Page 95 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The results of the modeling Scenario 2C are presented in the above table. These results indicate that at that loading scenario, most effluent parameters are approximately 50% below the permitted 7-DA effluent limitation. At 6.5 degrees C, which is projected to be the minimum November through March design temperature, the ammonia (NH3) concentration does increase to within ~40% of the permitted 7-DA effluent limitation. It is unlikely that the cold influent water temperature as it was projected that one year (1978) in 102 years of data sets would have achieved temperatures approaching those minimums. As shown in the above table, all effluent pH values achieved ~6.5. All Davidson Dr. WWTP modeling simulations for near and mid-term CIP improvements meet the planning MA and 7-DA effluent permit requirements. Interest was placed on investigation of processes beyond the planning period (i.e., MA Influent Flow Rate - Qi >16 MGD, i.e., 24 MGD). Preliminary modeling investigations were conducted on the use of integrated fixed film activated sludge (IFAS) within the existing boundary of the wastewater treatment plant (i.e., using the existing aeration basins, with additional clarification). The permit effluent limits are suspected that they would approach similar levels as identified within the current planning period. IFAS uses synthetic media either fluid suspended in the basin or fixed to allow for a place for biological growth to affix to (i.e., biomass) within an aeration basin. IFAS essentially allows for biomass to concentrate to a higher concentration (i.e., more bugs) to facilitate the treatment of additional loading (cBOD, NH3, etc.) within the same volume of aeration basin. The fixed film will sluff off periodically and combine with the MLSS to settle out in the secondary clarifiers. Biosolids are then wasted to solids processing facilities in a similar manner to conventional activated sludge Generally, the sluffed off MLSS achieves better settling characteristics than conventional activated sludges thus, in the case of Davidson Dr, could achieve a higher peak flow through the secondary process. Careful considerations would need to be given to biological loading to mitigate the impacts of cyclic extremes due to loading that can lead to excessive sluffing of biomass. Overall, this technology appears to be very promising for use at Davidson Dr. WWTP. These types of systems generally work better with lightly loaded influent, high secondary clarifier overflow rates and in instances where a small footprint is required. The downside to these systems is that they generally require the use of coarse bubble diffusion and require the aeration basin to operate at a higher DO which ultimately leads to higher energy consumption.
Page 96 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 3.2.4.1.3 SCENARIO 3 - BEYOND THE PLANNING PERIOD – DAVIDSON DR. WWTP Scenario 3 builds and replaces components from Scenario 1 and 2 through the addition of IFAS, anaerobic selector basins. Presented below is the near-term improvements CIP summary: 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) Scenario 2 (Mid-Term CIP) 2.1 Addition of sodium aluminate to sequester of orthophosphate 2.2 Addition of aluminum chlorohydrate (ACH) to aid in coagulant/ sequester orthophosphate 2.3 New RAS Pumps 2.4 Modify piping to WAS directly to continuous thickening (centrifugal) a. Dewatering unit sized for ~250 GPM 2.5 Thickened WAS conveyed to aerated sludge storage no. 1 2.6 Aerated sludge storage no. 1 a. Conversion of one (1) primary clarifier basin to aerated sludge storage 2.7 Convey from aerated sludge storage no. 1 to aerated sludge storage 2 2.8 Aerated sludge storage no. 2 a. Conversion of gravity thickeners (x2) to aerated sludge storage 2.9 Convey from aerated sludge storage no. 2 to new dewatering(centrifugal) a. Dewatering unit sized for ~150 GPM at 2500 dry-lbs./hr. 2.10 New sidestream treatment system (suspended air flotation) a. Addition of sodium aluminate to sequester orthophosphate Scenario 3 (Beyond Planning Period, Long-Term CIP) 3.1 Add additional headworks processes (physical unit processes) a. Additional screening b. Additional grit removal 3.2 Addition of anaerobic selector basins a. Additional conveyance piping/ splitter/ junction boxes
Page 97 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. b. Conversion of two (2) primary clarifier basin to anaerobic selectors 3.3 Conversion of existing anoxic basin to aerated/ mixed basin a. To be used as a swing basin b. Add new coarse air diffusers 3.4 Internal recycle pumping and conveyance piping a. ~96 MGD capacity (400%Qi) 3.5 Blower and aeration capacity increase up to 27,000 SCFM a. New blowers to accommodate additional aeration requirements b. New coarse bubble diffusers 3.6 IFAS aeration basin a. Addition of IFAS media b. New aeration laterals from new blowers c. Coarse bubble diffusers d. Modify RAS piping, add additional piping to convey RAS to anaerobic selection basins e. New RAS/ WAS pumping station (expand RAS Pumping) 3.7 WAS directly to continuous thickening (centrifugal), add additional thickening a. Each dewatering unit sized for ~250 GPM (total = 500 GPM) 3.8 Additional 100 ft secondary clarifier (x5) a. Increase total surface 39,270 ft2 (total = 76240 ft2) b. Additional conveyance piping/ splitter/ junction boxes 3.9 Additional tertiary filter (x1) a. Increase total tertiary filtration capacity by 48 MGD (total = 96 MGD) 3.10 Thickened WAS conveyed to aerated sludge storage no. 1 a. Increase aeration/ mixing capacity 3.11 Convey from aerated sludge storage no. 1 to aerated sludge storage 2 a. Increase TWAS pumping capacity 3.12 Aerated sludge storage no. 2 a. Increase aeration/ mixing capacity 3.13 Convey from aerated sludge storage no. 2 to new dewatering(centrifugal), add additional dewatering unit a. Dewatering unit sized for ~150 GPM at 2500 dry-lbs./hr. (total = 300 GPM at 5000 dry-lbs./hr. 3.14 New sidestream treatment system (suspended air flotation), add additional unit The proposed Scenario 3 modifications are planned to be completed as long-term CIP project.
