Filtration is the last step in the particle removal process. Although filter performance is a function of the coagulation, flocculation, and sedimentation processes, proper filter operation is needed to provide the high quality finished water required for this toolbox option. The following factors should be considered when optimizing or evaluating filtration performance.
7.4.4.1 Flow Split
Systems should evaluate the flow distribution to the filters to ensure there is an even load across all filters under the range of expected operating conditions (e.g., filter out of service, backwash).
7.4.4.2 Filter Beds
The filters should be operated with a design capacity that considers at least one filter as a reserve. The reserve filter is put on-line to maintain flow stability to the filters; if this is not possible, flow to the filters should be reduced. This will allow consistent flow when one filter is backwashed or taken out of service for maintenance.
Media loss or disturbance can lead to particles passing through the filters. The filter should be inspected on a regular basis to detect changes in the media. Media should be inspected to ensure depths of media are proper, the media are evenly distributed, and the size distribution
of the media are still to specifications. Media samples can be taken with a coring device or by excavation for the inspection. If media are lost or damaged, they should be replaced.
7.4.4.3 Backwashing
Backwashing is an integral part of the filtration process. Two important operating parameters for backwashing are the backwash flow rate and frequency of cycles. Other factors relating to backwash that affect filter effluent quality are hydraulic surges and filter start-up or “ripening.”
Flow rate
Systems should determine the appropriate flow that will clean the filter and prevent mudball formation, but will not upset the filter media and subject the underdrain to sudden momentary pressure increases. Typical flow rates are 15 to 20 gpm/ft2 which result in 15 to 30 percent bed expansion.
Frequency
Although the filter effluent turbidity is the indicator for pathogen control and the
determining factor for compliance, other operating parameters should be used to determine when backwash is needed. Emelko et al. (2000) performed filtration studies where pathogen
breakthrough occurred towards the end of the filter cycle before an increase in turbidity was detected. Their studies emphasize the need to evaluate and optimize backwashing cycles with respect to filter effluent water quality. Most systems use filtration time, headloss, effluent turbidity, or effluent particle counts to indicate when backwashing is needed. For improved process control, it may be beneficial to use all indicators.
Systems with multiple filters also should evaluate the hydraulic surges resulting from backwashing. The timing of individual filter backwash cycles should be considered with respect to the other filters, particularly adjacent filters. Consider the following two examples:
• If a large system with 50 filters backwashed 10 filters at the same time, this would cause a 20 percent increase in flow to the other filters. In this situation, the system could backwash fewer filters at one time or reduce the flow to the filters to avoid the filter overload.
• When one filter is backwashed, a hydraulic surge can be experienced by an adjacent filter.
LT2ESWTR Toolbox Guidance Manual 7-16 April 2010
Improving filter effluent during start-up
It is very important for systems to conduct a full evaluation of their backwashing process and operational variations to optimize the process. At the process optimization level, systems should minimize turbidity spikes in the filter effluent resulting from the backwashing process—it only takes a few high turbidity readings to cause non-compliance. The following operational practices may provide improved filter effluent during start-up:
• Ramping the backwash rate down in increments to allow better media gradation. • Resting a filter after backwash for several minutes or up to several hours before putting
the filter in service.
• Adding a polymer to the backwash water.
• Slowly increasing the hydraulic load on the filter as it is brought back on line.
7.4.4.4 Filter to Waste
During the beginning of a filter cycle the filter is “ripening” and the effluent turbidity is usually higher. To avoid sending this poorer quality water to the CFE stream, the filter effluent produced during the first few minutes of a filter cycle can be sent to waste (filter to waste) or recycled to the head of the plant. Some systems filter to waste or recycle until the filter effluent reaches the desired level of turbidity. Practicing filter to waste produces an overall higher quality water and may be necessary to maintain a CFE or IFE below 0.15 NTU.
7.4.4.5 Backwash Recycle
Plants that recycle the backwash water to the head of the plant should evaluate the impacts the backwash stream has on the coagulation, flocculation, and sedimentation processes. For example, the operator should know how the coagulation and flocculation processes need adjusting when there is a change in recycle flow. Ideally, the impacts of the recycle flow on these processes should be minimized.
For systems that recycle, the Filter Backwash Rule (FBR) requires spent filter backwash, thickener supernatant, or liquids from dewatering processes to be returned through all the
processes of a system’s existing conventional or direct filtration treatment train (40 CFR 141.76(c)). The rule allows for alternative recycle locations with state approval (40 CFR 141.76(c)).
7.4.4.6 Filter Assessments
Filter assessments can provide valuable information for optimizing the performance of a filter. The IESTWR and LT1ESWTR require systems to conduct an individual filter self-
assessment if a filter exceeds specified effluent turbidity criteria. However, systems seeking
Cryptosporidium treatment credit for lower finished water turbidity should also consider
conducting filter assessments to evaluate operating parameters and optimize filter performance. Chapter 5 of the IESWTR Turbidity Guidance Manual describes how to conduct an individual filter self- assessment.