W.G.J. van der Meer

Quantitative biofouling diagnosis in full scale nanofiltration and reverse osmosis installations.

Biofilm accumulation in nanofiltration and reverse osmosis membrane elements results in a relative increase of normalised pressure drop (DeltaNPD). However, an increase in DeltaNPD is not exclusively linked to biofouling. In order to quantify biofouling, the biomass parameters adenosine triphosphate (ATP), total cell count and heterotrophic plate count in membrane elements were investigated during membrane autopsies and compared with DeltaNPD in test rigs and 15 full scale investigations with different types of feed water. The combination of biomass related parameters ATP and total cell count in membrane elements seem to be suitable parameters for diagnosis of biofouling, whereas plate counts were not appropriate to assess biofouling. The applied DeltaNPD measurement was too insensitive for early detection of fouling. Measurements of biological parameters in the water were shown to be not appropriate in quantifying biofouling. Evidently, there is a need for a practical tool, sensitive pressure drop data and systematic research.

Pressure drop increase by biofilm accumulation in spiral wound RO and NF membrane systems: role of substrate concentration, flow velocity, substrate load and flow direction

In an earlier study, it was shown that biofouling predominantly is a feed spacer channel problem. In this article, pressure drop development and biofilm accumulation in membrane fouling simulators have been studied without permeate production as a function of the process parameters substrate concentration, linear flow velocity, substrate load and flow direction. At the applied substrate concentration range, 100–400 μg l−1 as acetate carbon, a higher concentration caused a faster and greater pressure drop increase and a greater accumulation of biomass. Within the range of linear flow velocities as applied in practice, a higher linear flow velocity resulted in a higher initial pressure drop in addition to a more rapid and greater pressure drop increase and biomass accumulation. Reduction of the linear flow velocity resulted in an instantaneous reduction of the pressure drop caused by the accumulated biomass, without changing the biofilm concentration. A higher substrate load (product of substrate concentration and flow velocity) was related to biomass accumulation. The effect of the same amount of accumulated biomass on the pressure drop increase was related to the linear flow velocity. A decrease of substrate load caused a gradual decline in time of both biomass concentration and pressure drop increase. It was concluded that the pressure drop increase over spiral wound reverse osmosis (RO) and nanofiltration (NF) membrane systems can be reduced by lowering both substrate load and linear flow velocity. There is a need for RO and NF systems with a low pressure drop increase irrespective of the biomass formation. Current efforts to control biofouling of spiral wound membranes focus in addition to pretreatment on membrane improvement. According to these authors, adaptation of the hydrodynamics, spacers and pressure vessel configuration offer promising alternatives. Additional approaches may be replacing heavily biofouled elements and flow direction reversal.

Impact of flow regime on pressure drop increase and biomass accumulation and morphology in membrane systems

Biomass accumulation and pressure drop development have been studied in membrane fouling simulators at different flow regimes. At linear flow velocities as applied in practice in spiral wound nanofiltration (NF) and reverse osmosis (RO) membranes, voluminous and filamentous biofilm structures developed in the feed spacer channel, causing a significant increase in feed channel pressure drop. Elevated shear by both single phase flow (water) and two phase flow (water with air sparging: bubble flow) caused biofilm filaments and a pressure drop increase. The amount of accumulated biomass was independent of the applied shear, depending on the substrate loading rate (product of substrate concentration and linear flow velocity) only. The biofilm streamers oscillated in the passing water. Bubble flow resulted in a more compact and less filamentous biofilm structure than single phase flow, causing a much lower pressure drop increase. The biofilm grown under low shear conditions was more easy to remove during water flushing compared to a biofilm grown under high shear. To control biofouling, biofilm structure may be adjusted using biofilm morphology engineering combined with biomass removal from membrane elements by periodic reverse flushing using modified feed spacers. Potential long and short term consequences of flow regimes on biofilm development are discussed. Flow regimes manipulate biofilm morphology affecting membrane performance, enabling new approaches to control biofouling.

Theoretical optimization of spiral-wound and capillary nanofiltration modules

The performance of spiral-wound (SPW) and capillary nanofiltration (NF) modules can be improved by optimizing the module configuration and operating conditions. To this effect, two mathematical models for spiral wound and capillary modules have been developed. Both models are based on the homogeneous solution model and take into account concentration polarization, difference in rejection of mono- and bivalent ions, and the pressure drop in the feed and permeate channel. In this paper the influence of the operating conditions (feed pressure, feed flow) and the module configuration (spiral wound, capillary, feed channel height, permeate channel height, porosity, capillary diameter) on the performance of the modules are presented. Compared to the optimized spiral-wound module, a further increase in performance of 100% can be achieved by optimizing the configuration and operation conditions of a capillary module.

