, dissolved organic matters, and other contaminants, thus reducing turbidity and associated taste and odor problems. ⬥ Suspended organic materials in water are filtered by the sand and are then slowly mineralized by microorganisms in the filter's biofilms. ⬥ Slow sand filters are a simple, reliable and easy to operate system. ⬥ Slow sand filters are operated without the use of coagulation. ⬥ Slow sand filters do not require highly trained operators, have minimal power requirements, and can tolerate reasonable hydraulic and solids shock loading. ⬥ Slow sand filters now are also being tested for pathogen control of nutrient solutions in hydroponic systems. ⬥ Appropriate technology for poor and isolated areas.
- 24. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 24 SPM NIWAS Advantages of Slow Sand Filter ❑ Advantages of slow sand filters (Contd.) ▪ Require little or no mechanical power, chemicals or replaceable parts, and they require minimal operator training and only periodic maintenance. ▪ Slow sand filters are recognized by the WHO as being a superior technology for the treatment of surface water sources in small water systems. According to the WHO "Under suitable circumstances, slow sand filtration may be not only the cheapest and simplest but also the most efficient method of water treatment.
- 25. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 25 SPM NIWAS Disadvantages of Slow Sand Filter ❑ Disadvantages of slow sand filters ▪ Large land is required ▪ Slow sand filters operate efficiently relatively low turbidity levels. In summer conditions with high microbial activity and in conditions when the raw water is turbid, blinding of the filters due to bio-clogging occurs more quickly by excessive amounts of algae and pre-treatment is required. Not very effective at removing color ▪ Intermittent operation of the filters may degrade the quality of the filter effluent by promoting anaerobic conditions within the filter bed. ▪ Filters requires periodical cleaning by scraping off a thin layer of sand from the surface of the filter bed. ▪ Filtration rate for slow sand filters is typically 50–100 times slower than that of granular media filters. ▪ Not very effective against all viruses and pathogens. ⬥ Unlike other water filtration technologies that produce water on demand, slow sand filters produce water at a slow, constant flow rate and are usually used in conjunction with a storage tank for peak usage. This slow rate is necessary for healthy development of the biological processes in the filter.
- 26. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 26 SPM NIWAS Design of Slow Sand Filter ❑ Design of slow sand filter ( Example) Problem: Design a SSF of population 7500, per capita supply of water @ 55lpcd. Rate of filtration 300 lit/hr/m2 . Assume max. demand 1.5 times of average demand. Sol. Average demand = 7500 x 55 = 412500 lit/day Max Demand = 1.5 x 412500 = 618750 lit/day = 619 m3 /day Rate of filtration 300 lit/hr/m2 = 7200 lit/day = 7.2 m3 /day So, filter area required = 618750/7200 = 86 sq. m say (say) Say 4 nos. filter to be used of which one will be kept as stand by so surface area required for each filter = 86/3 = 28.7 sq. m let us provide L = 2.5 B where L = length of the filter tank, B = Width of the filter tank So, 2.5 B2 = 28.7, B2 = 11.5 B= 3.4 m., L = 8.5 m , So area of each filter A = 28.9 m2 So provide 4 filters , each size 8.5 X 3.4 m . Of which 1 will be kept as standby. Area of 3 filters = 86.7 m2 Now total hourly demand = 618750/24 = 25782 lit/hr Depth of water in the filter = 25782/(300 x 86.7) = 1.0 m (say), Assuming free board 500 mm & 900 mm depth of media Depth of filter tank 2.40 m. So the tank dimension 8.5 x 3.4 x 2.4 m (LxBxH), 4 no. filter beds. v
- 27. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 27 SPM NIWAS Case Study ❑ Very old Slow Sand Filter in a Town in India still functioning ⬥ Uttarpara a rich cultural heritage municipal town in West Bengal, India, constructed a slow sand filter long back in the year 1895 after facing severe water quality issues due to contamination from muddy runoff of the primary source of drinking water, nearby river the Ganga , which often contained high levels of turbidity, bacteria, and organic matter. This led to frequent outbreaks of waterborne diseases such as diarrhea and typhoid. ⬥ To address the issue, a Slow Sand Filtration (SSF) system was introduced in the year 1895 as a cost-effective water treatment solution. ⬥ Design and Construction ⬥ Filter Bed: Constructed using locally available sand and gravel. ⬥ Schmutzdecke Formation: Allowed biological layer development over 3-4 weeks. ⬥ Flow Rate: Maintained at 0.2–0.5 m³/m²/hr for effective filtration. ⬥ Capacity: Designed to serve 1200 households, providing 100 liters per household per day.
- 28. Lecture 30: Slow Sand Filters for Small Canal or Surface Water 28 SPM NIWAS Case Study (contd.) ⬥ Operational Benefits ⬥ Water Quality Improvement: ■ 99% reduction in bacteria and pathogens (E. coli, cholera). ■ Reduced turbidity from 30 NTU to <1 NTU. ■ Improved taste and odor due to removal of organic matter. ⬥ Sustainability: ■ Low maintenance cost with minimal electricity use. ■ Community-led operation and monitoring. ■ No need for chemicals like chlorine. ⬥ Health Impact: ■ Waterborne diseases decreased by 80% within six months. ⬥ Challenges ⬥ Seasonal variation in water quality required adjustments in cleaning schedules. ⬥ Involvement of local women groups ensured regular maintenance.
- 29. SPM NIWAS 29 Thank you