14-Attachment C - Scope of Services-Exhibit 2 VDWWTP Tech Memo.pdf

Final Technical Memorandum

5405 Cypress Center Drive, Suite 250

Tampa, FL 33609

T: 813.371.9400

Prepared for: Hillsborough County – Water Resources Department

Project Title: Van Dyke Wastewater Treatment Plant (VDWWTP) Capacity Evaluation

Hillsborough Work Order No.: WWW-7-1133-17DS

Hillsborough CIP No.: C10300000

BC Project No.: 158545

Technical Memorandum [No. 3]

Subject: VDWWTP Capacity Evaluation –Final Summary Technical Memorandum (TM)

Date: November 18, 2022

To: Gita Iranipour, Ph.D.

From: Marie Burbano, Ph.D., PE

Copy to: Todd Bosso, PE

Anand Mody, PE

Duy Nguyen

Jose Jimenez, Ph.D., PE

Varun Srinivasan, Ph.D.

VDWWTP Capacity Evaluation

Table of Contents

List of Figures ........................................................................................................................................................... iii

List of Tables ............................................................................................................................................................. iv

Executive Summary

Section 1: Introduction

Section 2: Review of Existing Information

2.1 Influent Flows and Loads

Section 3: Hydraulic Capacity and Operations Assessment

3.1 Plant Baseline Hydraulic Model

3.2 Hydraulic Improvement Recommendations

Section 4: Process Capacity and Operations Assessment

4.1 Plant Baseline Process Model

4.2 Capacity Analysis

4.2.1 Dynamic Itinerary

4.2.2 Capacity Analysis

4.3 Process Improvement Recommendations

4.3.1 Increase Aeration Capacity

4.3.2 Increase Secondary Clarification Capacity

Section 5: Decommissioning Opinion of Probable Construction Cost (OPCC)

Section 6: Flow Diversion Opinion of Probable Construction Cost (OPCC)

Section 7: Recommendations

Attachment A: Decommissioning Opinion of Probable Construction Cost (OPCC)

Attachment B: Flow Diversion Opinion of Probable Construction Cost (OPCC)

Attachment C: Capacity Improvements Opinion of Probable Construction Cost (OPCC)

List of Figures

Figure 2-1.Daily, 30-day and 365-day moving average flows for the VDWWTP

Figure 2-2. Influent CBOD5 and TSS concentrations to the VDWWTP

Figure 2-3. Influent CBOD5 and TSS loads to the VDWWTP

Figure 2-4. Influent TKN and NH3-N concentrations to the VDWWTP

Figure 2-5. Influent TKN and NH3-N loads to the VDWWTP

Figure 2-6. Influent TP and OP concentrations for the VDWWTP

Figure 2-7. Influent TP and OP loads for the VDWWTP

Figure 3-1.Process Flow Schematic VDWWTP (1997)

Figure 3-2.Baseline Hydraulic Model of VDWWTP – 3.0 mgd PHF

Figure 3-3.Baseline Hydraulic Model of VDWWTP – 4.25 mgd PHF

Figure 3-4.Hydraulic Bottleneck between Headworks and Oxidation Ditch Structures

Figure 3-5.Hydraulic Model of VDWWTP – 4.25 mgd PHF (after improvements)

Figure 4-1. BioWin model schematic for VDWWTP used for Capacity Analysis

Figure 4-2. Comparison of Observed vs Modeled effluent nitrogen concentrations for the steady state validation

Figure 4-3. Comparison of Observed vs Modeled MLSS concentrations for the steady state validation

Figure 4-4. Comparison of Observed vs Modeled SRT for the steady state validation

Figure 4-5. Summary of Aeration Requirements for the Capacity Analysis Scenarios

Figure 4-6. Comparison of Maximum MLSS based on State Point Analysis with modeled MLSS concentrations for Secondary Clarification Capacity

Figure 4-7. Comparison of Peak Day Flow (MGD) allowable based on SPA with Actual Peak Day Flow

Figure 4-9. Position of additional floating aerators

Figure 4-11. Average and Maximum MLSS at Different Plant Capacity conditions

Figure 4-11. inDENSE Hydrocyclone technology (provided by World Water Works)

Figure 4-12. inDENSE Hydrocyclone skid system (provided by World Water Works)

Figure 4-13. State Point Analysis results for design conditions [peak hour flow of 4.25 MGD, MLSS of 3,100 mg/L, RAS of 2.0 MGD and SVI of 100 mL/g]

Figure 6-1. County Provided Flow Diversion FM Routes

List of Tables

Table ES.1 – Decommissioning Cost Summary

Table ES.2 – Flow Diversion Cost Summary

Table ES.3 – Capacity Improvements Summary and OPCC

Table ES.4 – Recommendations Summary and OPCC

Table 2-1. Summary of Influent Flows and Loads for the VDWWTP for the period of January 1, 2015 through April 30, 2022

Table 4-1. Summary of Influent Flows and Loads for Difference Scenarios for Capacity Analysis

Table 5.1 – Decommissioning Cost Summary

Table 6.1 – Flow Diversion Cost Summary

Table 7.1 – Capacity Improvements Summary and OPCC

Table 7.2 – Recommendations Summary and OPCC

Executive Summary The Van Dyke Wastewater Treatment Plant (VDWWTP) is owned and operated by Hillsborough County (the

County). The VDWWTP treats an annual average daily flow (AADF) of approximately 1.3 million gallons per day (MGD) and has a permitted capacity of 1.7 MGD AADF. The effluent is pumped into the County’s re-claimed water system located in the northwest portion of Hillsborough County. VDWWTP has experienced increased influent flow in recent years.

The County has plans to decommission the VDWWTP and replace it with a master pumping station to trans-fer the wastewater flow to the Northwest Regional Water Reclamation Facility (NWRWRF), via force main.

The planned decommissioning of VDWWTP anticipates completing several projects.

The County has initially estimated that the decommissioning projects will cost approximately $35 million.

The project is currently planned for completion approximately five (5) years from 2022. The County is con-cerned that the estimated decommissioning costs may exceed the amount available in the capital improve-ments budget and is investigating temporary improvements to defer decommissioning.

