PARTICLE
COUNlERS
normal backwashing (on profile prior to making any ~ ,n~I, time) should be inhibited. operatio nal c han ges . ilter Plant shutdown on filter I I Notice th e ve ry high II a,dSecc n~a~I 100 0 ' \V "" Cf outlet turbidity or head loss !"d l econda ,,IY Fllter I particle counts occurring I " ~000 I sh ould be activated t o whe n a prima ry and ----il '3000 pr eve nt t urbid water seconda1y filter come back carrying thro ugh to the 2000 - I Pia tonon line after a backwash. I \ reticulation. Carefu l T hese particle counts are J: d I Ji.. .i I .. planning is also required in ~ orders of magnitud e E::=tg g g : g 8 8 8 8 order to ensure that any g greater than the counts ;,; ~ ,!; ,!; ~ ~ ~ ~ = tu rbid water exiting the i!lg ~ ~ ~ ~ experienced during nonnal ~ ~ ~ ~ filters is disposed of to operation, and attem.pts waste, and not to the reticshould be made to signif- Figure 1. Particle counts/ml in the 2-15µm-s ize range before making operational changes. ulation , where it may pose icantly reduce or eliminate a health threat to consumers. these particle p eaks, in plant. Simple mathematics is required to Experience indicates that it is often order to achieve the target ofless than 200 calculate the following: difficult to effectively dump water, as particles/ml in the 2-15µm-size range, for existing control mechanisms and flow • The capacity of your backwash tank(s) 95% of the time. paths are generally not designed to cater • The capacity of your sludge handling Start by using gross visual means to for this kind of activity. facilities optimise plant performance • T he backwash rate(s) Compromises may be required The first thing to check is wheth er • The supernatant return rate Once a filter has been allowed to your filters are cleaning up effectively Once these figures are understood for proceed to particle breakthrough, you during backwashing. Watch and time each component of the plant, you know several backwashes under existing condimay fi nd that a single backwash is insufthe boundaries that you are working ficient to clean it effectively. If this occurs, tions. Record your observations, and also w ithin. E ntering informatio n into a record backwash rates by measuring the the filter should be backwashed again until spreadsheet is a useful way to represent rise rate of water in the filter du ring is comes clean, and a reduced maximum this information. For example, if your backwashing. A plot of level versus time filter run time should be estimated based backwash tank capacity is 25000L and the will allow you to calculate backwash rates. on the plant's ability to clean the filter in backwash rate is 1501/s, then you know a single backwash. You will find that Your observations may indicate a need that the maximum time that you can compromises such as this are required in to increase or decrease either the backwash backwash your filters is 2.75 minutes. rate or duration. Exercise caution if order to produce the highest quality water. Similarly, if the supernatant return rate is changing the backwash rate, as the rate Do not be ove rly su rprised if the 4 1/s, then it is going to take just over an must be high eno ugh to fluidise the bed, operating conditions that you eventually ho ur and a half to empty the sludge but not so high as to risk media loss over decide upon are far removed from the handling facility if you backwash for 2 .75 the launders. A long handled rake inserted original design chara cteristics of th e minutes, so you will have to w ait at least into the sand bed during a backwash w ill plant. Achieving design capacity may also this long before backwashing again. give a good indication of whether the bed be difficult, given that backwashing may Depending upon the quality of water is fluidising. If the rake does not fall to either consume more water, take longer entering you r plant, you may or may not the bottom of the filter under its own to complete, or be more frequent than be able to wait this Long before another weight (but remember to hold onto the before. Running at lower flow rates will backwash is required. You must underhandle!), then the bed is not effectively also impact the plant's capacity to produce stand these physical limitations before you fluidising . the maximu m amount of water, especially make any changes to the process. Without If you decide to make changes, it is if the plant already runs for extended this knowledge, the process or the fil ters important to make only one change at a periods of time each day. could be affected detrimentally, and it may time, and WAIT and use the particle Be patient - Eradicate or minimise take some time to recover. counter to observe the effects of each rapid flow changes If yo u already have filter ou tlet change. Compare the results after making It is really important to make only one turbidity trended at your plant, you should a change w ith previous res ults to change at a time, and make small changes. have an idea of w hen filter ou tlet determine w hether the cha nge has Use common sense to determine where turbidity begins to rise. This has tradi improved the filtered water quality. the 'big wins' are likely to be. If you make tionally been the trigger for backwashing. Determine the maximum run time of changes, carefully note down the date and H owever, particle breakthrough occurs your filters time of the change, then sit back and wait, BEFORE tu rbidity brea kthrough. It • How long can your filter run before and watch the trends to see what they tell makes sense therefore to determine when particle breakthrough occurs? you. Don't be tempted to 'fiddle' and particle breakthrough begins to occur, and make other changes at the plant until at • Can you still backwash the filter effecinitiate backwashes just prior to this. If you least two complete filtration cycles have tively if the filter reaches breakthrough? do not have filter outlet turbidity trended, elapsed. A filtration cycle is the time from To determine the answers to these or the filter outlet turbidity trend does not one backwash to the next of all filters questions (under relatively stable raw show a characteristic breakthrough curve, under normal operating conditions. wate r co nditions) requires patience, then 'pushing the limits' is required. To When the particle counts exiting the simple mathematics, and the discipline not do this without introducing confounding filter suddenly increase, the filter is said to alter more than one parameter at a time. variables requires careful coordination. to be 'shedding' particles. M ost particle Ideally, the plant should be allowed to run You w ill need to understand the shedding events are likely to be attributed uninterrupted for as long as possible, and physical limitations of your treatment . -LC- •
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48
WATERWORKS DECEMBER 2001
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