EPRI Assessment of Past EPRI FGD Water Characterizations 

Introduction 

The US EPA recently announced its intention to conduct a two-year study to determine 

whether the Steam Electric Categorical Effluent Guidelines should be revised. An element 

of EPA's two-year study will involve data collection at various sites to provide some data on 

waste streams from power utilities. To provide information to the EPA in its Effluent 

Guidelines Study, the Electric Power Research Institute (EPRI) is providing the results from 

its earlier research studies which attempted to characterize power plant FGD wastewater 

streams at a limited number of facilities. EPRI's efforts were research oriented, and the 

objectives of the various studies differed. The sampling protocols between the studies 

varied significantly and the FGD systems evaluated also differed. Because of these 

potential issues, EPRI believes it is critical to provide our perspectives on the results of these 

studies. 

The following document is an evaluation of the FGD wastewater data obtained during the 

subject EPRI field studies. The evaluation includes a critical assessment of the data quality 

and collection techniques based on insights gained during and after the studies. This 

evaluation reflects how representative the data set is of Steam Electric Utility industry's 

FGD wastewater as a whole, and includes key lessons learned in attempting to characterize 

power plant FGD wastewater accurately. 

Overview of EPRI FGD Studies 

In 1988, EPRI initiated the Power Plant Integrated System·Chemical Emissions Study 

(PISCES) project to characterize the distribution of trace substances in air and liquid streams 

from fossil-fuel-fired electric utility power plants. The original PISCES program included 

study of trace substance concentrations in internal and effluent waste water streams in 

fossil-fuel power plants. However, the PISCES sites gave only limited insight on 

wastewater from FGD systems. As described in our earlier March 9, 2007 assessment, at the 

time of the PISCES field studies, most power plants did not employ wet FGDs and only 3 of 

the 7 sites employed FGDs. Two sites (Sites D and F) had FGDs with once-through systems 

rather than recycling absorber slurry which is more representative of most, but not all, new 

FGDs. A third site (Site E) had FGDs built in 1982 and 1995 that were operated more 

conventionally, but only one sample was collected. Therefore, with the recent wave of new 

and planned FGD installations, EPRI recently conducted additional FGD studies to better 

characterize this wastewater stream. 

FGD Screening Study (1 01 0162) 

Report Overview 

As power plants began to install FGDs in response to the CAIR (Clean Air Interstate Rule), 

CA VR (Clean Air Visibility Ru1e), and CAMR (Clean Air Mercury Ru1e), EPRI initiated a 

"screening" study to evaluate the potential issues associated with FGD wastewater. The 

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results are summarized in the EPRI report- Flue Gas Desulfurization (FGD) Wastewater 

Characterization: Screening Study ("Screening Study"). 

This report summarizes and evaluates the results of grab samples of FGD wastewater from 

seven facilities collected in January to December 2005, and incorporates the earlier PISCES 

data from Site E. Samples were collected of the FGD wastewater at a point that the facility 

felt was representative of its final FGD wastewater entering the site's overall wastewater 

management (such as an ash pond). At sites that had no discharge of FGD wastewater 

(Sites P, RandY), they were asked to collect water from a location similar to where FGD 

wastewater would be discharged at other facilities. Sampling locations are described in 

Table 1-1 of this report. 

In order to minimize costs for this study, EPRI requested power plant personnel or other 

EPRI contractors on-site (primarily conducting flue gas measurements) to submit 

wastewater samples to a common laboratory for trace metals analyses. Samples were 

collected and sent unfiltered, unpreserved and on ice to the CH2M HILL Analytical 

Laboratory. Three samples were generated for each site: a "total" sample generated by 

shaking the liquid, a settled sample generated by decanting water one hour after the 

shaking, and a filtered sample of decanted water passed through a 0.45-micron filter. No 

data outliers were noted. No matrix interference was documented, as the laboratory 

addressed interference issues by diluting samples, up to 100 times, to keep the major 

chemical interferants (such as Ca, Mg, AI, Fe) in the range in which they could be 

quantified. 1his was defined by quarterly Linear Dynamic Range studies on the analytical 

instrument. If an element such as calcium was above this range, the sample was diluted 

until the element was within the range. Getting a result within this range was necessary so 

that the result could be used in inner-element correction factors for the trace metals. 

