The effect of sparging rate on transmembrane pressure and critical flux in an AnMBR.

Anaerobic membrane bioreactors (AnMBRs) have been shown to be successful units for the treatment of low strength wastewaters, however, the issue of membrane fouling is still a major problem in terms of economic viability. Biogas sparging has been shown to reduce fouling substantially, and hence this study monitored the effect of biogas sparging rate on an AnMBR. The critical flux under a sparging rate of 6 l per minute (LPM) was found to be 11.8 l m(-2) h(-1) (LMH), however, membrane hysteresis was found to have an effect on the critical flux, and where the AnMBR had previously been operated with a 2 LPM sparging rate, the critical flux fell to 7.2 LMH. The existence of a "critical sparging rate" was also investigated under the condition that 'there exists a sparging rate beyond which any further decrease in sparging rate will cause a dramatic rise in TMP'. For an AnMBR operating at a flux of 7.2 LMH the critical sparging rate was found to be 4 LPM.

Fouling cake layer in a submerged anaerobic membrane bioreactor treating saline wastewaters: curse or a blessing?

The treatment of inhibitory (saline) wastewaters is known to produce considerable amounts of soluble microbial products (SMPs), and this has been implicated in membrane fouling; the fate of these SMPs was of considerable interest in this work. This study also investigated the contribution of SMPs to membrane fouling of the; (a) cake layer/biofilm layer, (b) the compounds below the biofilm/cake layer and strongly attached to the surface of the membrane, (c) the compounds in the inner pores of the membrane, and (d) the membrane. It was found that the cake/biofilm layer was the main reason for fouling of the membrane. Interestingly, the bacteria attached to the cake/biofilm layer showed higher biodegradation rates compared with the bacteria in suspension. Moreover, the bacteria attached to the cake layer showed higher amounts of attached extracellular polysaccharides (EPS) compared with the bacteria in suspension, possibly due to accumulation of the released EPS from suspended biomass in the cake/biofilm layer. Molecular weight (MW) analysis of the effluent and reactor bulk showed that the cake layer can retain a large fraction of the SMPs in the reactor and prevent them from being released into the effluent. Hence, while cake layers lead to lower fluxes in submerged anaerobic membrane bioreactors (SAMBRS), and hence higher costs, they can improve the quality of the reactor effluent.

Are compatible solutes compatible with biological treatment of saline wastewater? Batch and continuous studies using submerged anaerobic membrane bioreactors (SAMBRs).

This study investigated fundamental mechanisms that anaerobic biomass employ to cope with salinity, and applied these findings to a continuous SAMBR. When anaerobic biomass was exposed to 20 and 40 g NaCl/L for 96 h, the main solute generated de novo by biomass was trehalose. When we separately introduced trehalose, N-acetyl-ß-lysine and potassium into a batch culture a slight decrease in sodium inhibition was observed. In contrast, the addition of 0.1 mM and 1 mM of glycine betaine dramatically improved the adaptation of anaerobic biomass to 35 g NaCl/L, and it continued to enhance the adaptation of biomass to the salt for the next three batch feedings without further addition. No shift in archaeal microbial diversity was found when anaerobic biomass was exposed in batch mode to 35 g NaCl/L for 360 h, and no changes were found when glycine betaine was added. The dominant species identified under these conditions were Methanosarcina mazeii and Methanosaeta sp. The addition of 5 mM glycine betaine to a continuous SAMBR at 12 h hydraulic retention time (HRT), and operation in batch mode for 2 days can significantly enhance saline (35 g NaCl/L) synthetic sewage degradation. In addition, the injection of 1 mM of glycine betaine into a SAMBR for five subsequent days also significantly enhanced dissolved organic carbon (DOC) removal from sewage under these conditions. The main compatible solutes generated by anaerobic biomass after 44 days exposure to 35 g NaCl/L in a SAMBR were N-acetyl-ß-lysine and glycine betaine. Finally, the addition of 1 mM glycine betaine to the medium was beneficial for anaerobic biomass in batch mode at 20 °C under saline and non saline conditions.

Identification of soluble microbial products (SMPs) from the fermentation and methanogenic phases of anaerobic digestion.

