Page 23 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 2.2.5 PLANNING AREA The planning area was identified in the 2016 Technical Memorandum developed by RJN Group, Inc and included as Appendix 6. There are no current identified plans to update or modify the existing planning area. 2.2.6 PLANNING PERIOD The established planning period has been defined to start January 1, 2019, and end December 31, 2040. Therefore, the total planning period is consisting of 22 years. The proposed capital improvements plan (CIP) is subdivided into three periods, near, mid and long. 2.2.7 MODELING PARAMETERS Initial modeling activities were performed in pursuit of prospective Capital Improvements Plan projects. Through staff discussions and reviewing previous reports, Davidson Dr. WWTP was identified as the most likely initial candidate for improvement. At Davidson Dr. WWTP several prospective improvements were initially identified. These improvements mostly center around improving the existing biosolids handling system while reducing the ammonia and T-P side streams generated from biosolids processing and dewatering. Also, it is possible to achieve luxury phosphorus uptake based on the influent parameters, thus minimizing chemical addition and reducing longterm WWTP costs. To maximize luxury phosphorus uptake, the primary clarifiers would need to be eliminated. Without primary sludge production, there is no advantage for the use of anaerobic digestion vs. aerobic digestion. Additionally, poor performance is currently observed from the existing anaerobic digestors because of the needed improvements to the process (as discussed in previous reports). Anaerobic digestion also produced high concentrations of ammonia and T-P that are recycled back to the headworks during decanting and dewatering activities. The existing compost facility is nearing capacity for biosolids disposal activities. Initial investigations were conducted into alternative biosolids disposal activities. The preliminary sludge production values are presented in Table 2.9.
Page 24 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 2.9 – Davidson Dr. WWTP Biosolids Production Final Disposal of Sludge Sludge Generation 2020 Annual Sludge Production Wet Sludge Weight (20% Solids) 12,756.49 TON Wet Sludge Volume 14,156.17 CY 2020 to 2040 Total Production (Annual) Annual Growth Rate 1 % Wet Sludge Weight 296,451 TON Wet Sludge Volume 328,979 CY Phosphorus Generation 2019 Annual Phosphorus Production Weight 1,212,76.94 lbs. 2020 to 2040 Total Production (Annual) Weight 2,818,379 lbs. The preliminary estimated costs for the various biosolids disposal options are presented in Table 2.10. Table 2.10 – Davidson Dr. WWTP Estimated Biosolids Disposal Costs Range Estimated Disposal Costs ($/ TON) 1 Land Application $39.50 to $56.00 Dryer $40.50 to $50.50 Composting $75.00 to $150.00 Landfilling $20.00 to $60.00 1 Not a total present worth cost analysis, including operations, maintenance, or other factors but only a preliminary investigation. Additional analysis will be included in the WWSMP. Preliminary investigations determined landfilling would be the likely lowest long-term biosolids disposal option for CHS. Initial modeling activities identified a series of needed improvements and corresponding estimated project costs. The goals of the modeling parameters were to identify the biological capacities and needs of Davidson Dr. WWTP. Additional hydraulic analysis and needs assessment will be completed during the development of the WWSMP. These projects and cost estimates are presented in the conclusions section of this report.
Page 25 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 2.2.8 PROCESS CAPACITIES The following loading rates and hydraulic capacities were taken from previous reports. Additional investigations will be conducted to confirm these hydraulic capacities. Table 2.11 – Davidson Dr. WWTP Treatment Processes and Capacities Headworks Screening Type Multi-Rake Opening 6 mm Angle 75 degree No. 2 Headworks Grit Removal Type Stacked Tray Vortex No. 2 Peak Day Capacity (Total) 48 MGD Primary Clarifier Dimension (diameter) 85 ft No. 3 Total Area 17023.5 SF Depth 10 ft Peak Day Flow Overflow Rate 1200 GPD/ SF Peak Day Flow Overflow Rate 20.38 MGD Anoxic Basin Dimension 30x40x21 ft No. 3 Total Volume 0.5655 MG Aerobic Basin Dimension 153x40x21 ft No. 3 Total Volume 2.884 MG Aeration Blowers Multi-Stage Centrifugal 2100 SCFM No. 4 Total (Firm) 6300 SCFM
Page 26 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 2.12 – Davidson Dr. WWTP Treatment Processes and Capacities Cont. Secondary Clarifier Dimension (diameter) 95 ft No. 3 Dimension (diameter) 100 ft No. 1 Total Area 29120 SF Depth 10 ft Average Day Overflow Rate 412 GPD/ SF Peak Day Flow Overflow Rate 700 GPD/ SF Peak Hour Overflow Rate 1000 GPD/ SF Peak Day Capacity (Total) 20.38 MGD Tertiary Filtration Type Media Surface Area 1008 SF No. 4 Peak Day Loading Rate Capacity (Firm) 4.58 gpm/ SF Peak Day Loading (Firm) 20 MGD Type Cloth No. 1 Peak Day Loading Rate Capacity 24 Peak Day Capacity (Total) 44 MGD UV Disinfection No. 2 Peak Day Capacity (Total) 44 MGD Biosolids handling capacities are not provided at this time. Additional investigations will be conducted to determine capacities and needs of the biosolids handling units. Capacities of the SW WWTP will be determined during the development of the WWSMP. 2.2.9 COLLECTION SYSTEM As discussed in an earlier sections of the report, CHS has completed approximately $70 million worth of improvements in the collection system during the last 5-years. SECAP projects identified, completed or eliminated because of I/I abatement are identified in Appendix 13. SECTION 2.3 - CONCLUSION This report performed preliminary technical investigations, reviewed pertinent reports, conducted initial ADEE discussions related to water quality permitting for the WWTPs, established initial criterial that will be used to
Page 27 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. develop future sections of the WWSMP, conducted preliminary Biowin™ modeling and developed near-term capital improvements projects. It is anticipated that minimal improvements are needed at the SW WWTP. The most likely improvement will be to improve the supplemental alkalinity and/ or pH adjustment storage and addition. Supplemental alkalinity and/ or pH adjustment will decrease the effluent ammonia risk of violation. It is also recommended that additional investigations be performed to determine the source of the influent ammonia spikes. It is anticipated that major improvements will be needed at the Davidson Dr. WWTP. The initial near-term CIP is presented in Table 2.13.
Page 28 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 2.13 – WWSMP Initial Near-Term CIP Project Description Estimated Capital Cost1 Collection System Project Gulpha Basin/ Fairway FM within the collection system. $2,264,000 Wastewater System Master Plan Complete remaining tasks associated with WWSMP. $300,000 Demolition Demolition and retrofit of existing facilities at Davidson Dr. WWTP. $635,000 Primary Clarifier Bypass Line Bypass primary clarifier to facilitate construction activities within the primary clarifiers. $632,000 Chemical Addition New chemical feed building, chemical storage and injection. $1,512,000 Aeration Basin and Blowers Upgrade New aeration and diffuser equipment and installation. $3,264,000 Aerated Sludge Storage Conversion of the primary clarifier basins to aerated sludge storage. $3,301,000 Sludge Thickening New sludge thickening facilities. $3,009,000 Sludge Dewatering New sludge dewatering facilities. $2,826,000 Secondary Clarifier New secondary clarifier. $1,766,000 1 August 2019 ENR CCI = 11311. The preliminary values presented in this section of the report were updated as they develop throughout the WWSMP project.
