Treatment Technology and Cost:
A Review of the Conventional Activated Sludge Cost Baseline

1. Purpose

Throughout the site selection committee process, the city’s engineer has cited construction cost as a primary factor in support of conventional activated sludge (CAS) technology for the new Lebanon wastewater treatment facility. The 60-acre minimum site threshold established by the committee is mathematically consistent with CAS at 20–50 MGD design capacity. That threshold has driven the scoring and potential elimination of candidate sites.

This document examines the completeness of the CAS cost framework on which that threshold depends. Specifically, it asks: does the published literature that supports a CAS cost advantage compare the correct CAS baseline — one capable of meeting Lebanon’s existing and anticipated permit requirements — against the alternatives? The answer, based on the peer-reviewed literature and Lebanon’s current NPDES permit, is that it does not. The cost comparison that has been presented to this committee is not an apples-to-apples comparison for Lebanon’s regulatory situation.

2. Lebanon’s Regulatory Baseline

The appropriate design basis for any cost comparison is the effluent quality the new facility must achieve. Two sections of NPDES Permit No. TN0028754, issued by TDEC on June 24, 2024, define that requirement.

2.1. Ammonia — Binding Numeric Limit (Permit Section 6.4)

Part 1 of the permit imposes the following effluent limits for ammonia nitrogen (NH₃-N) on the existing Lebanon facility:

The permit rationale (Section 6.4) explains the basis: “This is a best professional judgment limit reflecting that the treatment plant currently nitrifies ammonia and should continue to do so at the expanded design flow.” This limit is not speculative. It is an existing compliance obligation that will carry forward to any successor facility. Biological nitrification — the conversion of ammonia to nitrate through the nitrogen cycle — is a required process, not an optional design feature.

2.2. Total Nitrogen and Total Phosphorus — Monitoring Requirement and Regulatory Trajectory (Permit Section 6.6)

The permit does not yet impose numeric effluent limits for total nitrogen (TN) or total phosphorus (TP). However, Section 6.6 implements the Tennessee Nutrient Reduction Framework (NRF), requiring quarterly effluent characterization of TN and TP from all domestic wastewater dischargers. The permit rationale describes this explicitly as an adaptive management strategy: characterize loads now, use the data to set numeric reduction goals later.

Historical effluent data reported in the permit rationale shows Lebanon’s current plant discharging TN at concentrations ranging from 3.6 to 32.2 mg/L and TP from 0.9 to 4.6 mg/L across the prior permit term — figures consistent with a plant that performs no active biological nutrient removal beyond incidental effects of existing treatment processes.

A new facility designed today with a projected service life of 30–40 years will operate under multiple permit renewals. The NRF is explicit that numeric TN and TP limits will follow from the characterization data now being collected. Engineering practice for infrastructure of this scale and longevity requires designing to anticipated future permit requirements, not only current ones. The correct design baseline for the new Lebanon facility is full biological nutrient removal (BNR): nitrification and denitrification for nitrogen, plus phosphorus removal.

3. What the Published Cost Comparison Shows

The most directly applicable peer-reviewed cost comparison between MBR and CAS at Lebanon’s capacity range is Karim & Mark (2017), “A Preliminary Comparative Analysis of MBR and CAS Wastewater Treatment Systems,” International Journal of Water and Wastewater Treatment, 3(2). The study collected capital and O&M cost data from municipal treatment plants in the southeastern United States with capacities ranging from 8 to 36 MGD, covering Lebanon’s 20 MGD initial design capacity. All costs were normalized to 2016 dollars using RSMeans city cost indices.

The study’s regression equations for design and construction cost as a function of plant capacity (x, in MGD) are:

At Lebanon’s 20 MGD initial design capacity, these equations yield:

The regression crossover — the capacity at which MBR and CAS capital costs are equal — occurs at approximately 11 MGD. Above that threshold, CAS shows lower capital cost. This is the finding that underlies the engineer’s cost framing.

