W911SA-17-B-0026_Attachment_1_-_Final_Design_Engineering_Report.pdf

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Repair Digester (FY16) Fort McCoy, WI Final Design Engineering Report

Table of Contents Page TC-1

FINAL DESIGN ENGINEERING REPORT

FOR

REPAIR DIGESTER (FY16)

FORT MCCOY, WISCONSIN

TABLE OF CONTENTS

PART 1 - GENERAL PROJECT INFORMATION

1 PURPOSE

2 AUTHORIZATION

3 PROJECT DESCRIPTION

4 APPLICABLE CRITERIA

4.1 GENERAL APPLICABLE DESIGN CRITERIA

4.2 REQUIREMENTS

PART 2 – DESIGN REQUIREMENTS AND PROVISIONS

1 WASTEWATER SYSTEMS AND COLLECTION

1.1 DESIGN REFERENCES

1.2 GENERAL

1.3 FLOWS AND LOADING

1.3.1 Effluent Standards and Current Effluent Limits

1.3.2 Future Phosphorus Effluent Limits

1.3.3 Existing Influent Flow and Loading Analysis

1.3.4 Future Flow and Loading Considerations

1.4 EXISTING WWTP TREATMENT FACILITIES

1.4.1 Existing Treatment Facilities

1.4.2 Existing Sludge Handling System

1.4.3 Digester Repair Scope History

1.5 AEROBIC DIGESTER DESIGN

1.5.1 Selected Aerobic Digester Process Description

1.5.2 Aerobic Digester Components (Support Equipment)

1.6 PROCESS CONTROL

1.7 PROJECT CONSTRUCTION ACTIVITIES AND CONSIDERATIONS

1.7.1 Modifications to Existing Anaerobic Digestion System

1.7.2 Temporary Treatment during Construction/Contract Phasing

1.8 SITE APPLICATION APPROVAL FOR WASTEWATER TREATMENT SYSTEM

2 STRUCTURAL

2.1 PROJECT DESCRIPTION

2.2 DESIGN CRITERIA

2.3 TANK ANALYSIS

2.3.1 Soil Properties

2.3.2 Reinforcement Properties

2.3.3 Concrete Properties

2.4 TANK REPAIRS AND NEW INTERIOR COATING

2.5 NEW GEODESIC DOME

2.6 TANK WALLS INSIDE BUILDING

2.7 BLOWERS CONCRETE PAD

2.8 SUBMERSIBLE TANK PIPE SUPPORT

Table of Contents Page TC-2

3 MATERIALS (SOILS A)

3.1 PROJECT DESCRIPTION

3.2 UFGS SPECIFICATIONS USED

3.3 SPECIAL CONDITIONS

3.4 PRIOR TO PERFORMING REPAIRS

3.5 CONCRETE SPALL REPAIRS

3.6 CONCRETE CRACK REPAIRS

3.7 REPAIR WATERSTOPS

3.8 INTERIOR TANK COATINGS

3.9 PIPE COATINGS

4 MECHANICAL

4.1 PROJECT DESCRIPTION

4.2 DESIGN CRITERIA

4.3 PROJECT LOCATION

4.4 MECHANICAL SYSTEM

4.5 BLOWER EQUIPMENT CAPACITY

4.6 BLOWER LOCATION

4.7 COMPRESSED AIR PIPING

5 ELECTRICAL

5.1 DESIGN CRITERIA

5.1.1 Reference Publications

5.2 SCOPE

5.2.1 Design Systems

5.3 INTERIOR ELECTRICAL WORK

5.3.1 Panel boards and Motor Control Centers

5.3.2 Blower Control Panel

5.3.3 Variable Frequency Drives

5.4 EXTERIOR ELECTRICAL WORK

5.4.1 Power Distribution

5.4.2 Cathodic Protection

6 ENVIRONMENTAL PROTECTION AND COMPLIANCE

6.1 AIR POLLUTANT EMISSION SOURCES

6.1.1 Air Permit Emissions

6.1.2 Refrigerants

6.1.3 Demolition

6.2 WATER POLLUTANT SOURCES

6.2.1 Storm Water Discharge during Construction

6.2.2 Construction Waste Water

6.2.3 Dewatering of Ground Water

6.2.4 Sewer Mains, Lift Stations, Water Mains, and Fire Hydrants

6.2.5 Backflow Preventers

6.3 HISTORICAL, ARCHAELOGICAL, CULTURAL, AND BIOLOGICAL RESOURCES

6.4 MISCELLANEOUS ENVIRONMENTAL COMPLIANCE COMMON TO A CONSTRUCTION

SITE

6.4.1 Environmental Protection Plan

6.4.2 Environmental Permits

6.4.3 Special Environmental Protection

Table of Contents Page TC-3

APPENDICES

A – ENVIRONMENTAL CALCULATIONS

B – WPDES PERMIT

C – STRUCTURAL CALCULATIONS

D – MECHANICAL CALCULATIONS

Design Analysis Page 1 of 33

PART 1 - GENERAL PROJECT INFORMATION

1 PURPOSE

The purpose of this project is to repair and convert the existing Anaerobic Sludge digestion system into an Aerobic Sludge digestion system. Design development is currently underway for a near-future project (FY-17/18) for upgrade of the Ft. McCoy WWTP. That project will consist of removing the primary clarifiers, abandonment of the trickling filters and solids contact stabilization unit and replacement with a new extended aeration activated sludge treatment system.

This digester repair project will precede the plant upgrade due to the condition and urgency of repair needed on the existing anaerobic digester system. Conversion to an aerobic digestion process is a logical and compatible choice to complement the proposed activated sludge plant upgrade. This project is designed to meet Wisconsin Administrative Code NR110.26 and the “Recommended Standards for Wastewater Facilities” (Ten States Standards) design requirements for aerobic digesters and accounts for the design, operation and solids production of the proposed activated sludge treatment upgrade.

2 AUTHORIZATION

This FY 16 project is funded via Ft. McCoy Operations and Maintenance sources.