Page 29 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. CHAPTER 3 – BIOLOGICAL AND HYDRAULIC MODELING SECTION 3.1 - INTRODUCTION Biological treatment process is one of the most critical components of any wastewater treatment plant. Most of the upstream and downstream processes are implemented to improve biological treatment. Biological treatment is also usually the most expensive component of a wastewater treatment process therefore careful evaluation and approaches must be considered with any master plan development. Hydraulics are critical, indirectly to the successful treatment within any wastewater treatment plant. Influent, effluent, biosolids, etc. are all conveyed via hydraulically within the wastewater treatment plant and vital processes (headworks, clarification, tertiary filtration, etc.) rely on efficient solids/ liquid separation that is directed related to the hydraulic performance of the process. To aid in the development of the City of Hot Springs (CHS) Wastewater System Master Plan (WWSMP), biologically and hydraulic modeling (simulator) efforts are intended to be completed for Davidson Dr. Wastewater Treatment Plant (WWTP) and Southwest Wastewater Treatment Plant (SWWWTP). 3.1.1 PROCESS FLOW DIAGRAMS The simplified process flow diagrams for the existing WWTPs are presented in the following Figures.
Page 30 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.1 – Existing Davidson Dr. WWTP Simpl
0 of 291 prings, Arkansas | Crist Engineers, Inc. lified Process Flow Diagram – Liquid Treatment
Page 31 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.2 – Existing Davidson Dr. WWTP Simplif
1 of 291 prings, Arkansas | Crist Engineers, Inc. fied Process Flow Diagram – Biosolids Treatment
Page 32 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.3 - Existing Southwest (SW) WWTP Simp Figure 3.4 - Existing SWWWTP Simplified Pr
2 of 291 prings, Arkansas | Crist Engineers, Inc. plified Process Flow Diagram – Liquid Treatment rocess Flow Diagram – Biosolids Treatment
Page 33 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 3.1.2 INFLUENT CHARACTERISTICS Wastewater growth rates were determined from the Water System Master Plan study that was finalized in 2017. This master plan determined a 1% growth rate was acceptable and representative for the water distribution system area. Similarly, a 1% growth rate was also applied to the wastewater production to determine a 2040 planning year average day flow rate. Table 3.1 - Davidson Dr. WWTP Influent Flow Projections Parameters 1992 Design Flow 12 MGD 2040 Average Day (AD) Flow (2015 Report) 14.5 MGD 2040 Design Flow (2015 Report) 23.1 MGD 2018 Average Daily (AD) Flow 12.04 MGD Projected Growth Rate 1 % Minimum Average Monthly April Estimated Influent Temperature 15.1 Degrees C Minimum 7-DA April Estimated Influent Temperature 13.8 Degrees C Minimum 7-DA April Influent Design Temperature 13.3 Degrees C Minimum Average Monthly November to March Estimated Influent Temperature 10 Degrees C Minimum 7-DA November to March Influent Design Temperature 7.0 Degrees C Minimum 7-DA November to March Influent Design Temperature 6.5 Degrees C Design AD Annual Temperature 20.0 Degrees C Design 7-AD Maximum Temperature 28.0 Degrees C 2040 AD Projected Flow Rate 14.99 MGD 2040 AD Design Flow Rate 16 MGD 2040 7-DA Design Flow Rate 20 MGD Peak Day I/I Design Storm Event 94 MGD
Page 34 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 3.2 - SW WWTP Influent Flow Projections Parameters 2008 Design Flow 0.85 MGD 2018 Average Daily (AD) Flow 0.46 MGD Projected Growth Rate 1 % Minimum Average Monthly April Estimated Influent Temperature 15.1 Degrees C Minimum 7-DA April Estimated Influent Temperature 13.8 Degrees C Minimum 7-DA April Influent Design Temperature 13.3 Degrees C Minimum Average Monthly November to March Estimated Influent Temperature 10 Degrees C Minimum 7-DA November to March Estimated Influent Temperature 7.0 Degrees C Minimum 7-DA November to March Influent Design Temperature 6.5 Degrees C Design AD Annual Temperature 20.0 Degrees C Design 7-AD Maximum Temperature 28.0 Degrees C 2040 AD Projected Flow Rate 0.69 MGD 2040 Design AD Flow Rate 0.74 MGD 2040 Design 7-DA Flow Rate 0.925 MGD Peak Day I/I Design Storm Event 4.25 MGD Due to the infiltration and inflow (I/I) challenges that have historically plagued the City of Hot Springs, it is difficult to determine a peak or max month peaking factor. Additionally, the NPDES permit utilizes a 7-day average (DA) to represent peak effluent discharge conditions. Additionally, review of the historical and recent testing data does not indicate a traditional maximum month loading condition. When considering the transient nature of tourism and sporting activities associated with Hot Springs it is unlikely that a traditional maximum month would be observed in the data. However, it is possible to observe a maximum 7-DA during Arkansas Derby’s or Memorial Day weekend periods. In 2020, where detailed sampling and collection was conducted, a peak in loading or flow was not observed. In general, when influent flow increases the pollutant concentrations decrease proportionally. Peak flow conditions historically appear to be associated with I/I generated from storm event. It has been proposed that the City of Hot Springs may entertain receiving wastewater generated from the Lake Hamilton wastewater system. The anticipated amount is approximately 20,000 to 40,000 gallons per day that could be conveyed to the collection system for Davidson Dr. or SW WWTPs. The amount of wastewater generated from Lake Hamilton is
Page 35 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. included in the projection analysis for either system and will generate correspondingly minimal impact to either WWTP. To account for the potential under projections, data error or other unknowns, the 2040 design average day (AD) flow rate was determined to be 16 MGD (or an additional 6.7%). To determine the maximum 7-DA flow rate, a 1.25 factor was applied to the design AD flow rate. The 1.25 factor was taken from Wastewater Engineering Treatment and Resource Recovery, 5th Edition (Metcalf and Eddy et al.) for wastewater treatment plants of intermediate size. Influent parameters for Davidson Dr. WWTP were established through: 1. CHS staff collected, analyzed, and published data. An extensive laboratory testing regime was exceptionally performed by staff which greatly aided in the development of data set that was incorporated into a statistical analysis. 2. Previously reports or design approaches published by others. This information was used to evaluate and back-check against the statistically developed data to ensure similar loadings/ concentrations. The influent flow projections and characteristics for Davidson Dr. WWTP are presented in Table 3.3.
Page 36 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.3 - Davidson Dr. Influe Paramet 2018 Average Annual 2015 Report Max Month 2015 Report Average Month 1992 Design Parame cBOD 164 165 166 122 VSS1, 2 120.58 114.1 114.1 103.6 TSS3 172.26 163 163 148 TKN1, 4 20.57 17.71 19.57 14.13 Ammonia (as N) 14.4 12.4 13.7 9.89 Total P (as-P) 3.8 3.2 3.7 4.99 Nitrate (as-N) 0 0 0 0 pH1 6.57 6.57 6.57 6.57 Alkalinity1 125 125 125 125 Calcium1 30 30 30 30 Magnesium1 10 10 10 10 Dissolved Oxygen 0 0 0 0 1 Limited testing data, based on historical water quality parameters and ratios. Total Kjeldahl Nitrogen.
