Andrew A. Randall

Fouling control in activated sludge submerged hollow fiber membrane bioreactors

The goal of this study is to determine the impact of various operating factors on membrane fouling in activated sludge membrane bioreactor (MBR) process, typically used for water reclamation. In this process, ultrafiltration (UF) and microfiltration (MF) hollow fiber membranes, submerged in the bioreactor, provided a solid-liquid separation by replacing gravity settling. Activated sludge from a food wastewater treatment plant was inoculated to purify synthetic wastewater consisting of glucose and (NH&SO, as a source of carbon and nitrogen, respectively. The results clearly showed that membrane fouling, defined as permeate flux decline due to accumulation of substances within membrane pores and/or onto membrane surface, was greatly influenced by membrane type and module configuration. It was also found that the rate and extent of permeate flux decline increased with increasing suction pressure (or initial operating flux) and with decreasing air-scouring rate. The mixed liquor suspended solids (MLSS) concentrations, however, exhibited very little influence on permeate flux for the range of 3600-8400 mgk. Another important finding of this investigation was that non-continuous membrane operation significantly improved membrane productivity. This observation can be explained by the enhanced back transport of foulants under pressure relaxation. During non-suction periods, the foulants not irreversibly attached to the membrane surface, diffused away from the membrane surface because of concentration gradient. Furthermore, the effectiveness of air scouring was greatly enhanced in the absence of transmembrane suction pressure, resulting in higher removal of foulants accumulated on the membrane surface. The use of intermittent suction operation may not be economically feasible at large-scale, but it may offer an effective fouling control means for small-scale MBR processes treating wastewaters with high fouling potential.

ASSIMILABLE ORGANIC CARBON (AOC) AND BIODEGRADABLE DISSOLVED ORGANIC CARBON (BDOC): COMPLEMENTARY MEASUREMENTS

The objective of this study was to evaluate the necessity of measuring both assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC) as indicators of bacterial regrowth potential. AOC and BDOC have often been measured separately as indicators of bacterial regrowth, or together as indicators of bacterial regrowth and disinfection by-product formation potential, respectively. However, this study proposes that both AOC and BDOC should be used as complementary measurements of bacterial regrowth potential. In monitoring of full-scale membrane filtration, it was determined that nanofiltration (NF) removed over 90% of the BDOC while allowing the majority of the AOC through. Heterotrophic plate counts (HPC) remained low during the entire period of monitoring due to high additions of disinfectant residual. In a two-year monitoring of a water treatment plant that switched its treatment process from chlorination to chlorination and ozonation, it was observed that the plant effluent AOC increased by 127% while BDOC increased by 49% after the introduction of ozone. Even though AOC is a fraction of BDOC, measuring only one of these parameters can potentially under- or overestimate the bacterial regrowth potential of the water.

The behavior of membrane fouling initiation on the crossflow membrane bioreactor system

Abstract A membrane bioreactor (MBR) system (i.e. an activated sludge system where the clarifier has been replaced with membrane filtration) has many advantages compared with the conventional activated sludge process. However a serious problems that has prevented this technology from reaching its potential is membrane fouling. In this study, the biomass was cultivated under three conditions: normal (high dissolved oxygen, DO); bulking; and normal conditions with low DO concentration. Membrane filtration tests were performed with cellulose acetate (CA), sulfonated polyethersulfone (SPES) and polyethersulfone (PES) membranes, respectively. Relatively hydrophobic PES membranes fouled more seriously than hydrophilic CA membranes. This phenomenon was especially significant under bulking conditions. Ultimate flux was almost equal for all three membrane types however, since the PES and SPES started with higher initial fluxes. Effluent quality was not affected by membrane fouling/flux but did deteriorate at low DOs.

