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In an attempt to better understand and control seawater reverse osmosis (RO) membrane fouling, low-pressure (LPFD) and high-pressure fouling detectors (HPFD) were employed in an ex situ, sidestream configuration at a full-scale seawater desalination plant. These fouling detectors contained flat-sheet membrane and spacer materials extracted from the membrane modules installed at the plant. The high-pressure fouling detectors reasonably replicated full-scale plant permeate quality, permeate flux, and differential pressure, whereas low-pressure fouling detectors could only simulate differential pressure. When evaluating different pre-treatments, it was observed that activated carbon and chlorine dioxide (after chlorination, microfiltration and dechlorination) to be effective at preventing RO membrane fouling, whereas chlorination, microfiltration and dechlorination and the same sequence followed by ultraviolet irradiation were less effective. Accelerated fouling was found when dosing both colloidal and dissolved nutrients, but the colloidal nutrient caused more severe fouling through a combination of colloidal cake layer formation in addition to biofilm formation and growth. In some cases, differential pressure may be an adequate early warning fouling indicator, but in this study, the high-pressure fouling detector more accurately captured plant fouling impacts. Finally, full-scale seawater RO plant fouling impacts were sufficiently captured while monitoring only pressure, flow and electrical conductivity; therefore, the added cost and complexity of employing more sophisticated optical, electrical, ultrasonic or other methods in ex situ, side stream fouling detectors may not be justified. [ABSTRACT FROM AUTHOR]

A recent innovation in seawater desalination is the use of multi-stage and multi-pass combinations of nanofiltration (NF) and reverse osmosis (RO) membranes. One example of this approach is the “Long Beach method,” in which seawater passes through two different types of NF membranes to produce potable water. After several years of pilot studies comparing the performance of two-pass NF and single-pass RO systems, a number of membrane elements were sacrificed for autopsy analyses. The selected membranes represent different stages of operation including (1) new, (2) fouled, and (3) cleaned membranes. Used NF and RO spiral wound elements were removed from the first and last positions of the demonstration plant. Although operating data suggested no outward signs of membrane fouling – inorganic, organic, and bacterial accumulation were identified on all membranes. First pass RO and NF membranes contained similar amounts of deposited solids, while significantly fewer solids were found on second pass NF membranes. Viable, culturable marine bacteria were observed on all fouled and cleaned membranes, indicating that bacterial colonization of seawater NF/RO membranes was not (a) detected by plant performance monitoring devices, (b) prevented by microfiltration and chlorination, or (c) removed by chemical cleaning. Chemical cleaning recovered the measurable performance of both first pass RO and second pass NF membranes, but was relatively ineffective at removing deposited solids from first-pass NF membranes. Therefore, chemical-cleaning methods may need to be tailored and optimized more specifically for NF membranes used in seawater desalination. [ABSTRACT FROM AUTHOR]

A 6-month pilot-scale field study was conducted to investigate the performance of three polymeric ultra filtration membranes for tertiary filtration within a Southern California municipal water recycling plant. Commercial hydrophilic polyvinylidene fluoride, polyether sulfone, and PolyCera® flatsheet membranes were packaged identically into Spiral Monolith cross-flow back washable modules. The overall objective of the study was to evaluate the filtrate recovery, energy demand, and operating cost of the three membrane materials in achieving California Title 22 product water quality filtering secondary-treated sewage. All three membranes met California Title 22 filtered water quality criteria throughout the entire testing period due to their small characteristic pore size. The PolyCera membrane was the least fouling prone and, as a result, produced significant benefits in filtrate recovery, energy consumption, and operating cost due to less frequent and intense backwashing requirements. Filtration was maintained for over 5 weeks when operating the PolyCera membrane with an average flux of 33 LMH and filtrate backwashing every 25 min. During this long-term operation, the PolyCera system consumed, on average, 0.035 kWh at an operating cost of $0.016 for every cubic meter filtrate water produced, which is a net savings up to 46% compared to standard systems reported in literature. [ABSTRACT FROM AUTHOR]