Andrew Schuler | University of New Mexico (original) (raw)

Papers by Andrew Schuler

Proceedings of the Water Environment Federation, 2006

Computer simulation of activated sludge processes is a critical tool for design, operation, and t... more Computer simulation of activated sludge processes is a critical tool for design, operation, and troubleshooting, but simulating enhanced biological phosphorus removal (EBPR) systems has proven to be particularly challenging. This may be due in part to uncertainties in biokinetic models, but new research suggests it may also be due to deficiencies in conventional "lumped state" approaches to simulation, which model bulk concentrations of the polyphosphate accumulating organisms (PAOs) responsible for EBPR, as well as their microbial storage product (polyphosphate, glycogen, and polyhydroxyalkanoates) contents. A recently developed, alternative method for modeling biological treatment systems is the "distributed state" approach, which models individual bacteria as they move through a biological reactor system, rather than bulk concentrations. This approach predicts that PAO states (their microbial storage product contents) tend to diverge when reactors are completely mixed, and this can produce very different outcomes than those predicted by the conventional lumped approach. A MATLABbased distributed state program (DisSimulator 2.0) was applied to an A 2 O system and it was determined that distributed states tend to become more important with (1) shorter internal recycle ratios and (2) longer reactor hydraulic residence times. Consequently errors resulting from relying on lumped simulations are greatest in these conditions. This work illustrates that there appear to be interesting process phenomena related to changing state distributions that apparently cannot be accounted for by lumped simulations. These insights suggest that the continued advancement of the distributed simulator approach has the potential to improve design and operation of biological nutrient removal systems.

Water Environment Research, Nov 1, 2003

Laboratory‐scale sequencing batch reactors exhibiting enhanced biological phosphorus removal (EBP... more Laboratory‐scale sequencing batch reactors exhibiting enhanced biological phosphorus removal (EBPR) operated at different influent phosphorus/chemical oxygen demand (COD) ratios were analyzed to evaluate possible anaerobic sources of reducing equivalents. Assuming anaerobic glycogen degradation was the sole anaerobic reducing equivalent source, an anaerobic phase carbon balance showed that glycogen‐accumulating metabolism (GAM)‐dominated systems were nearly carbon‐balanced, but that polyphosphate‐accumulating metabolism (PAM)‐dominated systems had end‐anaerobic phase carbon deficits. An anaerobic‐phase reducing equivalent balance showed a reducing equivalent excess for the GAM‐dominated systems and a deficit for the PAM‐dominated systems, suggesting that glycogen degradation was not the sole reducing equivalent source for PAM. Reducing equivalent balances showed that metabolic models including complete anaerobic tricarboxylic acid (TCA) cycle activity, partial TCA cycle activity, and the glyoxylate bypass could provide the reducing equivalents required in PAM. Metabolic precursors produced in glycolysis, the TCA cycle, or modified versions of the TCA cycle could allow anaerobic growth and account for the PAM carbon deficits. The importance of considering both PAM and GAM activity in evaluating EBPR metabolic models was illustrated.

Chemical Engineering Journal, Oct 15, 2010

Biosolids sedimentation is a critical component of the activated sludge wastewater treatment proc... more Biosolids sedimentation is a critical component of the activated sludge wastewater treatment process. Seasonal variability of biomass settleability has been previously reported and linked to variable filament content in some studies, but others have reported seasonal variations without changes in filament content. Biomass density (mass per microbial floc volume, not including pore spaces) has recently been shown to vary substantially and to affect settleability in full-scale systems, but its potential role in seasonal variations has not been previously evaluated. Four full-scale activated sludge systems were monitored for density, filament content, and settleability for a year. Biomass density values were significantly higher in warm weather than in cold weather in all plants. Settleability was significantly worse in cold weather in three of the four systems, and the inverse of the buoyant density was correlated with settleability in these three systems. Filament content, on the other hand, exhibited seasonal variability and was correlated with settleability in only one of the four plants. Non-volatile suspended solids content was correlated with buoyant density and exhibited seasonal variability in all four systems. Biomass phosphorus content measurements suggested that seasonally variable enhanced biological phosphorus removal activity affected density in one of the systems. These results suggest that variable density plays a role in seasonally variable settleability in some full-scale wastewater treatment systems, they help to clarify previously unexplained reports of seasonally variable settleability that were independent of changes in filament content, and they provide the basis for development of strategies for improved performance.

