Draft Final Centreville WWTP Upgrade and Expansion PER Amendment July 2025

Exactly as extracted, unformatted — for checking against the original. The readable version is easier to read.

Centreville Wastewater Treatment Plant
ENR Upgrade and Expansion
T own of Centreville
Centreville, MD
July 2025
Preliminary Engineering Report Amendment

Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
Table of Contents
Table of Contents ........................................................................................................................................................ i
List of Tables ............................................................................................................................................................... i
List of Figures .............................................................................................................................................................. i
Appendices .................................................................................................................................................................. i
1 Introduction ..........................................................................................................................................................1
2 Outfall Extension .................................................................................................................................................1
3 Cost Estimate Amendment ..................................................................................................................................3
4 Life Cycle Cost Analysis ......................................................................................................................................5
5 Non-Monetary Evaluation ....................................................................................................................................6
6 Existing SBR Process Challenges ................................................................................................................... 10
6.1 Comparison: MBR vs. SBR Technology .................................................................................................. 10
6.2 Practicality of the Proposed MBR Design ................................................................................................ 10
6.3 Design Deficiencies of the Existing SBR System .................................................................................... 11
6.4 Long-Term Planning ................................................................................................................................ 11
7 Discussions with Environmental Interest Groups ............................................................................................. 11
8 Spray Field Application ..................................................................................................................................... 11
9 Projected ENR Sludge Production ................................................................................................................... 12
List of Tables
Table 1: Preliminary Construction Cost Estimate – Alternative 3 (MBR Activated Sludge) .......................................3
Table 2: Conceptual Construction Cost Estimates for Treatment Alternatives ..........................................................4
Table 3: Life Cycle Cost Analysis ...............................................................................................................................5
Table 4: Non-Monetary Evaluation of Treatment Alternatives ...................................................................................8
List of Figures
Figure 1: Centreville WWTP Outfall Extension Aerial ................................................................................................2
Appendices
Appendix A ENR Sludge Projections...................................................................................................................... 13
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
1 Introduction
In December 2024, Whitman, Requardt and Associates (WRA) prepared a preliminary engineering report (PER)
for the enhanced nutrient removal (ENR) upgrade and expansion of the Town of Centreville Wastewater
Treatment Plant (WWTP). In the PER, three treatment alternatives were presented and evaluated to select which
alternative best suits the goals of this project. Of the three treatment alternatives, Alternative 3 – Membrane
Bioreactor (MBR) Activated Sludge was recommended as the design basis for the upgrade and expansion of
Centreville WWTP.
Based on recent discussions between the Town and the Maryland Department of the Environment (MDE), this
amendment to the PER serves to provide supplemental information to support the recommendation of Alternative
3 – MBR Activated Sludge, as well as provide additional information on the proposed outfall extension to Corsica
River to accommodate the expanded discharge for the 1.0 million gallons per day (MGD) design flow capacity of
the Centreville WWTP. The cost estimate tables that were presented in the PER have been updated in this
amendment to include the projected costs to extend the outfall, as well as the cost to construct a 24-hour 1.0 MG
capacity shellfish protection tank that is anticipated to be required by a future treated effluent discharge permit
when extending the outfall into the Corsica River. Additionally, the non-monetary evaluation of the three treatment
alternatives has been updated in this amendment based on feedback from MDE. This amendment also provides
supplemental information regarding the discussions and coordination with public interest groups within Centreville
that have been on-going throughout the PER development in favor of applying membrane treatment technology.
Additional information is provided in this amendment regarding the operational challenges with the existing
sequencing batch reactor (SBR) process at Centreville WWTP and the limitations of expanding the existing spray
irrigation discharge, thereby driving the need for extending the existing plant outfall to allow for year-round surface
discharge in the Corsica River. Lastly, this amendment includes a projection of the additional sludge production
associated with ENR treatment levels.
2 Outfall Extension
As discussed in Section 6.1.5.1 of the PER, the Town is proposing to relocate the plant surface outfall from
Gravel Run to the Corsica River downstream of the Watson Road Bridge to expand surface discharge from the
winter months only (Gravel Run) to year-round (Corsica River). Year-round surface discharge, in conjunction with
maximum utilization of the existing spray irrigation fields during the warmer weather months (April to November),
will accommodate the full 1.0 MGD discharge of treated effluent.