Report Limitations 

In the Screening Study, the objective was to determine potential wastewater issues, and this 

work was conducted under a limited budget. The limitations in this Study include: 

o Available personnel (either site personnel or EPRI contractors conducting air 

studies) were used to collect FGD wastewater samples at the seven facilities; the 

water discharged to their larger wastewater management systems was targeted. 

o Because samples were collected by plant personnel, the quality of sampling is not 

known, and consistency of approach is not certain. Sample collectors were 

instructed to collect at least 2 gallons in a 5-gallon cubitainer and ship the sample to 

the laboratory. Samples were collected and shipped on ice to an off-site laboratory, 

where they were filtered several days after being collected which could have affected 

the partitioning between the solids and aqueous phases for various metals. 

o There were large variations between facilities' FGD systems, especially the solids 

separation and wastewater treatment. These variations in system configuration and 

sampling points led to widely varied samples. The 8 data sets represented vastly 

different levels of solids removal, ranging from -35 to 170,000 mg/L total suspended 

solids (TSS). 

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o The focus was on limestone-based FGDs to the exclusion of other types of FGD. 

However, there was no "big picture" design in determining the sites so that the set 

represented a range of coal types and power plant/FGD designs. Few new FGDs 

were yet installed and operational during the time of this study, so all the FGDs 

tested were older designs, i.e. began operation before 1996. 

o No FGDs were tested with a wastewater treabnent system specifically designed for 

trace metals removal. 

Due to these limitations, EPRI believes that these data should not be used by the Agency in 

its Effluent Guidelines Study. EPRI's primary concern is the partitioning between 

dissolved and particulate trace metals due to possible issues with laboratory and field 

filtration. The mercury data are the most uncertain, primarily because data exist that 

suggest different levels of mercury partitioning to the dissolved and particulate fractions. 

This is discussed in more details on the Sampling and Analytical Methods Section. While 

the total trace metal results are likely not impacted by the filtration issues, the variation in 

the 1SS levels between the eight samples could directly impact the total trace metal 

concentrations. 

Guidance Manual (1 013313) 

Report Overview 

To assist power plants in assessing future FGD wastewater management issues and to 

develop solutions, EPRI prepared EPRI Technical Manual: Guidance for Assessing Wastewater 

Impacts of FGD Scrubbers which provides an overall look at FGD wastewater including its 

composition, variables affecting composition, how to evaluate potential regulatory limits, 

FGD wastewater management options including how FGD wastewater will impact ash 

ponds, and other FGD operational issues. FGD wastewater characterization data from the 

Screening Study were used, and thus no new wastewater characterization data are 

presented. Laboratory treatability testing of FGD wastewater collected from four sites was 

described. These samples were from four of the sites in the Screening Study; however, the 

liquid was collected from a point in the FGD system that differed from the samples in the 

Screening Study, each was from a consistent point of the hydroclone overflow. Analyses of 

the water produced during treatabilty testing were done in the CH2M IDLL Analytical 

Laboratory. 

Report Limitations 

1his Guidance Manual incmporates the earlier data from the Screening Study, and thus 

there are no new FGD wastewater characterization data. The treatabilty testing results were 

limited by the small number of FGD wastewater samples available. One sample was from a 

small Chiyoda FGD system, the other three were limestone forced oxidation systems built 

between 1985 and 1995. 

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Fate of Mercury (1 009955) and Update on Enhanced Mercury Capture by Wet FGD 

(1012673) 

Report Overview 

To address issues with mercury flue gas "re-emissions" as well as potential issues with the 

management of mercury in the FGD solids and gypsum, EPRI conducted a series of 

investigations to characterize the fate of mercury in wet FGDs. This work is summarized in 

two EPRI reports: The Fate of Mercury Absorbed in Flue Gas Desulfurization (FGD) 

Systems and its update in Section 3 of Update on Enhanced Mercury Capture by Wet FGD: 

Teclurical Update. These reports summarized the status of a multi-plant assessment of 

mercury partitioning in FGD systems. Only mercury and cations and anions were reported 

in these reports. Results are presented for water at various points in the FGD system, 

though not specifically for the final wastewater produced. The information in the update is 

not the final product of the study, so some data areas are not yet complete. Two other 

studies are summarized in the Update, which focus on re-emission of mercury from FGD 

liquid back to FGD outlet flue gas. 

