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In the context of stringent effluent nitrogen limits, there appears to be no better alternative to Partial-denitrification Anammox (PdNA) for continuous flow BNR process upgrades or new construction. Anaerobic ammonia oxidation reduces the need for aerobic ammonia oxidation, drastically reducing required aerobic SRTs; this allows existing plants to increase capacity or dedicate existing aerobic volumes to anaerobic or anoxic volume and allows for designs with smaller nitrification safety factors for new construction. PdNA also reduces aeration requirements, alkalinity addition requirements, supplemental COD requirements, reducing O&M costs and driving sustainability. PdNA does not require anammox seeding from a sidestream process. Polishing PdNA has been demonstrated in full-scale deep bed sand filters for over 3 years (1) and has been demonstrated at pilot scales in MBBR, IFAS, and granular configurations. Startup of these processes appears to take 3 to 4 months (2) which is concordant with sidestream anammox process startup times. Finally, PdNA processes can achieve low effluent nitrogen concentrations (2,3,4). The aforementioned benefits of PdNA stem primarily from the incorporation of anammox into mainstream processes, even though Partial Nitritation Anammox (PNA) theoretically offers slightly improved benefits over PdNA. However, the challenge of maintaining NOB outselection in full-scale continuous flow processes has yet to be overcome (5),and PdNA processes configurations are compatible with PNA if mainstream NOB outselection becomes a reality. This paper explores these claims about PdNA through modelling two BNR processes: an A2O plant with a denitrification filter converted to a polishing PdNA filter and a 5-stage Bardenpho where integrated PdNA (IFAS) is installed in the second anoxic zone. The models were developed in SUMO using the recently developed partial denitrification (PdN) model (6), and the models were compared with existing full scale and pilot data to ensure reliability. Models in this study were built in SUMO with influent characterization shown in Table 1. Process configurations are shown in Figure 1 and Table 2. Both models incorporated suspended growth and biofilm modeling and used default kinetic parameter sets from the SUMO PdNA Model with minor changes. Both models were run to develop a baseline for each process. To incorporate PdNA, the A2O model was modified with AvN control to achieve a target ammonia/NOx ratio in the aeration effluent, which allowed the downstream methanol filter to be transitioned to PdNA via reduced methanol dosing. The 5-Stage Bardenpho process was also modified with AvN control to achieve a target ammonia/NOx ratio in the first aerobic zone effluent, and a biofilm component was added to the secondary anoxic zone to simulate IFAS which allowed PdNA via reduced methanol dosing.
Real-world examples referenced in the paper include sensor data, sample data, and batch test data from: the York River Treatment Plant (YRTP) full-scale process, a pilot-scale PdNA MBBR process, a pilot-scale PdNA IFAS process, a pilot-scale PdNA filter pilot, and pilot tests of fixed-film PdNA media. As anticipated from stoichiometry and previous full-scale and pilot experience, the models demonstrated the benefits of implementing polishing or integrated PdNA. A summary of these baseline and PdNA upgrade model results is given in Table 3. The A2O process baseline required a 10 day SRT to achieve full nitrification and performed as expected for a well designed plant. PdNA was implemented by reducing the aeration tank DO setpoint slightly (2.0 to 1.3) and reducing the total plant SRT (10 to 3.2 days), producing an aeration effluent with an NHx/NOx ratio close to 2.0. Methanol dosing was reduced to drive partial denitrification (PdN efficiency = 65%) in the filters, which resulted in sufficient ammonia and nitrite concentrations for anammox to grow. Startup of the PdNA process in the model required approximately 100 days, which reflects observed PdNA startup times (80-90 days) without seeding (2). The total plant SRT was reduced by 68% while also reducing the alkalinity required by 13% and the methanol required by 52%, demonstrating significant operational savings. Although the OUR in the aerobic reactor was reduced, the aeration requirements did not change significantly, due to changes in alpha factors based on MLSS; and total sludge production for the plant increased slightly, as a reduction in SRT drives higher observed yields. The results of this model aligned well with full-scale results from the YRTP filter performing year-round PdNA (Figure 2): ammonia removal via anammox was in the 2 – 3 mg N/L range and relative reductions in methanol requirements were similar. The 5-stage process baseline was similar to the A2O, requiring a 10 day SRT for full nitrification and achieved acceptable effluent N concentrations. PdNA was implemented by reducing the total plant SRT to 3.4 days, producing an aeration effluent with an NHx/NOx ratio close to 2.0. IFAS (fixed biofilm model) was added to the post anoxic zone and methanol dosing was decreased to drive partial denitrification (PdN efficiency = 58%). This results in sufficient ammonia and nitrite concentrations for anammox to grow, and given the same anammox kinetics were used, the startup time was similar to the A2O PdNA process (~100 days). Again, significant improvements were demonstrated: 66% reduction in SRT, 18% reduction in alkalinity required, and 55% reduction in methanol required. Aeration requirements were essentially unchanged and sludge production increased by 10% due to the large reduction in SRT. These model results aligned well with existing experimental work; the model did show slightly better effluent nitrogen performance than experimental PdNA IFAS has demonstrated (4), likely due to slightly higher PdN efficiency. HRSD is moving forward with construction of integrated PdNA in post anoxic zones at multiple full-scale plants. These results demonstrate not only the benefits of PdNA, but that the knowledge gained thus far about this process can be successfully incorporated into plant-wide models and used to model alternative designs for treatment plant upgrades and new construction. The examples here concentrate most of the gains from PdNA in to reduced SRT requirements, which would lead to increased plant capacity or increased plant efficiency (for plants lacking sufficient anaerobic or anoxic SRT), and still generates significant supplemental COD and alkalinity savings. If capacity is not a primary driver, AvN could be implemented via DO control, reducing aeration requirements and costs significantly and increasing SND, or any combination of reduced aerobic SRT, lower operating DO, transition of aerobic zones to anoxic/anaerobic, etc. Further research with these models will include influent variability (diurnal, wet weather, etc) and more detailed control mechanisms to deal with that variability. Additional modelling work will also focus on sizing and design loadings for these processes, as experimental work has consistently shown that higher loadings increase PdN efficiency and nitrogen removal rates.