Proceedings of the Water Environment Federation, 2017

Proceedings of the Water Environment Federation, 2007

Computer simulations are critical tools for design, operation, and troubleshooting of wastewater ... more Computer simulations are critical tools for design, operation, and troubleshooting of wastewater treatment systems, but predicting behaviors of nutrient removal systems can often be problematic. Conventional simulation programs utilize a "lumped" approach where process rates are calculated from biokinetic models using bulk concentrations of biomass and microbial storage products. In this paper, a new "distributed" approach to simulating activated sludge systems previously developed in our laboratory was expanded to predict the performance of a more complex enhanced biological phosphorus removal (EBPR) configurations, the 5-Stage Bardenpho process. Distributed-type simulations predicted worse EBPR performance than did lumped-type simulations. The effects of increasing the internal recycle in the 5-Stage Bardenpho process were to increase the nitrate concentration in the anoxic reactors, tending to decrease EBPR performance, and to simultaneously decrease differences in the state distributions between the linked aerobic and anoxic reactors. In the absence of nitrification, the latter effect was demonstrated to improve EBPR, which was in opposition to the former effect. There therefore appears to be a level of complexity linked to internal recycles in a 5-Stage Bardenpho process that is not accounted for by the lumped approach, and that whether increasing the internal recycle rate positively or negatively effects EBPR may depend on the specific system characteristics.

Water Research, May 1, 2007

Recent research has demonstrated that biomass density can vary in activated sludge systems, and t... more Recent research has demonstrated that biomass density can vary in activated sludge systems, and that this can affect biomass settleability. Other factors related to floc structure are well known to affect settleability, and these can also vary considerably across samples. A new method to isolate density effects on sedimentation was developed and applied based on the addition of commercially available, high and low density microspheres. Density was a linear function of microsphere dose in five full-scale samples, and it was negatively correlated with sludge volume index (SVI) values. Density effects on SVI were similar to previous reports where density varied with biomass polyphosphate content. A new method to calculate water content and floc volume was developed and applied, and water content was inversely correlated with density, polyphosphate, and nonvolatile suspended solids. These results suggest microsphere addition may be useful in future studies of biomass sedimentation and other properties.

Proceedings of the Water Environment Federation, 2005

ABSTRACT Computer simulation of activated sludge system performance is a critical tool for design... more ABSTRACT Computer simulation of activated sludge system performance is a critical tool for design, operation, and troubleshooting, and simulation of nutrient removal systems has proven to be particularly challenging. EBPR systems cycle bacteria through anaerobic and aerobic reactors, which selects for polyphosphate accumulating organisms (PAOs) containing several microbial storage products (polyphosphate, glycogen, and polyhydroxyalkanoates). Conventional simulation programs utilize a “lumped” approach where process rates are calculated using bulk concentrations of biomass and microbial storage products as inputs to sets of biokinetic equations. However, a newly developed activated sludge simulation program (DisSimulator) demonstrated that a range of PAO states (internal microbial storage product contents) is likely EBPR system, due to the variety of hydraulic residence times experienced by individual PAOs (Schuler, A.J., in press). The total calculated process rates calculated using the state distributions were considerably lower than those predicted using the lumped approach. The result was that lumped simulations consistently overestimated EBPR performance - this would tend to produce less conservative EBPR system designs than the distributed approach, and this could lead to undersized systems.In the current research, the effects of increasing the number of anaerobic and aerobic reactors in series on PAO states and EBPR performance were evaluated. It is known that as the number of reactors in series increases, the distribution of hydraulic residence times (HRTs) decreases and plug flow conditions are approached (uniform hydraulic residence time for all hydraulic elements), as is assumed in lumped simulations. Because HRT variation is the primary factor effecting PAO state distributions, there is a need to determine how changing hydraulic configurations affects EBPR performance with respect to state distributions. It was demonstrated that diversity decreased as the number of completely mixed reactors in series increased, and this was because hydraulic residence time distributions decreased with increasing numbers of reactors in series (plug flow was approached). Although increasing the number of reactors in series brought lumped and distributed predictions closer together, there were still large differences in these predictions, and so accounting for distributed states in full-scale systems is still likely to be important even in systems with several reactors in series. Based on these results, it appears that continued development of the distributed approach to activated sludge simulation has the potential to improve design and operation of biological nutrient removal systems.