The relocation of the outfall will require an additional 7,600 linear feet (LF) of 24-inch nominal pipe size, with
submerged discharge mixing diffusers, as shown in Figure 1. Installation of the new outfall piping will consist of
approximately 2,500 LF pipe trenching within the landside greenfields and 5,100 LF of directional drilling to install
the remaining outfall piping in Corsica River. The anticipated outfall location is around Earle Creek, however, this
is subject to further on-going mixing zone evaluations that are being coordinated with MDE and the Department of
Natural Resources (DNR), the latter with focus on thermal mixing and its potential impact on migratory fish and
spawning.
Page 1

Restricted Shellfish
Harvesting Area
5
4
Earle
3
Creek
2
1
Centreville
WWTP
Figure 1: Centreville WWTP Outfall Extension Aerial
X = TMDL Segment N Scale:1:14418
DISCLAIMER: Property information contained on this map is for reference purposes only and is NOT to be construed as a "legal description". The map scale displayed is not
= Proposed Outfall Alignment
accurate and serves as a general representation only.

Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
3 Cost Estimate Amendment
The following cost estimate tables are presented to include the preliminary cost estimate for the proposed outfall
extension and shellfish protection tank required to discharge the effluent design capacity (1.0 MGD). Costs that
have been added or updated since the submission of the PER are shown in red text. Table 1 serves to amend
Tables 1.2 and 8.3 in the PER and includes an estimated total construction cost for the selected treatment
alternative (Alternative 3 – MBR Activated Sludge). Table 2 serves to amend Table 6.11 from the PER and
includes an updated conceptual construction cost estimate comparison for the three treatment alternatives that
are presented and evaluated in the PER.
Table 1: Preliminary Construction Cost Estimate – Alternative 3 (MBR Activated Sludge)
Item No. Category Cost
1 Interior Demolition (Lab, Control, and Filter and Blower Buildings) $95,000
2 Influent Screening Expansion $825,000
3 Converting Influent Flow Equalization Tanks, Aerated, with Pumping $2,019,000
4 Methanol Facility $618,000
5 UV Disinfection System $642,000
6 Non-Potable Water System $54,000
7 Dewatering Facility $2,413,000
8 Covered Cake Storage Facility $835,000
9 Lab, Control, and Filter and Blower Buildings Refurbishments $617,000
10 Existing Tank Modifications $671,000
11 Miscellaneous Process Piping and Equipment $784,000
12 MBR Process Building, MBR Equipment and Controls $7,342,000
13 Aerobic Digester Tank and Equipment $78,000
14 Shellfish Protection Tank $1,715,000
15 Electrical $4,169,000
16 Site Civil, including Yard Piping and Demolition (15% Items 1-14) $2,806,000
17 Site SCADA (5% Items 1-14) $935,000
18 Outfall Extension $10,838,000
Subtotal $37,456,000
Design Contingency (30% of Subtotal) $11,237,000
Escalation to May 2028 (4%/year) $6,080,000
Total $54,773,000
Total (Low Range -20%) $43,818,000
Total (High Range +50%) $82,160,000
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
Table 2: Conceptual Construction Cost Estimates for Treatment Alternatives
Alternative 2:
Alternative 3: MBR
Alternative 1: SBR Conventional Activated
Activated Sludge
Sludge
Base Facilities
Interior Demolition (Lab, Control, and Filter and Blower Buildings) $ 95,000 $ 95,000 $ 95,000
Influent Screening Expansion $ 825,000 $ 825,000 $ 825,000
Methanol Facility $ 618,000 $ 618,000 $ 618,000
Non-Potable Water System $ 54,000 $ 54,000 $ 54,000
Dewatering Facility $ 2,413,000 $ 2,413,000 $ 2,413,000
Covered Cake Storage Facility $ 835,000 $ 835,000 $ 835,000
Lab Building Refurbishment $ 139,000 $ 139,000 $ 139,000
Control Building Refurbishment $ 130,000 $ 130,000 $ 130,000
Filter and Blower Building Refurbishment $ 348,000 $ 348,000 $ 348,000
Shellfish Protection Tank $ 1,715,000 $ 1,715,000 $ 1,715,000
Base Subtotal Cost $ 7,172,000 $ 7,172,000 $ 7,172,000
Facilities for ENR Alternatives
Influent Flow EQ Tank(s), Aerated, with Pumping $ 2,054,000 $ 2,019,000 $ 2,019,000
