Page 168 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. a. Age: ~32 years old b. Condition: Poor c. Efficiency: Poor d. No. of Units: 1 (1st stage and 2nd stage, in-series) e. Total Capacity: 18,600 dry-lbs./ day Figure 4.11 – Davidson Dr. WWTP Anaerobic Digestion The existing anaerobic digestion process is in poor condition and in need of major improvements. These improvements were estimated to be ~$5 million dollars in 2015 and when adjusted for inflation and market factors could be closer to $8 million dollars today. It also is detrimental to the overall biological nutrient removal targets discussed in other sections of the report. It is recommended that this unit process be abandoned, and due to the age, condition, intangibles, etc. not be repurposed. It is recommended that these structures be demolished and removed.
Page 169 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The cost of demolishing of these structures is presented in the mid-term CIPs. This unit process is intended to be replaced with aerated sludge storage as discussed in other sections of the report. The cost of aerated sludge storage is presented in the mid-term CIP improvements. 4.2.1.11 BELT FILTER PRESS DEWATERING 1. Treatment Type: Physical 2. Purpose: Dewatering, liquid solids separation 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~15 years old b. Condition: Acceptable c. Efficiency: Acceptable, processing capacity undersized for mid/ long-term d. No. of Units: 1 e. Total Capacity: 14,000 dry-lbs./ day
Page 170 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Figure 4.12 – Davidson Dr. WWTP Belt Filter Press The existing belt filter process is in acceptable condition and achieves acceptable performance when considering the purpose/ goals of the existing WWTP. The future biological nutrient removal goals of the WWTP rely on exceptional solids capture as discussed in other sections of the report. Additionally, the belt filter press is nearing its maximum capacity where currently, staff sometimes needs to operate the belt press 16 to 20 hours per day. The belt press currently does not contain any redundancy and is in a building that does not permit expansion. It is recommended that this unit process be abandoned in the mid-term. This unit process is intended to be replaced with centrifugal dewatering as discussed in other sections of the report. The cost of centrifugal dewatering is presented in the mid and long-term CIP improvements.
Page 171 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 4.2.1.12 SOLIDS DISPOSAL TO LANDFILL OR COMPOST This section is discussed in detail in other sections of the report. The current practice of landfill and composting has been recommended to continue in the near and mid-terms. Figure 4.13 – Davidson Dr. WWTP Biosolids Disposal
Page 172 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 4.2.2 SOUTHWEST (SW) WWTP SWWWTP is a traditional sequencing batch reactor wastewater treatment plant consisting of the following unit processes: Liquid Treatment 1. Headworks a. Screening 2. Sequencing Batch Reactor 3. Secondary Treatment a. Diffused Aeration b. Secondary Clarification 4. Tertiary Filtration 5. Step Aeration 6. UV Disinfection 7. Step Aeration Solids Treatment 1. Aerobic Digestion 2. Solids Disposal to Collection System of Davidson Dr. WWTP The simplified process flow diagrams for the SWWWTP are presented in the following Figures.
Page 173 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 4.14 - Existing Southwest (SW) WWTP Sim Figure 4.15 - Existing SWWWTP Simplified P
3 of 291 prings, Arkansas | Crist Engineers, Inc. mplified Process Flow Diagram – Liquid Treatment Process Flow Diagram – Biosolids Treatment
Page 174 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 4.2.2.1 SCREENING 1. Treatment Type: Physical 2. Purpose: Removal of solids, bar spacing 1 and 0.25 inches 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~15 years old b. Condition: Acceptable c. Efficiency: Acceptable d. No. of Units: 2 – 1 manual bar rack and 0.25 automatic bar rack e. Total Capacity: ~2,400 GPM Ea Figure 4.16 – SWWWTP Headworks Screen The existing screening system was installed in the 2005 with a nominal screen opening of 0.25 inch. The screens appear to be in acceptable condition and providing acceptable solids removal. Each screen is capable of conveying ~2,400 GPM (limited with influent pumping capacity of the Winkler Pump Station).
Page 175 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. There are no rehabilitations identified for this unit process during the planning period. 4.2.2.2 SEQUENCING BATCH REACTOR 1. Treatment Type: Biological 2. Purpose: Removal cBOD/ ammonia 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~15 years old b. Condition: Acceptable c. Efficiency: Acceptable d. No. of Units: 2 e. Total Capacity: ~0.85 MGD Figure 4.17 – SWWWTP Sequencing Batch Reactor The existing sequencing batch reactor was installed in the 2005 and appears to be in acceptable condition and providing acceptable biological treatment of contaminants. The aeration system appears to be sufficiently sized at this time. As discussed in other sections of the report,
Page 176 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. modeling activities performed identified that the existing treatment system was capable of treatment of 0.925 MGD (the projected 7-DA Design Flow Rate) with AD-Design influent loading. There are no rehabilitations identified for this unit process during the planning period. However, nitrification is inhibited at times due to low influent alkalinity. Therefore, it has been recommended in other sections of the report to include supplemental alkalinity addition. The cost of supplemental alkalinity addition is presented in the near-term CIP improvements. 4.2.2.3 TERTIARY FILTRATION 1. Treatment Type: Physical 2. Purpose: Removal TSS 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~15 years old b. Condition: Acceptable c. Efficiency: Acceptable d. No. of Units: 2 e. Total Capacity: ~2,000 GPM Ea
Page 177 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Figure 4.18 – SWWWTP Tertiary Filtration The existing tertiary filtration was installed in the 2005 and appears to be in acceptable condition and providing acceptable TSS removal. There are no rehabilitations identified for this unit process during the planning period. 4.2.2.4 STEP AERATION 1. Treatment Type: Chemical 2. Purpose: Addition of dissolved oxygen 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~15 years old b. Condition: Acceptable c. Efficiency: Acceptable d. No. of Units: 1 e. Total Capacity: ~2,000 GPM
Page 178 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Figure 4.19 – SWWWTP Step Aeration The existing step aeration system was installed in the 2005 and appears to be in acceptable condition and providing acceptable dissolved oxygen addition. There are no rehabilitations identified for this unit process during the planning period. 4.2.2.5 UV DISINFECTION 1. Treatment Type: Physical 2. Purpose: Inactivation of bacteria 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~2 years old b. Condition: Acceptable c. Efficiency: Acceptable d. No. of Units: 2 e. Total Capacity: 2000 GPM Ea.
