Complex feed water conditions require preconditioning prior to direct membrane filtration.  Pilot studies on complex feeds are recommended before designing a treatment system. Some feed water parameters are discussed below with observations from specific installations. 

Feedwater turbidity spikes are a problem at many treatment facilities, where normal contaminants levels are low but increase considerably for short periods, typically 1-5 days.  MF exhibits superior performance during turbidity spikes (Fig 1); however, increased fouling rate resulting in lower recovery and/or cleaning/backwash interval can occur.  This raises two questions for designers: How long does a spike have to be before it is considered a base condition, and what reduction in plant flow rate/recovery can be accepted during a spike?



Coagulant is often used in MF plants. Coagulant dosing can improve filterability by improving filter cake permeability and/or the removal of organics that cause fouling.  This was demonstrated at Scotland’s Invercannie Water Treatment Plant.

Invercannie’s 80MLD plant uses ozone and slow sand filtration prior to MF for cryptosporidium removal.  Although MF feed water is very low in TSS, high soluble TOC (up to 7mg/L) caused membrane fouling.  Fouling was minimized by replacing the ozone/sand filters with poly-aluminum chloride (PACl) dosing, 15 minutes’ contact time prior to MF and switching daily maintenance wash from hypochlorite to sulphuric acid as a precaution against higher levels of aluminum floc.

The type of coagulant can also have a significant effect on membrane performance, specifically on the fouling rate.  pH can also impact performance as demonstrated at a membrane plant in the north of England.  The PVDF membranes were dosed with coagulant at a pH of 4.5.  Initial operation at this pH resulted in a steady rise in trans-membrane pressure (TMP) over four weeks.  The pH of the coagulated water was then adjusted to pH 6.8 just prior to the membrane.  The rate of TMP rise was much lower than at the lower pH.  This was likely caused by a tendency for the floc particles to adhere more strongly near the membrane surface at low pH, resulting in a higher specific filter cake resistance. 

Algae can form a filter cake on the membrane surface, resulting in higher operating pressure and backwash frequency.  This was demonstrated at the 59-MLD membrane filtration plant at Ennerdale in the U.K.  The surface feed water at Ennerdale is subject to summer algae blooms, which cause an increase in operating TMP (Figure 2).  Chemical cleaning successfully maintains membrane performance during this period with return to lower TMP/Backwash frequency when the algae abates.



Iron and manganese is removed by MF, but only if the species are in the insoluble state.  Pretreatment with aeration, pH adjustment and possibly an oxidant such as sodium hypochlorite will precipitate iron so it can be easily removed by MF. For manganese removal, a stronger oxidant such as ozone, chlorine dioxide or potassium permanganate (KMnO4) can be used.  It is important, however, to avoid oxidation or precipitation steps occurring on or within the membrane itself.

At the 30MLD treatment plant at Banwell in southwest U.K., MF is combined with permanganate dosing, coagulant dosing (PACl @ 1mg/L as Al), pH control and two-stage retention to address feed water color, high manganese levels and algae.  KMnO4 was used to consistently remove manganese to <20μg/L. Ten minutes’ retention time at the natural pH of 8.0 is included before the coagulant dosing point.  Another 10 minutes is included after the coagulant dosing point with the pH controlled to 6.5-7.0.

Conclusion

Complex feed water conditions must be considered when designing an MF system.  Significant capital and operating cost savings can be made by sizing membrane plants to maintain low chemical consumption and high recovery during normal periods, and allowing a relaxation in throughput and operating cost-related parameters when feed conditions are at their extreme.  An MF system pilot test on complex feed waters can help in designing a system that consistently meets potable water quality expectations.

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