mle conversion process

MLE Conversion Process

July 14, 2016

Ryan Hendrix

Wastewater Operations Superintendent

Town of Christiansburg

Doug Urquhart P.E.

Project Engineer

CHA Consulting

Overview

Background

Facility Description

MLE Design

Operations

Lessons Learned

Conclusions

Facility Background

Current WWTF

• 6 MGD Design Capacity

• 2.5 MGD Average Flow

Wastewater Sources

• 9,600 Connections

• 21,500 Population

Facility Background

1980• 2 MGD WWTP; 1 MGD Average Flow

• Discharged to Crab Creek

• Aerobic Sludge Digestion

1989 Upgrade to 3 MGD• Added Additional Primary Clarifier

• Added Secondary Clarifier

• Changed to Anaerobic Digestion

1999 Upgrade to 4 MGD• Upgraded Headworks

• Aeration Basins Constructed

• Equalization Basin Constructed

• Discharge Relocated to New River

Description

Liquid Stream Treatment• Headworks/Grit Collection• Primary Clarifiers• Activated Sludge (MLR)• Secondary Clarifiers• UV Disinfection• Effluent Pump Station

Solids Treatment• Anaerobic Digestion• Combined Heat Power System • On-Site Storage• Land Application of Liquid Class B Biosolids

Aeration Basins

1.4 million gallons of aeration volume

Two 80-foot secondary clarifiers

Set up for tapered aeration

More diffusers in first cell (now anoxic)

Design per SCAT Regulations resulted in oversizing blowers (now use BioWin)

RAS influent flow-paced

Designed for future conversion to 5-Stage Bardenpho or MLE with common partition walls

Aeration Basin Design

ZONE 5 ZONE 4

ZONE 2ZONE 1

BASIN 2

ZONE 1

ZONE 4ZONE 5

INFLUENT CHANNEL

ZONE 2

ZON

E 3ZO

NE 3

BASIN 1

Primary Effluent

Q

RASQ

RASQ

Lime Feed

- Installed in 1996

- Used equipment

- Gravity feed

Design Goals

Influent Ammonia (NH3-N)

• 27 mg/L (average)

• 49 mg/L (maximum)

Effluent Toxicity

Reduce Operating Costs

• Alkalinity requirements 500 lb per day

Pending Freshwater Ammonia Criteria

Public Health Concerns

Conventional Biological Nitrification

Nitrification - Two step oxidation of ammonia (autotrophic bacteria)

Step 1: Oxidation of ammonia to nitrite ammonia oxidizing bacteria (AOB)

NH4+ + 1.5O2 NO2

- + 2H+ + H2O

Step 2: Oxidation of nitrite to nitrate by nitrite oxidizing bacteria (NOB)

NO2- + 0.5O2 NO3

- + H2O

Nitrite intermediate is short term in most wastewater, NOB growth faster than AOB

Autotrophic bacteria, Nitrosomonas (AOBs) and Nitrobacter (NOBs), are ubiquitous to wastewater (i.e. do not need to buy or dose bacteria)

Use HCO3- as carbon source for growth (alkalinity)

Per 1 lb of NH4-N oxidized need 4.57 lb oxygen and 7.14 lb of alkalinity as CaCO3

Heterotrophic Denitrification - Reduction of nitrate to nitrogen

NO3- + electron donor (COD) 0.5 N2 + OH-

Per 1 lb NO3-N reduced produce 3.57 lb of alkalinity as CaCO3

Nitrification Factors - Solids Residence Time (SRT)

SRTactual = MLVSS × Volume of Aeration Tank

QEFF× 𝑋EFF+QWAS× 𝑋WAS

Need to maintain minimum SRT for healthy nitrifying population

SRTmin =1

µ𝑚𝑎𝑥𝑁 −𝑏𝑁

If actual SRT > minimum SRT then complete nitrification can occur

SRT is temperature dependent:

10-16˚C SRTmin of 12-20 days

16-20˚C SRTmin of 8-10 days

At Christiansburg WWTF, started 15-20 days, now 8-12 days

Nitrification Factors - Inhibition

Nitrification by AOBs is limiting factor in ammonia removal

NH4+ + 1.5O2 NO2

- + 2H+ + H2O

Primary requirements to maintain healthy nitrification process

• Oxygen > 2 mg/L

• pH pH = 6.8 to 8.2

Other Factors• Ammonia < 100 mg/L (pH dependent)

