LAS REGLAS DEL JUEGO: REFUNDACIÓN Y PRESENTE

In the literature very little is said o f w hat constitutes the end of the ripening period. Is it achieving a quality target or a com plete end to the im provem ent process? Ripening is usually poorly defined, generally w ith no clear end point. Moran et al. (1993) sim ply defined it as the "im provem ent in effluent quality early in a filter run". For Tobiason et al. (1993) the ripening period was defined as the "period of declining effluent concentration to nearly complete rem oval". In this review o f the literature, where graphs have been examined, the second definition has been taken to signify the end of ripening, except where stated.

The literature shows there is not one universal ripening curve. Typical ripening curves show a rate of im provem ent that starts high and gradually declines. Ripening generally im plies that a ripened stage is reached. It is however not necessarily true that the best phase of filtratio n is an absolutely constant quality; there are inevitable fluctuations due to "random ", or not w ell understood.

breakthrough events, or breakthrough precipitated by flow variations, or even small variations due to measurement sensitivity. So at what point can ripening be said to be complete?

The end of the ripening period could be defined in a number of ways: when all filtrate quality parameters meet acceptable targets;

when a useful and easily measured parameter (i.e. well correlated w ith other water quality parameters) meets a target, i.e. in figure 2.3 which might be a low turbidity value;

when the filter achieves a performance target suggested as being likely to indicate satisfactory performance (e.g. log. removal or percentage removal value);

when no further improvement is noted (difficult to observe except after the event);

when no further significant improvement is observed (i.e. improvements becoming extremely small between readings, especially if close to the reliable sensitivity o f the instruments used); when a fixed proportion of the overall best removal has been reached (e.g. if the filter removed at best 90% of the load, then 90% of this value might be used, i.e. when 81% removal was reached;

when a time lim it had expired (likely to be used in filter to waste, but not necessarily directly indicative of the completion of ripening);

when the filtrate reaches a value which can be mixed with other filtrates and not compromise overall treatment plant performance (difficult to assess on a single filter pilot plant).

For primary filters the aim was to assist the main slow sand filtration treatment process so the first tw o approaches are not appropriate to the filters in this study. The last approach allows consideration of filter runs under the whole range of input water qualities and chemical treatments, and where the filtrate did not reach any recognised absolute target.

In laboratory studies influent water quality may be kept constant. In contrast, in actual water treatment practice the influent quality may change at any time. The variation in influent quality may

lead to difficulties interpreting filter ripening unless the filtrate quality remains constant or the removal is unaffected.

O f the few authors who have attempted to define the end of the ripening period Barnett et a/. (1992) had a target of 0.5 NTU, which was reached after a filter to waste period averaging roughly 0.5% of the total filter run time. Janssens et al. (1982) and François and Van Haute (1985) suggested

that ripening was achieved when the improving phase reached 0.2 JTU. The same value was also the point at which breakthrough was defined and filter runs were terminated. Watson (1990) used the EC guide level 0.4 NTU.

The value of 0.2 JTU was not the best quality that Janssens et a/. (1982) reported their filter could achieve, since values later in the filter run approached 0.1 JTU. This meant that their ripening point was not the point at which quality improvement stopped, so ripening was continuing. It is necessary to distinguish what is meant by ripening in this instance - reaching a target, or reaching the point of no further improvement. Can improvement in filtrate quality beyond the ripening point be considered as anything other than ripening? Their work presented a practical approach to ripening by considering the point at which an acceptable water quality was produced. A plant operator would not desire to run a protracted filter to waste period w hilst filtered water quality gradually reached a minimum, if much of the wasted water met filtered water quality targets. An area has been marked 4? and shaded on figure 2.3 to illustrate this point; Cü^ is the target filtrate quality, but it is not the minimum filtrate quality achieved.

Baumann (1989) reviewed the history of filtration from a health perspective and noted that the American Water Works Association had suggested a goal of 0.1 NTU for potable water leaving a works. Ripening would be completed when the quality of filtrate from a recently backwashed filter could be mixed w ith the water from other filters in operation w ithout causing elevated final water turbidities. The actual value of the ripening point would depend on the dilution. Plant operation w ould need to be controlled to avoid affecting overall plant turbidity; breakthrough, which could

result from poor flo w control or running filters too long, should be prevented to avoid an impact on the dilution water.

Al-Ani et al. (1986) could be considered to have given a definition of successful filtration applicable

to their studies on low turbidity waters. They said that if "raw water turbidity is 0.5 NTU and if removal of turbidity is 70 percent or greater, the probability is 0.85 that the removal o f Ciardia cysts

w ill exceed 99 percent."