Existing Tank Modifications $ 214,000 $ 1,450,000 $ 671,000
Clarifier Tanks, Equipment, and RAS Pumps - $ 3,497,000 -
Denitrification Filter Tanks, Equipment and Controls $ 3,112,000 $ 3,112,000 -
Miscellaneous Process Piping and Equipment $ 157,000 $ 235,000 $ 784,000
Additional SBR Tanks, Equipment and Controls $ 3,564,000 - -
Activated Sludge Equipment - $ 1,012,000 -
MBR Process Equipment and Controls, including MBR Process Building - - $ 7,342,000
Post EQ Tank and Equipment $ 78,000 - -
UV Disinfection System $ 642,000 $ 642,000 $ 642,000
Aerobic Digester Tank and Equipment $ 1,427,000 $ 78,000 $ 78,000
Alternative Subtotal Cost $ 11,248,000 $ 12,045,000 $ 11,536,000
Alternative Plus Base – Subtotal Construction Cost $ 18,420,000 $ 19,217,000 $ 18,708,000
Outfall Extension $ 10,838,000 $ 10,838,000 $ 10,838,000
Electrical $ 2,517,000 $ 2,722,000 $ 4,169,000
Site Civil, including Yard Piping and Demolition (15% Alternative + Base) $ 2,763,000 $ 2,883,000 $ 2,806,000
Site SCADA (5% Alternative + Base) $ 921,000 $ 961,000 $ 935,000
Subtotal $ 35,459,000 $ 36,621,000 $ 37,456,000
Contingency (30%) $ 10,638,000 $ 10,986,000 $ 11,237,000
Escalation to May 2028 (4%/year) $ 5,755,000 $ 5,944,000 $ 6,080,000
Grand Total Construction Cost $ 51,852,000 $ 53,551,000 $ 54,773,000
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
The capital costs of the three alternatives are within 6% from the least expensive Alternative 1 – SBR ($51.9
million), and the most expensive Alternative 3 – MBR Activated Sludge ($54.8 million). Although Alternative 3 –
MBR has the highest capital cost, the relative difference in costs between the three alternatives is small. In fact,
the capital cost of the MBR equipment alone is only approximately half of the total contingency cost for Alternative
3. For the purposes of this cost estimate comparison, all three alternatives have similar capital costs.
4 Life Cycle Cost Analysis
Table 3 below serves to amend Table 7.5 in the PER and includes an updated life cycle cost analysis (shown in
red) based on the updated WWTP capital costs presented in Section 3. Costs that have been updated since the
submission of the PER are shown in red text.
Table 3: Life Cycle Cost Analysis
Alternative 2 –
Alternative 1 – Alternative 3 – MBR
Conventional
SBR Expansion Activated Sludge
Activated Sludge
WWTP Capital Cost $51,852,000 $53,551,000 $54,773,000
Operating Cost
Annual Maintenance/Repair
$133,076 $161,208 $245,617
Costs(1)
Annual Electric Cost $ 55,157 $77,528 $84,877
Annual Burdened Labor $384,800 $395,200 $499,200
Annual Chemical Costs $336,886 $336,886 $336,886
Operating Cost Subtotal $909,919 $970,822 $1,166,580
Real Discount Rate(2) 2% 2% 2%
Project Life, years 20 20 20
Operating Cost Present Value $14,880,000 $15,880,000 $19,080,000
Present Worth $66,726,000 $69,431,000 $73,853,000
(1) Maintenance estimated at 2% of equipment cost for Alternatives 1 and 2 and 2.5% for Alternative 3 to
account for membrane replacement and cleaning chemicals
(2) December 2022 OMB Circular No. A-94
The life cycle cost analysis results in the present worth ranging from approximately $67 million for Alternative 1 up
to $74 million for Alternative 3. The results are within 10% of each other. Considering the variability in estimating
construction and operating costs, the three alternatives are similar in life cycle costs.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
5 Non-Monetary Evaluation
Life cycle costs include items of each alternative that have a dollar value. The value of the project will also be
influenced by factors that do not have a direct cost measure. The following criteria for the non-monetary
evaluation have been selected based on feedback from the Town of Centreville
• Leverages operators experience
• Operational simplicity
• Ease of maintenance
• Public acceptance
• Compatibility with water reuse
• Ability to evolve with future technologies
• Ability to upgrade treatment process in the future
• Available site space for future improvements
The non-monetary evaluation criteria are categorized and weighted as described below.
