Case Study

A membrane bioreactor lab measured how water wets pulp mill sludge and its membranes, and reported the results in three publications from 2021 to 2025.

Last Updated

October 18, 2026

Industry

Environmental engineering and wastewater research

Location

Thunder Bay, Ontario, Canada

Customer since

December 2018

Published with the Dropometer

2 peer reviewed papers and 1 PhD thesis

Instrument

Dropometer M-3, manual (current model shown)

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Drop shape method validated against a KRÜSS DSA100E in peer review Named as the Dropometer M-3 in a peer reviewed paper and a PhD thesis Two peer reviewed validation papers

Prof. Baoqiang Liao's group at Lakehead University turns pulp and paper mill sludge into biogas in a heated anaerobic membrane bioreactor. Membrane fouling decides how long such a system can run. The group added Dropometer water contact angle to its fouling tests on sludge and membranes. The values appear in a PhD thesis and two journal papers, with 3 µL drops stated in the thesis and the 2025 paper.

Manual Dropometer contact angle instrument with smartphone camera, sample stage and light source (current model)

Written by

Abhimanyu Bhandankar

Holds an MBA from Schulich School of Business and a BE in IT. He joined Droplet Lab in July 2019 and now leads sales and marketing.

CEO at Droplet Lab

Primary outcome: sludge wetting measured through three loading phases

Water contact angle of the reactor sludge at three organic loading rates

Thermophilic anaerobic membrane bioreactor, pulp mill primary sludge (Journal of Environmental Chemical Engineering, 2022)

Loading 2.2 kg TSS/m³·d 64.3 ± 3.2°

Loading 3.9 kg TSS/m³·d 24.6 ± 3.04°

Loading 1.5 kg TSS/m³·d 43.5 ± 5.8°

The reactor ran at 50 °C on primary sludge from a thermomechanical pulp mill. At a loading of 2.2 kg TSS/m³·d the sludge read 64.3 ± 3.2°. At 3.9 it fell to 24.6 ± 3.04°, and at 1.5 it rose again to 43.5 ± 5.8°. The 3.9 phase was also the one where pressure across the membrane reached 40.5 kPa in under 50 days. The paper reports both results side by side; it does not test a link between them.

64.3° → 24.6° sludge

Water contact angle of the reactor sludge as the organic loading rate rose from 2.2 to 3.9 kg TSS/m³·d.

39% lower

Membrane water contact angle after the 55 day solids retention time run, compared with an unused membrane.

3 µL water drops

Water drop volume stated in the thesis (about 3 µL, from a micropipette) and in the 2025 paper (3 µL, in triplicate).

3 publications

A PhD thesis (2021) and two peer reviewed papers (2022, 2024) report Dropometer contact angles.

Executive Summary

Who

Prof. Baoqiang Liao's group in the Department of Chemical Engineering and the Faculty of Natural Resources Management at Lakehead University, Thunder Bay, Ontario. The group develops anaerobic membrane bioreactors that turn pulp and paper mill sludge into biogas, with NSERC funding.

Problem

Membrane fouling limits how long these reactors run between cleanings. Fouling depends on the sludge and the membrane surface, and no single test explains it. The group needed a direct reading of how water wets the sludge and the membranes as loading rate and solids retention time changed.

Solution

A manual Dropometer M-3 for contact angle, surface tension and surface free energy, invoiced in December 2018. The thesis and the 2025 paper describe about 3 µL water drops on sludge and membrane samples, in triplicate in 2025. Contact angle sat alongside zeta potential, particle size, SEM, FTIR and XPS.

Time to Value

2018: first saw the Dropometer at the CSChE conference. 17 December 2018: instrument invoiced. January 2021: PhD thesis with Dropometer data. 11 March 2022: loading rate paper online (Journal of Environmental Chemical Engineering). 7 November 2024: solids retention time paper online (Separation and Purification Technology).

Results

Sludge contact angle moved from 64.3° to 24.6° and back to 43.5° across three loading rates. Membrane water contact angle fell 20%, 24% and 39% after runs at 32, 45 and 55 day solids retention times. The data appear in a PhD thesis and two peer reviewed papers.

64.3° → 24.6°

Sludge contact angle as loading rose

39%

Lower membrane contact angle after a 55 day run

3 µL

Water drop in the thesis and the 2025 paper

3

Publications with Dropometer data

Lakehead University, at a glance

Research focus

Anaerobic membrane bioreactors that turn pulp and paper mill sludge into biogas, and the membrane fouling that limits them.

Products / applications

A laboratory thermophilic submerged anaerobic membrane bioreactor treating primary sludge from a thermomechanical pulp mill, producing biogas with 52 to 60% methane.

Measurement stage

Characterising sludge and membranes during and after long reactor runs, at three loading rates and three solids retention times.

Users

A PhD researcher and two supervisors in Chemical Engineering and Natural Resources Management.

Materials and surfaces tested

Reactor sludge (mixed liquor) and flat sheet PVDF microfiltration membranes with 0.1 µm pores, unused and after each reactor run.

Key constraints

Reactor at 50 °C; wet, changing biological samples; membranes recovered only after runs lasting months.

