Optimizing the Cubuy Water Treatment Plant for Achieving the Long Term 2 
Enhanced Surface Water Treatment Rule Requirements 
 
Emeliz Torres Cabrera 
Civil Engineering 
Auristela Mueses, PhD 
Civil and Environmental Engineering Department 
Polytechnic University of Puerto Rico 
 
Abstract  The Long Term 2 Enhanced Surface between microbial pathogens and disinfection 
Water Treatment Rule is requiring additional byproducts (DBPs).  
removal or inactivation of Cryptosporidium Oocyst In order to improve the public health protection 
for public drinking water system.  The Cubuy Water through the control of microbial contaminants, it 
Treatment Plant needs to provide an additional one was created the Long Term 1 Enhanced Surface 
log removal/inactivation of Cryptosporidium.  This Water Treatment Rule (LT1ESWTR). Later on 
project has the main objective of determining the January 5, 2006 the USEPA published the Long 
most efficient alternative to achieve the Long Term Term 2 Enhanced Surface Water Treatment Rule 
2 Enhanced Surface Water Treatment Rule (LT2ESWTR), to address higher risk of public 
requirements. Three alternatives were considered water contamination with Cryptosporidium 
using different approaches for removal or microorganism, aiming for protection measures 
inactivation of the microorganisms.  Existing beyond those required for existing regulations [1]. 
facilities were evaluated to determine performance The LT2ESWTR has the objective of reducing 
limiting factors.  Field test were performed to microbial contamination, especially the protozoan 
identify possible operational or infrastructure parasite Cryptosporidium. The LT2ESWTR 
deficiency problems. The best alternative was requires source water monitoring at public water 
selected based on: Net Present Value, Reliability, system (PWS) that use surface water or ground 
Complexity, and Environmental Impacts.  The water under the direct influence of surface water 
recommended alternative includes filter plant (GWUDI).  Based on system size, systems need to 
optimization and disinfection using ultraviolet monitor for Cryptosporidium, E. coli, and turbidity 
reactors.  This alternative is capable of comply with [2]. Therefore, not all systems will need additional 
the new regulatory requirements and corrects other protection, and the decision as to which treatment 
Plant compliance problems. technique(s) should be implemented should be 
Key Terms  Cryptosporidium, Filter based on site specific conditions. 
Assessment, Long Term 2 Enhanced Surface Water Current regulations require filtered water 
Treatment Rule, Water Treatment Plant. systems to reduce source water Cryptosporidium 
levels by 2-log (99 percent). Recent data on 
INTRODUCTION Cryptosporidium infectivity and occurrence 
indicate that this treatment requirement is sufficient 
The Safe Drinking Water Act (SDWA) is the 
for most systems, but additional treatment is 
main federal law that protects the public health by 
necessary for certain higher risk systems [3]. These 
regulating the nation’s public drinking water 
higher risk systems include filtered water systems 
supply.  Later, the SDWA authorized the 
with high levels of Cryptosporidium in their raw 
Environmental Protection Agency (EPA) to set 
water sources and all unfiltered water systems, 
standards for potable water protection to work 
which do not treat for Cryptosporidium.  
together to ensure that these standards are 
In order to comply, the EPA and the Puerto 
complying. The 1996 amendments to the SDWA 
Rico Department of Health (PRDOH) have adopted 
require EPA to develop rules to balance the risks 
a multiple barrier approach for treating drinking 
water.  All Water Treatment Plants (WTP’s) in adequate flocculation process. Then the water 
Puerto Rico monitored there source water.  Based passes to the flocculation chamber, in which the 
on the EPA’s Source Water Monitoring Guidance slow mixing occurs to allow the formation of 
Manual for PWSs for the Final LT2ESWTR [4], bigger flocs that will be able to settle in the next 
water systems were divided into four groups process.  Following flocculation the water flows to 
depending on system size. This project is focused the sedimentation chambers, which are equipped 
on Cubuy WTP included in Group 4, which are with tube settlers. Then the water is conveyed 
those systems that serve less than 10,000 people. through surface collector launders to the filters. The 
For these systems two options were available to filters are dual media filters, consisting of 
monitor their raw water sources: conduct E. coli anthracite and sand media. The filtered water flows 
monitoring first and based on those results, the by gravity to the clear well, in which the post 
system may or may not need to conduct disinfection is performed. The facility uses chlorine 
Cryptosporidium monitoring; or systems may go for disinfection.  
directly to Cryptosporidium monitoring. The Puerto 
Rico Aqueduct and Sewer Authority (PRASA) METHODOLOGY 
selected option 2. 