The production and transformation of Soluble Microbial Products (SMPs) in biological treatment systems is complex, and their genesis and reasons for production are still unclear. SMPs are important since they constitute the main fraction of effluent COD (both aerobic and anaerobic), and hence are the main precursors for disinfection by-products (DBPs). In addition, they are a key component of fouling in membrane bioreactors. Hence, it is important to identify the chemical composition of SMPs, determine their origin, and understand what system parameters influence their production so we can possibly develop strategies to control their production. This study focuses on the production and identification of SMPs in an anaerobic batch process being fed a synthetic feed. To further understand the origins of SMPs, and how they are produced, we analysed the processes of fermentation and methanogenesis independently which has never been done in detail before. SMP concentration, molecular weight distribution and carbohydrate analyses were used to estimate the amount of SMPs in the supernatants. Gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-Time-of-Flight mass spectrometry (LC-ESI-Q-ToF) were used to identify many of the SMPs which have relative masses up to 2 kDa. Our results showed that fermentation released much higher SMP concentrations compared to methanogenesis, especially in the range of 70 k-1000 k Da and 106-1500 Da. Alkanes, alkenes, alcohols, acids, and nitrogen-compounds were the major group of compounds identified in the supernatant of both fermentation and methanogenesis, and 71% of the compounds identified were found in both phases of digestion. Results from LC-ESI-Q-ToF analysis identified components of the cell membrane, such as phosphatidylglycerol, phosphatidylethanolamine and phosphatidylserine, as well as other compounds such as flavonoids, acylglycerol, terpene and terpenoids, benzenoid, glyceride, steroid and steroid derivatives.

Saline sewage treatment using a submerged anaerobic membrane reactor (SAMBR): effects of activated carbon addition and biogas-sparging time.

This study investigated the performance of a submerged anaerobic membrane reactor (SAMBR) treating saline sewage under fluctuating concentrations of salinity (0-35g NaCl/L), at 8 and 20h HRT, with fluxes ranging from 5-8litres per square metre per hour (LMH). The SAMBRs attained a 99% removal of Dissolved Organic Carbon (DOC) with 35g NaCl/L, while removal inside the reactor was significantly lower (40-60% DOC). Even with a sudden drop in salinity overall removal recovered quickly, while the recovery inside the reactor took place at a slower rate. This highlights the positive effect of the membrane in preventing the presence of high molecular weight organics in the effluent while also retaining biomass inside the reactor so that they can rapidly acclimatize to salinity. The reduction of continuous biogas sparging to intervals of 10min ON and 5min OFF resulted in a slight increase in transmembrane pressure (TMP) by 0.025bar, but also resulted in an increase in effluent DOC removal and inside the SAMBR by 10% and 20%, respectively. The addition of powdered activated carbon (PAC) resulted in a decrease in the TMP by 0.070bar, and an increase in DOC removal in the reactor and effluent by 30% and 5%, respectively. The PAC dramatically decreased the high molecular weight organics in the reactor over a period of 72h. SEM pictures of the membrane and biomass before and after addition of PAC revealed a remarkable reduction of flocks on the membrane surface, and a reduction inside the reactor of soluble microbial products (SMPs). Finally, Energy Dispersive X-ray (EDX) analysis of the membranes pores and biofilm highlighted the absence of organic matter in the inner pores of the membrane.

Continuous treatment of the organic fraction of municipal solid waste in an anaerobic two-stage membrane process with liquid recycle.

The stability and performance of a two-stage anaerobic membrane process was investigated at different organic loading rates (OLRs) and Hydraulic Retention Times (HRTs) over 200 days. The Hydrolytic Reactor (HR) was fed with the Organic Fraction of Municipal Solid Waste (OFMSW), while the leachate from the HR was fed continuously to two Submerged Anaerobic Membrane Bioreactors (SAMBR1 and 2). The Total COD (TCOD) of the leachate varied over a wide range, typically between 4000 and 26,000 mg/L while the Soluble COD (SCOD) in the permeate was in the range 400-600 mg/L, achieving a COD removal greater than 90% at a HRT of 1.6-2.3 days in SAMBR1. The operation was not sustainable below this HRT due to a membrane flux limitation at 0.5-0.8L/m(2) h (LMH), which was linked to the increasing MLTSS. SCOD in the recycled permeate did not build up indicating a slow degradation of recalcitrants over time. SAMBR2 was run in parallel with SAMBR1 but its permeate was treated aerobically in an Aerobic Membrane Bioreactor (AMBR). The AMBR acted as a COD-polishing and ammonia removal step. About 26% of the recalcitrant SCOD from SAMBR2 could be aerobically degraded in the AMBR. In addition, 97.7 % of the ammonia-nitrogen was converted to nitrate in the AMBR at a maximum nitrogen-loading rate of 0.18 kg NH(4)(+)-N/m(3) day. GC-MS analysis was performed on the reactor effluents to determine their composition and what compounds were recalcitrant.