Page 179 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Figure 4.20 – SWWWTP UV Disinfection The existing UV disinfection system was installed in the 2005 and appears to be in acceptable condition and providing acceptable inactivation of bacteria. There are no rehabilitations identified for this unit process during the planning period. 4.2.2.6 STEP AERATION 1. Treatment Type: Chemical 2. Purpose: Addition of dissolved oxygen 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~15 years old b. Condition: Acceptable c. Efficiency: Acceptable d. No. of Units: 1 e. Total Capacity: ~2,000 GPM
Page 180 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The existing step aeration system was installed in the 2005 and appears to be in acceptable condition and providing acceptable dissolved oxygen addition. There are no rehabilitations identified for this unit process during the planning period. 4.2.2.7 AEROBIC DIGESTION 1. Treatment Type: Biological 2. Purpose: Digestion of secondary solids 3. Achieve Purpose: Yes 4. Physical Attributes a. Age: ~15 years old b. Condition: Acceptable c. Efficiency: Acceptable d. No. of Units: 1 e. Total Capacity: >700 dry-lbs./ day
Page 181 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Figure 4.21 – SWWWTP Aerobic Digestion The existing aerobic digestion basin was installed in the 2005 and appear to be in acceptable condition and providing acceptable digestion of secondary biosolids. The aerobic digester was created by purposing the 3rd sequencing batch reactor (SBR) basin in lieu of utilizing the 3rd SBR basin as redundancy. Therefore, it SWWWTP were to expand the aerobic digester would be eliminated. As it currently is configured, the aerobic digester is oversized for its function thus creating nutrient impacts at Davidson Dr. WWTP (when biosolids are hauled from SWWWTP to Davidson Dr. for further processing. There are no rehabilitations identified for this unit process during the planning period. 4.2.2.8 SOLIDS DISPOSAL TO COLLECTION SYSTEM OF DAVIDSON DR. WWTP The existing technique of disposal of biosolids generated from the SWWWTP into the collection system for Davidson Dr. WWTP has been recommended to be modified to dispose of biosolids directly into the aerated sludge storage. As discussed in other sections of the report this approach will place the biosolids into the solids processing portion of the Davidson Dr. WWTP directly.
Page 182 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. To address the receiving of the biosolids, a hauling receiving station has been included in the mid-term CIP improvements. Also, chemical sequestration of phosphorus has been also included in the receiving station to mitigate the impacts of nutrients on to the Davidson Dr. WWTP. The practice of hauling thickened liquid biosolids from SWWWTP to Davidson Dr. WWTP has been recommended to continue. SECTION 4.3 - CONCLUSION Most of the existing unit processes were considered during the development of other sections in the report. To conduct a thorough review of the biological and hydraulic capacities (existing and planned) each portion of the liquid and biosolids treatment section of the WWTPs must be evaluated. Generally, most of SWWWTP components were acceptable for biological and hydraulic and biosolids needs to achieve the desired effluent quality. Each component is currently approximately 15 years old and has not been recommended for replacement within the planning period (condition appears to be acceptable at this time). However, as equipment begins to degrade, it is recommended that each component be reevaluated at a future date to mitigate the risk of equipment failure causing permit violations. Proper planned and continuous maintenance of equipment will maximize the useful life of the equipment. Alternatively, at Davidson Dr. WWTP, most of the major equipment has been identified for improvement either to improve the hydraulic or biological capacity of the WWTP. Additionally, the biosolids handling has been recommended to be modified towards a scheme that minimizes nutrients while also minimizing costs related to disposal.
Page 183 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. CHAPTER 5 – BIOSOLIDS MANAGEMENT EVALUATION SECTION 5.1 – INTRODUCTION Biosolids are generated from biological and physical liquids separation processes within a wastewater treatment plant. Biosolids are generally discussed as two main categories, primary and secondary biosolids. Primary biosolids are generated by primary solids liquid separation technologies, with the most common being a primary clarifier. Primary clarification, as previously discussed, served historically as a vital function in removal of inert suspended solids, such as grit and screenings. The advent of improved headworks technologies, primary clarification is now predominantly used to remove influent TSS and cBOD for the development of primary biosolids. The remaining influent TSS and cBOD that isn’t removed from the influent travels downstream to biological treatment. Biomass contained within secondary processes (aeration basin/ secondary clarifier) utilizes the cBOD and TSS for cell maintenance and replication (growth). As the biomass grows through cell replication, biomass must be continuously pumped out of the process (i.e., “wasted”) to maintain the desired ratio of incoming cBOD (i.e., “food”) to biosolids (i.e., “mass”) necessary for optimizing treatment. The wasted biomass is commonly called waste activated sludge (WAS) or secondary biosolids. The City of Hot Springs (CHS) Wastewater System Master Plan (WWSMP) is intended to establish criteria to be used as the basis for the proposed Capital Improvements Plan (CIP). Biosolids processing is a significant expense to any wastewater treatment plant. This master plan will consider evaluating the management of biosolids produced by the Southwest and Davidson Dr. wastewater treatment plants (WWTPs). Currently, the Davidson Dr. WWTP is permitted to dispose of the biosolids to the composting facility or the landfill. Landfill is not specifically mentioned in the NPDES permit AR0033880 but through discussions/ meeting with ADEE, it is statutory allowed. The City of Hot Springs notified ADEE that if their biosolids production exceeds the permitted amount at the composting facility that it would then be hauled and disposed of into the Class 1 landfill located in Saline County. Currently, the SWWWTP is permitted (NPDES Permit AR0050148) to dispose of the biosolids to the Davidson Dr. WWTP. The City of Hot Springs Davidson Dr. Composting Facility is permitted by the Solid Waste Management Division of ADEE (permit no. 0396-SC-R1). This permit authorizes 2,222 cubic yards (CY) of biosolids, 21,600 CY of unscreened compost, 0 CY of racetrack straw and 80,000 CY of type Y compost material on-site at any point in time. It also states that biosolids/ sludge shall not be stored at the facility prior to composting and incorporated into the aerated state pile as soon as practicable. It also does not allow any other types of yard waste to be accepted at the compost facility except composting wood chips and sewage sludge.