In order to achieve the objectives of this 
Project, the following methodology will be 
DESCRIPTION OF EXISTING FACILITY 
followed. 
The CWTP is a conventional surface water 
Assessment of Plant Performance 
treatment plant, which provides drinking water for 
approximately 3,095 people in the Cubuy WTP The performance assessment uses historical 
service area.   This service area includes the data from plant records supplemented by data 
Maizales and Río Blanco sectors of Naguabo.  The collected during a filter assessment.  This data was 
CWP has a design capacity of 0.50 million of used to determine the status of the Cubuy WTP 
gallons per day (MGD), with an average flow of relative to achieving certain performance criteria to 
approximately 0.31 MGD. comply with the LT2ESWTR, and allows 
The raw water intake, known as “La Mina”, is identifying possible causes of non-compliance.  To 
located approximately 1 mile from the CWTP. The achieve compliance, the Cubuy WTP must 
intake has a single bar screen with the objective of demonstrate that it can produce consistent high 
retaining the larger suspended material, then the quality finished water, regardless of constant 
water is conveyed by gravity through an 8-inch raw changes in raw water characteristics.  The 
water pipe to the treatment facility.  performance assessment determines if major unit 
The CWTP is composed of two steel tanks treatment processes consistently perform at 
modules consisting of flocculation, sedimentation optimum levels to provide maximum multiple 
and filtration compartments, a holding tank and a barrier protection.  To achieve great result of the 
clear well.  In order to treat the sludge accumulated, assessment of the plant, various analyses of the 
the plant has a holding tank, a sludge thickener and plant operation and unit processes must be 
two sludge drying beds. The plant also has a dry performed. 
sludge storage area. Evaluation of Major Unit Processes 
Chlorine and primary polymers are injected 
The major unit process evaluation is an 
into the raw water line prior to entering the 
assessment of the treatment potential, from the 
flocculation unit.  A static mixer installed in the 
perspective of the capability of existing treatment 
pipe accomplished the rapid mixing necessary to 
processes to achieve enhanced performance levels.  
promote particle destabilization to promote an 
The major units were evaluated for capacity of each 
individual unit, and also for the capability to handle EVALUATION OF EXISTING FACILITY 
peak instantaneous flows [5].  The peak 
As part of this design project, an evaluation of 
instantaneous flow is generally determined from 
the plant’s current compliance status was 
historical data.  The major unit processes evaluated 
performed.  The compliance status evaluated 
was: flocculation, sedimentation, filtration and 
several parameters including haloacetic acids 
disinfection.  The evaluation was based on an 
(HAA ), turbidity, total organic carbon (TOC), and 
assessment of the existing unit process to meet 5
total trihalomethanes (TTHM).  The results were 
enhanced performance goals, not the goals that the 
compared with the EPA regulatory compliance data 
existing facility was designed to achieve. The 
in order to identify the improvements that are 
objective is to classify the treatment units in one of 
needed to achieve full compliance.  
three types of units. Type I: can manage the peak 
instantaneous flow; Type II: can manage the peak Total Carbon and Disinfection by Products 
instantaneous flow with operational changes; and 
Under Stage 1 of the Environmental Protection 
Type III: the units that will require infrastructure 
Agency, Disinfectants and Disinfection By-product 
improvements. 