Project Summary

The Technical Memorandum (TM) is intended to provide guidance for the County to perform additional engi-neering and analysis should the alternate improvements prove to be beneficial in deferring the decommis-sioning of the VDWWRF. The TM is a summary of this project set forth to evaluate the following:

• Establish a new cost opinion to complete the Van Dyke Decommissioning.

• Evaluate the current capacity of the VDWWTP to meet the permitted capacity (1.7 MGD AADF), identify potential hydraulic and process pinch points, and determine potential viable alternatives to verify the current permitted capacity of the VDWWTP and prepare cost opinions of the improvements.

• Evaluate the potential to increase the current VDWWTP average day treatment/permitted capacity, iden-tify potential hydraulic and process pinch points, and prepare the conceptual scope and project budget of the improvements.

• Prepare a cost opinion for the construction of two new force mains project to transfer a portion of the existing wastewater from the VDWWTP to the NWWRF.

VDWWTP Decommissioning OPCC

Table ES.1 below summarizes the costs associated with decommissioning the VDWWTP.

Table ES.1 – Decommissioning Cost Summary

Demolition Cost Summary1

Above Grade Structures/Site $3,228,953

Buried Pipe/Electrical $1,445,198

Ponds $6,456,987

Total Demolition Cost $11,131,138

Construction Cost Summary

Master Pump Station $9,525,371

Reclaimed Water Booster Pump Station $3,493,760

Wastewater Force Main $34,911,737

Total Construction Cost $47,930,868

Total Decommissioning Cost $59,062,006

1The demolition items can be phased depending on the County’s immediate needs.

VDWWTP Flow Diversion OPCC

Table ES.2 below summarizes the costs associated with diverting approximately 0.24 mgd AADF from the VDWWTP to the NWRWRF.

Table ES.2 – Flow Diversion Cost Summary

Phase 1 $918,000

Phase 2 $476,000

Review of Existing Information and Influent Flows and Loads

The project was initiated with a review of existing information, including influent flows and loads, hydraulics, the existing process model, and record drawings.

Daily data from January 1, 2010 through April 30, 2022 was analyzed to estimate historical influent flows and pollutant loadings for performing capacity analysis. Daily average flows were stable from 2010 through

2015 and then increased slowly from 2018 through 2022.For pollutant loads, 5-day carbonaceous biochem-ical oxygen demand (CBOD5), total suspended solids (TSS), total Kjeldahl nitrogen (TKN), ammonia-nitrogen

(NH3-N), total phosphorus (TP) and orthophosphate (OP) were reviewed. In general, concentrations of the pol-lutants have been stable throughout the entire period and showed no general trends. Increase in influent loads from 2018 through 2022 are primarily due to increase in influent flows.

It is understood that the current hydraulic loading peaking factor is 2.0 based on the facility re-rating in the permit in March 1999. That results in a peak flow of 3.4 mgd at the average daily flow of 1.7 mgd. The peak flow of 3.4 mgd remains the assumed plant capacity based on permit conditions. For the purposes of this capacity evaluation, a peaking factor of 2.5 was evaluated as a check to determine the impacts if the peak-ing factor was increased. This is a conservative assumption to provide information in terms of phasing of po-tential improvements. At the current average day permit condition of 1.7 mgd, a peaking factor of 2.5 will result in a peak hour flow of 4.25 mgd. It is understood that the influent flow to the plant may be hydrau-lically limited and this peak may not be realized.

Hydraulic Capacity and Operations Assessment

The liquid stream hydraulic modeling of the VDWWTP was developed using Visual Hydraulics (hydraulics soft-ware) for analyzing the current hydraulic conditions and proposing recommendations for hydraulic improve-ments. The VH model simulation indicated that the VDWWTP has bottlenecks above the PHF of 4.25 mgd

The VH model did result in observations that are contributing to significant head loss at the VDWWTP. How-ever, these observations and head loss even though significant in nature do not impact the VDWWTP facil-ity’s ability to hydraulically pass 3.0 MGD PHF. For the facility to pass the facility PHF of 4.25 mgd assuming the design flow of 1.7 mgd and the current 2.5 peaking factor, improvements are recommended. An option would be to increase the pipe capacity between the grit unit and the oxidation ditches. Additionally, as re-ported by the field staff and also as seen in the hydraulic evaluation, the filtration system is limited to a treatment capacity of between 2.8 - 3.0 mgd. Hence additional filtration capacity is also required under this option. An alternate option is to add a 500,000 gallon equalization tank that would store any flow in excess of 2.8 mgd and the facility would treat this excess flow during low flow periods. Additionally, flows in excess of 3.0 mgd could be sent to the reject ponds for treatment during low flow periods.

Also during BC’s facility site visit as well as while reviewing the facility’s process flow schematic and the VH model, it was observed that at the VDWWTP headworks structure, the only way to bypass the grit unit struc-ture is to manually close the gate to the grit unit and open the grit unit bypass gate. The facility operators indicated that under high flows scenario they would like an option where there is passive bypass around the grit unit. Therefore, BC recommends installing a weir gate at the location between the two gates thereby al-lowing a passive bypass over the weir gate under high flow conditions.

Process Capacity and Operations Assessment

The process simulator, BioWin® Version 6.2 (wastewater process simulation software), was used to simulate the VDWWTP oxidation ditch process along with the secondary clarifiers for use in the process capacity and operations assessment. Initially, a steady state model validation was conducted based on current maximum month flows and loads to confirm that the model matches observed historical data focusing mainly on matching mixed liquor suspended solids (MLSS) concentrations, solids retention time (SRT) and nitrogen re-moval.

Results of the capacity analysis showed the following:

• Aeration Capacity – The VDWWTP is just about able to meet the average aeration requirements un-der current conditions but does not have sufficient capacity to meet maximum demand.

• Secondary Clarifier Capacity – Under current conditions and peak daily flow, there would be times where the secondary clarifiers could be overloaded which would result in a secondary clarifier blan-ket accumulation and potential blanket washout based on the process model simulation with a 1 month dynamic itinerary including peak events. The main limiting factor for the secondary clarifiers at VDWWTP is the SVI.