Leverages Operators Experience
Operators currently at the Centreville WWTP have experience with operating the existing two-tank SBR
system. This criterion evaluates the complexity in training operators for each of the three treatment
alternatives and is assigned a weighting factor of 10%. It should be noted that the Town may consider
third-party contract operations regardless of which treatment process alternative selected. This would
also govern the following two criteria: operations and maintenance.
Operational Simplicity
Each of the three alternatives will have different day-to-day involvement for operators and will require
different levels of attention to maintain operation. Because some of the alternatives are more complicated
to operate, this criterion is also assigned a weighting factor of 10%.
Ease of Maintenance
The equipment for each treatment alternative must be accessible for maintenance once it is placed into
service. Equipment must also be reliable and have a low likelihood of experiencing upsets and
discharging unacceptable effluent, so maintenance may be kept at a minimum. This criterion considers
each alternative’s reliability, how often the equipment for each alternative will require servicing, as well as
the availability of replacement parts and how complex the equipment is to maintain. This criterion is
assigned a weighting factor of 15%.
Treatment Resilience
This category considers each treatment alternative’s ability to maintain reliable effluent quality as
environmental conditions vary, such as during wet weather events and significant fluctuations in
temperature. Treatment alternatives that are more resilient to changes in environmental conditions will be
assigned a higher score for this criterion. This criterion is assigned a weighting factor of 10%.
Public Acceptance
This category considers the public view and acceptance of the treatment technology alternatives from the
standpoint of maximizing effluent quality. The upgrade and expansion of the Centreville WWTP has been
presented to various public groups and stakeholders within the Centreville area, and the engagement and
feedback received from these groups is very strongly considered. Thus, this criterion is assigned a higher
weighting factor of 30%.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
Compatibility with Water Reuse
The Town of Centreville plans for future water reuse in some form using the WWTP effluent. This criterion
evaluates how each treatment alternative positions the Town to move towards potable water reuse in the
future. This includes how much expansion to the treatment process will be required in the future to meet
the effluent quality levels required for potable water reuse. This criterion has been assigned a weighting
factor of 10%.
Ability to Evolve with Future Technologies
It is important to the Town that the Centreville WWTP is positioned to incorporate future treatment
technologies and remain on the forefront for wastewater treatment. This category considers the flexibility
for each treatment alternative for incorporating future technologies. This criterion is assigned a weighting
factor of 5%.
Ability to Upgrade Treatment Process in the Future
Federal and state regulatory agencies may establish more stringent effluent quality requirements in the
future—therefore, this category considers how each treatment alternative can be upgraded to meet
potentially higher effluent quality in the future. This criterion is assigned a weighting factor of 5%.
Available Site Space for Future Improvements
Potential future expansion and development in the Town of Centreville, as included in the County
Comprehensive Water/Wastewater Plan, will require additional expansion of the WWTP beyond the
planned 1.0 MGD capacity. This category considers how much site space will be available after
construction of each treatment alternative. It is desired to maintain as much site space as possible for
future expansion efforts. This criterion is assigned a weighting factor of 5%.
In summary, each of the evaluation criteria noted above is weighted to reflect their relative importance to the
construction, and operation and maintenance of the treatment process (45%); public engagement (30%); and
future considerations (25%). Each alternative is assigned a score, 1 to 10, with 1 being the lowest or least
desirable and 10 being the highest or most desirable for a given criterion. The total score for each alternative is
the individual criteria score for the alternative times the assigned weight, summed up. Table 4 presents the final
criteria ranking tabulation and serves to amend Table 7.6 in the PER.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
Table 4: Non-Monetary Evaluation of Treatment Alternatives
Weight % 10 10 15 10 30 10 5 5 5
Ability to Expand Available Site Space
Leverages Operators Operational Compatibility with Ability to Evolve with Total
Ease of Maintenance Treatment Resilience Public Acceptance Treatment Process in for Future Rank
Experience Simplicity Water Reuse Future Technologies Score
the Future Improvements
Score Comments Score Comments Score Comments Score Comments Score Comments Score Comments Score Comments Score Comments Score Comments
The SBR The SBR This
process is The SBR tanks are alternative
Operators are more sensitive process is The SBR adaptable and requires the
familiar with The SBR The SBR to fluctuations viewed process will can be most amount
the existing process has process in temperature favorably by provide modified to of site space Requires
Alternative 1:
SBR a relatively requires and wet the public, but effluent employ and will be the relatively high
SBR 8 7 6 8 5 10 7 4 5 650 2
technology, low relatively low weather flows, is seen as an quality densified most amount of site
Expansion
but additional complexity maintenance with a higher older, suitable for technologies, challenging to disturbance.