Page 98 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.18 - Proposed Davidson Dr. WWTP Simplified Process Flo Term
8 of 291 prings, Arkansas | Crist Engineers, Inc. ow Diagram – Liquid Treatment – Beyond Planning Period (Longm CIP)
Page 99 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.19 - Proposed Davidson Dr. WWTP B
9 of 291 prings, Arkansas | Crist Engineers, Inc. Beyond the Planning Period – 24 MGD – IFAS
Page 100 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Scenarios 3A is also represented to include the following operational changes/ modifications to the Davidson Dr. WWTP, represented in the simulation as follows (Influent Flow Rate - Qi = 24 MGD) with the 2040 DA design influent parameters: 3A.1 Total Influent Alkalinity (magnesium hydroxide) a. Aerated swing basin i. Alkalinity = 162.5 mg/L as CaCO3 b. Unaerated swing basin (i.e., when denitrification occurring) i. Alkalinity = 125 mg/L as CaCO3 3A.2 Orthophosphate Sequester (sodium aluminate; influent/ aeration basin) a. Sodium aluminate (aluminum) = 0 lbs./ day (as Al+3) 3A.3 Coagulant (ACH; secondary clarifier/ tertiary filter) a. ACH (aluminum) = 0 lbs./ day (as Al+3) 3A.4 Return Activated Sludge (RAS) rate: a. QRAS = 0.43Qi to 0.45Qi b. QRAS Target = 0.44Qi 3A.5 Waste Activated Sludge (WAS) rate: a. QWAS = 0.0106Qi to 0.015Qi b. QWAS Target = 0.0112Qi 3A.6 Sludge Retention Time (SRT) a. SRT = 6 to 13.5 days b. SRT Target = 9.5 days 3A.7 Swing Basin Mixed Liquor Volatile Suspended Solids (MLVSS) a. MLSS = 2,900 to 3,100 mg/L b. MLSS Target = 3,000 mg/L 3A.8 Swing Basin Residual Dissolved Oxygen (DO) a. DO = 2 to 3 mg/L (water temperature <20 degrees C) b. DO = 0 mg/L (water temperature >20 degrees C) i. Internal Recycle = 4Qi 3A.9 Swing Basin Residual Dissolved Oxygen (DO) a. Airflow = 2,900 to 3,100 SCFM 3A.10 IFAS Aeration Basin Residual Dissolved Oxygen (DO) a. DO = 4 mg/L (water temperature <15.1 degrees C) b. DO = 3 mg/L (water temperature >15.1 degrees C) 3A.11 Mixed Liquor Suspended Solids (MLSS) to Secondary Clarifier a. MLSS = 4,500 to 4,750 mg/L b. MLSS Target = 4,600 mg/L 3A.12 IFAS Aeration Basin Airflow Rate a. Airflow = 5,800 to 13,800 SCFM 3A.13 Thickener Target Performance a. Total Solids = 3 to 3.5% b. Solids Capture = 95% 3A.14 Aerated Sludge Storage No. 1 Residual DO a. DO Target = 2.0 mg/L
Page 101 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. b. Supplemental Alkalinity = 1271 lbs./ day as CaCO3 3A.15 Aerated Sludge Storage No. 2 Residual DO a. DO Target = 2.0 mg/L 3A.16 Dewatering Performance a. Total Solids = 22 to 24% b. Solids Capture = 95% 3A.17 Sidestream Treatment Performance a. Total Solids = 2.5% b. Sodium Aluminate (aluminum) = 0 lbs./ day (as Al+3) c. Solids Capture = 96% An important consideration is that Scenario 3A modeling scenario/ operation includes dewatering operation, planned for 3 days per week. Table 3.29 – Davidson Dr. WWTP Projected Effluent Quality – Beyond the Planning Period – 24 MGD – IFAS (Operational Dewatering) Parameter 2018 to 2023 MA Permit Limits MA Planning Permit Limits 7-DA Permit Planning Limits Average Parameter Deviation from Limit (%) Minimum Parameter Deviation from Limit (%) Flow 24 24 24 MGD Water Temperature 10 15.1 20 Degrees C cBOD 10 10 15 2.69 2.37 2.49 mg/L -74.8% -73.1% TSS 15 15 22.5 3.47 3.02 3.01 mg/L -78.9% -76.9% Ammonia (as N) April 3.6 4.1 10.1 - 1.30 - mg/L -68.3% -68.3% May to October 3.6 3.6 5.4 - 1.30 1.06 mg/L -67.2% -63.9% November to March 10 10 15 3.45 - - mg/L -65.5% -65.5% Total P (as-P) 1 0.7 1.1 0.292 0.322 0.412 mg/L -51.4% -41.4% Nitrate (as-N) Report Report Report 3.96 5.44 1.731 mg/L - - Nitrite (as-N) Report Report Report 0.45 0.37 0.271 mg/L - - Nitrate + Nitrite (asN) Report Report Report 4.41 5.81 21 mg/L - - pH 6 to 9 6 to 9 6 to 9 6.64 6.59 6.49 - -9.6% -8.2% 1 Denitrification associated with swing basin. 2 Modeling does not and cannot completely account for dewatering days. Based on mass balance, it is suspected that T-P values could be under reported by approximately 35%. Therefore, multiply effluent T-P value by 1.35 to obtain a closer estimation of effluent T-P values during operational dewatering.
