Walter van der Meer

The role of blocking and cake filtration in MBR fouling

Abstract Membrane fouling in a side stream biomass separation MBR pilot plant was investigated. Constant flux filtration (18–721/M2h) was employed. Air was continuously supplied to the MBR system with the feed (sludge) to flush the membrane surface, and backwashing was applied every 5–10 min for 8 s to control membrane fouling. Although the duration of pore blocking was generally short (completed in 8 s at a flux of 52 llm2h), blocking resistance (mainly irreversible blocking resistance) was the main cause of membrane fouling. However, the resistance of the filter cake also played an important role, particularly when the backwashing interval was extended to 10 min. In terms of fouling reversibility, blocking resistance was not completely reversible by backwashing, especially at higher fluxes (e.g. 69 I/m2h), and frequent chemical cleaning (once every week at 401/m2h) was required. However, cake filtration was easily reversible via a combination of backwashing and sludge/air flushing of the membrane surface. Finally, a simple method to identify both irreversible and reversible blocking resistance and filter cake resistance was proposed.

Biological black water treatment combined with membrane separation.

Separate treatment of black (toilet) water offers the possibility to recover energy and nutrients. In this study three combinations of biological treatment and membrane filtration were compared for their biological and membrane performance and nutrient conservation: a UASB followed by effluent membrane filtration, an anaerobic MBR and an aerobic MBR. Methane production in the anaerobic systems was lower than expected. Sludge production was highest in the aerobic MBR, followed by the anaerobic MBR and the UASB-membrane system. The level of nutrient conservation in the effluent was high in all three treatment systems, which is beneficial for their recovery from the effluent. Membrane treatment guaranteed an effluent which is free of suspended and colloidal matter. However, the concentration of soluble COD in the effluent still was relatively high and this may seriously hamper subsequent nutrient recovery by physical-chemical processes. The membrane filtration behaviour of the three systems was very different, and seemed to be dominated by the concentration of colloidals in the membrane feed. In general, membrane fouling was the lowest in the aerobic MBR, followed by the membranes used for UASB effluent filtration and the anaerobic MBR.

Direct capillary nanofiltration — a new high-grade purification concept☆

Capillary nanofiltration (NF) enables raw water to be treated in a single step to produce high quality permeate. The permeate can be used as process water for the industry. Trial studies show that a stable operation is achievable using capillary NF without pretreatment (in other words direct NF), that the technique saves energy, that there is a low use of chemicals and that the permeate is of a high quality. The capillary NF membrane module combines the favorable cleaning properties of capillary ultrafiltration membranes with the favorable separation properties of NF membranes in terms of the removal of bacteria, viruses, color, hardness and pesticides.

Comparison of the modeling approach between membrane bioreactor and conventional activated sludge processes.

Activated sludge models (ASM) have been developed and largely applied in conventional activated sludge (CAS) systems. The applicability of ASM to model membrane bioreactors (MBR) and the differences in modeling approaches have not been studied in detail. A laboratory-scale MBR was modeled using ASM2d. It was found that the ASM2d model structure can still be used for MBR modeling. There are significant differences related to ASM modeling. First, a lower maximum specific growth rate for MBR nitrifiers was estimated. Independent experiments demonstrated that this might be attributed to the inhibition effect of soluble microbial products (SMP) at elevated concentration. Second, a greater biomass affinity to oxygen and ammonium was found, which was probably related to smaller MBR sludge flocs. Finally, the membrane throughput during membrane backwashing/relaxation can be normalized and the membrane can be modeled as a continuous flow-through point separator. This simplicity has only a minor effect on ASM simulation results; however, it significantly improved simulation speed.