3 PROJECT DESCRIPTION

This project includes the upgrade of the existing anaerobic digestion system of the Wastewater Treatment Plant (WWTP) at Ft. McCoy, WI. The WWTP is located at 2280 Buckley Ct., Fort McCoy, WI. The upgrade consist of the conversion of the existing anaerobic digester and the sludge holding tank into an aerobic sludge digestion system. The modifications for conversion to aerobic digestion will include the following:

a. Removal of the existing anaerobic digester floating gas cover and associated mixing equipment

b. Removal of all gas collection, handling and disposal piping and associated equipment

c. Cleaning and concrete repairs to Structure 60 the (existing digester) and Structure 62

(sludge holding tank) concrete walls and surfaces

d. Installation of a new protective coating/lining system to the interior concrete surfaces of

Structures 60 and 62

e. Installation of a new jet aeration system, consisting of a submersible mixing pump and nozzle diffuser piping within both digesters. Air to the jet aeration system will be supplied by three new externally-located positive displacement blowers equipped with variable frequency drives.

f. Installation of a new floating decant system for each digester

g. Installation of dissolved oxygen and liquid level sensing equipment within each digester

The existing waste activated sludge (WAS) pumps, primary sludge pumps, recirculation pump and sludge transfer pump will be reused. The two existing gas-fired boilers will be left in place for heating the building and will continue to be supplied by natural gas. The spiral heat exchanger, while not required for proper aerobic digestion, will remain in place. The existing dissolved air

Design Analysis Page 2 of 33 flotation thickener (DAFT) unit will continue to be used when necessary to reduce the volume requirements for digester storage during peak loading conditions. All other existing treatment processes will remain in-place and un-altered for this project.

4 APPLICABLE CRITERIA

4.1 GENERAL APPLICABLE DESIGN CRITERIA

Refer to individual design disciplines sections of this DA for the applicable general and industry design criteria used in the design. References materials, including calculations, catalog cut sheets, and other supporting documentation are provided in the appendices.

4.2 REQUIREMENTS

The Ft. McCoy WWTP existing anaerobic digester and holding sludge tank and mixing equipment were most recently modified around 1997. The anaerobic floating gas cover is in need of major structural rehabilitation/replacement and the integral gas mixing system is no longer functional.

Gas production from the facility has never been reliable or predictable and is not an economical source of gas supply for heating. The methane gas produced is currently wasted in the existing methane burner. Both tank structures (digester and sludge holding tank) also require minor concrete wall crack repairs and application of a protective coating/lining system to extend serviceability of the units (the original WWTP digester tanks were built in the 1950’s).

Design Analysis Page 3 of 33

PART 2 – DESIGN REQUIREMENTS AND

PROVISIONS

1 WASTEWATER SYSTEMS AND COLLECTION

1.1 DESIGN REFERENCES

The following references were used in preparing the water supply and wastewater disposal design:

GREAT LAKES-UPPER MISSISSIPPI RIVER BOARD OF STATE PUBLIC HEALTH AND

ENVIRONMENTAL MANAGERS

GLUMRB (2014) Recommended Standards for Sewage Works

STATE OF WISCONSIN ADMINISTRATIVE -DEPARTMENT OF NATURAL RESOURCES (NR)

CODE NR 110 (2014) Sewerage Systems Design, and Construction Criteria

CODE NR 108.04(2)(f) (2014) Plan Submittal Requirements Guidance

CODE NR 281.41 (2014) Wastewater Systems Plan Review for Municipal and other Non-Industrial Systems

WASTEWATER ENGINEERING TREATMENT AND RESOURCE RECOVERY METCALF &

EDDY/AECOM (Fifth Edition)

1.2 GENERAL

This section discusses the design requirements to support the operations of Ft. McCoy Wastewater Treatment Plant (WWTP) Digestion System. The modifications of the existing Anaerobic Digestion System into an aerobic sludge digestion system includes:

a. Removal of the existing anaerobic digester floating gas cover and associated mixing equipment

b. Removal of all gas collection, handling and disposal piping and associated equipment

c. Cleaning and concrete repairs to Structure 60 the (existing digester) and Structure 62

(sludge holding tank) concrete walls and surfaces

d. Installation of a new protective coating/lining system to the interior concrete surfaces of

Structures 60 and 62

e. Installation of a new jet aeration system, consisting of a submersible mixing pump and nozzle diffuser piping within both digesters. Air to the jet aeration system will be supplied by three new externally-located positive displacement blowers equipped with variable frequency drives.

f. Installation of a new floating decant system for each digester

g. Installation of dissolved oxygen and liquid level sensing equipment within each digester

1.3 FLOWS AND LOADING

Ft. McCoy is a military installation facility where resident and non-resident populations are seasonal and can vary widely from year to year. The transient troop load varies across the year, Design Analysis Page 4 of 33 with an average monthly population of 15,000. The WWTP influent flows varies throughout the year, reflecting lower flows during the winter and higher flows during the summer month season, which coincides with the training period where the population and usage is expected to be the highest. Deployments can also significantly impact both resident and transient populations. Fort McCoy’s WWTP was most recently upgraded in 1997 and was designed for an average daily flow of 1.32 million gallons per day (MGD).

1.3.1 Effluent Standards and Current Effluent Limits

Treated wastewater from the Ft. McCoy WWTP is currently discharged to the La Crosse River in the Upper La Crosse River Watershed. Ft. McCoy‘s WPDES discharge permit (Permit No. WI- 0022420-07-0 issued by the State of Wisconsin Department of Natural Resources (WDNR) in July I, 2013) will expire June 30, 2018. Ft. McCoy WPDES’s permit currently specifies the following limits for treated wastewater that is directly discharged to the Upper La Crosse River Watershed:

The following table summarizes the current discharge parameters.