Effect of fluctuations in salinity on anaerobic biomass and production of soluble microbial products (SMP(s)).

This study investigated the acclimation potential of batch fed anaerobic biomass with salinities of 0-50 gNaCl l(-1). Anaerobic biomass was acclimatized to salinities up to 20 gNaCl l(-1)over a period of 35 days, with 3 consecutive feedings. After this period the biomass was subjected to non-saline conditions to simulate fluctuating feed compositions. High activity was obtained after the first exposure to non saline conditions for biomass previously exposed to 30 gNaCl l(-1). Short exposure (2-48 h) to high salinity (40 gNaCl l(-1)) did not reduce biomass activity when it was re-subjected to normal conditions. The sensitivity of each anaerobic bacterial group showed that propionate utilisers were the most affected by sudden changes in salinity. Using size exclusion chromatography (SEC) it was found that biomass exposed to concentrations of salinity above 30 gNaCl(-1) produced higher molecular weight soluble microbial products (SMPs) which were present in the culture for longer periods than the control indicating that the effluent was more difficult to degrade. With the sudden removal of salinity anaerobic biomass can easily readapt to normal conditions without any high MW compounds being produced. These findings highlight the fact that anaerobic biomass is able to overcome sharp decreases in salinity in contrast with aerobic biomass as reported in the literature.

Anaerobic digestion of the organic fraction of municipal solid waste in a two-stage membrane process.

A batch of the Organic Fraction of Municipal Solid Waste (OFMSW) was treated in a two-step process with effluent recirculation comprising a novel hydrolytic reactor (HR) followed by a Submerged Anaerobic Membrane Bioreactor (SAMBR) operating at a stable permeate flux of 5.6 L/m(2) hr (LMH). A soluble COD removal higher than 95% was obtained from the SAMBR. The soluble COD as well as the Total Suspended Solids (TSS) did not build up due to efficient hydrolysis inside the SAMBR, and no VFA accumulation occurred due to the complete retention of methanogens by the membrane as well as the formation of syntrophic associations. Because of the microfiltration membrane in the second reactor a stabilized leachate was obtained from the very first days of the treatment and the highly stable process enabled shorter treatment periods compared to traditional leach bed processes. This experiment showed that the recycle of the stabilised leachate does not lead to a build up of SCOD. Size exclusion chromatography analysis confirmed that high molecular weight compounds were completely degraded and did not appear in the SAMBR permeate, and that low molecular weight fulvic-like and medium MW material were present in the permeate of the SAMBR but their concentration remained stable with time.

Biomass acclimatisation and adaptation during start-up of a submerged anaerobic membrane bioreactor (SAMBR).

The effects of biomass acclimatisation and adaptation during the start-up of three 3-litre submerged anaerobic membrane bioreactors (SAMBRs) were studied for the treatment of a sucrose-meat extract based medium strength wastewater (4 gCOD l(-1)). At 30 hours hydraulic retention time, the SAMBRs achieved more than 90% chemical oxygen demand (COD) removal with two different types of initial sludge--one previously adapted in a SAMBR for low strength treatment (SAMBR A), while the other was acclimatised in low shear continuous stirred tank reactors (CSTRs; mixture of biomass from two CSTR reactors fed on sucrose and/or acetate and propionate: SAMBR B--gassed at 5 litres per minute, SAMBR C at 2 litres per minute = 1.2 m3 m(-2) h(-1)). Although acclimatisation (enrichment of sludge adapted in a SAMBR for sewage treatment) took longer (100 days) than adaptation (enrichment and adaptation of sludge to SAMBR conditions) (39 days), the methane potential of the biomass improved substantially from its initial values. For successful start-up of SAMBRs a low initial loading rate, low biogas sparging rate and long acclimatisation and adaptation times were necessary to develop strong bacterial associations for interspecies electron transfer (SAMBRs A and C). The shorter adaptation time in SAMBR B resulted in permanent deterioration of performance, probably because of an insufficient population of methanogens and acetogens, and a reduction in floc size during overloaded conditions. The low value of flux (2 litres per square metre per hour) during high strength treatment was attributed to fine colloids, higher amounts of SMP and a high biomass concentration inside the reactor.

