1
n i 1log (x )i mean of "log x " n i
=
• ∑=
⎧ ⎨ ⎩ ⎫ ⎬ ⎭ =10
10
Summary of methods to determine numerical value of DOP credit
Two alternatives related to median and two alternatives related to the geometric mean are the preferred methods for establishing the numerical value of the DOP credit using spore data or other Cryptosporidium surrogates. Of these, either approach using median are simplest and most straightforward to determine. The two approaches related to geometric mean (see equations 4.4 or 4.7) are also suitable, though the “log of geometric mean” approach as outlined in equation 4.4 may be complicated under certain situations described above. The “log of means” approach outlined in equation 4.2 is identical to the approach proposed by USEPA in the Microbial Toolbox Guidance Manual. This approach is the only approach described above that can be demonstrated as clearly unsuitable for use in establishing the numerical value of the DOP credit, unless complicated provisions are included by USEPA to help identify and manage data outliers and censored data. Instead of requiring development of complicated guidance needed for other approaches, USEPA should explicitly recommend either of the median approaches to establish the numerical value of the DOP credit. If these other approaches are also listed, they should be described as alternatives to the median approaches and the guidance manual should adequately outline the limitations of these other approaches so states and utilities reviewing the guidance manual can make informed decisions.
Whatever method is adopted, use of data collected at least once a week for one year should be sufficient to establish the numerical value of the DOP credit. Sample collection for periods longer than one year or at frequencies greater than once a week should not be required by the states or USEPA, however utilities that voluntarily choose to collect more data should be allowed to do so. Samples should be collected at regular, evenly spaced intervals throughout an entire 52 week period, or multiples of 52 weeks when more than one year of data is used.
However, utilities should be allowed some discretion on sampling dates and number of samples, especially if more than 50 samples are collected. For example, 50 out of 52 samples for weekly sampling dates or about 350 daily samples excluding holidays seems reasonable. In addition, while utilities collecting samples once a week, for example, should be encouraged to collect samples on the same day each week, utilities should be allowed some discretion in establishing sample collection dates as needed due to holidays, vacations, or other issues related to availability of sample collection personnel.
Using full-scale spore monitoring to evaluate treatment performance other than DOP
Utilities performing spore monitoring to evaluate performance of treatment facilities may get some value out of collecting samples from raw water and intermediate points in the process. However, most utilities will focus most of their attention on finished water spore levels as an indicator of performance of the filters in particular, and the entire treatment process in general. For example, since spores are more sensitive than particle counts or turbidity and reflect more closely factors that may impact Cryptosporidium removal, monitoring of filtered water spore levels like that illustrated in Figure 4.8 can be used to illustrate improvements over time due to changes in infrastructure, operational practices, or management strategies. This greater sensitivity may allow demonstration of treatment enhancements for costly but difficult to quantify improvements, such as operational practices and management policies. Figure 4.8 shows progressive improvement over time at one facility from 1997 to 2002 due to refinements in operational practices. Chapter 6 includes spore monitoring data to illustrate treatment performance during different stages at lime softening plants.
Pilot-Scale Microbial Challenge Studies
A DOP pilot-scale challenge study would involve measuring removal of spiked Cryptosporidium or appropriate surrogate in a 2 to 5 gpm pilot facility mimicking the full-scale treatment plant. Source water for the pilot facility should be provided from the location representative of where the Cryptosporidium bin assignment samples are/were collected. Although explicit guidance on pilot studies is not available from USEPA, the following would be reasonable for most facilities:
• Challenge studies conducted for two (2) weeks during each of four consecutive quarters
• The study should be preceded by two (2) weeks of side-by-side studies comparing performance of full-scale facility to pilot plant operated under identical conditions (it must be shown that full-scale performance equals or exceeds pilot-scale performance through the final filtration stage)
• Monitoring of raw, spiked, and finished water grab samples to include turbidity, particle counts, and Cryptosporidium (or other microbial indicator)
• On-line, continuous turbidity and particle count monitoring of individual filter and combined filter effluent is recommended for the benefit of the utility so they can evaluate operations of the pilot facility, but this monitoring should not be a requirement or condition for the credit
• Pilot-study protocol to be negotiated with, and approved by the State.