In the Fate of Mercury study (1009955 and 1012673), samples were collected by EPRI's 

contractor, URS. Samples were collected and filtered on-site using a 0.7-micron filter. The 

focus of these two reports are on mercury since the objective was to understand the mercury 

chemistry in the FGD water management system as to optimize flue gas mercury removal as 

well as to assist in managing the mercury in the FGD solids. 

Samples were collected from several locations in the FGD system from seven sites reported 

in the first report (1009955). Sample location varied between sites due to differences in the 

plants. Liquid sample locations included FGD absorber slurry, thickener overflow and 

underflow, primary and secondary hydroclone overflow and underflow, and "other liquor" 

(belt filter filtrate, pond discharge water, and wastewater treatment plant effluent). Samples 

were collected from a total of ten FGD systems (at seven sites) reported in the update 

(1012673). Results are presented for the absorber liquor. Additional information from this 

evaluation will be presented in later reports. Samples were collected, filtered on-site, 

preserved and sent to URS laboratories for mercury and other conventional water 

parameters analyses. 

Report Limitations 

The key objective was to understand the FGD mercury chemistry, and thus the results are 

primarily limited to mercury. The sites were selected to represent a range of FGD designs 

and operating conditions, such as reagent type, oxidation mode, fines blowdown, and coal 

type. The limitations in the sites selected are that most of the FGDs are older design (only 

one built after 1996), and only one of the sites was tested with the SCR in operation. Note 

four sites were tested in the non-ozone season with the SCR off-line. For the one power 

plant with the SCR on-line, this site measured flue gas mercury removals of about 35% - as 

compared to other EPRI, DOE, EPA, et. al. studies that indicate that for bituminous coals, 

the SCR/FGD combination were able to achieve ~85% flue gas removals. Thus EPRI 

believes that the tested SCR site may not be representative of other bituminous plants with 

SCR and wet FGD. 

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The Update 1012673 sununarizes the results of an evaluation of varying field filtration and 

preservation techniques; however this evaluation did not include the limitation of 

laboratory-filtered samples. 

Sampling Methods, Locations, and Overall Representativeness 

For the FGD wastewater characterization studies, EPRI partnered with various host utilities 

that voluntarily participated. Because of this, the sites sampled were based in large part on 

a willingness to participate. Few of the newer FGDs were yet installed and operational 

during these studies, so most of the FGDs tested were older designs, i.e. only one of the 

FGDs tested began operation after 1997. Therefore, the overall representativeness is focused 

on older generation FGD systems, and not the newer systems being installed this decade. 

To help encourage participation in these wastewater characterization studies, EPRI agreed 

to not identify the name of the utility and the power plant. A description of the FGD studies 

is provided in Table 1. 

Table 1 

Summary Description of EPRI FGD Wastewater Characterization Sites 

FGD Liquid- Year 

Solid FGD 

Sampling Separation SCR Online 

srte Location Process TypeofFGD FGD Mfr. ? Coal 

Screening_ Studr_£10101621 

E* Sludge fixation Thickener Umestone, Babcock& N Bituminous 1982 & 

process inhibited Wilcox 1995 

overflow and oxidation with (B&W) 

waste sludge fixation 

p Downstream of Thickener Limestone, natural CE y Bituminous 1996 

FGD blowdown oxidation (Combustion 

holding tank, Engineering) 

upstream of 

thickener 

R Between FGD Thickener Limestone, natural MHI y Bituminous 1983 

blowdown holding oxidation 

tank and thickener 

s· From primary Primary Limestone forced Company· y Bituminous 1985 

hydroclone hydroclone oxidation designed (B&W 

overflow, before (water also assistance on 

sent to ash pond wasted from the upgrades • 

gypsum trays) 

dewatering 

press) 

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Table 1 

Summary Description of EPRI FGD Wastewater Characterization Sites 

FGD Liquid- Year 

Solid FGD 

Sampling Separa'lion SCR Online 

Site Location Process Type of FGD FGDMfr. ? Coal 

T No distinct chloride Primary Limestone ex-situ ABB y Bituminous 1990 

purge stream. hydroclone forced oxidation 

Collected from 

tank which 

contains a mixture 

of -60% primary 

hydroclone 

overflow, belt rinse 

water, and 

hydroclone 

underHow water. 

