ATTACHMENT 1 Sun Prairie - FPA 2023-11-15-Public Review.pdf

City of Sun Prairie Water Pollution Control Facility

3040 Bailey Road, Sun Prairie, Wisconsin

2023 Facility Plan Amendment

November 2023

Public Review Copy

Prepared by:

Donohue & Associates, Inc.

3311 Weeden Creek Road, Sheboygan, WI 53081 donohue-associates.com

Donohue Project No.: 14238

City of Sun Prairie Water Pollution Control Facility November 2023

2023 Facility Plan Amendment – Public Review Copy Donohue & Associates, Inc.

TABLE OF CONTENTS

1. Executive Summary

1.1 Background

1.2 Drivers

1.2.1 Solids Train Conditions and Capacities

1.2.2 Biosolids Program Resiliency and Sustainability

1.3 Alternatives

1.4 Recommendation

2. Background

3. Current and Future Conditions

3.1 Population

3.2 Historical Data and Future Forecasts

3.3 Current Flows and Loadings

3.4 Current Solids Production

3.5 Future Flows and Loadings

3.6 Future Solids Production

3.7 Solids Process Capacities

3.7.1 WAS Gravity Belt Thickening

3.7.2 Dewatering Belt Filter Press

3.7.3 Dewatered Biosolids Storage

3.8 Biogas Storage and Handling

4. Biosolids Program Alternatives

4.1 Drying Technologies

4.1.1 Solar Drying

4.1.2 Thermal Drying

4.1.3 Dehumidification Drying

4.2 ELODE Dewatering

4.3 Anoxic Thermophilic Pretreatment

4.4 Feasible Alternatives Screening

4.4.1 Status Quo

4.4.2 Enhanced Dewatering (ELODE)

4.4.3 ELODE + Solar Drying

4.4.4 ATP + ELODE + Solar Drying

4.4.5 Thermal Drying

4.4.6 Solar + Thermal Drying

4.4.7 Dehumidification Drying

4.4.8 Solar + Dehumidification Drying

4.5 Feasible alternatives screening summary

5. Alternatives analysis

5.1 Ancillary Improvements

5.2 Alternative 1: ELODE + Solar Dryer

5.3 Alternative 2: ATP + ELODE + Solar Dryer

5.4 Alternative 3: Thermal Dryer

5.5 Alternative 4: Solar + Thermal Dryers

5.6 Alternative Comparison

6. Environmental Assessment

7. Implementation Plan

7.1 Schedule

7.2 Funding Strategy

7.3 User Rate Implications

7.4 Public Participation and City Council Approval

LIST OF TABLES

Table 1-1 Alternative Valves Scores

Table 3-1 Historical and Projected Service Area Populations

Table 3-2 Current Flows and Loadings

Table 3-3 Current Solids Production

Table 3-4 Future Flows and Loadings

Table 3-5 Future Solids Production

Table 3-6 Current and Future Solids Process Capacities

Table 4-1 Biosolids Metals Restrictions

Table 4-2 Class B Site Criteria by Land Application Method

Table 4-3 Solar Drying Advantages and Disadvantages

Table 4-4 Class A Thermal Drying Advantages and Disadvantages

Table 4-5 Dehumidification Drying Advantages and Disadvantages

Table 4-6 ELODE Advantages and Disadvantages

Table 4-7 ATP Advantages and Disadvantages

Table 4-8 Non-Economic Factors and Scoring System

Table 4-9 Status Quo Non-Economic Score

Table 4-10 Enhanced Dewatering Non-Economic Score

Table 4-11 ELODE and Solar Drying Non-Economic Score

Table 4-12 ATP, ELODE, and Solar Drying Non-Economic Score

Table 4-13 Thermal Drying Non-Economic Score

Table 4-14 Solar and Thermal Drying Non-Economic Score

Table 4-15 Dehumidification Drying Non-Economic Score

Table 4-16 Solar and Dehumidification Drying Non-Economic Score

Table 4-17 Non-Economic Scoring Summary

Table 5-1 ELODE Year 2045 Design Parameters

Table 5-2 Solar Dryer Year 2045 Design Parameters

Table 5-3 ATP Year 2045 Design Parameters

Table 5-4 Thermal Drying Year 2045 Design Parameters

Table 5-5 Solar + Thermal Drying Year 2045 Design Parameters

Table 5-6 Alternative Values Scores

LIST OF FIGURES

Figure 1-1 Alternative Scores and 20-Year Total Present Worth Costs

Figure 1-2 User Rate Cost Comparison

Figure 1-3 Implementation Timeline

Figure 3-1 Current and Future Solids Process Capacity Analysis

Figure 4-1 Solar Drying Illustration

Figure 4-2 Dehumidification Dryer Cross-Section

Figure 4-3 ELODE Overview

Figure 4-4 ELODE Cake Dewatering

Figure 4-5 ELODE Installation (Henderson, KY)

Figure 4-6 ATP Overview

Figure 4-7 Four Reactor ATP (Holland, MI)

Figure 4-8 Non-Economic Scores

Figure 5-1 Sun Prairie Biogas Raw Production

Figure 5-2 Alternative Scores and 20-Year Total Present Worth Costs

Figure 7-1 Implementation Timeline

Figure 7-2 Sewer Rates (10k-50k pop and nearby)

APPENDICES

Appendix A – Alternative Layouts

Appendix B – Alternatives Opinion of Probable Construction Cost

Appendix C – XXXX, 2023 City Council meeting presentation

ABBREVIATIONS

ac acre

ATP anoxic thermophilic pretreatment

BFP belt filter press

BOD biochemical oxygen demand

CARPC Capital Area Regional Planning Commission cfd cubic feet per day

CFR 40 Code of Federal Regulations

CIP Capital Improvement Plan

City City of Sun Prairie cy cubic yard

EPA Environmental Protection Agency

EQ Exceptional Quality

Facility Water Pollution Control Facility ft feet gal gallon

GBT gravity belt thickener hrs hours

HRT hydraulic retention time

HSW high strength waste

Kcf 1,000 cubic feet kg kilogram kWh kilowatt hours lbs pounds mgd million gallons per day mg milligram

