www.jacobs.com | worldwide

Lessons Learned from the Design & Operation of the First Full- Scale Post-Aerobic Digester (PAD)

Samuel Jeyanayagam, PhD, PE, BCEE, WEF FellowVice President & Regional WW Solutions Lead – US North

Bruce Johnson, PE, BCEE, IWA FellowWastewater Treatment Technology Fellow

29 Aug. 2018

Safety Moment: Motion Induced Blindness

Motion Induced Blindness

• When driving, if you fix your gaze on the road straight ahead, you may suffer a momentary loss of peripheral vision & may not see a vehicle or person approaching from the side.

• Always use the “eyes moving” technique when driving, to reduce the risk of motion induced blindness.

Use “Eyes Moving” Technique

Learning Objectives

• Explain the Post Aerobic Digestion (PAD) process• Describe the benefits of PAD• Recognize drivers for adopting PAD• Identify what it would take to implement PAD

Presentation Outline

• What is PAD?• Benefits of PAD• Case Studies• Take Home Messages

What is Post Aerobic Digestion (PAD)?

• Aerobic digestion after mesophilic anaerobic digestion

Aerobic DigesterAnaerobic

Digester

DewateringWAS + Primary Sludge

History• First investigated by Prof. John Novak (Virginia Tech)• Further developed by him working with Dr. Sudhir

Murthy (DCWater Innovations Team)

Licensing rights are owned by Ovivo

• PAD tank is just an aerobic digester with respect to equipment

• PAD sludge age:– Typically, 5 to 10 days (driven by N removal requirements) – Longer SRT (20 d) provides higher VSR; but requires larger tank.

• Can reuse existing tankage• Can also serve as solids holding prior to dewatering• Heat generating process - digester contents are typically

around 35oC– May require cooling during warm summer conditions

Features of the PAD Process

Benefits of PAD

1. Additional volatile solids removal (VSR)2. Sidestream treatment3. Struvite stabilization4. Enhanced dewaterability & reduced polymer use5. Reduced biosolids odor

• Provides an additional 10 to 40% VSR across PAD

• Overall VSR (anaerobic + aerobic) can be >70%

• Reduced solids handling requirements and costs

* Novak et. al. (2009)

Volatile Solids Reduction (VSR)

From Novak et. al (2009)

Avg. VSR =50% in meso. anaerobic digester; 20 d SRT

Avg. VSR = 62% in PAD

DC Water Results

• They either remain attached to the cell's outer surface, or are secreted into the bulk liquid.

• EPS can constitute 50% to 90% of a biofilm's total organic matter

• Measured as VS

The Fate of Exocellular Polymer Substances is Central to PAD Performance

Some EPS, (e.g. those containing proteins called lectins) are degraded only aerobically

• Exocellular polymer substances (EPS) are natural polymers of high molecular weight secreted by microorganisms into their environment.

• EPSs are mostly composed of: Polysaccharides & Proteins DNA, Lipids, & Humic substances

• EPS are the construction material of bacterial cells

• Anaerobic digestion releases significant amounts of ammonia & phosphate– Dewatering can return nutrient loads back to the head of the plant

• Low Flow ≈ 1% of influent flow• >800 mg/L Ammonia-N (15-40% of Infl. TN Load)• 100 – 700 mg/L Ortho-P (5-40% of Infl. TP Load)

• PAD can be operated to remove ammonia & nitrate:– Continuous aeration: Nitrification only (requires alkalinity addition). Ammonia

converted to nitrate, which will be in the centrate– Intermittent aeration or Low DO: Simultaneous nitrification denitrification (SND).

Remove ammonia & nitrate.• Benefits:

– Reduced recycle loads – Nitrifier bioaugmentation

Sidestream Treatment

At DC Water: • Ammonia removal from 2,500 mg/L to < 200 mg/L

at a 6 day SRT and 50/50 Aer/Non-aerated cycle• Nitrate < 200 mg/L

• Struvite is Magnesium Ammonium Phosphate (MAP)• Forms because:

– Anaerobic digestion releases: Mg, NH3, & PO4

– Turbulence releases CO2 causing pH increase & struvite formation

• Nuisance struvite scaling in pipes & equipment can be an operational nightmare

• PAD stabilizes struvite formation in the high solids environment of the PAD tank– Struvite crystals are incorporated into the biosolids

stream & prevented from scaling piping & equipment– Down time & mitigation costs avoided– Phosphorus removal achieved

Struvite Stabilization

Chemical structure of struvite

• EPS binds substantial amount of water in multiple structured layers.