Rule, a treatment technique is established for the 
Review and Trend Charting of Plant Operating removal of TOC.  By reducing the level of TOC, 
Records the formation of the Disinfection by Products 
(DBP’s) will also be reduced.  The treatment 
Trend graphs were prepared based on the 
technique is enhanced coagulation and it applies, 
maximum daily turbidities, from one year of data of 
depending on the TOC and alkalinity levels in the 
Cubuy WTP operating records.  Turbidity data 
raw water source, to public water systems using 
were collected for the raw water, settled water, 
conventional filtration.   
finished water, and individual filter effluents. A 
Figure 1 shows the average removal ratio of 
performance potential graph is used to evaluate 
TOC per trimester. The federal regulations requires 
major unit processes.  The potential graph will 
to the public drinking water system to inform the 
show the flow capacity of each major unit; this 
running annual average ratio. The regulation 
would be compared to the instantaneous peak flow 
requires that systems maintain through the year a 
of the plant.  Maximum data from the most recent 
removal ratio equal or greater than 1. Based on 
one year were used, to prepare trend charts since 
these values, the annual running average from 
the goal is to assess the integrity of each barrier at 
January 2010 to December 2011 is 1.10. There was 
the peak time.   
no data available for the second trimester of 2011. 
The CT, concentration of chlorine per contact 
time, was calculated to evaluate efficiency of 
disinfestation and inactivation for Giardia lamblia 
and virus. 
In addition, a filter assessment was performed 
to evaluate each filter to identify anything that is 
hindering the performance of a non-optimized 
filter.  Different tests can be performed to evaluate 
the performance of each filter: Visual inspection of 
the media condition, Evaluating backwash protocol, 
Visual backwash observation, Bed expansion test, 
Rise rate test, and Backwash water turbidity profile  
Figure 1 
are some. 
Removal Ratio for TOC in Cubuy WTP 
Other parameters evaluated were HAA5 and 
THHM.  The regulatory maximum contaminant 
levels (MCL) of these compounds are 0.08 parts per 
million (ppm) and 0.06 ppm for TTHM and HAA5, 
respectively.  
Figure 2 shows the HAA5 data obtained during 
period 2010 to 2011.  It can be observed, that 
compared to the MCL the parameter was achieved 
consistently over these periods.  However, it is 
important to point out that the second trimester of 
 
the two periods evaluated overpassed the 75 percent 
of the MCL.  This requirement goal is established Figure 3 
TTHM Data for Cubuy WTP 
in order to notice that the results are close to reach 
the MCL and in case that in a near future the Turbidity 
regulation change, it could be possible that the 
Turbidity is a measure of the cloudiness of 
Plant would not achieve compliance.  The reason 
water and it is a key parameter in determining water 
that the results of this parameter are higher than the 
quality.  The turbidity limits established by law are 
requirement goal could be because the season of the 
0.3 Nephelometric Turbidity Unit (NTU) 95% of 
year; the second trimester comprise the months 
the time, and not exceeding 1 NTU at any time, 
from April to June which are considered months of 
except for the first 15 minutes after a filter 
high precipitation.   
backwash. 
Despite the fact that there are regulatory limits, 
it is preferable to measure the Plant’s performance 
with stricter parameters, such as optimization limits 
proposed in the EPA Handbook CPE Manual [5]. 
Some of EPA’s optimization limits considered for 
the purpose of this evaluation are:   
 For a facility to be optimized, the combined 
filter effluent should be under 0.1 NTU 95% of 
the time and not exceeding 0.3 NTU at any 
 moment.   
Figure 2  Over filter effluent should be less than 2 NTU 
HAA5 Data for Cubuy WTP when raw water turbidity is 10 NTU or greater.  
If the average raw water turbidity is less than 
TTHM were also evaluated for the period of 
10 NTU, then the limit is less than 1 NTU for 
2010 to 2011.  Figure 3 shows the data obtained for 
the clarified water. 
TTHM.  This figure shows that the plant did not 
exceed the MCL.  However, the third trimester of Daily turbidity data from the plant was 
2011 the plant exceeded the requirement goal of provided by PRASA for the period 2012 through 
TTHM with a value of 0.0758 parts per million 2013.  Figure 4 shows a turbidity profile for the 
(ppm).  As previously mentioned, this requirement Cubuy WTP.  This figure shows turbidity values for 
goal is to indicate that the results are near the MCL.  raw water, over filters, combined filter effluent 
compared with the sedimentation effluent goal, 
regulatory requirement and with the effluent goal.  
