Improving the performance of anaerobic digesters using hydraulic mixing

Raj ParthasarathyProfessor (Chemical Eng.,), RMIT University

Acknowledgements:Staff: Prof. Sati Bhattacharya, Dr. Maazuza Othman, Assoc. Prof. Nicky Eshtiaghi, Dr. Pradipto BhattacharyaPhD students: Dr. Siew Cheng Low, Dr. Stephen Kennedy, Mr. James McLeod 

Improving mixing in anaerobic digesters (AD)…

Dead zones (inactive volume) in anaerobic digesters

• Formed due to poor mixing

• Poor mixing leads to channelling, solids settling, dead zones and digester failure

• Dead zone volume can be up to 77% of AD volume (Bello-Mendoza and Sharratt, 1998)

• Funded by ARC (Linkage) grant, Melbourne Water and South East Water

Hydraulically  Mixed AD

References:Bello‐Mendoza, R., and Sharratt, P. N.,(1998), Modeling the effects of imperfect mixing on the performance of anaerobic reactors for sewage sludge treatment, Journal of Chemical Technology & Biotechnology 71(2):121 – 130.

Inactive volume (dead zones) determination in hydraulically mixed vessels…

Primary sludge

Secondary sludge

Yield stress shear‐thinning non‐Newtonian fluid

Transparent xanthan gum solution – Model fluid for digester sludge

P1P2

P3P4

Electrical Resistance Tomography t = 60 min; Nt = 13.2; 0.3 wt% XGKT solution

Downward facing nozzle

Conductivity maps

Inactive volume Active volume

Active volume

Inactive volume determination – nozzle orientation effect…

Inactive volume reduction with time

Upward facing nozzle

0.5 % XG solutionVi = 0.81 m/s 

Role of suction‐injection interaction on active volume growth

Suction‐injection interactions can be used to enhance active volume in AD hydraulic mixing

DF NozzleUF Nozzle

Active volume

Active volume

Active volume

Active volume

Experimental setup for sludge digestion involving hydraulic mixing…

• 9.5L - total reactor volume

• 7.25L - effective volume

• Mesophilic conditions (37°C)

• Batch mode, 21 days• Hydraulically mixed at three

specific power inputs P/V (SPI)• 0.46 W/m3

• 2.48 W/m3

• 6.00 W/m3

Full details of experimental set up in:Process intensification of anaerobic digestion: Influence on mixing and process performance, James McLeod, Maazuza Othman, Rajarathinam Parthasarathy, Bioresource Technology - 2019, Vol.274, Pages 533-540

Upwards facing nozzle Downwards facing nozzle

Reactor nozzle configurations, dimensions shown in mm

Sludge average homogeneity index…,

,

where: • HIn - homogeneity index at time n• TSapp,n - apparent total solids measured from

the piping loop at time n• TSHom,21 - homogenized reactor total solids at

the conclusion of the experiment

• Upwards facing (UF) nozzle produced better homogeneity

• Short-circuiting was only observed when the injection and suction nozzles were on the same vertical plane (DF nozzle)

• Short-circuiting prevents the reintegration of scum layers

UF nozzle

UF nozzle

UF nozzle

DF nozzle

DF nozzle

DF nozzle

SPI = 0.046 W/m3

SPI = 2.48 W/m3

SPI = 6.00 W/m3

Biogas production …

SPI = 0.46 W/m3

SPI = 2.48 W/m3

SPI = 6.00 W/m3

UF nozzle

UF nozzle

UF nozzle

DF nozzle

DF nozzle

DF nozzle

• At 0.46 W/m3, the greater homogeneity aided gas production

• At 2.48 W/m3, short circuiting in the DF nozzle case resulted in lower suspended solids and excessive mixing

• At 6.00 W/m3, short circuiting in the DF nozzle case prevented the reintegration of the scum layer, providing a region free from damaging shear forces

SPI = P/V (W/m3)

Process intensification in ADs…

Option Do Nothing New Infrastructure

Process intensification

Cost  0 $32M  $22M

Capacity none +25% +160%

‐ (5‐7 years) (30‐45 years)

Risk High Low High

Capacity expansion of AD to handle increased inflow in a plant operating at capacity

Risk of process intensification• AD sludge with solids concentration > 2.5 

wt% with non‐Newtonian rheology with perceived yield stress

0

100

200

300

400

500

600

700

800

0.00 1.50 3.00 4.50 6.00 7.50Cum

ulat

ive

Bio

gas

afte

r 20

days

(N

ml/g

VS)

P/V (W/m^3)

Traditional mixing criteria used in 

digester design – P/V (W/m3)

0

100

200

300

400

500

600

700

800

0 0.1 0.2

Cum

ulat

ive

Bio

gas

afte

r 20

days

(Nm

l/gVS

)

P/MS W/kgTS

Alternative design criteria –P/MS (W/kg TS)

UF nozzle

Optimum SPI

3.4, 4.4, and 4.9% w/w

Process intensification – Producing ‘more with less’

Next thinking… Process Intensification in WWTP• Anaerobic digester is a not a waste treatment vessel but a biological reactor used for

maximising biogas production• Process intensification is used in process industries reactors to produce more (products) with

less (energy and raw materials)Question: Is process intensification of AD possible using

• Thickened sludge as feed (rheological complexity, dead zones)• Innovative mixing techniques• Hydraulic mixing to maximise active volume by optimising the suction and

injection flow fields interaction• Intermittent mixing

Full details of experimental set up in:Process intensification of anaerobic digestion: Influence on mixing and process performance, James McLeod, Maazuza Othman, Rajarathinam Parthasarathy, Bioresource Technology - 2019, Vol.274, Pages 533-540