Based on the results of the capacity analysis and operations assessment, the following two process improve-ments are recommended.

• Increase Aeration Capacity – BC recommends adding four additional floating aerators (two per oxida-tion ditch) with a capacity of 25 HP/aerator with local variable frequency drives (VFD) to control the power delivered by each pump. This will add an additional 100 HP to the aeration capacity at the

VDWWTP. The VFDs will provide the facility with the flexibility to individually control the power of each of the additional pumps. This additional capacity will be sufficient to meet the peak aeration de-mands for the facility under current conditions.

• Increase Clarification Capacity – In order to improve the settleability of the activated sludge, the ad-dition of hydrocyclone-based selective wasting (inDENSE™) in the secondary treatment processes is recommended. New waste activated sludge (WAS) pumps would be included with this selective wast-ing improvement. The continuous wasting of lighter particles/flocs including filaments improves the

SVI of activated sludge, thus maintaining the treatment efficiency and increasing clarification capac-ity. Based on the observations of SVIs at the plant, it appears that hydrocyclones would be effective in increasing the settleability of the solids. Additional testing and investigation of the benefits of the hydrocyclones would be appropriate to evaluate specific benefits.

VDWWTP Capacity Cost Summary

Table ES.3 summarizes the recommendations and costs associated with increasing the capacity at the

VDWWTP.

Table ES.3 – Capacity Improvements Summary and OPCC

Association for the Advancement of Cost Engineering International (AACE) Class 5 Estimate (Range -50% to +100%)

Improvements Summary

Opinion of Probable

Construction Cost Range

(estimated cost)

Aeration Capacity Increase aeration capacity by 100 hp with four additional floating aerators (two per oxidation ditch) with a capacity of 25 HP/aerator.

$460K to $1.8M ($910K estimated)

Clarification Capacity Increase clarification capacity by improving the settleabil-ity of the activated sludge with hydrocyclone-based selec-tive wasting.

$710K to $2.9M ($1.4M estimated)

RAS Pumps Increase RAS pumping capacity to 2.0 mgd by replacing the four existing RAS pumps with four new Discflo pumps.

$700K to $2.7M (1.3M estimated)

Flow Equalization Implement flow attenuation capabilities with the addition of a 0.5 MGD flow equalization system.

$2.9M to $12M ($5.9M estimated)

Grit Unit Bypass Weir Increase headworks flow capacity under high-flow condi-tions with a passive weir gate bypass around the grit unit.

$23K to $90K ($45K estimated)

Parallel Ox Ditch Inf. Pipeline Add 16” parallel pipeline between grit units and oxidation ditches.

$100K to $400K ($200K estimated)

Filter Improvementsa Add disc filters for 1.5 mgd added filtration capacity to meet 4.25 mgd PHF.

$1M to $4M ($2M estimated)

Electric Capacity

Increase electric capacity to accommodate the proposed improvements. This estimate used a previous electric evaluation (VDWWTP Generator Replacement Evaluation, May 2020) as a baseline to expand upon.

$4M to $16M ($7.9M estimated) a If granular media filters are required based on current County requirements and an exception cannot be made, the cost is assumed to approximately double and the footprint will be approximately 40% greater than disc filters.

VDWWTP Recommendations Cost Summary

Table ES.4 provides a summary of combinations of improvements that are needed to achieve flow capacity goals. The recommendations listed include the list of improvements and the total OPCC (Class 5). Electrical improvements are listed as a percent of the process improvements or with the new electrical service build-ing. A 30% added cost for electrical was included if the new electrical service building is not constructed.

This is an assumption that will need to be verified.

Table ES.4 – Recommendations Summary and OPCC

Association for the Advancement of Cost Engineering International (AACE) Class 5 Estimate (Range -50% to +100%)

Option Recommendations

OPCC Flow Capacity

With Assumed Electrical Upgrades

(elect = 30% of process improvements)b

With New Electrical Service Building

Annual average daily flow

(AADF), mgd

Peak hour flow (PHF), mgd

0 Do nothinga $0 $0 1.2 3.0d

Aeration Capacity Clarifier Capacity

RAS Pumps Grit Unit Bypass Weir

Flow Equalization

$6.2M to $25M ($12.5M estimated)

$8.7M to $35M ($17.5M estimated)

1.7 4.25

Aeration Capacity Clarifier Capacity

RAS Pumps Grit Unit Bypass Weir

Parallel Ox. Ditch Inf. Pipeline Filtration Improvements

$3.8M to $15.4M ($7.7M estimated)

$6.9M to $27.7M ($13.8M estimated)

1.7 4.25

Aeration Capacity Clarifier Capacity

RAS Pumps

$2.4M to $9.6M (4.8M estimated)

$5.8M to $23.1M ($11.5M estimated)

1.4 3.5d

4 Flow Diversionc $700K to $2.8M

($1.4M estimated)

1.2 3.0 a Based on capacity evaluation analysis. Note that permitted capacity is 1.7 mgd AADF and current AADF is 1.24 mgd.

b Assume electrical upgrades will be ~30% of the process improvements.

c AADF entering the VDWWTP will be reduced by 0.24 mgd.

d Filters maximum PHF per field performance is observed to be approximately 2.8 mgd. Greater than this flow can be achieved using reject storage ponds for the highest peaks. Effluent pumping and distribution system would need fur-ther evaluation if filter capacity is increased.

Section 1: Introduction The Van Dyke Wastewater Treatment Plant (VDWWTP) is owned and operated by Hillsborough County (the

County). The VDWWTP treats an annual average daily flow (AADF) of approximately 1.42 million gallons per day (MGD) and has a permitted capacity of 1.7 MGD AADF. The effluent is pumped into the County’s re-claimed water system located in the northwest portion of Hillsborough County. VDWWTP has experienced increased influent flow in recent years and expects a continued trend due to planned developments.

The County has plans to decommission the VDWWTP and replace it with a master pumping station to trans-fer the wastewater flow to the Northwest Regional Water Reclamation Facility (NWRWRF), via force main.

