 | Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SKAT, 1996) |
 |  | Part 2: Design, construction and operation of roughing filters |
| | 14. Operation and maintenance of roughing filters |
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14.3 Flow control
A 24-hour continuous filter operation makes maximum use of the installations. Continuous and constant flow conditions usually improve treatment plant performance and reduce the required structure size. However, gravity flow is usually necessary for such ideal situations. Continuous flow might not be possible in water supply schemes where the raw water has to be pumped. When pumping is required, the treatment plant might be staffed for 8 or 16 hours a day, depending on whether one or two shifts are available.
Intermittent slow sand filter operation is not recommended for quality reasons. In order not to affect the biological activities in the slow sand filter, this filter can be operated at a declining filtration rate in pumped systems during the unstaffed period of the day. In practice, the stock of supernatant water is drained through the filter at a continuously declining flow rate during the night and in the morning hours, the filter is refilled with pretreated water to reassume normal operation. Such an operation calls for special provisions as pretreated water for the slow sand filter has to be provided at higher pumping rates.
Roughing filters are mainly physical filters. They are less affected by flow interruptions as they do not depend on a continuous supply of nutrients as biological filters. Hence, intermittent operation can be applied without causing a significant deterioration of the prefiltrate, provided smooth restarting of filter operation is observed. Declining filter operation rate of roughing filters to supply slow sand filters with a constant flow is not advisable due to the relatively small water volume stored in the prefilters. The most favourable option in a pumped scheme is the provision of a raw water balancing tank which allows continuous filter operation. Removal of the coarse solids is a positive side effect of such a tank. The different tank volumes required for a 100 m³/d plant are illustrated in Fig. 50.
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A Goat and a Bag of Sweet Potatoes Ngondzen was the fourth water supply system I visited on that day and Ndzenshwai was the last scheme before reaching the hotel in Kumbo, a small district town in West Africa. Tired from the heat and dust, I really looked forward to the evening shower and probably also the entire crew in the four-wheel drive project car. As we entered Nongdzen at rather high speed, the project coordinator suggested to see the caretaker of the treatment plant at once in order not to lose too much fume. Neither Mr Boniface, the local technician, nor I had any objections to his suggestion. As we crossed the junction next to the church we saw a large crowd of people. We assumed that a wedding was being held and drove on to the caretaker's house. However, the chairman of the water committee ran after our car and tried desperately to attract our attention. This is when our schedule got mixed up. The crowd in front of the church was not celebrating a wedding but had been waiting for us for three hours. We reversed the car and drove it to the church, where a local group of drummers started pounding on their instruments as we were directed to chairs in front of the community hall. The welcoming address was well-prepared, typed on a piece of paper bearing three official stamps, and read by Mr Patrick, the chairman of the water committee. Two dancing groups performed traditional dances after his speech. Thereafter, a colourful cap was placed on my head and I was nominated chief of the village and loudly applauded by the entire community. While another dancing group was starting its performance, a traditional gift commonly donated to chiefs was conferred on me - a goat! Headed by Mrs Dominica, chairperson of the women's group, a cozen women started to dance and deposit a bag filled with sweet potatoes in front of my feet. Overwhelmed by so much of honour, I addressed a word of thanks to the villagers and stressed the importance of maintenance in a water supply scheme. Mr Boniface, the local technician, was sitting quietly next to me during the entire ceremony. We were then served local dishes and brews in the community hall and more toasts were exchanged. It was late afternoon as we climbed into our project car, the goat attached onto the roof rack and the bag of sweet potatoes loaded in the rear. After cheerfully waving goodbye to the villagers, we left Ngondzen. However, the new chief who was sitting next to Mr Boniface felt embarrassed because he had won, during the two-hour celebration, all the praise for the efforts made by the local technician to mobilise the community, to organise construction material and to supervise this self-help project - a work which had kept him busy for the last two years. We looked at each other without exchanging a word but the message was clear. The goat and the bag of sweet potatoes were unloaded in
front of his home before we drove on to our hotel under a shining
moon. |
The flow through roughing filters is controlled by a flow control device at the inlet and by a fixed weir at the outlet as illustrated and recommended in Fig. 32 on page IX-7. This is also true for the recommended inlet controlled slow sand filters which compensate the progressive headloss development by a gradual increase of the supernatant water level. For gravity schemes, constant feeding is maintained by a more or less fixed position of the valve in the supply pipe and a subsequent overflow in the distributor box. For pumped schemes with a raw water tank, the flow to the treatment plant is regulated by a mechanical flow rate device as shown in Fig. 33. These two main possibilities are illustrated in Fig. 21 on page Vl-9.
V-notch weirs are generally used for discharge measurements. Permanently installed V-notch weirs or transportable equipment used for flow control are described in Annex 2. The flow rate through each filter should be routinely controlled once a day if V-notch weirs are installed, and with transportable equipment at least twice a week according to the monitoring programme outlined in Annex 8.
Filter resistance in roughing filters is minimal and hardly reduces the flow through the filter. The headloss increases to a few centimetres in well-operated roughing filters. Filter resistance builds up along the entire filter bed as roughing filters act as space filters. It is reduced to its initial value by efficient and regular filter flushing. However, intake and dynamic filters can build up considerable filter resistance as they mainly act as surface filters. Since the headloss in intake filters can increase to 20 - 30 cm within a week [48], the flow through the filter has to be adjusted by gradual opening of the valve located in the effluent pipe. Dynamic filters have to produce by definition a high filter resistance within a short time during periods of high raw water turbidity. This will clog the filter bed and prevent highly turbid raw water from flowing to the subsequent filters. Intake filters are usually cleaned once a week, and dynamic filters after every high turbidity peak.
Filter resistance can easily be determined by measuring the level of the free water tables in the inlet and outlet chamber of roughing filters. The effluent's weir crest level can be used as reference (0-level). Gauging rods fixed to the walls of these two chambers will facilitate the respective measurements.
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