Direct capillary nanofiltration for surface water

Abstract Capillary nanofiltration (NF) enables raw water to be treated in a single step to produce high-quality permeate. The permeate can be used as process water for industry. Trial studies show that a stable operation is achievable using capillary N F without pre-treatment (in other words direct NF), that the technique saves energy, that there is a low use of chemicals and that the permeate is of a high quality. The capillary NF membrane module combines the favourable cleaning properties of capillary ultrafiltration membranes with the favourable separation properties of NF membranes in terms of the removal of bacteria, viruses, colour, hardness and pesticides. This study describes both the technical as economical backgrounds of the application of direct capillary NF.

Optiflux®: from innovation to realisation**

In order to optimise brackish water desalination, Vitens and DHV have developed a hydraulic optimised pressure vessel (Optiflux®), which reduces the pressure loss significantly and thereby increases the productivity of the membranes by 15-20%. The Optiflux® concept is applied full scale in the Vitens water treatment plant Engelse Werk at Zwolle, The Netherlands. The plant is currently under construction and the production of drinking water is expected to start in the first half of 2003. Currently, seawater desalination plants tend to operate at a higher flux and possibly at a lower recovery (due to the introduction of energy recovery from the concentrate flow), which will lead to a higher flow per pressure vessel, resulting in higher hydraulic losses. These developments will result in an increasing potential for the application of Optiflux® in seawater desalination plants. Based on current practice as well as the expected developments in desalination the authors believe that the full scale implementation of Optiflux® will have significant spin off in future brackish and seawater desalination plants. Introduction Currently, nanofiltration and ultra low pressure Reverse Osmosis (RO) plants are commonly equipped with spiral wound membrane elements. These membrane elements are captured in pressure vessels with a narrow fit between pressure vessels and membrane elements. In order to minimize the costs, 6 to 7 elements are placed in series (pressure vessel length: 21-24 ft). The vessels are linked to each other to create so called stages. This was an optimal design for RO plants operating at high feed pressures (30-60 bar). Pressure losses caused by the serial linkage were in the order of 1,5 bar and not significant compared to the feed pressure. Hydraulic optimal design In 1997, a thesis study was performed to determine the hydraulic optimal design with respect to staging and the number of membrane elements per pressure vessel. This study was based on fresh and brackish feed water. The main results of this study are presented in figure 1. Figure 1: Main results study

Removal of polar organic micropollutants by pilot-scale reverse osmosis drinking water treatment

The robustness of reverse osmosis (RO) against polar organic micropollutants (MPs) was investigated in pilot-scale drinking water treatment. Experiments were carried in hypoxic conditions to treat a raw anaerobic riverbank filtrate spiked with a mixture of thirty model compounds. The chemicals were selected from scientific literature data based on their relevance for the quality of freshwater systems, RO permeate and drinking water. MPs passage and the influence of permeate flux were evaluated with a typical low-pressure RO membrane and quantified by liquid chromatography coupled to high-resolution mass spectrometry. A strong inverse correlation between size and passage of neutral hydrophilic compounds was observed. This correlation was weaker for moderately hydrophobic MPs. Anionic MPs displayed nearly no passage due to electrostatic repulsion with the negatively charged membrane surface, whereas breakthrough of small cationic MPs could be observed. The passage figures observed for the investigated set of MPs ranged from less than 1%-25%. Statistical analysis was performed to evaluate the relationship between physicochemical properties and passage. The effects of permeate flux were more pronounced for small neutral MPs, which displayed a higher passage after a pressure drop.

Multiple dynamic Al-based floc layers on ultrafiltration membrane surfaces for humic acid and reservoir water fouling reduction