• Objectives of testing should be to establish performance under routine or typical conditions (not “worst-case” scenarios – see later discussion), with “typical” performance established by monitoring removal of Cryptosporidium (or approved surrogate) in combined filter effluent (not in individual filters – see discussion below)
• DOP credit established in pilot studies will be applicable to full-scale plant at full- scale flow rates consistent with unit process loading rates used in pilot-scale unit processes (flocculation, clarification, filtration) during DOP evaluation, unless otherwise negotiated with the State
The above discussion proposes an aggressive schedule incorporating two weeks of quarterly studies, although a less aggressive schedule would be appropriate under most conditions. For example, a few weeks of one-time testing would be appropriate for many facilities, and would be consistent with existing performance testing requirements for some states. However, Chapter 12 of the draft MTGM outlines an extravagantly expensive and overly
aggressive program that includes 52 weeks of testing (Table 12.3, page 12-13 of drat MTGM). Furthermore, the objective of testing should be to represent performance under routine or typical conditions, not “worst-case” conditions emphasized in the MTGM. This is consistent with the characteristics of the LT2ESWTR framework which incorporates typical Cryptosporidium occurrence, viability, and infectivity along with typical treatment performance to establish overall risk to drinking water consumer. In addition, “typical” treatment performance is reflected by measuring treatment performance from raw (plus spike) through combined filter effluent, not raw through each individual pilot filter. This is consistent with the requirements for the DOP credit using spores, and is also consistent with Section 12.5.2.1 of the MTGM, although Table 12.3 on the previous page of the guidance manual implies the opposite.
The DOP credit established in pilot studies should be applicable to the full-scale plant at full-scale flow rates consistent with unit process loading rates used in pilot-scale unit processes (flocculation, clarification, filtration) during the DOP evaluation, unless otherwise negotiated with the State. For example, a pilot study which achieves >4.5 log removal of Cryptosporidium using the State approved protocol when pilot filters were operated at 8 gpm/sf means that the full-scale plant should be certified for a total credit of at least 4.5, not the 3.0 automatic credit, as long as the full-scale plant filtration rate is <8 gpm/sf. This does not mean the State is required to certify the full-scale plant for filtration rates up to 8 gpm/sf. However, it does mean that as long as the State establishes the maximum filtration rate for the plant anywhere below 8 gpm/sf (e.g., 4.5 gpm/sf), the State can not award a credit lower than 4.5.
Spore monitoring results can also be used to evaluate performance of pilot-scale facilities, in particular comparison of pilot- versus full-scale performance, like results shown in Figure 4.9. Most water treatment professionals with experience interpreting pilot study data have observed similar results numerous times demonstrating that pilot-scale facilities do not overestimate full-scale treatment performance in areas such as removal of spores and other particulates. In fact, results like those in Figure 4.9 demonstrating better spore or particulate removal in full-scale clarifiers and filters than in analogous pilot-scale facilities are commonly observed.
Other DOP issues
Some DOP issues were inadequately addressed or not addressed at all in the Draft Toolbox Guidance Manual for the LT2ESWTR. These issues are discussed separately below: costs, maintenance of toolbox credits, process specific studies, and potential DOP penalty.
Costs
The cost for the DOP credit using either aerobic spores or pilot-scale microbial challenge studies would not be dependent upon the size of the facility being demonstrated. For example, a full-scale spore monitoring study will include the same number of samples collected at the same sampling frequency whether a facility is 2 mgd or 200 mgd (one raw and one finished water sample, once per week for a year is proposed for both). A pilot-scale facility will similarly be about the same size (~5 gpm) and will include about the same duration of testing (about 2 weeks of testing per quarter for one year is reasonable) associated with full-scale facilities of a variety of sizes (the one difference perhaps is that larger utilities may have enough in-house staff to design, build, and operate the pilot whereas a smaller utility may not be able to do it without hiring an outside consultant). Therefore, the DOP credit will be cost-effective for a wide variety of facility sizes, and will be even more cost-effective for larger facilities.