MHI median household income

NE non-economic

NH3-N Ammonia

No/Low BITP/A Broad Incidental Take Permit/Authorization for No/Low Impact Activities

OPCC opinion of probable construction cost ppd pounds per day

PTS primary thickened sludge

TAS thickened activated sludge

TBD To be determined

TP total phosphorous

TPV total present value

TPW total present worth

TS total solids

TSS total suspended solids

USEPA United States Environmental Protection Agency

VS volatile solids

WAS Waste activated sludge

WDNR Wisconsin Department of Natural Resources

WPDES Wisconsin Pollutant Discharge Elimination System

WPFC Water Pollution Control Facility

YR year

$M Million dollars

1. EXECUTIVE SUMMARY

The City of Sun Prairie (City) owns and operates a Water Pollution Control Facility (WPCF or Facility). On average, the Facility treats more than three million gallons of wastewater daily. It discharges the treated effluent, in accordance with the WPDES Permit, water back to the environment via Koshkonong Creek.

1.1 BACKGROUND

A wastewater treatment facility can be divided into two large halves: [1] the half that treats and recovers the water – the liquid train – and [2] the half that treats and recovers the solids – the solids train. The City completed facilities planning for its WPCF in 2018. The 2018 Facilities Plan (Facilities Plan) recommended near-term liquid train improvements and identified some potential solids train improvements for future implementation. The Wisconsin Department of Natural Resources (WDNR) approved the 2018 Facilities Plan. The City implemented the recommended and approved liquid train improvements.

Concurrent with the design of the liquid train improvements, the City produced a 2019 Facilities Plan Amendment. The WDNR approved that Amendment, which deliberately and strategically deferred further evaluation of solids train improvements until the liquid train improvements were operational and the associated solids production well understood.

Consistent with the 2018 Facilities Plan and the 2019 Facilities Plan Amendment, this Facility Plan Amendment focuses on the solids train. This Amendment evaluates the existing WPCF’s solids train processes and strategies and evaluates alternative solids processes and strategies to address current or near-term shortcomings, targeting year 2045 for future capacity. It ultimately recommends the City implement solids train improvements that will provide a reliable, robust, and cost-effective biosolids program for current and future generations.

1.2 DRIVERS

This Facility Plan Amendment documents an evaluation of alternatives to address two broad concerns: [1] the condition and capacities of the solids train processes and [2] the resiliency and sustainability of the biosolids reuse program.

1.2.1 SOLIDS TRAIN CONDITIONS AND CAPACITIES

Anaerobic digestion is the process the WPCF uses to stabilize the solids and produce biosolids suitable for reuse. The process that thickens solids upstream of anaerobic digestion is nearing the end of its reliable service life, lacks adequate redundancy, and lacks adequate Year 2045 capacity. The anaerobic digester covers that maintain anaerobic conditions and store methane-laden biogas are nearing the end of their reliable service lives. The boiler and heating system that heats the anaerobic digestion process is nearing the end of its reliable service life. The biosolids thickening process is nearing the end of its reliable service life, lacks adequate redundancy, and lacks adequate Year 2045 capacity. The biosolids storage structure lacks adequate capacity for the upstream solids processes. Storage is discussed more in the next section.

1.2.2 BIOSOLIDS PROGRAM RESILIENCY AND SUSTAINABILITY

The process of converting wastewater to clean water generates solids. The WDNR, Environmental Protection Agency (EPA), and Water Environment Federation (WEF) refer to these solids as biosolids.

Today, the Facility annually produces roughly 5,000 wet tons of biosolids. In Year 2045, the Facility is projected to annually produce roughly 7,000 wet tons of biosolids – 40% more than today’s production rate.

Regulators categorize biosolids from domestic wastewater treatment facilities as Class A or Class B. The WPCF produces Class B biosolids. As such, the City’s reuse program is heavily regulated by the WDNR via the WPDES Permit. Class B biosolids are restricted to landfilling or bulk application to agricultural land, forest, or reclamation sites. All the Facility’s biosolids are applied to agricultural land.

The City’s agricultural land application program must [1] comply with strict site restrictions that limit crop harvest schedules and public/livestock access or contact and [2] accommodate modern agricultural practices. Landowners have historically accepted Class B biosolids as a nutrient supplement; however, it is increasingly difficult for treatment facility owners and/or their contractors to haul and land apply biosolids on WDNR-approved fields during increasingly short spring and fall hauling seasons. The City of Sun Prairie and its regulated Class B land application program faces a growing challenge unique to its location: it must compete with the Madison Metropolitan Sewerage District and other treatment facility owners for a limited and diminishing number of suitable agricultural fields in proximity to the WPCF. These factors have placed stress on biosolids storage and reuse efforts at the WPCF. This stress is forecasted to increase significantly as development continues at a rapid pace in the service area and region.

The existing biosolids storage building at the WPCF is too small for a resilient and flexible Class B biosolids reuse program, offering roughly half the desired volume today and less than 40% of the desired volume in Year 2045. Insufficient storage jeopardizes [1] cost effectiveness, [2] sustainable nutrient reuse, and [3] WPDES Permit compliance. If storage is full, the operating staff could be forced to landfill biosolids at a considerable cost, diverting the recovered nutrients from reuse use on agricultural fields and burying them in a landfill. In lieu of landfilling, the operating staff could adjust WPCF operations to retain solids in the liquid train; however, this will degrade the quality of the effluent – the clean water discharged to the local watershed – and could cause WPDES Permit violations and excessive nutrient loadings to the watershed.