DNF process for operators. of equipment. likelihood of conventional future water such as further
is needed. suspended treatment reuse. aerobic upgrade the
solids flushing process. granular treatment
out of the SBR. sludge. capacity.
Operators will
be able to
transfer This
The activated
knowledge of alternative
The The activated sludge basins
the existing The requires a
activated sludge are adaptable
SBR process activated moderate
sludge The activated The activated process is and can
to learn the sludge amount of site
process is sludge sludge process viewed employ other
Alternative 2: new process will space to Requires
moderately process is relatively favorably by technologies,
Conventional conventional provide construct relatively low
5 6 complex for 6 requires 9 resilient to 5 the public, but 10 7 such as 8 5 640 3
Activated activated effluent additional amount of site
operators, relatively low fluctuations in is also an integrated
Sludge sludge quality activated disturbance.
including maintenance environmental older fixed film
process suitable for sludge basins
clarifies and equipment. conditions. conventional (IFAS), or
though future water for future
return sludge treatment PdNA (Partial
clarifiers and reuse. upgrade or
pumping. process. deNitrification-
DNF employ other
Anammox)
processes technologies.
add more
complexity.
The MBR
The MBR There is strong
process
process has a public
configuration
The MBR low sensitivity perception of
is similar to The MBR The MBR
process has to fluctuations exceptional
Alt 2 but process process will Similar to Alt 2
more in effluent quality
Alternative 3: without requires provide technologies Requires
complex environmental with a
MBR clarifiers and higher level of effluent with the added Similar to Alt least amount
5 5 equipment 5 10 conditions and 10 membrane 10 9 8 7 795 1
Activated DNFs. maintenance quality benefit of 2. of site
that requires is more technology as
Sludge However, the of more suitable for membrane disturbance.
higher resilient during is
membrane complex future water treatment.
operational wet weather demonstrated
facility equipment. reuse.
attention. events and at the smaller
requires more
temperature Queenstown
advanced
variations. “sister plant”.
training.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
Based on the results of the evaluation, of the three treatment alternatives evaluated, Alternative 3 – MBR
Activated Sludge shows the highest score. Weighing the non-monetary score and the relatively small difference in
both construction cost and life cycle cost, Alternative 3 remains the preferred and recommended alternative. As
mentioned in the PER the MBR process equipment is typically vendor specific in terms of layouts, ancillary
equipment selection and power requirements. As described in more detail in the PER, this would be addressed
during the design phase through vendor evaluation and pre-selection.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
6 Existing SBR Process Challenges
As the Town recently discussed with MDE, the following summarizes the Town’s key considerations with respect
to the existing SBR system and proposed MBR process.
6.1 Comparison: MBR vs. SBR Technology
Sequencing Batch Reactors (SBR)
SBRs have been widely used with consistent performance but present several operational challenges, as has
been experienced by the Town.
• Treatment efficiency can significantly be impacted by seasonal temperature fluctuations, which can
negatively affect settleability, nutrient removal, and phosphorus reduction. After such temperature shifts, it
typically takes several weeks for the system to stabilize and return to optimal performance.
• Meeting ENR limits with an SBR will typically require enhanced filtration systems and additional chemical
inputs, increasing operational complexity. These operational needs demand more intensive monitoring
and frequent manual adjustments by plant operators.
The SBRs also present challenges to the ENR upgrade in the construction sequencing. The existing SBR based
treatment system is near its practical operating limit with both tanks online. This does not provide for any
opportunity to take one SBR tank offline for refurbishment or upgrades until a new SBR tank is first constructed
and online. The existing SBR tanks could then be taken offline one at a time and refurbished. The new SBR tank
would also have to be at the same hydraulic grade as the existing SBR tanks, limiting where the tank can be
located.