Only two membrane modules per pressure vessel? Hydraulic optimization of spiral-wound membrane filtration plants☆

Abstract In order to optimize the performance of NF and RO membrane filtration plants, the influence of the plant design and operating conditions has been studied. For this purpose a simplified mathematical model has been developed, which takes into account the difference in rejection of monovalent and bivalent ions, and the hydraulics of SPW membrane modules. This study clearly shows that an increase in permeate productivity of 20% can be achieved, by lowering the number of membrane modules (N

Comparison of Particle-Associated Bacteria from a Drinking Water Treatment Plant and Distribution Reservoirs with Different Water Sources

This study assessed the characteristics of and changes in the suspended particles and the associated bacteria in an unchlorinated drinking water distribution system and its reservoirs with different water sources. The results show that particle-associated bacteria (PAB) were present at a level of 0.8–4.5 × 103 cells ml−1 with a biological activity of 0.01–0.04 ng l−1 ATP. Different PAB communities in the waters produced from different sources were revealed by a 16S rRNA-based pyrosequencing analysis. The quantified biomass underestimation due to the multiple cells attached per particle was ≥ 85%. The distribution of the biologically stable water increased the number of cells per particle (from 48 to 90) but had minor effects on the PAB community. Significant changes were observed at the mixing reservoir. Our results show the characteristics of and changes in suspended PAB during distribution, and highlight the significance of suspended PAB in the distribution system, because suspended PAB can lead to a considerable underestimation of biomass, and because they exist as biofilm, which has a greater mobility than pipe-wall biofilm and therefore presents a greater risk, given the higher probability that it will reach the customers’ taps and be ingested.

Mathematical model of nanofiltration systems

The present design of nanofiltration systems is based mostly on the design of reverse osmosis systems including such aspects as Christmas tree configurations, six spiral-wound modules per pressure vessel, and no recirculation of the brine. However, the feed and osmotic pressure of NF systems are much lower compared to RO systems due to the lower salt concentration of the feed and the lower rejection for monovalent ions. Therefore the hydraulic pressure losses in NF systems are no longer negligible as they are in RO systems. Thus the configuration of NF systems could be different from RO systems. In order to improve the performance of NF systems by optimizing the configuration and operating conditions, a mathematical model has been developed. The model describes the mass transfer through the membranes by using the homogeneous solution model, which is improved by including the concentration polarization. Hydraulic pressure losses are calculated using a modified Darcy-Weisbach equation. In order to optimize the performance of NF systems, the influence of recirculation on recovery, rejection, permeate production per element, and energy consumption has been studied. First results of this study showed that for a given recovery, a one-stage installation with recirculation produces more permeate per element than a two-stage installation without recirculation. However, application of recirculation leads to a slightly increasing energy consumption and permeate concentration.

Innovative design of nano- and ultrafiltration plants

Abstract The productivity of nanofiltration plants can be significantly improved by installing a reduced number of membrane elements serially in pressure vessels [1]. A reduced number of elements per pressure vessel results in a reduction of the hydraulic pressure losses, but more pressure vessels (with a shorter length) have to be applied. The water supply companies Friesland (WLF), Oost-Brabant (WOB) and the consultant DHV Water have developed a new pressure vessel and interconnector for nanofiltration and ultra low pressure reverse osmosis plants. This new pressure vessel in combination with the new interconnector allows optimization of the system hydraulics, without the need for extra pressure vessels. Compared to a traditional design with 6 elements serially placed in pressure vessels, an annual cost reduction of approximately 10–15% for membranes and pressure vessels can be achieved with the new design. In ultrafiltration plants, pressure vessels contribute to approximately 4% of the total construction costs. Since pressure vessels are principally not needed for pressure hold because of the low feed pressures applied, the pressure vessels can be replaced by the new interconnectors. Approximately 2% of the total construction costs can be saved in this way. Water supply company Friesland applies the new pressure vessel in an ultra low pressure reverse osmosis plant, that is started-up this year. Water supply company Oost-Brabant is testing the new interconnector for application in ultrafiltration plants.

Surface water treatment with Zenon microfiltration membranes: minimisation of energy and chemical use☆

Abstract In Nijmegen (The Netherlands) groundwater is withdrawn by NUON to produce drinking water (which is also used for industrial purposes). To be able to meet the future needs for water, NUON will need fo find other sources, to prevent exceeding the limits given in the groundwater licenses. An opportunity could be the scenario in which Philips Semiconductors (a client of NUON with a considerable water use applied in the production of semiconductors) is supplied with treated surface water instead of treated groundwater (i.e. drinking water). To investigate the technical feasibility of this scenario a pilot investigation, using Zenon membrane, has been performed. Dosing air underneath the membranes has proven to be necessary to control fouling. To be able to judge the consequences for the ultrapure treatment plant of Philips (with which the supplied NUON drinking water is treated) a hyperfiltration installation was part of the pilot plant. Analysis afterwards showed mainly biological fouling on the HF membranes. It is expected that a more careful cleaning of the piping work before operation will substantially reduce this type of fouling. Based on the reliability and the constant quality of the treated water the conclusion has been drawn that the switch from treated groundwater to treated surface water is technically feasible.

Visualization of hydraulic conditions inside the feed channel of Reverse Osmosis: A practical comparison of velocity between empty and spacer-filled channel.

It is widely accepted that our understanding about the membrane process increases by investigation of the hydraulic conditions of membranes. While numerical studies have been broadly used for this purpose, the experimental studies of a comparable resolution are scarce. In this study, we compared the pressure drop, the temporal and the spatial velocity maps of a spacer-filled channel and an empty channel of the same size to determine the effect of presence of the feeds spacer on hydraulic conditions. The velocity maps are obtained experimentally by using of the Particle Image Velocimetry (PIV) technique. Application of the feed spacer caused 2-8.5 higher pressure drop increase in the experimental conditions in this research. The flow had a spatial distribution in the form of a unimodal symmetric curve of normal distribution in the empty channel and a bimodal asymmetric curve in the spacer-filled channel. The bimodal curve indicates the presence of high- and low-velocity zones. Additionally, the low-velocity zones showed also a lower variation of velocity in time, which indicates the high fouling potential of these locations. The results from this study may be uses for validation of numerical studies.