• Results in gel-like consistency, which is difficult to extract during dewatering.

• PAD destroys EPS & allows:– More water to be removed– Dryer cake solids to be produced– Lower polymer consumption– Cost savings to be achieved

Dark rods are bacteria

Light areas around the cells are EPS

Dewaterability Enhancement

• Spokane County WRF (Spokane, Washington, USA)• Northern Treatment Plant (Denver, Colorado, USA)• Boulder WWTP (Boulder, Colorado, USA

Case Studies

• Greenfield, 8 MGD average, MBR facility• First started treating wastewater in late 2011• First full scale PAD plant in North America

Spokane County Regional Water Reclamation Facility (WRF)

Centrifuge

Centrate

AnaerobicDigesters

Waste Solids

Raw

Chemically Enhanced Primary Treatment

Step Feed Bioreactor

Membranes

PAD

Spokane County Regional WRF

• Two parallel mesophilic anaerobic digesters– Approx. 15 day SRT

• PAD– 5 to 10 days SRT with coarse bubble aeration– Provides additional VSR – Doubles as a solids storage tank when dewatering is not possible

(variable level)– Removes liquid phase inorganic nitrogen to less than 50 mg/L– Integral part of the nutrient removal system

Spokane Regional WRF PAD System

Anaerobic Digestion

Thickening & Dewatering

CoGen & Gas Storage

Aerobic Digestion

Spokane Solids Handling System

Biosolids handled decreased when PAD was in operation

PAD effluent NH3 decreased when PAD was in operation

PAD Performance at the Spokane Regional WRF

Avg. rate of cumulative solids production decreased by 43% when PAD was in operation

Decreased by 12% when PAD was in operation

PAD Performance at the Spokane Regional WRF

Operational DataAverages - Dec 2011 to Jan 2017

Ammonia-N TKN Nitrate/Nitrite-NAnaerobic Digester, mg/L ~900 ~2000

PAD, mg/L ~20 ~40

Removal Efficiency (PAD), % 98% 96% (TIN)

No supplement carbon or alkalinity addition needed

PAD Performance at the Spokane Regional WRF

Overcoming Challenges at the First PAD Startup

• Temperature:– Air cooling system– Sprinklers (evaporative cooling) on aluminum lid– Automation and controls

• Foam:– Control temperature & process stability– Foam overflow system– Refining control over aeration cycles

• pH control – sulfuric acid addition

Drivers for PAD• Sidestream Nitrogen Removal• Struvite Stabilization• Potential Dewatering Benefits

Northern Treatment PlantDenver, CO.

24 mgdDrivers for PAD• Use of Existing Infrastructure• Sidestream Nitrogen Removal• Reduce Disposal Cost• Struvite Stabilization• Potential Dewatering Benefits

Boulder WWTPBoulder, CO.

25 mgd

• Baseline (No Sidestream Treatment) offers lowest capital cost

• Sidestream Treatment with PAD offers the lowest annual cost

• Equivalent life cycle costs for Baseline, PAD, and Anammox:– Consider PAD when using bio-P; TN removal; additional VSR;

and nuisance struvite mitigation are desired– Consider Anammox when N removal is desired and energy

minimization are desired.

Relative Costs

• PAD is a new technology with significant benefits.• Key drivers for implementing PAD include the need to:

– Achieve additional VSR.– Provide chemical-free (alkalinity & carbon) sidestream treatment – Minimize struvite scaling potential.– Reduce cost of solids handling (higher cake solids & lower polymer use).

• PAD represents a relatively simple process conversion. If tankage is available, it can potentially provide a quick pay-back.

• Because PAD is a young technology, utilities interested in implementing PAD should stay abreast of lessons learned at operating facilities to fully understand plant-wide impacts.

Take Home Messages

28

Questions?

www.jacobs.com | worldwideDecember 27, 2018© Copyright Jacobs

Samuel Jeyanayagam, PhD, PE, BCEE, WEF FellowVice President & Regional WW Solutions Lead – US NorthSamuel.eyanayagam@Jacobs.com

Bruce Johnson, PE, BCEE, IWA FellowWastewater Treatment Technology Fellow

Lessons Learned from the Design & Operation of the First Full- Scale Post-Aerobic Digester (PAD)

29 Aug. 2018