6 of 291 prings, Arkansas | Crist Engineers, Inc. ent Characteristics Projections ters eters 2020 January through June Average Day 2020 April Maximum Day 2040 AD Design 2040 7-DA Design 93 118 164 205 mg/L 82.6 75 121 151 mg/L 118 107 173 216 mg/L 14.47 17.00 20.57 25.71 mg/L 10.13 11.9 14.4 18.0 mg/L 2.42 3.5 3.8 4.75 mg/L 0 0 0 0 mg/L 7.01 7.11 6.57 6.57 65 75 125 65 mg/L 30 30 30 30 mg/L 10 10 10 10 mg/L 0 0 0 0 mg/L 2 VSS = Volatile suspended solids. 3 TSS = Total Suspended Solids 4 TKN =
Page 37 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.4 - SW WWTP Influen Param 2018 Average Annual 2018 Max Monthly 2008 Design Parameters cBOD 242.21 293.33 250 VSS1, 2 124.90 177.60 240 TSS3 156.12 222 300 TKN1, 4 39.18 57.59 50 Ammonia (as N) 27.43 40.32 35 Total P (as-P) 5.39 6.78 - Nitrate (as-N) 0 0 0 pH1 6.57 6.57 6.57 Alkalinity1 125 125 125 Calcium1 30 30 30 Magnesium1 10 10 10 Dissolved Oxygen 0 0 0 1 Limited testing data, based on historical water quality parameters an Solids 4 TKN = Total Kjeldahl Nitrogen
7 of 291 prings, Arkansas | Crist Engineers, Inc. nt Characteristics Projections meters 2020 January through June Average Day 2020 April Maximum Day 2040 AD Design 2040 7-DA Design 150.29 208.82 242.21 302.77 mg/L 152.23 276 124.90 156 mg/L 190.29 345 156.12 195 mg/L 37.49 31.80 39.18 51.17 mg/L 25.12 26.40 27.43 34.30 mg/L 3.93 5.20 5.39 6.74 mg/L 0 0 0 0 mg/L 7.01 7.11 6.57 6.57 65 75 125 65 mg/L 30 30 30 30 mg/L 10 10 10 10 mg/L 0 0 0 0 mg/L nd ratios. 2 VSS = Volatile suspended solids. 3 TSS = Total Suspended
Page 38 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Similarly, as with the projected flow rate, to account for the potential under projections, data error or other unknowns, a 1.25 factor was applied to the design AD influent characteristics to determine the maximum 7-DA influent characteristics. The 1.25 factor was taken from Wastewater Engineering Treatment and Resource Recovery, 5th Edition (Metcalf and Eddy et al.) for wastewater treatment plants of intermediate size. Wastewater generated from the City of Hot Springs collection system is generally lower in carbonaceous biochemical oxygen demand (cBOD), total suspended solids (TSS), ammonia (NH3-N) and total phosphorus (T-P) than what may be anticipated regionally. The reduced loadings of these main wastewater constituents are generally considered an advantage to wastewater treatment as it generally reduces the require infrastructure needed to meet effluent limitations. The majority of the discussions related to influent characteristics will center primarily around Davidson Dr. WWTP because it is the largest and most costly infrastructure item in the wastewater system. It is also near its original design capacity thus requiring the majority of the capital improvement funds. Wastewater, after treatment at the Davidson Dr. WWTP, is required to meet the water quality characteristics as presented in Table 3.5. SW WWTP influent characteristics are similar to Davidson Dr. WWTP and are presented in Table 3.5.
Page 39 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 3.5 – Davidson Dr. WWTP NPDES Effluent Limit Summary 2018 to 2023 Monthly Average (MA) Permit Limits 7 Day Average (7-DA) Planning Permit Limits Future Monthly Average (MA) Permit Limits Future 7 Day Average (7-DA) Planning Permit Limits Carbonaceous Biochemical Oxygen Demand (cBOD5) cBOD5 (mg/L) 10 15 10 15 cBOD5 (lbs./ day) 1000 - 2001.62 - Total Suspended Solids (TSS) TSS (mg/L) 15 22.5 15 22.5 TSS (lbs./ day) 1500 - 3002.42 - Ammonia (as N) (NH3-N) April (mg/L) 3.6 8.9 4.1 10.1 April (lbs./ day 360.3 - 820.72 - May to October (mg/L) 3.6 7.5 3.6 5.4 May to October (lbs./ day) 360.3 - 720.62 - November to March (mg/L) 10 15 10 15 November to March (lbs./ day) 1000 - 2001.62 - Total Phosphorus (T-P) Total P (as-P) (mg/L) 11 Report 0.7 1.1 Total P (as-P) (lbs./ day) 100.1 - 140.12 220.22 Nitrate/ Nitrate/ Nitrogen Nitrate + Nitrite - Nitrogen (mg/L) Report Report Report Report Nitrate + Nitrite - Nitrogen (lbs./ day) Report - Report Report Other pH 6 to 9 6 to 9 6 to 9 6 to 9 Chronic Whole Effluent Toxicity (WET) Report Report Report Report Dissolved Oxygen (mg/L) January to December 5 5 - - May to October - - 5 5 November to March - - 5.6 5.6 Fecal Coliform (colonies/ 100 mL) May to September 200 400 200 400 October to April 1000 2000 1000 2000 1 Concentration based on permit estimated average daily flow of 12 MGD. 2 Mass calculated using a future permit estimated average daily flow of 24 MGD.