Page 102 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. It is recommended that the preliminary modeling analysis for Scenario 3 be re-visited after completion of the near and mid-term CIP improvements. This will allow for the model to be recalibrated prior to future determination of this Scenario. It is also recommended that peak loading scenarios and alternative temperatures be revisited with future analysis. 3.2.4.2 SOUTHWEST (SW) WWTP 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: 3.2.4.2.1 SCENARIO 1 Scenario 1 (Near-Term Improvements CIP) 1.1 Process elimination of: a. lime feed for supplementary alkalinity; 1.2 Addition of magnesium hydroxide for supplemental alkalinity 1.3 Modifications to program/ control strategy to accommodate anaerobic and anoxic sequences These modifications are planned to be completed in the near-term. Alternatives considered included conveying wastewater from SWWWTP to Davidson Dr. WWTP through the existing collection system. This was ultimately not further evaluated because of the sunk asset cost of the SWWWTP. This alternative can be revisited when SWWWTP needs major process upgrades or a capacity expansion. Also, the needs at Davidson Dr. WWTP in the near and mid-terms far outweigh any potential improvements at SWWWTP. SWWWTP can however be modified to optimize treatment through slight changes to the process. These changes do not require additional tankage or equipment but can be performed through modifications to the control strategies. A presentation was given on the subject to CHS staff in May 2020. Currently, the main issues at SWWWTP are ammonia, alkalinity and pH. Generally, the amount of alkalinity incoming into the wastewater treatment plant isn’t sufficient for nitrification, therefore supplemental alkalinity is needed.
Page 103 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 3.30 - SWWWTP Alkalinity Accounting Parameter Value Unit Nitrification Influent Ammonia (Design) 27.43 mg/L Effluent Ammonia (Target) 0 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 194.48 mg/L Excess1 30.00 mg/L Alkalinity Deficit as CaCO3 100 mg/L Total Alkalinity as CaCO3 225 mg/L Total Alkalinity as CaCO3 4.50 mmol/ L Denitrification Nitrate Available 26.43 mg/L Alkalinity Stoichiometrically Gained 3.57 mg alkalinity/ mg nitrate Effluent Nitrate 10 mg alkalinity/ mg nitrate Total Alkalinity Gained as CaCO3 58 mg/L Net (Nitrification - Denitrification) Needed Alkalinity 42 mg/L Needed Alkalinity Reduction 58% - Total Alkalinity as CaCO3 167 mg/L Total Alkalinity as CaCO3 3.34 mmol/ L 1 Excess is provided to ensure complete nitrification, appears to be greater variation in influent ammonia concentrations at SWWWTP than at Davidson Dr. WWTP. However, as with the Davidson Dr. WWTP, supplemental alkalinity is critical to manage to obtain proper ammonia removal. SWWWTP has increased hydraulic retention time (HRT), i.e., influent flow/ total tank volume which allows for consideration of an additional process step (denitrification). Utilizing this denitrification step, when the water temperature is in excess of 15.1 degrees C can reduce the needed amount of supplemental alkalinity needed. As described in the above table, the net reduction of needed chemical is ~58%.
Page 104 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.20 - Proposed SWWWTP Simplified Process Figure 3.21 - Proposed SWWWTP Simplified Process F
4 of 291 prings, Arkansas | Crist Engineers, Inc. s Flow Diagram – Liquid Treatment (Near-Term CIP) Flow Diagram – Biosolids Treatment (Near-Term CIP)
Page 105 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.22 - SWWWTP Scenario 1
5 of 291 prings, Arkansas | Crist Engineers, Inc. 1A, 1B, 1C Simulator Configuration
Page 106 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Scenarios 1 (A, B, C) is also represented to include the following operational changes/ modifications to the SWWWTP, represented in the simulation as follows: Scenario 1A Influent Flow Rate - Qi = 0.74 MGD AD Design Influent Loading Conditions Scenario 1B Influent Flow Rate - Qi = 0.74 MGD 7-DA Design Influent Loading Conditions Scenario 1C Influent Flow Rate - Qi = 0.925 MGD AD Design Influent Loading Conditions 1.1 Total Influent Alkalinity (magnesium hydroxide) a. Alkalinity = 225 mg/L as CaCO3 (water temperature < 15.1 degrees C) b. Alkalinity = 167 mg/L as CaCO3 (water temperature > degrees 15.1 C) 1.2 Sequencing Batch Reactor (Total Sequence = 12 hrs.): a. Fill Sequence i. Duration = 3 to 3.2 hrs. b. Anaerobic Sequence (Fill/ Anaerobic Concurrent) i. Duration = 3 to 3.5 hrs. c. Aerobic Sequence i. Duration = 3.25 to 5.75 hrs. ii. DO = 2 mg/L d. Anoxic Sequence i. Duration = 0 to 2 hrs. e. Aerobic Sequence i. Duration = 0 to 0.75 hrs. ii. DO = 2 mg/L f. Settle/ Decant Sequence i. Duration = 2.75 to 3.75 hrs. g. WAS Sequence (Decant/ WAS Concurrent) i. Duration = 1.5 hrs. ii. 20 to 30 GPM h. Sludge Retention Time (SRT) i. SRT = 10 to 15 days ii. SRT Target = 11 days (for T-P < 2 mg/L, water temperature > 15.1 degrees C) i. Mixed Liquor Volatile Suspended Solids (MLVSS)