Introduction
Membrane bioreactors provide robust removal of pathogens, as proven through ample studies, and demonstrations; this is attributed to a variety of mechanisms related to biological treatment in addition to the physical exclusion of the membrane itself. Difficulty in monitoring pathogen removal is a challenge in the implementation of potable reuse applications. (WHO 2017) Previous work covers baseline testing and assessment of MBR system performance with breaches in membrane integrity through intentional damage (Katz et al, 2021); this paper will continue forward and address the impact of standard operating procedures that can create instability in compromised systems as demonstrated through intentional damage. This includes conditions such as cleaning and pressure decay testing, where operating procedures may temporarily dislodge solids that have been beneficially plugging fiber breaches. Various tests were conducted under different operating modes and with varying levels of fiber damage. The monitoring of MBR system performance and understanding the impacts on effluent quality is critical to the successful operation of MBR systems in advanced water recycling. To address this, the following are methods of monitoring that were explored:
Turbidity - Turbidity monitoring is the standard for MBR permeate quality and has been adopted as the basis for monitoring in various potable water reuse guidance documents. It is a continuous online measurement that is a strong threshold limit for excellent system performance. (WaterSecure 2017)
#Solids-LRV - A method called 'Solids-LRV' was introduced to estimate the removal of pathogens including protozoa, bacteria and virus. The method is based on the standard method for TSS enumeration with a modification for a low-solids stream. The advantage of this modification is the resolution for TSS can be extended by 3 orders of magnitude thereby allowing the measurement of a high level of removal across the MBR system. The method offers the ability to directly measure the removal of particles across the MBR, is corrected to account for the high volumetric concentration factor in the system, and accounts for all pathogen removal mechanisms due to the selected filter size. (Katz et al., 2018; Katz et al,. 2020)
Pathogens - Pathogen reduction is critical in potable reuse systems and gaining credit for pathogen reduction across unit operations is core to the design of multi-barrier treatment schemes. The pathogens that are typically regulated for reduction in these systems are Protozoa and Virus. As an initial indicator of pathogens, measurements of E. Coli and Total Coliforms were made.
Experimental Testing was done at an MBR pilot unit in Ontario, Canada where raw sewage was screened and fed to the MBR system. Baseline information was gathered on the unit under various conditions, and then membranes were then intentionally damaged with increasingly more significant damage. The impacts on the system effluent were monitored after the damage events and after a variety of stress-events such as maintenance cleaning and pressure decay testing, to assess any instability that these events would create on the self-healing of the fibers. A summary of the experimental conditions and treatment set-up is shown in Table 1.
Results
The baseline data was consistent and no significant differences were seen between the different operating conditions. The average turbidity was 0.04 NTU and Solids-LRV was an average of 4.7 log reduction of solids through the MBR system. Baseline E. Coli log reductions were 7.5 and Total Coliform reductions were 7.4 in operation with relax.
Figure 1 shows the system performance after two maintenance cleans (MC), with 7 fiber cuts occurring in between the two cleans. Figure 2 and 3 both show the system performance following different amounts of fiber damage (15 and 60 cuts, respectively), each followed by a pressure decay test (PDT) and maintenance clean to the system. In each of the tests, maintenance cleaning and pressure decay testing both lead to instability in the membrane performance when the fibers were damaged. This instability of effluent quality is seen in both the increased turbidity and the decreased LRV values (Solids and E.Coli LRV). The amplitude of these spikes was impacted by the amount of damage to the system, with the biggest instability occurring when the damage was highest. The system also took the longest time to recover from stress when the amount of damage was the higher, however in all cases, all of the monitored parameters returned to baseline values within a few hours. Due to the nature of the test, the Solids-LRV values are generally reported on a 24-hr frequency, however the E. Coli was measured and reported at a greater frequency, including samples before the stress condition (fiber cutting, maintenance clean, or pressure decay test), at the beginning and end of the first production cycle after the stress condition and the production cycle 1 hour after the stress condition. This is interesting to see because the first samples after the stress condition show high variability, dropping as low as 2.9 in some cases, but the system quickly recovers from this stress, and returns to high levels of E. Coli removal. This shows direct correlation to the turbidity, which also recovers within a few production cycles.
Conclusions
Turbidity, E. Coli removal, and Solids-LRV all showed a strong correlation to each other across the varying levels of damage, demonstrating that there is promise in Solids-LRV being a complement to turbidity to augment system monitoring. This will provide a stronger indication of pathogen reduction and an additional monitoring point for MBR system performance. Where pathogen removal is a critical parameter for MBR performance, system design, permitting and monitoring should consider operational conditions which increase instability of the system, such as limiting the need to regularly use pressure decay monitoring.

Background
The adsorption/bio-oxidation (A/B) process has been gaining traction to be adopted in wastewater treatment plants (WWTP). It offers several advantages like energy savings due to reduced aeration in both stages, higher carbon redirection to the solids stream through the A-stage, and smaller footprint due to the reduced residence times in the A-stage and higher removal rates in the B-stage [1]. The alternating activated adsorption (AAA) is a novel A-stage system that retrofits the primary clarifiers. AAA integrates biological activity with the primary treatment to capture more carbon [2]. Full scale AAA system has been implemented in Strass WWTP in Austria. In previous work, it was demonstrated that AAA system has the potential to outcompete other A-stage systems in terms of carbon removal, redirection, and capture when treating raw wastewater [3]. Carbon redirection can be considered the key advantage of the A-stage systems [4,5]. Nonetheless, in addition to carbon redirection, A-stage significantly influence the B-stage by determining the B-stage influent's carbon characteristics. This includes carbon concentration, carbon fractionation, carbon to nitrogen ratio, and readily biodegradable carbon to phosphorus ratio. In addition, increased phosphorus assimilation has been reported in A-stage systems which affects the downstream phosphorus process. Such effects cannot be overlooked and shall be balanced with carbon redirection to maintain sustainable wastewater treatment. Unfortunately, previous studies focused on developing new A-stage systems [2,6,7] and studying the effect of solids retention time (SRT) on carbon redirection at multiple scales and for different processes [4,8,9]. However, the influence of other operational parameters (i.e., hydraulic retention time (HRT) and dissolved oxygen (DO) control) was rarely discussed.
This study aims at demonstrating the impact of the SRT, HRT, and DO on the performance of lab-scale AAA system. This is an attempt to evaluate the capacity to control an A-stage system to balance carbon redirection with other objectives including effluent quality in terms of overall removal and effluent fraction, and phosphorus removal.
Methodology
Multiple lab scale reactors (with working volume of 1.5L) were operated and fed with synthetic wastewater with concentrations of 470±46 mgCOD/L. The synthetic influent has different sources of organics and nutrients (i.e., soluble and particulate) to simulate real wastewater complexity [4]. The system was originally seeded with sludge collected from full scale activated sludge system. To evaluate HRT and SRT effect, Four SRTs (0.5, 1, 2, and 4 days) and three HRTs (1, 2, and 4 hours) were applied. Moreover, three DO levels were applied; Low DO (<0.5 mg/L), moderate DO (0.5-2 mg/L), and High DO (above 2 mg/L). Other than those three parameters, all other operational parameters were identical. Samples were periodically collected to monitor and quantify systems' performance. Measurements and calculations were conducted over the time interval between two sludge samples. Over this duration, effluent and wastage sludge were discharged to storage tanks from which samples were collected. Meanwhile, at least, two influent samples were collected to assure its consistency and representation of the influent over the same period.
