Black & Veatch’s Frank Rogalla looks at filter maintenance and operations
This article is the second part of a summary presenting the findings of information gathered from filtration plants at five sites in the UK, six filter units in Australia and 49 North American plants.
Filter to waste
FTW or rewash, involves wasting water from the first portion of a filter run to prevent discharge into the clearwell of the initial high-turbidity water produced by the filter. Some plants carry out FTW for a fixed period of time, but this practice may not achieve the goals. The important concept is that FTW is managed on the basis of the filtered water quality (whether particle counting or turbidity measurement is used). The length of time for FTW is site-specific and may vary with source water quality or pre-treatment approach.
At some plants the filter to waste piping may not be adequate to carry away the wasted filtrate at the full filtration rate. Increasing the filtration rate after filter to waste is ended could cause a spike or increase in turbidity, so careful management of the rate change is required. The use of a slow FTW rate simply delays ripening and leads to the risk of a hydraulic shock when the filter comes back into service, potentially causing turbidity or particle breakthrough.
At the end of FTW, a brief deterioration of filtered water quality may occur, even if FTW is done at the full filtration rate, because of the change of direction of water to the clearwell. Unless the turbidity of filtered water is measured continuously, both in the wasting mode and the production mode, operators will not know if increasing the filtration rate at the end of FTW has had an adverse effect on water quality. A well-designed system should ensure the opening of the filter outlet valve and the closure of the FTW valve can be achieved smoothly and with no overall rate change to water coming out of the filter.
Allowing filter media to ‘settle-in’ after backwashing, for a time of 15mins-24 or 48 hours has been done at some plants and some operators believe this helps minimise the initial turbidity spike when the filter is started. In this strategy, after backwashing is completed, settled water is introduced to the filter box and held for a period of time before filtration begins.
Delayed filter starts were evaluated at three full-scale plants and found to reduce particle counts during the FRP by 35-42% and reduced the peak particle count by over 50%. The turbidity peak from a filter rested for four hours was about half that of a filter placed into service immediately, and the FRP for the rested filter was 45% shorter.
Resting filters before starting a new run is not a cure-all, as four plants participating in the AWWARF project reported using delayed starts but did not consistently control their initial turbidity spike. Nor can filters be rested for an unlimited length of time. Microbiological problems can develop if filters are out of service for an excessive time and then returned to service without being backwashed again.
At some plants, the initial turbidity spike is managed by starting the filter at a low filtration rate and gradually increasing the rate over a period of time, such as 15-30mins. Nevertheless, results for using a slow or gradual start to attain better quality during filter start-up are mixed. While slow start makes some impact, removing a proportion of a large number of particles, or allowing them to pass over a longer time period, it still leaves a large number of particles. Trimming the height of a peak may enable a utility to meet its maximum turbidity goals, while not necessarily meeting the spirit of those goals.
Coagulant or Polymer in Backwash Water
The addition of coagulant chemical or polymer to backwash water during a portion of the filter wash can successfully help to control the FRS. Addition of coagulant or polymer to backwash water helps to condition the media and the remaining suspended solids – within the media and the water over the filter media – that were not washed out before the end of the backwashing.
At the end of a filter backwash, low turbidity, unused backwash water (typically, finished water) remains in the piping, the underdrains and the support media below the filter. If the chemical addition is not stopped before the end of backwashing there is a risk that unacceptable concentrations of coagulant metal ion or polymer may come out of the filter, unless FTW is employed.
Coagulant dosing may cause pH to fall in some waters. Plant staff should be able to calculate when to stop dosing from plans of the filters and data on the backwash flow rate.
No clear consensus on ways to determine coagulant or polymer type and dosage and the duration of dosing was found in the literature or in the survey of utilities. Pilot-plant testing and careful evaluation of a single full-scale filter, equipped with FTW, are two ways to evaluate addition of coagulant or polymer.
Settled water entering the filter box
Some water utilities control the FRS by adding coagulant or cationic polymer to the settled water as it refills the filter box after backwashing. Success has been attained with an alum slug dose that is the equivalent of about 4mg/l in all of the water above the filter media after the filter box has refilled. Mixing of the slug dose takes place because it is added during the early phase of refilling the filter, when turbulence is high in the water above the washwater trough.
Pilot plant studies where an excess of coagulant was introduced into the water for 10-40mins at the start of the filter run indicated this could reduce the ripening peak and duration. Too high a dosage or too long a duration ultimately resulted in higher turbidity values than those attained by use of a constant coagulant dosage.
Thames Water undertook a short-term trial at a conventional treatment works where clarified water passed to six dual-media filters, consisting of 200mm depth of 1.2-2.5mm anthracite over 500mm depth of 0.5-1.0mm sand. The filters operated at a nominal rate of 5m/h (2gpm/SF). The FTW ran for 22mins at approximately 2m/h (0.8gpm/SF). Three additional, nominal coagulant doses of approximately 1.2, 2.9 and 4.8mg/l as Fe were tested on one day. The continuous turbidity monitoring was allowed to follow the initial peak turbidity value and ripening. Each of the three filters receiving extra Fe reached 0.10ntu more quickly than the three that were not dosed with extra coagulant. The filter with 4.8mg/l reached 0.1ntu before the end of the 22-minute FTW period.
Adding an excess of coagulant as the filter refills could help to condition the media and destabilise any charges on the particles left in the filter or the supernatant after the backwash. It is less likely coagulant will pass into the filtered water when added during the filter box refill, as long as turbidity remains low and is easy to test. The impact of extra coagulant on pH should be checked.
Operators who try the slug dosing method or a short-term continuous feed are advised to start at low doses of coagulant or polymer and gradually increase the dose if no positive effect is seen in the filter effluent quality. Furthermore, to develop solid evidence that the procedure is effective, they should perform alternate filter runs with added coagulant or polymer and filter runs having no added coagulant or polymer. A common mistake to be avoided is the assumption that ‘if a little bit is good, a lot is much better.’ Overdosing either an inorganic coagulant or a polymer after backwash could have serious consequences beyond filtrate quality, it may cause problems of short filter runs and/or mudball formation