Solids train processes that produce Exceptional Quality (EQ) Class A biosolids enhance end-use flexibilities, largely eliminating the regulations and associated logistical challenges that encumber Class B programs.

In effect, a Class A EQ process and product would dramatically reduce many of the biosolids reuse program stresses.

1.3 ALTERNATIVES

This Facilities Plan Amendment considered a wide spectrum of Class B and Class A EQ alternatives that, to varying degrees, address the program drivers outlined above. All alternatives were evaluated based the five non-economic categories listed below.

1. Safety, Operability, and Maintenance

2. Resiliency, Stability, Robustness

3. Quality of Life

4. Environmental Stewardship, Sustainability

5. Longevity and Flexibility

Each non-economic category was assigned a weighting factor that determined how much influence a particular category score would have on the overall selection decision. The weighting factors generally reflected the overall goals and priorities of the WPCF. Categories were scored between 1 and 5 in 0.5-point increments, with 1 being least favorable and 5 being most favorable. Alternatives with a non-economic score of 3.5 or higher were deemed suitable and developed. The suitable alternatives were evaluated based on a combination of their economic and non-economic merits. The economic consideration considered their capital costs, operating costs, and 20-year total present values.

Four alternatives were deemed suitable and advanced to the composite (economic and non-economic) evaluation. Those four alternatives are listed and briefly described below. Each would address the solids train conditions and capacities drivers and, to varying degrees, the biosolids program resiliency and sustainability drivers.

• Alternative 1: ELODE + Solar Dryer – This alternative would use an innovative advanced dewatering technology (ELODE) to dramatically, cost effectively, and sustainably reduce the biosolids’ water content from roughly 85% to 55%. Reducing the water content early in the treatment process reduces the size of downstream processes, and the energy required to evaporate or haul water. Biosolids discharged from the ELODE process would sustainably and cost effectively dried in a solar dryer. The water content of the biosolids exiting the solar dryer would be roughly 10%. Without ELODE and the solar dryer, the WPCF is forecasted to produce roughly 7,000 wet tons per year in the Year 2045. With ELODE and the solar dryer, this would be reduced to 1,200 wet tons per year. In theory, a solar dryer could produce a Class A product with the right design features, operating strategy, testing regimen, and ultimately WDNR approval. Because the WDNR has never given Class A approval of a solar dryer and obtaining Class A approval would be very challenging at the Sun Prairie latitude, the evaluation assumed this alternative would produce a Class B product.

• Alternative 2: ATP + ELODE + Solar Dryer – This alternative would produce a Class A product by employing the anoxic thermophilic pretreatment (ATP) process upstream of anaerobic digestion.

It would also use ELODE advanced dewatering and a solar dryer to cost effectively and sustainably produce a dry Class A product with a water content of roughly 10%. Instead of producing roughly 7,000 wet tons of a heavily regulated Class B product in Year 2045, this alternative would produce roughly 1,200 wet tons of a largely unregulated Class A EQ product. The Class A EQ product would be given away or, potentially, sold to local agricultural or landscaping operations, either eliminating the annual cost of hauling and land application or generating a small annual revenue stream.

• Alternative 3: Thermal Drying – This alternative would produce a Class A product by employing a thermal drying process. The thermal dryer would burn some biogas and supplemental natural gas to elevate the temperature of the biosolids and evaporate all but 10% of the water. Like Alternative 2, this alternative would produce roughly 1,200 wet tons of a largely unregulated Class A EQ product which could be given away or sold to local agricultural or landscaping operations.

Relative to Alternative 2, this alternative is more complex, requires more operation and maintenance effort, and, because of its reliance on natural gas, has a higher carbon footprint.

Moreover, this alternative could not use the federal solar drying grant funding that is already awarded to the City.

• Alternative 4: Solar + Thermal Drying – This alternative would employ solar and thermal dryers in series. Because it is challenging for a solar dryer to consistently produce a Class A product, a thermal dryer would be the final drying step to consistently produce a Class A EQ product. Like Alternative 2, this alternative would produce roughly 1,200 wet tons of a largely unregulated Class A EQ product which could be given away or sold to local agricultural or landscaping operations.

This Alternative is more complex, requires more operation and maintenance effort, and, because of its reliance on natural gas and the decarbonization of the electric grid, has a higher carbon footprint than Alternative 2.

Figure 1-1 shows non-economic scores and 20-year total present values. Table 1-1 shows the value scores, calculated as follows: non-economic score / 20-year total present value (in $M) x 100.

Figure 1-1 Alternative Scores and 20-Year Total Present Worth Costs

Table 1-1 Alternative Valves Scores

Alternative

NE

Score

TPV

($M)

Value Score (NE Score/TPV x 100)

1 – ELODE + Solar 3.6 36.8 9.9

2 – ATP + ELODE + Solar 4.6 41.7 11.1

3 – Thermal Dryer 3.5 47.2 7.4

4 – Solar + Thermal Dryers 3.5 57.8 6.1

Alternative 2 offers the most value: it is an operationally simple, safe, cost-effective, and sustainable strategy to [1] produce a dry Class A EQ product consistently and reliably, [2] satisfy all the program drivers outlined previously, and [3] recover and reuse valuable nutrients in perfect harmony with the City’s sustainability program. Alternative 2 has the highest value score with a 20-year total present value within 15% of the next alternative, while producing a Class A solids product.

1.4 RECOMMENDATION

Based on the results of the economic and non-economic evaluation, WPCF staff and Donohue recommend the City implement Alternative 2: ATP + ELODE + Solar. Figure 1-3 shows the recommended schedule.

The City will fund this project using [1] reserve funds, [2] a $3.2M grant for the solar dryer, and [3] a low-interest loan from the Clean Water Fund. Renewable energy related funding measures such as available grant/rebate programs and Focus on Energy will be used wherever possible.

