Maltos, Rudy

Rudy Maltos, an adventurous PhD graduate from Colorado School of Mines, he applies his passion and excitement to the intricate world of process engineering at Metro Water Recovery. When hes not refining densification processes for activated sludge or optimizing phosphorus recovery techniques, youll find him enthusiastically supporting the Denver Nuggets or navigating Colorados white water rapids. Rudys passion for applied research and zest for adventure make him a dynamic force in both his professional and personal pursuits.

Titles from this speaker

Combining Metabolic, Kinetic and Physical Selection to Achieve Full-Scale Continuous Flow Densified Activated Sludge and Nutrient Removal at Robert W. Hite Treatment Facility

Abstract

Introduction: Metro Water Recovery's (Metro) Robert W. Hite Treatment Facility (Hite) in Denver, CO is permitted for a maximum month (MM) flow of 220 mgd and has two parallel liquids treatment trains including the North Secondary (NSEC). The NSEC utilizes a modified Ludzack Ettinger (MLE) process with a sidestream anaerobic reactor (SAR) to achieve nutrient removal. In 2018, Metro's facility plan identified a need for increased aerobic solids retention time (aSRT) to meet future potential total nitrogen (TN) and total phosphorus (TP) regulations (Colorado Regulation 31). To avoid costly expansion, Metro commissioned a full-scale demonstration train (NAB2) to trial densified activated sludge (DAS). This work documents findings from the full-scale demonstration facility and discusses how Metro is translating DAS into a full-scale solution at the RWHTF. Specifically, this paper will discuss factors that have led to the development and long-term operation of DAS with biological and physical selection, as well as provide insights into how particle size distribution (PSD) may be impacted by operational parameters and influence nutrient removal. Materials and Methods: DAS facility: The DAS demonstration facility (denoted as NAB2) consisted of an isolated, paired aeration basin and secondary clarifier from NSEC. Description of the facility is provided elsewhere (Avila et al., 2021). Metro operated NAB2 to test multiple distinct metabolic and kinetic selector configurations, with two primary configurations, Fig 1. A 'control' train was also operated without physical and biological selection. Routine influent/eff concentrations of organics, solids and nutrient content were determined using Standard Methods for the Examination of Water and Wastewater. PSD of the MLSS, hydrocyclone UF, and hydrocyclone OF were routinely characterized. Settling Column Testing: Settling column testing of bulk DAS ML was conducted on multiple occasions to determine the Vesilind equation settling coefficients (V0 & k) as well as the PSD. Additionally, settling properties for particle size fractions: <200µm, 200-600µm, and >600µm were characterized across a concentration range of approximately 1,500 mg/L to 6,000 mg/L. Activity Testing: Batch scale activity testing of bulk mixed liquor and three particle size fractions was used to evaluate nutrient removal rates of activated sludge in 5-gallon reactors. Analytes (NH4, NO3, NO2, and PO4) were measured with HACH TNT kits. Hydrocyclone Nozzle Testing: Hydrocyclone UF nozzle sizes ranging from 15 to 25 mm were individually tested at a target pressure of 30 psi. Flow measurements from the hydrocyclone feed and OF were recorded via flow meter. Total suspended solids (TSS) and PSD samples were collected from the RAS, OF, and UF. Process and Capacity Modeling - Data from NAB2 testing were used to develop, calibrate, and validate AB2 specific and a whole plant process model in BioWinTM and computational fluid dynamics (CFD) models of the NSEC secondary clarifiers. These models establish system capacity of existing infrastructure and design criteria for full-scale implementation. Results and Discussion: Impact of biological selection on densification: PSD of NAB2 biomass during configuration 1 (SAR-MLE) mode) stabilized into a uni-modal distribution, with median size increasing from 100 to 200 um respectively (Fig 2A). The average SVI30 for the NAB2 versus control basins were 102 mL/g and 153 mL/g respectively. PSD of NAB2 biomass during configuration 2 (SAR-A2O) mode reflected a bi-model distribution (Fig 2B). The average SVI30 for the test versus control basins were 81 mL/g and 173 mL/g respectively. Cumulatively, results suggest that improved settleability is observed when the mass fraction of granules (particles >212 um) in the sludge exceeds 15%. This was observed on multiple occasions when NAB2 operated in SAR-A2O mode with anaerobic food to microorganism ratio (F/M) being sustained between 1.0 and 3.0 lb sCOD/lb MLVSS-day. Impacts on PSD on Settling Performance and Biomass Activity: Settling velocity and compressibility were observed to increase as the particle size fraction increased (Table 1). Biomass activity results indicated that larger particle size fractions had lower denitrification and nitrification rates versus smaller particle size fractions while Bio-P activity was highest in particles greater than 200 um. These results indicate that minimizing particles sizes greater than 600 um may allow Metro to exploit high nitrification/denitrification/Bio-P rates and with superior settling and compressibility (Quoc et al., 2021). Influence of Physical Selective Pressures on PSD in NAB2 (Fig 3): Results indicated that mass flow through the UF could be reduced when the nozzle size decreases (Fig 2). This phenomenon can potentially be exploited to craft a wasting strategy focused on developing and keeping a target PSD. DAS impacts on Capacity: Stress testing indicated that NAB2 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) while still maintaining reliable blanket control and stable effluent TSS (Fig 4). Whole plant process and CFD modeling indicated that the densified scenario could allow Metro to successfully treat 2040 flows without needing to invest in additional aeration basins or secondary clarifiers, allowing for savings of $50 million dollars.