Parameter Limit Type Limits and Units

Flow Rate Continuous mgd BOD5 Monthly Avg 30 mg/l BOD5 Weekly Avg 45 mg/l BOD5 Weekly Avg (May- Oct) 438 lb/day BOD5 Weekly Avg (Nov- April) 855 lb/day Suspended Solids, Total (TSS) Monthly Avg 30 mg/l Suspended Solids, Total (TSS) Weekly Avg 45 mg/l Suspended Solids, Total (TSS) Weekly Avg (May- Oct) 438 lb/day Suspended Solids, Total (TSS) Weekly Avg (Nov- April) 855 lb/day Total Phosphorus Monthly Avg 1.0 mg/l pH Daily Min and Daily Max 6.0 su to 9.0 su Dissolved Oxygen (DO) Daily Min 7.0 mg/l Fecal Coliform (March-Sept) Geometric Mean 400 # per 100 ml

1.3.2 Future Phosphorus Effluent Limits

Wisconsin promulgated water quality standards for phosphorus in December 2010 under Wisconsin Administrative Code NR217. The existing effluent monthly average permit limitation for total phosphorus is currently 1.0 mg/l. This is an interim limit throughout the term of the current permit (expires June 30, 2018). The final phosphorus limit will become 0.075 mg/l and

0.83 lbs/d (6 month average) and 0.225 mg/l (monthly average). Phosphorus reduction is currently achieved by the chemical addition of ferric chloride to the contact tank effluent upstream of the final clarifiers. The ferric reacts with the phosphorus, settles in the final clarifier, and is removed in the final clarifier sludge.

1.3.3 Existing Influent Flow and Loading Analysis.

An analysis of existing plant flow records was undertaken for this project in an effort to identify influent flows and organic loadings to determine and validate aerobic digester volume requirements. Influent flow and loading results were then compared to the values used as the 1997 Plant Upgrade design parameters.

Influent data from WWTP operating records and discharge monitoring reports (DMRs) for January

Design Analysis Page 5 of 33

2011 through December 2015 period were used in the analysis. For this project, the primary data gleaned from this information was the daily influent flow volumes (million gallons, MG) and daily influent Biochemical Oxygen Demand (BOD5 in terms of lbs. per day) and Total Suspended Solids (TSS, in terms of lbs. per day) and maximum and minimum flow rates for each day of that period, and the daily loading to the plant.

1.3.3.1 Influent Flow

The analysis of the existing plant flow records for the period evaluated indicates a wide range of flows due to the seasonal variations in resident and non-resident populations and changes in activity levels at Ft. McCoy as it varies from year to year. For the 2011-2015 period reviewed, the daily influent flow ranged from a minimum flow of 0.02 MGD to a maximum flow of 1.26 MGD, with an average for the period of 0.34 MGD. By comparison these flows are much less than the corresponding 1997 design values for average daily influent flow value of 1.32 MGD and peak influent daily flow of 2.257 MGD. Graphical representations of these results are illustrated in Appendix A.

1.3.3.2 Influent Loading

The analysis of the existing influent BOD5 and TSS loading during the period of study followed a similar variation in values due to the wide range in resident and non-resident populations and changes in activity levels at Ft. McCoy from year to year. BOD5 ranged from very low (1 lbs. per day) to a maximum of 2,984 lbs. per day, with the average of 674 lbs. per day over the period.

TSS ranged from very low (13 lbs. per day) to a maximum of 4,837 lbs. per day, with the average of 803 lbs. per day. Graphical representations of these results are also illustrated in Appendix A.

The following table presents a summary of the design and actual influent loading parameters from the 2011-2015 study period and the original 1997 design values.

1.3.4 Future Flow and Loading Considerations

Fort McCoy’s projected future flows and loadings were evaluated in a separate study, WWT System Evaluation Report conducted by AECOM in August 2013. Based on that study and consultation with Ft. McCoy staff, the installation is planning to maintain their current training load of approximately 100,000 soldiers per year, taking into consideration that most soldiers are not permanently stationed at the Fort. Training durations will vary from a few days to several weeks.

This training loading was also confirmed with Ft McCoy based on their projected spreadsheet for Fort McCoy Facility Support Capacity Chart for soldier population.

Ft. McCoy does have future plans for an increase in housing development on post (from 55 existing units to an additional 79 for a total of 134 housing units). The study estimated a total additional future flow of 0.017 MGD to the WWTP (assuming 70 gallons per day per capita (gpcd) x 3 people x 79 units). The study indicated that since this represents approximately three (3%) percent increase of the existing flow over the past five years, this amount was considered insignificant, particularly considering that current flows and loadings are well within the 1997 design basis values.

1.3.4.1 2016 Design Influent Flow and Loading

The analysis of daily influent flow and BOD5 and TSS loading over the period of 2011-2015, as well as examination of projected future expansion or staffing plans at Fort McCoy, concludes that the 1997 Plant Upgrade design values were, and are projected to be, conservative values on which to base WWTP upgrades for 2016 and the upcoming 20-year planning period. A comparison and summary of this data is provided below.

Design Analysis Page 6 of 33

Parameter 1997 Upgrade Design

Values 2011-2015 Data

Results

2016 Upgrade Design Values

Population:

Average monthly population 15,000 15,000

Maximum population 20,000 20,000

Estimated minimum monthly population 2,500 2,500

Influent Flows:

Monthly average flow

1.32 MGD

(15,000 pop. x 88 gpcd, based upon summer peak populations)

0.34 MGD 1.32 MGD

Monthly maximum flow

1.875 MGD

(15,000 pop. x 125 gpcd)

0.654 MGD

(June 2014)

1.875 MGD

Daily maximum flow

2.257 MGD

(1.32 MGD x 1.71 peak day factor)

1.26 MGD 2.257 MGD

Peak Hourly flowrate 2.31 MGD 2.31 MGD

Instantaneous peak flow 3.96 MGD (1.32 MGD x 3.0 peak factor)

3.96 MGD

Estimated minimum monthly flow

0.300 MGD

(based upon existing plant flows)

0.124 MGD

(November 2012) 0.124 MGD

Influent BOD:

Monthly Average BOD 1500 lbs./day (136 mg/l)

(based upon 15,000 pop. x 0.10 lb.

BOD/pop./day)

674 lbs./day (238 mg/l @ 0.34

MGD)

1500 lbs./day (136 mg/l)

Monthly Maximum BOD 2000 lbs./day (128 mg/l)

(based upon 20,000 pop. x 0.10 lb.