The average residential user cost will increase approximate $9.10 per month or about $110 per year, maintain affordable, comparable rates with utilities of similar size as well as Sun Prairie’s neighbors.

The figure below indicates the current and future projected rates, as well as the rates of neighboring communities of any size, and similar sized facilities across the state. The City’s diligent efforts to invest wisely in the facility have resulted in rates at the lower end of its peers and neighbors.

This allows this investment in the future of solids handling at the facility without placing undue burden on its ratepayers. Expressing the sewer rate as a fraction of the community’s MHI firmly establishes the relatively low cost of treatment relative to its peers, particularly when considering the extremely high discharge standards to which the facility is held.

Figure 1-2 User Rate Cost Comparison

The schedule below depicts an anticipated implementation timeline targeting a late 2024 bidding period, using the winter season to begin administration of the construction project in advance of spring/summer construction.

Figure 1-3 Implementation Timeline

2. BACKGROUND

The City of Sun Prairie (City) completed facilities planning for its Water Pollution Control Facility (WPCF) in 2018. The 2018 Facilities Plan (Facilities Plan) recommended near-term liquid train improvements and identified some potential solids train improvements for future implementation. The Wisconsin Department of Natural Resources (WDNR) approved the 2018 Facilities Plan. The City implemented the recommended and approved liquid train improvements.

Concurrent with the design of the liquid train improvements, the City produced a 2019 Facility Plan Amendment. The WDNR approved that Amendment, which deliberately and strategically deferred further evaluation of solids train improvements until the liquid train improvements were operational and the associated solids production well understood.

Consistent with the 2018 Facility Plan and the 2019 Facility Plan Amendment, this Facility Plan Amendment focuses on the solids train. The recent liquid train improvements have been operational for more than a year. This Amendment evaluates the existing WPCF’s solids train processes and strategies and evaluates alternative solids processes and strategies to address current or near-term shortcomings.

It ultimately recommends the City implement solids train improvements that will provide a reliable, robust, and cost-effective biosolids program for current and future generations.

After the WDNR approves this Facility Plan Amendment, the City will be eligible to use the Wisconsin Clean Water Fund to finance the capital improvements and submit reviewable plans and specifications to the WDNR for review, approval, and construction authorization.

3. CURRENT AND FUTURE CONDITIONS

3.1 POPULATION

The Wisconsin Administrative Code requires wastewater facilities planning use population projections produced by the Capital Area Regional Planning Commission (CARPC). The 2018 Facilities Plan projected the Year 2038 service area population using projections provided CARPC. This Amendment uses the same procedure to calculate the Year 2045 population. Table 3-1 shows historical populations from the United States Census and CARPC projections. Year 2022 and Year 2045 populations were calculated by interpolating the CARPC projections.

Table 3-1 Historical and Projected Service Area Populations

3.2 HISTORICAL DATA AND FUTURE FORECASTS

Flows, loadings, and solid production from late 2019 to late 2021 proved atypical and unrepresentative of future conditions due to several internal and external factors: [1] water usage and loadings varied in response to COVID-19 and [2] the WPCF’s typical operating procedures were modified to accommodate the liquid train improvements project. Flows, loadings, and treatment facility operations stabilized in early 2022.

Future flows and loadings are forecasted using the annual average values from 2022, the WDNR-approved peaking factors from the 2018 Facilities Plan, the CARPC-provided Year 2022 population, and the CARPC-projected Year 2045 population. Future solids production values are forecasted using the annual average values from 2022, peaking factors from the plant-wide solids balance, the CARPC-provided Year 2022 population, and the CARPC-projected Year 2045 population. Solids production values should be refined during the subsequent preliminary design phase using data from Years 2022 and 2023.

Year US Census CARPC

2010 29,364

2020 35,967 35,428 [1]

2022 36,631 [2, 3]

2035 44,450 [1]

2045 50,465 [2, 3]

2050 53,472 [1]

Notes

1 - Value provided by CARPC

2 - Calculated by interpolation

3 - Used to calculate future flows, loadings, and solids production values

3.3 CURRENT FLOWS AND LOADINGS

Table 3-2 shows current flows and loadings.

Table 3-2 Current Flows and Loadings

3.4 CURRENT SOLIDS PRODUCTION

Table 3-3 shows current solids production values.

Table 3-3 Current Solids Production

3.5 FUTURE FLOWS AND LOADINGS

Table 3-4 provides Year 2045 flows and loadings.

Table 3-4 Future Flows and Loadings

Maximum Maximum

Parameter Units Average Month Week

Flow mgd 3.19 5.20 6.02

BOD ppd 8,192 11,141 16,060

TSS ppd 7,560 11,415 14,515

NH3-N ppd 739 1,157 1,370

TP ppd 147 266 364

Maximum Maximum

Parameter Units Average Month Week

Primary Sludge ppd 5,034 7,623 9,567

Waste Activated Sludge ppd 2,905 4,113 6,071

Total Digester Feed ppd 7,851 11,594 15,488

Total Digester Feed gpd 15,913 23,437 31,511

Total Digested Solids ppd 4,148 6,792 9,039

Maximum Maximum

Parameter Units Average Month Week

Flow mgd 4.40 7.17 8.31

BOD ppd 11,293 15,359 22,139

TSS ppd 10,422 15,736 20,010

NH3-N ppd 1,019 1,596 1,889

TP ppd 203 367 502

3.6 FUTURE SOLIDS PRODUCTION

Table 3-5 shows Year 2045 solids production values.

Table 3-5 Future Solids Production

3.7 SOLIDS PROCESS CAPACITIES

Table 3-6 and Figure 3-1 shows current and future solids process capacities. Dewatered biosolids storage is overloaded today, assuming the NR110 minimum 180 days of storage is adequate. The 2018 Facilities Plan recommended a far more reliable and resilient 330 days of storage. The existing cake storage structure falls far short of this capacity of objective. The WAS thickening, dewatering, and storage processes will be overloaded in Year 2045.