u At reclaim tank Thickener Limestone, natural ABB N Sub- 1992 

(which is after the oxidation Bituminous 

thickener which 

removes nearly all 

solids) 

y Downstream end Gypsum stacking Limestone, forced Chlyoda and N Bituminous 1992 

of the 3rd gypsum pond oxidation owner 

stacking pond 

z· From tank Overflow from Limestone, ABB/Aistom N Bituminous 1995 

beforeWWTP primary forced oxidation 

(overflow from hydroclone 

the clarifier) goes to a 

clarifier 

The Fate of Mercury Absorbed In Flue Gas Desulfurization (FGO) Systems (1009955) from Table 2-1 

1 Thickener, Mg-lime, ex-situ Wheelabrator y High-S 1980 

hydrocyclones, forced oxidation /Pullman- Bituminous 

belt filter Kellogg 

4 Thickener Mg-lime with Marsulexl y High-S 1976, 

combined Chemico Bituminous 77 

particulate 

removal, 

inhibited 

oxidation 

6 ** Hydroclones, Limestone, GEES I/ y High-S 1994 

two stage, belt forced oxidation Marsulex Bituminous 

filter 

7 Hydroclones, Limestone, GEES I/ y High-S 1995 

belt filter forced oxidation Marsulex Bituminous 

8 Thickener, Limestone, Wheelabrator N TX lignite 1985 

drum filter inhibited /UOP 

oxidation 

9 ** Thickener, Umestone, Whee!abrator y High-S 1977 

drum filter inhibited /UOP Bituminous 

oxidation 

10 Thickener, Limestone, CE/Aistom N Sub- 1992 

belt filter natural oxidation Bituminous 

Urzdate on Enhanced Mercua. CaQture br: Wet FGD £1012673l from Table 3-1 

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Table 1 

Summary Description of EPRI FGD Wastewater Characterization Sites 

FGD LiquidSolid 

Sampling Separation 

Site Location Process 

Thickener 

. Thickener 

1---- Sampled FGD 

absorber liquor, Thickener 

as well as 

overflow and 

* underflow of Hydroclone, twoliquid-solid stage - separation Hydroclone, one- devices stage 

* Hydroclone, onestage 

.. Hydroclones, 

two-stage; belt 

filter 

Notes: 

ABB - Asea Brown Boveri 

B&W = Babcock and Wilcox Company 

DOE = U.S. Department of Energy 

FGD = flue gas desulfurization 

MG = million gallons 

MHI - Mitsubishi Heavy Industries 

TypeofFGD 

Mg-lime, 

External forced 

oxidation 

Limestone, 

inhibited 

oxidation 

Limestone, natural 

oxidation 

Limestone, forced 

oxidation 

Limestone, forced 

oxidation 

Limestone, forced 

oxidation 

Limestone, 

forced oxidation 

SCR 

FGD Mfr. ? Coal 

GEESI/ N High-S 

Marsulex Bituminous 

B&W N High-S 

Bituminous 

Pullman- N SubKellogg/ Bituminous 

Wheelabmtor 

ABB/Aistom N Medium-S 

Bituminous 

ABB!Aistom N Medium-S 

Bituminous 

American Air N High-S 

Filter Bituminous 

GEES I/ y High-S 

Marsulex Bituminous 

PRB = Powder River Basin 

S =sulfur 

WWTP = wastewater treatment plant 

Year 

FGD 

Online 

1994 

1995 

1979/ 

1980 

1994/ 

1995 

2001 

1982/ 

1985 

1994 

* - Site tested in both the Screening Study 

and Fate of Mercury Studies 

** - Site tested in both the 2005 and 2007 

Fate of Mercury Studies. 

Water management varies widely between FGD systems, and therefore there is a wide 

range in the quantity and characteristics of the wastewater generated. In evaluating 

whether the FGD sites studied represent FGDs across the industry as a whole, it is useful to 

understand what factors may affect FGD wastewater (for full discussion, see Section 2 of the 

Guidance Manualr 1013313). 

• Fuel characteristics: Variation in coal characteristics can affect FGD wastewater metals 

content, especially for those metals likely to significantly partition from the coal to the 

flue gas vapor phase entering the FGD. Coal chloride level (specifically the ratio of coal 

chloride to sulfur) typically drives the amount of FGD wastewater produced as FGD 

operations are often controlled based on chloride levels. 