Page 107 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. i. MVLSS = 2,000 to 2,750 mg/L ii. MVLSS Target = 2,300 mg/L 1.3 Aerobic Digester a. DO Target = 2.0 mg/L b. Supplemental Alkalinity = 40 lbs./ day as CaCO3 An important consideration to note is Scenario 1 (A, B, C) includes 7 day per week (52 days per year) hauling of liquid sludge included in the analysis. This approach was done to simplify the model. However, in practice, hauling sequence from onsite sludge storage could occur (based on volume) ~ 1 day per week. Each hauling sequence day would haul/ convey approximately 12,000 gallons at ~3% solids (or three (3) 4000-gallon truck loads). To alleviate conveying SWWWTP biosolids through the collection system, it is recommended that the ~12,000 gallons per week of biosolids be conveyed directly into the aerated sludge storage no. 1, located at Davidson Dr. WWTP. When the biosolids handling project is completed (Mid-Term) at Davidson Dr. WWTP it will include a receiving station for biosolids hauled as a liquid from SWWWTP. It should be noted that when hauling activities occur, it is anticipated that approximately 189 lbs. of T-P with 118 lbs. (~62%) being orthophosphate will be delivered to aerated sludge storage no. 1. To alleviate the concern of an instantaneous spike of orthophosphate in Davidson Dr. WWTP biosolids processing it is recommended that sodium aluminate be fed into the receiving station when hauling activities are occurring. Figure 3.23 - Proposed SWWWTP Simplified Process Flow Diagram – Biosolids Treatment (Mid-Term CIP)
Page 10 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.31 – SWWWTP Projecte Parameter 2020 to 2025 MA Permit Limits MA Planning Permit Limits 7-DA Permit Planning Limits 2040 AD 2040 Flow 16 16 16 Water Temperature 6.5 7 10 cBOD November to April 5 5 7.5 - - - May to October 10 10 15 1.6 1.6 1.4 TSS 15 15 22.5 5.75 6.4 7 Ammonia (as N) April 5.2 5.2 5.2 - - - May to October 2 2 3 - - - November to March 6 6 9 0.6 0.6 0.6 Total P (as-P) Report Report Report 0.4 0.79 1.9 Nitrate (as-N) Report Report Report 1.66 2.7 5.25 Nitrite (as-N) Report Report Report 3.31 2.41 0.06 Nitrate + Nitrite (asN) Report Report Report 4.96 5.1 5.31 pH 6 to 9 6 to 9 6 to 9 6.6 6.6 6.6 1 WET testing may be impacted when concentrations of nitrite exceed 0.5 m
8 of 291 prings, Arkansas | Crist Engineers, Inc. d Effluent Quality – Scenario 1A Scenario 1A Design Flow Rate (QAD) = 0.74 MGD 0 AD Design Influent Parameters Average Parameter Deviation from Limit (%) Minimum Parameter Deviation from Limit (%) 16 16 16 16 16 MGD 13.3 13.8 15.1 20 28 Degrees C - - 1.4 1.4 1.2 mg/L -73.3% -72.0% 1.9 1.6 1.4 1.4 1.2 mg/L -84.9% -81.0% 12 9.75 7.4 7.4 6.6 mg/L -48.1% -20.0% 0.6 0.5 0.45 - - mg/L -90.1% -88.5% - - 0.45 0.45 0.4 mg/L -78.3% -77.5% 0.6 0.5 - - - mg/L -90.3% -90.0% 1.6 1.1 0.78 1.2 1.4 mg/L - - 5 3.2 3.2 3.11 3.4 3.7 mg/L - - 6 0.06 0.06 0.07 0.06 0.05 mg/L - - 1 3.26 3.26 3.18 3.46 3.75 mg/L - - 6.65 6.57 6.4 6.4 6.45 - -8.9% -6.7% mg/L.
Page 109 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The results of the modeling Scenario 1A are presented in the above table. These results indicate that at that loading scenario, most effluent parameters are well below the permitted monthly average (MA) effluent limitation. The effluent pH is also approximately within 10% of the permitted MA effluent limitation but generally, with the capability to add supplemental alkalinity, the effluent pH is unlikely to be less than 6.5.
Page 110 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.32 – SWWWTP Projecte Parameter 2020 to 2025 MA Permit Limits MA Planning Permit Limits 7-DA Permit Planning Limits 2040 AD 2040 Flow 0.74 0.74 0.74 Water Temperature 6.5 7 10 cBOD November to April 5 5 7.5 - - - May to October 10 10 15 2.60 2.40 2.20 TSS 15 15 22.5 9.75 9.75 9.50 Ammonia (as N) April 5.2 5.2 5.2 - - - May to October 2 2 3 - - - November to March 6 6 9 4.00 2.75 0.65 Total P (as-P) Report Report Report 0.50 0.40 0.50 Nitrate (as-N) Report Report Report 0.24 1.05 2.91 Nitrite (as-N) Report Report Report 3.201 2.701 1.561 Nitrate + Nitrite (asN) Report Report Report 3.44 3.75 4.47 pH 6 to 9 6 to 9 6 to 9 6.70 6.70 6.70 1 WET testing may be impacted when concentrations of nitrite exceed 0.5 mg/L
0 of 291 prings, Arkansas | Crist Engineers, Inc. d Effluent Quality – Scenario 1B Scenario 1B Design Flow Rate (QAD) = 0.74 MGD 7-DA Design Influent Parameters Average Parameter Deviation from Limit (%) Minimum Parameter Deviation from Limit (%) 0.74 0.74 0.74 0.74 0.74 MGD 13.3 13.8 15.1 20 28 Degrees C - - 2.10 1.80 1.60 mg/L -75.6% -72.0% 2.20 2.10 2.10 1.80 1.60 mg/L -85.8% -82.7% 10.75 10.50 8.50 7.75 7.50 mg/L -58.9% -52.2% 0.51 0.60 0.50 - - mg/L -89.7% -88.5% - - 0.50 0.45 0.40 mg/L -85.0% -83.3% 0.51 0.60 - - - mg/L -81.1% -55.6% 1.20 1.30 0.40 0.38 0.35 mg/L - - 4.80 4.81 1.25 2.22 1.60 mg/L - - 0.07 0.07 1.251 0.05 0.06 mg/L - - 4.87 4.88 2.50 2.27 1.66 mg/L - - 6.70 6.70 6.50 6.60 6.65 - -10.9% -8.3% L.
Page 111 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The results of the modeling Scenario 1B are presented in the above table. These results indicate that at that loading scenario, most effluent parameters are well below the permitted 7- DA effluent limitation. The effluent pH is also approximately within 10% of the permitted 7-DA effluent limitation but generally, with the capability to add supplemental alkalinity, the effluent pH is unlikely to be less than 6.5.