Page 184 Wastewater System Master Plan (WWSMP) – Hot Sp The existing Davidson Dr. WWTP currently processes biosolids as d Figure 5.1 - Existing Davidson Dr. WWTP Simplifi
4 of 291 prings, Arkansas | Crist Engineers, Inc. escribed in Figure 5.1. ied Process Flow Diagram – Biosolids Treatment
Page 185 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The existing anaerobic digesters need repair and are currently underperforming. The estimated calibrated model volatile reduction is approximately 15%. The current total estimated biosolids produced by Davidson Dr. WWTP, (12 MGD average annual flow) based on the existing model calibration is approximately 22,000 CY per annual. It is estimated that the existing belt press achieves approximately 15.6% TS. Current hauling activities consist of utilizing an existing ~8 CY dump truck to haul biosolids from the Davidson Dr. WWTP to mainly convey biosolids to the compost facility. This approach requires approximately 8 truckloads per day be hauled from Davidson Dr. WWTP to compost or contract hauling/ disposal to the Class 1 landfill. Composting activities are very site specific and highly dependent upon the environmental conditions. If the facility experiences colder or wetter than traditional conditions the biosolids processing capacity of the facility decreases. Alternatively, with warmer/ dryer conditions the composting facility can process more biosolids. Also, the estimated number of windrows are contingent upon the total turn around period, which is the time that it takes for pre-processing, attainment of time-temperature, post processing. In the case of Davidson Dr. an average annual turnaround period for each windrow is approximately 35 days. Generally, the shorter the turnaround period the more biosolids that can be processed. This is the average amount of time required to achieve Class A biosolids requirements. It is suspected that the underperforming anaerobic digesters provide for increased levels of volatile solids (VS) thus generating an increased amount of biological activity within the windrow leading to achievement of the required temperature sooner than if the biosolids contained lower amount of VS. The higher concentrations of VS ultimately lead to a shorter turn around period. Based on communications with onsite composting staff, the following site-specific parameters are presented in Table 5.1. Table 5.1 - Existing Davidson Dr. Biosolids – Composting Facility Processing and Storage Capacity Existing Davidson Dr. Biosolids – Composting Facility – Windrow Processing 2019 Total Impervious Area 5.16 Acres Volume of Biosolids Per Windrow 240 CY Annual Number of Windrows1 46 - Total Annual Processing Capacity 11,040 wet-CY Available Storage (at any instant) 2,222 wet-CY Total Annual Capacity 13,262 wet-CY Total Annual Capacity 11,951 wet-tons 1 Average annual windows is highly dependent on environmental conditions.
Page 186 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. As shown in the above table, the existing composting facility can compost approximately 13,262 CY per annual. Finished compost is provided to the general public at no charge. At different times of the year, based on demand, compost may or may not be loaded for a charge (~$20/ truck load). From communications with staff, it appears that February through May is the busy season for the compost facility as customers are preparing their gardens for planting season. Based on the processing capacity and annual budget, the total cost analysis for processing is presented in Table 5.2. Table 5.2 - Existing Davidson Dr. Biosolids – Composting Facility Cost Analysis Existing Davidson Dr. Biosolids – Composting Facility – Windrow Processing 2021 Annual Cost (est.) $832,000 Cost per Wet CY (including storage) $63 $/ wet CY Cost per Wet Ton $70 $/ wet-ton Cost per Dry Ton @ 15.6% TS $446.28 $/ dry-ton As shown in the above table the average annual current processing cost is approximately $446.28/ dry ton. The remaining unclassified biosolids can then be transported to a Class 1 landfill. The total annual amount estimated to be transported to the landfill is presented in Table 5.3. Table 5.3 - Existing Davidson Dr. Biosolids – Volume and Weight to Landfill Existing Davidson Dr. Biosolids to Landfill Annual Volume to Landfill 8,738 wet-CY Annual Weight to Landfill 7,874 wet-tons As shown in the above table, the existing amount of biosolids conveyed and disposed into the landfill is approximately 8,738 CY per annual. Table 5.4 - Existing Davidson Dr. Biosolids – Landfill Transportation and Tipping Costs Existing Davidson Dr. Biosolids to Landfill 2021 Annual Cost (est.) $905,514 2021 Transport and Tipping Costs $104 $/ wet-CY 2021 Transport and Tipping Costs $115 $/ wet-ton Cost per Dry Ton @ 15.6% TS $737.18 $/ dry-ton
Page 187 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. As shown in the above table the average annual current transportation and tipping costs are approximately $737.18/ dry-ton. The existing total estimated annual cost for biosolids handling is presented in Table 5.33.
Page 188 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. SECTION 5.2 – DISCUSSION 5.2.1 FINAL DISPOSAL OPTIONS When evaluated biosolids processing, it is best to consider the ultimate or final disposal or intended use of the biosolids first and then work backwards. Biosolids for final disposal are generally subdivided into three (3) categories: Class A, Class B and unclassified. Biosolids resulting from WWTPs are subject to the standards set in place by 40 CFR Part 503. EPA/ ADEE regulates the treatment, use and disposal of biosolids. More specifically, this regulation establishes standards for biosolids when applied to land, disposed of in a landfill, or incinerated. EPA 40 CFR Part 503 regulates the quality of the pathogenic quality and vector attraction as well as amounts of heavy metals found within the biosolids. Additional permit specific regulations related to nutrients may also be considered. The purpose of the regulation is to minimize the risk to the general public and environment through processing, handling and disposal of biosolids. Class B biosolids are treated to a minimum level to allow them to be disposed on land with restricted uses. Treatment is conducted to a minimal acceptable level of pathogenic level of the geometric mean of 7 samples result in a fecal coliform concentration of 2,000,000 colony forming units (CFU)/ g of total solids (TS). To achieve this level, biosolids must be processed through approved processes that significantly reduce pathogens (PSRP). These approved processes are as presented in Table 5.5. Table 5.5 - Processes that Significantly Reduce Pathogens (PSRP) Process Definition Aerobic Digestion Biosolids are agitated with air or oxygen to maintain aerobic conditions for a mean cell residence time (MCRT) and temperature between 40 days at 20oC and 60 days at 15o C Air Drying Biosolids are dried on sand beds or on paved or unpaved basins for a minimum of 3 months. During 2 of the 3 months, the ambient average temperature exceeds 0oC Anaerobic Digestion Biosolids are treated in the absences of air between an MCRT of 15 days at temperatures of 35 to 55o C and an MCRT of 60 days at a temperature of 20oC. Times and temperature between these endpoints may be calculated by linear interpolation Composting Using either within-vessel, static aerated pile or windrow composting, the temperature of the biosolids is raised to 40oC or higher for 5 days. For 4 hours during the 5 days period the temperature in the compost pile should exceed 55oC. Lime Stabilization Sufficient lime is added to raise the pH of the biosolids to pH 12 and maintained for 2 hours of contact.