BOD/pop./day)

1914 lbs./day (June 2011)

2000 lbs./day (128 mg/l)

Daily Maximum BOD 4400 lbs./day (234 mg/l)

(based upon 20,000 pop. x 0.22 lb BOD/pop./day)

2984 lbs./day (16 June 2011, 477 mg/l

@ 0.75 MGD)

4400 lbs./day (234 mg/l)

Monthly Minimum BOD 153 lbs./day

(December 2015) 153 lbs./day

Total Suspended Solids (TSS)

Monthly Average TSS 1650 lbs./day (150 mg/l)

(based upon 15,000 pop. x 0.11 lb.

TSS/pop./day)

803 lbs./day (283 mg/l @ 0.34 MGD)

1650 lbs./day (150 mg/l)

Monthly Maximum TSS 2200 lbs./day (141 mg/l)

(based upon 20,000 pop. x 0.11 lb.

TSS/pop./day)

1830 lbs./day (July 2011)

2200 lbs./day (141 mg/l)

Daily Maximum TSS 4800 lbs./day (234 mg/l)

(based upon 20,000 pop. x 0.24 lb.

TSS/pop./day)

4837 lbs./day (11 March 2015, 2320 mg/l @ 0.25 MGD)

4800 lbs./day (234 mg/l)

Monthly Minimum TSS 162 lbs./day

(December 2015) 162 lbs./day

Design Analysis Page 7 of 33

1.4 EXISTING WWTP TREATMENT FACILITIES

1.4.1 Existing Treatment Facilities

The Fort McCoy WWTP was most recently upgraded in 1997 and designed for an average day flow of 1.32 million gallons per day (MGD). The existing WWTP provides primary and secondary treatment. Preliminary treatment consist of influent screening, grit removal and dewatering, raw wastewater pumping, and primary clarification. Secondary treatment is provided by trickling filters, solids contact stabilization and final clarification. Ferric chloride feed for phosphorus removal is added just upstream of the final clarifier. Disinfection is provided with an ultraviolet radiation system (UV). Sludge is digested in an anaerobic digester. Temporary liquid digested sludge storage is provided in adjacent sludge holding tank. Sludge is dried either on sludge drying beds or freezing drying beds. Disposal of liquid digested sludge and dewatered sludge can also be contracted for hauling and disposal at land application sites.

Three major buildings (Headworks, Mechanical and Service), in addition to the structures/tanks related to the wastewater treatment process, support the Ft. McCoy WWTP. The existing Headworks Building houses the preliminary treatment equipment and raw wastewater pumps.

The existing Mechanical Building includes sludge pumps, digester gas handling equipment, aeration blowers and the emergency power generator. The existing Service Building includes a garage, office and plant laboratory.

The Ft. McCoy WWTP is satisfactorily treating wastewater within current effluent permit standards. This is largely in part due to the regular maintenance and housekeeping practices followed and the operating knowledge of the treatment plant personnel. Most of the current operating plant equipment is from the 1997 upgrade project. Plant hardware and components have generally met their practical design operating life and are showing signs of age in some areas from normal wear and usage, especially those components that have not been replaced or rebuilt since the 1997 upgrade project. The plant has historically undergone major upgrades/modifications and optimization efforts to extend its operating life and to comply with effluent permit standards from the original plant located on the same site. The internal evaluation study conducted by AECOM in August 2013, stated that in general there are no major concerns with process equipment not performing well, or requirements for additional maintenance beyond normal preventative maintenance activities.

While the current unit process equipment at the WWTP is generally in good condition and operating well, the evaluation report recommended optimization, upgrade or replacement of components of the current WWTP unit process that fall into the following categories:

• Equipment required for process/treatment improvement

• Replacement because of end-of-service-life

• Equipment required to prevent operational deficiencies

Additionally, new elements or process units are identified that are required to meet current or anticipated effluent requirements. This includes namely improving nitrogen and phosphorus removals. Plans are currently underway (in an FY-17 Project Order, referred to as Phase II Upgrade) for removal of the primary clarifiers, trickling filters and solids contact basin with replacement by a new extended aeration activated sludge process (oxidation ditch). A separate project (not part of the FY-17 Project Order) will include a tertiary treatment process for improvements to phosphorus removal. The proposed oxidation ditch process will enhance biological phosphorus removal to practical limits of approximately 1 mg/L, however additional consideration will be required for further reduction below permit limits. With the exception of the solids handling to support the proposed oxidation ditch, most of the remaining components of the existing treatment plant process will remain.

The proposed oxidation ditch and plant modifications support the conversion of the existing

Design Analysis Page 8 of 33 anaerobic digester to an aerobic system. Due to the condition of the existing anaerobic digester lid and methane gas handling system that conversion will be undertaken immediately with this current project (FY-16 Project Order or Phase I).

1.4.2 Existing Sludge Handling System

The Ft. McCoy WWTP current sludge stabilization system includes one anaerobic digester and one sludge holding tank as well as a supporting sludge digester control building. The sludge piping, waste activated sludge (WAS) pumps, recirculation pumps, transfer pumps, methane gas handling and heating/boiler are located at the Mechanical building. Originally, the WWTP design included two digester tanks built in the 1940’s. During the Ft. McCoy WWTP upgrade in 1997, one anaerobic digester tank was reused and all the anaerobic digester equipment system replaced while the second digester tank was converted into a digested sludge holding tank. The anaerobic digester components have been in service since the 1997 upgrade and are in poor condition. This includes the degradation of the floating gas cover and failure of the mixing system that relies on gas production. Mixing is currently provided by operation of the sludge recirculation pump(s). Methane recovery for digester heating has always been inadequate for this WWTP and is currently out-of-service. Instead methane gas produced is periodically wasted to the flame burner.

The primary components of the existing anaerobic digester system include the following:

1.4.2.1 Anaerobic Digester Tank

The digester is a concrete circular unit of 55-foot inside diameter with a wall height of 21.5 feet.