Findings
At SRT of 0.5 and 1 day, comparable carbon redirection was attained while higher oxidation (Figure 1A) took place at SRT of 1 day which resulted in higher TCOD removal. Further increase in the SRT to 2 and 4 days resulted in higher oxidation which increased from 27±7% of the influent COD at SRT of 1 day to above 40% at 2 and 4 days. This increase was at the expense of the COD redirection which decreased to 22±8% from 45±8% at SRT of 1 and 2 days, respectively. No significant difference in the removal efficiency of the influent filtered flocculated COD (ffCOD) were observed at different SRTs (Figure 1B). Yet, particulate COD (pCOD) removal peaked at SRT of 1 day. It can be seen from Figure 1C that SRT did not affect phosphorus removal which averaged around 60%. Interestingly, such a value is much higher than values reported in the literature [9,12] and correlated with ffCOD removal. Finally, SRT is not reliable to control effluent fractions as, for example, ffCOD was always above 20% (Figure 1D). Increasing HRT from 2 to 4 hours has much lower effect on the redirection of the influent COD, as shown in Figure 2A. The average COD redirection only decreased from 45% to 36%. Meanwhile, such an increase was associated with an obvious increase in ffCOD removal (Figure 2B). Accordingly, HRT better controls the process's effluent fractions. As seen in Figure 2D, ffCOD fraction was decreased from more than 40% to less than 10% by HRT increase. On the other hand, pCOD removal declined at higher HRT which can be referred to the lower organics loading rates negatively affecting sludge settleability [14]. Interestingly, more than 30% increase in phosphorus removal was attained at HRT of 4 hours compared with 2 hours reaching 79±6%. Lastly, DO influence was evaluated at short HRT (1hour) and SRT (1day). It can be seen that the influent COD oxidation and removal decreased in correlation with DO concentration. Whereas, a drop in COD redirection was observed at low DO while similar redirection was attained at moderate and high DO (Figure 3A). As demonstrated in Figure 3B, the ffCOD fraction decreased in the effluent with DO increase while the pCOD fraction increased. The differences are not drastic between different DO levels which can be referred to the limited SRT and HRT which might have hampered COD uptake. Meanwhile, phosphorus removal was found to be the same regardless of DO applied averaging at 60%. Again, this might be referred to the limited retention times which did not allow for DO effect to be pronounced. COD oxidation and redirection are strongly correlated when only SRT is changing (Figure 4A). This can elucidate why SRT control results in higher redirection but the system fails to achieve higher oxidation (mainly due to soluble fraction removal) in the meantime. Yet, the lower association of between oxidation and redirection when HRT and DO (Figures 4B and C) is changing implies that better control on effluent COD fraction and achieving high removal and high redirection is possible.
Research significance
This study is the first to investigate the impact of SRT, HRT, and DO on the AAA system and DO and HRT effect on the A-stage process. It can be concluded that high phosphorous and ffCOD can be attained in the effluent while acceptable levels of redirection are maintained relying on HRT extension and keeping SRT less than 1 day. Yet, higher redirection is attained at lower HRT. It is demonstrated that, unlike SRT, HRT and DO have more flexibility to be changed to control the carbon fractions and P concentration of the B-stage influent.

The Boston Water and Sewer Commission (BWSC) owns and operates a wastewater and stormwater collection system that consists of 1,455 linear miles of sewer, eight pumping stations, 47,413 manholes, 35,934 catch basins, 243 outfalls, and 202 tide gates. In 2014, the Massachusetts Department of Environmental Protection (MassDEP), under 314 CMR 12.00, mandated that municipalities must complete an infiltration and inflow (I/I) analysis of their system and develop a targeted Sanitary Sewer Evaluation Study (SSES) in areas with excessive infiltration and inflow (I/I). BWSC developed a city-wide I/I Analysis Report in May 2017 to achieve compliance with 314 CMR 12.00. BWSC has performed several efforts to improve the overall operation of the system, including sewer separation, combined sewer overflow (CSO) reduction, sanitary sewer overflow (SSO) reduction, and rehabilitation work throughout the system. Since 2001, BWSC has commissioned multiple investigative studies in the Mattapan neighborhood of Boston to locate sources of I/I. These studies have implemented a multitude of field investigation programs including smoke testing, building inspections, dyed water testing, closed circuit television inspection, and manhole inspections. Prior to 2019, nearly 80% of the neighborhood had been smoke tested and every building was inspected for external sources of inflow. Despite these extensive investigations for inflow sources, flow metering conducted as part of the City-wide I/I Analysis Report completed in May of 2017 showed that many sewersheds within the Mattapan neighborhood still have a direct response to rainfall, with R-values greater than 10%. Therefore, in 2020, the BWSC embarked on a SSES to investigate the Mattapan neighborhood as well as portions of Hyde Park, Roslindale, Roxbury, and South Dorchester. The project team devised an alternative SSES strategy to identify previously undocumented sources of infiltration and inflow. As the typical investigative techniques to locate inflow sources, such as building inspections and smoke testing, had already been exhausted, a new and 'outside the box' approach was necessary. A dye water flooding program to investigate the impact of service lateral rainfall induced infiltration and inflow (RDI/I) was included in the Mattapan SSES implementation plan. Dye water flooding involves applying dyed water to the ground surface directly above a service lateral to simulate rainfall as shown in Figure 1. The dye and applied water infiltrate into the ground, potentially into the sewer system as shown in Figure 2. The mainline sewer is monitored at the connection between the mainline sewer and the targeted service lateral for the presence of dye with the use of a CCTV camera. The presence of dye or an increase in flow indicates a hydraulic connection between water percolating through the ground surface and defects in the service lateral. Over a two-week period in October 2021, the project team and its field teaming partner implemented the dye water flooding pilot program with the goal of investigating a total of 50 properties. Access issues, weather, application techniques, and public relations were only some of the difficulties faced during the planning and execution of the program. During the execution of the pilot program, the project team made observations and adjustments in the field to adapt the intended approach to simulate real world conditions while determining the contributions of service laterals to RDI/I. As shown in Table 1, a total of 44 investigations were completed, and approximately 30% of properties exhibited a hydraulic connection between water percolating through the ground surface and the service lateral itself.
Based on the findings of this Mattapan SSES Dye Water Flooding Pilot Program, service lateral dye water flooding has proven to be a worthwhile investigation technique under certain conditions. The purpose of this paper is to discuss the challenges and lessons learned from the implementation of the pilot program. Topics covered will include public outreach, public relations, rainfall simulation calculations, critical success factors, and recommended conditions for the implementation of this innovative investigation technique.

Introduction to One Water Honolulu
In December 2020, the Mayor of Honolulu signed a One Water Ordinance to address climate change concerns and codify the responsibilities of the previously-established City of Honolulu Office of Resilience (City), including developing and implementing a climate adaptation plan to proactively prepare for the physical and economic impacts of climate change and organize a One Water Panel of city water, wastewater, transportation and other agencies to build consensus on One Water policies and projects that the City should prioritize. The Resilience Office is also tasked with implementing a coastal and water program, a climate resilience and equity program, and a food security and sustainability program. The coastal and water program will coordinate actions related to mitigation and adaptation, protecting coastal areas, promoting resiliency and natural infrastructure development, and integrating water and urban forest management. This will include developing a coastal monitoring program, participating in the National Flood Insurance Program's Community Rating System, tracking water use by agencies, and providing technical assistance to departments and agencies.
Flooding in Mapunapuna
One of the most critical climate resiliency projects identified by the One Water Panel is the Mapunapuna area of Honolulu, HI, a low-lying industrial neighborhood (see Figure 1) built on fill, that is subject to subsidence and periodic flooding caused by a combination of heavy rainfall and storm drain system backups during high tides, especially king tide events. Commercial and industrial properties in the area have dealt with nuisance flooding over many decades, and the Honolulu Board of Water Supply has experienced more than 50 water main breaks in this area over the past 30 years, primarily due to corrosion and settlement. The nuisance flooding, which also impacts other buried utilities and infrastructure, roads, emergency vehicle access, and property usage, will continue to worsen with sea level rise (see Figure 2).