As shown in Figure 4, the Cubuy WTP experienced 
high turbidity variable spikes in its raw water (pink 
line) during this period.  A WTP should be capable 
of managing these spikes when operated properly.  
The 95 percentile of the turbidity values for raw 
water is 54.22 NTU.  Therefore the sedimentation 
effluent should be less than 2 NTU.  As 
demonstrated in the figure, although the CWTP 
frequently achieved values less than 2 NTU in the 
over filters samples (dark red line), some 
exceedances above this limit were observed.  The 
Figure 5 
combined filter effluent (green line) shows values 
Turbidity Monthly Turbidity Compliance Percentages 
of less than 0.3 NTU.  It is important to state that 
Bacteriology 
the 95 percentile of combined filter effluent values 
resulted in 0.3 NTU, which is the regulatory The Total Coliform Rule (TCR) is the Federal 
turbidity requirement.  However, the combined regulation under the SDWA that sets MCLs and 
filter effluent does not achieve the optimized goal monitoring requirements for certain biological 
of 0.1 NTU 95 percent of time.  Also various spikes contaminants [6].  It requires every public water 
are over the sedimentation effluent goal. system to periodically collect samples and analyze 
them for bacteria called coliforms.   Total coliforms 
are a group of closely related bacteria that are 
generally harmless.  Their presence in drinking 
water suggests that there has been a breach, failure, 
or other change in the integrity of the water system 
and pathogens may have entered into drinking 
water.  The number of routine samples required 
each month, quarter, or year depends on the system 
size and source water.  This information is 
established by the EPA on the Total Coliform Rule.  
According to this Rule, Cubuy WTP requires 3 
Figure 4 
Turbidity Profile of Cubuy WTP samples per month. These samples are taken in 
predetermined points in the distribution systems. 
Also the monthly turbidity compliance 
These points are determined in cooperation with the 
percentages from the CWTP for the period between 
Puerto Rico Department of Health. The 
2010 through 2011 were also obtained from 
bacteriology results of Cubuy WTP were analyzed 
PRASA.  Figure 5 shows the monthly turbidity 
for the periods of 2010 through 2011, resulting in 
removal percentages.  It can be observed that the 
positive samples ranged between 2–8.  The system 
plant achieved 100% compliance for all the period. 
incurred in violations in two months, January and 
There was no data available for the months of 
February of 2010. 
January through March of 2010. 
Potential Performance Graph 
To assess the Plant’s capacity for managing the 
peak instantaneous flow a potential performance 
graph (PPG) was generated.   Figure 6 shows the 
PPG of the Cubuy WTP, showing a peak 
instantaneous flow of 0.5 mgd.   
values for the 3-log inactivation of Giardia lamblia 
Cysts (CT 99.9) and the 4-log inactivation for 
viruses (CT 99.99) are 157.7 and 2.4, respectively.  
These values are taken from the CPE Manual based 
on the pH and temperature of the Plant, as 
established by EPA.  
To compare the compliance of the actual CT 
value for the facility with the value required by 
EPA, a ratio is calculated. If the ratio is greater or 
 equal to 1.0, adequate disinfection is provided. 
Figure 6 
Table 1 
Cubuy WTP Potential Performance Graph 
CT Calculation of Cubuy WTP Existing Conditions 
As shown on Figure 6, it can be concluded that Floculation Sedimentation Filtration Distribution Tank
Volume 12,925.44 26,198.00 12,925.44 75,000.00
the flocculation and filtration units processes has Flow rate 
347.22 347.22 347.22 347.22
the capacity to handle the peak flow of the Plant, (gpm)
Theoretical 
however the sedimentation and disinfection 37 75 37 216Detention 
Baffling Unbaffled (mix Unbaffled (mix 
processes does not have capacity to treat the peak Average Average
Condition flow) flow)
flow. The disinfection unit is capable of treat only Factor 0.5 0.1 0.5 0.1
T  (min) 18.6 7.5 18.6 21.6
the 68% of the plant’s peak flow. This means that 10
Disinfectant 0.1 0.8
capital improvements are necessary to be able to pH 7.6
Temp (°C) 23
treat the 0.5 MGD. CT Value 2.6 32
CT Total 35
CT 99.9 157.7
CT Evaluation CT 99.99 2.4
Giardia lamblia Ratio (99.9) 0.22
The CT factor shall be calculated to all surface Viruses Ratio (99.99) 14.49  
water system or groundwater systems under the It can be observed in Table 1, the CT value for 
direct influence of surface water.  CT represents the Cubuy WTP is 35. This ratio value is under the CT 
concentration of free chlorine residual in mg/l in a required for 3-log inactivation for Giardia lamblia 
contact time in minutes. Once the actual CT is Cysts, but complies with the 4-log inactivation for 
calculated, this number must be compared to a viruses.     