Taken in isolation, this finding supports CAS on capital cost at Lebanon’s scale. This document does not dispute the arithmetic. It disputes whether the comparison is valid for Lebanon’s regulatory situation.

4. The Baseline Problem: Karim’s CAS Is Not BNR-CAS

Biological nutrient removal requires additional treatment capability beyond standard secondary CAS. Full BNR-CAS adds anaerobic and anoxic treatment zones for nitrogen removal, internal recirculation systems, supplemental carbon dosing where needed, and chemical or enhanced biological systems for phosphorus control. These components add capital cost and operational complexity relative to basic secondary CAS. A cost comparison between BNR-capable MBR and basic CAS does not establish which technology is more economical for a facility that must achieve BNR.

Two features of Karim & Mark (2017) confirm that the CAS plants in the study dataset are not BNR-CAS.

4.1. Internal Evidence: Performance Data

Karim & Mark conducted statistical analysis (t-test and F-test) of effluent quality from local MBR and CAS plants for 2015 operating data. The parameters analyzed included CBOD₅, COD, TSS, ammonium (NH₄), total phosphorus (TP), and VSS. The paper reports:

The t-test results for NH₄ show t = −10.45 with p = 2.6 × 10⁻²³ — an extremely large and statistically significant difference. Similarly, TP shows t = −19.3 with p = 5.4 × 10⁻⁶³.

These results are inconsistent with the CAS plants having been optimized BNR systems. Full BNR-CAS routinely achieves ammonia concentrations below 5 mg/L and TP below 2 mg/L. MBR achieves below 1 mg/L for ammonia. A statistically significant gap of the magnitude Karim reports — highlighted as a principal finding of the study — indicates the CAS plants in the dataset were not performing nitrogen and phosphorus removal at a level consistent with BNR design. They were performing secondary treatment, with incidental nutrient effects, not controlled biological nutrient removal.

4.2. Internal Evidence: The Authors’ Own Framing

Karim & Mark state in the introduction:

The authors treat nutrient removal requirements as a special condition under which MBR becomes cost-competitive. This framing only makes sense if the general cost comparison — the one that shows CAS cheaper above 11 MGD — does not reflect BNR-capable CAS. If BNR-CAS were the default comparator, there would be no distinct “situations which require nutrient limits”; the comparison would simply be BNR-MBR versus BNR-CAS across the board.

The authors are explicitly telling the reader: the cost crossover finding applies to a general secondary-treatment comparison, and the economics shift when nutrient limits apply. Lebanon’s permit already imposes nutrient limits. Lebanon is one of the “situations” the authors are referring to.

5. Cost Implications of the Correct Baseline

5.1. Capital Cost

Karim’s ~$174 million CAS figure at 20 MGD reflects basic secondary CAS, not BNR-capable CAS. Adding full BNR capability to a CAS plant — the anaerobic and anoxic zones, internal recirculation pumping, carbon dosing, and phosphorus removal systems — increases both capital and operational cost. The magnitude of this premium depends on influent characteristics, receiving water limits, and plant-specific design, and is not established by any study in the petitioners’ possession for a 20 MGD facility. What is established is that the ~$174 million figure understates Lebanon’s actual CAS capital requirement, and the published crossover point of 11 MGD was derived from a comparison that does not reflect Lebanon’s regulatory situation.

No published study available to the petitioners directly compares BNR-CAS against BNR-MBR at the 20 MGD scale. DeCarolis et al. (2007, WEFTEC), the second primary source in the petitioners’ possession, confirms that its MBR cost estimates reflect full BNR — specifically, BOD < 2 mg/L, ammonia < 1 mg-N/L, nitrate < 10 mg-N/L, and total phosphorus < 2 mg-P/L. However, DeCarolis does not provide a CAS comparator and covers only 1–5 MGD capacity. The absence of a published BNR-CAS vs. BNR-MBR study at 20 MGD is itself informative: it means the cost basis for a CAS recommendation at Lebanon’s scale and regulatory requirement has not been established in the public record.