• Liquid/Solid separation technology: There is a wide range of liquid/solid separation 

devices in use for removing solids from the recirculated FGD liquid. These can produce 

wastewater with very different solids content. 

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• Oxidation and Organic add additives: Limited data suggest that organic acid additives 

(e.g. dibasic acid or DBA) may affect the speciation of selenium. Most, if not all, new 

FGDs use forced oxidation which may affect selenium speciation. 

• Gypsum management: If gypsum produced in an FGD is sold for wallboard 

production, it will likely be washed, sending more fine particulate matter to the FGD 

wastewater stream. 

• Reagant and/ or source water characteristics. Lime, limestone, and other reagents will 

introduce varying amounts of solids and trace metals. Most, if not all, new FGDs use 

limestone. The impact of the trace metal loadings in the reagent (e.g. limestone) on the 

trace metal loadings in the wastewater is not known. 

Our review of the available data and comparison with the overall industry identified the 

following gaps and analytical needs. 

• Most of the EPRI FGD field study systems began operation before 1997. FGD design 

has evolved over the past 30 years, e.g. high-efficiency S02 removal absorber designs as 

well as employing hydroclones instead of thickeners for solids separation. Newer FGDs 

have much tighter water balances than the older technologies and therefore tend to 

discharge at much lower flow rates. 

• For the data available, none of the EPRI FGD sites included a wastewater treatment 

system designed specifically for trace metals removal. Two of the later Fate of Mercury 

Study sites did sample at various locations around the wastewater treatment system and 

these analyses are on-going. 

• It is critical to understand the impact of on-site filtration as compared with laboratory 

filtration, but there has been no study yet making this comparison. This could be 

important for mercury because it may have different partitioning between dissolved and 

particulate based on the limited available data with field filtering versus those with lab 

filtering. No data are currently available to assess if other metals had differences in 

partitioning between the two filtration methods. 

• A mass balance of all streams into and out of a FGD wastewater system should be done 

to better understand partitioning of metals that pass through an FGD system. 

Sampling and Analytical Methods: Lessons Learned 

The FGD field studies provide useful insights in conducting future sampling and analytical 

studies. Guidance has been provided to EPA in a separate report "Sampling and Analytical 

Plan Guidance for Water Characterization of Coal-Fired Steam Electric Utility Facilities" 

dated March 2007, "Supplemental Information: Sampling Coal-Fired Electric Power Plant 

Wastewaters" dated May 17, 2007, and "Summary of Power Plant Wastewater Matrix 

Interferences and Suggested Remedies" dated March 30, 2007. Information on mercury 

partitioning and arsenic and selenium speciation specific to the FGD studies is presented 

below. 

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Mercury Partitioning 

The partitioning of mercury within the FGD system is important, since mercury that 

partitions into solids will likely be removed in the gypsum stream and in the solids 

separated out of the FGD wastewater stream. Therefore the mass of mercury in the 

wastewater stream discharged from the FGD would be much lower than the total mass 

captured by the FGD. The various studies described herein each collected some information 

towards understanding partitioning of mercury, however, differences in the project 

objective and approach between the studies makes it difficult to draw definitive 

conclusions. The plants visited had different types of FGD systems, differing power plant 

designs such as SCR operation, differing coals, and different water management 

approaches. Sample locations also differed, as is shown in Tables 1 and 2. 

Of potential importance were the differences in sampling methods. One study included 

field filtering while the other study used lab filtering. Lab filtering simplified the sampling 

effort and may have reduced the likelihood of contamination. However, the several days 

that the sample was allowed to sit may have allowed mercury to partition from dissolved 

into particulate form in shipment. 

The total mercury in FGD wastewater was relatively similar between the studies, as is 

shown in Table 2. However, the partitioning between liquid and solid phase varied widely. 

In the Screening Study, nearly all mercury in the wastewater was in particulate form. 

However, in the Fate of Mercury Study, samples were filtered on-site- and indicated a wide 

range of mercury in the particulate fraction, varying from 20 to 99%. This difference in the 

mercury partitioning may be related to the differences in the filtration approach and/ or 

differences in the power plants tested. Thus EPRibelieves the dissolved/particulate 

mercury data from the Screening Study includes significant uncertainty and recommends to 

the Agency that these results not be used in its Effluent Guidelines Study. 