The integration of adsorbents with ultrafiltration (UF) membranes is a promising method for alleviating membrane fouling and reducing land use. However, adsorbents typically are only injected into the membrane tank once, resulting in a single dynamic protection layer and low removal efficiency over long-term operation. In addition, the granular adsorbents used can cause membrane surface damage. To overcome these disadvantages, we injected inexpensive and loose aluminum (Al)-based flocs directly into a membrane tank with bottom aeration in the presence of humic acid (HA) or raw water taken from the Miyun Reservoir (Beijing, China). Results showed that the flocs were well suspended in the membrane tank, and multiple dynamic floc protection layers were formed (sandwich-like) on the membrane surface with multiple batch injections. Higher frequency floc injections resulted in better floc utilization efficiency and less severe membrane fouling. With continuous injection, acid solutions demonstrated better performance in removing HA molecules, especially those with small molecular weight, and in alleviating membrane fouling compared with the use of high aeration rate or polyacrylamide injection. This was attributed to the small particle size, large specific surface area, and high zeta potential of the flocs. Additionally, excellent UF membrane performance was exhibited by reservoir water with continuous injection and acid solution. Based on the outstanding UF membrane performance, this innovative integrated filtration with loose Al-based flocs has great application potential for water treatment.

Further developing the bacterial growth potential method for ultra-pure drinking water produced by remineralization of reverse osmosis permeate

Ensuring the biological stability of drinking water is essential for modern drinking water supply. To understand and manage the biological stability, it is critical that the bacterial growth in drinking water can be measured. Nowadays, advance treatment technologies, such as reverse osmosis (RO), are increasingly applied in drinking water purification where the produced water is characterized by low levels of nutrients and cell counts. The challenge is, therefore, how to measure the low bacterial growth potential (BGP) of such ultra-pure water using the available methods which were originally developed for conventionally treated drinking water. In this study, we proposed a protocol to assess BGP of ultra-pure drinking water produced by RO and post-treatment (including remineralization). Natural bacterial consortium from conventional drinking water was added to all water samples during this study to ensure the presence of a wide range of bacterial strains. The method development included developing an ultra-pure blank with high reproducibility to lower the detection limit of the BGP method (50 ± 20 × 103 intact cells/mL) compared with conventional blanks such as bottled spring water, deep groundwater treated by aeration and slow sand filtrate of surface water supply. The ultra-low blank consists of RO permeate after adjusting its pH and essential mineral content under controlled laboratory conditions to ensure carbon limitation. Regarding the test protocol, inoculum concentrations of >10 × 103 intact cells/mL may have a significant contribution to the measured low levels of BGP. Pasteurization of water samples before measuring BGP is necessary to ensure reliable bacterial growth curves. The optimized method was used to assess BGP of ultra-pure drinking water produced by RO membranes and post-treatment (including remineralization), where the BGP has decreased more than 6-fold to a level of 90 ± 20 × 103 intact cells/mL compared with conventionally treated water (630 ± 70 × 103 intact cells/mL).

Preferential binding between intracellular organic matters and Al 13 polymer to enhance coagulation performance

Coagulation is the best available method for removing intracellular organic matter (IOM), which is released from algae cells and is an important precursor to disinfection by-products in drinking water treatment. To gain insight into the best strategy to optimize IOM removal, the coagulation performance of two Al salts, i.e., aluminum chloride (AlCl3) and polyaluminum chloride (PACl, containing 81.2% Al13), was investigated to illuminate the effect of Al species distribution on IOM removal. PACl showed better removal efficiency than AlCl3 with regard to the removal of turbidity and dissolved organic carbon (DOC), owing to the higher charge neutralization effect and greater stability of pre-formed Al13 species. High pressure size exclusion chromatography analysis indicated that the superiority of PACl in DOC removal could be ascribed to the higher binding affinity between Al13 polymer and the low and medium molecular weight (MW) fractions of IOM. The results of differential log-transformed absorbance at 254 and 350 nm indicated more significant formation of complexes between AlCl3 and IOM, which benefits the removal of tryptophan-like proteins thereafter. Additionally, PACl showed more significant superiority compared to AlCl3 in the removal of <5 kDa and hydrophilic fractions, which are widely viewed as the most difficult to remove by coagulation. This study provides insight into the interactions between Al species and IOM, and advances the optimization of coagulation for the removal of IOM in eutrophic water.