Methods for Managing Filter Ripening Spikes
By David K. Hardy, Utah Valley Plant Manager
Central Utah Water Conservancy District
Phase IV Excellence in Water Treatment Award recipient - 2003
Since the early days of surface water treatment and granular filtration, water professionals have been challenged with turbidity spikes associated with filter startups; these events are called ripening spikes. Over the years, methods and theories have been developed as to why this happens and what can be done to reduce or eliminate these spikes. Some methods are more effective than others, depending upon plant specific conditions. A variety of methods for reducing ripening spikes are listed below, in the order that the author has found to be the most reliable and effective. Note that every water filtration plant is unique and the unique properties of a given plant may impact the selected method applied to reduce ripening spikes.
1. Filter to Waste: The filter flows to waste until it meets the utility’s water quality goals before being placed in service. Filter to waste capabilities may not exist at all filtration plants.
2. Extended Terminal Subfluidized Wash (ETSW): Accomplished by lowering the terminal backwash flow rate to a subfluidized flow long enough to rinse one filter volume of water (also referred to as, ‘Rinse to Waste’).
3. Filter Seeding: The addition of coagulant to the backwash header during the ETSW cycle.
4. Backwash Duration: Leaving a certain amount of turbidity or solids in the bed to promote faster ripening.
5. Filter Resting: Allowing a filter time to rest after backwash before it is placed back into service.
The intent of this article is to discuss the proper methods of utilizing Filter to Waste as a viable way to improve filter performance. In future Tech Tips, “Filter Seeding” and “ETSW” will be discussed in detail as additional options that the author has found to be effective methods of managing ripening spikes. These methods will be particularly helpful to those utilities that do not have the ability to perform Filter to Waste at their facilities.
Why do Ripening Spikes Occur?
For this discussion, it is assumed that plant chemistry is optimal and is not part of the issue with filter performance. However, if the optimization status of plant chemistry and chemical dose is unknown, it is suggested that utility staff further investigate this area during the process of examining filter ripening spikes.
Ripening spikes occur due to remnant particles that remain in the filter box following the backwash cycle. What is meant by ‘remnant particles’? During a typical backwash cycle, the filter is subjected to some sort of surface wash or air scour cycle to break up the filter media and allow for a more complete detachment of trapped particles that have accumulated in filter beds and on the surface of the media itself. Following the air scour, the backwash cycle is ramped up to a ‘high backwash rate’ to achieve the fluidization and expansion of the filter media and to wash the accumulated solids from the filter bed. [Hint: A good rule of thumb for media expansion is about 25 – 30%.] During the high backwash rate period, wash water pushes out the suspended particles and removes almost all of the solids from the filter bay. But because the filter media is fluidized, media particles continue to collide causing additional scouring action and the detachment of microscopic particles. At the end of the backwash, if the high backwash rate is reduced to zero without a sub-fluidized (or low flow) step in the backwash cycle, remnant particles remain within the filter box, no matter how visibly clean the filter appears at the end of the backwash. These particles are ‘stable’ and do not filter out at the beginning of the filter run which creates the ripening spike that is observed. The discussion below describes how Filter to Waste, if performed properly, can eliminate the ripening spike and transition the filter into service smoothly.
Filter to Waste
Filter to Waste was developed to allow operators the ability to waste (or recycle) the filter effluent until water quality leaving the filter matches the plant’s turbidity goal, before the filter is placed into service. Ninety percent of all particles in the filter run are typically passed within the ripening phase. Refer to Figure 1 for a filter profile that includes the Filter to Waste cycle. It is recommended that utilities develop goals for Individual Filter Effluent performance, so that a clear target exists for returning filters to service after Filter to Waste is completed ( ≤ 0.10 NTU is suggested – this is the Phase IV Partnership for Safe Water performance goal). The Utah Valley Water Treatment Plant has adopted the “Four Commandments of Filter to Waste”. These commandments are described below. Utah Valley Water Treatment Plant staff has found that by following these guidelines, the use of filter to waste can become a valuable tool in managing ripening spikes.
“Thou Shalt Filter to Waste at Full Rate”
The primary principle of Filter to Waste is to move enough water through the filter to remove remnant particles and to replace the remaining backwash water in the filter box with coagulated water to allow for ripening. Ideally, Filter to Waste should provide the ability to flow water through the filter at full rate, meaning the same rate that it will be at when the filter goes online (e.g., if all other plant filters are running at 4.0 gpm/ft2, then the filter that is wasting should also flow at that rate). This allows the filter to be conditioned and ready to be placed into service at the current plant filtration rate. This will help to eliminate the ‘dreaded second spike’ that can occur if, for example, the filter has been wasted at 1.0 gpm/ft2 and then goes online at 5 gpm/ft2. The second spike is caused by hydraulic changes through the filter bed, not remnant particles. If a plant does not have the capability of full rate filter to waste, a utility may wish to consider some of the other options to enhance the ability to manage ripening spikes.
“Thou Shalt Not Bother Thy Neighboring Filters”
Whether in the process of filtering to waste or backwashing a filter, care should be taken to avoid changing flows through all other filters that are online. If a filter planned to be backwashed is to be replaced by a fresh filter, time the closing and opening of the influent valves to occur at that same moment so that the filter in waste mode takes in the same amount of water from the filter being taken out of service [Hint: Rotate filters – always have a filter in reserve to rotate in service as one is being removed for backwash. This practice will help to reduce hydraulic bumps caused by changing flows through the filters.]
“Thou Shalt Not Put a Filter Online Unless it is Below 0.10 NTU”
The benefits of keeping individual filter effluent turbidity below 0.10 NTU have been well documented and this has been adopted as the Partnership for Safe Water Phase IV optimization goal. It is recommended that utilities establish strict goals for filter effluent water quality to continuously strive for improved performance in this area. Consider each filter as an individual barrier or process and hold each filter to meeting the utility-established goals. If this goal seems unreachable at the current time, adopt it anyway; you will discover ways and means to achieve these goals! Remember that, “A goal not written is only a wish!”
“Thou Shalt Transition from Filter to Waste to Online in a Bumpless Fashion”
When the Filter to Waste cycle is complete, it is important to put the filter into service in a way that makes a smooth transition. Time the closing of the Filter to Waste valve with the opening of the effluent valve. This will allow flow through the filter to continue without interruption and will avoid the common ‘second spike’ caused by flow changes through the filter.
For filtration plants with the ability to utilize Filter to Waste, applying the principles described above may help to minimize ripening spikes and improve performance. Plants that do not have Filter to Waste abilities should look for additional tips in an upcoming issue of the Partnership for Safe Water Newsletter. The Partnership for Safe Water reminds readers that every plant is unique and different. The effectiveness of ideas and suggestions presented in this article may vary from one plant to another.