To meet future nitrogen and phosphorus regulations without constructing new infrastructure, Metro Water Recovery conducted a full-scale demonstration of densified activated sludge (DAS) at its Robert W. Hite Treatment Facility. This study evaluated the DAS system, focusing on metabolic and kinetic selector configurations, particle size distribution, and nutrient removal efficiency. Results include impacts of configurations, particle size, and nozzle size and projected capital cost savings.

SpeakerMaltos, Rudy

Presentation time

09:00:00

09:30:00

Session time

08:30:00

10:00:00

SessionFull-scale Intensification Experience

Session number510

Session locationRoom 343

TopicFacility Operations and Maintenance, Intermediate Level, Municipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Maltos, Rudy, Rauch-Williams, Tanja, Freedman, Daniel, Cavanaugh, Liam, Scopp, Anna, Priest, Ryan, Griborio, Alonso, Mayer, Alyssa, Noteboom, Haley, Latimer, Ron, Sun, Yewei, Khunjar, Wendell

Author(s)R. Maltos1, T. Rauch-Williams1, D. Freedman1, L.M. Cavanaugh1, A. Scopp2, R. Priest2, A. Griborio3, A.J. Mayer4, H. Noteboom5, R.J. Latimer6, Y. Sun7, W. Khunjar8

Author affiliation(s)1Metro Water Recovery, CO, 2Hazen and Sawyer, CO, 3Hazen & Sawyer, FL, 4Hazen & Sawyer, OH, 5Hazen & Sawyer, NY, 6Hazen and Sawyer, GA, 7Hazen and Sawyer, VA, 8Hazen & Sawyer PC, VA

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159645

Volume / Issue

Content sourceWEFTEC

Word count24

Densified Activated Sludge: Successful Analytical Methods for Process Performance Monitoring