Treatment of oilfield produced water by waste stabilization ponds.

Produced water (PW) from oil wells can serve as an alternative water resource for agriculture if the main pollutants (hydrocarbons and heavy metals) can be removed to below irrigation standards. Waste stabilization ponds seem like a promising solution for PW treatment, especially in the Middle East where solar radiation is high and land is available. In this work, hydrocarbon removal from PW in a biological waste stabilization pond was examined at lab-scale followed by an intermittent slow sand filter. The system was run for 300 days and removed around 90% of the oil in the pond, and 95% after the sand filter. COD removal was about 80% in the pond effluent, and 85% after the filter. The system was tested under various operational modes and found to be stable to shock loads. Installation of oil booms and decantation of surface oil seem to be important in order to maintain good system performance over time.

Chromatographic characterization of dissolved organics in effluents from two anaerobic reactors treating synthetic wastewater.

This paper presents results on the quantification and chromatographic characterization of soluble microbial products (SMP) accumulated in two laboratory-scale reactors: a submerged anaerobic membrane reactor (SAMBR or MBR), and an anaerobic CSTR. The results obtained under steady-state conditions show that 2.1% of the substrate was channelled into the production of SMP in the CSTR, whilst in the SAMBR this was estimated to be 25%. Chromatographic characterization showed that more hydrophobic and high MW organics that absorb at 254 nm were detected in the SAMBR supernatant than in the CSTR. A comparison of chromatograms suggest that the release of extracellular polymers (ECP) and cell lysis may be important sources of SMP in the SAMBR. Electrophoresis results confirmed that there was more soluble protein inside the SAMBR, and showed that the release of ECP by shear or hydrolysis seemed to have contributed to the production of protein-like SMP in both systems.

Bioaugmentation and its application in wastewater treatment: A review.

Bioaugmentation (the process of adding selected strains/mixed cultures to wastewater reactors to improve the catabolism of specific compounds, e.g. refractory organics, or overall COD) is a promising technique to solve practical problems in wastewater treatment plants, and enhance removal efficiency. The potential of this option can now be enhanced in order to take advantage of important advances in the fields of microbial ecology, molecular biology, immobilization techniques and advanced bioreactor design. Reports on bioaugmentation in WWT show the difficulties in evaluating the potential parameters involved, leading frequently to inconclusive outcomes. Many studies have been carried out on the basis of trial-and-error approaches, and it has been reported that reactors bioaugmented with pure cultures often fail to perform as well as the pure cultures under laboratory conditions. As an interesting technical challenge, the feasibility of bioaugmentation should ultimately be assessed by data from field implementation, and this review highlights several promising areas to explore in the future.

Optimization-based methodology for the development of wastewater facilities for energy and nutrient recovery.

A paradigm shift is currently underway from an attitude that considers wastewater streams as a waste to be treated, to a proactive interest in recovering materials and energy from these streams. This paper is concerned with the development and application of a systematic, model-based methodology for the development of wastewater resource recovery systems that are both economically attractive and sustainable. With the array of available treatment and recovery options growing steadily, a superstructure modeling approach based on rigorous mathematical optimization appears to be a natural approach for tackling these problems. The development of reliable, yet simple, performance and cost models is a key issue with this approach in order to allow for a reliable solution based on global optimization. We argue that commercial wastewater simulators can be used to derive such models, and we illustrate this approach with a simple resource recovery system. The results show that the proposed methodology is computationally tractable, thereby supporting its application as a decision support system for selection of promising resource recovery systems whose development is worth pursuing.

Cytochrome P450 immobilisation as a route to bioremediation/biocatalysis.

The diverse substrate specificity of the cytochrome P450 (P450; CYP) enzyme superfamily offers the opportunity to develop enzymatic systems for environmental detoxification and biotransformations of drugs, pesticides and fine chemicals. Here we report on the immobilisation of a fusion protein between plant cytochrome P450-71B1 (CYP71B1) and its electron donor, plant NADPH cytochrome P450 reductase using an oil-in-water macro-emulsion, termed polyaphron, which contains a proportion of internal organic phase (phi) greater than 0.74. Efficiency of P450 immobilisation was greater than 85%, and in this state enzymatic activity could be measured for more than 24 h at 15 degrees C. Chlortoluron, a recalcitrant herbicide pollutant in the environment, was shown to be metabolised, with the major metabolite (N-monodemethylated chlortoluron) being separated from the substrate due to partitioning into the aqueous phase. The turnovers exhibited superactivity compared with those obtained using free enzyme located in membranes prepared following heterologous expression in Saccharomyces cerevisiae and Escherichia coli. The potential to exploit the unprecedented catalytic diversity of the P450 superfamily in biocatalysis is discussed.