Proceedings of the Water Environment Federation, 2004

Current state-of-the-art activated sludge simulation programs assume average, or "lumped," system... more Current state-of-the-art activated sludge simulation programs assume average, or "lumped," system characteristics, rather than accounting for the distribution of individual bacterial states (such as internal concentrations of the microbial storage products polyphosphate, glycogen, and polyhydroxybutyrate) likely to occur in a given population. These individual states are hypothesized to vary because hydrodynamic variability produces a variety of possible histories in terms of upstream reactor residence times. This is particularly true in systems containing completely mixed reactors, where residence times vary across hydraulic elements. A MATLABbased bioreactor simulation program was built based on a standard biokinetic model (ASM2) to predict the potential effects of distributed states on simulation of the enhanced biological phosphorus removal (EBPR) process. A simple two reactor, anaerobic-aerobic EBPR system was simulated assuming either distributed or lumped characteristics. Lumped simulations consistently overestimated EBPR performance compared to distributed simulations, potentially leading to underestimation of required reactor sizes required for successful EBPR. The results demonstrated that improvements to activated sludge systems design and process control are possible by accounting for the distributed characteristics of their resident populations. Further research is needed for the development of the distributed simulation approach for its practical application.

Water Science and Technology, Aug 1, 2002

Laboratory-scale sequencing batch reactors exhibiting enhanced biological phosphorus removal were... more Laboratory-scale sequencing batch reactors exhibiting enhanced biological phosphorus removal were analyzed for pH effects on anaerobic phosphorus (P) release, glycogen degradation, and acetate uptake. Samples with non-soluble P/total suspended solids values of either 0.13-0.14 mg/mg (HP) or 0.065-0.075 mg/mg (LP) were analyzed in anaerobic batch tests with excess acetate addition at pH values ranging from 5.2 to 9.5. A polyphosphate-accumulating metabolism (PAM) had a competitive advantage over a glycogen-accumulating metabolism (GAM) at pH > 7.0. Maximum acetate uptake rates by the HP and LP samples occurred at pH values 8.0 and 6.9, respectively. Anaerobic P release/acetate uptake increased with increasing pH at rates similar to previously reported values. Glycogen degradation/acetate uptake decreased with increasing pH above pH 7, which disagreed with previous reports that glycogen degradation/acetate increased or was unaffected by increasing pH. The results suggested that the acetate uptake mechanisms of GAM and PAM may be different.

Water Research, Apr 1, 2007

A recently developed method to manipulate biomass density based on the addition of synthetic micr... more A recently developed method to manipulate biomass density based on the addition of synthetic microspheres was used to isolate density effects on zone (hindered) settling velocities in samples from full-scale activated sludge plants over a range biomass densities and concentrations. Settling velocities increased by as much as a factor of five as density was increased by microsphere addition. The effects of density on the Vesilind sedimentation model parameters (V ¼ V o e ÀkX , where V is the settling velocity, X is the biomass concentration, and V o and k are constants) were evaluated. The parameter V o was positively correlated with density for values greater than approximately 1.02 g/mL, while k values were approximately constant in this range. New models were developed for settling velocity as a function of both density and biomass concentration, and these may be useful for future incorporation with clarifier models to improve predictions of system performance, particularly when biomass characteristics known to affect density are variable, such as polyphosphate and nonvolatile suspended solids content.

Environmental Science & Technology, Feb 2, 2007

Both variable biomass density and floc structure were determined to affect the settleability of m... more Both variable biomass density and floc structure were determined to affect the settleability of microbial biomass produced during biological wastewater treatment (activated sludge). Average biomass density varied from 1.022 to 1.056 g/mL in a survey of 17 full-scale biological wastewater treatment systems with a variety of configurations. Biomass settleability was correlated with density in samples with higher filament contents and/or more open floc structures, but settleability was independent of density in biomass with lower filament contents and more rounded floc structures. Biomass density increased with polyphosphate content, and enhanced biological phosphorus removal (EBPR) plants had higher density and better settleability than non-EBPR plants, including two systems that converted to EBPR during the course of this study. Density also increased with increasing nonvolatile suspended solids content, which was linked both to polyphosphate and to increasing solids residence times. Both density and floc structure should be considered in future analyses of activated sludge settleability, and it may be possible to improve system performance by adopting a new set of operational and design strategies to increase density.