Page 112 Wastewater System Master Plan (WWSMP) – Hot Sp Table 3.33 – SWWWTP Projecte Parameter 2020 to 2025 MA Permit Limits MA Planning Permit Limits 7-DA Permit Planning Limits 2040 7-DA 204 Flow 0.925 0.925 0.925 Water Temperature 6.5 7 10 cBOD November to April 5 5 7.5 - - - May to October 10 10 15 2.75 2.75 2.75 TSS 15 15 22.5 10.00 10.25 9.75 Ammonia (as N) April 5.2 5.2 5.2 - - - May to October 2 2 3 - - - November to March 6 6 9 1.40 0.90 0.60 Total P (as-P) Report Report Report 0.30 0.30 0.30 Nitrate (as-N) Report Report Report 4.30 4.20 4.50 Nitrite (as-N) Report Report Report 1.901 2.101 1.801 Nitrate + Nitrite (asN) Report Report Report 6.20 6.30 6.30 pH 6 to 9 6 to 9 6 to 9 6.60 6.60 6.60 1 WET testing may be impacted when concentrations of nitrite exceed 0.5 mg/L.
2 of 291 prings, Arkansas | Crist Engineers, Inc. d Effluent Quality – Scenario 1C Scenario 1C Design Flow Rate (Q7-DA) = 0.925 MGD 40 AD Design Influent Parameters Average Parameter Deviation from Limit (%) Minimum Parameter Deviation from Limit (%) 0.925 0.925 0.925 0.925 0.925 MGD 13.3 13.8 15.1 20 28 Degrees C - - 2.75 2.60 2.50 mg/L -65.1% -63.3% 3.00 3.00 2.75 2.60 2.50 mg/L -81.6% -80.0% 11.75 12.00 11.75 11.25 11.50 mg/L -51.0% -46.7% 0.55 0.60 0.50 - - mg/L -89.4% -88.5% - - 0.50 0.40 0.40 mg/L -85.6% -83.3% 0.55 0.60 - - - mg/L -91.0% -84.4% 0.40 0.50 0.50 0.70 0.60 mg/L - - 2.20 2.20 2.90 4.50 0.30 mg/L - - 1.751 1.751 1.201 0.08 0.02 mg/L - - 3.95 3.95 4.10 4.58 0.32 mg/L - - 6.60 6.60 6.60 6.60 6.70 - -10.2% -10.0%
Page 113 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The results of the modeling Scenario 1C are presented in the above table. These results indicate that at that loading scenario, most effluent parameters are well below the permitted 7- DA effluent limitation. The effluent parameters would also meet the permitted MA effluent limitation under this scenario. To account for the increased influent ammonia loading mass of this scenario (25% increase), the magnesium hydroxide feed mass was correspondingly increased thus ensuring an effluent pH more than 6.5. 3.2.5 HYDRAULIC SIMULATOR Visual Hydraulics by Innovative Hydraulics is a software that uses adopted existing open channel flow equations to establish gradients within complex hydraulic system. This is accomplished through downstream to upstream water surface calculations through structural elements (i.e., weirs, pipes, orifices, fittings, et al.). The model can also perform more complex iterations to determine flow splitting and management of flow paths through overflows or flow control. As with the biological simulator, these developed water surfaces can then be used by the user to make engineering judgements and decisions. This allows for multiple model calculations to be completed simultaneously and at a much faster rate than the user could calculate via hand calculations. Checks are randomly completed using hand calculations to minimize the potential for error to develop through a simulation. 3.2.5.1 HYDRAULIC MODELING CALIBRATION During model calibrations most of the calibration activities focused on field measured elevations vs. published as-constructed water surface elevations. Actual weir and critical structure elevations were verified with field surveying activities and modifications were completed as they related to the structural element found downstream. The acceptable range of results to the published water surface elevations contained within the hydraulic profile were within 1 ft of modeled elevation. Information about the existing plant was gathered through records drawings of the original construction as well as record drawings of improvements to different plant components. A flow of 40 million gallons per day (MGD) was used in the Davidson Dr. WWTP simulation (1992 hydraulic capacity). Since there had been additional improvements to the headworks and disinfection systems after 1992, an updated hydraulic profile of the treatment plant needed to be constructed so that the model could give an accurate representation of current conditions. Using a compilation of notes and updates, a new hydraulic profile was created that details the quantity and size of each process in the treatment plant. The new profile was drawn out on paper first to ensure accuracy and then drawn in AutoCAD for reference. Once the updated hydraulic profile was agreed upon, the model was built in the Visual Hydraulics software. This software creates hydraulic profiles by using a selected control point and calculating upstream of that point. Full pipe flow, various weirs, open channel flow and other components are all options of inputs in this software. Given the flow, size, friction coefficients and potential losses through bends or fittings, total head loss is calculated for that
Page 114 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. section. This head loss is cumulative from the control point, generally a starting water elevation, to either the next control point or the beginning of the plant. Since this software’s primary function is simulating head loss through a plant given a certain flow, the different plant processes are represented in a simple structural form, rather than fully designing the specifics of every process. Focus was given to the Davidson Dr. WWTP because of the proposed hydraulic improvements needed to minimize the risk of overflow from the flow equalization basin and/ or conveyance of excess flow through secondary process leading to biological challenges. Limited stress testing or field water surface verifications were completed with this exercise. The reason is that at this time the Davidson Dr. WWTP has limited ability to measure influent, side stream flow rates as well as elevated effluent flow rates. However, it is recommended that the hydraulic model be revisited after improvements to confirm water surface elevations. SWWWTP did not receive the same level of analysis because there are limited hydraulic improvements anticipated at this WWTP. This WWTP has an existing capacity (4.25 MGD) is approximately 9x of their existing (0.46 MGD) average daily flow rate. However, when compared to the planned design average daily flow rate (0.85 MGD) this ratio decreases to 5x. When compared to Davidson Dr. WWTP the existing capacity (54 MGD) is approximately 4x of their existing (12 MGD) average daily flow rate. The modeling simulators become complex quickly. Therefore, to view the entire model within this document it becomes challenging and requires it to be subdivided into three portions. 