The hydrodynamics of a Graesser ("raining bucket") contactor with a reverse micellar phase.

A variety of contactor types have been assessed for the liquid-liquid extraction of proteins using reversed micelles; however, many of these contactors suffer from drawbacks such as emulsion formation and poor mass transfer performance. In this study, a small (1.25 L) Graesser "raining bucket" contactor was assessed for use with this system since it has the potential to ameliorate many of these problems. The aim of the work was to evaluate the hydrodynamics of the contactor in order to use this information for future work on mass transfer performance. Hydrodynamic characteristics such as the axial mixing coefficient were determined by residence time distribution studies using a tracer injection method. The effect of rotor speed and flow rate of each phase on axial mixing was investigated, and as a result of its unusual structure, i.e., falling/rising sheet, the interfacial mass transfer area in the Graesser was determined by image analysis. It was found that rotor speed had more influence on the axial mixing coefficient in the aqueous phase than in the reverse micellar phase. The axial mixing coefficient in each phase increased by increasing the flow rate of the same phase. The images obtained in a dropping cell showed that under the conditions of this study (3 rpm, 22 degrees C), the bucket pours one phase through the other in the form of a curtain or sheet. A new image technique was developed to determine the interfacial area of both phases, and it was found that the specific area was 8.6 m(2)/m(3), which was higher than in a spray column but considerably lower than in a RDC or a Graesser run at high rotational speed (50 rpm) without the addition of a surfactant.

Soluble microbial products formation in anaerobic chemostats in the presence of toxic compounds.

Anaerobic chemostats fed on glucose (approximately 10 g chemical oxygen demand (COD)/L) were used to investigate the effects of toxicity on soluble microbial product (SMP) formation. Addition of the toxic compounds chloroform and chromium increased the net accumulation of SMP, despite reducing the percentage of SMP in the effluent due to the overwhelming production of volatile fatty acids (VFAs). In the reactor spiked with chloroform the normalized accumulation of SMP (SMP/So) increased from 2% to 8%, whereas in the reactor spiked with Cr (VI) the SMP/So ratio reached as high as 20% after the spike, and in both cases SMP net accumulation was proportional to the concentration of toxicant. After the chloroform and chromium spikes biomass seemed to produce more extra cellular polymeric substances (EPS) suggesting that this might have helped them to cope with the stress. Chromatography results indicate that some of the high MW compounds present in the SMP might have been due to EPS release into the bulk solution, and that other compounds, probably released as a result of cell lysis, were also present. Hydrolysis of EPS did not seem to contribute to SMP accumulation in the presence of toxic compounds, and DNA analysis suggested that cell lysis products was an important contribution to SMP accumulation, in the presence of chromium.

Hydrogen production in anaerobic reactors during shock loads--influence of formate production and H2 kinetics.

In this article the role of hydrogen as a process monitoring tool in methanogenic systems was studied by considering the influence of several key system parameters. Hydrogen production was found to be influenced mainly by the inocula's source pH, and varied only slightly with external pH and HCO3- levels. When an inoculum adapted to above neutral conditions (pH > 7) was shocked, reducing equivalents were selectively channelled through formate, while high hydrogen production was noticed with acidically (pH < 6.5) adapted inocula. The results also revealed that the production of hydrogen or formate during shock loads was not strongly associated with microbial morphology (granules or flocs) as high electron fluxes were possible through either during acidogenesis. Shock load experiments in continuous reactors revealed that neither hydrogen nor formate accumulated to any significant degree, nevertheless digester recovery took a long time due to the slow kinetics of volatile fatty acid degradation. Selective formate production under neutral pH environments, coupled with high hydrogenotrophic activity, was found to be responsible for the dampened hydrogen response during the early phases of gradually shocked systems (step change). Based on these results it appears that the role of hydrogen as a process monitoring tool has been overemphasised in the literature.

Nitrification of high strength ammonia wastewaters: comparative study of immobilisation media.