The planned decommissioning of VDWWTP anticipates the following projects:

• Demolish VDWWTP wastewater treatment equipment and associated infrastructure and maintain the reclaimed storage and associated pumping infrastructure

• New force main to transfer flow to NWRWRF

• New master pump station located at VDWWTP (approximately 2 MGD Average Day Flow) with associated electrical improvements

• New reclaimed water booster pump station (located at a mid-point between NWRWRF and VDWWTP)

The County has initially estimated that the decommissioning projects will cost approximately $35 million.

The project is currently planned for completion approximately five (5) years from 2022. The County is con-cerned that the estimated decommissioning costs may exceed the amount available in the capital improve-ments budget.

This project was set forth to evaluate the following:

• Establish a new cost opinion to complete the Van Dyke Decommissioning as identified above.

• Evaluate the current capacity of the VDWWTP to meet the permitted capacity (1.7 MGD AADF), identify potential hydraulic and process pinch points, and determine potential viable alternatives to verify the current permitted capacity of the VDWWTP and prepare cost opinions of the improvements.

• Evaluate the potential to increase the current VDWWTP average day treatment/permitted capacity, iden-tify potential hydraulic and process pinch points, and determine potential viable alternatives to increase the treatment/permitted capacity of the VDWWTP and prepare the conceptual scope, project budget and schedule of the improvements.

• Prepare a cost opinion for the construction of two new force mains project to transfer existing wastewater from the VDWWTP to the NWWRF.

This TM presents the results of the evaluation in the following sections.

• Section 1 – Introduction

• Section 2 – Review of Existing Information

• Section 3 – Hydraulic Capacity and Operations Assessment

• Section 4 – Process Capacity and Operations Assessment

• Section 5 – Decommissioning OPCC

• Section 6 – Flow Diversion OPCC

• Section 5 – Recommendations

Section 2: Review of Existing Information The project was initiated with a review of existing information, including influent flows and loads, hydraulics, the existing process model, and record drawings.

The information review focused on the following items that were used in the evaluation.

Reports

March 2022, Hillsborough County Public Utility VDWWTP Reclaimed Water Operating Protocol [12 pages]

June 2017, FDEP Permit No. FLA012234-010-DW1P/NR to operate VDWWTP [79 pages]

March 1999, FDEP Permit No. FLA012234-00-01 permit revision for VDWWTP to change capacity from 1.5 mgd to 1.7 mgd [379 pages]

Sept. 2010, Envirodyne Systems, Inc. mechanical aerators equipment description [5 pages]

Sept. 2010, Envirodyne Systems, Inc. mechanical aerators equipment specification [3 pages]

Jan 2010 to May 2022 Operational Data [influent, process, rotor aerators, solids, effluent, sludge hauling, chemicals]

BioWin® Model and Users Manual

Record Drawings

1986 – Sub Regional WWTP for Van Dyke Resource Preservation Venture [69 sheets]

1990 – Davco Deep-bed, Dual Media Filter Retrofitting [6 sheets]

1997 – Class 1 Modifications [43 sheets]

2005 – Reclaimed Water Storage Tank and PS Improvements [23 sheets]

2007 – RW Storage Tank and PS Improvements [10 sheets]

2012 – Aeration & Clarifiers [44 sheets]

2012 – Chlorine Conversion [15 sheets]

2012 – Reject Pond PS Upgrade [3 sheets]

2014 – Headworks Rehab [48 sheets]

2017 – Reject Piping Filter Bypass [15 sheets]

2020 – Sludge Holding-Thickening (Mobile Dewatering Unit) [1 sheet]

2021 – Reject System Rehab and Configuration [5 sheets]

2.1 Influent Flows and Loads

Daily data from January 1, 2010 through April 30, 2022 was analyzed to estimate historical influent flows and pollutant loadings for performing capacity analysis. Hourly flow data from January 1, 2018 through April

30, 2022 was also analyzed to determine peak hourly flow rates and peaking factors. The historical daily, 30-day and 365-day moving average flows for the VDWWTP is shown in Figure 2-1. Daily average flows were stable from 2010 through 2015 and then increased slowly from 2018 through 2022. Daily flows were also highly variable from 2010 through 2015 but then had a lower deviation from the average from 2015 on-wards. In terms of peak daily flows, there was a significant peak flow event in 2017 with a peak daily flow of

2.67 million gallons per day (MGD) which aligned with Hurricane Irma.

Figure 2-1.Daily, 30-day and 365-day moving average flows for the VDWWTP

For pollutant loads, 5-day carbonaceous biochemical oxygen demand (CBOD5), total suspended solids (TSS), total Kjeldahl nitrogen (TKN), ammonia-nitrogen (NH3-N), total phosphorus (TP) and orthophosphate (OP) were available on a 1-2 days per week frequency. A summary of the available pollutant concentrations and loadings are shown in Figure 2-2 through Figure 2-7. In general, concentrations of the pollutants have been stable throughout the entire period and showed no general trends. Increase in influent loads from 2018 through 2022 are primarily due to increase in influent flows.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

F lo w

M G

D

Inf Avg Daily Flow Annual Average 30 per. Mov. Avg. (Inf Avg Daily Flow)

Figure 2-2. Influent CBOD5 and TSS concentrations to the VDWWTP

C o n ce n tr a ti o n m g /L

Inf Comp (CBOD5) Inf Comp (TSS)

30 per. Mov. Avg. (Inf Comp (CBOD5)) 30 per. Mov. Avg. (Inf Comp (TSS))

Figure 2-3. Influent CBOD5 and TSS loads to the VDWWTP

1,000

2,000

3,000

4,000

5,000

6,000 Lo a d lb

/d a y

Inf Comp (CBOD5) Inf Comp (TSS)

30-Day Avg Inf Comp (CBOD5) 30-Day Avg Inf Comp (TSS)

Figure 2-4. Influent TKN and NH3-N concentrations to the VDWWTP

C o n ce n tr a ti o n m g /L

Inf Comp (TKN) Inf Comp (NH3-N)

30 per. Mov. Avg. (Inf Comp (TKN)) 30 per. Mov. Avg. (Inf Comp (NH3-N))

