| Surface water treatment by roughing filters - A design, Construction and Operation manual (1996) |
|Part 1: General aspects of roughing filter application|
" In the earliest days of the human race, water was taken as found. It might be pure and abundant, plentiful but muddy, scarce but good, or both scarce and bad. To get more or better water, man moved to other sources rather than transport better water to his own location or to try to improve the quality of water at hand". This cited text marks the beginning of Baker's epilogue in "The Quest for Pure Water" , a reference book he started compiling at the beginning of this century and which was finalised in the 1940s. Baker continues by saying " Man's earliest standards of quality were few: freedom from mud, taste and odour". However, an increase in man-made water pollution, the development of technical and public health science, as well as the consumers' greater need for clean water contributed to the development of the water purification technology.
At the beginning of the 19th century, the first water treatment plants for public water supplies were constructed in Britain and France. They generally comprised settling basins followed by gravel and sand filters. In the course of time, slow sand filters were developed as an efficient water treatment process, and used by many water authorities at the end of last century. By this time however, the Industrial Revolution came up with the "mechanical" filters as rapid sand filters were initially called. The growing water demand and the subsequent discovery of chlorine to disinfect the water enhanced the use of rapid sand filters. In 1940, there were about 2,275 rapid filter plants in the United States as opposed to about 100 slow sand filter plants. Another outstanding feature with regard to the water treatment technology was the use of aluminium and iron salts as coagulants in water treatment. Since the beginning of this century, coagulation and flocculation combined with sedimentation, rapid filtration, and final chlorination are now commonly used in water treatment. This treatment combination is now usually regarded as conventional.
Water treatment plants are either built in situ, usually as reinforced concrete structures, or installed as package plants manufactured by the water industry. Fig. 1 illustrates the extensive use of chemicals in conventional water treatment. Colloidal matter has to be destabilised by coagulants, such as aluminium sulphate or ferrous sulphate, possibly in combination with lime dosage for pH adjustment and polymers or polyelectrolytes to improve flocculation. As rapid filters do not significantly improve the microbiological water quality, chlorine has to be used as final treatment step to produce water which is safe for consumption. Finally, the numerous chemicals added may also have changed the chemical water characteristics. The treated water, which may either be corrosive or deposit-forming, could greatly harm the distribution system. Consequently, the treated water often has to be stabilised with a final dose of lime.
Conventional water treatment also requires a substantial input of energy and mechanical equipment. Frequently, the raw water has to be pumped through the different treatment stages. Flocculation requires energy inputs for hydraulic or mechanical mixing, sludge removal in sedimentation tanks is often carried out with mechanical scrapers, and rapid sand filters are backwashed for filter cleaning. Dosing pumps are necessary for adequate chemical application. In brief, conventional treatment calls for an extensive use of power-driven, mechanical and often sophisticated equipment.
A reliable and efficient operation of a conventional water treatment plant is a demanding task. A continuous supply of different chemicals must be guaranteed, spare parts of mechanical equipment must be stocked or easily available, and the treatment plant operated by well-trained and skilled personnel. The local infrastructure should support maintenance and repair of treatment plant components. However, these criteria are hardly ever met by local conditions prevailing in rural areas of developing countries.
Wagner states in the preface of the manual "Upgrading Water Treatment Plants" , which is the result of a WHO working group on operation and maintenance established in the 1990s: "In the majority of plants, especially in the less developed countries, much of the expensive equipment does not operate properly due to lack of understanding or disregard of maintenance and operation recommendations". Only a few plants are designed on the basis of bench and pilot plant testing. The need for careful design is most urgent in countries with the least resources. However, design studies are in fact considered a luxury rather than a necessity in these countries. The most widely encountered deficiency in water treatment is the application of coagulants to raw water. Incorrect dilution of the solution, inadequate doses and inappropriate dosing are the most common mistakes. Difficulties are also experienced with the flocculation step. Uncontrolled energy inputs result in small floes of low settleability. Sedimentation tanks are often not well-designed; short circuiting and incorrect water abstraction lead to poor clarification and overloading of the subsequent filters. These in turn cannot be backwashed properly and produce filtered water of high turbidity. Finally, poorly or inoperative chlorination equipment is commonplace in rural water treatment plants in developing countries, as the equipment usually originates from industrialised countries and, hence, foreign exchange is required to purchase these installations and spares. The described difficulties encountered with conventional water treatment will result in the production of water of erratic quality which is often unsafe for consumption.