minimum CT value set by EPA.  This value is used 
Filter Assessment 
to illustrate the effectiveness of chlorine to 
inactivate Giardia cysts. The objective of the filter assessment is to 
To effectively determine the CT value for understand limiting factors affecting the filtration 
Cubuy WTP, the facility was divided by chlorine process [7]. This unit is particularly important 
injection points. The first group includes the because is the final barrier for removing 
flocculation and sedimentation units, while the microorganism of the treatment plant. The test 
second group includes the filters and the performed to the Cubuy WTP included 
distribution tank. Once the contact time is instantaneous turbidity profile through treatment 
calculated (volume divided by the flow rate), it is processes, filter backwash assessment and filter 
necessary to adjust this time with a baffling factor. recovery evaluation.  It was intend to measure the 
The baffling factor to be assigned is based on the media expansion of the filter during backwash 
description of CPE Manual, 1998. The actual CT is evaluation. This test consists in measuring the 
the sum of the CT’s obtained for group 1 and 2. expansion of the filter media during the high rate 
Table 1 shows the calculations performed for filter backwash. However it could not be completed 
the CT evaluation in each unit processes. The CT because after completed the back wash process the 
water was turbid and the measuring device cannot turbidity measurements are less than 0.15 NTU in 
be appreciated.  The assessment was performed to at least 95 percent of the values recorded in each 
filter No. 1. month, excluding the 15 minute period following 
Proper maintenance of filters is essential to return to service from a filter backwash; and no 
preserve the integrity of the filter as constructed. individual filter has an IFE measured turbidity 
Technical resources consider a filter to be greater than 0.30 NTU in two consecutive 
adequately clean when the spent backwash water measurements taken 15 minutes apart.  This 
turbidity is in the range of 10-15 NTU [8]. alternative will focus on meeting the LT2ESWTR 
The backwash process was performed on requirements by the optimization of the combined 
automatically, and lasted only five minutes.  It was and individual filter performance. 
observed mudballs were formed on the top of the Based on the PPG included in Figure 6, the 
media. The presence of mudballs in the filter media existing filters have a treatment capacity of 0.83 
is an indication of inadequate filter backwashing MGD, which results in the need to provide an 
over time.  The backwash process resulted on a additional filter unit to achieve the desired 4.0 
2
turbidity of 21.3 NTU, therefore after finished the gpm/ft  with one unit out of service because with 
backwash the filter was unclean. only one filter the filtration ratio is approximately 
2
Another criterion to assess plant optimization 4.82 gpm/ft  which is higher than the desired. By 
is the filter recovery period. This period is providing an additional filter of the same 
comprised by the 15 minutes following a filter dimensions of the existing units (9 ft long by 8 ft 
backwash. Ideally it is required that the filter wide), the maximum capacity of the filters would 
recuperate in 15 minutes. During this period it is be 2.21 MGD. With the new filter unit, the loading 
desirable that the turbidity in the filtered water does rate of the filters at the maximum flow of 0.5 MGD 
2
not exceed 0.3 NTU and that it stabilizes at less would be 1.60 gpm/ft  with all units in operation, 
2
than 0.1 NTU in less than 15 minutes [9].  and 2.41 gpm/ft  with one unit out of service. These 
The turbidity resulted on approximately 0.25 loading rates result in a robust system that should 
NTU which is higher than the expected stabilization be able to achieve the additional 1.0-log removal 
value of 0.1 NTU. credit by meeting the CFE and IFE turbidity 
criterion included in the LT2ESWTR. To optimize 
EVALUATION OF ALTERNATIVES the system, the existing filters should also be 
rehabilitated and upgraded as part of the alternative.  