Table 3-6 Current and Future Solids Process Capacities

Maximum Maximum

Parameter Units Average Month Week

Primary Sludge ppd 6,940 10,509 13,188

Waste Activated Sludge ppd 4,005 5,669 8,369

Total Digester Feed ppd 10,823 15,983 21,350

Total Digester Feed gpd 21,937 32,310 43,440

Total Digested Solids ppd 5,718 9,363 12,460

Unit Process

Operating

Condition Units

Capacity

Objective

Year

% of

Capacity

Year

% of

Capacity

WAS Storage Maximum

Day gal 410,000 110,000 27% 310,000 76%

WAS Gravity Belt Thickener Maximum

Week hrs/day 7.0 4.1 59% 8.0 114%

Anaerobic Digestion - HRT Maximum

Month days 15 40 38% 21 72%

Anaerobic Digestion - VS Loading Maximum

Month ppd-

VS/kcf 150 73 49% 138 92%

Dewatering Belt Filter Press Maximum

Week hrs/day 8.0 3.9 48% 8.3 104%

Dewatered Biosolids Storage - 180 d

Average

Loading, 180 days cy 2,751 2,792 101% 3,849 140%

Dewatered Biosolids Storage - 330 d

Average

Loading, 330 days cy 2,751 5,118 186% 7,056 257%

Figure 3-1 Current and Future Solids Process Capacity Analysis

3.7.1 WAS GRAVITY BELT THICKENING

The lone gravity belt thickener (GBT) does not have sufficient capacity for the Year 2045. This important unit process also lacks redundancy. All alternatives developed and evaluated later this Amendment will enhance WAS thickening capacity and reliability by [1] removing the existing end-of-life GBT and [2] adding two properly sized GBTs. Although the marketplace offers alternative thickening devices, the Owner wants to continue using GBTs because this technology has proven to be reliable, simple to operate, simple to maintain, and cost effective.

3.7.2 DEWATERING BELT FILTER PRESS

The lone belt filter press (BFP) does not have sufficient capacity for the Year 2045. This important unit process also lacks redundancy. All alternatives developed and evaluated later this Amendment will enhance dewatering capacity and reliability by [1] removing the existing end-of-life BFP and [2] adding two properly sized BFPs. Although the marketplace offers alternative thickening devices, the Owner wants to continue using BFPs because this technology has proven to be reliable, simple to operate, simple to maintain, and cost effective.

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

220%

240%

260%

WAS Storage WAS Gravity Belt Thickener

Anaerobic Digestion - HRT

Anaerobic Digestion - VS

Loading

Dewatering Belt Filter Press

Dewatered Biosolids Storage -

180 d

Dewatered Biosolids Storage -

330 d

% Capacity

Solids Process

Current YR 2045 Capacity

3.7.3 DEWATERED BIOSOLIDS STORAGE

The existing cake storage structure does not have sufficient capacity, providing less than 180 days storage at the current cake production. It falls far short of the 330-day storage objective. All alternatives developed and evaluated later in this Amendment consider strategies to address this shortcoming.

3.8 BIOGAS STORAGE AND HANDLING

The existing anaerobic digesters operate effectively and have sufficient capacity for current and future loadings. The digester covers are worn and need rehabilitation. The digester covers are scheduled for rehabilitation as part of the City’s operation and maintenance efforts in conjunction with its Capital

Improvements Plan. All alternatives developed and evaluated later in this Amendment include cover maintenance.

Biogas is collected from the anaerobic digesters and burned in combination boiler-heat exchangers to heat the sludge and buildings. Excess biogas is flared. All alternatives developed and evaluated later in this Amendment include beneficial biogas reuse, reinforcing the City’s commitment to using its sustainable, renewable resources.

4. BIOSOLIDS PROGRAM ALTERNATIVES

The USEPA refers to biosolids as the nutrient-rich organic materials that are produced when treating domestic or municipal wastewater at a wastewater treatment facility. When sufficiently treated and processed, these residual solids can be recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth. Based on more than a decade of scientific study and a research-based risk assessment, the USEPA established regulations for the final use and disposal of biosolids in 40 Code of Federal Regulations (CFR), Part 503 (1993) and adopted a national policy to promote beneficial reuse through land application. The WDNR has taken the same position regarding beneficial reuse. NR 204 states the following: the beneficial use of biosolids and recycling them to the land as a fertilizer or soil conditioner is encouraged, rather than disposing of them through incineration or landfilling. The WDNR directly regulates biosolids through the WPDES Permit program.

Biosolids are classified as either Class A or B. Class A solids can be designated as Exceptional Quality (EQ) if concentrations of specific metals are less than the values stated in 40 CFR, Part 503 and NR 204. Metals requirements are summarized in Table 4-1. Metals concentrations from biosolids generated at the facility in recent years (2020 – 2022) satisfy the Class A EQ requirements.

Table 4-1 Biosolids Metals Restrictions

Metal Class A EQ Concentrations

(mg/Kg) Class A or B Ceiling

Concentrations (mg/Kg) Lifetime Cumulative Limit for

Class B (lbs/ac)

Arsenic 41 75 36

Cadmium 39 85 34

Copper 1,500 4,300 1,339

Lead 300 840 268

Mercury 17 57 15

Molybdenum - 1 75 - 1

Nickel 420 420 375

Selenium 100 100 89

Zinc 2,800 7,500 2,500

1USEPA is revising

Class B Biosolids

Class B biosolids are produced by a process that reduces pathogens to a level that significantly reduces the risk to the public. The City uses anaerobic digestion to produce a Class B biosolids product. These solids are restricted to landfilling or bulk application to agricultural land, forest, or reclamation sites. The product is not suitable for unregulated distribution to the agricultural community, landscapers, or public.