Assessment Methodology
The City has been addressing the flooding problem by installing backflow prevention devices ('duckbill valves') on the stormwater outfalls to block backflow from the ocean and stream during high tide and large storm events. With the looming threat of climate change and sea level rise, where climate models show a possible increase in sea level of over three feet by mid-century, more adaptations are needed if Mapunapuna wants to remain a viable industrial hub. Across the U.S., coastal cities like Miami and Boston have wrangled with similar flooding issues due to sea level rise. Options to address these issues include: armoring, or building grey and green flood barriers to protect infrastructure; raising or adapting the low-lying infrastructure to work in a flooded environment by elevating infrastructure or installing pump systems to remove water or allowing flooding on first floors of buildings; or relocating or retreating at risk infrastructure and population areas, where people or facilities may be relocated to higher elevations or some low-lying areas may be abandoned altogether. Many communities are considering a combination of these to cost-effectively mitigate existing flooding problems and adaptation for future climate conditions. As a catalyst project for One Water Honolulu, an initiative that promotes valuing all water forms and finding efficiencies and co-benefits of collaboration between agencies, a multi-agency Working Group considered important issues associated with climate adaptation and resiliency in Mapunapuna, including impacts on internal and external stakeholders and evaluating the cost, benefits, and potential timelines for adaptation development. The solutions investigated include: 1) building a flood resilience barrier for physical separation from the flooding potential of Moanalua Stream through the construction of additional backflow prevention devices and raised roads or a floodwall along the stream and southern end of the neighborhood, 2) a pumping system that can address the worse areas of flooding, and 3) the potential of raising streets and utilities to keep important infrastructure out of the reach of flood waters. These solutions were evaluated in the context of flood stages of the Moanalua Stream and potential sea level rise using FEMA watershed models, the potential benefits and the costs of the solutions are presented, and the timeline for implementation is explored.
Findings
Based on the economic benefits and cost analysis, the implementation of a floodwall and stormwater pumping system was not found to be feasible. But there are additional considerations and variables that should be weighed in any decision to move forward with flood mitigation or retreat. An adaptive management approach, shown in Figure 3, was taken to target solutions and decisions that will mitigate flooding for critical areas in the near-term to extend the useful life of the area as well planned and proposed studies that would be used to support these decisions. Long-term, the impact of climate change will be re-evaluated and the approach will be refined with a phased approach for armoring, raising and relocating combinations for large mitigation projects in Mapunapuna. The alternatives and mitigation/adaptation plan will be refined through the public participation process. Any successful solution for the Mapunapuna area will require the active participation of local stakeholders, including landowners and tenants along with public and private funding to implement the mitigation and adaptation measures. The approach regarding when and how to engage stakeholders in the findings of numerous studies was explored by the Working Group as well.

INTRODUCTION Many anaerobic digestors have not been fully driven to its stable performance potential. Despite the many merits, the anaerobic digestion (AD) process has several drawbacks such as low hydrolysis rates, unstable at high organic loading rate, and release of toxic/corrosive hydrogen sulfide (H2S). Suboptimal AD performance may mean under-utilised biogas production, foaming events, or poorly dewatered cake. Traditional AD processes will benefit from a facelift that maximizes biogas production, enhancing process stability and broadening substrate treatability without major infrastructure modification. Moreover, elevated concentrations of H2S in the biogas is often a limitation for downstream processing of biogas that typically requires expensive gas conditioning before it is used to generate energy. This facelift could be given by implementing micro-aeration, a process-integrated technology that is defined as a system with zero concentration of oxygen present in the sludge and limited (trace) consumption of oxygen. Bench-scale and full-scale tests of organic waste digestion processes using micro-aeration technology was undertaken. The tests confirmed that this process-integrated technology has potential to use minimum plant upgrading while contributing to the sustainability and economic efficiency of the energy recovery process as operated for example at water resource recovery facilities (WRRFs). Pre-treatment processes such as THP is installed progressively to improve the bioavailability of the organic substrate in the digester and bust the methane production. Despite the advantages of a high-thermal pre-treatment processes, microbes in the feedstock have been killed-off resulting in a different, and potentially a reduced diverse microbial community in the digesters, which could make the AD process vulnerable. Biogas generated from anaerobic digestion of sludge typically requires gas conditioning before it is used to generate energy as it can create maintenance related corrosion problems in combustion engines, and the release of SOx in flue gases. Micro-aeration has been researched and used in industrial biogas plants with H2S removal efficiency of more than 90% in most cases (Jenicek et al., 2017, Diaz et al., 2017, Kraakman et al., 2019). Moreover, micro-aeration has shown to improve the degradability of COD and volatile suspended solids in some cases and enrich the diversity of the microbial community potentially improving process stability. OBJECTIVE The main objective of this presentation is to share information about micro-aeration and explain how it has potential to enhance the digestion process and improve the biogas quality. The presentation will include bench-scale research and full-scale testing data as well as several lessons learned. METHODOLOGY Three main full-scale case studies at different WRRFs are used to illustrate the potential and the challenges when implementing micro-aeration technology. For these case studies micro-aeration was tested over a period of several months and up to one year. In all cases, air was injected in the sludge recirculation pipeline after the recirculation pump using fine bubble air injection nozzles. The biogas H2S concentrations and sludge characteristics such as pH, alkalinity, total solids, volatile solids were measured daily using industry standard methods, while the biogas production and sludge feed were measured continuously. Separately, bench-scale testing focused on investigating benefits from micro-aeration on digestion stability and performance. 20-L bench-scale digestion testing used Napier grass (5 months old, a typical energy crop) with and without micro-aeration with cattle manure-derived inoculum were performed at OLR of 5.0 g VS/L/day. Microbial community structure and function were investigated by the analysis of 16S rRNA gene and meta-genomics sequencing. RESULTS Bench-scale research at an organic loading rate of 5 g volatile solids (VS)/L/day showed increased digester stability and performance with micro-aeration, operating at residual acetic acid concentrations of 3.0 g/L, compared to instable control at 9.2 g/L without aeration introduction as shown in Figure 1 (Nguyen et all., 2019). The test digester with micro-aeration also showed an increase in methane yield. 16S rRNA gene sequencing analysis shows increase in relative abundance of facultative bacteria with micro-aeration, while the dominance of hydrogenotrophic methanogens remained similar with and without micro-aeration (Wu et all., 2021). Full-scale micro-aeration test were performed mainly as a means to reduce H2S concentration in the biogas. An example of the reduction of the H2S concentration in the biogas before and after introducing the micro-aeration process is that the H2S concentration in the produced biogas dropped by more than 80% from 5,000 ppmv to less than 1,000 ppmv in the first case study; the second case study showed a decreased H2S concentrations from 1,000 ppmv to 200 ppmv (see Figure 2). The presence of small amounts of oxygen in the anaerobic digestion process in the full-scale test did not impact the biomethane productivity when the amount of air injected is limited. In fact, these full-scale tests (like other bench-scale studies) revealed that, besides efficiently desulfurizing biogas, micro-aeration can potentially improve organic matter degradation during municipal sludge digestion. However, not all attempts of implementing microaeration have resulted in consistent H2S removal; the third case study showed that the overall average was only 40% suspected to be due to limitations in oxygen transfer. The potential capital cost savings as well as operational cost savings that micro-aeration technology can provide is substantial (see Table 1). Lessons learned to be shared as part of this manuscript and presentation are related to: - method and locations for micro-air injection - full-scale implementation design including safety measures - potential risk of elemental sulfur accumulation in downstream equipment - process model development to support process understanding and optimization CONCLUSION From the full-scale tests and related bench-scale research, it can be concluded that micro-aeration technology holds great potential for being an effective gas conditioning technology and enhance the digestion process, while requiring minimal capital investment and operating costs. Micro-aeration would further contribute to the sustainability and economic efficiency of the energy recovery process of organic waste digestion as operated for example at WRRFs.