Figure 2-5. Influent TKN and NH3-N loads to the VDWWTP

1,000

Lo a d lb /d a y

Inf Comp (TKN) Inf Comp (NH3-N)

30-Day Avg Inf Comp (TKN) 30-Day Avg Inf Comp (NH3-N)

Figure 2-6. Influent TP and OP concentrations for the VDWWTP

C o n ce n tr a ti o n m g /L

Inf Comp (TP) Inf Comp (PO-4)

30 per. Mov. Avg. (Inf Comp (TP)) 30 per. Mov. Avg. (Inf Comp (PO-4))

Figure 2-7. Influent TP and OP loads for the VDWWTP

Based on the historical trends summarized above, influent flows and loadings for the period of 2015-2022 were used as the basis for the capacity analysis (Table 2-1). Since daily flow data was available from flows, rolling averages were used to determine maximum month (30-day) flows. For pollutant loads, since daily data was not available to use rolling averages to calculate peaking factors, a percentile approach was taken to calculate peaking factors. Maximum month loads were determined at a 92nd percentile value, and maxi-mum day loads were determined at a 99.7th percentile value. Based on a peak day and peak hour flow as outlined in Table 2-1, the peaking factors were calculated as 2.5 and 2.3 for peak hourly and daily flow re-spectively. It is important to note that the peak daily flow of 2.7 MGD is based on the peak event during Hur-ricane Irma. It is understood that the current hydraulic loading peaking factor is 2.0 based on the facility re-rating in the permit in March 1999. That results in a peak flow of 3.4 mgd at the average daily flow of 1.7 mgd. The peak flow of 3.4 mgd remains the assumed plant capacity based on permit conditions. For the pur-poses of this capacity evaluation, a peaking factor of 2.5 was evaluated as a check to determine the impacts if the peaking factor was increased. This is a conservative assumption to provide information in terms of phasing of potential improvements. At the current average day permit condition of 1.7 mgd, a peaking factor of 2.5 will result in a peak hour flow of 4.25 mgd. It is understood that the influent flow to the plant may be hydraulically limited and this peak may not be realized.

Lo a d lb /d a y

Inf Comp (TP) Inf Comp (PO-4)

30-Day Avg Inf Comp (TP) 30-Day Avg Inf Comp (PO-4)

Table 2-1. Summary of Influent Flows and Loads for the VDWWTP for the period of January 1, 2015 through April 30, 2022

Influent Peak Hour Peak Day Maximum Month Annual Average Design Average

Flow (MGD) 3.6 2.7 1.5 1.3 1.70

CBOD5 (lb/d) -- 3,720 2,830 2,210 3,545

TSS (lb/d) -- 4,200 3,050 2,250 3,545

TKN (lb/d) -- 780 650 520 425

NH3-N (lb/d) -- 700 500 420 --

TP (lb/d) -- 90 70 60 142

OP (lb/d) -- 75 55 45 --

Section 3: Hydraulic Capacity and Operations Assessment The liquid stream hydraulic modeling of the VDWWTP was developed using Visual Hydraulics (hydraulics soft-ware) for analyzing the current hydraulic conditions. Visual Hydraulics (VH) is a commercially licensed soft-ware developed by Innovative Hydraulics. VH provides one-dimensional hydraulic profile modeling that em-ploys a standard stepwise calculation method to calculate water surface levels by calculating the hydraulic grade line (HGL) through a WWTP. VH includes pre-calibrated/pre-programmed hydraulic losses for most hy-draulic features within a gravity flow hydraulic scenario. These hydraulic features include full and partial flow in pipes, open channels, weirs, flumes, orifices/gates, launders, racks/screens, and filter media. The model-ing software also provides the ability to analyze flow splits, parallel flow paths and flow combinations and to create unique flow loss situations via programmer-entered loss characteristics and dimensional criteria.

VH was employed to create the hydraulic profile to identify potential hydraulic bottlenecks associated with the VDWWTP under existing conditions, to understand operations, and to provide recommendations regard-ing possible upgrades for increased capacity, up to the facility’s permitted capacity. The results of the hy-draulic model evaluation are presented in this section.

3.1 Plant Baseline Hydraulic Model

The hydraulic model for VDWWTP was constructed for the entire plant, using the process flow schematic from the 1997 VDWWTP facility improvements and then subsequent improvements as listed in Section 2. VH model particularly analyzed through the most conservative hydraulic path in the facility, including all existing elements. HGL calculations were initiated from a known water surface elevation (56.46 feet) at the down-stream end of the plant—CCT effluent box—and progressed hydraulically upstream against the wastewater flow through the VDWWTP. At each hydraulic feature, the HGL was calculated accounting for frictional and minor losses in both open channels and pressurized conduits. As the VDWWTP hydraulic grade is not all gravity, effluent from the secondary clarifiers ends up in a wet well from where it is pumped up to the filters, two separate profiles were developed.

The objectives of the hydraulic analysis included the following:

• Identify potential hydraulic bottlenecks associated under 3.0 – 4.25 million gallons per day (mgd) peak hourly facility flow (PHF) scenario (using 1.7 mgd average daily flow, permitted capacity, and 2.5 multi-plier results in 4.25 mgd PHF for the VDWWTP).

• Recommend hydraulic improvements, if necessary, that could pass the peak hourly flow hydraulic capac-ity for VDWWTP

The following major modeling parameters were used:

• A process unit was determined to be able to hydraulically pass the modeled flows if the unit maintained an adequate amount of freeboard during peak flows (> 6 inches) and did not submerge any flow-control-ling or flow-splitting devices.

• All process units and trains are in service

• Typical yet conservative friction and minor head loss coefficients for the pipe material, pipe size, pipe age and fitting types were used

• The RAS flow rates was assumed to be 2.0 mgd as derived from the process evaluation (Section 4 of this TM).

• Infrastructure components (i.e., aeration basins, secondary clarifiers, CCTs) and hydraulic connection and control elements (i.e., piping, channels, weirs, and splitting structures) were added to the model to take into consideration practical constraints at the facility.

