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

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Facili

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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 ??