Page 40 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Planning limits were established through request and notification from the ADEE as included in the Appendix 1a. The constituents of note identified in the current and planning limits that impact decision making are the April ammonia (NH3 as N) concentration and the total phosphorus (T-P) concentration. Table 3.6 – Southwest WWTP NPDES Effluent Limit Summary 2015 to 2020 Monthly Average (MA) Permit Limits 2015 to 2020 7 Day Average (7- DA) Planning Permit Limits 2020 to 2025 Monthly Average (MA) Permit Limits 2020 to 2025 7 Day Average (7- DA) Planning Permit Limits Carbonaceous Biochemical Oxygen Demand (cBOD5) May to October (mg/L) 5 7.5 5 7.5 May to October (lbs./ day) 35.5 - 35.5 - November to April (mg/L) 10 15 10 15 November to April (lbs./ day) 70.9 - 70.9 - Total Suspended Solids (TSS) TSS (mg/L) 15 22.5 15 22.5 TSS (lbs./ day) 106.3 - 106.3 - Ammonia (as N) (NH3-N) April (mg/L) 5.2 5.2 5.2 5.2 April (lbs./ day 36.9 - 36.9 - May to October (mg/L) 2 3 2 3 May to October (lbs./ day) 14.2 - 14.2 - November to March (mg/L) 6 9 6 9 November to March (lbs./ day) 42.5 - 42.5 - Total Phosphorus (T-P) Total P (as-P) (mg/L) Report Report Report Report Total P (as-P) (lbs./ day) Report - Report - Nitrate/ Nitrate/ Nitrogen Nitrate + Nitrite - Nitrogen (mg/L) Report Report Report Report Nitrate + Nitrite - Nitrogen (lbs./ day) Report - Report - Other pH 6 to 9 6 to 9 6 to 9 6 to 9 Chronic Whole Effluent Toxicity (WET) Report Report Report Report Dissolved Oxygen (mg/L) May to October 7 7 7 7 November to April 6 6 6 6 Fecal Coliform (colonies/ 100 mL)
Page 41 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. May to September 200 400 200 400 October to April 1000 2000 1000 2000 The planning limits were established based on the current discharge location for each WWTP. It was suggested to consider the option of abandoning Davidson Dr. WWTP, utilizing the equalization pond and installing a pump station to convey wastewater to a wastewater treatment plant located in an alternative location. Limited investigations were performed related to this alternative because of the challenges associated with existing topography. The collection system that conveys wastewater to Davidson Dr. is primarily conveyed via force main from various drainage basins within the City of Hot Springs. Therefore, relocating the wastewater treatment plant would require extensive re-pumping from the Davidson Dr. WWTP site to an alternative site. Due to the configuration of the drainage topography, potential sites would be located downstream of the existing site. The current discharge location for Davidson Dr. WWTP is located between Lake Hamilton and Lake Catherine. It would likely be a detriment to the existing permit limits if the discharge were relocated closer to Lake Catherine from its current location. To relocate the discharge to a location that would have an advantage for discharge limits, it would likely need to be relocated downstream of Remmel Dam. That would place a potential new WWTP site approximately 7 miles from the existing site. This distance would make relocation of the wastewater treatment plant cost prohibitive. 3.1.3 STOICHIOMETRIC DISCUSSIONS The purposes of the following discussions are to establish the fundamentals associated with biological treatment for creating the foundations for the understandings and recommendations that are to be applied to the Davidson Dr. WWTP. cBOD and ammonia are to be removed biologically at the Davidson Dr. WWTP to the levels identified within the current and future permits. The majority of total phosphorus is to be removed biologically and polished through chemical/ physical methods. 3.1.3.1 BIOLOGICAL CBOD UTILIZATION PROCESS The aerobic heterotrophic bacteria (AHB) biologically degrade cBOD using oxygen as an electron acceptor to produce carbon dioxide and water.
Page 42 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Figure 3.5 – Glucose Utilization Biochemical Reaction Figure 3.6 – Biomass Production Represented as a Biochemical Reaction In the above reaction, cBOD is represented as glucose for simplicity purposes. The pathway that municipal wastewater’s complex cBOD wastes are degraded are unknown. For the purposes of discussions, the experimental growth yield of AHB’s is 0.45 lbs. VSS per lbs. of cBOD (or COD) oxidized. For biochemical reactions to occur, AHB’s require 1.1 lbs. of O2 per lbs. of cBOD oxidized as measured experimentally. The growth rates and require oxygen will vary depending on WWTP operational factors. The decay/ death rate AHBs can vary but generally is considered 0.15 lbs. biomass/ lbs. biomass per day. Degradation of cBOD (commonly called “food”) is used by the biomass (commonly called “mass”) for cell maintenance and reproduction (i.e., producing more biomass). When there is a lack of cBOD or food for the ratio of biomass or mass present in a specific system, endogenous decay will occur. During endogenous utilization phase of an activated sludge system, predation of biomass occurs simultaneous cBOD reduction. During endogenous utilization phase the oxygen needs increase to an experimentally determined 1.8 lbs. of O2 per lbs. of cBOD. The biomass growth phases curve is presented in Figure 3.7. Figure 3.7 – Biomass Growth Phases with cBOD Utilization Image found at http://files.dep.state.pa.us/Water/BSDW/OperatorCertification/TrainingModules/ww15_act_sludge_1_wb.pdf on May 7, 2020
Page 43 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. This graph represents a general cross section of the growth stages of the biomass system when presented as a batch. For the purposes of WWTP, the growth rate of biomass has three phases of importance: log growth, declining growth, and endogenous phases. Log growth occurs as the initial reproduction rate of the biomass increase because there is excess cBOD is present. Declining growth begins to occur because the ratio of the biomass present exceeds the amount of food present (i.e., no more excess food present). The endogenous phase occurs when most of the cBOD has been utilized thus leading to a significant excess of biomass. At this point biomass predation occurs thus decreasing the biomass amount found within a system. Also presented in Table 3.7 are the terms high rate, conventional and extended aeration. These terms are commonly used in wastewater operations to represent the desired range on the growth curve where the average biomass is maintained. Biomass continues to grow throughout the life cycles presented in the above graph, however, to maintain a specific point on the growth curve, operations staff will waste biomass at a specific rate where new biomass grows. This approach leads to an average biomass age or commonly referred to sludge retention time (SRT). A specific SRT will then correspond to a location on the growth curve based on the food to mass ratio. The specific food to mass (F/M) ratio relates the amount of food at any given instant of time with the biomass present within the biological system. As discussed previously, when excess food or a high F/M ratio are present, this is considered a high-rate treatment system. Declining F/M ratios correspond to declining growth stage associated with conventional wastewater treatment operation. Finally, endogenous utilization occurs with much lower F/M ratios. Another important aspect of biological treatment is the hydraulic retention time (HRT). HRT is the time or duration that food (or cBOD) is in contact with mass (or biomass) in the presence of oxygen (i.e., electron acceptor). With a lower or reduced HRT, the biomass has less opportunity to consume the cBOD thus requiring higher (i.e., quicker) utilization by the biomass. Table 3.7 – High Rate, Conventional and Extended Aeration Operational Modes Comparison High Rate Conventional Extended Aeration 1 F/M (lbs. cBOD/ lbs. MLVSS) 1.5 to 2 .2 to .4 .04 to 0.15 2 SRT (days) .5 to 2 3 to 15 20 to 40 3 HRT (hrs.) .5 to 1 3 to 6 18 to 30 1F/M = food to mass ratio. 2SRT = Sludge Retention Time 3HRT = Hydraulic Retention Time As previously discussed, wastewater treatment plants operate as continuous flow systems, where food (cBOD) is continuously being fed into the aeration basin while the mass continues to grow (i.e., age). Biological treatment support systems are used to maintain the desired operational range that provides the desired treatment goals. These support systems maintain a