Introduction
Conventional design of enhanced biological phosphorus removal (EBPR) focuses on providing volatile fatty acids (VFAs) to phosphate accumulating organisms (PAOs) in an influent anaerobic zone. As an alternative, sidestream EBPR (S2EBPR) diverts a portion of RAS flow to a longer hydraulic retention time (HRT) sidestream reactor. In this sidestream reactor, biomass and particulate COD is fermented to generate the VFA required for PAO metabolism. In a strict RAS fermentation reactor, this results in selection of PAO metabolism without direct use of influent readily biodegradable COD (rbCOD) or VFA. In a carbon focused wastewater paradigm, this provides increased flexibility for COD diversion and capture; use of influent rbCOD for nitrogen removal; and potentially more stable phosphorus removal due to a lack of sensitivity to influent COD characteristics. In addition, moving the PAO selection to the sidestream provides increased infrastructure flexibility for incorporation of EBPR into existing facilities. The Wisconsin Rapids Wastewater Treatment Plant (WRWWTP) began pilot testing of RAS fermentation in April 2021. The facility moved through a series of operational setpoints (Figure 1) to optimize phosphorus removal. One of the keys was the addition of a cranberry waste to provide additional carbon for RAS fermentation. By the end of the testing period, effluent total phosphorus was averaging less than 0.4 mg/L with no chemical addition (Figure 2).
Carbon Balance Exploration
To better understand why the S2EBRP configuration was working at the WRWWTP, a series of bench-scale tests were completed to explore the carbon balance for the facility. Batch RAS fermentation rate tests were completed as previously described (WRF Project UR13). The fermentation rate tests provided an indication of the amount of soluble biodegradable COD (sbCOD) produced per mass of volatile solids in the fermenter. On average, a fermentation rate of 1.5 mg sbCOD/g vss-hr was observed over the testing period (Figure 3). This is a critical piece of information, as it provides insight into how much carbon can be released from RAS fermentation. Phosphorus release and uptake rate testing (Figure 4) provided insight into the use of carbon by PAOs in the system. As EBPR was established, the sbCOD:P ratio was reduced to 13:1 (Figure 5).
This information provides insight into the ratio of carbon required by PAOs in the S2EBPR reactor. The last set of batch experiments explored the phosphorus release rate of the ecology with various external carbon sources. The full-scale system was fed a cranberry juice wastewater, with was dominated by sugars. During bench scale testing, the cranberry juice exhibited the highest release rate of the three carbon sources tested (Figure 6). This difference is likely driven by an adaption of the ecology to the carbon source, and not specifically tied to the cranberry juice. Although, cranberry juice is well noted for its overall health benefits, and the antioxidants from Wisconsin grown cranberries may provide additional benefits to PAOs. But this detail was not explored during this research. The bench scale testing provided the information to complete the carbon mass balance for an S2EBPR system (Figure 7). Using the measured rates, the sbCOD deficiency can be calculated. If a deficiency is occurring, a reduced level of effluent phosphorus removal performance would be anticipated. If the deficiency is zero, or if extra carbon is available in the RAS fermenter, effluent phosphorus concentrations should be reduced. As shown in Figure 8, the mass balance around carbon correlated to observed effluent phosphorus performance.
Benefits and Significance
This presentation will provide a detailed review of an extensive testing of S2EBPR. The following benefits will be provided to the audience: - High-level review of bench testing protocols - Value proposition for adding bench testing to operational data for EBPR and S2EBPR facilities - Outline of how to complete a carbon balance for an S2EBPR facility - Target values for fermentation and sbCOD:TP in S2EBPR facilities These results will be significant for anyone planning, designing, or operating an EBPR facility.

1. INTRODUCTION
Potable reuse with reverse osmosis (RO) presents brine disposal challenges for inland communities. Carbon-based advanced treatment (CBAT) eliminates these brine disposal challenges. CBAT trains often include combinations of the following processes: ozonation, biologically activate carbon (BAC) filtration, granular activated carbon (GAC), coagulation, microfiltration (MF)/ultrafiltration (UF), UV disinfection, and/or UV advanced oxidation processes (UV/AOP). Much research has been done on chemical removal by the processes used in CBAT. Nevertheless, demonstration projects continue to provide meaningful information about new or overlooked chemicals and their occurrence or removal in wastewater effluent. The concentrations of chemical pollutants vary between wastewater treatment plants (WWTPs) based on industrial point sources or different consumer patterns. WWTPs also have site-specific differences in water quality parameters that impact treatment efficiency [e.g., nitrite consuming the ozone dose or total organic carbon (TOC) competing with trace organics for sorption sites on GAC]. Thus, piloting is necessary at planned reuse sites to (1) demonstrate the safety of reuse to the public and regulators, (2) fine-tune process design, and (3) identify key site-specific health- and performance-based indicators. In 2020, Colorado Springs Utilities (CSU), the Colorado School of Mines (CSM), and Carollo Engineers, Inc. collaborated to design and build a mobile Direct Potable Reuse (DPR) demonstration trailer. The PureWater Colorado demonstration trailer has served multiple purposes: public engagement, research, piloting for Colorado utilities considering reuse, and informing state-level reuse policy. The CBAT process in the trailer includes ozonation, BAC, GAC, MF, UV/H2O2, and chlorination. PureWater Colorado was commissioned in June 2021. Routine sampling is ongoing at multiple points through the multi-barrier, CBAT process. Several evaluations have been conducted as part of this project, but this presentation will focus on the identification and removal of health- and performance-based indicators.