Abstract

Introduction The densified activated sludge (DAS) process is a relatively new procedure that Water Resource Recovery Facilities (WRRFs) are adding to improve treatment efficiency. Certain inherent properties of DAS, however, can cause interferences with conventional laboratory methods. Overlooking these inherent granule properties can result in compromised laboratory results and misinformation for DAS process monitoring and operation. Metro Water Recovery's (Metro's) process engineers, operations, and laboratory worked together throughout a 2-year DAS demonstration project to identify, refine, and modify appropriate analytical methods for DAS. This presentation will summarize four of Metro's analytical methods developed for DAS as described below. Methods Metro initially assessed the applicability of standardized and newer wastewater methods for settling velocity1, TSS2, microscopy3, and PSD4 to DAS. Over the duration of the DAS pilot operation, Metro adjusted these conventional methods in response to analytical problems and errors noted in the datasets as discussed in the following. Particle Settling Characteristics Conventional Method Issues (CMI) Sludge volume index (SVI) is a routine method that has been used to assess the settleability of sludge for over 90 years5. Settling velocity is a variation of the SVI method which provides valuable process information for DAS, specifically in the first 6 minutes when much of the DAS settling occurs1. Despite the value in settling velocity, there is some inherent variability from the need for nearly immediate readings before a clear defined front has formed making it difficult to accurately measure settling. Modified Procedures (MP) Metro defined three method adjustments to improve the reproducibility of the method: 1) using flashlights to aid in the readings, 2) standardizing the method for readings between laboratory staff to ensure consistency, and 3) optimizing sample homogenization time to clarify front formation. Metro will present images and videos to illustrate these findings and share recommendations to standardize this method in other laboratories. Total suspended solids (TSS) CMI TSS is a commonly used laboratory method to assess solids inventory, solids residence time, wasting rates, and granulation efficiency. Due to the rapid settling of DAS, measuring the exact sludge volume for TSS using graduated cylinders and standard pipettes can underestimate TSS. Metro developed modified analytical protocols for accurate TSS measurements. DAS can also cause online and handheld TSS measurements to systematically underestimate true concentrations due to particle settling and stratification in the basins. MP Metro's laboratory modified their standard operating procedures (SOPs) for DAS to overcome interference due to DAS settling. Using wide-bore pipettes to measure and transfer sample to filter from the sample container (rather than graduated cylinders), better maintains granules in suspension through the sample volume measurement process. The fact that TSS online and handheld probes read lower values compared to laboratory TSS controls was overcome by determining instrument calibration reference factors (ratio of laboratory values to probe readings) and correction factors used to adjust online readings. These adjustment methods resulted in more consistent readings closer to the true TSS (lab) values. Microscopy CMI Microscopy is used to assess sludge properties such as microbial community and filament abundance6. It is an equally important tool for DAS providing details on the size and morphology of the granules, density, and filaments. Appropriate techniques for wet mount slide preparation ensures the structure of granules is not compromised and quality images are produced. MP Metro developed a modified wet mount procedure for DAS analysis, emphasizing how to place the cover slip to avoid DAS compression. Metro will demonstrate this method in the presentation. Figure 1 shows the granules in-tact which prevents errors and Figure 2 shows flattened granules. Particle Size Distribution (PSD) CMI Particle size distribution (PSD) is not a common wastewater analysis method but can be extremely important when working with granular sludge. It is beneficial to quickly quantify the size distribution of the granules being produced so operational changes can be made to maintain the appropriately sized granules and sufficient nutrient removal. Sieving and PSD via instrumentation were assessed by Metro. MP Sieving is a common method for PSD analysis, but it is slow, labor intensive, prone to bias, and only provides information on the discrete sieve sizes used. While PSD measured with instrumentation using light diffraction methods is more of a qualitative method, Metro demonstrated that PSD analysis by instrumentation provides reliable, reproducible PSD results (Figure 3). This is done in less than ten minutes per sample allowing for timely data for operational decisions, whereas the sieving procedure can take between 30 minutes and two hours. Conclusions Metro's team has identified important method modifications that laboratories should consider when analyzing the characteristics of DAS. By validating and adopting these recommendations in other laboratories, operators and process engineers will increase confidence in the data being used

The densified activated sludge (DAS) process allows water resource recovery facilities to improve treatment capacity or effluent quality. However, certain properties of DAS can cause interferences with laboratory methods leading to misinformation for DAS operational decision-making. Metro Water Recovery modified four analytical methods to improve results including, (1) particle settling characteristics, (2) total suspended solids, (3) microscopy, and (4) particle size distribution.

