Microbial Optimization Goals – Then and Now

By Larry DeMers and Bob Hegg – Process Applications, Inc.

The microbial optimization goals provide the basis for the national Area Wide Optimization Program.  These goals were introduced to the AWOP states at the beginning of the program and were used to encourage a different level of performance from surface water treatment plants than the regulatory requirements at the time.  However, the initial development of these goals goes back even further in time.  The first Composite Correction Program handbook published by EPA in 1991 for water treatment optimization does not specifically list performance goals.  Performance is discussed under the “Conducting Performance Assessment” section of the CPE methodology and reference is made to achieving 2 NTU from sedimentation basins and 0.1 NTU from filters.  Included in a discussion in filter performance after a backwash, acceptable performance is described as a turbidity increase of 0.2 to 0.3 NTU for less than 10 minutes after a backwash.  Moving on to the 1998 edition of the Composite Correction Handbook, Chapter 2 is devoted to protection of public health from microbial pathogens, and the research basis for the optimization goals and specific performance goals are described within the chapter.  The turbidity performance goals, which all AWOP participants are familiar with, are summarized below.

Individual Sedimentation Basin Performance

  • Settled water turbidity less than 1 NTU 95 percent of the time when annual average raw water turbidity is less than or equal to 10 NTU.  This goal increases to 2 NTU when the annual average raw water turbidity is greater than 10 NTU.

Individual Filter Effluent Performance (IFE)

  • Filtered water turbidity ≤ 0.1 NTU 95 % of the time (excluding 15 minute period following backwashes) based on maximum values recorded during 4-hour time increments.
  • Maximum filtered water turbidity of 0.3 NTU.
  • Initiate backwash after turbidity breakthrough has occurred and before turbidity exceeds 0.1 NTU.
  • Maximum filtered water turbidity following backwash of < 0.3 NTU.
  • Maximum backwash recovery period of 15 minutes (i.e., return to < 0.1 NTU).

Although specific goals were not established for combined filter effluent turbidity (CFE) in the 1998 edition of the handbook, the same goals as with IFE are applied to CFE when interpreting performance at this location.  This point is demonstrated in the handbook in the CPE methodology section on assessing plant performance.

Further discussion on interpreting the filter backwash recovery performance goals occurs in Chapter 4 under supplemental data collection.  This section states that the same goals (i.e., limit turbidity spike to < 0.3 NTU and recover to ≤ 0.1 NTU within 15 minutes) should be used to assess filters with filter-to-waste capability.  The 15 minute recovery period starts when the filter begins filtering during the filter-to-waste period.  The rationale for this approach is that the filter-to-waste period is a key indicator of a plant’s process control, and monitoring of performance should start immediately after the filter is placed back in service.  This description is important to remember, since NOLT has made changes to this interpretation that are currently being applied during Targeted Performance Improvement (TPI) activities (e.g., CPEs, PBT), and they will be discussed later in this article.

Since the last update of the turbidity performance goals in 1998, further research using higher resolution turbidimeters and particle counters have confirmed the validity of these goals and provided the basis for refinements.  One specific refinement is the use of two significant figures when referring to the filtration goals (i.e., 0.10 in place of 0.1 NTU, 0.30 in place of 0.3 NTU).  One specific research project that is included in the “Why Optimize” presentation during PBT references work conducted by Emelko (Water Quality Technology Conference, 2000) on Cryposporidium removal during filtration.  The researchers were able to demonstrate 5 to 6 log removal during optimized, stable filter operation, and the measured turbidity during this period of operation was approximately 0.04 NTU.  At the end of the filter run the log removal decreased to 2 to 3 log with a corresponding turbidity increase to approximately 0.10 NTU.  Advances in turbidimeter resolution also support the refinement in turbidity readings.  A common low range process turbidimeter currently used in water treatment plants (i.e., Hach 1720E) has an accuracy of ± 2 % of the reading or ± 0.015 NTU from 0 to 40 NTU.

A recent refinement to the sedimentation and filtration optimization goals was reported in the August 2009 edition of AWOP News.  This article described a recommended approach for establishing the frequency of data collection for continuous reading turbidimeters when pursuing process optimization.  For sedimentation basins a frequency of at least 15 minutes is recommended.  For individual filters and combined filter effluent at least a 1 minute frequency is recommended.  Additional information on this refinement can be found in the article.

For the last microbial goal refinement discussed in this article, revisions to the filter-to-waste performance goal will be reviewed.  The performance goals for plants without filter-to-waste capability remain the same, other than the change to using two significant figures.  These plants should strive to limit their turbidity spike following backwash to < 0.30 NTU and should achieve ≤ 0.10 NTU within 15 minutes of return to service.  Recent experience from PBT plants implementing special studies has shown that filter backwash spikes can be reduced substantially through use of practices such as filter rest periods and the extended terminal subfluidization wash (ETSW).  In many cases filter spikes can be reduced to < 0.10 NTU.  For filters with filter-to-waste capability the performance goals have changed to the following:

  • Minimize the turbidity spike during the filter-to-waste period (i.e., record the highest turbidity and direct optimization efforts at minimizing this value).
  • Return the filter to service at ≤ 0.10 NTU.

This refinement does not establish a maximum turbidity value or length of time to the filter-to-waste period.  The primary reason for this change is that, under the revised goal, filters are not returned to service until the turbidity is ≤ 0.10 NTU thus limiting the need to establish specific maximum turbidity and duration goals for water that is going to waste.  The revised goal recommends that plant operators monitor performance during filter-to-waste and minimize the turbidity spike during this period, a practice that has not been pursued by plants in the past but is a key activity during PBT.  The magnitude of the spike during the filter-to-waste period can be used as a relative indicator of filter conditioning prior to the filter going into service.  Most plant operators have a desire to keep filter-to-waste periods short, since they want to minimize wasting production water.  Consequently, operators are motivated to achieve ≤ 0.10 NTU as quickly as possible during filter-to-waste.

An example post filter backwash assessment from a recent Oregon PBT session is shown in the chart below.  These turbidity data describe the performance of a filter during the filter-to-waste period.  During the initial part of the period the turbidity reflects the quality of the backwash water exiting the filter.  Once this water is removed, settled water passes through the filter, and the maximum turbidity of 0.13 NTU occurs at about 12 minutes into the period.  A turbidity of 0.10 NTU is reached between 17 to 18 minutes.  At this time the plant operator changed from filter-to-waste to filter-to-clearwell operation.

It is important to understand that the initial lag in the turbidity response can be impacted by the size of the filter, including the underdrain volume, as well as the filter-to-waste rate.  Ideally, the filter-to-waste rate is similar to the filtration rate; however, this is not always the case for plants with different size waste piping.  It is also important to understand that the potential exists to minimize the turbidity spike and duration of the filter-to-waste period described by the performance in the chart.  As described previously, many PBT operators have been able to utilize the special study approach to maintain the turbidity spike during filter-to-waste at ≤ 0.10 NTU.

Almost 20 years after the publication of the Composite Correction Program handbook for surface water treatment plants, the basis for the turbidity goals remains in place.  The refinements described in this article have been made based on industry research, changes in instrumentation capability, and the considerable amount of experience gained through implementing AWOP activities.

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