VH model was run under the PHF condition of 3.0 and 4.25 mgd through the VDWWTP. This model run in-cluded a RAS return rate of 2.0 mgd. Figure 3-1 shows the VDWWTP process flow schematic from the 1997 facility improvements project.

Figure 3-1.Process Flow Schematic VDWWTP (1997)

Various improvements subsequent to the 1997 facility improvements such as the headworks rehabilitation, aeration and clarifier upgrades, filters upgrades, and others as listed in Section 2 were included in the VH model; however not shown in Figure 3-1. With these improvements included the existing facility’s process components, the VH model simulation indicated that the VDWWTP can hydraulically pass the targeted PHF of

3.0 mgd; however, indicated that there are hydraulic issues associated with the 4.25 mgd PHF scenario. The hydraulic grade line elevations for the VDWWTP under the PHF 3.0 mgd flow condition as calculated by the

VH model is shown in Figure 3-2. The hydraulic grade line elevations for the VDWWTP under the PHF 4.25 mgd flow condition as calculated by the VH model is shown in Figure 3-3.

Figure 3-2.Baseline Hydraulic Model of VDWWTP – 3.0 mgd PHF

Figure 3-3.Baseline Hydraulic Model of VDWWTP – 4.25 mgd PHF

As shown in Figure 3-3, piping from the headworks to the oxidation ditches reduces from 16 inches down to

12 inches before it splits between the two oxidation ditches. This is creating a significant head loss resulting in the headworks overflowing (leaving ~3-inches of freeboard). The filters also appear to create a significant headloss under the 4.25 mgd PHF. The County operations staff confirmed this bottleneck and confirmed that the filters have performance issues under any flow conditions higher than 2.8 mgd. The County, how-ever, mentioned that they manage flows greater than 2.8 mgd by diverting the excess flow to the reject stor-age ponds and treat it back when the flows are back lower to 2.8 mgd or less.

3.2 Hydraulic Improvement Recommendations

As mentioned in Section 3.1 the VDWWTP can hydraulically pass approximately 3.0 mgd with the current process components and control elements (except for filter which have a reported field capacity of approxi-mately 2.8 mgd). However, during the development of the VH model below are the list of model observations that should be noted as they are contributing to significant head loss at the VDWWTP.

• Approximately 30 ft of 12-inch PVC C900 piping after the headworks structure leading up to the two oxidation ditches (see Figure 3-3). This common header piping carries the facility PHF of 3.0 mgd.

This results in a flow velocity of about 5.91 ft/s and 1.57 ft of head loss (see Figure 3-2). This signifi-cant head loss can be reduced by replacing this piping with a larger piping (16-inch piping).

Figure 3-4.Hydraulic Bottleneck between Headworks and Oxidation Ditch Structures

• Oxidation ditch effluent weir is set at EL 66.00. This results in about 2 ft of head loss. This is under-stood to be required due to aeration process requirement.

• Secondary clarifier weir is set at EL 62.92. This results in about 1.3 – 1.5 ft of head loss. This again is understood to be required due to process requirements.

• Filtration system has significant head loss around it. The clear well weir is set at EL 66.75 whereas the top of the chlorine contact chamber is set at EL 58.46 which could result in overflow at the chlo-rine contact chamber.

The above observations and head loss even though significant in nature do not impact the VDWWTP facility’s ability to hydraulically pass 3.0 mgd PHF (except for filters which has 2.8 mgd capacity). Hence no hydraulic improvements are recommended. Under the 4.25 mgd flow conditions, the hydraulic pipeline bottleneck be-tween the grit unit and the oxidation ditches significantly increases the headloss and could possibly cause overflows at the headworks (assuming the 3rd manual bar screen channel is not available for use). To allevi-ate this hydraulic bottleneck, a replacement of the 14- and 12– inch pipeline is recommended. As seen in

Figure 3-5, once additional pipeline capacity is added (either replacing the 14- and 12-inch piping or adding a new parallel 16-inch piping), the hydraulic bottleneck can be alleviated. Additionally, it is also recom-mended to add approximately 1.5 mgd of additional filtration capacity to alleviate the treatment limitations at the filtration system.

Figure 3-5.Hydraulic Model of VDWWTP – 4.25 mgd PHF (after improvements)

Another alternate that can be considered in lieu of the above improvements is adding an equalization tank that would store any wastewater flows in excess of 2.8 mgd and then during low flow conditions the wastewater from the equalization tank can be treated at the facility. Preliminary calculations indicate that an equalization capacity of 500,000 gallons will be required based on equalizing to a peak hourly flow of 2.8 mgd and AADF of 1.7 mgd. Detailed analysis and design will be required to determine actual equalization capacity requirements and fill/drain schemes.

Additionally, during BC’s facility site visit as well as while reviewing the facility’s process flow schematic and the VH model, it was observed that at the VDWWTP headworks structure, the only way to bypass the grit unit structure is to manually close the gate to the grit unit and open the grit unit bypass gate. The facility opera-tors indicated that under high flows scenario they would like an option where there is passive bypass around the grit unit. BC recommends installing a weir gate at the location between the two gates (see Figure 3-4) thereby allowing a passive bypass over the weir gate under high flow conditions.

Section 4: Process Capacity and Operations Assessment

4.1 Plant Baseline Process Model

The process simulator, BioWin® Version 6.2 (wastewater process simulation software), was used to simulate the VDWWTP oxidation ditch process along with the secondary clarifiers. BioWin, a proprietary product of En-viroSim, Ltd., Canada, is an advanced biological activated sludge simulator with the ability to model various physical and chemical processes. BioWin allows the prediction of complex biological interactions using vari-ous mechanistic and empirical models to represent material transformations and pollutant removals in the plant for both liquid and solids process streams. It enables the user to simulate carbonaceous oxidation and the fate of nutrients in the activated sludge treatment process. A previously calibrated BioWin model was available from previous efforts by University of South Florida (USF) that was adapted for the purposes of this analysis. The following changes were made to the model:

• Both North and South trains were combined into 1 train for modeling purposes. This assumes equal flow split between the North and South trains.

• Default wastewater characteristics were used except for changes to unbiodegradable particulate

COD, Ammonia/TKN and OP/TP ratios based on matching to observed data.