SpeakerLove, Natalie

Presentation time

14:30:00

15:00:00

Session time

13:30:00

15:00:00

SessionInnovative Approaches to Address Traditional Operational Challenges

Session number211

Session locationRoom 344

TopicFacility Operations and Maintenance, Intermediate Level, Municipal Wastewater Treatment Design, Nutrients

Author(s)

Love, Natalie, Bruhn, Ashley, Fielder, Tracy, Caley, Cassandra, Maltos, Rudy

Author(s)N. Love1, A. Bruhn1, T. Fielder1, C. Caley1, R. Maltos1

Author affiliation(s)1Metro Water Recovery, CO

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159643

Volume / Issue

Content sourceWEFTEC

Word count11

Evaluating Full-Scale Impacts of Activated Sludge Densification on Disinfection Efficacy: A WRF Tailored Collaboration

Abstract

Background Water resource recovery facilities (WRRFs) around the world are constantly faced with significant challenges 1, 2. It is estimated that WRRFs are operating at an average of 81% of their design capacity, while 15% of systems are at or have exceeded that threshold3. As the most prevalent biological nutrient removal (BNR) process, any improvements to conventional activated sludge (CAS) processes are highly desirable in our industry4. However, upgrading CAS trains is costly and alternative technologies often tend to be more energy-intensive, expensive1, and time consuming to install, commission, and operate. Thus, the use of physical selectors (e.g., hydrocyclones) for biomass densification has emerged as a front-runner for increasing secondary clarifier capacity and improving nutrient removal. Objective and Approach Despite the many studies exploring the advantages of densified activated sludge (DAS) systems compared to CAS systems, DAS impacts on microbial communities and fecal indicator bacteria (FIB) are unknown2, 5. In 2023, the Water Research Foundation funded a tailored collaboration with the Metro Water Recovery (MWR) in Denver. The collaboration aimed to investigate FIB removal and disinfection kinetics. This study focused on CAS and DAS treatment trains at three North American full-scale operating WRRFs (Figure 1). The WRF project aims to: A) Compare E. coli concentrations over a 15-month period and investigate factors influencing differences (e.g., granulation size distribution, floc/granule mass fraction, floc/granule aSRT, microscopy, COD, Turbidity, TSS, NH3, pH, Temperature, Color, UV-254, TKN, TP, TOC). B) Determine how changes in Secondary Effluent (SE) properties between DAS and CAS affect disinfection efficacy using chlorine, peracetic acid (PAA), performic acid (PFA) and ultraviolet disinfection (UV) seasonally (n=12), with the same water quality parameters listed in Objective A. C) Assess impacts of operational variables on DAS and provide utilities with recommendations. These can include how to utilize modeling and statistics to improve performance using demand and decay values and mechanistic disinfection efficacy models D) Determine if additional analyses (e.g., protozoa counts, granule sieving analyses) provide operational value to utilities adopting DAS. Results Data collected for one year at the MWR Hite Treatment Facility highlights the statistically significant difference in E. coli concentrations between the CAS and the DAS trains (Figure 2). This could be explained by a variety of variables that have not yet been evaluated. FIB counts may be impacted by the predator protozoan community5, but this has yet to be supported by data (Figure 3). While percent mass fraction (Figure 4) and seasonality (Figure 5) have been hypothesized to impact DAS E. coli counts6, this has not been shown by the preliminary data collected, which highlights the importance of a more detailed study to explain the differences observed . Summary DAS is able to be implemented with low capital cost while increasing the capacity of CAS systems. However, there is little data on how SE from DAS systems impact downstream disinfection processes. This WRF project will evaluate the impact DAS systems have on disinfection processes compared to CAS systems. Data collection and testing for this project will begin in April 2024.