Page 189 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Additionally, Class B biosolids must meet one of the vector attraction reduction (VAR) requirements as described in Table 5.6. Table 5.6 - Vector Attraction Reduction (VAR) Requirements Option Performance 1 At least 38% reduction in volatile solids during biosolids treatment 2 Less than 17% additional volatile solids loss during bench-scale anaerobic batch digestion of biosolids for 40 additional days at 30 to 37oC. 3 Less than 15% additional volatile solids reduction during bench-scale aerobic batch digestion for 30 additional days at 20oC. 4 Specific Oxygen Uptake Rate (SOUR) at 20oC is 1.5 mg O2/hour-gram total biosolids solids. 5 Aerobic treatment of the biosolids for at least 14 days over 40oC with an average temperature of over 45oC. 6 Addition of sufficient alkali to raise the pH to at least 12 at 25oC and maintain a pH of 12 for 2 hour and a pH of 11.5 for 22 additional hours. 7 Percent solids of 75% prior to mixing with other materials. 8 Biosolids of 90% prior to mixing with other materials. 9 Biosolids is injected in soil so that no significant amount of biosolids is present on the land surface 1 hour after injection, except Class A biosolids which must be injected within 8 hours after pathogen reduction process. 10 Biosolids is incorporated into the soil within 6 hours after application to land. Class A biosolids must be applied to the land surface within 8 hours after the pathogen reduction process and must be incorporated within 6 hours after application. Typically, the most common methods to achieve Class B biosolids is to use aerobic or anaerobic digestion processes to achieve the PSRP and Option 1 VAR. 5.2.1.1 ANAEROBIC DIGESTION Anaerobic digestion involves the decomposition of organic matter and the reduction of inorganic matter in the absence of oxygen. To achieve anaerobic digestion three reactions
Page 190 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. must occur: hydrolysis, fermentation, and methanogenesis. Hydrolysis transforms complex and particulate materials into simple and soluble materials whereas fermentation is the further processing of these products into organic acids. Methanogenesis occurs when organic acids are converted into methane and other gaseous products which form biogas. The parameters important to anaerobic digestion of biosolids are solids retention time, hydraulic retention time, mixing/ contact, temperature, alkalinity, pH, the presence of inhibitory substances, and bioavailability of nutrients and trace metals. At most wastewater treatment plants that utilize anaerobic digestion the critical parameters that most closely relate to performance of the process are VSS/ cBOD (loading/ food), mixing/ contact, temperature, alkalinity and pH. Anaerobic digestion through the PSRP and VAR of the biosolids generates biogas which can be used for beneficial use. Biogas is most often collected and used to heat the incoming biosolids to maintain the desired temperature for PSRP, VAR and biogas production. Biogas is not pure methane but contains undesirable products such as moisture, carbon dioxide, hydrogen sulfide, siloxanes, et al. which decreases the thermal efficiency as well as generates challenges for use in a boiler/ heat exchanger system. During periods of excess biogas, it will be commonly flared or in some cases used to generate electricity using a co-generation facility. Alternatively, biogas can also be scrubbed to remove undesirables to generate natural gas thus permitting its compression and distribution into the natural gas distribution system for use by other residences or businesses. As discussed in previous sections, maintaining primary clarification and/ or primary filtration was considered. However, the main issue with this approach is that these processes produce primary sludge that is generally handled using anaerobic digestion. As discussed, improvements to this process were estimated to be approximately $5 million (in 2015 dollars) in the 2015 report (adjusted to 2021 = $6.25 million). Additionally, primary clarification is an existing hydraulic bottleneck within Davidson Dr. 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. Therefore, to improve the primary clarification and anaerobic digestion would cost between $9 to $11 million. It is suspected that an additional $3 to $5 million would be needed for co-generation or biogas scrubbing and/ or conveyance (i.e., compression) into the natural gas distribution system. As also discussed in previous sections, gas production is proportional to the amount of influent VSS/ cBOD. Davidson Dr. WWTP ranges from approximately 150 to 200 mg/L or 65% of the necessary concentration needed to make the process feasible. While gas collection and use as fuel has been suggested for the Davidson Dr. WWTP, there is simply not enough loading in the influent to justify the use of anaerobic digestion for generation of biogas. As discussed in other sections, primary clarification (desired for anaerobic digestion) impedes biological luxury phosphorus uptake. Additionally, anaerobic digestion creates an environment where phosphorus accumulating organisms (PAO) to release their stored phosphate, thus generating decant or wash waters high in total phosphorus (T-P). This type of process/
Page 191 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. approach is counterproductive to the requirements of the effluent permit and ultimate purpose of the Davidson Dr. WWTP.