The MBR tanks would be constructed separately from the SBRs. It would be commissioned, tested and started up
independently of the SBR system. The MBR therefore provides greater flexibility in site layout, construction
sequencing, and consistency in meeting the discharge permit.
Membrane Bioreactors (MBR)
Activated sludge MBR systems are continuous flow with less sensitivity to temperature variations, maintaining
more consistent performance throughout the year.
• By design, MBR integrates biological treatment with membrane filtration, producing a higher-quality
effluent without the need for additional downstream processes.
• As future regulations tighten beyond ENR standards, MBRs offer flexibility through modular upgrades,
including the potential addition of Granular Activated Carbon (GAC) and Reverse Osmosis (RO) systems.
This modularity makes MBR systems a forward-compatible and technologically advanced solution for
evolving wastewater treatment needs.
6.2 Practicality of the Proposed MBR Design
Practical Advantages of Implementing the Proposed MBR System
• Flow Equalization and Emergency Storage: The current SBR system lacks basic infrastructure such as
flow equalization and emergency storage. This forces operators to dose chemicals based on peak flow
rates rather than average conditions, resulting in wasted chemicals and elevated operating costs.
The MBR design includes dedicated influent flow equalization and reuses existing SBR tankage for this
purpose, eliminating the need for overdosing and supplementing 24-hour emergency storage.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
• Operational Flexibility, Constructability and Reliability: The proposed MBR system allows the current
SBR to remain operational during construction. Once online, the MBR can assume full operation with
reduced complexity and greater reliability. Critical maintenance tasks—such as taking tanks offline—will
become feasible without disrupting plant performance, something the current system cannot
accommodate.
6.3 Design Deficiencies of the Existing SBR System
Poor Tank Design: All tanks (SBRs, Post Equalization, Digester) have flat floors, making solids removal
inefficient and maintenance more difficult and time-consuming. Sloped floors would allow for proper cleaning and
minimize downtime.
Vulnerable Equipment Placement: The central control CPU is placed in a high-corrosion environment near
chemical and UV systems, which is accelerating hardware deterioration and threatens long-term system reliability.
The Motor Control Cabinet is located in the blower room, where excessive summer heat leads to frequent relay
trips, disrupting pumps and actuators essential to plant operations.
Inadequate Filtration for SBR Discharge: The Disc Filter is undersized for the intermittent flow pattern typical of
SBR systems, creating a critical chokepoint that threatens the plant’s ability to meet discharge requirements
during high-flow events.
6.4 Long-Term Planning
The current SBR system has reached its functional limits. It performs well only under ideal conditions, which
require constant vigilance and intensive labor from skilled operators. Any unexpected condition—whether due to
temperature, flow, or equipment limitations—can throw the system out of balance and risk permit compliance.
The proposed MBR system is not just a replacement; it is a strategic investment in the future of the Centreville
WWTP. It eases the burden on operators, increases system resiliency, reduces chemical and energy costs, and
prepares the plant for future regulatory and environmental standards. MBR represents the most cost-effective and
sustainable path forward with improved automation, modular upgrade paths, and better environmental outcomes.
7 Discussions with Environmental Interest Groups
The Town, as well as MDE, has actively engaged with the various stake holder groups, such as the Corsica River
Conservancy and ShoreRivers group. The Town has presented the upgrade and expansion project to the groups
and participated in outreach meetings. The feedback received from the groups regarding the proposed application
of a membrane technology for the plant upgrade and expansion has been positive, and with the expectation that
the effluent quality will exceed that of the other two alternatives considered. A change in technology selection, that
might be perceived as producing a lesser effluent quality, could undermine the positive collaboration with these
groups over the past 12-18 months.
8 Spray Field Application
One of the advantages of using an MBR treatment process is the increase in the maximum hydraulic application
rate in accordance with COMAR for MBR specific effluent quality for onsite sewage disposal systems. Under
favorable spray field conditions this could allow for more effluent flow to be diverted and applied to the existing
spray fields. This would free up existing lagoon storage to better manage flows during unfavorable conditions.
This would not eliminate the need for a new outfall for the plant expansion but could potentially reduce the amount
of flow that would be otherwise discharged to Corsica River.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
9 Projected ENR Sludge Production
The additional sludge projected to be generated under ENR treatment levels are estimated to represent about
212151%%% of the total sludge. This has been summarized in a separate memorandum shared with MDE and included in
this PER amendment as Appendix A.