Table 2. Mercury Partitioning Comparison Between Studies 

Parameter Fate of Mercury Study Screening Study 

~otal Mercury''" (J.'9/L) p.s to 79 (median "" 17) 1 to 48 (median = 7) 

Partitioning % of mercury in 20 to 99% solids in absorber liquid for the S2% to 99% particulate* 

solid or particulate form in limestone forced-oxidation systems 

FGD wastewater or absorber ~6 to 99% in the other systems iquid 

Method of filtering Field filtered with a 0.7 micron filter Lab filtered with a 0.45 micron filter 

Sampling location that best pyertlow from the dewatering device such !sites were asked to sample the 

represented wastewater from as hydroclone or thickener ~stewater produced and sent to 

FGDsystem reatment. For several sites, where 

r.vastewater was not discharged, the 

~ample was taken from a location 

~ere wastewater could be blown 

~own. 

SCR? pnly tested one FGD with an SCR on-line ~ sites had SCRs, 4 did not 

*- Particulate fraction of for those samples with typical suspended solids loading (two sites had <40mg/L TSS 

and correspondingly had low percent particulate mercury: 5% and 56%) 

••- It should be noted that this set of data come from varvina sample locations and using different methods for 

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filtration so care is needed in comparing the data points. 

Analytical Procedures including Arsenic and Selenium Speciation 

A finding of the Screening Study was that forced oxidation systems may produce selenium 

primarily in the oxidized selenate form, while other FGDs produced the reduced form 

selenite. This would be significant as selenate is more difficult to treat, though less toxic, 

then selenite. 

The arsenic and selenium speciation methods used in the FGD studies varied. In the 

Screening Study, Sites Rand U used the inductively coupled plasma mass spectromeiry (ICICP-MS) method, while Sites Sand T used the hydride method (EPA 1632). See the "EPRI 

Assessment of the PISCES Water Characterization Studies" letter dated March 9, 2007 for 

further discussion of speciation methods. Further research is needed to confirm these 

arsenic and selenium speciation trends. 

Key Observations and Lessons Learned 

There are a number of potential variables that may impact FGD wastewater, including coal 

properties, FGD type, source water, and FGD operation. There is potentially very wide 

variability in FGD wastewater characteristics as coal usage changes and due to cycles of 

concentration used for blowdown in FGD system operations. Because a number of new 

FGDs are now being installed, it is important to understand how older (i.e. 1980's vintage) 

and current FGD designs may impact wastewater characterization. In addition to the S02-

removal efficiencies, FGD systems may vary widely in the way water is managed in terms 

of solids removal (percent solids), method of solids removal (hydroclones, thickeners, and 

settling ponds), how the blowdown is managed to control chlorides and/ or fines, where the 

blow down stream is taken, and trace metals treatment (settling pond, iron coprecipitation, 

constructed wetlands). All of these factors may result in a wide range of wastewater 

characteristics between facilities. 

The EPRI studies to characterize FGD wastewaters have been of limited scope and duration 

(typically 1 to 3 samples each). Therefore, conclusions based on the studies may not be 

generally applicable to the rest of the industry, and in fact, none of the collected samples are 

fully representative of the newer FGD technologies currently being built. The key lessons 

learned during the studies include: 

• Sampling: It is vitally important to design and document proper sampling and analysis 

protocols in advance of any FGD sampling. It is imperative to understand the operation 

of the system in order to collect representative wastewater samples. If planning with 

site personnel is limited to the phone, it is challenging and should be done with process 

flow diagrams or other tools to clarify sampling locations. Further, it is recommended 

that fuhrre samples be collected of wastewater at the discharge from the FGD (as was 

done in the Screening Study), but also at a point that can be standardized between 

facilities to allow cross-plant comparisons. 

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• I 
• Analysis: Analysis of FGD wastewater is very complicated due to matrix interferences 

as was discussed in the March 30, 2007 EPRI "Summary of Power Plant Wastewater 

Matrix Interferences and Suggested Remedies" memorandum to EPA. 

• Mercury: The various studies employed different sampling and filtration techniques, 

and tested different sites. The number of variables makes the results difficult to 

interpret and compare across the various sites. Based on the limited available data, 

laboratory filtration may impact the partitioning of mercury between the dissolved and 

particulate fractions. Additional research is needed to better evaluate the various 

filtration approaches. 

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