The existing operating side water depth (SDW) is 18.5-foot, providing an approximate volume of 328,000 gallons or 44,000 cu ft. This digester is used for sludge stabilization and solids destruction. The top cover is a floating steel frame digester gasholder type, initially designed to with a gas mixing system (which is no longer operational). A sludge recirculation pump is currently used for mixing, heating (in a heat exchanger) and transfer of sludge contents within the digester.

1.4.2.2 Sludge Holding Tank

The sludge holding tank is a concrete circular unit of 55-foot inside diameter with a wall height of

21.5 feet. The existing operating side water depth is 16.5.-foot for an approximate volume of 292,000 gallons or 39,035 cu ft. The sludge is mixed utilizing the three stationary nozzles within the tank and the sludge transfer pump within the adjacent Mechanical Building. The holding tank is covered with an aluminum domed roof. Digested sludge can also be removed for liquid disposal at the truck fill stand or directed to the sludge drying beds or freeze-drying beds by the sludge transfer pump(s).

1.4.2.3 Sludge Digester Piping Building area

A small building with a footprint of approximately 10- ft wide x 8- ft long is constructed between the digester and the sludge holding tank. This building houses the following sludge process lines:

6-in supernatant and overflow pipe, 4-in and 6-in sludge feed pipe, 8-in sludge drawoff pipe, and 4-in digester gas drawoff pipe. These sludge and gas lines run from/to digester/holding tank and to/from the Mechanical Building.

1.4.2.4 Mechanical Building

The basement of the Mechanical Building, approximately 36 ft- 6-in wide x 39 ft-10-in long area, houses:

a. Three (3) return activated pumps (RAS)- pumps draw activated sludge direct from final clarifier

Design Analysis Page 9 of 33

b. One (1) waste activated pump (WAS)- pump WAS from RAS pump header line to influent wetwell and combined with primary sludge

c. Two (2) sludge recirculation pumps – recirculates sludge to and from the digesters and through the hot water heat exchanger and also to provide the gas mixing

d. One (1) sludge transfer pump – transfer sludge from digester to sludge holding tank, transfer digested sludge or holding tank sludge to truck loading station to the sand or the freeze drying beds

e. Process piping for the feeding, recirculation and discharging of the sludge; process piping for sludge feed/withdrawal and supernatant withdrawal

f. Methane gas line system and heat exchanger system

g. Two (2) boilers – used for the heat exchanger (for digester sludge heating) and to heat the building

h. Dissolved Air Flotation thickener (DAFT) - designed to thicken (reducing the liquid volume) the waste activated sludge up to 4% prior to pumping to the anaerobic digester.

1.4.2.5 Digester Waste Gas Flame Burners

The waste gas burner is the original burner system, it consists of a burner and a 4-inch methane gas line from the Mechanical building. This unit is currently the only disposal method for elimination of methane gas produced by the anaerobic digestion process.

1.4.3 Digester Repair Scope History

Deficiencies, repairs and upgrades for the existing anaerobic digestion system were identified in the Ft. McCoy WWT System Evaluation Report (prepared by AECOM, dated August 2013).

Recommendations for plant improvements included the replacement of the existing trickling filter systems and solids contact basin with an extended aeration oxidation ditch and conversion from anaerobic digestion to aerobic digestion for solids handling. Concerns for the condition and operability of the existing anaerobic digester (gas cover lid, mixing system and gas recovery) prompted Ft. McCoy to program a project for repair and replacement to the anaerobic digester system, Repair Digester and Sludge holding tanks (FY 16 - Phase I) prior to the extended aeration plant upgrade project proposed in FY 17 (Phase II).

Following acceptance of the original project order (for floating gas cover replacement and other repairs/replacements associated with the anaerobic digester) an alternative approach was identified that would preserve the efforts and expenditures in the Phase I project for usefulness in the Phase II upgrade project. The original project order scope, considered an interim means to extend the serviceability of the anaerobic digester system, was intended a stop-gap measure until WWTP Alteration Project Order (Phase II) modifications were completed. The alternative approach, identified and preferred as the most effective/economical solution, was to undertake the conversion of the existing anaerobic digester and the sludge holding tank into an aerobic sludge digestion system as the FY 16 Phase I project. That approach is the basis for this project as presented and detailed in the information that follows.

1.5 AEROBIC DIGESTER DESIGN

1.5.1 Selected Aerobic Digester Process Description

This project consists of conversion of the existing anaerobic digester system into an aerobic digestion system and is designed in anticipation of treating the expected waste activated sludge loadings generated by the near-future (FY 17/18) WWTP upgrade to an extended aeration activated sludge process (oxidation ditch). That project is in currently under design by the Omaha District Corps of Engineers.

A multiple channel (three channel) oxidation ditch is proposed for the WWTP upgrade. That

Design Analysis Page 10 of 33 system will be designed with three (3) concentric rings, incorporating the ability to provide full biological treatment within permitted limits while providing total nitrogen removal (nitrification/denitrification) and maximizing biological phosphorus removal to reduce the tertiary phosphorus treatment requirements to achieve future phosphorus permit limits.

Aerobic digestion was selected as the sludge stabilization process to destroy the biologically-degradable organic components of the sludge. The aerobic digestion was selected based upon the aged aerobic sludge that will be produced by the future oxidation ditch system. It was also the process identified in the future upgrade project for Ft. McCoy WWTP Alteration - FY 17.

Overall the aerobic digester system is less expensive to build and operate than anaerobic digesters.

The total design waste activated sludge load used for this project incorporates both the expected waste activated sludge produced by the extended aeration plant upgrade as well as the sludge currently generated from existing phosphorus removal (currently utilizing ferric chloride chemical treatment).