Removal of assimilable organic carbon and biodegradable dissolved organic carbon by reverse osmosis and nanofiltration membranes

Abstract The main objective of this study was to evaluate the effectiveness of reverse osmosis (RO) and nanofiltration (NF), under various solution chemistries, on bacterial regrowth potential as quantified by assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC). The bench-scale experiments, using tap groundwater spiked with acetate as organic carbon, revealed that AOC removals by RO/NF membranes were strongly dependent on charge repulsion. AOC removals were greater at conditions of low ionic strength and low hardness, and were slightly higher at high pH values. BDOC removals by NF membrane also increased with decreasing hardness and ionic strength, and increasing pH. However, the RO membrane showed less dependence on feed solution chemistry for BDOC removal, suggesting that BDOC removal was determined by the combined effect of both size exclusion and charge repulsion. The bench-scale observations were compared to a full-scale drinking water treatment plant that used nanofiltration as a primary treatment process. From full-scale operation, it was observed that nanofiltration was a very effective means to reduce BDOC, but conversely, did not reject the bulk of raw water AOC. The high BDOC rejection by NF membranes at full scale can be explained by size exclusion, since a significant fraction of BDOC in raw surficial ground water consists of compounds, such as humic and fulvic acids, which are larger than the pores of NF membranes. The insignificant AOC rejection observed in the full-scale system was probably due to the low pH, high hardness, and high ionic strength (TDS) of the raw groundwater combined with acid addition during pretreatment. These solution environments repress the electrostatic interaction between charged organic compounds and membranes, allowing passage of small molecular weight compounds and thus reducing AOC rejection.

A biochemical hypothesis explaining the response of enhanced biological phosphorus removal biomass to organic substrates.

Anaerobic/aerobic batch experiments were conducted with a variety of volatile fatty (VFAs) and amino acids on two sequencing batch reactor populations displaying enhanced biological phosphorus removal. The batch experiments were consistent between the two systems and with the past literature: acetic and isovaleric acid were the most efficient substrates, and propionic acid was the least efficient of the 2-5 carbon VFAs (lack of acclimation was ruled out). A survey of the engineering and biochemical literature revealed that both acetic and isovaleric acid resulted in a negative reaction redox balance (i.e. it requires reducing equivalents such as NADH2) during their biotransformation to polyhydroxyalkanoates (PHAs). In addition, the survey indicated that acetic and isovaleric acid resulted in 3HB rather than 3HV or 3H2MV formation. Two possible hypotheses were put forward for evaluation: (1) it was hypothesized that a negative intracellular redox balance might result in higher PHA content since PHA biosynthesis could be sustained under anaerobic conditions (no NADH2 build up), and/or (2) it was hypothesized that 3HB resulted in greater P-uptake than other PHA forms such as 3HV.

Induction of phosphorus removal in an enhanced biological phosphorus removal bacterial population

Abstract Volatile fatty acids (VFAs) resulting from prefermentation of influent glucose were used to cultivate a bacterial population capable of enhanced biological phosphorus removal (EBPR) in two identical anerobic/aerobic sequencing batch reactors (SBRs). An identical SBR receiving starch, which did not readily preferment, established only marginal EBPR. The Starch SBR population did not respond in batch tests to any of the substrates studied. In batch tests for the glucose SBR populations the two to five carbon VFAs, except propionic acid, induced greater inorganic phosphate (Pi) removal. Succinic acid also improved removals. Branched VFAs were superior to their linear isomers. Isovaleric acid improved Pi removal the most consistently, and at lower molar concentrations than any other VFA. The C2 and C5 alcohols did not have a significant effect on Pi removal, and neither did formate or methanol. The C3 and C4 alcohols did result in relatively small but consistent improvements in removals. Glucose, as well as amino acid rich synthetic wastewater, were both extremely detrimental to Pi removal. Fructose and starch did not have the same detrimental effect as glucose.

Polyhydroxyalkanoates form potentially a key aspect of aerobic phosphorus uptake in enhanced biological phosphorus removal.