Class B biosolids carry strict site restrictions that limit crop harvest schedules and public/livestock access or contact. These factors have placed increasing stress on storage and disposal efforts at the facility in recent years.

Table 4-2 summarizes other Class B site restrictions (NR 204) to protect groundwater, surface water, public access, and public perception.

Table 4-2 Class B Site Criteria by Land Application Method

Criteria Surface Tillage Injection

Depth to Bedrock (ft) 3 3 3

Depth to High Groundwater (ft) 3 3 3

Allowable Slope (%) <6 0 - 12 0 - 12

Distance to Well: Community or School (ft) 1,000 1,000 1,000

Distance to Well: Private Potable (ft) 250 250 250

Distance to Well: Irrigation (ft) 50 50 50

School or Healthcare Facilities (ft) 1,000 1,000 1,000

Property Lines (ft) 50 25 25

Stream, Lake, or Wetland (ft): 0 to 6% Slope 200 150 100

Stream, Lake, or Wetland (ft): 6 to 12% Slope Not Allowed 200 150

Soil Permeability (in/hr) 0.2 – 6 0 – 6 0 - 6

The City’s current Class B biosolids program is subject to many factors outside their control:

Seasonal weather patterns constrict operations timeframes for solids disposal via land application. Often the window is on the order of two weeks, or narrower depending on weather conditions and seasonal weight limits on County Highways. “Mother nature” is in charge of all Class B land application programs that rely on agricultural fields. Recent weather trends have proven problematic for both spring and fall hauling and land application seasons.

Nutrient management plans and modern agricultural practices combine to limit available acres on which apply biosolids. Many agricultural landowners are required or choose to monitor nutrients such as nitrogen and phosphorus through routine soil samples and documentation of crop rotations. This has increased the cost, effort, and challenge to land apply biosolids on agricultural fields. Facilities need more fields than they did previously. Many of these fields are farther from the Facility, increasing hauling costs, labor requirements, competition for suitable sites, and other logistical challenges related to finding, getting approval of, getting to, and getting on suitable Class B land application sites. Agricultural practices tending towards no-till cultivation further limit the available acres on which solids injection is favorable to landowners.

Future application rates are likely to be limited by agronomic phosphorus loadings to better protect surface waters. This requirement would roughly triple the required acreage and time to land apply.

Limiting application rates to agronomic phosphorus loadings is likely to cause another distressing challenge: acceptance by the agricultural community. Applying biosolids at an acceptable phosphorus loading will dramatically reduce the nitrogen loading to roughly 1/3 of historical or present-day loadings.

Landowners are likely to decide biosolids application no longer provides adequate value as the fields will need supplemental nitrogen fertilizer after the biosolids are applied and biosolids application delays planting and compacts the soil.

Although current Class B biosolids regulations are science based, future regulations may be defined by public opinion. High-profile stories involving Class B biosolids have prompted public discourse regarding the longevity of Class B regulations and the potential for future Federal regulations eliminating Class B and allowing only Class A. Such a change would not be driven by science, but rather by one or several sensational news stories and/or effective activist groups that change public perceptions and acceptance.

Neither the USEPA nor the WDNR require facilities to produce a Class A product. It’s quite the opposite as both agencies endorse and advocate for Class B land application. Nevertheless, facilities making a significant biosolids program investment might be wise to consider the potential for future regulations that are more restrictive.

The City has contracted privately for biosolids transportation and land application on WDNR-approved agricultural fields. Landowners have historically accepted Class B biosolids as a nutrient supplement;

however, it is increasingly difficult for WPCF owners and/or their contractors to haul, and land apply biosolids on WDNR-approved fields during increasingly short spring and fall hauling seasons. Moreover, it is becoming increasingly difficult to find fields in the area that can satisfy the nutrient management plan and WDNR requirements for topography, soil type, and separation from groundwater. The City will face additional significant challenges if/when land application rates are limited to agronomic phosphorus loadings.

In addition to changing agricultural practices and regulations, the City of Sun Prairie and a Class B land application program faces a growing challenge that is unique to its location: it must compete with the Madison Metropolitan Sewerage District and other treatment facility owners for a limited and diminishing number of suitable agricultural fields in proximity to the treatment facility.

Class A Biosolids

Class A biosolids undergo a process to further reduce pathogens to a level that is nearly non-detectable compared to native soil and environment, eliminating crop harvest and public access restrictions. Class A EQ biosolids can be distributed to and used by the public as a fertilizer product and have minimal regulatory requirements. In addition to agriculture, these end products can be used for landscaping, horticulture, and other public uses.

Due to the additional processing required to reduce pathogen levels, the initial cost to produce Class A biosolids is often higher than the cost to produce Class B biosolids. Relaxed regulations, however, offer the potential to reduce annual costs for biosolids disposal. The increasingly troublesome and restrictive logistics inherent with large Class B land application programs are prompting an increasing number of facilities to invest in Class A technologies, particularly those that produce a dry Class A EQ product that can be distributed unhindered.

In addition to reducing land application restrictions and challenges, drying dramatically reduces biosolids volumes, weights, and, in turn, vehicle traffic to/from a Facility and on City streets. EPA regulations codify the parameters constituting the requirements to produce a Class A product. Biosolids drying is often used to produce Class A biosolids, but other methods have been successfully used for many years.

With increasing costs, land availability, and weather dependency causing concern for Class B biosolids land application, Class A production has become increasingly popular. Potential Class A disposal options include public distribution, non-metallic mining reclamation, compost collaboration, golf course fertilizer, agriculture fertilizer, and biomass fuel. Implementing a Class A biosolids process could allow the WPCF to produce a beneficial reuse product that eliminates the cost to haul and apply the product.