Three alternatives were evaluated to identify 
Rehabilitation activities shall include: replacement 
improvements in the Cubuy WTP to provide the 
of underdrain, filter media and addition of an air 
1.0-log removal requirement for Cryptosporidium 
scouring system, along with instrumentation to 
of the LT2ESWTR. In addition, other rehabilitation 
provide the operator the necessary information to 
activities not associated with the LT2ESWTR have 
optimize the operation of these units. 
been included.  
To address non-compliance events recorded for 
Alternative No. 1 – Optimization of Existing the HHA5 and TTHM, it is recommended to reduce 
Facility the disinfection achieved by pre-chlorination in the 
The LT2ESWTR provides a 0.5-log removal facility and provide additional contact time to 
credit if the combined filter effluent (CFE) turbidity achieve a higher degree of disinfection via post-
measurements taken for any month are less than chlorination reducing the bacteriology. At the 
0.15 NTU in at least 95 percent of the maximum flow of 0.5 MGD, and considering a 
measurements. An additional 0.5-log removal credit temperature of 23°C, pH of 7.6 and chlorine 
is awarded if the individual filter effluent (IFE) concentration of 0.8 mg/l, the required CT to 
achieve a 3-log inactivation of Giardia with post-
chlorination only would be 157.7, resulting in a inactivation requirements, resulting in a reduction 
contact volume of approximately 100,000 gallons.  of the contact tank volume required to meet with 
Therefore the construction of a new tank of 0.1 disinfection requirements. Even with a UV system, 
million gallons it is recommendable.  Table 2 chlorine use will still be required in the facility, 
shows the cost estimate of the Alternative No. 1. mainly to achieve virus inactivation, and to 
maintain the minimum required residual chlorine in 
Table 2 
Alternative 1 – Cost Estimate the distribution system.  Table 3 shows the cost 
estimate of the Alternative No. 2.  
Items Cost
New Tube Settlers in Sedimentation Basin $    36,100.00 Table 3 
Alternative 2 – Cost Estimate 
New Continuous Sludge Removal System $    48,100.00
New Filter $    36,100.00 Items Cost
Rehabilitation of existing Filters $    72,200.00 New Tube Settlers in Sedimentation Basin $    36,100.00
New Contact / Distribution Tank (0.1 MG) $ 1 30,000.00 New Continuous Sludge Removal System $    48,100.00
New Control System and Instrumentation $    18,100.00 New Filter $    36,100.00
Sub-Total $  340,600.00 Rehabilitation of existing Filters $    72,200.00
Site work & Yard Piping (5%) $    17,100.00 New Contact / Distribution Tank (0.1 MG) $ 1 30,000.00
Electrical Work (8%) $    27,300.00 New Control System and Instrumentation $    18,100.00
Construction Cost Sub-Total $ 3 85,000.00 New UV System $    60,100.00
Overhead & Profit (18%) $    69,300.00 Sub-Total $  400,700.00
Site work & Yard Piping (5%) $    20,100.00
Total Construction Cost $  454,300.00
Electrical Work (8%) $    32,100.00
Total Investment Cost $  708,800.00  Construction Cost Sub-Total $  452,900.00
Overhead & Profit (18%) $    81,600.00
Alternative No. 2 – Optimization of Existing 
Total Construction Cost $ 5 34,500.00
Facility + UV System Total Investment Cost $ 8 33,900.00  
This alternative considers the same 
improvements explained on Alternative No. 1, but Alternative No. 3 – Optimization of Existing 
also includes installation of a UV reactor upstream Facility + Membrane System 
of the new contact / distribution tank. The This alternative is similar to Alternative No. 1 
improvements proposed in Alternative 1 are but also includes installation of a pre-engineered 
necessary because the water quality needed in the membrane package plant for polishing to the 
UV influent is important to avoid reactor fouling combined filter effluent. The membrane system will 
due to raw water contaminants. The UV system will have the capacity of treating the peak flow of 0.5 
have the capacity of treating the peak flow of 0.5 MGD. Membrane systems can receive 1.0 log 
MGD, and should provide enough dose to meet the removal credit after challenging testing 
required 1.0 log removal credit.  This unit will be demonstrated with integrity tests. 
