4/22/2017

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Pilot-scale Evaluation of AirPrex for Phosphorus Management

JTAC

Innovation Program—Phosphorus Initiative

4 Angles

1

2

3

4

Liquid stream

TP removal

SolidsTP

removal

2

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Phosphorus Initiative: Solids Stream Phosphorus Removal

GOALTo find the most effective and sustainable phosphorus management approach through an intensive study phase of phosphorus recovery technologies under the phosphorus initiative

Internal Sidestreams

Return Activated Sludge (RAS)

Secondary Treatment

Centrate (recycle flow)

Primary Treatment

Digester

Centrifuge

Phosphorus Recovery

4

Less than 1% of the flow but 25% of the Phosphorus Load

4/22/2017

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DIGESTATE RECOVERY PILOT WORKPhosphorus Recovery Pilot Work

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Effective and Sustainable Phosphorus Management

5Measures for Success

1. Break the phosphorus recycle loop

2. Biosolids dewatering costs

3. Struvite‐scale issues for O&M 

4. Phosphorus loading on soils  

5. Recovery of a finite resource

1 lb of phosphorus equates to 8 lbs of struvite.7,000 lbs of phosphorus enter the RWHTF each day!

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AirPrex Pilot Testing

Hypothesis:Will create a controlled environment to precipitate struvite from District solids to simultaneously address:

Break P-Recycle Loop

Biosolids Dewatering

Struvite Issues

Phosphorus Index

Product Recovery 7

Proven Innovation

8

Full Scale Installations:

5 in Germany

4 in the Netherlands

1 Belgium

1 in China

Other US Pilot Investigations:

Miami, FL

Stevens Point, WI

Fox River, IL

Fond du Lac, WI

Meridian, Idaho

Pima County, AZ

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How Does AirPrex Work?

Anaerobic Digestion

Centrifuge

Centrate

Biosolids

AirPrex

Struvite

Digester effluent is fed to AirPrex reactorReactor is aerated which strips the CO2 from the reactor and raises the pH

Magnesium is dosed to the reactor causing struvite to 

precipitate 

CO2 Mg

Struvite settles and is pumped out and cleaned

AirPrex effluent, stripped of phosphorus, is sent to dewatering centrifuges

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AirPrex Pilot

• Pilot onsite from June 6th through August 3rd

• Reactor operated continuously at a flow of 11 gpm

• Centrisys CS10-4 centrifuge operated 6 – 8 hours per day

• Mg:P molar dosing ratio varied between 0.7:1 – 1.7:1

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Sampling and Testing Protocol

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• Weekly water quality analyses• Daily dewatering analyses• Over 3000 water and solids analyses!

Parameter Reactor CentrifugeInfluent Underflow Effluent Influent Centrate Cake

pH Temperature Total Solids (TS) Volatile Solids (TVS) Total Ca, Mg, Fe, K, Na Dissolved Ca, Mg, Fe, K, Na Orthophosphate Total Phosphorus Water Extractable Phosphorus Acid‐cake Struvite Test Ammonium  Conductivity Total Alkalinity CODEPS

Modeling

• Thermodynamic modeling to model nuisance struvite formation potential upstream and downstream of the digesters and recovery technologies

• Struvite Tool (Office of Water Programs, Sacramento, California)

• Visual Minteq (KTH, Sweden)

• BioWin

• Effects of reduction in recycle phosphorus would have on Bio P performance 12

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Driver 1: Phosphorus Recycle Control

0

100

200

300

Typical Untreated Mg:P0.7:1

Mg:P1.4:1

Mg:P1.7:1

Phosphorus, m

g/L

Orthophosphorus Particulate Phosphorus

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• OP and TP were observed to decrease in the centrate as the Mg:P molar dosing ratio increased to 1.4:1

• At 1.7:1 Mg:P molar ratio, OP was lowest while TP increased –potentially due to fines loss

Driver 1: Phosphorus Recycle Control

0

100

200

300

400

500

0

0.05

0.1

0.15

0.2

0.25

0.3

No Recovery Digestate

Centrate Recycle Soluble 

P, m

g/L

Secondary Effluent 

Soluble P, m

g/L

Secondary Effluent Soluble P Centrate Recycle Soluble P

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• Reducing recycle soluble phosphorus concentrations from 400 mg P/L to 30 mg P/L would result in a decrease in secondary effluent OP 

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Driver 2: Dewatering Performance

50.0

55.0

60.0

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.0

0

10

20

30

40

50

60

70

Jan‐15 Feb‐15 Apr‐15 May‐15 Jul‐15 Sep‐15 Oct‐15 Dec‐15 Feb‐16 Mar‐16 May‐16

Cap

ture Efficiency (%)

Chemical Pounds per Dry Ton / Cake 

%TS

Cake %TS Polymer lbs/DT Fe lbs/DT Capture Efficiency

40% of Facility  100% of Facility in EBPR

Intentional EBPR Turned Off

While more economically sustainable than chemical precipitation, Bio P does have significant impacts on full scale operations

