Recovery  cleaning  of  submerged  MBR  system  

Background  Copenhagen   Chemicals   had   been   in   dialogue  with   a   German  manufacturer   and   supplier   of  MBR  systems  about  our   range  membrane   cleaners,   including  our   specialised  MBR  cleaners.  The  MBR  systems  supplied  by  the  manufacturer  are  based  on  submerged  membranes,  and  are  normally   cleaned   by  NaOCl   solution   filled   into   the   permeate   side   of   the  membranes.   Other  than   this,   the   systems   are   normally   subjected   to   a   full   recovery   clean   using  NaOCL,   caustic  and/or  a  specialised  membrane  cleaner  1  to  2   times  per  year.  These  recovery  cleanings  are  done  by  emptying  the  biological  tank  and  filling  it  with  a  cleaning  solution  that  is  recirculated  in  forward  flow  for  about  1  hour.    The  manufacturer  had  a  pressing  problem  with  a  2.000  man  MBR  unit  placed  on  a  cruise  ship,  where  the  membranes  could  not  be  recovered  using  this  standard  approach.  The  production  had   fallen   to   such   a   low   level,   that   the  wastewater  plant   could  not   keep  up  with   the  water  flowing  to  the  system.  The  production  was  down  to  below  50%  of  design.    Copenhagen   Chemicals   was   asked   to   supply   our   newly   developed  MBR  membrane   cleaner  MBR  KLEAN  210  to  see  if  this  could  help  restore  the  membrane  permeability.  

Description  of  system  The  system  is  a  submerged  type  MBR  system  using  plate  membranes.  The  system  is  designed  to  treat  wastewater  from  2.000  people.  The  unit   is  placed  on  a  cruise  ship,  and  therefor  has  limited  possibilities  to  store  any  excess  wastewater  that  the  system  cannot  treat.  Below  is  a  principal  drawing  of  the  system  design  (bio-­‐reactor  only).  

 

   

Description  of  the  problem  The   system  had   so   far   been   cleaned   regularly   by  backwashing   the  modules   in  place  with   a  NaOCl   solution.   The   backwashing   had   been   performed   from   the   inside   of   the   plate  membranes  with  pressure.  Due  to   the   fact   that   the  membranes   touch  each  other  during  the  backwashing  procedure  nearly  no   solution  entered   the  biological   tank.  However  due   to   the  increased  fouling  the  liquid  could  not  flow  through  the  membrane.    Each  block  was  cleaned  monthly  by   this  procedure  with  a  1000ppm  NaOCl  solution  but   the  cleanings  did  not   yield   the  desired   results.  Gradually   the  permeability   got   lower   eventually  (after   about   one   year)   making   it   impossible   to   treat   all   wastewater   due   to   the   low  permeability.    The  customer  was  searching  for  a  new  way  to  perform  cleaning  as  an  additional  procedure  to  increase  the  permeability  on  a  half  year  basis  if  necessary.      The   fouling   observed   was   all   over   the   membranes   resulting   in   a   bad   performance   of   the  aeration   and   following   accumulation  of   particles   on   the  downside  of   the  membrane  blocks.  This  caused  further  blockages  on  the  plates,  which  again  further  reduced  the  effectiveness  of  the  air  scouring.  Partly  there  was  also  a  kind  of  caking  observed.  Sludge  accumulated  between  the  plates  and  compressed.  This  caking  could  be  removed  very  easily  with  some  fresh  water  during  a  manual  inspection  of  the  tank.  The  following  pictures  illustrate  the  fouling  issues.  

   An  analysis  of  a  piece  of  membrane  showed  a  dark  brown  discoloration  caused  by  pectines  and  humic  substances.  Partly  the  membrane  was  smooth  and  not  assigned;  partly  there  were  encrustations,  which   could   be   removed   very   easily.   These   encrustations  were   identified   as  bioorganic  and  insoluble  in  water.    The  deposits  was  not  only  on  the  surface  but  also  inside  the  membrane  structure,  infiltrating  and  impregnating  the  membrane.          

     The  following  pictures  show  a  new  membrane  in  comparison  to  a  blocked  one.  

 

Copenhagen  Chemicals’  solution  to  the  problem    Copenhagen   Chemicals   proposed   a   solution,   where   a   full   recovery   cleaning   would   be  performed  using  one  of  our  specialised  MBR  Membrane  cleaners.  Due  to  the  low  pH  tolerance  of  the  membranes  and  the  difficulty  in  handling  acid  dosing  on  the  ship,  we  proposed  to  use  MBR  KLEAN  210  in  a  1,5%  solution.  The  tank  was  emptied  and  filled  with  70  m3  of  water  that  was  added  1.050  kg  MBR  KLEAN  210.  A  cleaning  was  performed  recirculating  the  cleaning  solution  for  about  1.5  hours  through  the  membranes  in  normal  flow  direction.  

Results  The   result   of   the   cleaning   was   impressive.   The   permeability   was   practically   doubled  increasing  from  below  120  to  above  225  (see  below  pictures  of  the  permeability  before  and  after  the  cleaning  (black  line)).  This   result   was   really   good   for   our   customer,   practically   restoring   their   unit’s   production  capacity  to  design  value.  The  unit  can  now  easily  handle  the  wastewater  produced  by  the  ship.  Directly  after  the  cleaning  the  diffusers  was  also  changed  to  keep  the  results  of  the  cleaning  success  going  for  as  long  as  possible.  

     

 

 

     

   

Conclusion  • MBR  KLEAN  210  gave  much  better  performance  than  any  other  tested  product.  • MBR  KLEAN  210  gave  no  foaming  issues  and  no  compatibility  issues.  • MBR  KLEAN  210  gave  no  concerns  regarding  corrosion  in  the  tank,  as  experienced  

when  using  NaOCl.  • The  customer  will  continue  to  use  MBR  KLEAN  210  for  bi-­‐annual  cleanings.  • The  results  indicate  possibility  to  optimise  on  the  concentration  of  MBR  KMLEAN  210,  

lowering  the  concentration  thereby  further  reducing  cost.