1.5.1.1 Aerobic Digester Design Criteria

The design parameters used for analysis and design were based upon the following (and supported by the attached calculation sheets):

• Ten State Standards- 2014 (Page 80-11, Table 85.31),

• Wisconsin State - Chapter NR 110.26 (Para. 6, page 107)

• Metcalf & Eddy, Waster Engineering Treatment and Resource Recovery - 5th Edition

(page 1544)

• WEF MOP 8/ASCE 76, Design of Municipal Wastewater Treatment Plants , 5th Edition

(Table 13-44-Design Criteria for Aerobic Digesters)

1.5.1.2 Design Parameters

Design Average Daily Flow (Q) 1.32 MGD Peak hourly Flow (QP) 2.31 MGD Monthly Average Flow 0.34 MGD

Minimum Liquid Temp (winter) 10o C (50o F) Maximum Liquid Temp (summer) 20o C (68o F)

Average Daily BOD 136 mg/L 1500 lb/d Average Daily TSS 150 mg/L 1650 lb/d

Solids Retention Time (SRT) 60 days Winter conditions VS Reduction: 40% @ 60 days (Using Fig. 13-35, Metcalf & Eddy) Summer conditions VS Reduction: 48% @ 60 days

Volatile Solids (VSS) 70% Oxygen required/lb of volatile solids destroyed 2 lb O2/lb VSS destroyed

Specific Gravity of Sludge 1.03

Assumed Clarifier Sludge Solids concentration 10,000 mg/L (1%) Thickened WAS Sludge Solids from DAFT 30,000 mg/L (3%)

Total Volume of WAS Sludge to be disposed @ 1% concentration:

Design Flow (1.32 MGD) 18,835 gal/d (2,518 cf/d)) Max Aver Flow (0.34 MGD) 8,235 gal/d (1,101 cf/d)

Design Analysis Page 11 of 33

Total Volume of WAS Sludge to be disposed from DAFT@ 3% concentration:

Design Flow (1.32 MGD) 6,278 gal/d (839 cf/d) Max Aver Flow (0.34 MGD) 2,745 gal/d (367 cf/d)

1.5.1.3 Volume and Oxygen Requirements

The solids retention time (SRT) or detention time needed to achieve 40% VSS reduction @ 10o C temperature governed the digesters estimated volume of 160,529 gal (21,461 cf) using Eq. 13-25, Metcalf & Eddy, page 1546. An additional 10% volume was provided to allow seed for the next batch. Further an additional 25% volume was provided for supernatant separation (per 2014 Ten State Standards for Wastewater Facilities, Page 80-11,Table 85.31). The designed volume will be 220,778 gal (29, 509 cf) per digester. The existing Volume per Digester tank:

Side water Operating Depth (SDR) 17.5 ft Freeboard 3 ft Diameter 15 ft Volume per Tank 310,997 gal (41,577 cf)

Therefore, the existing tanks have adequate capacity for the SRT necessary to achieve 40% VS reduction.

The O2 requirement was determined using 2.0 lb O2/lb solids destroyed, during the winter and summer time (see calculations)

O2 required @ Design Flow (1.32 MGD) Winter 906 lb O2/d Summer 1,087 lb O2/d

O2 required @ Max Aver Flow (0.34 MGD) Winter 396 lb O2/d Summer 475 lb O2/d

The Standard Oxygen Demand (SOR lb O2/d during winter conditions (govern) :

SOR @ Design Flow (1.32 MGD):

Winter 1,603 lb O2/d

SOR lb O2/d @ Max Aver Flow (0.34 MGD) Winter 701 lb O2/d

Therefore, the volume of air required per day at Standard Conditions using Jet Aerator type of aerobic mixing, based on an oxygen transfer efficiency of 24% as per manufacturer is:

Air required @ Design Flow (1.32 MGD):

Winter 267 cfm

Air required @ Max Aver Flow (0.34 MGD) Winter 117 cfm

1.5.1.4 Sludge Production

The amount of sludge generated to the aerobic digesters was estimated using the amount of total sludge that will be generated from the future conversion of the WWTP to an Extended Aeration Process using an Oxidation Ditch. The total sludge produced by the Oxidation Ditch process was calculated and compared to manufacturer calculations for an Orbal 3-Channel Oxidation Ditch.

Design Analysis Page 12 of 33

The three (3)-channel oxidation ditch design will be designed to provide biological treatment for nitrogen and phosphorus removals. Biological phosphorus removals to the practical limit of 1.0 mg/l total phosphorus (TP) and less than 10 mg/l for total Nitrogen (TN) are design objectives of the ditch design. The oxidation ditch will aid significantly when compared to the existing plant in reducing the phosphorus levels requiring chemical phosphorus removal. For design purposes however, the total sludge production for design of the aerobic digesters includes the sludge allowance for current chemical phosphorus removal. The total sludge waste produced (WAS) for the future oxidation ditch system was estimated at the design average flow of 1.32 MGD and the current monthly average flow of 0.34 MGD.

The total sludge produced (WAS), including the chemical sludge from phosphorus removal (without biological phosphorus treatment) was estimated to be 1,618 lb/day (dry solids) at the design flow of 1.32 MGD. At the current monthly average flow of 0.34 MGD the total sludge produced was estimated to be 707 lb/day as indicated below:

Total WAS TSS Sludge = Sludge from Final Clarifiers + additional WAS TSS from P chemical removal

Design Flow (1.32 MGD): Total WAS TSS Sludge = 1,368 lb/day + 250 lb/day = 1,618 lb/day Aver Flow (0.34 MGD): Total WAS TSS Sludge = 643 lb/day + 64 lb/day = 707 lb/day

1.5.1.5 Aerobic Digester Volume Analysis

An evaluation of the storage volume available for aerobic digestion of sludge at the Ft. McCoy WWTP facility was performed to verify adequacy of the existing digester and sludge holding tank volume if both units were converted to aerobic digesters. The design criteria for aerobic digestion performance requirements is derived from 40 CFR Part 503-Standards for the Use or Disposal of Sewage Sludge. That criteria identifies two primary treatment consideration, pathogen reduction and vector attraction reduction.

Pathogen Reduction - To achieve a Class B sludge a Process to Significantly Reduce Pathogens (PSRPs) must be utilized to achieve a minimum volatile suspended solids (VSS) reduction of 40%. For an aerobic digester that requires providing a digester unit(s) of adequate storage volume to provide a mean cell residence time (MCRT) between 40 days at 20° C (68° F) and 60 days at 15° C (59° F).