A DOP spore study is expected to involve a minimum of 50 weeks of two paired spore samples (plant influent and effluent) per week. Median cost for spore analysis from a survey of eight commercial laboratories is about $60/sample. The resulting analytical cost is about $6,000 for the entire study. Cost for data interpretation and report preparation, would increase the total cost to about $60,000 for the entire study. Facilities performing the analyses in-house could reduce costs even further. Many facilities may already possess most of the required equipment (an incubator, an autoclave, a membrane filtration apparatus, and a microscope), and may only need to purchase a shaking water bath capable of achieving and maintaining at least 90°C ($3,000 to $6,000) or a stirring hotplate ($300-$1,200) capable of achieving the same temperatures. The analytical technique does not require special training or certification, for example like is needed for Cryptosporidium analyses, for personnel familiar with standard microbial analyses, in particular the membrane filtration technique for coliform analyses. For a pilot-scale study, a 5-gpm pilot plant with all required hardware and instrumentation, plus
design, construction, operation, monitoring, data evaluation, and report preparation would require $600,000 or less. Consequently, even the latter pilot-scale DOP cost is more cost effective than some other toolbox credits for facilities capable of achieving up to 4.0 log of Cryptosporidium removal (or approved surrogates), and becomes even more cost effective for larger facilities. The guidance manual should clearly indicate that spores are an accepted surrogate for Cryptosporidium spiking.
Maintenance of DOP credit
Once established, the DOP credit should be retained as long as the facility maintains compliance with the IESWTR. Determination of alternative compliance criteria, and requirements to comply with these alternate criteria, are not appropriate. The DOP credit establishes a correction or adjustment to the Cryptosporidium protection capability of an existing treatment facility in compliance with the IESWTR. Whereas a utility in compliance with the IESWTR is allowed a minimum 3.0 credit (2.5 for direct filtration) without having to prove what the true treatment capability of the existing system really is, once a utility completes a DOP study and proves what the actual capability of the system is when it is in compliance with the IESWTR, it should not be required to establish and attain a compliance requirement beyond IESWTR compliance.
Process specific demonstration studies
DOP studies can be applied to entire treatment processes (raw to finished), to intermediate segments of the treatment process (e.g., pre-sedimentation effluent to finished water), or to individual treatment processes (second stage of two filter process). Due to practical limitations resulting from decreased ability to mathematically demonstrate DOP credit when process influent levels are lower, the DOP approach using ambient spores becomes less useful when the starting point for the process gets later and later in the process. Consequently, DOP studies using full-scale spore monitoring work best when measured from raw water to some later point in the treatment process. Pilot-scale challenge studies can be used instead of full-scale spore monitoring studies under conditions when influent ambient spore levels are too low, whether this is due to low raw water levels or due to low ambient influent levels at points in the process after previous stages in treatment.
One unit process that is potentially particularly well suited for DOP studies using spore monitoring is RBF. DOP studies for RBF systems can be performed at full-scale pilot facilities, for example a monitoring well/collector representing a future monitoring well network or other river bank, river bottom, lake bank, or lake bottom collection system similar to the pilot well/collector. This facility will not be able to use microbial challenge studies, but can measure removal of ambient Cryptosporidium indicators, such as aerobic spores. DOP studies for RBF are unique in that these are the only opportunities to use DOP to establish UBT credits. Therefore, a facility that is ineligible for an automatic RBF credit for one reason or another (media gradation, distance from surface water source, etc.) can receive whatever credit can be proven in a DOP study, and the first 1.0 of this credit can be used for UBT and the remainder can be used for the additional 1.0 or 1.5 credits needed for bins 3 and 4, respectively. Once the pilot well/collector establishes the Cryptosporidium protection capability of the riverbank in the vicinity of the well/collector, the utility will be allowed to construct the RBF system outlined in the State approved DOP protocol and the facility will be awarded the credit demonstrated in the pilot well without need of further testing (analogous to the alternate intake indirect “credit” in the toolbox). This credit will be useable until the second round of Cryptosporidium bin assignment samples, at which time the new bin assignment sample location will be the combined discharge from the RBF system.
