Evaluation of Conventional Activated Sludge Compared to Membrane Bioreactors

Transcription

1 Evaluation of Conventional Activated Sludge Compared to Membrane Bioreactors Short Course on Membrane Bioreactors 3/22/06 R. Shane Trussell, Ph.D., P.E.

2 Outline Introduction Process Design Effluent Water Quality Peak Flows Mixed Liquor Properties Conclusions

3 Outline Introduction Process Design Effluent Water Quality Peak Flows Mixed Liquor Properties Conclusions

4 Introduction Biological processes have become the preferred municipal wastewater treatment process Activated Sludge Process (ASP) has developed into a mature process over the past century Membrane Bioreactor (MBR) process is relatively new to wastewater treatment with the concept of direct sludge filtration emerging four decades ago

5 Membrane Bioreactor (MBR) Modified activated sludge process UF/MF membrane Two configurations External (EMBR) Submerged (SMBR) Introduction

6 Flow Schemes for the MBR and Conventional Activated Sludge Process Conventional Aeration Basin Secondary Clarifier Microfiltration WASTE Backwash Water Tertiary Treated Wastewater Primary Treated Wastewater

7 Flow Schemes for the MBR and Conventional Activated Sludge Process Conventional Aeration Basin Secondary Clarifier Microfiltration WASTE Backwash Water Tertiary Treated Wastewater Aeration Basin Primary Treated Wastewater (Equivalent to a 1-3 mm screen) WASTE MBR Tertiary Quality Wastewater

8 Submerged MBR (SMBR) Q Aeration Basin Effluent Primary Treated Wastewater WASTE Solids Recycle Q R = 3-5 x Q Waste Activated Sludge

9 External MBR (EMBR) Aeration Basin Effluent Q Primary Treated Wastewater Solids Recycle Q R = x Q Waste Activated Sludge

10 Outline Introduction Process Design Effluent Water Quality Peak Flows Mixed Liquor Properties Conclusions

11 Process Design MBRs combine activated sludge technology with membrane filtration to expand the normal operating region MBRs can be designed at higher MLSS concentrations because they are not affected by the limitations of gravity sedimentation for solid-liquid liquid separation SMBRs are typically designed for MLSS concentrations g/l

12 Advantages of High MLSS SMBRs operate at 2 to >6 times ASP MLSS concentrations Higher MLSS concentrations translate into: Longer SRT same HRT, or Shorter HRT same SRT

13 Longer SRT Designs For a given HRT, the SMBR process can operate at 2 to >6 times the SRT of ASP Traditional SMBR design has been to operate at conventional HRTs with long SRTs (i.e. > 20 days) Long SRTs have the following advantages: Complete nitrification can occur even in cold climates Reduced biological sludge production Complete oxidation of influent organics Possibility that slow growing microorganisms can degrade persistent organics

14 Shorter HRT Designs In general, for a given SRT, the SMBR process can treat wastewater in 1/2 to 1/4 the HRT of ASP Short HRTs have the following advantages: Reduce overall plant footprint Capital cost savings from reduced land and tank volume Concept of shorter HRTs brings about one of the principle limitations of SMBRs compared to ASP Minimum SRT There is a minimum SRT where membrane fouling becomes rapid A general design guideline is target the minimum SRT for nitrification plus an additional safety factor Some manufacturers have established their own lower limit at 12 days

15 Effect of SRT on Steady-State State Fouling Rate MCRT, d HRT = 1 h 3.0 y = 1.661x R 2 = HRT = 4 h F/M, g COD/g VSS. d

16 Process Design Key difference is in solid-liquid liquid separation ASP is not sensitive to low SRTs and can successfully operate in a conventional mode SMBRs are sensitive to low SRTs and compact designs can result in increased membrane fouling rates This difference in solid-liquid liquid separation also makes pretreatment imperative Fine screening is an absolute must in SMBRs The MBR community has been and is still learning how important reliable screening equipment is

17 Process Design Higher MLSS concentrations influence the oxygen transfer efficiency Oxygen transfer from coarse bubble aeration required for membrane agitation needs to be considered Krampe and Kauth, 2002

18 Outline Introduction Process Design Effluent Water Quality Peak Flows Mixed Liquor Properties Conclusions

19 Effluent Water Quality Biological process applied to oxidize organics and remove nutrients Principle difference is solid-liquid liquid separation mechanism Membrane provides a more consistent, higher quality effluent