Page 44 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. specific range on the biomass growth phases that achieves the desired cBOD utilization as shown in Figure 3.7. To maintain a continuous treatment, mass must be recycled back into the system for continuous treatment while excess biomass must be taken out of the system. These operations systems are called return activated sludge (RAS) and waste activated sludge (WAS). RAS pumps move biomass from a secondary clarifier back to the beginning of the aeration basin. WAS pumps convey biomass out of the treatment system to biosolids processing. These systems perform the main function of maintaining the desired F/M and SRT of a WWTP. 3.1.3.2 BIOLOGICAL AMMONIA UTILIZATION PROCESS Biological ammonia utilization processes or commonly called nitrification, biologically occurs in a two-step process. The first (1) step is called nitritation where ammonia is converted to nitrite in the following biochemical reaction: Figure 3.8 – Ammonia to Nitrite Biochemical Reaction Nitritation is performed by a specific subset of aerobic autotrophic bacteria (AAB) contained within the MLVSS are called ammonia-oxidizing bacteria (AOB). To allow for this specific subset of bacteria to proliferate, careful consideration must be given to the SRT, average amount of ammonia oxidized daily and aeration basin volume. The experimental growth yield of AOB’s is 0.12 lbs. VSS per lbs. of NH3-N oxidized. For biochemical reactions to occur, AOB’s require 3.21 lbs. of O2 per lbs. of NH3-N oxidized and 7.09 lbs. of alkalinity (as CaCO3) per lbs. of NH3-N (as measured experimentally). The second (2) step is called nitrite utilization where nitrite is converted from nitrite to nitrate in the following biochemical reaction Figure 3.9 – Nitrite to Nitrate Biochemical Reaction This process is performed by a specific subset of aerobic autotrophic bacteria (AAB) contained within the MLVSS call nitrite-oxidizing bacteria (NOB). The experimental growth yield of NOB’s is 0.04 lbs. VSS per lbs. of NO2-N oxidized. For biochemical reactions to occur, NOB’s require 1.11 lbs. of O2 per lbs. of NO2-N oxidized as measured experimentally. The decay/ death rate of both the AOBs and NOBs are similar with an experimentally measured value of 0.15 lbs. biomass/ lbs. biomass per day. The interesting note is that NOBs grow yield is much lower than that of AOBs. Therefore, NOBs (specific subset of the overall MLVSS) will be found at much lower concentrations than AOBs
Page 45 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. within the biological system. Low water temperatures, biological activity is reduced which slows the utilization rate of nitrite by the NOBs. Additionally, high flow rates reduce the HRT of the aeration basin, which in turn reduces the time that NOBs have to convert nitrite to nitrate (i.e., reduce the contact time between nitrite and NOBs). When low temperature and high flow occur, an increase in effluent nitrite will be generally the first indication that the biological nitrification process is beginning to breakdown. An increase in effluent nitrite concentration is also important consideration for chronic whole effluent toxicity. Nitrite has a 24-hour LC50 toxicity of between 0.5 to 2 mg/L for most fish species, depending on co-factors, such as pH, chloride concentration and dissolved oxygen level. 3.1.3.3 BIOLOGICAL PHOSPHORUS UTILIZATION PROCESS Phosphorus is a macro-nutrient that is required for biological growth and production. It is traditionally identified as a limiting nutrient in biological systems because it will limit that amount of biological production even if excess cBOD and nitrogen are present. This is one of the main reasons why T-P can be a regulated effluent parameter (as it this case it is regulated) to discourage algae growth within the receiving water. The City of Hot Springs influent does not contain excessive amounts of T-P. In general, most traditional wastewater influent contains approximately 25 to 30 lbs. cBOD per lbs. T-P. When ratios are less than 25 lbs. cBOD per lbs. T-P, investigations should be completed to determine the source of the excessive influent T-P. If sources are determined within the collection system, it may be possible to address through pre-treatment program. Current influent values indicate that the City of Hot Springs at the Davidson Dr. WWTP contain on average 3.8 mg/L of T-P which equates to a ratio of 43 lbs. cBOD per lbs. T-P. However, those ratios may change in the future. Due to the drinking water challenges that occurred in Flint, Michigan, and elsewhere, in October 2019 the EPA proposed he Long-Term Revisions to the Lead and Copper Rule (LTRLCR). The proposed rule establishes the following: a. Identifying areas most impacted b. Identify Areas (“find and fix”) c. Strengthening treatment requirements d. Replacing Lead Service Lines (LSLs) e. Increasing sampling reliability f. Improving risk communication g. Protecting Children in Schools
Page 46 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The component of importance, as related to the T-P effluent compliance for the City of Hot Springs is the “strengthening treatment requirements”. Under this section of the new proposed rule, compliance level is the same with LTRLCR as they are for the current Lead and Copper Rule (LCR) through establishing a 90th percentile of the distribution system must not exceed 0.015 mg/L for lead and 1.3 mg/L for copper. However, it establishes a new action level of 0.010 mg/L for lead where the utility must address through corrosion control if exceedance occurs. Considering City of Hot Springs supply water is electrochemically corrosive, it may be necessary to increase the total phosphorus addition at the water treatment plants. This approach could impact the overall influent concentrations found at the wastewater treatment plants by leading to high concentrations of T-P which will ultimately decrease the cBOD: T-P ratio. As the proposed rule becomes final, the rule’s full impact can be understood leading to site specific strategies for compliance. In general, the most cost-effective approach to remove T-P, when the cBOD: T-P ratios are greater than 25:1, is for it to be removed biologically. When ratios are less than 25:1, the most cost-effective approach is to determine the source of excessive T-P and remove it from the waste stream prior to entering the collection system. Other possible solutions may be to reuse wastewater effluent for restricted or unrestricted watering of green spaces or for industrial use. Removing wastewater flow from the discharge (i.e., removing T-P), removes the T-P mass that is discharge to the receiving stream. Water reuse can also create a new revenue stream for utilities while at the same time creating a consistent supply of water that is not subject to drought restrictions for a customer may be attractive to some communities. Generally, chemical addition is the highest cost solution for removal of T-P from wastewater. This is the existing method for T-P removal at the Davidson Dr. WWTP. Sodium aluminate is currently used to precipitate T-P for removal via biosolids processing. As previously discussed, phosphorus is a limiting nutrient, that is required for cell maintenance and growth by AHBs and AABs. During traditional aerobic treatment operations, it can be anticipated that approximately 0.015 lbs. T-P/ lbs. of biomass will be generally utilized for cell respiration. At this rate, it is anticipated that approximately 40 to 60% of the influent T-P will be utilized within traditional aerobic treatment processes. However, it is possible to create an environment that encourages luxury phosphorus uptake or enhanced biological phosphorus removal (EBPR) by a specific subset of biomass conveniently called phosphorus accumulating organisms (PAOs). PAO’s are facultative anaerobes that are favored when an environment is created absent of nitrate, nitrite and oxygen electron acceptors that allows them to consume readily biodegradable BOD (or COD) in the form of volatile fatty acids (VFAs). The experimental growth yield of PAO’s is 0.45 lbs. VSS per lbs. of acetate cBOD which is similar to the growth rates of AHBs. Even with similar growth kinetics, the total mass amount of PAOs that are found in an EBPR process is limited by the amount of VFAs and nitrate, nitrite and oxygen electron acceptors found within their environment. Therefore, the PAOs are a small subset of organisms found that may only represent 10 to 20%
Page 47 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. of the biomass. The advantage is that the PAOs can store up to 15 times more phosphorus within their cell walls than AHBs. An anaerobic selector (subset of an anoxic zone) is used to favor the growth of the PAOs. This is an anaerobic environment that they favor so the amount of T-P found within their cell walls is similar to what is found in other microorganisms. However, as they transition into an oxic or aerobic zone of the treatment process, which is not their favored environment, they will absorb excess T-P.