• Sulfate < 500 mg/L

• Chromium (VI) < 0.25 mg/L

• Nickel < 0.25 mg/L

• Zinc < 0.50 mg/L

• Copper < 0.10 mg/L

Nitrifying bacteria are very dependent on pH

Nitrification reduces alkalinity by 7.14 lb per pound of NH4+ oxidized

Phosphorus removal chemicals also reduce alkalinity

• 5.3 - 13.5 lbs Alkalinity as CaCO3 per lb Fe added

• 6.0 - 9.0 lbs Alkalinity as CaCO3 per lb Al added

Nitrification Factors – pH and Alkalinity

Reference: Gradyand Lim, 1980

Modified Ludzack-Ettinger (MLE)

Effective BNR process, pre-anoxic and adaptable for existing activated sludge facilities

Internal recycle (2-4 times influent flow) provides BOD for denitrification in pre-anoxic reactor

Anoxic AerobicInfluent Effluent

SecondaryClarifier

Sludge

Return Activated Sludge

Anoxic AerobicInfluent Effluent

SecondaryClarifier

Sludge

Return Activated Sludge

Internal Recycle

WWTP Simulation(Existing)

Aeration 1.1

Aeration 2.1

Secondary Clarifier Effleunt

Aeration 1.2 Aeration 1.3 Aeration 1.4 Aeration 1.5


Primary Clarifier Effluent

WAS

WWTP Simulation(Proposed MLE)

Aeration 1.1

Aeration 2.1

Secondary Clarifier Effleunt



Primary Clarifier Effluent

WAS

WWTP Simulation

Mixed Liquor Return (MLR) from Zone 5 to Zone 1

Simulated varying recycle rates

• 100% Recycle Rate:

TN: 21.6 mg/L

NH3-N: 1.1 mg/L


TN: 22.2 mg/L

NH3-N: 1.4 mg/L


TN: 22.6 mg/L

NH3-N: 1.6 mg/L

Influent Alkalinity = 250 mg/L

Effluent Alkalinity = 100 mg/L

No lime addition required with simulation

MLE Conversion

ZONE 5 ZONE 4

ZONE 2ZONE 1 (pre-anoxic)

BASIN 2

ZONE 1 (pre-anoxic)

ZONE 4ZONE 5

INFLUENT CHANNEL

ZONE 2

ZON

E 3ZO

NE 3

BASIN 1

WINDOWPUMP

WINDOWPUMP MIXER

MIXER

Primary Effluent

Q

RASQ

RASQ

MLE Conversion Design

MLE Conversion Installation

Anoxic Mixers (2011)

• Two 2.5 HP submersible mixers

• $28,000 (total installation cost)


Window Pumps (2014)

• Two 2.5 HP Window pumps

• 3600 gpm at 1.1’ TDH

• Variable speed

• $43,000 (total project)

Results

Effluent Nitrate (as N)

• 12 mg/L (before MLE Conversion)

• 6 mg/L (after MLE Conversion)

Lime feed turned off July 2014

Annual savings of ~$24,000 (4.0 year ROI)

Results - Nitrate

0

2

4

6

8

10

12

14

16

3/31/2014 06/03/14 7/30/2014 10/29/2014 2/9/2015 6/18/2015 1/11/2016

Nit

rate

(as

N)

mg

/L

NRS OFF 7/17/16

WINDOW PUMPSINSTALLED (7/1/14)

LIME FEEDSHUT OFF (7/22/14)

Results - MLSS

0

50

100

150

200

250

300

350

400

450

500

0

500

1000

1500

2000

2500

3000

3500

4000

4500

10/18/2012 5/6/2013 11/22/2013 6/10/2014 12/27/2014 7/15/2015 1/31/2016 8/18/2016 3/6/2017

SRT

and

SV

I

MLS

S (m

g/L)

MLSS SVI SRT

Optimization

Pumps flow paced

• 4.4 to 5.0 MGD Recycle Flow

• 250% to 300% Recycle to Influent Ratio

• Changed from influent flow-paced to effluent flow-paced

Controlling Recycle Flows

Minimizing D.O. carryover in MLR

Increase use of equalization basins

Lessons Learned

Blowers are even more oversized now

Low cost lime system saved money but difficult to operate

SRT took some fine tuning

Accommodating Wet Weather

• Effluent flow pacing over influent flow pacing

Need for additional Process Control Testing

• ORP, Nitrate & Ammonia Analyzers, etc.

Future Projects

Turbo Blowers

DO and Ammonia Control

Cell 5 Mixers

• No aeration, use as “swing zone”

Improved Diurnal Flow Control

Questions and Comments

mle conversion process

Documents