• Internal recycle flow was reduced to 25 MGD based on observed total nitrogen (TN) removal.

Figure 4-1. BioWin model schematic for VDWWTP used for Capacity Analysis

A steady state model validation was conducted based on current maximum month flows and loads (Table

2-1) to confirm that the model matches observed historical data focusing mainly on matching mixed liquor suspended solids (MLSS) concentrations, solids retention time (SRT) and nitrogen removal as shown in Fig-ure 4-2 through Figure 4-4. In conclusion, the BioWin model predicted the historical performance of the facil-ity based on the selected input data.

Figure 4-2. Comparison of Observed vs Modeled effluent nitrogen concentrations for the steady state validation

Figure 4-3. Comparison of Observed vs Modeled MLSS concentrations for the steady state validation

1.0

2.0

3.0

4.0

5.0

6.0

TAN TN

C o n ce n tr a ti o n m g -N

/L

Observed Modeled

MLSS

C o n ce n tr a ti o n m g /L

Observed Modeled

Figure 4-4. Comparison of Observed vs Modeled SRT for the steady state validation

4.2 Capacity Analysis

Secondary treatment capacity is governed by several factors including influent flows and loadings, permit limits, sludge quality, temperature, aeration requirements and plant operations. For this analysis, the exist-ing secondary treatment capacity at VDWWTP was based on the following criteria:

1. Ability to meet average, maximum month, and peak aeration demands.

2. Maintain solids loading rate (SLR) below the maximum SLR as determined by state point analysis

(SPA).

The capacity analysis was conducted at two different influent flows and loadings scenarios:

1. Scenario 1 - Current influent flows and loads.

2. Scenario 2 – Original design influent flows and loads.

The analysis also assumes that both North and South trains are in service, all the secondary clarifiers are in service, and influent wastewater characteristics continue to match the current conditions. The solids reten-tion time (SRT) was assumed to be 10 days based on historical data. However, to control mixed liquor con-centrations, BC recommends to reduce the SRT to 6.0 - 8.0 days.

4.2.1 Dynamic Itinerary

A 1-month daily dynamic itinerary was developed for each of the influent flows and loadings scenarios listed above based on historical plant data and wastewater characteristics. Influent TSS, TKN and total phospho-rous (TP) loads were calculated based on wastewater characteristics using CBOD5:TSS, CBOD5:TKN and

CBOD5:TP ratios. Peaking factors were assumed to be the same as current conditions for all three scenar-ios. For scenario 2, annual average influent flow was calculated based on assumed CBOD5 loads and as-sumed design CBOD5 concentration of 250 mg/L.

2.0

4.0

6.0

8.0

10.0

12.0

SRT

S R

T d a y s)

Observed Modeled

Table 4-1. Summary of Influent Flows and Loads for Difference Scenarios for Capacity Analysis

Parameter Current Loading Design Capacity

Influent Flow – Annual Average (MGD) 1.26 1.70

Influent Flow – Peak Daily (MGD) 2.65 3.57

Influent CBOD5 Concentration (mg/L) 2101 2502

Influent CBOD5 Load – Annual Average (lb/day) 2,200 3,550

Influent CBOD5 Load – Maximum Month (lb/day) 2,650 4,260

1 Influent CBOD5 concentration of 210 mg/L based on historical average concentrations.

2 Influent CBOD5 concentration of 250 mg/L based on original design concentrations.

4.2.2 Capacity Analysis

4.2.2.1 Aeration Capacity

A critical limitation in biological wastewater treatment is the aeration capacity, specifically the ability to meet the aeration demands imposed by the influent wastewater under warm weather average and peak aeration demands. The VDWWTP, currently, has four brush aerators with 50 horsepower (HP) capacity and a floating

20 HP surface aerator that is used as needed. The floating aerator is only used periodically and it is moved around the oxidation ditch to meet local aeration demands. For the purposes of this analysis, it was as-sumed that the aeration capacity of the facility is 200 HP (not counting with the 20-HP floating aerator) with a design oxygen transfer efficiency of 1.8 lb-O2/hp-hr based on BC’s experience with similar brush aerators.

A summary of the results of the analysis are shown in Figure 4-5. Based on a current available capacity of

200 HP, the existing brush aerators at VDWWTP can meet the minimum and average aeration demands un-der current design conditions. However, at maximum loading conditions, the existing brush aerators do not have adequate capacity, resulting in low dissolved oxygen conditions and possibly affecting the treatment performance, resulting in high effluent ammonia concentrations in the final effluent. At design capacity , the

VDWWTP does not have adequate aeration capacity to meet the projected loading demands. Hence, addi-tional aeration capacity must be provided.

Figure 4-5. Summary of Aeration Requirements for the Capacity Analysis Scenarios

4.2.2.2 Secondary Clarifier Capacity

The VDWWTP currently has two 50-ft secondary clarifiers with a sidewater depth of 12 feet with a total area of approximately 4,000 ft2. Return activated sludge (RAS) and waste activated sludge (WAS) pumping is ac-complished by four 730,000 gallons per day (GPD) pumps. For the purposes of this analysis, it was assumed that two of the pumps (1 pump per pumping station) are used for RAS pumping providing a firm and total capacity of 1.1 MGD capacity since spare pumps are available if needed. Note that this flow is less than the flow assumed in the hydraulic capacity analysis. In the process analysis, the RAS pump flowrate is not a lim-iting factor and this assumption is based on conversations with the VDWWTP staff. The hydraulic capacity analysis used a higher flow to verify that the hydraulics were not limiting.

