Outline Introduction Membrane Issues Other Issues

Transcription

1 Outline Introduction Membrane Issues Other Issues

2 Outline Introduction Membrane Issues Other Issues Aeration Foam and Colloids Pretreatment Research needs

3 Aeration For Biomass

4 Oxygen Transfer Most commonly cited problem for operating at high MLSS conditions Really because this issue is a still a design engineer s s concern Membrane risks or issues are transferred to manufacturer Currently, aeration basin designs for MBRs are generally between 8 and 10 g/l Why 8 to 10 g/l and not 20 to 30 g/l? Because there is great concern and discussion about the alpha factor for MBR designs

5 Definitions Oxygen transfer rate (OTR) represents the transfer rate into water OTR depends on the mass transfer coefficient k L a and the concentration difference: OTR =k ac ( * C) cleanwater L α factor relates k L a sludge / k L a cleanwater β factor relates C*sludge/C*cleanwater α factor is main concern, although this is still a very active area of MBR research

6 Wide Variation in α Values MLSS concentrations below 10 g/l helps to eliminate concerns with low α value Adapted from Germain et al., 2007 Water Research

7 Not The Whole Story! We do not understand what impacts α yet Wastewater and mixed liquor constituents are extremely important Common for ASP to have α values vary along the length of a plug flow reactor (increasing α with increasing length of the reactor) Researchers have only been able to correlate α with MLSS concentration to date But there is a very wide variation for the same MLSS concentration Continued research will be help define important factors contributing to α and thus, help re-define MLSS designs

8 Dissolved Oxygen Levels Some manufacturers market a lower reactor DO concentration An attempt to balance out the extra air costs for the membranes There are negative impacts to low DO conc. There is some literature that indicates DO concentrations impact membrane fouling Higher DO leads to a lower sludge fouling propensity (Kang et. Al, 2003) Anaerobic conditions are known to deflocculate (Wilen et. Al, 2004) Anoxic conditions have a higher fouling proprensity than aerobic (Jang et. Al, 2006) Low DO concentrations encourage bulking filaments

9 Aeration for Membranes

10 Oxygen Transfer High Oxygen Transfer Efficiency Desired for cost effectively providing dissolved oxygen to biomass Requires small bubbles for high K L a Coarse bubble aeration used by most MBR manufacturers has 50% the OTE of fine air US Filter s JetTech technology should allow improved oxygen transfer - getting close to the OTE of fine air (75-80%) But you lose the effectiveness of coarse bubble air scour Bubble rise velocity, U L, is proportional to its size But Large bubbles provide better lift and more effectly resuspend rejected material

11 R M R F R C Jss = to membrane V L = away from membrane Jss V L (rapid fouling)

12 Operating Conditions Reactor MCRT, d HRT, h Target MLSS, g/l G Value, 1/s SMBR CMAS Lo CMAS Hi G= P Vµ

13 Particle Size Distribution- CMAS Lo vs. SMBR CMAS Lo CMAS Lo Frequency SMBR Characteristic Length, µm SMBR

14 Particle Size Distribution- CMAS Hi vs. SMBR CMAS Hi Frequency SMBR Characteristic Length, µm

15 Colloidal Material CMAS Hi CMAS Lo SMBR MCRT, d

16 Activated Sludge Dispersion

17 Extracellular Polymeric Substances CMAS Hi CMAS Lo SMBR MCRT, d

18 Filamentous Microorganisms CMAS Hi CMAS Lo SMBR Total Filamentous Microorganisms (intersections/gtss) 8.1x x x10 7 Nocardioform Fraction 13% 26% 85%

19 Sludge Properties Conclusions Particle Size Distribution ( micron) CMAS Lo had a bimodal distribution CMAS Hi and SMBR had similar distributions Colloidal Material (<10 micron) Higher levels in SMBR Extracellular Polymer Substances Lower levels in SMBR Filamentous Microorganisms Higher levels in SMBR

20 Biological Foaming - Nocardioform

21 Illustration of Mechanical Foam Control Foam Liquid ML Recycle Line D = 0.1 m Foam Foam Liquid Coarse Aeration On Membrane Tank Intermittent Aeration (10s on/10s off) ML Recycle Pump Liquid Aeration Tank D = m Foam Liquid Coarse Aeration Off

22 Foaming Events

23 Sprayer System

24 Biological Foaming Much more than a process nuisance Removes biomass from the reactor Dramatically shifts F/M in the reactor Often correlates with membrane fouling MBRs are the perfect trapping environment Recommend ALL sludge wasting be done from the surface Only place foaming filaments can survive is on the surface Membrane tanks are particularly prone to biological foaming due to the aeration Recommend overflowing the membrane tank continuously Controlling foaming filaments by surface wasting does not control bulking filaments

25 Pretreatment

26 Pretreatment Adequate pretreatment is essential Protects the membranes from damage Protects the membrane from requiring manual cleaning Allows membrane fouling to be controlled as designed In 1999, only 3 mm screens required for all MBRs Now, MBR manufacturers are attempting to separate themselves from the others by claiming reduced screenings requirements (2 vs 3 mm)

27 Pretreatment Problem remains for all - this inert material (hair, fibers, and grit) will accumulate in the reactor Grit will settle at the bottom below the air distributors and form piles Best operational results are obtained with high quality pretreatment (0.8 mm) Minimizes handling of the membranes and exposure to cleaning/repair procedures The need for such advanced screening has significant negatives Labor intensive Require a lot of washing and compacting of this material Not pleasant to deal with When it makes economic sense (> 5 MGD), primary treatment is highly recommended Reduce the load on screenings - eases operation Prescreen becomes insurance policy

28 Pretreatment The standard prescreen type that is proven successful are: Mesh Punch hole Its imperative that there is no bypass or opportunity for solids carryover Also imperative that the screen loading not be too aggressive Frequently results in the facility backing up sewage and spills Most commonly employed prescreen devices are Internally fed rotating drum screen In channel rotating drum screen Rotating brush screen Traveling band screen

29 Research Needs So much progress, but so much left to do!

30 Research Needs Means of improving and controlling sludge filterability Sludge properties completely control fouling Investigate more thoroughly the benefits of chemical additives Focus on biological reactor design as well What are the measures of filterability? How do we measure filterability Although we don t t need the sludge to settle, we do need it to filter Improve our understanding of how to control α values for aeration designs Are we using all of the tools we have available This commonly cited as the reasons MBRs cannot be more compact or carry higher MLSS concentrations I believe that Praxair and other companies like this are focusing on this issue and will make good progress

31 Research Needs Energy reduction designs Air scouring is imperative in the high solids environment of MBRs To date, this is what prevents conventional wastewater utilities from really embracing MBRs Zenon has made significant strides in this respect, but total aeration demand is 75% of the energy for an MBR and about 1/2 of that air is for the membranes Minimize shear and floc breakage Koch Puron also has some unique ideas on how to minimize membrane air requirements Management of peak flows Important for MBR manufacturers to be honest about the significance of this issue Potentially, when a peak membrane flux is required, the system could not maintain for even 1 h Need to think about possible membrane related solutions

32 Thank you