3.2.5.2 DAVIDSON DR. WWTP The model begins with the 48-inch pipe conveying raw sewage to the screens. It should be noted here that at the plant there is a weir wall here at elevation 386; the water that overtops this weir due to excessive flow is conveyed to the equalization pond. From the 48-inch raw sewage pipe, flow is split between two screens and conveyed to a 7 ft x7 ft concrete channel and conveyed to the grit chambers. As previously mentioned, this software takes a simplistic structural approach to model and simulate the water surface elevation throughout the plant. The two (2) grit chambers are simply modeled as weirs at elevation 382.50, as this is the exiting weir elevation of these structures. Flow is combined and conveyed by a 5 ft x 6.5 ft concrete channel to a 42-inch pipe where it is distributed from a box by three (3) 30-inch pipes to the primary clarifiers. These primary clarifiers are modeled as weirs with the elevation at 378.33. After topping the weirs, 30-inch pipes convey the water back to the distribution box. From the box, a 42-inch pipe takes flow to a junction box, represented by a manhole in the software, where a 54-inch pipe then takes the primary effluent to the aeration basins. A 4 ft x 24.2 ft channel helps to distribute flow evenly to each of the three (3) anoxic zones. A weir with an elevation of 376.3 splits water into these zones. The anoxic and aerobic zones are both represented as 40 ft x 24.2 ft deep channels. The anoxic zone being 30 ft long and the aerobic zone being 150 ft long. Flow is combined into single channel where it overtops a first weir at elevation 375.65, and then a second weir at elevation 374.9 that divides flow evenly to
Page 115 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. be conveyed to the secondary clarifiers. Return activated sludge (RAS) flow (at approximately 40% of the influent flow) is brought back into the aeration tanks as a sidestream flow. MLSS is then conveyed through 30 inch and 24-inch pipes to the secondary clarifiers. The size reduction, as well as any of significant bends or fittings are considered in this software. The secondary clarifiers are modeled as weirs at elevation 372.05. Flow is combined to a junction box and then conveyed by a 48-inch pipe to the tertiary filters. The tertiary filters are represented through media shape, depth, porosity, and particle size, along with the filtered surface area. Filter effluent flows into a sump with an exit weir is at elevation 346.74. A 36-inch pipe then conveys filter effluent to a junction box and onto another 36-inch junction at the beginning of the UV disinfection building. Two (2) 3 ft x3 ft openings at elevation 330.00 divide the effluent into two (2) channels. The first section of the channel is 5.12 ft wide and 11.48 ft deep with an 82.5 ft tank length which then transitions into 7.66 ft deep channels where filter effluent is exposed to the UV light for disinfection. Disinfected wastewater then spills over a level control weir at elevation 334.00. Wastewater effluent flow is then measured using a 4 ft wide Parshall flume. After exiting the flume, wastewater effluent drops into a junction box where it is conveyance to Lake Catherine through a 36-inch outfall pipe.
Page 116 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.24 - Davidson Dr. Calibration Scenario H
6 of 291 prings, Arkansas | Crist Engineers, Inc. Hydraulic Simulator Configuration – Upper Reach
Page 117 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.25 - Davidson Dr. Calibration Scenario H
7 of 291 prings, Arkansas | Crist Engineers, Inc. ydraulic Simulator Configuration – Middle Reach
Page 11 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.26 - Davidson Dr. Calibration Scenario H
8 of 291 prings, Arkansas | Crist Engineers, Inc. Hydraulic Simulator Configuration – Lower Reach
Page 119 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 3.2.5.3 SWWWTP The SWWWTP simulation was developed and evaluated using plans from 2005 with a process design flow of 0.85 MGD. A 16-inch line transports raw sewage to/ through each screen. Another 16-inch line then takes the screened wastewater to each SBR tank(s). Each tank is modeled as an open channel with the dimensions 106 ft long by 50 ft wide and 22.50 ft deep, with a volume of 881,705 gallons ea. Flow was split equally between the SBR tanks. A 24-inch pipe takes this effluent to a 16-inch pipe and then run through the tertiary filter. Tertiary filter effluent is transported by a 16-inch pipe to a step aeration. This process is modeled as a 4 ft weir where the filter effluent over tops and begins to cascade down the stair, and then a channel with the downstream invert elevation being 534 to simulate the stair step. A 24-inch pipe transports the water into a 4 ft x 4 ft channel at elevation 529. The water level raises to 531.18 and goes through the 2 ft x 31 ft UV channel. The disinfected water drops into a 4 ft x 4 ft channel at elevation 527.00. Each of these are modeled as open channel with the specific dimensions and elevations. After the drop, a 24-inch pipe transports the water to a 3 ft x 4 ft channel at elevation 527.00. The water level raises to 529.50 where it flows through a 1.50 ft Parshall flume. Then a 24-inch pipe conveys water to a final step aeration, modeled as an open channel with the downstream elevation at 509.00. Finally, a 24-inch outfall pipe takes water to Little Mazarn Creek.