Due to legislative pressures, sludge production and processing in the UK will increase substantially in the future resulting in a supernatant liquid high in ammonia (500-1000 mg l-1) and "hard" COD (approximately 500 mg l-1). A small footprint reactor is required to effectively nitrify this effluent, and the aim of this work was to compare a number of immobilisation media under a variety of conditions in order to determine which media held the most promise for future development. Laboratory-scale continuously stirred tank reactors containing freely suspended and immobilised biomass were operated with a high-strength synthetic ammonia wastewater (500 mg N l-1) to determine the nitrification rates at various temperatures, and ammonia and COD loadings. COD:NH3 ratios in sludge liquors vary widely depending on the treatment processes employed, and therefore ratios of 1:1 and 2:1 were tested as being fairly typical. The freely suspended nitrifiers were washed out of the reactors at a 1 d hydraulic retention time (HRT), whereas the reactors containing adsorption particles (Linpor and Kaldnes) and PVA-encapsulated nitrifiers continued partially nitrifying down to 12 h, and oxygen addition enhanced nitrification. A decrease in temperature from 25 to 16 degrees C only caused a small (10%) decrease in nitrification in the immobilised cell reactors, demonstrating that nitrification was mass transfer rather than kinetically controlled. A reduction in nitrification occurred when glucose (500 mg l-1) was added to the feed due to the growth of a heterotrophic population. The adsorbed biomass reactors lost 35% of nitrification compared to only 7% with PVA, and it appears that the colonisation of PVA by heterotrophs is more difficult than for Linpor and Kaldnes. Respiration rates for all particles increased with time in the reactors, and nitrifiers immobilised in PVA retained approximately 40% of their viability after immobilisation. Volumetric nitrification rates were generally higher for the PVA reactor than for Linpor and Kaldnes, and were: suspended biomass reactor: 0.36; Linpor: 0.57; Kaldnes: 0.53 and PVA: 0.70 kg N m-3-reactor d-1 for a 25% reactor fill. These equate to 2.28, 4.24 and 3.97 g N m-2-media d-1 for Linpor, Kaldnes and PVA respectively, hence other reactor fill rates for Kaldnes warrant further investigation. However, the PVA particles with the highest nitrification rates under all conditions showed promise as an immobilisation medium, and are amenable to further optimisation for the nitrification of high-strength ammonia wastewaters.

The effect of organic and hydraulic shock loads on the production of soluble microbial products in anaerobic digesters.

Anaerobic chemostats were used to investigate the effects of organic and hydraulic shock loads on the production of soluble microbial products (SMP). Production of SMP was found to increase during glucose spikes, reaching up 15% of the influent chemical oxygen demand. These SMP appear to be utilization-associated products produced as a result of the temporarily high organic load, and chemical analysis and ultrafiltration experiments revealed that most of these compounds are difficult to identify and that the majority of them are present in the low molecular weight (MW) range. Production of SMP also increased when the hydraulic retention time was reduced from 15 to 3 days, and an increase in DNA concentration in the bulk solution suggested enhanced cell lysis. Although the cause of lysis was not clear, it is believed that most of the SMP produced under such conditions were biomass-associated products following cell death. While the majority of these compounds lay in the low MW range, as much as 35% were found to have MWs greater than 1 kDa. During the period when the anaerobic chemostat was fed no alkalinity and the pH remained lower than 6.5 for more than a week, a slightly higher production of SMP and a shift in the MW distribution towards the production of higher MW SMP was observed.

Modeling of soluble microbial products in anaerobic digestion: the effect of feed strength and composition.

Continuously stirred tank reactors (CSTRs) and fill-and-draw reactors were used to investigate soluble microbial product (SMP) production during anaerobic digestion. Continuously stirred tank reactors with a glucose feed at three different strengths (5, 10, and 20 g chemical oxygen demand [COD]/L) and fill-and-draw reactors with four different feed compositions (glucose, glycerol, lactic acid, and starch at 10 g COD/L) were used with a solids retention time (SRT) of 15 days. The SMP production ranged from 102 to 588 mg COD/L for the glucose-fed CSTRs and between 157 and 1192 mg COD/L for the fill-and-draw reactors and was found to increase with increasing influent COD (SO). Normalized production of SMPs (SMP/So) ranged from 1.4 to 3% for the CSTRs and 12.0, 1.7, 14.7, and 3.6% for the glucose-, glycerol-, lactic acid-, and starch-fed reactors, respectively. A model incorporating SMP production and degradation was fitted to results from carbon-14 tracer studies in all of the reactors. The best-fit parameters from this model revealed that the type of SMP that dominates in any particular system depends not only on the strength of the feed but also on the composition of the feed and the type of reactor.