4.1 DRYING TECHNOLOGIES

4.1.1 SOLAR DRYING

Solar drying uses solar radiation to evaporate water from biosolids. This is depicted in Figure 4-1. Sludge is distributed uniformly across the floor of building with a glass housing that looks and functions like a conventional “greenhouse.” Traveling agitators mix the biosolids to maintain aerobic conditions, renew the exchange surface area, and gradually move increasingly dry sludge from one end of the greenhouse to the other. Solar energy is the main mechanism of drying; however, air or floor heating can be used to expedite drying. Intensive ventilation moves saturated air out of the housing.

Figure 4-1 Solar Drying Illustration

Solar drying can dry biosolids to approximately 90%. Depending on the time of year, solar availability, and State-specific regulatory requirements, solar drying has the potential to produce a Class A product. Dried product can be transferred to storage or directly hauled for reuse. Table 4-3lists advantages and disadvantages of solar drying.

Table 4-3 Solar Drying Advantages and Disadvantages

Advantages Disadvantages

• Safety

• Simplicity

• Low carbon footprint

• Enhances storage capacity by reducing volume and adding volume

• Large spatial footprint*

• Low drying rate*

• Few installations

• Class A certification challenges

*As compared to other drying technologies installed in the City’s climate

4.1.2 THERMAL DRYING

Thermal dryers are well-proven technologies adept at producing Class A and Class A EQ biosolids. Thermal dryers are direct or indirect. Direct dryers heat the sludge by direct contact between the biosolids and hot gas. Indirect dryers heat the sludge using heat transferred from the encased heating fluid to the biosolids via conduction and radiation. Thermal dryers can achieve a wide range of product moisture content, ranging from 35-90+% solids depending on the dryer technology, design, and operation.

Some thermal dryers operate at high temperatures – 900°F or more. Examples include rotary drum and fluidized bed dryers. These technologies produce a pelletized product that is more valuable for resale;

however, the high temperature dryer systems are more expensive and require more safety precautions.

Low temperature dryers include belt dryers, paddle/screw dryers, and thin film dryers. These systems generate a product that is less consistent in shape and size. Low or medium temperature dryers tend to be more economical for small to mid-size plants. They are also generally considered to be simpler and require fewer safety precautions. Table 4-4 lists advantages and disadvantages of Class A thermal drying.

Table 4-4 Class A Thermal Drying Advantages and Disadvantages

Advantages Disadvantages

• Large volume reduction

• Enhances end-use flexibility

• Reduces weather-related encumbrances that hinder Class B land application

• Capable of producing high-quality pellet suitable for commercial use

• Capital cost

• Complexity

• Safety

• Energy consumption

• Staffing requirements

4.1.3 DEHUMIDIFICATION DRYING

With dehumidification dryers air is heated using reject heat from a refrigeration compression cycle to the desired drying temperature, then circulated over the product. An integral condenser condenses moisture removed from the biosolids via the air stream. The cool, dry air from the condenser passes over a hot coil system and is recirculated over the biosolids. Very little or no external makeup is used in the system, reducing heating demand relative to a direct fired thermal dryer. This air recycling can also concentrate volatilized compounds with eventual precipitation on process surfaces, degrading efficiency and causing maintenance issues over time.

Figure 4-2 shows a cross section of a dehumidification dryer. Unlike other dryer technologies, dehumidification drying does not require a separate heating plant, boiler, burner, or thermal fluid system.

The evaporative capacity for dehumidification dryers is 55-4,400 lbs-water/hr. The WDNR has recently deemed a dehumidification dryer capable of producing a Class A product based on time-temperature.

Table 4-5 lists advantages and disadvantages of dehumidification drying.

Figure 4-2 Dehumidification Dryer Cross-Section

Table 4-5 Dehumidification Drying Advantages and Disadvantages

• Simple

• Low emissions

• Low carbon footprint if the electricity that provides the drying heat originates from a renewable source – e.g., solar, hydro, or biogas

• Large footprint

• Lower capacity relative to thermal dryers

• Few installations

4.2 ELODE DEWATERING

ELODE (Electro Osmosis Dehydrator) is an enhanced dewatering technology using electrical manipulation of particle charges to remove free and absorbed water. Direct current is applied to a low-pressure drum;

negatively charged sludge particles migrate to the positively charged anode drum and positively charged water moves to the negatively charge cathode caterpillar. ELODE can efficiently increase total solids content from ±15% to over 40% without chemicals or pressurized mechanical dewatering. Sun Prairie participated in a bench test in 2023. The ELODE increased the solids content from 12.7% to 44%. Figure 4-3 shows a ELODE process schematic. Figure 4-4 and Figure 4-5 show photographs from the Donohue-designed ELODE installation at Henderson, KY. Table 4-6 lists advantages and disadvantages of ELODE dewatering.

Figure 4-3 ELODE Overview

Figure 4-4 ELODE Cake Dewatering

Figure 4-5 ELODE Installation (Henderson, KY)

Table 4-6 ELODE Advantages and Disadvantages

• Excellent volume reduction

• Compact: small footprint

• Simple operation, easy startup/shutdown

• Fast processing speed

• Safety: no dust or high temperatures

• No chemical consumption

• Low energy consumption

• Low carbon footprint if the electricity that provides the drying heat originates from a renewable source – e.g., solar, hydro, or biogas

• High energy consumption

• Few installations

• Few manufacturers

• Periodic drum maintenance

4.3 ANOXIC THERMOPHILIC PRETREATMENT

Anoxic Thermophilic Pretreatment (ATP) is a digestion pretreatment process that reduces pathogens to meet the Class A time and temperature requirements. The ATP system operates in batches. A tank is fed primary thickened sludge (PTS) and thickened activated sludge (TAS) before holding that material for the required duration at the required temperature. After the specified time, the contents are transferred to digestion. The ATP residence time is typically 24 hours. During the heat/hold phase the heat exchanger provides recirculated heat. Figure 4-6 shows an overview of an ATP process upstream of anaerobic digestion. The system requires three tanks, heat exchanges, pumps, piping, valves, and controls. The control system runs and monitors the system. Table 4-7 lists the ATP advantages and disadvantages.