Page 120 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.27 – SWWWTP Calibration Scenario Hy Figure 3.28 - SWWWTP Calibration Scenario Hyd
0 of 291 prings, Arkansas | Crist Engineers, Inc. ydraulic Simulator Configuration – Upper Reach draulic Simulator Configuration – Middle Reach 1
Page 12 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.29 - SWWWTP Calibration Scenario Hyd Figure 3.30 - SWWWTP Calibration Scenario Hy
1 of 291 prings, Arkansas | Crist Engineers, Inc. draulic Simulator Configuration – Middle Reach 2 ydraulic Simulator Configuration – Lower Reach
Page 122 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 3.2.5.4 DAVIDSON DR. WWTP HYDRAULIC MODELING RESULTS Within the software, an operating condition description can be selected for each component. This will notify the user if the component is under normal condition or if it has been compromised in some manner. The first warning is at the primary clarifiers modeled as weirs. With 40 MGD flowing through the system, each of the weirs is seeing 33.3% (13.3 MGD) flow over it. The warning is that the downstream water elevation is higher than the head over the weir under free flow conditions. The weir elevation is 378.33, while the water elevation over the weir is 378.99 and the downstream water elevation is 378.97. The primary clarifier weirs are effectively submerged at this point and not able to operate correctly. Through discussions with CHS staff this scenario does occur during high flow events. It was initially observed that the primary clarifiers are one of the main hydraulic constraints observed within the Davidson Dr. WWTP. The next warning is at the tertiary filters whereas the calculated head loss through the filter is greater than ~ 2 ft (2.34 ft). It should also be noted that the original plans dictated that water levels flowing through the filters would be controlled at 366.00. It should also be noted that multiple, back-to-back storm event can fill up the flow equalization basin (FEB) risking overtopping. The hydraulic simulator does not represent the FEB and/ or the volume of raw wastewater that can be stored within its unit process. The hydraulic simulator evaluates flow as it travels through the unit processes with associated water elevations. The hydraulic simulator also does not address decision making as it relates to flow diversion to the FEB and/ or the corresponding decision as to when to bring raw wastewater stored in the FEB back to into the Davidson Dr. WWTP. Historically, when the volume gets to a certain elevation within the FEB, raw wastewater will flow by gravity back from the FEB and then be conveyed around headworks, grit removal, secondary and tertiary treatment processes (allowed by permit). This influent wastewater flow does pass through the FEB which provides settling that essentially acts as a very large primary clarifier. Concurrently, influent flow is conveyed through the headworks and secondary, tertiary treatment processes. These two wastewater streams aggregate in the UV disinfection structure where they are disinfected, passed through flow measurement, and conveyed to the outfall. This approach has historically been a successful as long as there is sufficient time period between storm events to allow for the WWTP to process enough volume from the FEB to accommodate the next storm event induced incoming flow. When there isn’t enough time between storm events however, the maximum that can be processed through the existing Davidson Dr. WWTP is approximately 54 MGD. The main hydraulic limiting factor for that maximum flow rate through the existing WWTP is the outfall pipe. To alleviate this hydraulic limiting factor, it is recommended that the outfall pipe be replaced and upsized from a 36 inch to a 54-inch pipe. A 54-inch pipe would increase the outfall capability to 96 MGD.
Page 123 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. However, in the near-term, it is recommended that the Davidson Dr. WWTP be capable of processing ~24 MGD through secondary processes and 48 MGD through screening, grit removal and tertiary filter. The aggregated flow will be disinfected through UV disinfection, flow measured and discharged. This approach would increase the total overall processing capacity from 54 MGD to 72 MGD. In the near-term, to accomplish the processing of 24 MGD through secondary treatment, primary clarification will be eliminated thus eliminating a hydraulic bottleneck. Additionally, it should be noted that during periods where flows exceeded 16 to 20 MGD, the primary clarifier’s removal performance is greatly diminished. This hydraulic approach allows for wastewater in the near-term, to be conveyed directly from screening/ grit removal to secondary treatment (i.e., anoxic, aeration and secondary clarifiers). Conveyance piping of 48 inch would be used to convey wastewater from screening/ grit removal to flow control box or commonly called homeplate. Figure 3.31 presents an overall view of the total processes with corresponding conveyance piping. The overall view is subdivided into areas so as to permit a more detailed view of the processes and conveyance piping. Figure 3.32 is a presentation of area 1, which schematically identified headworks and primary treatment.