Page 48 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. SECTION 3.2 – DISCUSSION 3.2.1 INFLUENT WATER TEMPERATURE 3.2.1.1 NOVEMBER 1 TO MARCH 31 AMMONIA (NH-3 AS N) The November through end of March ammonia concentration is an important design/ decision making parameter because the cold-water temperature experienced during these months will retard biological activities, specifically, the AOBs/ NOBs. Therefore, during extreme cold water temperature periods minimal nitrification may occur. Careful water temperature considerations should be given to the observed minimum water temperature that occurs during the winter months. 3.2.1.2 APRIL 1 – 30 AMMONIA (NH-3 AS N) The April ammonia concentration is an important design/ decision making parameter because the concentration decreases from 10 mg/L to 3.6 mg/L (future 4.1 mg/L) in the case of Davidson Dr. WWTP. The 2.7x (times) reduction that occurs impacts the nitrification needs of the WWTP. As discussed, water temperature is the critical component to determine the rate of nitrification that occurs to achieve compliance. In the case of SWWWTP the decrease isn’t as dramatic with a drop from 6 mg/L to 5.2 mg/L. Careful considerations should be given to the minimum water temperature that is likely to be observed during the month of April. Influent water temperature as discussed in previous sections, has a critical impact on the utilization rate of the AOBs/ NOBs. To compensate for the reduced utilization rate that occurs with colder water temperatures, the concentration of the AOBs/ NOBs (i.e., MLVSS) can be increased by increasing the hydraulic retention time (i.e., volume of the aeration basin(s)) or increasing the SRT. Increasing the SRT can have unintended impacts with traditional activated sludge therefore, traditionally the AOBs/ NOBs concentration is increased by adding volume to the aeration basins. This approach also increases the HRT, thus providing more contact time between the AOBs/ NOBs and the ammonia/ nitrite. However, the approach at the Davidson Dr. WWTP is to maximize the existing volume of the aeration basin. To maximize the existing volume the minimum temperature becomes a critical design parameter. Limited influent water temperature data exists for the Davidson Dr. WWTP. Another consideration is that even if there were influent water temperature data it would likely only data to the ~1970s. Therefore, interpolations and estimates will need to be used to determine a design minimum influent water temperature. NOAA (STA USC00033466; HOT SPRINGS 1 NNE, AR US) has maintained a weather station in the City of Hot Springs since 1875 and is currently located near the Hot Springs Ranger Station near Reserve St and Beech St. Minimum and maximum daily air temperature data exists for 1891, 1893, 1907 to 1916, 1930 to 2020 (exclude 2019), or a total of 102 data sets.
Page 49 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The focus of the records associated with this station are centered around the recorded minimum and maximum daily temperatures from January 1 to April 30. The data review centered on January 1 to March 31 minimum and maximum daily temperatures where the coldest known years were determined by summing the minimum and maximum daily temperature (excluding leap days on leap years). The lower sums were considered colder air temperature periods which inherently would lead to lower influent water temperatures. To calibrate the data set air temperatures to water temperatures, a review was conducted related to Little Rock Water Reclamation WWTPs (Adams Field and Fourche Creek) as well as Hot Springs Davidson Dr. measured influent water temperature values. The calibration is based on 2020 and 2014 influent water temperatures observed at these facilities. These values along with projected are presented in Table 3.8.
Page 50 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.8 – Corresponding A Jan 1 to March 31 A Hot Springs Air Temp. Max + Min Daily Temp. Hot Springs Air Temp. Max + Min Daily Temp. % Deviation from 2020 Davidson Dr. 7 Day Average Minimum Influent Water Temp. (Degrees C) Davidson Dr. Average Monthly Minimum Influent Water Temp. (Degrees C) Average 8463 2020 8203 - 15.81 17.11 2014 7474 -8.9% 14.92 - 2010 7598 -7.4% 15.12 - 2008 8045 -1.9% 15.92 - 1996 8121 -1.0% 16.02 - 1993 8186 -0.2% 16.12 - 1978 6875 -16.2% 13.82 15.12 1 Field measured values. 2 Projected or estimated values.
0 of 291 prings, Arkansas | Crist Engineers, Inc. Air and Water Temperatures April 1 to April 30 Jan 1 to March 31 Little Rock Water Reclamation - Fouche Creek WWTP Influent Water Temp. Little Rock Water Reclamation - Adams Field WWTP Influent Water Temp. Davidson Dr. 7 Day Average Minimum Influent Water Temp. (Degrees C) Davidson Dr. Average Monthly Minimum Influent Water Temp. (Degrees C) Little Rock Water Reclamation - Fouche Creek WWTP Influent Water Temp. Little Rock Water Reclamation - Adams Field WWTP Influent Water Temp. 16.51 171 - - 121 13.51 17 17 7.62 - 9 9 - - 7.72 - - - - - 8.12 - - - - - 8.22 - - - - - 8.22 - - - - - 7.02 102 - -
Page 51 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. A review of the air temperature data concluded that 1978 Jan 1 through April 30 was the coldest cumulative air temperatures experienced for Hot Springs, AR. Also, other cold air temperature years within the last 50 years have been identified as 2014, 2010, 2008, 1996 and 1993 (all years with below average air temperatures). To project or estimate the minimum influent water temperatures at Davidson Dr. WWTP, the calibrated temperature was first adjusted based on the slight influent temperature observed between the Little Rock Water Reclamation WWTPs and Davidson Dr. WWTP and then proportionally (or linearly) lowered. This does not account for rainfall, infiltration/ inflow (I&I), new water supply sources and other factors that may impact the influent water temperature at the Davidson Dr. WWTP. To account for potential errors, the design minimum water temperatures were further reduced by 0.5 degrees C. Therefore, the minimum design influent 7-DA temperature is 13.3 degrees C for April and 6.5 degrees C for November through March at Davidson and SW WWTPs. 3.2.2 PRIMARY CLARIFICATION/ PRIMARY SLUDGE/ ANAEROBIC DIGESTION The purpose of primary clarification is to use physical process (i.e., sedimentation) to remove TSS and cBOD. Primary clarification historically served the vital function of removal of inert products such as grit and screenings. With the advent of improved screening and grit removal technologies, primary clarifiers use for removal of insoluble materials has primarily become obsolete. The approach of TSS and cBOD removal minimizes secondary treatment process while producing primary sludge that can be then used in the anaerobic digestion process to biologically produce methane. Methane is a desirable byproduct of wastewater treatment that can be used to generate revenue or offset operational costs for the utility. Volume reduction and concurrent stabilization of biosolids (to achieve Class B for land application) occurs during anaerobic digestion. Factors the directly impact the anaerobic digestion process performance are influent concentrations of cBOD and TSS. The readily biodegradable portion of the influent TSS, called the volatile suspended solids (VSS) coupled with the cBOD produced during primary clarification provides the food or mass to the anaerobic digestor. Biomass within the anaerobic digestor converts first the VSS to soluble compounds through a process called hydrolysis. These hydrolyzed compounds and cBOD (removed by the primary clarifier) are then converted into intermediate products through a process called fermentation or acetogenesis. The resulting intermediate products (acetate, acetic acid, volatile fatty acids, carbon dioxide, etc.) consume alkalinity and suppress the pH. A specific subset of desirable biomass then converts the intermediate products to methane, carbon dioxide and water through a process call methanogenesis. Other competing reduction biochemical reduction reactions occur concurrently with methanogenesis that produce other gasses, including nitrogen gas, hydrogen, hydrogen sulfide, et al. whereas the entire gas complex is called biogas. Biogas is then commonly used in an onsite boiler to heat the incoming sludge to the desired temperature (~37 degrees C) to maximize methane production as well as achieve the Pathogen Reduction Requirements (PRR)/ Vector Attraction Reduction (VAR) associated with Class B Biosolids