5.2. Operation and Maintenance Cost

Karim & Mark provide O&M cost regression equations for both technologies:

At 20 MGD, these equations yield annual O&M costs of approximately $11.6 million for CAS and $8.0 million for MBR — a difference of approximately $3.6 million per year, consistently favoring MBR at every capacity in the study range. The paper notes: “CAS unit cost for O&M is about 1.5 times of MBR unit cost for O&M. Therefore, in terms of O&M costs, MBR seems to be a clear choice of selection.”

Over a 30-year design period, the O&M differential accumulates to approximately $108 million in favor of MBR (in 2016 dollars, undiscounted). Over a 40-year period, approximately $144 million. These are not marginal differences. They are the primary reason Karim finds MBR cost-effective over the long term and documents a break-even point at 67 years for his dataset — a break-even derived, as established above, by comparing BNR-MBR against a non-BNR CAS baseline. For the correct comparison (BNR-MBR against BNR-CAS), the capital gap is smaller and the O&M gap is at minimum the same or wider, both of which move the break-even earlier — potentially within Lebanon’s expected facility service life.

Critically, the O&M comparison in Karim does not yet reflect the additional operational burden of BNR in the CAS baseline. BNR-CAS requires closer process control, chemical addition, and more complex sludge management than basic secondary CAS. O&M costs for BNR-CAS exceed those for basic CAS, while MBR’s O&M advantage is already established against the less expensive basic CAS baseline. Substituting BNR-CAS into the comparison widens the O&M differential further in MBR’s favor.

5.3. Footprint as a Cost Factor

Karim & Mark document footprint regression equations as a function of plant capacity:

At 20 MGD initial design capacity, these equations yield:

These are process-only footprints and do not include buffer, access roads, biosolids handling, or site infrastructure. The city engineer’s 60-acre minimum appears to include those elements. Applying the same proportional relationship, MBR’s process footprint at 20 MGD is approximately 43% of CAS — a differential that does not disappear when buffer is added, it merely scales.

Land cost is a real component of total project cost that does not appear in any technology cost comparison the committee has received. Wilson County land values for parcels of the required size are not negligible. A technology requiring 25–35 fewer net acres of usable land than CAS at 20 MGD carries a land cost advantage that belongs in any complete cost comparison. It has not been quantified for this project on the public record.

6. Summary of the Comparison Gap

7. Formal Request

The petitioners do not assert that MBR or any alternative technology is definitively less expensive than BNR-CAS for a 20 MGD facility in Lebanon, Tennessee. That conclusion requires a site- and permit-specific engineering cost analysis that does not exist in the public record. The petitioners assert that the inverse conclusion — that CAS is definitively less expensive — has not been established either, because the comparison that has been presented does not reflect Lebanon’s regulatory requirements.

Before any candidate site is eliminated on the basis of acreage thresholds derived from an undisclosed technology selection, the petitioners formally request that the committee require the city’s engineer to disclose and enter into the public record:

• The specific treatment technology and effluent quality specification (including ammonia, total nitrogen, and total phosphorus targets) upon which the 60-acre site threshold is based.
• A documented capital cost comparison between the selected technology and at minimum one alternative BNR-capable technology (MBR, MBBR, or aerobic granular sludge) designed to achieve the same effluent quality standard at 20 MGD initial and 40–50 MGD ultimate capacity.
• Annual O&M cost estimates for each technology over a 30-year period at the specified design capacity, including the additional O&M burden of biological nutrient removal for any CAS-based option.
• An estimate of land acquisition cost as a function of the process footprint differential between compared technologies.
• A sensitivity analysis showing whether the cost conclusion is robust to a ±20% variation in capital cost estimates, given the uncertainty inherent in conceptual-level engineering cost modeling.

The committee has stated on the record that harder questions come in the second phase. The technology cost question is not a second-phase question. It is the question that defines the acreage threshold that is driving first-phase elimination decisions today. Answering it requires only a disclosure from the engineer who prepared the cost framework this committee is relying upon.