Densification has emerged as a cost-effective means to increase the capacity of conventional activated sludge (CAS) processes at WRRFs. While many studies have explored the advantages of densified activated sludge (DAS) systems, its impacts on disinfection are unknown. This project will compare the E. coli concentrations between full-scale CAS and DAS systems; will compare disinfectants by performing disinfection dose response testing; and will develop efficacy models based upon the results.

SpeakerHilts, Brian

Presentation time

14:30:00

15:00:00

Session time

13:30:00

15:00:00

SessionEvaluating Plantwide Impacts of AGS and DAS

Session number606

Session locationRoom 343

TopicIntermediate Level, Municipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Hilts, Brian, Goldman, Josh, Maltos, Rudy, Maal-Bared, Rasha, Kaymak, Baris

Author(s)B.A. Hilts1, J. Goldman2, R. Maltos3, R. Maal-Bared4, B. Kaymak5

Author affiliation(s)1CDM Smith, UT, 2Metro Wastewater Recovery, 3Metro Water Recovery, 4CDM Smith, 5CDM Smith

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159717

Volume / Issue

Content sourceWEFTEC

Word count15

Description: Refining Phosphorus Recovery: Practical Improvements for Water Resource Recovery...

Refining Phosphorus Recovery: Practical Improvements for Water Resource Recovery Facilities