For this analysis, it was assumed that the design sludge volume index (SVI) was 230 mL/g (based on the av-erage historical performance value). BC also assumes a standard derating factor of 20 percent on secondary clarifiers capacity to account for non-ideal hydrodynamic conditions not accounted for when state point anal-ysis is performed. Based on the selected RAS flow of 1.1 MGD, the design SVI of 230 mL/g and two second-ary clarifiers operational, the design solids loading rate should not exceed 17 lb/ft2-day. State point was used to estimate the maximum allowable MLSS to handle the projected peak flow while maintain the design maximum SLR and results are summarized in

Current Design Capacity

A e ra ti o n

R e q u ir e m e n ts

H

P

Average Maximum Minimum

Current Capacity = 200 HP

Figure 4-6. This figure also includes the maximum and average MLSS predicted by BioWin for each scenario at 6-day and 10-day SRT. At current conditions, the maximum allowable MLSS, based on SPA is 2,300 mg/L, while based on BioWin modeling, the maximum MLSS at current conditions is 2,400 mg/L. This would mean that, under current conditions and peak daily flow, there would be times during the event where the second-ary clarifiers could be overloaded which would result in a secondary clarifier blanket accumulation and po-tential blanket washout. For the design capacity conditions, the maximum allowable MLSS is 1,700 mg/L and the BioWin predicted results are all in excess of 1,700 mg/L; hence, the secondary clarifiers do not have adequate capacity to handle this condition. The main limiting factor for the secondary clarifiers at VDWWTP is the SVI. A lower SVI would result in better settling characteristics and an increased clarification capacity at the VDWWTP.

Figure 4-6. Comparison of Maximum MLSS based on State Point Analysis with modeled MLSS concentrations for Sec-ondary Clarification Capacity

Current Design Capacity

C o n ce n tr a ti o n m g /L

Maximum MLSS (based on SPA) Average MLSS (6d) Average MLSS (10d) Maximum MLSS (6d) Maximum MLSS (10d)

Current Design Capacity

C o n ce n tr a ti o n m g /L

Maximum MLSS (based on SPA) Average MLSS (6d) Average MLSS (10d) Maximum MLSS (6d) Maximum MLSS (10d)

If the facility were to maintain the modeled predicted MLSS and current SVI of 230 mL/g, at current condi-tions, the maximum allowable peak day flow through the secondary clarifiers would be 2.30 MGD against the actual peak day flow which is 2.67 MGD (Figure 4-7). The secondary clarifiers do not have adequate ca-pacity to handle the design capacity condition due to high SVI, elevated MLSS and limited RAS capacity.

Figure 4-7. Comparison of Peak Day Flow (MGD) allowable based on SPA with Actual Peak Day Flow

It should be noted that, it is standard practice to use a 90th percentile value which would be 320 mL/g

(based on historical performance). This would mean, that if we were to assume a 90th percentile value, the analysis would indicate a lower capacity than shown in this analysis.

4.3 Process Improvement Recommendations

Based on the capacity analysis performed in Section 4.2.2, the following process improvements are recom-mended to meet the design capacity requirements listed in Table 4-1.

4.3.1 Increase Aeration Capacity

The current aeration capacity (200 HP) at the VDWWTP is insufficient to meet peak aeration demands under current conditions. BC recommends adding four additional floating aerators (two per oxidation ditch) with a capacity of 25 HP/aerator with local variable frequency drives (VFD) to control the power delivered by each pump. This will add an additional 100 HP to the aeration capacity at the VDWWTP to handle the design ca-pacity conditions. The VFDs will provide the facility with the flexibility to individually control the power of each of the additional pumps. This additional capacity will be sufficient to meet the peak aeration demands for the facility under current conditions and design conditions. The additional aerators should be added in Z2 and Z3 as shown in Figure 4-8. If there is a need to increase the aeration capacity at VDWWTP beyond its current rated capacity of 1.7 MGD, additional aeration capacity would be needed by the addition of more floating aerators or bigger floating aerators.

0.5

1.5

2.5

3.5

Scenario 1 Scenario 2 Scenario 3

P e a k

D a y F lo w

M

G D

PDF (based on SPA) (MGD) PDF (MGD)

Figure 4-8. Position of additional floating aerators

4.3.2 Increase Secondary Clarification Capacity

As presented in Section 4.2.2.2, the secondary clarification system at VDWWTP is limited by the concentra-tion of the mixed liquor and current SVI. Even though increasing the aeration capacity of the plant could re-sult in SVI improvements, the results would not be sufficient to significantly increase the clarification capac-ity at VDWWTP. Figure 4-11 present the average and maximum MLSS concentrations predicted by the model at different plant capacity values at two SRT conditions, 6 days and 10 days. As previously mentioned, BC recommends reducing the SRT to about 6 days as the flows to VDWWTP increase to control the biomass in-ventory and to reduce the MLSS to the secondary clarifiers. Hence, for this evaluation, it was assumed that the SRT of 6 days would be adopted in the future by plant staff.

Figure 4-11. Average and Maximum MLSS at Different Plant Capacity conditions

1.2 1.7 2 2.5

M

LS

S m g

/L

Plant Capacity (MGD)

Average MLSS (6d) Average MLSS (6d) Average MLSS (10d) Maximum MLSS (10d)

To improve the settleability of the activated sludge, the addition of hydrocyclone-based selective wasting (in-

DENSE™) in the secondary treatment process is recommended, Figure 4-12. By using centrifugal force, a hydrocyclone separates lighter solids and denser solids from the WAS. The overflow of the hydrocyclone con-tains the lighter solids, which are wasted, whereas the underflow contains the denser solids, which are re-turned to the aeration basins. The continuous wasting of lighter particles/flocs including filaments improves the SVI of activated sludge, thus maintaining the treatment efficiency. The typical flow split of the cyclones consists of 65 to 80 percent overflow and 20 to 35 percent underflow. For this, it is recommended the instal-lation of a four 44 gallons per minute (GPM) hydrocyclone system. Figure 4-13 shows an example drawing for a four hydrocyclone skid system.

The new hydrocyclone system will tap into the existing WAS line, and pump to the hydrocyclones, with the aid of a small booster pump. It is assumed that the current WAS pumps are not capable of providing the re-quired discharge head of 23 to 30 psi (maximum of 35 psi) that is required for the hydrocyclones to operate.

Figure 4-11. inDENSE Hydrocyclone technology (provided by World Water Works)

Figure 4-12. inDENSE Hydrocyclone skid system (provided by World Water Works)

The implementation of hydrocyclones at similar oxidation ditch facilities have resulted in SVI reduction from…