Page 124 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.31 - Davidson Dr. Existing and Proposed Simplified
4 of 291 prings, Arkansas | Crist Engineers, Inc. Process Flow Diagram – Hydraulic Improvements – Overall
Page 125 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.32 - Davidson Dr. Existing and Proposed Simplified
5 of 291 prings, Arkansas | Crist Engineers, Inc. d Process Flow Diagram – Hydraulic Improvements – Area 1
Page 126 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. As shown in the above figure, it is recommended that influent pass through screening and grit removal before being conveyed to the FEB. This will allow influent flow to be processed during normal high flow events through the peak flow line while still maintaining the storage capability within the FEB. This approach will allow for operations to better manage multiple storm events that thus decreasing the risk of over toping the FEB as well as eliminating the hydraulic bottleneck presented by the primary clarifiers (as previously discussed). There is also a biological function to this approach through reducing the duration and volume of stored raw wastewater will in turn reduce the production of ammonia associated with anaerobic reactions within the FEB. The FEB will still serve a vital function that it will receive the first or initial flush from storm events where the cBOD and ammonia is elevated to a sufficient level where peak flow conveyance around secondary processes will cause a permit effluent excursion. Use of the FEB during the vast majority of the rain events (exception very large storms) will serve more of a water quality function rather than the water quantity function. Initially influent flow above ~20 MGD will pass through screening and grit removal to the peak flow junction box where it will be diverted to the FEB. This initial FEB diversion flow is only anticipated to occur for ~ 2 hours and estimated to be approximately 2 to 4 MG of total flow. As soon as the anticipated aggregated flow based on the influent quality and processed secondary quality will maintain permit compliance it is to be diverted from the FEB to the tertiary filter. Once the peak flow conveyance occurs, the flow to the secondary clarifiers can be increased from ~ 20 to ~ 24 MGD (25.8 MGD). After the initial influent flow diversion, periods where up to 48 MGD of raw wastewater would be processed through screening, grit removal, by which ~24 MGD (25.8 MGD) would be processed through secondary treatment. The remaining difference of up to ~24 MGD (22.2 MGD) would be conveyed directly to tertiary treatment where it would be aggregated with ~24 MGD (25.8 MGD) of secondary clarified effluent. The total 48 MGD would then be processed through tertiary filtration, UV disinfection, flow measurement and conveyed to the outfall. It should be noted that the new tertiary filtration is proposed in the near-term to be constructed using cloth media filtration technology with a total processing capacity of 24 MGD each for a total processing capacity of 48 MGD. Also, UV disinfection process is proposed in the near-term to be increased to a total processing capacity of 75 MGD. Alternatively, during periods where the FEB water level is in an elevated position, an additional ~24 MGD (25.8 MGD) worth of FEB volume (i.e., primary settled wastewater) could be conveyed to the peak flow junction box where it would be aggregated with an additional 48 MGD screened/ de-gritted raw wastewater. At this point, 48 MGD would be conveyed to the tertiary filters and ~24 MGD (25.8 MGD) would be conveyed to secondary treatment. The ~24 MGD (25.8 MGD) of clarified secondary effluent would then be conveyed to UV disinfection, without passing through tertiary filtration where it would aggregate with 48 MGD of tertiary filtration effluent. The total ~72 MGD (73.8 MGD) would then aggregate in the UV disinfection structure (total proposed near-term capacity is 75 MGD) where they would then be disinfected, passed through flow measurement, and conveyed to the outfall. In the long-term, a total of 94 MGD of influent wastewater would directly be processed through screening and grit removal. This will be accomplished through the addition of 2 – 24 MGD
Page 127 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. screens and 2 – 20 MGD grit removal processing units. It should be noted that the existing grit removal system is sized for 20 MGD each with 95% removal of >105-micron particles. When the flow exceeds 20 MGD each, the removal performance decreases slightly but does not impede flow. Future unit may or may not also be sized in a similar manner but will have the hydraulic conveyance capacity of at least 96 MGD. Once the headworks is sized to accept the design storm event (94 MGD), all the flow can first be processed through screening and grit removal before being conveyed to the peak flow junction box where flow can then be conveyed to secondary and/ or to the FEB. This approach will minimize the amount of debris that is sent to the FEB during normal and abnormal high flow events. Alternatively, flow can then be split at the peak flow junction box where ~48 MGD (53.3 MGD) is conveyed to secondary treatment and ~48 MGD (41.2 MGD) is then conveyed to the tertiary filter junction box. After the high flow event, the initial (~2 to 4 MG) and/ or excess flow (influent flow rate above 48 MGD) influent flows captured within the FEB would be pumped back to the headworks of the wastewater treatment plant where it would be aggregated with average day influent flows for processing through secondary. As presented in Figure 3.33, screened and de-gritted wastewater is conveyed to secondary processes where it is treated and clarified before conveyance to downstream processes.
Page 12 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 3.33 - Davidson Dr. Existing and Proposed Simplified
8 of 291 prings, Arkansas | Crist Engineers, Inc. d Process Flow Diagram – Hydraulic Improvements – Area 2
Page 129 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Additional 36-inch conveyance piping will be used to convey mixed liquor suspended solids (MLSS) from the aeration basin to secondary clarifier no. 5 and 6 flow splitter box. Secondary clarifier no. 5 is proposed to be near-term improvement whereas secondary clarifier no. 6 is proposed to be a long-term improvement. It should also be noted that it has been recommended that secondary clarifier’s 1, 2, 3 and 4 be rehabilitated beyond the mid-term improvement period. Each proposed new secondary clarifier will be 100 ft diameter with a peak day hydraulic capacity of 5.5 MGD. From there 30” conveyance piping will then convey MLSS to secondary clarifier no. 5. RAS piping from secondary clarifier no. 5 to the RAS/ WAS pumping station is planned to be a 14 inch. Secondary clarifier no. 6 and associated piping from the splitter box/ RAS piping is not planned to be installed with the near-term improvements (planned to be a long-term improvement). Clarified effluent from secondary clarifier no. 5 is to then be conveyed by a 24-inch pipe to the splitter box where it is then aggregated with clarified effluent from long-term proposed secondary clarifier no. 6. Future combined clarified effluent will then be conveyed via a 30-inch pipe to an existing clarifier blend box where clarified effluent from secondary clarifiers no. 1, 2, 3 and 4 are aggregated for conveyance through a 42- inch pipe to the proposed new tertiary filters. The total combined clarified effluent flow is also proposed in the near-term to be measured at this point. In the long-term it is proposed that additional 100 ft diameter secondary clarifiers 7 and 8 to be sequenced and installed with their own separate MLSS, clarified effluent and RAS piping in a similar approach as secondary clarifiers 5 and 6. However, a common splitter box is proposed to be used to split incoming/ outgoing flows between secondary clarifiers 7, 8, 9 and 10. Longterm proposed 100 ft diameter secondary clarifiers 9 and 10 piping is also proposed to be approached in a similar manner to allow for hydraulically balanced flow. A proposed long-term 36-inch conveyance pipe with flow measurement is also proposed to convey future clarified effluent to the tertiary filters. The near and long-terms hydraulic improvements approach within secondary treatment eliminates the existing tertiary filters (sand media) hydraulic bottleneck while simultaneously increasing the hydraulic capacity from ~20 MGD to ~24 MGD (25.8 MGD). As presented in Figure 3.34, clarified secondary effluent and peak flow is then conveyed, treated, and discharged to the Lake Catherine.