Page 192 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 5.2.1.2 AEROBIC DIGESTION Aerobic digestion is typically used to achieve the PSRP and VAR requirements while simultaneously reducing the biosolids disposal mass. Aerobic digestion is an extension of the aeration basin that allows for biosolids to endogenously respiration. Wasted activated sludge through bio-chemical reactions convert biosolids to carbon dioxide and water. Aerobic digesters require aeration thus will utilize significant amounts of electricity. The benefit is that this process achieves Class B biosolids requirements while reducing the total final amount that is required to be disposed. This process has historically been utilized at WWTPs with flows generally less than ten (10) MGD because of the capital cost and energy requirements for operation. Aerobic digestion would increase the wash water (or side stream) concentration of T-P significantly (3x) or 250 to 300 lbs./ day. Aerobic digestion to achieve Class B biosolids, utilizing (Option 1 VAR strategy) would elevate the effluent T-P concentration to approximately 1.3 to 1.75 mg/L during dewatering activities therefore challenging the current MA permit limit of 1 mg/L and future MA planning limit of 0.7 mg/L. To accommodate for this increased concentration of T-P found in the side stream, additional sequestering coagulant, sodium aluminate, would be required to reduce the concentration to within compliance. Additionally, digestion, through endogenous nitrification, produces high concentrations of nitrate. This nitrate, when recycled back to the head of the Davidson Dr. WWTP will impede the anaerobic process necessary for luxury T-P uptake. As presented in the following table, the nutrient removal performance increases with decreasing sludge storage/ aerobic digestion. Table 5.7 - Davidson Dr. WWTP Aerobic Sludge Handling Biological Performance Parameter Sludge Storage (0 days) Sludge Storage (7 days) Aerobic Digestion (15 days) Anoxic Basin PO4-P (mg/L) 7.05 6.51 6.34 Nitrate (mg/L) 0 0.01 0.01 Aeration Basin PO4-P (mg/L) 0.28 0.81 1.3 Nitrate (mg/L) 5.42 5.18 5.19 Effluent T-P (mg/L) 0.38 0.92 1.41 Nitrate (mg/L) 5.42 5.18 5.19 5.2.1.3 ALKALINE STABILIZATION Alkaline stabilization has also been used to achieve PSRP and VAR requirements. This is typically accomplished through the addition of lime to achieve the desired pH (with associated time). This high pH will inactivate the viruses, bacteria, and other microorganisms that can cause putrefaction, offensive odors, and vector attraction.
Page 193 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. High levels of ammonia and trimethylamine (TMA) are generated during this alkaline stabilization process. These dangerous and odorous gases as well as the cost associated with the addition lime (direct cost of lime as well as the additional solids production) have led to this practice falling out of favor with most utilities. Class A biosolids, like Class B biosolids are required to achieve the VAR requirements as well as achieve a much lower pathogenic level with either Salmonella species less than 3 most probable number (MPN) per 4 gram of dry sludge solids or for the fecal coliforms to be less than 1,000 MPN/ g of total solids (TS). Approved processes to further reduce pathogens (PFRP) are presented in Table 5.8. Table 5.8 - Processes to Further Reduce Pathogens (PFRP) Process Definition Composting Using either within-vessel or static aerated pile composting, the temperature of the biosolids is maintained at 55o C or higher for 3 days. Using windrow composting, the temperature of the wastewater sludge is maintained at 55o C of higher for 15 days or longer. During this period a minimum of five windrow turnings is required. Heat Drying Dewatered biosolids are dried by direct or indirect contact with hot gases to reduce the moisture content to 10% or lower. Either the temperature of biosolids particles exceed 80oC or the wet bulb temperature of the gas stream in contact with the biosolids as the biosolids leave the dryer exceed 80oC. Heat Treatment Liquid biosolids are heated to a temperature of 180oC or higher for 30 min. Thermophilic Aerobic Digestion Liquid biosolids are agitated with air or oxygen to maintain aerobic conditions and the MCRT is 10 days at 55 to 60oC. Beta Ray Irradiation Biosolids are irradiated with beta rays from an accelerator at dosages of at least 1.0 megarad (Mrad) at room temperature (~20oC). Gamma Ray Irradiation Biosolids are irradiated with gamma rays from certain isotopes such as 60 cobalt or 135 Cesium at dosages of at least 1.0 Mrad at room temperature (~20oC). Pasteurization The temperature of the biosolids is maintained at 70oC or higher for at least 30 min.
Page 194 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The most common process to achieve PFRP requirements is composting. These biosolids can then be used without restriction in applications to land, commonly used on farms, lawns, gardens, etc. It is commonly given or sold back to the public for their own use however they see fit. Alternatively, there are processes that utilize alkaline stabilization (lime) to generate sufficient heat for pasteurization to achieve Class A biosolids classification. Also heat drying has also been commonly utilized it some applications for the production of Class A biosolids. Most other PFRP techniques not commonly used and require site specific needs to make them economically viable. Unclassified (or un-stabilized) biosolids are those that do not meet the criteria to be considered Class A or Class B biosolids. The common disposal of unclassified biosolids is in a secured Class 1 landfill where dewatered biosolids is mixed with municipal solids waste and covered daily. The most common biosolids disposal process is Class B land application. This method has historically been the most cost-effective approach to final disposal because of land availability and proximity to the wastewater treatment plant. In addition, the requirements related to the quality of the biosolids, there are also land application restrictions. Table 5.9 presents the land application restrictions of Class B biosolids.