Page 52 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. production. If excess methane is produced it can also be used in a generator to produce electricity (co-generation facility) or scrubbed for addition into the natural gas supply system. The amount of methane produced is directly related to the amount of VSS and cBOD that can be produced through the primary clarification process and subsequent the amount of influent VSS and cBOD. The amount of VSS and cBOD that can be produced from primary clarification process at the Davidson Dr. WWTP is poor to marginal for the ultimate production of methane. Generally, the average influent cBOD and TSS should range from 250 mg/L to 350 mg/L to generate a cost benefit associated with methane production. Historical information shows that the average influent cBOD and TSS at the Davidson Dr. WWTP ranges from approximately 150 to 200 mg/L or 65% of the necessary concentration needed to make the process feasible. Digestion processes whether aerobic or anaerobic will inherently produce phosphorus through handling (thickening or dewatering) and endogenous respiration. This phosphorus is then recycled back to the beginning of the process through what is commonly called side streams. Side streams are generated when the bulk biosolids are processed. Historically, these side streams were overlooked and discounted because they appeared to have little impact on the effluent discharge limits. However, with lower discharge permit requirements that now include nutrient removal (phosphorus and nitrogen), side streams have to be handled with more care. The most common and generally the lowest cost strategy is to employ highly efficient processes for thickening and dewatering. Mechanical thickening (i.e., centrifugal) generally has a much higher solids capture rate than gravity thickening. Also, centrifuge dewatering generally achieves a high rate of solids capture during dewatering. Also, the greater the volatile solids reduction that occurs through endogenous respiration during digestion will increase the amount of phosphorus found in side streams during solids handling. These side streams can be a significant source of phosphate that ultimately increases the instantaneous effluent T-P concentrations. To mitigate the amount of phosphorus generated during endogenous respiration, the most effective strategy is to minimize the amount of digestion that occurs during biosolids processing. Other potential solutions, such as side stream treatment, create another treatment system (usually chemical/ physical) that increases capital/ operational costs. These types of side stream systems also can create complications associated with operations because of their operation is intermittent and generate high instantaneous flow rates. Initial model runs (preliminary) were completed, prior to model calibration, to determine the impact of aerobic digestion of biosolids and the associated increases in T-P found within the effluent.
Page 53 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 3.9 – Aerobic Digester vs. Aerated Sludge Storage Value Unit Temperature 20 degrees C Influent Flow1 16 MGD Aerobic Digester Aerated Sludge Storage Supplemental Alkalinity Needed 2342 1004 lbs./ day VSS Reduction 43.8 20 % HRT 41 2.73 days Aeration 5222 2489 SCFM TSS to Dewatering 15107 19359 lbs./ day Washwater Dewatering Sidestream Total P (as-P) 175 73.77 mg/L Total P (as-P) 292 123 lbs./ day Nitrate + Nitrite (as-N) 254 109 mg/L Nitrate + Nitrite (as-N) 424 182 lbs./ day pH 7.29 7.07 Flow 0.2 0.2 MGD Effluent cBOD 2.65 2.66 mg/L TSS 4.42 4.45 mg/L Ammonia (as N) 0.74 0.72 mg/L Nitrate + Nitrite (as-N) 7.17 7.16 mg/L Total P (as-P) 2 2.83 0.9 mg/L Total P (as-P) 2 378 120 lbs./ day pH 6.6 6.55 1 Model runs completed using 2025 Maximum Day influent parameters. 2 Sodium aluminate addition not included in model runs. As shown in the above table, the concentration of T-P increases significantly in the effluent (3x). To accommodate aerobic digestion would require additional tankage, equipment, and much higher operational cost through the consumption of electricity. This would result in a much more expensive biosolids handling process.
Page 54 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Also, another driving force typically for digestion, as discussed, is for achieving the PRR/ VAR associated with Class B Biosolids. In the case Hot Springs, Class B Biosolids production is not necessarily required because their permitted activities are to dispose of the biosolids at the composting facility or at a secured Class 1 Landfill. In the case of composting biosolids, PRR/ VAR requirements are achieved at the composting facility to generate Class A Biosolids (greater PRR than Class B Biosolids) for use by the public. When disposal into a Class 1 Landfill, PRR/ VAR requirements are not necessary because the biosolids is incorporated/ covered in accordance with the landfill permit requirements. There is not currently a reason to achieve Class B Biosolids through digestion. 3.2.3 BIOLOGICAL MODELING CALIBRATION Calibration of the Davidson Dr. WWTP and the Southwest WWTP biological models was completed using data provided using the existing WWTP configurations. Considering the individual variables that can impact real-world performance, variation and deviation from the model was anticipated. Modeling calibration primarily was focused on achieving steady-state analysis while achieving general trends for output data. The model configurations and arrangements are presented in Figure 3.10 and 3.11. The Davidson Dr. WWTP is a continuous-type flow process therefore allowing for the use of steady-state type simulation. The SWWWTP is a batch-type flow process therefore a dynamic simulation must be used.
Page 55 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.10 - Davidson Dr. Calibration Sce
5 of 291 prings, Arkansas | Crist Engineers, Inc. enario Biological Simulator Configuration
Page 56 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.11 - SWWWTP Calibration Scen
6 of 291 prings, Arkansas | Crist Engineers, Inc. nario Biological Simulator Configuration
Page 57 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The data provided by the City of Hot Springs through 2019 was limited and has significant deviations at times. This can mostly be attributed to the high flow events that have occurred during wet weather. Additionally, at times there may not be sufficient amounts of influent alkalinity for the amount of nitrification required and errors were noted within the model. Slight adjustments were made to some of the model parameters for sight specific calibration. The results of the calibration are presented in Table 3.10.