Figure 4.10 and Table 4.5 summarize spore removal in a pair of wells associated with an existing RBF system. Mean Cryptosporidium occurrence in the river source is >0.8 oocysts/L, which is close to the bin 3 threshold of 1.0 oocysts/L. Giardia occurrence in the river is also high. Yet, no Giardia or Cryptosporidium have been detected in the RBF wells. Spore monitoring data indicates that the spore removal capability for the RBF process at this location is at least 4.0 log. Since the wells are existing, this RBF system is not eligible for a direct RBF credit, although it will receive what amounts to an indirect credit of 1.0 credit since the river sample would put the facility in bin 2, at minimum, but collecting bin assignment sample from the RBF system, as mandated by the LT2ESWTR, will ensure assignment to bin 1 at this facility. However, if these were pilot wells for a proposed RBF system, results from spore monitoring would justify a credit >2.5 log (including 1.0 credit of UBT), sufficient for any bin assignment
Potential DOP penalty
DOP studies inherently provide conservative estimates of the true treatment performance of the full-scale facilities being evaluated. For example, the DOP credit established using full- scale aerobic spore monitoring includes an inherent safety factor because the numerical value of the Cryptosporidium removal credit will be assumed to be equal to the measured spore removal, even though Cryptosporidium removal in literature studies is always greater than spore removal (see later discussion). Similarly, pilot-scale microbial challenge studies will be conservative indicators of full-scale Cryptosporidium removal (especially if conservative Cryptosporidium indicators like spores are used) because widespread experience of water treatment practitioners with experience using pilot studies indicates that full-scale clarification and filtration facilities routinely provide greater removal of particulates, turbidity, and microbial indicators than do pilot-scale systems. In addition, even if this was not a common observation for pilot facilities, the USEPA requirements for the DOP credit using pilot studies include a precaution requiring the verification of the relationship between pilot-scale and full-scale facilities associated with the DOP study by requiring a period of comparison where both are operated under identical conditions (same influent raw water, similar coagulation conditions, filter and clarifier loading rates, etc.).
A provision in the draft rule (section IV.C.17) and in Chapter 12 of the microbial toolbox guidance manual allows State’s the discretion to potentially penalize facilities that try a DOP study and end up mathematically demonstrating a total credit that is less than the automatic credits allowed by the Rule. Therefore, for example, a State can choose to award a 3.3 credit to a facility that qualifies for 3.5 total automatic credits if the utility was only able to mathematically demonstrate 3.3 log removal in a DOP spore study or pilot study.
However, it is difficult to see any benefit from inclusion of this potential penalty since the automatic credit is based on a national mean, not on individual performance. The use of national mean was purposely done by USEPA and deducting credits at an individual plant basis is contrary to that approach. Several huge potential negative consequences are associated with inclusion of this penalty provision, including the potential to increase costs and nuisance for facilities attempting to achieve the DOP credit using spores, and the potential to deter facilities that may have benefited from using the credit from even trying for the credit.
In summary, the prospective penalty USEPA is attempting to incorporate as part of the DOP credit requirements demonstrates a lack of understanding of the conservative nature of proposed demonstration studies for the DOP credit (full-scale spore monitoring or pilot-scale microbial challenge studies). This potential penalty is not expected to be applied to many, if any, facilities since most that will try for the credit are expected to demonstrate sufficient removal in pilot-scale microbial challenge studies or full-scale studies. However, the greatest potential impact of this potential penalty is that the mere prospect that a utility could be penalized for trying to achieve a DOP credit will deter a number of utilize from even trying for the credit, even though most facilities will probably be able to demonstrate great enough Cryptosporidium removal capabilities that they not only would not be subject to a penalty, but would be able to easily establish a higher credit for their existing facilities using the DOP credit. Therefore, a