Water Research, Mar 1, 2010

Integrated fixed film activated sludge (IFAS) is an increasingly popular modification of conventi... more Integrated fixed film activated sludge (IFAS) is an increasingly popular modification of conventional activated sludge, consisting of the addition of solid media to bioreactors to create hybrid attached/suspended growth systems. While the benefits of this technology for improvement of nitrification and other functions are well-demonstrated, little is known about its effects on biomass settleability. These effects were evaluated in parallel, independent wastewater treatment trains, with and without IFAS media, both at the pilot (at two solids residence times) and full scales. While all samples demonstrated good settleability, the Control (non-IFAS) systems consistently demonstrated small but significant (p < 0.05) improvements in settleability relative to the IFAS trains. Differences in biomass densities were identified as likely contributing factors, with Control suspended phase density > IFAS suspended phase density > IFAS attached phase (biofilm) density. Polyphosphate content (as non-soluble phosphorus) was well-correlated with density. This suggested that the attached phases had relatively low densities because of their lack of anaerobic/aerobic cycling and consequent low content of polyphosphate-accumulating organisms, and that differences in enhanced biological phosphorus removal performance between the IFAS and non-IFAS systems were likely related to the observed differences in density and settleability for the suspended phases. Decreases in solids retention times from 8 to 4 days resulted in improved settleability and increased density in all suspended phases, which was related to increased phosphorus content in the biomass, while no significant changes in density and phosphorus content were observed in attached phases.

Water Research, Nov 1, 2007

An often-noted feature of activated sludge settleability is the existence of a threshold content ... more An often-noted feature of activated sludge settleability is the existence of a threshold content of filamentous bacteria, below which filaments have a relatively small effect on settleability, and above which settleability is much more sensitive to filament content. This concept has historically been based on an apparent inflection point in settleability/filament content curves. However, this was demonstrated to be an artifact of data presentation on semilogarithmic scales. Analyses of several published data sets indicated that there was little or no evidence of a threshold effect when data were presented on linear scales, particularly around the 150 mL/g sludge volume index (SVI) ''threshold''. Rather, there appears to be a generally continuous (and often nearly linear) relationship between filament content and SVI, suggesting that the threshold effect, as commonly perceived, is not accurate. A modified empirical model of filament content and settleability was also presented.

Proceedings of the Water Environment Federation, 2015

Water Environment Research, May 1, 2008

Conventional wastewater treatment simulation programs use a ''lumped'' approach, where process ra... more Conventional wastewater treatment simulation programs use a ''lumped'' approach, where process rates are calculated using bulk concentrations of biomass and microbial storage products. A recently developed distributed, or agent-based, approach, where individual bacteria are modeled to account for their potentially variable hydraulic experiences, was applied to the 5-stage Bardenpho process, a type of enhanced biological phosphorus removal (EBPR) that includes internal recycle flows, which were hypothesized to affect distributed state development. Consistent with previous results, the EBPR predicted performance was worse according to the distributed approach than the lumped approach. In addition, increasing the internal recycle rate increased the anoxic reactor nitrate concentrations, tending to decrease EBPR performance. However, in the distributed approach, differences in the state distributions in internal recycle-linked reactors decreased with increasing recycle flow, tending to improve EBPR. These phenomena tend to have simultaneous and opposite effects on EBPR. The net effect will depend largely on the specific systems and the nitrate concentration in anoxic reactors. Water Environ. Res., 80, 454 (2008).

Biotechnology and Bioengineering, 2007

Distributed state-type simulations (based on modeling of individual bacteria as they move through... more Distributed state-type simulations (based on modeling of individual bacteria as they move through a reactor system) predicted a greater sensitivity of enhanced biological phosphorus removal (EBPR) performance to endogenous degradation than did conventional, &amp;amp;amp;amp;amp;quot;lumped&amp;amp;amp;amp;amp;quot;-type simulations (based on average biomass compositions). Recent research has indicated that the variable hydraulic residence times experienced by individual microbial storage product accumulating bacteria in systems with completely mixed reactors tend to produce populations with diverse microbial storage product contents (distributed states). Endogenous degradation in EBPR systems is of particular interest because the polyphosphate accumulating organisms (PAOs) responsible for EBPR rely on the accumulation of three different storage products that may be endogenously degraded. Simulations indicated that as endogenous degradation rates of microbial storage products were increased, EBPR performance decreased more rapidly according to the distributed approach than according to the lumped approach. State profile analysis demonstrated that as these rates increased, the population fraction with depleted storage products also increased, and this tended to increase the error in calculated biokinetic rates by the lumped approach. Simulations based on recently reported endogenous rate coefficients also suggested large differences between distributed and lumped predictions of EBPR performance. These results demonstrated that endogenous decay processes may play a more important role in EBPR than predicted by the lumped approach. This suggests a need for further research to determine endogenous process rates, and for incorporation of this information to distributed-type simulators, as this should lead to improved accuracy of EBPR simulations.