20 Effluent Water Quality Membrane provides an absolute barrier and effluent quality is no longer a concern. TSS, mg/l Turbidity, NTU Total Coliform, #/100 ml BOD 5, mg/l ASP Effluent <30 2 to >10 10,000 to 100,000 <2 to 30 MBR Effluent ND (<2) <0.2 ND to 100 ND (<2)

21 Effluent Water Quality MBR eliminates the need for monitoring sludge settleability as an operational parameter» Effluent quality is not dependent on operations» Not necessary to determine TSS/VSS concentrations to maintain desired SRT» Can use fixed waste rate SRT=V/Q WAS

22 Effluent Water Quality Public Health Benefit membranes increase the distance between reclamation and the risk of microbial disease» pathogens are removed by size exclusion not a highly selective chemical or photochemical reaction» pathogens can be rejected, not just reduced Results from operating MBR plants:

23 Indigenous Coliphage Primary Effluent Reactor #1 Reactor #2 Reactor #1 (Non-Detect) Reactor #2 (Non-Detect) Tertiary 1.E+06 Start -up Period New Membranes Reactor # 2 1.E+05 1.E+04 Repaired Integrity 1.E+03 1.E+02 1.E+01 1.E+00 1.E Hours of Operation

24 Effluent Water Quality MBR Effluent Allows Modern Objectives to be Realized Ideal for UV disinfection» All particulate matter and suspended solids that can interfere with UV have been rejected at membrane barrier» High percent transmissivity (>70%)» Dose of 80 mj/cm 2 adequate for MBR effluent, while 100 mj/cm 2 required for granular filtered wastewater Ideal pretreatment process for reducing TDS» Suitable for direct feed to RO» Chloramine residual is required

25 Aqua 2000 Bureau II Study [Filmtec BW , low pressure TFC RO membranes] Net Operating Pressure Temperature Feed TDS = 1200 mg/l Plant shutdow n Net Operating Pressure, psi Temperature, C weeks Time of Operation, h

26 Outline Introduction Process Design Effluent Water Quality Peak Flows Mixed Liquor Properties Conclusions

27 Peak Flows Peak flows are well addressed in ASP, but can be troublesome for MBRs Membranes are designed for a certain throughput (design flux) MBRs are typically limited to a peaking factor of 1.5Q Dependent on design flux (aggressive or conservative) temperature, and mixed liquor conditions (aggressive or conservative), ASP is capable of sustaining larger peak flows (>2.5Q) for longer periods of time Possible deterioration in effluent quality

28 Peak Flows MBR designs for large peak flows consider the following solutions: Additional membrane area for peak flow service Flow equalization tanks (frequently primary eff) Flux enhancing polymers or coagulant addition Currently, the most conservative and cost effective solution is typically some kind of flow equalization Flux enhancing polymers and coagulant addition are showing great promise, but research on long-term effects is needed

29 Peak Flows Peak flows will become less of a disadvantage for MBRs as membrane costs continue to decline and our understanding of conditions affecting membrane fouling increase

30 Outline Introduction Process Design Effluent Water Quality Peak Flows Mixed Liquor Properties Conclusions

31 Mixed Liquor Properties Mixed liquor properties are important because they determine how easily a sludge can be filtered through a membrane, settled or dewatered Differences in solid-liquid liquid separation apply different selective pressures ASP requires a biology that flocculates and settles well to remain in the system MBRs retain all biomass, even single cells in the mixed liquor

32 Mixed Liquor Properties Merlo et al. (2004) revealed some key findings comparing SMBR and ASP under steady state conditions for 2, 3, 4, 5, 10-d SRTs SMBR has higher colloidal content SMBR has higher filament concentrations Both SMBR/ASP particle size distribution (excluding colloidal - i.e. >2 µm) was controlled exclusively by mixing intensity, G

33 Particle Size Distribution ASP Hi vs.. SMBR CMAS ASP Hi Frequency SMBR Characteristic Length, µm

34 Outline Introduction Process Design Effluent Water Quality Peak Flows Mixed Liquor Properties Conclusions

35 Conclusions SMBRs have advantages compared to ASP (compact, high quality effluent, high MLSS concentrations) SMBRs have disadvantages compared to ASP (low SRT limit, peak flow issues) Mixed liquor properties are different in SMBRs compared to the ASP because of the reactor conditions Engineers have been studying mixed liquor properties to improve the settleability of ASP Future of the SMBR process will be studying mixed liquor properties that improve filterability

36 Questions?