Figure 4-7 shows a four-reactor ATP system Donohue designed for Holland, MI.

Figure 4-6 ATP Overview

Table 4-7 ATP Advantages and Disadvantages

• The batch operation reliably and consistently produces a Class A product. Every particle of biosolids satisfies the required time-temperature.

• Enhances downstream digestion performance and gas production

• Control system makes operation simple

• Process layout lends itself to the WPCF

• Requires more heat input than conventional anaerobic digestion; however, heat recovery minimizes the amount of this heat

• Additional maintenance and installed equipment

Existing Anaerobic Digestion

Figure 4-7 Four Reactor ATP (Holland, MI)

4.4 FEASIBLE ALTERNATIVES SCREENING

The evaluated technologies were assembled into functional groups – holistic biosolids alternatives – feasible for operation and installation at the WPCF. The alternatives provide outcomes on a spectrum of desirability for multiple factors like operability, energy-intensity, initial cost, or lifetime cost. The alternatives evaluation was designed to identify an alternative that provides the most favorable long-term value to the City and its rate payers.

The evaluation considered capital costs, operating costs, 20-year present value costs, and five non-economic categories. The five non-economic categories are listed below and described in Table 4-8.

6. Safety, Operability, and Maintenance

7. Resiliency, Stability, Robustness

8. Quality of Life

9. Environmental Stewardship, Sustainability

10. Longevity and Flexibility

Each category was assigned a weighting factor. The weighting factors determined how much influence that category would have on the overall selection decision, and generally reflected the overall goals and priorities of the WPCF. Categories were scored between 1 and 5 in 0.5-point increments, with 1 being least favorable and 5 being most favorable. Alternatives with a feasibility score of 3.5 or higher were deemed suitable and developed further to support an opinion of probable construction cost (OPCC).

Table 4-8 Non-Economic Factors and Scoring System

Score

Evaluation Factors

1 3 5

Safety, Operability, and Maintenance

Many pieces of equipment, proprietary parts, hot parts

Many parts, proprietary parts not many parts, little proprietary parts or complicated

Process Stability Very difficult /unproven with lots of variability

Pretty consistent product, but requires a bit of time to make it so consistent product, not much effort to get it the same all the time

Process Redundancy and Operational Flexibility

Several points of failure between intermediate processes and final product with major impact to operations expected even for routine maintenance.

Single point of failure between intermediate processes and final product with bypass available or operations easily mitigated.

Redundant systems available, no impact to operations expected.

Quality of Life routine/required exposure to chemicals, odors, etc. – status quo trucking on local roadways, chemical deliveries, etc.

infrequent chemical exposure, contained/treated odors – reduced trucking by volume reduction fully contained odors, no exposure to harsh chemicals

– dried material with minimal trucking or beneficial reuse into another product

Environmental Stewardship and Sustainability

Dependent on outside resources, with availability of the City’s control – generates significant trucking

Partially powered/fueled by resources available at the facility – reduced trucking and resulting GHG emissions

“net zero” sustainable process (or could be with future additional efforts) – minimal trucking or onsite disposal available

Regulatory Flexibility

Single point of disposal Multiple points of disposal Multiple types and points of disposal

Unit Process Compatibility the flow path is awkward, unproven or requires significant manipulation to be workable (e.g. DSL blending to make extrusion possible) the flow path is straightforward, doesn’t require significant pumping or manipulation and is logical the flow path is straightforward, logical and doesn’t require much/any additional unit processes from existing

4.4.1 STATUS QUO

This alternative would maintain the WPCF’s current practices: [1] anaerobic digestion, [2] mechanical dewatering, and [3] regulated land application of a Class B product at approximately 15% TS. Additional storage would be constructed, and the WPCF would remain reliant on contract hauling and land application. Heavy, diesel truck traffic would continue at present levels, increasing as biosolids production increases. Staff would have to increase current efforts to maintain relationships with landowners and secure others to maintain enough land to sustain its regulated Class B land application program.

The total non-economic score is 2.1 (Table 4-9). This alternative was eliminated from further consideration because: [1] its non-economic score was less than 3.5, [2] it is poorly aligned with the ethos of the community and WPCF as a carbon-based, energy intensive, labor-intensive process, and [3] it would require a large sunk-cost capital investment addressing none of the ever-increasing challenges associated with a high-water-content, high-volume Class B biosolids program. Development in and around Dane

County will continue to eliminate suitable land application fields and, with the growing regional population, the competition for a diminishing number of fields will accelerate with time.

Table 4-9 Status Quo Non-Economic Score

Categories Weight Score

Safety, Operability, and Maintenance 10 5.0

Process Stability 20 2.5

Process Redundancy and Operational Flexibility 20 1.0

Quality of Life 10 1.0

Environmental Stewardship and Sustainability 15 1.0

Regulatory Flexibility 15 1.0

Unit Process Compatibility 10 5.0

Total 100 2.1

4.4.2 ENHANCED DEWATERING (ELODE)

This alternative would maintain its current operations for solids generation and disposal but install an enhanced dewatering unit (ELODE) as an intermediate step between dewatering and storage. ELODE would reduce the total biosolids volume by approximately one-third, significantly reducing trucking and land requirements for disposal. Electricity usage would increase, but chemical usage will decrease as the BFP operation would not need to be optimized. The end product would be Class B at 40-50% TS.

The total non-economic score is 3.1 (Table 4-10).