Objectiveness demands that earlier experienced operational difficulties with slow sand filters have to be mentioned at this point. Initially, slow sand filters were developed to combat the cholera and typhus epidemics in Europe last century. On account of its simplicity and low-cost, the slow sand filter concept was then indiscriminately exported to developing countries in the early days of technical cooperation. Slow sand filters operate perfectly well with raw water of low turbidity as generally encountered in European surface waters. However, raw water quality in tropical climates can vary considerably, especially as regards turbidity and solid matter load. Therefore, this direct transfer of technology has proved inadequate. The inability of slow sand filters to sustain adequate filter runs when subject to high turbidity loads became obvious. Worldwide practical experience revealed that the slow sand filter design concept was often misunderstood, the use of pretreatment processes, such as plain sedimentation or flocculation and sedimentation, were either inefficient or unreliable as well as inappropriate, and that operation and maintenance deficiencies contributed to the poor performance of slow sand filters. In the early 1960s in Brazil, for example, the communities were not adequately trained in slow sand filter operation, thus causing a high failure rate of the slow sand filters . In Cameroon on the other hand, slow sand filters were operated adequately twenty years ago. However, due to the raw water quality deterioration caused by progressive deforestation of the catchment areas, these filters faced increasing operational difficulties which required treatment plant rehabilitation . Finally, an evaluation of the performance of four slow sand filter plants carried out in India in 1993 revealed that its current design, construction and operation, including source protection, is far from being satisfactory and often leads to poor filter performance .
Successful projects call for a multidisciplinary approach requiring various types of inputs. Sociocultural, institutional, and natural conditions must be considered along with financial and technical aspects. The synthesis of all these inputs lead to appropriate and sustainable solutions. This manual focuses mainly on technical aspects and gives answers to perhaps the least difficult problems. From the technical viewpoint, development of the roughing filter technology has contributed towards an efficient and reliable slow sand filter operation appropriate for rural water supply schemes in developing countries.
A Blue "White Elephant"
A Blue "White Elephant"
William, the driver of the project car, and I were heading north of the capital of a country in West Africa. The midday sun was beating down on the paved road, the air was vibrating and I felt drowsy from the heat. We were nearing Ndikinimeki, a small administrative centre of the province. Suddenly, I spotted a few dark blue dots in a banana plantation about 150 m off the main road. I ordered William to stop the car at once, which he did some 30 m further on. We drove back to a small path leading to the plantation where some people were waiting for transport to Ndikinimeki I asked them to see the treatment plant manager but he was in town.
The first 100 m we drove to the treatment plant, but had to leave the car next to a bridge and walk the last 50 m. The main gate was locked, so we climbed over it and stood on a plot which had originally been the treatment plant premises but had now been partly converted into farmland. Nice banana trees were growing on the fertile and humid soil located along the river.
The treatment plant consisted of about seven large ship containers standing on small concrete foundation blocks. All the containers were painted blue and had large doors. We climbed on one of the containers to study the treatment scheme. The plant was apparently designed as conventional treatment scheme comprising prechlorination, aeration, coagulation, pH correction, flocculation, sedimentation, rapid sand filtration, pH control, and disinfection. However, wafer was not flowing through the different treatment stages. Only the cascades and the sedimentation tanks were partly filled with rain water which had collected during the wet season and had ended a few weeks ago.
We rejoined the ground and tried to fight our way through the vegetation and pipe fittings scattered all over the plot. We managed to reach one of the side doors which we opened and were horror-stricken by what we saw. Corroded dosing pumps were still on the containers, some were falling to pieces, electric cables from the switchboard were hanging loosely from wall to wan and, in the far end of the container, we discovered a pair of sandals focally called flip flop. As we opened another container used as storeroom for the chemicals, a few lizards disappeared through corroded holes and two meagre bags of alum sulphate were lying in a corner. The last container contained the general switchboard. Two red bulbs were still burning and the small display indicated 004 382 pumping hours. Hence, the plant was about two years in operation if water was pumped for six hours a day.
William was rather angry as we left this place of "quick money". He realised that this represented a big loss to his country. He calculated that with the same amount of money about fifty sturdy roughing and slow sand filter plants could have been constructed, providing some income to local contractors as well as a good and durable investment for public welfare. At this point, we passed a large European-style villa with blue window shutters, located in a large lawn and enclosed in high walls ...