Vector Attraction Reduction - Vector attraction for aerobic digestion is demonstrated when the treatment results in 38% or greater VSS reduction, or when the specific oxygen uptake rate (SOUR) at 20° C is less than or equal to 1.5 mg of oxygen per hour per gram of total solids, or when additional volatile solids reduction during bench-scale aerobic batch digestion for 30 additional days at 20° C is less than 15%.

The combined volumes of the existing digester and sludge holding tank were examined in this evaluation. Each of these units are of identical size and will be converted for use as aerobic digesters. The overflow piping in the current digester tank will be moved to match the existing sludge holding tank. Therefore the total operating level (beginning at the sidewall) will be 17.5 feet, providing a total volume of 310,997 gallons per tank.

Two plant flow scenarios were used in the evaluation. The first is at the current 0.34 MGD average daily flow and the second is at the 1.32 MGD design flow. Sludge production and daily wasting rates were based upon the continuous loop reactor (CLR) basin that will be provided in the Phase II WWTP upgrade. The solids concentration of the sludge pumped to the digesters is estimated at 1% (10,000 mg/l). The total daily volume of 1% sludge is estimated at 18,835 gallons for a design flow of 1.32 MGD and 8,235 gallons for an average flow of 0.34 MGD (providing consideration for the sludge produced by the existing ferric chloride addition for chemical phosphorus removal). A summary of the sludge wasting rates is provided below:

Design Analysis Page 13 of 33

Plant

Flowrate

(MGD)

Daily Sludge Production (lbs./day)

Sludge Produced by Ferric Chloride Phosphorous

Treatment (lbs./day)

Total Sludge Wasting Rate

(lbs./day)

Hydraulic Sludge Wasting Rate (assuming 1% solids from clarifier and s.g. of 1.03) (gallons per day)

0.34 643 64 707 8,235 (9,424 during decant)

1.32 1,368 250 1,618 18,835 (21,556 during decant)

Structures 60 and 62 will both be converted to aerobic digesters. The converted aerobic digester system is based upon a batch-flow mode of operation. The batch operation involves a number of steps; sludge filling with intermittent supernatant removal/sludge thickening, digestion, and digested sludge removal. Each of the two existing digesters tanks are used to accommodate the batch cycle.

One digester will be “on- line” (in the filling/decanting mode) accepting the WAS flow, while the other digester will be “off”-line” (in the filled and digesting mode or the emptying/emptied mode following digestion). The “on-line” digester will be fed intermittently by the existing WAS sludge pumps in the Mechanical Building basement. The sludge will be fed periodically during the day.

Gravity thickening of sludge can be accommodated within the digesters during the filling process by periodically interrupting the aeration/mixing equipment and allowing the sludge to settle. The interruption will be such that settling can occur without allowing the sludge contents to become anoxic. The supernatant that will form at the top of the digester following the quiescent period can then be manually decanted, via a floating decanter mechanism provided within each digester.

The supernatant will be returned to the influent plant flow upstream of the headworks building.

Decanting would begin once the level is over 7 feet above the bottom of the vertical sidewall (which is where the lowest current decant draw-off level is located). Decanting will return a portion (assumed to be 50% of the daily wasted volume for analysis) every day back to the plant influent.

Periodic decanting will help to consolidate the sludge volume remaining in the digester and result in a greater net volume of solids stored, thereby extending the fill-time of the digester resulting in increased MCRT for the sludge. It is estimated that a 1% (10,000 mg/l) TSS incoming WAS to the digester can be consolidated to a 2% sludge by decanting. The decant liquid was assumed to contain 2,000 mg/l TSS, therefore in order for the total pounds of required solids to be wasted or removed from the CLR a slight increase in the actual hydraulic sludge wasting rate would occur when decanting is provided. That rate is reflected in the analysis as indicated in the previous table.

The total cycle time calculation includes the time it takes to fill the digester, including the effects of intermittent decanting, the time it can be “off-line” undergoing digestion (or draining for drying once the digestion is complete) before the subsequent digester is filled. A spreadsheet (see calculations, Appendix A) was utilized to determine the MCRT available with the volumes provided in Structures 60 and 62. The spreadsheet incorporates filling with periodic decanting (once the 7-foot depth is reached) of one digester until completely filled and taken “off-line” for digestion. Once the first digester is completely filled the WAS is directed to the second digester which undergoes the same filling and decanting process. The maximum time available for the first unit to be “off-line” is the fill time of the second unit. An allowance for the time (assumed 2 days) it takes to pump out the first unit must also be considered. The MCRT for the first unit is therefore the MCRT calculated for the fill period of that unit plus the fill time for the second unit (less the pump-out period of the first unit).

Under the stated assumptions at the current monthly average flow of 0.34 MGD, the fill time is 53 days (incorporating decanting to consolidate the WAS from 1% to 2%), yielding a calculated MCRT of 30 days. The 53 days it takes to fill the second unit would provide an additional MCRT

Design Analysis Page 14 of 33 of 53 days, but in consideration of the pump-out period of 2 days for the first unit, 51 days are available for further digestion, yielding a maximum MCRT of 81 days (30 + 51). This is greater than 60 days required to achieve a minimum of 40% reduction of VS during winter conditions (15° C sludge temperature).

The evaluation indicates that under current average daily flow there is adequate volume provided in the two units to provide 60 days MCRT at 15° C, however that is NOT the case for the design daily flow as shown below:

Plant Flowrate

(MGD)

Days to Reach 1st Decant

Level

Total Days to Fill One Unit (including decant period)

Calculated MCRT (days) in 1st Unit

Fill Period for 2nd Unit (1st Unit Full and Digesting)

(days)

Total MCRT of 1st Unit

(allowing 2 days to pump to drying beds)

0.34 16 53 30 53 81

1.32 7 23 13 23 34

At 1.32 MGD consolidation to 2% TSS of the sludge does not provide sufficient MCRT in the volumes provided by the existing Structures 60 and 62. Additional volume is required to achieve the MCRT necessary for the VSS reduction or greater consolidation of the sludge, beyond the 2% TSS utilizing decanting, with the available storage were considered two potential alternatives as identified below:

ALTERNATIVE 1: This would require an additional digester that would be slightly larger than the existing unit. Providing another unit of the same size would provide an additional 22 days (fill time for that unit) of MCRT or 53 days (30+23).

