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Services provided in NY as T&B Engineering, PC
Northeast Onsite Wastewater Short Course & Exhibition April 6th, 2016
Erin K. Moore, PE
No-Holds-Barred Match:
RBC vs. MBR
Overview
■ Evaluation target
■ Historic use of technology
■ Technology differentiators
■ Case studies
■ Design considerations
The Battlefield
The Battlefield
RBCs vs. MBRs
Rotating Biological Contactors
(RBC)
■ Invented in Europe in 1960 – Used in US late 1970’s to present
■ Paired with clarification for
secondary treatment
■ Additional reactors for
denitrification
Membrane Bioreactor
(MBR)
■ Invented in mid 1960’s – Used in US mid-1990’s to present
■ Capable of secondary &
tertiary treatment
RBC Principals of Operation
■ Rotating media aerates waste
■ Attached growth on media removes BOD &
nitrogen
■ Excess growth falls off RBC and is settled
MBR Principals of Operation
■ Same principal as activated
sludge process
■ Membrane filters replace clarifiers
RBC vs. MBR
Similarities
Both technologies
proven performers
Both widely used
throughout NE
Differences
Site Constraints
Effluent Quality
Energy Usage
Sludge Production
Operational Complexity
High Flow Management
Process Reliability
0.0
0.5
1.0
1.5
2.0
2.5
3.0
BOD TSS TN
An
nu
al E
xce
da
nce
s p
er
Facili
ty
Facility Exceedance Comparison
RBC MBRNote:
1. Analysis based on MassDEP DMR records analysis of 100 RBC and 7 MBR facilities over one calendar year.
2. 10 out of 18 TN exceedances for MBR treatment were observed at one facility.
Process Reliability
RBCs had period of lower quality
components resulting in early failure
and less confidence in performance - Not current concern, but stigma is still present
MBR membranes are susceptible to
fouling with high FOG and fibers - Need appropriate pretreatment
Site Constraints
MBRs will have smaller footprint than
RBCs for the same average flow rate
Design Factor RBC MBR
Large Site
Small Site
Sample Design Comparison
System Component RBC MBR
Preliminary Treatment N/A 6,000 gal
Primary Treatment 28,000 gal N/A
Flow Equalization N/A 32,000 gal
Anoxic Tank N/A 20,300 gal
Aeration Tank N/A 21,400 gal
RBC or Membrane Tank 42,000 gal 15,400 gal
Secondary Clarifiers 46,000 gal N/A
Sludge Storage 40,000 gal 16,300 gal
Tertiary Treatment 24,000 gal N/A
Total: 180,000 gal 111,400 gal
Required Process Volumes for 100,000 gpd System
Note: Plants designed to achieve a discharge permit of 30/30/10 for BOD/TSS/TN.
Effluent Quality
Nitrogen & Phosphorus Removal Required?
Lower Lifecycle cost with MBRs
Design Factor RBC MBR
Nutrient Removal
or Effluent Reuse
BOD/TSS/
Ammonia Limits
Lifecycle Cost – Capital
– Electricity
– Sludge Production
– Chemical Usage
– Equipment Replacement
Lifecycle Cost Factors
Design Factor RBC MBR
Less Electricity
Lower
Maintenance Cost
Less Sludge
MBRs have higher energy consumption
MBRs have higher maintenance costs
RBCs produce more sludge
Permeate pumps and
blowers drive MBR
energy consumption
Effluent Quality
MBR Higher Quality Effluent = Greater Protection of
Disposal Area
Design Factor RBC MBR
Subsurface
Disposal Protection
Operational Complexity
MBRs More Complicated, But Highly Automated
RBCs More Operator Labor – Less Maintenance Labor
RBCs May Have Lower Grade Operator Requirement
Design Factor RBC MBR
Higher
Complexity &
Automation
Less
Maintenance
Lower Grade
Operator
Highly Variable Flows
RBCs better suited to handle highly variable flows
Highly Variable Flows = High MBR Equalization Cost
Design Factor RBC MBR
Highly Variable
Flows
Low Peaks or I&I
Impacts
Case Study 1 – Hyde Park, NY
■ RBC Plant built in 1980’s
■ Permitted Flow: 132,000 gpd
■ Average Flow: 70,000 gpd
Case Study 1 – Hyde Park, NY
■ Site Constraints: Large Parcel
Case Study 1 – Hyde Park, NY
Effluent Quality – BOD: <5 mg/L
– TSS: <10 mg/L
– Ammonia: < 2 mg/L
– No Nitrogen or Phosphorus Limits
– Surface Discharge
Operations – Some Operator Grade Requirement Concerns
High I/I Flow Impacts – Daily Average Flows >400,000 gpd
Case Study 1 – Hyde Park, NY
Design Factor RBC MBR
Large Site
Effluent Quality
Operations
Concerns
Highly Variable
Flows
Plant rehabilitation with continued use of RBCs was
recommended for this facility
RBC Design Considerations
■ Consider Load Cells – Determined weight of contactor to
determine thickness of biomass
■ Baffles – Can existing tank support baffles to
create additional treatment stages
■ Enclosures – Easy media observation vs.
humidity and HVAC concerns
– Maintenance needs
■ Site Topography – Can layout be adjusted to avoid
pumping? Process pumping
lessens cost effectiveness.
Case Study 2 – Wayland, MA
■ WWTP built in 1960’s
■ Permitted Flow: 52,000 gpd
Case Study 2 – Wayland, MA
■ Site Constraints:
¾ acre parcel with
close neighbors
Case Study 2 – Wayland, MA
Effluent Quality – BOD: <5 mg/L
– TSS: <10 mg/L
– Nitrogen: < 10 mg/L
– Phosphorus: <0.1 mg/l
– Surface Discharge to wetland
Operations – No staffing concerns
Low flow variability – Low pressure collection system
Case Study 2 – Wayland, MA
Design Factor RBC MBR
Small Site
Effluent Quality
No Operational
Concerns
Low Flow
Variation
New MBR Plant was recommended for this location
Case Study 2 – Wayland, MA
MBR Features – Minimal aesthetic impact to
adjacent development
– Entire system in building for
odor/noise control
– 2 mm fine screens
– Kubota flat plate membranes » Reduced fouling
» Stacked plates reduce
cleaning air
Anoxic Zone & RAS Pumps
Pre-aeration Tank
MBR Tank
Case Study 2 – Wayland, MA
Influent BOD Loads – Design = 320 mg/l
– Observed 600 to 1,500 mg/l
– Typical effluent <5 mg/l
Noise – Blower turn down during
startup created resonance
– Piping changes resolved
problems
Program changes increased loads
Overnight blower turndown noise issue
MBR Design Considerations
■ Influent Screening – 2-3 mm perforated plate
recommended
■ Flow Equalization – EQ is often less expensive than
more membranes to handle peaks
■ Surface Wasting – Near surface sludge wasting
allows solids & FOG removal
■ Cleaning & Process Aeration – Cleaning & process air systems
vary between vendors and are not
all equal in terms of
energy/efficiency
Erin Moore, P.E., Project Manager, Tighe & Bond, Inc.
Thank You!
Questions ??
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