Driver 2: Dewatering Performance

• Cost Centers: $ Polymer consumption

$ Wet mass of biosolids for hauling and dispersal

Tracked cake total solids and polymer consumption Polymer dose varied in 5 active pound/dry ton increments Higher average centrifuge hydraulic pressures for AirPrex

treated

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Solids Correction – Dry Mass

CentrifugeAirprex Apx Eff Biosolids

Cen

trate

MgCl2

fp

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NH4MgPO4●6H2O Generated in AirPrex

Total solids ∆ ∗

Fraction of struvite in biosolids matrix ‐ fb

Digestate

Product

Fraction of struvite fines in centrate ‐ fcFraction of struvite that settles and is pumped out as product ‐ fp

fp + fc + fb = 1

fb

fc

Solids Correction – Dry Mass

CentrifugeAirPrex Apx Eff Biosolids

Cen

trate

fp

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Digestate

Product

fb

fc

Acidification tests performed on dewatered biosolids to estimate total struvite

Untreated cake struvite content ~5.3% AirPrex treated cake struvite content ~11.7% ~6.4% increase in struvite - 80% retention in cake

Assumed negligible based on centrate TP at optimal operation

~0

~80%

~20%

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Solids Correction

NH4MgPO4●6H2OAmmonia and water evaporate from struvite in 

the temperature range of 55 oC to 250 oC

Traditional TS concentration test measures a portion of struvite mass as water, underestimating total solids in sample

NH4 + 6H2O = 51.4% of struvite mass

This affects metrics traditionally used for assessing sludge dewaterability

Solids Correction Method

Corrected TS concentration measurement Ongoing work to determine actual extent of struvite evaporation Assumed 51.4% loss of struvite due to evaporation

Digestate TS correction:% % ∗ 1 0.053 ∗ 0.514

• AirPrex effluent TS correction:% % ∗ 1 0.117 ∗ 0.514

• Wet tons calculation:

%20

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AirPrex - DewaterabilityComplete Dataset

15%

17%

19%

21%

23%

25%

27%

29%

0 1 2 3 4 5 6 7

Cake % Total Solid

s (Uncorrected for Struvite)

Polymer Dose (Active Pounds per Hour)

AirPrex Effluent

Digestate

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297 data points analyzed

8.9 % reduction in wet tons hauled

AirPrex – DewaterabilitySorted - 10 gpm, 90% OP removal

15%

17%

19%

21%

23%

25%

27%

29%

0 1 2 3 4 5

Cake % Total Solid

s(Uncorrected for Struvite)

Polymer Dose (Active Pounds per Hour)

AirPrex 

Effluent

Digestate

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61 data points analyzed

12.44% reduction in wet tons hauled

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AirPrex – DewaterabilityPaired Dataset

15%

17%

19%

21%

23%

25%

27%

29%

0 1 2 3 4 5

CAKE % TOTA

L SO

LIDS 

(UNCORREC

TED FOR STR

UVITE) 

POLYMER DOSE (ACTIVE POUNDS PER HOUR)

AirPrex 

Digestate

23

20 data points analyzed

8.7% reduction in wet tons hauled

17.6% decrease in polymer consumption

Sensitivity Analysis of Assumptions

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• Assumptions on product recovery and evaporated TS mass do impact the estimation of wet tons hauled per day

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AirPrex Biosolids Dewatering Costs

Approximately 15–20% decrease in polymer use

8.7% reduction of biosolids hauled

Untreated Biosolids21 Hauled Truckloads 

After AirPrex Treatment19 Hauled Truckloads

Difference of 2 truckloads per day or 730 truckloads per year!

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Driver 3: Nuisance Struvite Reduction

0

200

400

600

800

1000

1200

1400

6/9 6/19 6/29 7/9 7/19 7/29

Pre AirPrex, Post AirPrex, and 

Centrate 

Struvite Production, lb/d

Untreated Digestate Airprex Treated Digestate

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Significant reduction of struvite mass predicted between untreated and AirPrex treated 

0

100

200

300

400

500

600

700

800

6/9 6/19 6/29 7/9 7/19 7/29

Pre AirPrex, Post AirPrex, and 

Centrate 

Struvite Production, lb/d

Untreated Centrate Treated Centrate

Reduced Mg Feed Reduced Mg Feed

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Driver 3: Digester Struvite Reduction

0

5,000

10,000

15,000

20,000

No Recovery AirPrex Treated

Struvite Production in 

Digesters, ppd P

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• Modeling efforts in BioWin show a reduction in struvite production from 18,000 lb/d to 13,000 lb/d with digestate phosphorus recovery

Driver 5: Product Recovery

150 tons of phosphorus recovered annually

Potential distribution

Composters

Fertilizer manufacturers

Potential uses

Turf grass

Alfalfa

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Driver 5: Product RecoveryStruvite Product - Metals Analysis

Pollutant"High Quality"

Pollutant ConcentrationLimits (mg/kg)

Pilot MAP Results(ppm)

Lab Analysis from Previous Pilot (ppm)

Arsenic 41 BDL2

Cadmium 89 BDL BDLCopper 1500 71 19Lead 300 18.9

Mercury 17 1.7Molybdenum1 < 75 0.8

Nickel 420 BDL BDLSelenium 100 BDL

Zinc 2800 33 26

1. There is currently no limit for Molybdenum for High Quality while rules are awaiting EPA investigation. Ceiling limit is  75 ppm. 