Page 58 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.10 - Davidson Dr WWTP Biolo Parameter 2018 to 2023 MA Permit Limits MA Planning Permit Limits 7-DA Permit Planning Limits Flow Water Temperature cBOD 10 10 15 TSS 15 15 22.5 Ammon April 3.6 4.1 10.1 May to October 3.6 3.6 5.4 November to March 10 10 15 Total P (as-P) 1 0.7 1.1 Nitrate (as-N) Report Report Report Nitrite (as-N) Report Report Report Nitrate + Nitrite (as-N) Report Report Report pH 6 to 9 6 to 9 6 to 9
8 of 291 prings, Arkansas | Crist Engineers, Inc. ogical Simulation Calibration Results Existing 12 12 12 12 12 MGD 10 12.5 15 20 28 Degrees C 2.91 2.58 2.29 2.03 1.89 mg/L 7.09 5.98 4.97 4.24 4.21 mg/L ia (as N) 1.74 1.13 0.81 - - mg/L - - 0.81 0.48 0.26 mg/L 1.74 1.13 0.81 - - mg/L 1.98 1.96 1.98 2.01 2.06 mg/L 8.42 9.08 9.47 9.88 10.21 mg/L 0.62 0.33 0.22 0.13 0.07 mg/L 9.04 9.41 9.69 10.01 10.28 mg/L 6.52 6.51 6.5 6.5 6.52 -
Page 59 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.11 - SWWWTP Biologica Parameter 2020 to 2025 MA Permit Limits MA Planning Permit Limits 7-DA Permit Planning Limits Flow Water Temperature cBOD May to October 5 5 7.5 November to April 10 10 15 TSS 15 15 22.5 Ammonia (as N) April 5.2 5.2 5.2 May to October 2 2 3 November to March 6 6 9 Total Phosphorus (as-P) Report Report Report Nitrate (as-N) Report Report Report Nitrite (as-N) Report Report Report Nitrate + Nitrite (as-N) Report Report Report pH 6 to 9 6 to 9 6 to 9
9 of 291 prings, Arkansas | Crist Engineers, Inc. al Simulation Calibration Results Existing 0.46 0.46 0.46 0.46 0.46 MGD 10 12.5 15 20 28 Degrees C - - 1.00 0.75 0.75 mg/L 1.25 1.25 1.00 - - mg/L 5.25 5.25 5.25 5.25 5.25 mg/L 0.31 0.30 0.28 - - mg/L - - 0.28 0.25 0.25 mg/L 0.31 0.30 0.28 - - mg/L 3.4 3.55 3.66 3.70 3.70 mg/L 16.25 16.75 17 17.5 18.1 mg/L 0.046 0.045 0.04 0.04 0.038 mg/L 13.6 16.8 17.04 17.54 18.14 mg/L 6.45 6.45 6.45 6.45 6.5 -
Page 60 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. It is important to note that the T-P effluent values indicated in the above table do not include sodium aluminate addition. With sodium aluminate addition the effluent T-P values would be reduced further, as demonstrated in the full-scale WWTP. The model calibration, however, was primarily focused on the biological performance of the simulation. Generally, the biological performance of the simulation was within +/- 20% of what has historically been observed, exclusionary of deviations from wet-weather events. The calibration step was important to demonstrate the BioWin simulator can replicate results similar to those observed at the Davidson Dr. WWTP and those observed at SWWWTP. However, as stated, these results should be carefully qualified because of the variables and calculations involved in this type of simulation. It is also important to note that the proposed strategy of the future Davidson Dr. WWTP is to eliminate and modify process components to such an extent that the result fundamentally changes the function of the existing WWTP (i.e., eliminate primary clarification and anaerobic digestion). Therefore, there isn’t any existing real results that can be used to compare against the proposed simulation until after the modifications have been completed. The results of the proposed modeling simulations should also be carefully qualified similarly as the modeling calibration. Alternatively, it is anticipated that minimal process modifications will be necessary at SWWWTP. However, when modeling a sequencing batch reactor (SBR) it is necessary to conduct dynamic simulations. Dynamic simulations require an increased level of interpolation when compared to steady-state simulations. 3.2.4 BIOLOGICAL SIMULATOR BioWin developed by EnviroSim Associated LTD. is to be used for biological modeling activities. BioWin is a simulator that was derived from the Activated Sludge Model No. 1 (ASM1) and the Wentzel et al. model for polyP organisms developed in the early 1990’s. The simulator components utilize macro-mathematic methods to represent complex biological system. Additional mathematical methods were employed that utilize chemical and physical parameters associated with a wastewater treatment plant. When using hand calculations or computer simulators, complex biochemical reactions and associated populations are represented by a set of generalized equations. These equations provide with reasonable accuracy desired inputs/ outputs that can then be used by the user to make engineering judgements and decisions. Kinetic, mass transfer, Monod, liquid separation, filtration, and other principle equation models are incorporated into the simulator. This allows for multiple model calculations to be completed simultaneously and at a much faster rate than the user could calculate via hand calculations. To accommodate model uncertainty and error, it is best for the user to view the outputs as ranges while applying appropriate significant digits. Also, it may be necessary to conduct stress testing or book ending, statistical analysis, Monte-Carlo method exercises to determine the
Page 61 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. sensitivity of various model inputs. The lack of historical data and the significant changes that are likely to occur, modeling activities for this application focused on stress testing/ booking ending approach. 3.2.4.1 DAVIDSON DR. WWTP 3.2.4.1.1 SCENARIO 1 Scenario 1 (including 1A, 1B and 1C) is represented to include the following proposed structural changes/ modifications to the Davidson Dr. WWTP, represented in the simulation, as follow: Scenario 1 (Near-Term Improvements CIP) 1.1 Process elimination of: a. lime feed for supplementary alkalinity; b. primary clarification and production of primary sludge; c. anaerobic digestion 1.2 Addition of magnesium hydroxide for supplemental alkalinity 1.3 Upgrade/ increase airflow rates to aeration basin 1.4 Additional 100 ft secondary clarifier a. Increase total surface 7,854 ft2 (total = 36,970 ft2) 1.5 New tertiary filtration 1.6 WAS directly to existing dewatering (BFP) a. New WAS Pumping 1.7 New dewatered biosolids hauling trailer (x2)/ heavy haul tractor (x1) These modifications are planned to be completed in the near-term. Alternatives considered included maintaining primary clarification and/ or primary filtration. The main challenge with this approach is that they produce primary sludge that then has to be handled. As previously discussed, the handling of primary sludge would require improvements to the anaerobic digestion process which was estimated to be approximately $5 million (in 2015 dollars) in the 2015 report. Additionally, primary clarification is an existing hydraulic limiting process of the WWTP. To achieve reasonable treatment performance, two additional primary clarifiers (similar sized) would need to be added to the existing process to achieve a hydraulic capacity equal to the secondary processes. Each primary clarifier was estimated to be approximately $1.5 to $2 million. Alternative secondary processes that were considered required additional tankage (i.e., A2O process, Bardenpho, extended aeration, etc.) and/ or integrated fixed film activated sludge (IFAS) requires additional capital cost. However, it should be noted that additional tankage and/ or IFAS was considered for expansions beyond the planning period. Chemical addition is critical for the success of the Davidson Dr. WWTP. As discussed in previous sections, sufficient alkalinity is necessary for nitrification (i.e., the treatment of ammonia). The alkalinity needed for nitrification is identified in the following table:
Page 62 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 3.12 - Davidson Dr. Alkalinity Accounting Parameter Value Unit Nitrification Influent Ammonia (Design) 14.4 mg/L Effluent Ammonia (Target) 0.6 mg/L Influent Alkalinity (Design) as CaCO3 125 mg/L Alkalinity Stoichiometrically Needed as CaCO3 7.09 mg alkalinity/ mg ammonia Total Alkalinity Needed as CaCO3 97.84 mg/L Excess - mg/L Alkalinity Deficit as CaCO3 1 97.5 mg/L Total Alkalinity as CaCO3 223 mg/L Total Alkalinity as CaCO3 4.45 mmol/ L 1 Corrected, to minimize amount required for chemical addition.