constructed after the existing combined filter This unit will be constructed after the existing 
effluent.  This alternative could provide a 1.0-log combined filter effluent. This alternative could 
removal credit; which should be sufficient to meet provide a 1.0-log removal credit; which should be 
the LT2ESWTR requirements for the Cubuy WTP, sufficient to meet the LT2ESWTR requirements for 
even if the plant fails to meet the CFE or IFE the Cubuy WTP, even if the plant fails to meet the 
turbidity criterions for filter optimization described CFE or IFE turbidity criterions for filter 
under Alternative No.1. optimization described under Alternative No. 1.  
An advantage of the UV disinfection system This provides a safety factor in terms of compliance 
would meet both Giardia and Cryptosporidium with the new regulatory requirements making easier 
the plant operation.  Table 4 shows the cost alternative to use the UV system as the 
estimate of the Alternative No. 3. additional barrier to meet the LT2ESWTR 
requirements resulted in the lower NPV, 
Table 4 
Alternative 3 – Cost Estimate followed by the membrane system. 
 Reliability - This criterion considered the 
Items Cost
reliability of the system to obtain the credits 
New Tube Settlers in Sedimentation Basin $        36,100.00
New Continuous Sludge Removal System $        48,100.00 required by the LT2ESWTR. The most reliable 
New Filter $        36,100.00 alternatives were the membrane and UV, even 
Rehabilitation of existing Filters $        72,200.00 if the plant fails to meet the IFE and CFE 
New Contact / Distribution Tank (0.1 MG) $     130,000.00 turbidities on any given month. Finally, the 
New Control System and Instrumentation $        18,100.00 filter optimization alternative is the least 
New Membrane System $     480,800.00 favored since the compliance is dependent on 
Sub-Total $     821,400.00 the monthly results of the IFE and CFE 
Site work & Yard Piping (5%) $        41,100.00
turbidities, without having an alternate mean to 
Electrical Work (8%) $        65,800.00
obtain removal credits. 
Construction Cost Sub-Total $     928,300.00
Overhead & Profit (18%) $     167,100.00  Complexity - For this criterion the highest 
Total Construction Cost $  1,095,400.00 ranking was awarded to the filter optimization 
Total Investment Cost $ 1 ,708,900.00 alternative since it represent the lowest amount  
of construction activities for this reason less 
SELECTION OF ALTERNATIVES complicated. When considering the magnitude 
of the construction activities, Alternative No.1 
In order to encourage the objectivity and 
would be followed by Alternative No. 2 the 
uniformity of the analysis, the mean weighted 
installation of the UV system and finally 
average method has been used for the selection of 
Alternative No. 3 the membrane system 
the recommended alternative.  The criteria 
because it requires the construction of the unit 
considered in the comparison were: Net Present 
and will need a challenging test to receive the 
Value (NPV); Which alternative provides greater 
treatment credit. 
reliability to the system; Complexity of the 
 Environmental Impacts - For this criterion, the 
proposed alternative in terms of both construction 
highest rankings were issued to the filter 
and operation; The impact that the project will have 
optimization and the UV system, since this 
on the environment. 
alternatives contemplates the construction of 
A scale from 0 thru 5 was used, 5 meaning 
new structures outside of the existing PRASA 
fully compliance to the criterion and 0 no 
property limit but when compared with 
compliance. 
Alternative No.3 required the less additional 
A description of the items that were considered 
space, in areas that have not being impacted.  
for each of the criterion used for the evaluation is 
The Alternative No.3 received the lower 
presented below: 
ranking because it will impact an additional 
 Net Present Value - As expected, after 
area.  All alternatives will require the 
analyzing capital and O&M costs for each 
demolition of existing structures and an 
alternative it was concluded that the filter 
assessment to determine the presence of 
optimization alternative presented the best 
asbestos and lead based paint. 