Abstract

Introduction Metro Water Recovery, based in Denver, Colorado, serves 2.2 million population equivalents through the Robert W. Hite Treatment Facility (RWHTF) and the Northern Treatment Plant. RWHTF has a design capacity of 220 MGD and treats an average daily flow of 130 MGD. The facility's solids handling complex includes gravity thickening, grease processing, dissolved air flotation thickening of waste activated sludge, two-phase mesophilic anaerobic digestion, centrifuge dewatering, phosphorus recovery, and biosolids management for storage, hauling, and land application. In 2016, Metro piloted the MagPrex post-digestion phosphorus recovery system (PREX) to address challenges posed by enhanced biological phosphorus removal (EBPR). EBPR results in high concentrations of soluble phosphorus in the digesters, which combines with magnesium and ammonia to form nuisance struvite, causing scaling in equipment, lowering dewatering efficiency, and upcycling phosphorus through the treatment process. In response to stricter effluent TP limits imposed by the Colorado Department of Public Health and Environment (CDPHE), Metro commissioned the world's largest PREX reactor in October 2020. This system allows RWHTF to reliably meet a TP target of 0.7 mg/L, with fewer operational disruptions associated with nuisance struvite formation. Phosphorus Recovery System Overview The PREX reactor is a completely mixed system receiving 600-800 GPM of digester effluent with orthophosphate (OP) concentrations between 350-450 mg/L. PREX quickly transforms OP into struvite by adding magnesium chloride and raising pH through air stripping via coarse air bubble diffusers . The reactor can harvest struvite as a reusable product, with large crystals settling in the reactor cone for separation, Figure 1. Since startup, the reactor has achieved >90% OP conversion, enabling Metro to maintain effluent TP concentrations below 0.4 mg/L. Optimization Objectives While the PREX system has successfully achieved the required OP conversion, opportunities for further optimization quickly became apparent. Initially, only 2% of the struvite formed in the PREX reactor was captured for reuse, and maintenance costs to address nuisance struvite downstream remain high (Figure 2), estimated at over $9,000 per month in labor alone. This study aims to enhance the system design and assess the downstream impacts on centrifuge dewatering, struvite scaling in PREX effluent, and centrate piping. The optimization efforts are guided by three key objectives: 1. Maximize Struvite Recovery: Increase recovery beyond the initial average of 300 lbs/day to meet the reactor's estimated potential of 2,000 lbs/day. This effort investigates changes to process operation and configuration to increase struvite recovery. 2. Minimize Downstream Struvite Accumulation: Address scaling in the PREX effluent and centrate system. Although anerobic digestors have observed a reduction of nuisance struvite, portions of PREX effluent system require a high pressure cleaning every three weeks to remove struvite accumulation. 3. Optimize Residual Magnesium for Dewatering: Evaluation of residual magnesium from the PREX reactor to improve dewatering efficiency by reducing polymer demand and lowering chemical costs. Methods The PREX reactor was operated under various conditions, including adjustments to hydraulic retention time (HRT), pH, and Mg:OP ratios. A 3D multi-phase turbulent model was developed to refine operational settings and predict particle settling efficiency. To assess struvite scaling, ion mass balances were performed on centrate samples collected at two points: upstream at the centrifuge discharge and downstream at the outlet to the sidestream deammonification reactor. These samples were analyzed for OP, TP, magnesium (Mg), and pH. Additionally, soluble and total magnesium concentrations were measured in the PREX effluent and centrate. The relationship between these concentrations, polymer demand, and dewatering performance was explored. Conductivity and zeta potential measurements were collected as potential proxies for cation concentrations, enabling real-time monitoring of dewatering performance. Results 1.Struvite Recovery Optimization The 3D CFD model revealed that only 10% of struvite crystals under 250 Î¼m in size could be captured under typical airflow conditions (Figure 3). Particles with a diameter >700 um had an average recovery rate of 80%. However, over 50% of the struvite produced by PREX was less than 250 Î¼m, significantly limiting recovery. Improvements included adjusting classifier operation such as classifier bed height , introducing dilution water, and recycling classifier overflow. These changes increased struvite recovery from 300 to 1,200 lbs/day. 2.Minimizing Downstream Struvite Accumulation While the PREX system significantly reduced nuisance struvite formation in the digesters, accumulation persisted in the PREX effluent piping due to the presence of small struvite crystals and turbulent flow, which caused localized pH shifts and the precipitation of residual OP, Mg, and NH4. Polyvinylidene fluoride (PVDF) liners are being installed in the effluent lines to reduce turbulence and mitigate scaling. Ongoing ion mass balance sampling will provide further insights, with results to be presented at the conference. 3.Residual Magnesium and Dewatering Performance Maintaining residual magnesium concentrations of 50-60 mg/L reduced normalized dewatering polymer usage per flow by 39%, resulting in chemical cost savings of $800,000 annually (from $2.9 million to $2.1 million) (Figure 4). Conductivity and zeta potential measurements strongly correlated with cation concentrations, indicating their potential use for real-time monitoring and future optimization. Conclusions The optimization of Metro's PREX system demonstrates the potential for substantial improvements in phosphorus recovery and operational reliability. Struvite recovery increased fourfold, highlighting the importance of fine-tuning operational parameters and integrating recycling strategies. Addressing scaling in effluent lines through material upgrades and improved chemistry management will further enhance long-term system reliability. Optimizing magnesium levels improved dewatering performance and delivered substantial chemical cost savings. Potential real-time monitoring using conductivity and zeta potential as proxies for cation concentrations offers an innovative approach to operational control and performance optimization. As phosphorus regulations become more stringent, these advancements position Metro as a leader in sustainable resource recovery. These findings provide valuable insights for other utilities aiming to enhance phosphorus recovery, minimize operational disruptions, and reduce chemical usage, advancing the next generation of phosphorus management for water resource recovery facilities.

This paper was presented at the WEF Residuals & Biosolids and Innovations in Treatment Technology Joint Conference, May 6-9, 2025.

SpeakerMaltos, Rudy

Presentation time

11:25:00

11:45:00

Session time

10:45:00

11:45:00

SessionOptimizing Resource Recovery: Biogas and Nutrient Reuse

Session number14

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicAnaerobic Digestion, Biogas Utilization, Cogeneration, Energy recovery, Greenhouse Gases, Innovative Technology, Nitrogen Removal, Process Intensification