2. METHODS Health-based indicators are defined as specific chemicals that occur at concentrations with human health relevance at the influent to the reuse system-regardless of their difficulty to remove or federal regulatory status (Drewes et al. 2018; Anderson et al. 2010). Health-based indicators specific to the CSU installation were selected using the procedure recommended by a Science Advisory Panel to the state of California to select which trace organics to monitor in potable reuse. Maximum concentrations of each chemical at the pilot influent were divided by Monitoring Threshold Levels (MTLs). These MTLs are based on drinking water guidelines or toxicity information from a variety of sources including the California Notification Levels and the Australian reuse guidelines. Since the focus of the above Science Advisory Panel was on organics, concentrations of metals and inorganics were divided by their USEPA Primary Maximum Contaminant Levels (MCLs). Chemicals with a maximum measured value to MTL ratio (max/MTL) would be necessary to remove to protect the public health, either through treatment or source control. Chemicals with max/MTL greater than 0.1 would be recommended for removal to maintain a desirable margin of safety. Performance-based indicators are defined as specific chemicals that are useful for monitoring whether a treatment process is performing as designed-regardless of their toxicity (NWRI 2019; Thompson and Dickenson 2020). Performance-based indicators specific to CSU were selected using the procedure recommended by a National Water Research Institute (NWRI) Independent Expert Advisory Panel to the state of Colorado. Chemicals were identified that would have 'sweet spot' moderate removal for each process. Ideally, the chemical would still be detectable after the process when the process is functioning properly, but the chemical's removal would drop noticeably during a malfunction. Among chemicals that met these criteria, the one with highest median concentration divided by its Method Reporting Limit (MRL) was selected, since this chemical could be most precisely quantified.
3. RESULTS Three health-based indicators with max/MTL > 1 were identified in the CBAT influent at CSU: the industrial chemical quinoline, the antibiotic amoxicillin, and the perfluoroalkyl substance (PFAS) perfluorooctanoic acid (PFOA) (Table 1). Fortunately, these three chemicals have so far been removed to below detection by the combination of treatment process, though this should continue to be monitored as the adsorptive capacity of the GAC diminishes over time. All measured chemicals were below their MTL or MCL in the CBAT purified water, through nitrate was closest to its limit. This demonstrates the importance of effective, consistent nutrient removal upstream of reuse (Hill et al. 2018). Based on the criteria described in Methods, the pharmaceutical primidone or the pesticide diuron would be suitable performance-based indicators for ozonation at CSU. Primidone would also be the best performance-based indicator for GAC (Figure 1). While no organic chemicals were consistently detected after GAC as of the time of this writing, N-nitrosodimethylamine (NDMA) could be an appropriate performance-based indicator for UV/H2O2, based on (1) its detection after BAF (Figure 1), (2) its known rapid breakthrough in GAC, (3) its resistance to oxidation, and (4) its moderately high removal by UV (Glover et al. 2019; Thompson and Dickenson 2020).
4. CONCLUSIONS AND NEXT STEPS Highest priority health-based indicators and ideal performance-based indicators identified in this study differed from those identified in technical reports or peer-reviewed literature conducted at other locations. For example, Drewes et al. (2018) recommended health-based indicators NDMA, NMOR, and 1,4-dioxane, and performance-based indicators gemfibrozil, sulfamethoxazole, iohexol, and sucralose. This exemplifies the importance of site-specific piloting and indicator selection. Fundamental to the mobile aspect of this project is that when piloting is complete at CSU, the PureWater Colorado trailer will become available for other Colorado utilities considering potable reuse.

Introduction
Amidst progressively tightening effluent nutrient limits over the next 20 years (Figure 1), Metro Water Recovery (Metro) is exploring secondary treatment technologies at the Robert W. Hite Treatment Facility (RWHTF) to examine potential opportunities to leverage the existing infrastructure to increase capacity, provide flexibility to meet future effluent nutrient limits, improve reliability while minimizing footprint, reduce chemicals and energy, and reduce complexity of operation. The RWHTF, located in Denver, Colorado, is comprised of two independent liquid stream processes (North Treatment Complex and South Treatment Complex) where preliminary, primary, secondary, and disinfection treatment occurs. The North Secondary Complex (NSEC) is rated for 106 million gallons per day (MGD) while the South Secondary Complex (SSEC) is rated for 114 MGD, for a total treatment capacity of 220 MGD. The RWHTF currently achieves annual median effluent limits of total inorganic nitrogen (TIN) < 15 mg/L and total phosphorus (TP) < 1 mg/L. Secondary treatment at the RWHTF currently utilizes a combination of biological, chemical, and physical processes to remove solids, carbon, nitrogen, and phosphorus from primary effluent. Both NSEC and SSEC employ a Modified Ludzack-Ettinger (MLE) process for nitrogen removal with flexibility to operate in an enhanced biological phosphorus removal (EBPR) mode using a three-stage A2O (anaerobic, anoxic, oxic) configuration. The existing NSEC capacity is effectively limited by the secondary clarifiers, while the SSEC capacity is limited by aeration basin capacity. Both systems are currently designed with aggressive minimum aerobic SRTs (3 to 5 days), which presents challenges for meeting future effluent ammonia and TIN standards. Additionally, settleability issues limit the ability to reliably operate at MLSS values greater than 3,000 mg/L, particularly in the NSEC where the clarifiers are notably shallow (10-foot sidewater depth, compared to a 14-foot sidewater depth in the SSEC). Although limited, both NSEC and SSEC have available space for expansion; however, the facility is landlocked and quickly reaching build-out. For NSEC, there is space to build two additional aeration basins and two additional 130-ft clarifiers. Similarly, for SSEC, there is space to build two additional aeration basins and two additional 140-ft clarifiers. Yet, substantial capital investment would be required for such expansions: $80M for NSEC and $67M for SSEC. Moving into the future, technologies that allow Metro to cost effectively increase aerobic solids retention time while improving sludge settleability would significantly benefit secondary treatment capacity at the RWHTF without the need for additional aeration and clarification infrastructure and the associated capital expenditure. More specifically, Metro identified the need to increase aerobic solids retention (aSRT) to between 6 and 8 days to meet nitrogen removal targets at future flow and load conditions. Densified activated sludge (DAS) was identified as a promising technology that may support intensification and allow Metro to meet aSRT requirements while minimizing, if not eliminating, the need for additional aeration basins and secondary clarifiers. Metro commissioned a full-scale continuous flow DAS demonstration train in 2018 to: i) assess the feasibility of achieving densification, ii) inform treatment capacity rating and expected performance of full-scale DAS, and iii) gain insights into long term DAS operations. This work documents how findings from the full-scale demonstration are being leveraged to maximize treatment capacity of the existing infrastructure.
Methods
The full-scale DAS demonstration (denoted as 'test') facility consisted of an isolated, paired aeration basin (2.05 million gallons (MG)) and secondary clarifier (13,270 ft2) that was equipped with a dedicated primary effluent pumping station designed to mimic full-scale diurnal flow patterns. The test facility was equipped with a hydrocyclone skid comprised of eight 10 m3/h hydrocyclones that was used for physical selective wasting. The test facility has been in operation since May 2018 and been used to test eight (8) distinct metabolic and kinetic selector configurations. Metro has also operated 'control' trains that do not have similar metabolic, kinetic, and physical selection capabilities.
Sampling and Data Collection
- Over the course of the demonstration operation, field sampling/analysis was performed and operational data was collected to characterize performance and inform subsequent process modeling. Modeling and Conceptual Design - Data from testing were used to calibrate and validate whole plant process models and computational fluid dynamics (CFD) models of the RWHTF secondary clarifiers. These calibrated models were used to determine the capacity of the existing infrastructure under different settling and aSRT conditions as well as assess performance and risk mitigation strategies for addressing treatment capacity needs. A conceptual design of full-scale DAS was developed and will be discussed in the paper and presentation.