ALTERNATIVE 2: The second option would be to utilize the DAFT unit and to thicken the sludge from the clarifiers prior to going to the digesters. This significantly reduces the volume discharged to the digesters and extends the fill time. The hydraulic wasting rate to the digesters would be 6,278 gallons per day at 3% solids concentration. Assuming there would be no decanting under this option, the total time to fill one digester would be 49 days, providing a MCRT of 24.5 days. Adding the fill time for the second unit, would increase the MCRT in the first unit to

71.5 days (73.5 days less two days to pump out the digested solids to the drying beds).

Utilizing the existing DAFT unit was determined to be the most feasible and economical solution in lieu of constructing another digester, considering the favorable condition of the DAFT unit to the cost to construct another digester with associated support equipment. This is further supported considering that current average plant flowrates are far less than the 1.32 MGD design flow.

1.5.1.6 Existing Dissolved Air Floatation Unit

The existing DAFT unit was provided as part of the Ft McCoy WWTP upgrade in 1997 to provide the capability to thicken waste activated sludge (WAS) up to 4% without the use of chemical flotation aids. The unit was designed to operate on a continuous basis. The system was designed to pump an average flow of 10 gpm of WAS to the DAFT on regular intervals set by a timer. The 4-inch WAS feed line to the DAFT was also connected to the RAS pumps to provide adequate flow so it can be occasionally flushed due to the low velocity (less than 2 fps). The thickened sludge is discharged to an existing sump where it is pumped to the digester. The DAFT unit installed (Envirex DAFT Model 10) can operate at a flow up to 20 gpm, and about 42 lbs. per hour of waste solids to provide 4% thickened sludge. The historical loading to the anaerobic digestion process has not required the use of this unit since it was installed.

The condition and rated capacity of the existing DAFT unit was assessed by the manufacturer, Envirex, on April 26, 2016. Since the unit has minimal (if any) use on it since installation, the

Design Analysis Page 15 of 33 condition assessment resulted in recommendations for minor maintenance repairs to restore full operational capability of this unit. The repair work for the DAFT unit is not part of this project and will be coordinated directly by Ft. McCoy under separate O&M activities.

The DAFT manufacturer has stated that the existing DAFT unit is rated for and capable of handling the anticipated future final clarifier sludge WAS loading following the plant upgrade to the extended aeration oxidation ditch system. That estimated hydraulic flow rate is 18,835 gallons per day (13.1 gpm over 24-hours), assuming a final clarifier sludge draw-off concentration of 1% solids @ 1.03 specific gravity (s.g.). The basis for this estimate is from the projected sludge waste rate of 1,618 lbs. per day at the design flow rate of 1.32 MGD by the future oxidation ditch system.

Thickening waste sludge prior to filling the digester will proportionately reduce the total volume of liquid sludge. This extends the solids retention time (SRT) in the available digester storage to achieve the necessary volatile suspended solids (VSS) reduction for pathogen and vector attraction reductions. Achieving at least a 3% solids thickened sludge would reduce the 18,835 gallons per day (at 1% from the clarifier underflow) to 6,278 gallons per day into the digesters which significantly extends the fill time and achieves the desired SRT. Sludge thickening will be required during winter conditions as the plant flow reaches the design plant flow of 1.32 MGD.

Under current plant conditions (0.34 MGD average daily flow) there should be no need to thicken sludge with the DAFT unit. Periodic decanting of supernatant during the fill cycle should achieve adequate thickening to extend the storage volume availability.

1.5.2 Aerobic Digester Components (Support Equipment)

The new aerobic digestion system will be provided with the following major components:

1.5.2.1 Air-Mixing Equipment

Two types of aeration systems, coarse bubble diffusers and jet aeration, were considered to provide oxygen and mixing within the digesters. Minimum design requirements for oxygen demand and mixing are based upon Wisconsin State, Chapter NR 110.26 (Para. 6). For an air-only combined aeration and mixing system (i.e. coarse bubble diffusors) the mixing horsepower typically is much greater than that required for the aeration benefit. Whereas mixing by mechanical methods is typically more efficient. Jet aeration systems offer the benefit of combining both aeration and mixing at a lower total horsepower, resulting in a more efficient system with greater potential for controlling each component (aeration and mixing) separately to further optimize operational flexibility. A summary of the requirements for each system is provided below:

(1) Coarse bubble diffusers

• Requires a 100 HP blower (one per digester), based upon the controlling criteria for mixing (the aeration requirement alone is much less). A total of three blowers would be required to provide dedicated supply to each of two digester, with one serving as backup.

• Requires continuous operation of the blowers to provide continuous mixing, regardless of the aeration demand.

(2) Jet Aeration

• Utilizes air/liquid in the jet aeration manifold within the digester tank to provide aeration and mixing.

• Aeration is provided by a 30 HP blower (one per digester) dedicated to provide the oxygen source. A total of three blowers would be required to provide dedicated supply to each of two digester, with one serving as backup.

• Mixing is provided by a 25 HP submersible pump (one per digester) located within the digester supplying the jet motive force to propel and mix the sludge.

Design Analysis Page 16 of 33

• Total digester horsepower requirement per digester is 55 HP.

Not only is the total horsepower requirements lower for the jet aeration system, that system also provides greater flexibility in independent adjustment of the aeration and mixing to optimize treatment needs. Blower speed can be made adjustable with a variable frequency drive and can be reduced when dissolved oxygen demand decreases. The jet motive pump can operate continuously (will not be furnished with variable frequency drive) to keep the contents mixed, or intermittently stopped for periodic decanting.

The components of the Jet Aeration Mixing equipment for each digester as designed and coordinated with manufacturer includes:

• Jet Aeration and mixing manifolds with 12-in stainless steel sludge recirculation line with 6-in submerged air piping, 8 nozzle assemblies, back flush riser valve and supports

• One 25 HP Submersible Jet Mixing Pump with discharge fitting and lifting chain &…