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Centreville WWTP ENR Upgrade and Expansion
Preliminary Engineering Report Amendment
Centreville, MD
Appendix A
ENR Sludge Projections
Page 13

MEMORANDUM
Date: July 14, 2025
To: Matthew Marshall Work Order Number: 14375-000
From: D. Nixson
Subject: ENR Related Sludge Increase Project: Centreville WWTP ENR Upgrade
CC: Carolyn Brinkley, Town of Centreville
This memorandum summarizes the additional sludge that is expected to be generated at the Centreville wastewater
treatment plant (WWTP) in Centreville, MD due to the upgrade to enhanced nutrient removal (ENR) levels of
treatment. With the current treatment process and recent flows and loads to the WWTP, the plant is adding poly-
aluminum chloride (PACl) to meet the permitted monthly effluent total phosphorus concentration of 1.0 mg-P/L.
Using BioWin® wastewater computer modeling developed by EnviroSim, of the MBR process, at a 1.0 MGD annual
average design flow and average influent loads described in the Preliminary Engineering Report (PER), the ENR
treatment process is expected to produce a total 1,300 dry lbs of sludge per day.
To provide ENR levels of treatment, the effluent phosphorus concentration will be reduced to 0.3 mg-P/L. Therefore,
the ENR upgrade will reduce the effluent phosphorus concentration from 1.0 mg/L down to 0.3 mg-P/L, a reduction of
0.7 mg-P/L. At the 1.0 MGD annual average flow this reduction in phosphorus translates to
1 MGD * 0.7 mg-P/L * 8.34 = 5.8 lbs phosphorus/day.
To attain the 0.3 mg-P/L effluent concentration is expected to require addition of 15 moles or more of aluminum for
each mole of phosphorus precipitated. Using a 15:1 aluminum to phosphorus molar ratio, and converting the 5.8 lbs
phosphorus to moles:
5.8 lbs-P / 31 lbs-P/mole-P = 0.19 moles-P
And then using the 15:1 ratio to find the number of moles of aluminum needed:
0.19 moles-P * 15 moles-Al/1 mole-P = 2.9 moles-Al
The chemical reaction for the aluminum from the aluminum chloride with the phosphate:
Al3+ + H PO + 2HCO-  AlPO + 2CO + 2H O
2 4 3 4 2 2
Indicates a 1:1 molar ratio of aluminum ions to aluminum phosphate created:
0.19 moles-Al produces 0.19 moles AlPO
4
Converting from moles to weight:
0.19 moles AlPO * 122 lbs / mole AlPO = 23 lbs AlPO
4 4 4
The chemical reaction for the excess 2.71 moles aluminum ions from the polyaluminum chloride to the aluminum
hydroxide precipitate:
Al3+ + 3 H O Al(OH) + 3 H+
2 3
Indicates a 1:1 molar ratio of aluminum ions to aluminum hydroxide created:
2.71 moles-Al = 2.71 moles-Al(OH)
3
Converting from moles aluminum hydroxide to weight:
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July 14, 2025 Page 2 14375-000
2.71 moles-Al(OH) * 78 lbs/mole- Al(OH) = 211 lbs of aluminum hydroxide precipitate, i.e., chemical sludge,
3 3
per day will be produced.
In addition, the addition of methanol will produce additional biological sludge. An estimated yield of 1.3 mg/L of
volatile solids will be produced for each mg/L of nitrate removed when utilizing methanol as an external source of
carbon for the denitrification reaction. Upgrading to ENR levels of treatment will require an additional 4 mg/L of nitrate
removal. Therefore 44 dry lbs of sludge per day will be produced.
Combining the chemical and denitrification sludges :
23 lbs AlPO + 211 lbs Al(OH) + 44 dry lbs of biological sludge = 278 dry lbs sludge per day.
4 3
Based on the 1,300 lbs/day total sludge produced at 1 MGD based on BioWin modeling of the ENR upgraded
process, the ENR related sludges represent :
278 lbs ENR sludge/day / 1,300 lbs total sludge/day = 21% of the total sludge.
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David R. Nixson, PE
N:\14375-000\Engineering\Corresp\Memos\ENR Sludge Production Memo 2025_07_14.docx