Page 195 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. Table 5.9 - Land Application Restrictions for Class B Biosolids Process Definition Restrictions for Harvesting of Crops and Turf Food crops with harvested parts that touch the biosolids/ soil mixture and are total above ground shall not be harvested for 14 months after application of biosolids. Food crops with harvested parts below the land surface where biosolids remains on the land surface for 4 months or longer prior to incorporation into the soil shall not be harvested for 20 months after biosolids application. Food crops with harvested parts below the land surface where biosolids remains on the land surface for 4 months or longer prior to incorporation into the soil shall not be harvested for 38 months after biosolids application. Food crops, feed crops and fiber crops, whose edible parts do not touch the surface of the soil, shall not be harvested for 30 days after biosolids application. Turf grown on land where biosolids is applied shall not be harvested for 1 year after application of the biosolids where the harvested turf is placed on either land with a high potential for public exposure or a lawn, unless otherwise specified by the permitting authority. Restrictions for the Grazing of Animals Animals shall not graze on land for 30 days after application of biosolids to the land. Restrictions for Public Contact Access to land with a high potential for public exposure, such as a park or ball field, is restricted for 1 year after biosolids application. Examples of restricted access include posting with no trespassing signs or fencing. Access to land with a low potential for public exposure (e.g., private farmland) is restricted for 30 days after biosolids application. An example of restricted access is remoteness. Also, when considering land application, the amount of climate, including rainfall amounts and topography must be considered. Generally, most land application permits issued by ADEE require that Class B biosolids may not be applied to wet or frozen ground. This minimizes the potential for runoff from the land application sites. Considering the amount of rainfall that Arkansas receives this causes another challenge when considering storage and/or duration of land application. The other primary consideration is that Class B biosolids cannot be land applied (unless through subsurface injection) with slopes greater than 6% (topography). Also, the type of geology (soil
Page 196 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. conditions), subsurface water conditions (distance to groundwater), proximity to receiving streams or bodies of water can impact the likely success of Class B land application. Other factors that significantly impact land application locations is aesthetic and economic factors including odors, transportation availability, land use, population density, and property values. 5.2.2 ALTERNATIVES CONSIDERED FOR DISPOSAL Ultimate disposal of biosolids can be distilled down to two alternative approaches: (1) intended use or (2) intended disposal. If the biosolids has an intended use than a specific process approach can be tailored to achieve the intended use requirements. An example of intended use would be to sell or provide a low cost biosolids product to the public for their unrestricted use in an efficient and cost-effective manner. Alternatively, intended disposal approach is to devise a scheme that discard biosolids efficiently and at as low of a cost to the customer. Hot Springs with its two wastewater treatment plants and current permitted disposal approaches are in a unique position to consider four (4) biosolids handling schemes and are presented in Table 5.10. Table 5.10 - Biosolids Disposal Alternatives for Hot Springs WWTPs Approach Scheme Description Final Placement Intended Use Drying Using mechanical drying equipment to achieve Option 8 VAR requirements while simultaneously achieving the Heat Drying requirements of PFRP to meet Class A biosolids. Unrestricted use for land application, farming activities for production of food or feed crops. Composting Using windrow composting equipment to achieve Option 5 VAR requirements while simultaneously achieving the Composting requirements of PFRP to meet Class A biosolids. Unrestricted use for land application, used by the general public for gardening, soil amendment, et al. Intended Disposal Land Application Using Option 1 or 4 VAR while simultaneously utilizing Aerobic Digestion PSRP to then purchase or identify surface property for the disposal of Class B biosolids. Restricted use for land application, farming activities for production of food or feed crops. Landfill Identify Class 1 landfills for the disposal of unclassified biosolids. Incorporation into a municipal solids waste (MSW) Class 1 landfill. Other processes such as thermal pasteurization, alkaline stabilization, autothermal thermophilic aerobic digestion (ATAD) were preliminarily considered but not selected for further analysis due to the size and specific site conditions associated with Hot Springs WWTPs.
Page 197 Wastewater System Master Plan (WWSMP) – Hot Sp Table 5.11 - Biosolids Disposal Alternatives C Scheme Description Benefits Drying 1. Implement sludge thickening and aerated storage to allow for continuous wasting from biological processes and interruption of dewatering equipment. 2.Use mechanical dewatering equipment to generate biosolids 15 to 25% total solids (TS) 3. Mechanical drying equipment removes liquid from unclassified biosolids to achieve at least 90% total solids (TS). 4. Covered, dried biosolids are then immediately transported to unrestricted land application site for disposed. 1.Generates a biosol can be used for un application. 2. Contains undigeste in ammonia, desira incorporation/ am for crop productio 3.Desirable soil ame farmers, generally customer for dispo 4. 90% TS reduces the for hauling to the s site. 5. Contained/ advanc the treatment of b
7 of 291 prings, Arkansas | Crist Engineers, Inc. Comparison for Davidson Dr. WWTP – Drying Drawbacks lids product that restricted land ed biosolids, rich able product for endment to soil n. ndment from easy to identify ose. e mass/ volume soil amendment ced process for biosolids. 1.Dried biosolids and associated dust is flammable. 2.Waste moisture/ exhaust gas will be odorous and require treatment. 3. Immediate disposal required, otherwise absorb moisture, and become odorous, vector attraction, and require re-processing. 4. Expensive capital cost. 5.High energy usage. 6. Complicated operation of equipment and scheme. 7. Land application dependent on weather conditions, challenging disposal during cold, wet periods.’ 8. Produces an undesirable product for use by the general public. 9. Requires alternative disposal (landfill, incineration) during periods when unable for land application. 10. Requires compliance with EPA 40 CFR Part 503. 11. Possible disposal locations likely within the Ouachita River watershed, ultimately land application may lead to T-P non-point source runoff into discharge receiving stream anyways.
Page 19 Wastewater System Master Plan (WWSMP) – Hot Sp Table 5.12 - Biosolids Disposal Alternatives Com Scheme Description Benefits Composting 1. Implement sludge thickening and aerated storage to allow for continuous wasting from biological processes and interruption of dewatering equipment. 2.Use mechanical dewatering equipment to generate biosolids 15 to 25% TS. 3.Dewatered biosolids are transported (covered) to the composting facility for further processing. 4. Storage of dewatered biosolids only as permitted. 5.Off-site wood products material is brought into the compost facility for chipping. 6.Dewatering biosolids are mixed with wood chips and arranged in a windrow to create the biological conditions for thermal stabilization. 7.Windrow composting piles must be arranged in a pile for at least 15 to 20 days (achieve 55 oC for 15 days; 14 days over 40 oC with an average above 45oC). Turn windrow compost piles at least 5 times. 8. Separate bulking material (wood chips) from the stabilized biosolids (commonly called compost at this point). 9. Store compost to allow for interruption of taking by the public or final disposal by the utility. 1.Generate a bio that can be us and unrestrict application. 2. Biosolids mass through biode volatile suspen 3. Beneficial use product for us community. H intangibles val public. 4. Relatively simp operation (i.e. technical chall
8 of 291 prings, Arkansas | Crist Engineers, Inc. mparison for Davidson Dr. WWTP – Composting Drawbacks osolids product ed by the public ed land s reduction gradation of nded solids. of a waste e within the Has a high lue with the ple and easy to , limited enges). 1. Process highly dependent on climate, difficult to maintain or achieve thermal stabilization during cold or wet periods. 2. Requires continuous input/ source of bulking material (wood chips) and saw dust cannot be used. 3. Manual labor intensive. 4. Large expensive equipment used for processing (front end loaders, windrow machines, screeners, et al. 5. Requires a significant amount of available land for processing. 6. Requires impervious area with stormwater drainage to the Davidson Dr. WWTP. Increases nutrient and suspended solids loading to the WWTP. 7. Requires large, covered storage of unclassified biosolids and Class A biosolids to prevent re-wetting. 8. May require at times compliance with EPA 40 CFR Part 503. 9. Possible disposal locations likely within the Ouachita River watershed, ultimately land application may lead to T-P non-point source runoff into discharge receiving stream.
