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| 1. Greenfield or new water treatment plants based on Tube Settler technology / clarifiers read more... |
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| 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
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| Research Paper & Presentation |
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| Hyderabad , Andhra Pradesh, INDIA |
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| Augmentation of existing water treatment plants |
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| Introduction : |
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In the year 2000, the plant was already over drawing by about 30 mld , and was being operated under overloading condition at 180 mld. Since the raw water turbidity of the source is low (less than 20 NTU through out the year) the overloading was possible. However the filters were facing problems due to shortened length of service cycles. Therefore it was not possible to take out any one filter for maintenance purpose. Even isolating one filter bed for backwash created problems. In the year 2002, it was decided to convert four out of twelve beds to high rate filters to treat additional flow of 30 to 40 mld so as not to affect the performance.
The existing filter battery was designed as variable declining rate filter operation. The beds were mono grade (deep bed) sand filters. The size of each bed was 10.40m x 9.70m and was catering to 15.0 mld flow. Each bed was further divided in to two sections of 10.40m x 4.20m each. Each bed had a central wash-water gullet separating the two sections. The average rate of filtration was 6000 lph/sqm. However the manifold pit of filter underdrain system was common to both the sections. The side water depth in the filter bed was 4.0m. The underdrain was of lateral-manifold type. The total height of the media was 1.20m (0.80m sand and 0.40m of graded gravel). The height of the central gullet was 1.40m from bottom of the filter. For surface wash, a side gutter was provided, so that when operated with the backwash, it pushed the water towards central gullet. The beds were provided with air-scour and hard wash as independent operations. The capacity of backwash tank was 450 cum and original rate of backwash was 500 lpm/sqm. Each filter had one inlet gate, one wash-water disposal gate, one filtered water outlet valve and one wash-water inlet valve.
There was an existing problem of excessive air release towards inlet side of filter bed at beginning of the air scour. This was due to the fact that air piping was connected at one end of manifold pit and there was not much space in the manifold for pressure equalisation. Otherwise the performance of existing filter battery was satisfactory. It was decided to convert four beds in to dual media filters with an average rate of filtration of 12,000 lph/sqm, each catering to 30 mld flow.
The conversion of filter bed demanded following modifications. The main modifications and the reasons are listed below in brief.
1. It was required to divide (physically separate) the filter bed in two sections for purpose of backwash since there was limitation of size of wash-water header (dia.600 mm) and wash-water tank. This was done by opening the top slab of manifold pit and by constructing concrete plugging blocks at two ends. The modifications ensured separation of sections for the backwash purpose. This ensured the backwash rate of 900-1000 lpm/sqm for one section. The capacity of backwash tank was insufficient for washing of both the sections for duration of 10 min wash. Hence high rate pumps were installed to fill the backwash tank in 10-15 minutes.
2. Two separate sections necessitated two separate outlet valves (diam. 350mm each) and two separate washwater inlet valves (one for each section, dia. 600mm each). This was done by making the opening ( diam. 600mm) in the side wall of filter bed. The additional valves and piping were integrated with the existing piping, air and wash header.
3. A new Filter outlet chamber with a weir was required to be constructed in the pipe gallery to receive the water from one of the newly created filter sections. The chamber further conveyed the filter water to common collection channel. It was necessary to raise the level of filter outlet weir in the existing chamber so as to avoid the possibility of de-watering of bed.
4. The existing laterals were of asbestos cement pipe (dia.80mm). They were connected to manifold pit by a specially casted concrete �Tee�. All the laterals were replaced with rigid PVC pipes (6 kg/sqcm) along with the concrete tees. The perforations to the laterals were provided as 0.5% of bed area, as against 0.35% in the original design.
5. Air header pipe of 150mm diam. was extended along the length of manifold pit from the top. Each lateral (concrete tee) was connected to air distributor pipe through a nozzle. The nozzle size was restricted to create pressure drop to ensure uniform distribution of air throughout the bed.
6. The central gullet was extended from original height of 1.40m to 2.10m to accommodate the dual media configuration and the space for media expansion. The graded gravel of size 2-50 mm was provided at bottom up to height 0.60m. Quartzite sand (E.S. 0.50mm, U.C.1.5), fine media, consisted of depth 0.35m. The coarse layer consisted of crushed coconut shell media (substitute to anthracite coal) of size 1-2mm. The depth of this layer was 0.40m. The top of central gutter was 0.75m above top of the media.
7. The dual media filters necessitated incorporation of cross-troughs. The side wash gutter, which was in the original design, was dismantled as it was projecting above the media inside the bed. For each section, five numbers of carbon steel cross troughs were provided at a uniform spacing.
8. Fortunately no changes were required to filter inlet gate and washwater disposal gate as they were of adequate size even for the augmented flow.
The modifications to the filter bed were completed in three months. The newly converted bed was put in to operation in November 2002. The performance was found to be satisfactory with filtrate quality of 0.50 to 0.80 NTU. The service cycles were registered in the range of 48 to 72 hr. Alum was used as a coagulant and no filter aid was used.
The cost of conversion of one bed was INR 2.25 million (0.0450 million US dollars). The modifications were extensive and exhaustive. As it was difficult to construct and integrate new filter beds at site, there was no alternative but to modify the existing beds. The modifications were done only to two beds, as it was not possible to draw more water through the gravity raw water main
 Fig - 1, Fig - 2 |
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| Acknowledgements : |
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| 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. |
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