Fouling of membranes in membrane bioreactors for wastewater treatment: Planktonic bacteria can have a significant contribution.

Membrane bioreactors (MBRs) for wastewater treatment show great potentials in the sustainable development of urban environments. However, fouling of membranes remains the largest challenge of MBR technology. Dissolved extracellular polymeric substances (EPS) are often assumed be the main foulant in MBRs. However, single bacterial cells are often erroneously measured as EPS in traditional spectrophotometric analysis of EPS in activated sludge, so we hypothesized that single cells in many cases could be the true foulants in MBRs for wastewater treatment. To study this, raw MBR sludge and sludge supernatant with varying concentrations of planktonic cells were filtered on microfiltration (MF) membranes, and we found a direct correlation between the cell count and rate of flux decline. Addition of planktonic cells to fresh MBR sludge dramatically increased the flux decline. The identity of the most abundant planktonic cells in a full-scale MBR water resource recovery facility was determined by DNA fingerprinting. Many of these genera are known to be abundant in influent wastewater suggesting that the influent bacterial cells may have a direct effect on the fouling propensity in MBR systems. This new knowledge may lead to new anti-fouling strategies targeting incoming planktonic bacteria from the wastewater feed. PRACTITIONER POINTS: Planktonic cells constituted up to 60% of the total protein content of "soluble extracellular polymeric substances" in membrane bioreactor sludge. Planktonic cells are hidden under a surrogate concentration of extracellular polymeric substances which is often associated with fouling. Membrane fouling rate is directly proportional to amount of free planktonic cells suspended in sludge. Several influent bacterial genera are enriched in the water phase of membrane bioreactor sludge. Removing these may mitigate fouling.

Industrial Wastewater Treatment by Nanofiltration-a Case Study on the Anodizing Industry.

The anodizing industry generates several alkaline and acidic wastewater streams often with high concentrations of heavy metals. In this study, nanofiltration (NF) was used to treat wastewater from individual baths, i.e., wastewater from color rinse, alkaline pickling rinse, acidic pickling rinse and anodizing rinse, as well as a mixture of all the wastewater streams. The experiments were carried out by using a commercial membrane (NF99HF) exhibiting pure water permeability of 10 L/(m2·h·bar). For all wastewater streams except one, pH was adjusted to bring it within the recommended pH limits of the membrane, whereby part of the heavy metals precipitated and was removed. The NF of the color rinse offered high-quality permeate (heavy metals below detection limit) and high permeability (9 L/(m2·h·bar)), whereas the nanofiltration of the alkaline pickling rinse exhibited no permeability. The NF of the acidic pickling rinse showed a permeability of 3.1-4.1 L/(m2·h·bar), but low ion rejection (7%-13%). NF of the neutralized mixed wastewater, after the removal of precipitate, produced high-quality permeate with a stable permeability of 1 L/(m2·h·bar). Treatment of the mixed wastewater is therefore the best option if the water has to be discharged. If the water has to be reused, the permeate conductivity in the color rinse and anodizing rinse baths have been reduced significantly, so the treatment of these streams may then be a better option.

Filtration properties of activated sludge in municipal MBR wastewater treatment plants are related to microbial community structure.

In the conventional activated sludge process, a number of important parameters determining the efficiency of settling and dewatering are often linked to specific groups of bacteria in the sludge--namely floc size, residual turbidity, shear sensitivity and composition of extracellular polymeric substances (EPS). In membrane bioreactors (MBRs) the nature of solids separation at the membrane has much in common with sludge dewaterability but less is known about the effect of specific microbial groups on the sludge characteristics that affect this process. In this study, six full-scale MBR plants were investigated to identify correlations between sludge filterability, sludge characteristics, and microbial community structure. The microbial community structure was described by quantitative fluorescence in situ hybridization and sludge filterability by a low-pressure filtration method. A strong correlation between the degree of flocculation (ratio between floc size and residual turbidity) and sludge filterability at low pressure was found. A good balance between EPS and cations in the sludge correlated with good flocculation, relatively large sludge flocs, and low amounts of small particles and single cells in the bulk phase (measured as residual turbidity), all leading to a good filterability. Floc properties could also be linked to the microbial community structure. Bacterial species forming strong microcolonies such as Nitrospira and Accumulibacter were present in plants with good flocculation and filtration properties, while few strong microcolonies and many filamentous bacteria in the plants correlated with poor flocculation and filtration problems. In conclusion this study extends the hitherto accepted perception that plant operation affects floc properties which affects fouling. Additionally, plant operation also affects species composition, which affects floc properties and in the end fouling propensity.

Modeling cake buildup under TMP-step filtration in a membrane bioreactor: cake compressibility is significant.

Fouling is inevitable in membrane bioreactors (MBRs) due to the complex nature of activated sludge, which contains a broad variety of potential foulants. Filter cakes that build up from sludge particles are traditionally highly compressible due to both the deformation of the individual sludge particles and the rearrangement of these particles in the cake. However, this phenomenon has been little examined in studies of fouling mechanisms in MBR systems. This study examines the properties of the cake layer, modeling the cake buildup and specific cake resistance (α), including compressibility, in terms of pressure-dependent α. The changes in fouling resistance during transmembrane pressure (TMP)-step filtration in an MBR setup were simulated using an empirical pressure dependence of the specific cake resistance and a simple mass balance model. The total change in fouling resistance in each TMP step could be divided into an initial rapid change in specific cake resistance due to filter cake compression followed by simple cake buildup. By including cake compression in this simple model, the model fitted the data with high precision. We demonstrated that compressibility should be considered when describing cake fouling in MBRs.