Page 199 Wastewater System Master Plan (WWSMP) – Hot Sp Table 5.13 - Biosolids Disposal Alternatives Compa Scheme Description Benefits Land Application 1. Implement sludge thickening and aerated digestion to allow for continuous wasting from biological processes and interruption of dewatering equipment. 2. Aerobic digester(s) to achieve a total MCRT of 40 days at 20oC, however, the total days can be reduced to 15 to 20 days at 20oC to achieve at least 38% volatile reduction (i.e., Option 1 VAR strategy). 3. Biosolids mass reduction through biodegradation of volatile suspended solids. 4.Use mechanical dewatering equipment to generate biosolids 15 to 25% TS. 5.Dewatered biosolids are transported (covered) to the restricted land application site for immediate incorporation into the soil. 1. Total solids reduc 20,500 lbs./ day t or ~25% reductio mass and/ or amo that is required fo intended disposa 2. Achieves Class B b storage and/ or tr minimizing the ve and/ or odor pote 3. Increase in dewat (i.e., increased TS polymer, et al.).
9 of 291 prings, Arkansas | Crist Engineers, Inc. arison for Davidson Dr. WWTP – Land Application Drawbacks ction, from ~ to ~15,500 lbs./ day n. Reduces the ount of hauling or land application/ l. biosolids prior to ransportation, thus ector attraction ential. tering performance S with similar 1. Increased capital cost with large sludge storage basins (i.e., aerobic digesters) and increased blower capacity (i.e., increased hp and SCFM requirements). 2. Increased wash water T-P concentration. 3. Increased consumption of alkalinity (i.e., magnesium hydroxide) and sequestering coagulant (i.e., sodium aluminate). 4. Increased concentration of nitrate recycled back to influent; disruption of anaerobic process required for luxury T-P uptake by PAOs. 5. Compliance with EPA 40 CFR Part 503. 6. May require dewatered biosolids storage for storage during periods where biosolids cannot be land applied due to wet or frozen ground. 7. Alternatively, during periods where land application cannot be accomplished, biosolids could be disposed of into the Class 1 landfill located in Saline County. 8. Possible disposal locations likely within the Ouachita River watershed, ultimately land application may lead to T-P non-point source runoff into discharge receiving stream. 9. Class B biosolids less desirable for farmer’s crops because of the reduced ammonia concentration. 10. Property values, offset from adjacent landowners, odors, intangibles et al.
Page 200 Wastewater System Master Plan (WWSMP) – Hot Sp Table 5.14 - Biosolids Disposal Alternatives C Scheme Description Benefits Landfill 1. Implement sludge thickening and aerated sludge storage to allow for continuous wasting from biological processes and interruption of dewatering equipment. 2.Use mechanical dewatering equipment to generate biosolids 15 to 25% TS, permitting passage of the paint-filter test. 3.Dewatered biosolids are transported (covered) to the Class 1 landfill located in Saline County. 1. Ease of operation 2. Low labor or staff 3.Hauling/ handling accomplished by 4. Simple approach biosolids. 5. Limited process im variations. 6. Planned, budgete 7. Limited capital co compared to othe alternatives. 8. Sequestration of limits non-point s 9. Compliance with Minimizes the po requirements ass generated from in 10. Likely lowe compared to othe alternatives. 11. Encourage prior to transport disposal of water
0 of 291 prings, Arkansas | Crist Engineers, Inc. omparison for Davidson Dr. WWTP – Landfill Drawbacks n. fing requirements. g/ disposal can be an outside entity. to intended disposal of mpacts from climatological ed, consistent cost. ost and investment when er intended use or disposal nutrients within landfill (i.e., source pollution potential). 40 CFR Part 503 is limited. tential pre-treatment ociated with metals ndustries. est carbon footprint when er intended use or disposal es a highly dewatered biosolid ting (i.e., paying tipping fee for ). 1. Tipping fees or disposal costs within the landfill. 2.Unknown or potential contractual fee increases from landfill (i.e., utility does not control landfill operation). 3.Hauling/ handling/ disposal can be accomplished by an outside entity. 4.Non-beneficial use of a waste product (i.e., public use, growing of crops, et al.). Has a low intangibles value with the public.