NPV, which is lower than the other alternatives 
Table 5 presents the ranking gave to 
that considered the filter optimization with 
alternatives for all the evaluated criteria, and 
additional barriers to meet the LT2ESWTR 
compared the obtained weighted average by 
requirements. Among the other alternatives, the 
criteria. 
Table 5 credit even if the IFE and CFE turbidities are not in 
Evaluation of Alternatives 
compliance with the LT2ESWTR requirements. 
Criterion Weight Alternative Score Weighted Score
I 5 1.25 The total construction cost for the recommended 
NPV 0.25 II 3 0.75
III 1 0.25 alternative was estimated at $534,500, with an 
I 1 0.3 investment cost of $833,900 which is obtained 
Reliability 0.3 II 5 1.5
III 5 1.5 multiplying the Total Construction Cost by a factor 
I 4 0.8
Complexity 0.2 II 3 0.6 of 1.56. 
III 1 0.2
Environmental I 3 0.75 The selected alternative provides solution to 
0.25 II 3 0.75
Impact III 2 0.5  the problems identified in the Cubuy WTP; which 
are exceedances in bacteriology, DBP’s and not 
Table 6 shows that Alternative No. 2 
compliance with CT requirements. Also, the facility 
(Optimization of Existing Facility + UV System) 
does not achieve optimization goals. The selected 
obtained the highest ranking, with a total score of 
alternative will provide the required 
3.6, closely followed by Alternative No. 1 
Cryptosporidium oocysts removal required by the 
(Optimization of Existing Facility), with a total 
ESWTRLT2, and will improve the treatment 
score of 3.1. As mentioned previously, Alternative 
process, which will result in the solution to the 
No. 2 would allow the plant to meet the 
identified problems. This alternative offers the most 
LT2ESWTR even if the plant fails to meet the IFE 
reliable and practical solution. Also, when 
and CFE turbidity limits for any given month, thus 
considering that with time, the regulation will 
resulting in a highly reliable system. 
become stricter, UV disinfection is an alternative 
Table 6 that have the capability of be expanded with minor 
Evaluation of Results 
changes that will not represent a high investment 
Alternative Total Score from PRASA. 
I 3.1
II 3.6
III 2.5 REFERENCES  
[1] EPA, “Long Term 2 Enhanced Surface Water Treatment 
CONCLUSION AND RECOMMENDATIONS Rule Toolbox Guidance Manual”, 2010. 
The objective of this design project was to [2] EPA, “LT2ESWTR Source Water Monitoring for Systems 
Serving Less Than 10,000 People Factsheet”, 2006. 
identify the improvements required in the Cubuy 
WTP to bring the plant into compliance with the [3] Idaho Department of Environmental Quality, 
“Implementation Guidance for the Long-Term 2 Enhanced 
LT2ESWTR and other enforcement activities 
Surface Water Treatment Rule”, 2007. 
applicable to the facility.  The analysis included 
[4] EPA, Fact Sheet - Long Term 2 Enhanced Surface Water 
gathering raw water data of the facility to determine 
Treatment Rule, 2005. 
if the existing sizing of the treatment units is 
[5] EPA, “Optimizing Water Treatment Plant Performance 
adequate and an evaluation of the plant’s current 
Using the Composite Correction Program”, 1998. 
compliance status.   
[6] EPA, “Total Coliform Rule”, 2010. 
After evaluating each of the alternatives, it was 
determined that the Optimization of Existing [7] Bureau of Water, “Filter Assessment Manual”, 2003. 
Facility with the UV system, which resulted in the [8] Kawamura, Susumu, “Integrated Design and Operation of 
second lowest NPV, provided a higher degree of Water Treatment Facilities” Second Edition, John Wiley 
and Sons, Inc., 2000. 
reliability.   As a result, it is recommended to 
implement the construction of a project to optimize [9] AWWA, “Optimum Backwash of Dual Media Filters and 
Granular Activated Carbon Filter-Adsorbers with Air 
the operation of the filters, while providing a UV 
Scour”, 1990. 
system that would guarantee the required 1.0-log