Results and Discussion
The test train had significantly improved settling characteristics versus the control train (Table 1). Results from stress testing confirmed that the test train could be loaded at surface overflow rates (SORs) > 700 gpd/sf and solids loading rates (SLRs) > 60 pounds per day per square foot (ppd/sf) and still maintain reliable blanket control and stable effluent TSS (Figure 2). The control train was observed to experience failure if SLR exceeded 35 ppd/sf. Results from the whole plant evaluation indicated the following: - The capacity of the existing infrastructure is projected to be exceeded between 2030 and 2032 under historical settling conditions (non-DAS condition). Up to 4 new treatment trains would be required to meet the projected 2040 flow and load conditions while supporting increasingly stringent nutrient limits. - Operating in the highly densified mode (i.e.,75th percentile SVI < 80 mL/g) reflective of test train performance is expected to provide the necessary capacity within the existing infrastructure through 2040 without the need for any additional aeration basins or secondary clarifiers. Under this condition, Metro can operate at an 8 day aSRT and maintain one aeration basin and secondary clarifier out of service. - Sensitivity analyses indicated for a 75th percentile SVI of 100 mL/g, the 2040 flow/load and nutrient removal requirements can be achieved by either operating at a 7 day aSRT with one basin and clarifier out of service OR operating a 8 day aSRT with all units in service. - Additional risk mitigation strategies available to Metro, in the event that full-scale DAS 75th percentile SVI exceeds 100 mL/g include increasing metabolic and kinetic selective pressure through modifications to reactor basins and implementing technologies to selectively increase aSRT. Cumulatively this work illustrates that the combination of metabolic, kinetic, and physical selection can be used to successfully achieve continuous flow densification. Additionally, by leveraging DAS, Metro will be able to offset secondary treatment expansion costs associated with up to 4 aeration basins and secondary clarifiers (estimated at $147M). Full details regarding the model calibration and utilization as well as the design basis for the full-scale facility will be provided in the paper and presentation.

Background and Objectives
The Great Lakes Water Authority (GLWA) Water Resource Recovery Facility (WRRF) serves 3.1 million residences in Southeast Michigan. Treating a combined sewage inflow up to 3,560,000 m3/day (940 million gallons per day, MGD) it is one of the largest treatment facilities in the world. The secondary treatment process consists of four limited-nitrifying High Purity Oxygen (HPO) activated sludge bioreactors, consisting of eight cells in two bioreactors and ten cells in the other two bioreactors, followed by 25 circular secondary clarifiers. Phosphorus removal is achieved by a combination of ferric chloride addition to influent wastewater prior to primary treatment along with coincidental biological phosphorus removal (bio-P) occurring in the HPO system. Influent wastewater characteristics are highly variable, likely due to sedimentation and biological activity occurring in the large volume and high residence time of the combined sewer system.
This paper summarizes the results of over four years of work: (1) characterizing variations in the influent wastewater characteristics, (2) documenting the unique bio-P performance of the system resulting principally from fermentation occurring in the secondary clarifiers (a form of side-stream fermentation), and (3) consolidating these results in a plant-wide model characterizing chemical (ferric chloride) and biological phosphorus removal along with impacts of solids handling recycles. These results should be of interest to those working to achieve effective phosphorus removal along with those dealing with extreme wet weather flow conditions.
Methodology
Experimental work consisted of extensive analysis of 8-2/3 years of liquid stream operating data (from January 2013 to August 31, 2021), coupled with influent wastewater characterization, full-scale bioreactor profiles, batch reactor testing, and jar testing. Process modeling was conducted using SUMO21 (Dynamita), calibrated as depicted in Figure 1. Figure 2 is the developed model from this study to reflect actual operating conditions for modeling historical performance (i.e. calibration). Batch reactor tests conducted in the late summer of 2018, early winter of 2020, and early winter of 2021 assisted with model calibration. The tests were performed to simulate the anaerobic and aerobic periods occurring in the full-scale HPO bioreactors and within the secondary clarifier sludge blanket. Appropriate quantities of return activated sludge (RAS) or concentrated mixed liquor were combined with primary effluent, then sampled for measuring ortho-phosphorus, soluble COD (sCOD), and oxygen uptake rate (OUR). An initial half-hour anaerobic period was followed by two-hour aerobic period per one cycle of batch tests. In some cases, a further two-hour anaerobic period was added to simulate residence time within the secondary clarifier to measure bio-P's performance in different seasonal conditions. In 2021, another set of batch reactor was operated under strict anaerobic conditions for 72 hours to measure phosphorus release rate and possible phosphorus release amounts. Wastewater characteristics over the entire wastewater treatment region were separately analyzed for 5-days biological oxygen demand (BOD5), total chemical oxygen demand (COD), sCOD, total phosphorus (TP), total SP (TSP), total suspended solids (TSS), volatile suspended solids (VSS), and influent volatile fatty acid (VFA) concentration. A year-long intensive sampling phase evaluated particulate, colloidal, and dissolved biodegradable, and non-biodegradable organic matter.
Results
Influent fractionation is a crucial component of calibrating the model and predicting bio-P performance from modeling because biodegradable organic amounts (including soluble particulate forms) largely affect microbes' activities in the system, and it is generally assumed the fraction of biodegradable organic matter from influent VSS as 0.625. Although this assumption returns a reasonable estimation, it cannot capture the nature of fluctuating influent wastewater, resulting in deviations in model prediction. An interesting correlation was observed from the dataset (Figure 3), showing a robust correlation between influent BOD/VSS concentration and its removal ratio over primary clarifiers, dBOD/dVSS (here dBOD and dVSS refer to the removed fractions, respectively, of the two components). This leads to estimate fraction of biodegradable particulate organic matter from influent VSS, allowing reasonable approximation of daily varying wastewater characteristics for simulation. The modeling results are displayed in Figure 4 where MLVSS was used as an indicator for the calibration performance over the input mass balance. The bio-P performance was calibrated using batch test results (Figure 5), and Figure 6 compares the results predicted by the calibrated model. We find that influent VFA concentrations were insufficient to support bio-P activity, and the initial unaerated zones were not large enough to either supply or produce the VFA's needed for PAO growth. However, we found that sludge fermentation in the secondary clarifiers did provide sufficient VFA's for PAO growth. Analysis of the model results identified three general factors affecting bio-P performance: (1) For the GLWA WRRF, the data suggests that an SRT of 1.8 days or shorter at a wastewater temperature of about 10 °C leads to PAO washout, and subsequently to elevated effluent TSP. (2) During warm periods, even small amounts of nitrite and nitrate generated by nitrification adversely impact bio-P because they act as an alternative electron acceptor in the secondary clarifier sludge blanket, resulting in reduced fermentation and VFA production for PAO uptake and phosphorus release, which suppresses the PAO population. (3) A sufficient sludge blanket depth must be maintained to achieve sufficient fermentation and VFA production for PAO growth. Conclusion Influent characteristics are utilized to estimate time varying influent particulate organic matter fraction, which improves model prediction performance. The calibrated modeling results suggested operational protocols for improving bio-P performance, these include: (1) seasonal adjustment of SRT (range of 2.0 to 2.5 days) in response to wastewater temperature variations to avoid PAO washout during low-temperature conditions and to minimize nitrification during warmer periods, (2) maintaining a secondary clarifier sludge blanket of 3.3 feet. Minor physical modifications are also suggested, including the addition of mixers to the bioreactor's initial zones and the diversion of pure oxygen feed around these aeration deck stages to improve bio-P performance via the establishment of deeper fermentation conditions. The fermentation also could be increased by directing RAS to the first stage of the aeration deck and using step feed capabilities to direct influent to the second stage. The results of suggestions show the improved bio-P indicated in Figure 7.