Page 201 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. For discussion purposes, dried biosolids can be used as a fuel source for incineration to generate electricity. The logistics, permitting and infrastructure for that option, unless an incinerator existing within the area can be complex thus require a detailed specific analysis that is beyond the original intent of this study. Also, liquid biosolids could be pumped from the wastewater treatment facility to the Davidson Dr. Composting Facility for processing. This has been discussed as a possible approach to minimize the biosolids storage and processing footprint at the Davidson Dr. WWTP. Once dewatered at the Davidson Dr. Composting Facility, biosolids could be used to generate compost or hauled to the landfill for further processing, this approach, all though more costly, could provide maximum flexibility for future expansions or approaches to processing of biosolids. Composting does not necessarily have to be accomplished through using a windrow. Alternatively, in-vessel (rotating screen type) and static pile were not considered for further evaluation because of: 5. the amount of biosolids processing (i.e., size of facility), 6. additional equipment that would be required (aeration/ vacuum), 7. challenges associated with operational control of the process, 8. and the desirability of the finished Class A biosolids product. The in-vessel or static pile approach allows for a reduced period of biosolids processing. It is suspected that this reduced period could ultimately achieve the thermal requirements for stabilization, however, it may not allow for sufficient time to degrade/ convert the volatile solids to generate the desirable compost consistency. Production of Class A biosolids for the intended use by the public (either as a giveaway or for sale) must achieve a product that the public will want. The type of process or disruption thereof, climatological factors or other intermediates can disrupt the generation of desirable compost thus leading to an undesirable product (i.e., aesthetically unwanted but yet achieves the requirements of the PFRP and VAR) that now cannot be sold or given away. Examples of undesirable composting material is the consistence it too wet/ heavy and difficult to handle or spread, too many inorganics (i.e., plastics), odorous when rewetted, thick or muddy consistency, too many volatiles, et al. This can then create a disposal liability for the utility as they must now determine a location or identify a customer that will accept the undesirable product. It is highly suspected that an invessel or static pile approach with undigested biosolids would create an increased risk of production of an undesirable product. In the Ouachita Mountains region of Arkansas, land application poses another challenge for identifying land application sites as a significant portion of potential available land exceeds 6% slope. The simplest approach for biosolids land application sites is for the application site to be used for grazing of animals. This approach allows for maximum flexibility of the utility for land application of biosolids as for it allows for animals to use with site within 30 days after incorporation. Under this approach, the agronomic approach is to grow Alfalfa, for
Page 202 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. consumption by grazing animals. Alfalfa has one of the highest T-P uptakes of any crop, thus maximizing the agronomic application which reduces the necessary required land. A review of the aerial view of Hot Springs location and adjacent properties quickly reveals limited contiguous available land meeting the offset from adjacent landowners, property values, slope and Alfalfa growing criteria. It is likely that the Class B dewatered biosolids will need to be transported at a minimum to an area within the US I-30 corridor for land application.
Page 203 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 5.2 – Hot Springs, AR Su
3 of 291 prings, Arkansas | Crist Engineers, Inc. urrounding Arial Google Image
Page 204 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. As shown, in the above imagery, the lighter green/ brown colors are non-wooded areas predominantly being used for agricultural purposes. There are some properties that may be available for use for land application adjacent to Hot Springs (i.e., toward the SW) but upon closer inspection reveal multiple smaller properties (i.e., < 100 acres). The US I-30 corridor has larger, contiguous tracks of land. Currently, a Class 4 landfill existing in Garland County (operated by Garland County). Considerations were given to permitting and operating a Class 1 landfill at this location to minimize the distance for disposal (i.e., lower fuel/ maintenance cost associated with transportation of biosolids) as well as minimizing the tipping fee through participation of the Hot Spring utilities within its permitting/ expansion to a Class 1 landfill. Although it may be feasible to consider, it was not pursued due to intangibles associated with this option. Also, there are other landfill disposal locations that were not included in the cost analysis but may be considered depending on the economics associated with the tipping fee(s). There are Class 1 landfills located in Howard and Hempstead counties as well as two (2) Class 1 landfills located in Pulaski County. The Class 1 landfill located in Saline County is the closest distance to the Davidson Dr. WWTP.
Page 205 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 5.3 – Active Class 1 and Cla
5 of 291 prings, Arkansas | Crist Engineers, Inc. ass 4 Landfills in Arkansas (2019v)
Page 206 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. 5.2.3 DAVIDSON DR. WWTP ANALYSIS To compare the proposed intended use and intended disposal options, we first must quantify the estimated biosolids production. Biosolids production is based on the results of the biological modeling of the various near, mid and long-term scenarios. All of the scenarios do not include costs associated with biosolids processing (except where appropriate, i.e., aerobic digestion) associated with WAS, thickening, TWAS, dewatering and dewatered biosolids conveyance into the truck. These costs were not included because they are consistent and similar among all the biosolids intended use or disposal options. 5.2.3.1 SCENARIO 1A Scenario 1 improvements at Davidson Dr. WWTP include the following: Scenario 1 (Near-Term Improvements CIP) 1.8 Process elimination of: a. lime feed for supplementary alkalinity b. primary clarification and production of primary sludge c. anaerobic digestion 1.9 Addition of magnesium hydroxide for supplemental alkalinity 1.10 Upgrade/ increase airflow rates to aeration basin 1.11 Additional 100 ft secondary clarifier a. Increase total surface 7,854 ft2 (total = 36,970 ft2) 1.12 New tertiary filtration 1.13 WAS directly to existing dewatering (BFP) a. New WAS Pumping It is proposed with Scenario 1A to convey waste activated sludge (WAS) directly from the secondary clarifiers to the existing dewatering unit. Wasting and dewatering are planned to be completed approximately 20 hours per day, 7 days per week under this near-term approach. This is not ideal solution because it requires almost continuous operation of the dewatering equipment and does not allow for downtime of the equipment for maintenance or servicing. However, this approach/ strategy does permit biosolids improvements to be improved in the mid-term rather than the near-term. It will allow for continuous conveyance of WAS to the dewatering unit. This approach should maximize the existing dewatering performance (i.e., continuous and consistent) as well as improve the biological efficiency of the near-term proposed secondary treatment process. This approach also permits for mid-term improvements to be completed with minimal interruption to the near-term biosolids handling process while also allowing for the elimination of the anaerobic digesters. The general influent parameters of Scenario 1A are AD design flow rate (QAD) = 16 MGD (6.7% excess of the projected 2040 average daily influent flow) with the 2040 AD design influent parameters.
Page 207 of 291 Wastewater System Master Plan (WWSMP) – Hot Springs, Arkansas | Crist Engineers, Inc. The simplified process flow diagram and associated biosolids production values are presented in the following figure and table.
Page 20 Wastewater System Master Plan (WWSMP) – Hot Sp Figure 5.4 - Proposed Davidson Dr. WWTP Simplified Pro
8 of 291 prings, Arkansas | Crist Engineers, Inc. ocess Flow Diagram – Solids Treatment (Near-Term CIP)