Very late reply here.   For ...

Very late reply here.   For me  DMF with media  of a   bio supporting  type eg Pumice or Expanded Clay is nearly always better solution than UF/MF .

a) Are there really any large size UF SWRO plants that  do not suffer bad biofouling of the RO .  DMF with deep biosupport media and dechlorination   before DWF  should reduce the biofouling on RO  to  level with far fewer CIP annual actions

b)  Claimed longer RO membrane life with UF/MF not reality due to biofouling needing frequent CIP actions. Biofouling causes of lack of RO membrane performance/life always excluded from RO membrane guarantees.

c)  Membrane integrity a major problem for several UF/MF  plants for seawater RO .   

d)  DMF  tendency now to operate with no acid , and average iron dose of only  around 1- 2 ppm. This the chemical saving/sludge treatment saving /Environmental argument is  present but  not very strong .   

e)  Gravity DMF  needs only about  12-16  filter cells for a 150 MLD RO plant,  there will be backwashing  only once every 30-50 hours of each  train  using approx. 5 actuated valves .      A   UF/MF system will use  20-30  trains each with backwashing  occurring every 30-45 mins using 5 actuate valves .  UF/MF  has to be far more mechanically unreliable.

f)  UF/MF valves often have some leakage , thus needs constant dosing of the filtrate with SMBS to prevent oxidation of the RO.

g)  Too often the spent  chlorine and other chemicals   used for CEB/CIP  actions of the UF/MF are not neutralised to save cost and discharged to sea  (yes neutralisation facilities provided but not used) if there is not effective environmental monitoring .

h) The UF/MF membranes life  is very uncertain 3-7 years  with many (but not all) of the suppliers. One UF/Mf supplier has a proper demonstrated  10 year life. This is also linked to the membrane integrity issue.  

I) The UF/MF modules  are often bespoke for the Rack systems of the supplier , this makes them  more  subject to  issues when they become  obsolete  and also leads to sole source vendor issues,  (price exploitation , and issue when they leave the business).

J)  DMF plants do not need a building . Yes some designers  put them in the building to prevent algae growth, but I believe why bother , let the algae grow on the  top surface of the DMF  and remove the nutrients needed for biofouling of the RO.  The DMF will remove most (not all) of  this additional algae any way, better to remove the soluble and fine colloidal nutrients.        

k)   The  UF /MF needs to have effective pressure screen pretreatment . Some say 100 microns to protect against barnacles,  other say  50 microns to protect against Mussels.  Other are now allowing 150-300 microns , but it is very unclear the effect of fine  sand /silt abrasion on the membrane.   Perth II desal has published (San Diego 2015) a MF membrane  integrity  problem caused by very small spinicules .