Discussion
The effect of cessation of ferric chloride addition to the influent wastewater was simulated by adjusting PE, TP, and TSP concentrations to account for historical chemical phosphorus removal. Figure 8 presents estimated effluent TP level in comparison to revised influent TP concentration assuming ferric chloride is not added to the influent wastewater. The results indicate that plant effluent TP discharge limits can be met by developing these suggested conditions, suggesting that ferric chloride addition could be reduced or eliminated while sustaining required performance.

Introduction and Background
Carbon transformations and bioavailability impact overall nutrient removal as well as sludge settleability. Bioavailability of organic carbon is a known limiting factor for simultaneous N and P removal. A significant fraction of organics in municipal wastewater is in the particulate and colloidal form (slowly biodegradable COD, sbCOD) that must be hydrolyzed to readily biodegradable COD (rbCOD) before it can be utilized by microorganisms. Hydrolysis is the rate limiting step in denitrification, as the process is slower than heterotrophic growth (Inset et. al, 2003). For simultaneous nitrification and denitrification (SND) systems, hydrolysis of particulate COD during aerobic zones can increase the carbon availability for denitrification. Prior to aeration, some hydrolysis will also occur within anaerobic selectors, where sbCOD can be converted into intracellular carbon storage, such as poly-b-hydroxybutyrate (PHB). In subsequent aerated zones, internally stored carbon can be utilized for denitrification. Aerobic hydrolysis rates can be measured using batch or semi-continuous respirometry methods and curve fitting to surface-saturation-type rate equations (Inset et. al, 2003). These methods are able to estimate the maximum hydrolysis rate for sbCOD when oxygen is not limiting. However, these rates do not reflect observed hydrolysis rates at low DO or anaerobic conditions, which are favorable for anaerobic uptake of carbon and for SND. As part of a Water Research Foundation study (WRF 5083), laboratory methods are being developed to estimate hydrolysis rates under aerobic and anaerobic conditions. Hydrolysis coefficients have been estimated using respirometry (oxygen uptake rate (OUR)), and curve fitting (see Figure 1 by Insel et al., 2003). This method is being adapted for low DO and anaerobic conditions in the Sturm lab at KU, with the nitrate utilization rate (NUR) being used instead of OUR. Internally stored carbon, as PHB, is also being measured within sludge acclimated to low DO BNR conditions. Experimental rates for hydrolysis under anaerobic and low DO conditions will be presented along with modeling results that demonstrate how the hydrolysis rates in sidestream fermentation reactors affect the design for advanced BNR systems. Experimental rates are being determined from pilot reactors fed with primary clarifier effluent and acclimated to low DO setpoints (< 0.5 mg/L). The laboratory research contributes to the larger WRF 5083 project and will be used to evaluate model calibration of full-scale treatment plants.
Materials and Methods
Pilot Operation Two identical 200-L continuous flow pilot reactors were operated at a flowrate of 575 L/day at the Kansas River WWTP, KS, US. The pilots were fed with primary effluent wastewater that passed through two anaerobic selector zones and three aerobic zones prior to settling. Aeration and mixing were achieved with porous air stones and axial flow impellers to maintain a DO setpoint between 0.3 - 0.5 mg/L in the first train and 1.5 - 2.0 mg/L in the second train.
Experimental Hydrolysis Rate Determination Activity batch tests were used for evaluating hydrolysis rates: the conventional oxygen utilization rate (OUR) test for aerobic hydrolysis and the nitrate utilization rate (NUR) test for anaerobic hydrolysis (Ekema et. al, 1986). Oxygen uptake data was measured with a fiber-optic oxygen sensor (FireSting -PRO, PyroScience) for the aerobic OUR test. NUR tests were completed by adding 10 mg-N/L of nitrate while maintaining anoxic conditions, as observed by DO readings less than 0.1 mg/L. Then, the mixed liquor was spiked with a carbon source > 400 mg COD/L, so that the NUR values were at the maximum hydrolysis rates and were not affected by differences in rbCOD (Melcer, 2004). Nitrate concentration was observed over time with a nitrate sensor (1006 Nitrate ISE, YSI) and confirmed with ion chromatography analysis. Mixed Liquor Volatile Suspended Solids (MLVSS) measurements were completed according to standard methods (APHA, 1992.) Hydrolysis parameters are fitted for both aerobic and anaerobic hydrolysis according to the multiple experiment curve fitting procedure outlined by Insel et al., 2003 (see Figure 1).
Sensitivity Analysis of Hydrolysis Rates for Sidestream Fermentation (Modeling) In BioWin, aerobic hydrolysis of particulate substrate to readily degradable complex substrate is the product of a hydrolysis rate constant (2.1 d-1), heterotrophic biomass concentration, and a Monod expression for the ratio of particulate substrate to biomass COD. The hydrolysis rate is reduced under anoxic and anaerobic conditions by applying an efficiency factor of 0.28 and 0.04, respectively. However, based on modeling of conventional and sidestream EBPR processes, this model formulation has been re-evaluated and modified to a standard product inhibition model where the hydrolysis rate only decreases as rbCOD accumulates. That is, the hydrolysis rate is not reduced for unaerated conditions unless required for calibration purposes using an efficiency factor. The results from this work will be used to confirm this new model formulation and refine the hydrolysis kinetics including the product inhibition and efficiency factors.
Results
Aerobic hydrolysis parameters can be obtained from OUR respirometry batch tests. This method is being adapted for NUR. Initial results are presented in Table 1, with three carbon sources tested (nutrient broth, acetate, and potato starch), which represented different sbCOD concentrations. These initial tests were used to validate that anoxic conditions are maintained in the experimental set-up (Figure 2) and that increasing sbCOD concentrations yielded decreasing NURs, as expected. Continuing research will adjust the S/X ratios of complex wastewaters with a ratio of pCOD and rbCOD. Recycled activated sludge fed to a sidestream fermenter is also being tested as a model complex wastewater that undergoes hydrolysis. This wastewater, obtained from the Wakarusa, KS wastewater treatment plant, is typical of RAS fermenters being calibrated in the WRF 5083 study.
Conclusion and Significance
Aerobic hydrolysis is routinely modeled in process simulators, but recent efforts to calibrate sidestream fermenters for EBPR have required adjustments to the hydrolysis function. The results from this work will be used to confirm new model approaches for anaerobic hydrolysis.