Eighteen anaerobic/aerobic batch experiments were conducted with a variety of volatile fatty acids (VFAs) on a sequencing batch reactor (SBR) population displaying enhanced biological phosphorus removal (EBPR). A statistically significant (P << 0.01 for all variables) correlation between aerobic phosphorus uptake and polyhydroxyalkanoates (PHAs) quantity and form was observed. The results suggest that poly-3-hydroxy-butyrate (3HB) results in significantly higher aerobic phosphorus (P) uptake per unit mmoles as carbon (mmoles-C) than poly-3-hydroxy-valerate (3HV). The results showed that acetic and isovaleric acids resulted in higher P removals (relative to propionic and valeric acids) during EBPR batch experiments not because of higher PHAs quantity, but largely because the predominant type was 3HB rather than 3HV. In contrast propionic and valeric acids resulted in 3HV, and showed much lower aerobic P uptake per unit PHAs.

Anaerobic metabolic models for phosphorus- and glycogen-accumulating organisms with mixed acetic and propionic acids as carbon sources.

With acetate or propionate as the sole carbon source, anaerobic metabolic models describing phosphorus- and glycogen-accumulating organisms (PAO and GAO) have been developed in the literature. However, comprehensive models are in need for the description of PAO and GAO behaviors with mixed acetic and propionic acids as carbon sources since they are the two main volatile fatty acids (VFA) that coexist in real wastewater. Two metabolic models were proposed to characterize the anaerobic stoichiometry of PAO and GAO, respectively, and two groups of sequencing batch reactors (i.e. 5 PAO-SBRs and 5 GAO-SBRs) with different propionic to acetic acid ratios were used for the validation of the models. The experimental data indicated that polyhydroxyalkanoates were synthesized via random condensation in GAO cells, whereas the semi-selective/semi-random pathway was used for the integration of acetyl-CoA and propionyl-CoA in PAO cells. When the VFA was pure acetic or propionic acid, the proposed PAO (or GAO) model reverted back to the reported acetate or propionate PAO (or GAO) model. Results also showed that the energy required for the transportation of 1C-mol VFA across the membrane of both PAO and GAO cells was independent of the propionate/acetate ratio.

Reductive dechlorination of tetrachloroethylene by soil sulfate-reducing microbes under various electron donor conditions.

Biotransformation of tetrachloroethylene (PCE) by soil sulfate-reducing bacteria was investigated using acetate, lactate and methanol as electron donors. Results show that the effectiveness of biotransformation of PCE by soil sulfate-reducing bacterial is dependent upon the type of electron donor used.

Modeling of organic substrate transformation in the high-rate activated sludge process

This study describes the development of a modified activated sludge model No.1 framework to describe the organic substrate transformation in the high-rate activated sludge (HRAS) process. New process mechanisms for dual soluble substrate utilization, production of extracellular polymeric substances (EPS), absorption of soluble substrate (storage), and adsorption of colloidal substrate were included in the modified model. Data from two HRAS pilot plants were investigated to calibrate and to validate the proposed model for HRAS systems. A subdivision of readily biodegradable soluble substrate into a slow and fast fraction were included to allow accurate description of effluent soluble chemical oxygen demand (COD) in HRAS versus longer solids retention time (SRT) systems. The modified model incorporates production of EPS and storage polymers as part of the aerobic growth transformation process on the soluble substrate and transformation processes for flocculation of colloidal COD to particulate COD. The adsorbed organics are then converted through hydrolysis to the slowly biodegradable soluble fraction. Two soluble substrate models were evaluated during this study, i.e., the dual substrate and the diauxic models. Both models used two state variables for biodegradable soluble substrate (SBf and SBs) and a single biomass population. The A-stage pilot typically removed 63% of the soluble substrate (SB) at an SRT <0.13 d and 79% at SRT of 0.23 d. In comparison, the dual substrate model predicted 58% removal at the lower SRT and 78% at the higher SRT, with the diauxic model predicting 32% and 70% removals, respectively. Overall, the dual substrate model provided better results than the diauxic model and therefore it was adopted during this study. The dual substrate model successfully described the higher effluent soluble COD observed in the HRAS systems due to the partial removal of SBs, which is almost completely removed in higher SRT systems.