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Water treatment

80%. Only one excavation

80%. Only one excavation point in the middle section of each fraction gravel cell was used to calculate the retained solids in the HGF units of Cocharcas. After observing the uneven distribution of the retained solids in the filter media in the cases of La Cuesta and Compin, Pardón (1989) considers that the 40% value could be an overestimation of the cleaning efficiency of the HGF of Cocharcas. Due to the large volumes of water involved, the time required for the repeated cycles of inundation, drainage, and refilling of the units, together with the required additional hydraulic structures, Pardón (1989) does recommend to study alternative cleaning procedures involving lower drainage velocities. It was encouraging to note that Pardón (1989) also observed that the concentration of solids in the drainage became higher as long as the solids load increased inside the units. In fact, in 1987 the first drainage of the first gravel fraction had 1.22 gl -1 , and the second 0.18 on a subsequent day, and two years later, 5.18 gl -1 first, and 3.22 and 2.89 in the subsequent days. All values were referred to filter load equivalents (σ v ). This tendency of increasing amount of deposit removal with increasing filters loads may contribute to improve global filter runs between the costly and troublesome manual cleaning of the HGF units. In Ethiopia, a plant designed in 1990 was built to treat 18.7 ls -1 (67.3 m 3 h -1 ) of river water with an average turbidity of 80 to 120 NTU, and peak values over 2000 NTU (Shenkut, 1996). The plant consists of two sedimentation tanks, six HGF, and four SSF. The design filtration rate in the HGF units is 2 mh -1 . Each HGF unit, without hydraulic cleaning facilities, has a total filter bed length of 21 m distributed in four compartments. The first 8m length with gravel size in the range of 22-19 mm and the last 3m length with gravel size in the range of 6-4 mm. The HGF units served 3-4 years before manual cleaning became necessary. It took one month to dig out the filter materials and to replace it. The capital investment in the plant serving 50,000 people was 735,000 USD of 1990 (9.4% in settling, 31.4% in HGF, and 59.2% in SSF). Then the capital investment per ls -1 was 39,300 USD, and per inhabitant was 14.7 USD while the operation cost is about 600 USD per month. The cost of cleaning the HGF was 2,000 USD. On the basis of bench and pilot scale studies, using mainly K-clay suspensions and short filter cells, Wegelin (1986) published his first tentative design guidelines for HGF as pretreatment stage of SSF. Later, reporting part of the experience with full-scale units, Wegelin and Mbwette (1989) published a revised version of these guidelines (table 2.10). Table 2.10 Tentative design guidelines for HGF (Wegelin, 1986; Wegelin and Mbwette, 1989) Maximum suspended solids concentration in influent water, C 0 (mgl -1 ) Parameter >300 300-100 150

Some points of discussion about HGF during the 1990s. At the beginning or during the present research work discussions have existed about some aspects of HGF. They include the criterion of using maximum turbidities or suspended solids values to design the units, the filter medium length; the cleaning procedures of the units; the provision for head losses; and the hydraulic behaviour of the units versus their removal efficiencies. In the Andean Region the highland rivers tend to present rather low average levels of solids concentration and only during short periods of the hydrological cycles these values are high. Consequently, alternative criteria to the maximum concentration values should be explored to design shorter and even more economically competitive HGF units. These criteria may include the use of the DyGF technology to deal with the sharp peaks of solids or to reduce the filtration rate during periods of heavy rainfalls in the catchment areas. The filter length is a critical dimension in designing HGF, since an appropriate balance is necessary between capital investment and running cost. Filter length affects filter efficiency and silt storage capacity. It seems that the initial approach was mainly oriented to ensure high silt storage and produced too large filtering beds. Even in countries with low cost of labour, shorter filter lengths seem more reasonable if the cleaning of the units can be accomplished in a more efficient way, and as long as the required coarse filter efficiency is not compromised. Therefore, during the 1990s it became pertinent and relevant to explore, under local conditions, the possibility to extend the tendency showed in the later version of the tentative design guidelines (table 2.10). Standard design practice has allowed for 30 cm of gravel above the effluent flow weir level to take into account headlosses along the filter run. Based on his observations with full-scale plants in Peru (HGF units operating at