Iglesia de Santa María de Mave (palencia)

In an introduction to filter backwashing Cleasby (1990) described four methods: i) upwash w ith full fluidisation, on its own and ii) supplemented by surface washing; and backwashing assisted by air scour, w ith the air either iii) preceding a water wash or iv) combined w ith it. Water alone was the

least effective cleaning method and combined air scour and sub-fluidising water washing was the most effective. Methods ii) and iii) were of similar, intermediate, effectiveness (Cleasby, 1990). The assistance provided by surface washing was demonstrated by Kawamura (1975a). Cleasby et a/. (1975) showed the benefit of preliminary air scour over water-only washing, but said that this method could not prevent long term accumulation of material coating the sand grains. For dual media filters Cleasby et a/. (1975) suggested that further auxiliary washing should be provided at the media interface. The danger of media loss when air scour is used, and ways it may be avoided, were discussed by Cleasby (1990). One effect of air scour was to break up large floes and allow their removal by lower water rates (Adin and Hatukai, 1991)

a. BACKWASHING WITH WATER ALONE

In the US backwashing has traditionally used a fluidising upwash, w ith bed expansion of 15-30% (Cleasby, 1990), often assisted by a surface water scour or mechanical rakes. Amirtharajah (1978) and Quaye (1976), reviewing the history of backwashing, said that Hulbert and Herring (1929) had found 50% bed expansion was necessary for effective backwashing, w hilst Hudson (1935) and Baylis (1937, 1959) required additional surface washing to eliminate mud-ball formation. Quaye (1976) did not regard 50% bed expansion for optimum backwashing as theoretically justified. Amirtharajah (1978) and Quaye (1976) said it was necessary to set up conditions for maximum hydrodynamic shear to optimise removal of deposits. These conditions were met at expanded bed porosities of 0.65-0.70 (Amirtharajah, 1978) or 0.75 and 0.78 for UK anthracite and sand respectively (Quaye, 1976). Amirtharajah (1978) said that his porosity values corresponded to the old "rule of thumb" bed expansions of 40-50%. Cleasby (1990) recommended that backwashing with full bed fluidisation should be achieved with a water rate of 1.3 (velocity of minimum fluidisation), based on a V^f calculation using the dgo sieve size, therefore allow ing the coarsest media grains to be mobile.

Ives (1982) was concerned that Amirtharajah's (1978) w ork indicated that 70-80% bed expansions were required, and that these were "too high for practical use." He did not state why, but reasons

T h e is th e p o in t at w h ic h th e pressure d ro p m easu re d as w a te r flo w s up th ro u g h a b e d o f filte r m e d ia stops in c re a s in g , b e in g e q u a l to th e b u o y a n t w e ig h t o f th e m e d ia , a n d th e b e d starts to e x p a n d (C le a s b y , 1 9 9 0 ).

could have Included the volume of wash water required, problems of filter floor design, backwash pump sizing, clean wash water pipework diameter and holding tank volume, and dirty wash water treatment and disposal, with their consequent impact on construction and operating costs.

b. BACKWASHING WITH AIR ASSISTANCE

Amirtharajah (1978) noted that European tradition had been to utilise air scour, either follow ed by a water velocity sufficient to fluidise the media marginally (British practice), or simultaneously with water (mainland Europe), but concluded that the most effective way of washing a filter might involve a simultaneous air scour and sub-fluidising water wash.

Cleasby (1990) summarised typical backwash air and water flow rates. In Britain for 0.5 mm ES sand he said air flows ranged from 1 8 - 3 6 m.h ’, and water from 1 2 - 2 0 m.h ’ . In the United States for dual media with 1.0 mm ES anthracite and 0.5 mm ES sand air rates of 55 - 91 m.h ’ and fluidising water rates of 37 - 49 m.h ’ were typical. For CASBW w ith 1.0 mm ES sand he reported air rates of 27 - 73 m.h ’ and water at 15 m.h \

The development of theory and practice of combined air and water backwashing have been described by Amirtharajah (1978, 1984, 1993), Fitzpatrick (1990) and Amirtharajah et al. (1991a).

Combining air scour with sub-fluidising rates of upflowing backwash water produced the conditions, descriptively termed "collapse-pulsing", which resulted in the optimum removal of deposits from filter media. The efficacy of collapse-pulsing backwashing has been demonstrated by Amirtharajah (1984, 1993), Regan and Amirtharajah (1984), Amirtharajah et al. (1991a), Addicks

(1991) and Fitzpatrick (1990, 1993).

Linear regressions of water flow rate as a percentage of against air flow rate under conditions of collapse-pulsing with sand and sand/anthracite filters were produced by Amirtharajah et al. (1991a).

Lower water rates required higher air rates and vice versa. It was recommended that air rates should

fall in the range 30 to 135 m.h'L At the lower end of the air flow range water rates should be 40 to 60% of V^f. At higher air flow rates water flows should be in the region of 25 to 45% of V^,.

Amirtharajah (1984) presented a general equation for predicting collapse-pulsing conditions, generated from soil mechanics theory, of the form:

+ ( i v ) = *

where a and b = dimension less constants for a particular system Qa = air flow rate

V = water flow rate

= velocity of minimum fluidisation

Amirtharajah (1984) presented experimental data that produced a linear regression close to his prediction with a high correlation coefficient (r = 0.88).

Visual observations, at laboratory scale, by Fitzpatrick (1990, 1993) showed optimum cleaning of filters dosed with kaolin suspensions occurred under conditions of collapse-pulsing. Practical confirmation of the effectiveness of backwashing using collapse-pulsing has been reported by Amirtharajah et al. (1991a) and Amirtharajah (1993) at pilot plant scale and full scale where

collapse-pulsing conditions produced optimum cleaning in sand, dual media and G AC filters.