2. Levels were below detection limit

Struvite product produced by Airprex is well below limits of “high quality” pollutant concentrations

Driver 5: Product RecoveryStruvite Product - Pathogen Testing

Fecal Coliforms Drying Test (MPN/g‐TS)

Sample Date Initial 24hr 48hr

8/2/2016 31 26 65

Class A: maximum density of fecal coliform < 1000 MPN per gram total solids (dry‐weight basis)

2 Samples were taken for pathogen testing using fecal coliforms July 19: 1000 MPN/g TS August 2: Investigate fecal coliform with drying time 

Current testing of product shows Airprex struvite may be within Class A categorization for biosolids

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AirPrex Pilot Testing Results

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PhosphorusRecycleControl Reliable OPandTPReduction

Biosolids Dewaterability 

Polymer ~17%polymerreduction

TruckHauls ~8.7%reductionhauledmass

Struvite Reduction 

Digesters ~25%reductiondigesterstruvite

Dewatering Significantreductionindewateringnuisancestruvite

Phosphorus IndexAccumulationofphosphorusinbiosolids

Product Recovery   25%‐ 35%productrecovery

PHOSPHORUS WAS STRIPPING AND CENTRATE RECOVERY PILOT WORK

Phosphorus Recovery Pilot Work

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Ostara PearlFirst installation 2008

CalgaryEdmonton

Saskatoon

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How Ostara+WASSTRIP Works

ThickeningDewatering

Anaerobic Digestion

WASSTRIPProcess

Struvite

Biosolids

Centrate

Phosphorus‐stripped WAS from the WASSTRIP reactor is thickened

Phosphorus is released into the liquid stream and separated from the biosolids

Low P

High P

Centrate from dewatering is high in ammonia and is combined with liquid stream from WASSTRIP

Caustic

Mg

Caustic is added to raise the pH

Magnesium is dosed to the reactor, causing struvite to precipitate 

Struvite pearls settle and are pumped out and cleaned

Ostara Pearl effluent, stripped of phosphorus, is recycled back to mainstream

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Phosphorus Stripping Pilot Test

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Pilot October 2016 to ongoing

Evaluate features and benefits of WAS phosphorus stripping

Dewatering performance

Nuisance struvite reduction 

Batch WAS phosphorus stripping reactor with thickening press

Phosphorus Stripping Pilot Test

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Control Train receives TWAS, test train receives stripped TWAS

Two separate activated sludge trains allows for centrate recycle to be fed to North; control train reflects bio‐P with recycle control

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Nuisance Struvite and Dewaterability Improvements Project

0

50

100

150

200

250

300

350

400

450

0

0.05

0.1

0.15

0.2

0.25

0.3

No Recovery Digestate Centrate &Stripped Filtrate

Cen

trat

e R

ecyc

le S

olu

ble

P,

mg

/L

Sec

on

dar

y E

fflu

ent

So

lub

le P

, m

g/L

Secondary Effluent Soluble P Centrate Recycle Soluble P

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<0.1

Digester Nuisance Struvite Production

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0

4,000

8,000

12,000

16,000

20,000

No Recovery Digestate Centrate &Stripped Filtrate

Struvite Production in

 Digesters, lb/d

Anaerobic Digestion

Method of determination: • Mass balance on measured analytes• BioWin modeling• XRD analysis

25% 45%

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Biosolids Production

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0

100

200

300

400

500

No Recovery Digestate Centrate & StrippedFiltrate

Biosolid

s Production, tpd

Dry Mass, dtpd Total Mass, wtpd

Preliminary Pilot Testing Results

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DigestatePhosphorusStripping

PhosphorusRecycleControl

Reliable OPandTPreduction

Assessingneedforlamellaplatesettlers

Biosolids Dewaterability 

Polymer ~17%polymerreduction Awaitingadditionaltesting

TruckHauls~8.7%reductionhauledmass

~6.6%reductionhauledmass

Struvite Reduction 

Digesters~25%reductiondigesterstruvite

~55%reductiondigesterstruvite

DewateringSignificantreductionindewateringnuisancestruviteduetolimitingOP

SignificantreductionindewateringnuisancestruviteduetolimitingMg

Phosphorus IndexImprovementoverchemicalsequestration

Improvementoverchemicalsequestration

Product Recovery  25%‐ 35%productrecovery

70%+productrecovery

4/22/2017

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Next Steps

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Complete WAS phosphorus stripping digester pilot

Complete Business Case Evaluation for phosphorus recovery systems

Select a path forward for near‐term implementation