The challenge: membranes that foul while turning mill sludge into energy

Fouling depends on the sludge and the membrane surface, and no single test explains it.

What the lab was building

Primary sludge makes up about 70% of the sludge from a typical Canadian pulp and paper mill. Liao's group treats it in a heated anaerobic membrane bioreactor that makes biogas. A submerged membrane holds back the solids and lets treated liquid through. The longer the membrane stays clean, the longer the reactor runs well. Paper and board mills also feed the supply chain in our Packaging and Containers industry guide.

Why fouling is hard to read

Most of the resistance comes from a cake and gel layer on the membrane, about 97% in the 2025 study. That layer depends on sludge concentration, particle size, surface charge and surface chemistry. No single test explains it, so the group combined zeta potential, particle size, SEM, FTIR, XPS and contact angle. Our use case on membrane hydrophobicity and fouling characterization explains where wetting fits. Some labs test wet membranes with the captive bubble method; this group used sessile water drops.

What was at stake

In the 2022 study, the highest loading rate pushed the pressure across the membrane to 40.5 kPa in under 50 days. In the 2025 study, full cleaning recovered 94.3% of the flux after a 32 day solids retention time, but only 84.4% and 85.5% after 45 and 55 days.

What success would require

A direct number for how water wets the sludge and the membrane, taken on small samples and repeated across months of operation. It had to sit next to the other surface tests, and be reported with its spread so readers could judge it. Our guides to contact angle measurement and reproducible contact angle measurement cover the method and its checks.

The insight: put a number on surface wetting next to the other fouling tests

What Was Deployed

A manual Dropometer contact angle and surface tension kit, model M-3, invoiced on 17 December 2018. It came with contact angle, surface tension and surface free energy modules. Here is the method as each source reports it. The PhD thesis names the Dropometer M-3, uses pure water and drops of about 3 µL from a micropipette, and averages each specimen (our analysis of the Bokhary PhD thesis). The 2022 paper names the Dropometer M-3 with pure water as the probe liquid, but gives no drop volume (our analysis of the 2022 organic loading rate paper). The 2025 paper credits a sessile drop method from Droplet Smart Tech Inc., our company name, with 3 µL water drops in triplicate (our analysis of the 2025 solids retention time paper).

What Made it a Fit

One manual kit for contact angle, surface tension and surface free energy.

Small 3 µL water drops on small sludge and membrane samples.

Owned by the lab, so water contact angle could be repeated across a thesis and two papers from 2021 to 2024.

Drop shape method validated against a KRÜSS DSA100E in Review of Scientific Instruments and Colloids and Surfaces A.

The Origin

From a conference meeting to three publications

Prof. Baoqiang Liao first saw the Dropometer at the CSChE conference in 2018. His lab then ordered a manual Dropometer M-3 with contact angle, surface tension and surface free energy. It was invoiced on 17 December 2018. The group was running a heated anaerobic membrane bioreactor on primary sludge from a local thermomechanical pulp mill. Membrane fouling decided how long each run could last, so the group measured the sludge and the membranes in many ways. Water contact angle became one of those tests. The first Dropometer data appeared in a PhD thesis in January 2021. Two peer reviewed papers followed, published online in March 2022 and November 2024. For labs weighing a similar setup, our guide to the best contact angle goniometer for advanced research compares the options.

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Before vs After

Metric	Before	After
Sludge contact angle, loading 2.2 to 3.9 kg TSS/m³·d	64.3 ± 3.2°	24.6 ± 3.04°
Sludge contact angle, loading 3.9 to 1.5 kg TSS/m³·d	24.6 ± 3.04°	43.5 ± 5.8°
Average pressure across the membrane, loading 2.2 to 3.9	8.2 ± 3.3 kPa	16.5 ± 9.1 kPa
Sludge contact angle, solids retention time 32 to 55 days (thesis)	49.3 to 62.4°	19.1 to 26.7°
Membrane water contact angle, unused vs after 55 day run	Unused membrane	39% lower
Flux recovered after full cleaning, 32 vs 55 day run	94.3%	85.5%

Rollout timeline

Timeline (high level)

2018: Conference and order

Prof. Liao sees the Dropometer at the CSChE 2018 conference
Manual Dropometer M-3 with contact angle, surface tension and surface free energy, invoiced on 17 December 2018

2019 to January 2021: Thesis work

Sludge contact angle at three solids retention times, drops of about 3 µL from a micropipette (thesis Chapter V)
Sludge contact angle at three organic loading rates (thesis Chapter VII)
PhD thesis dated January 2021

March 2022: Loading rate paper

Sludge contact angle at three organic loading rates, reported in Table 2
Published online on 11 March 2022 (Journal of Environmental Chemical Engineering)

March 2023: Software update

Free software update offered for the lab's instrument
Prof. Liao asked a graduate student to confirm the setup

November 2024: Solids retention time paper

Membrane and sludge contact angle with 3 µL water drops, in triplicate, at 32, 45 and 55 days
Published online on 7 November 2024 (Separation and Purification Technology, 2025 issue)

October 2026: Open dataset invitation

Droplet Lab invites the group to contribute drop images and metadata to its open contact angle dataset

Training and SOP updates

Not reported in the papers. The thesis describes the routine: pure water as the probe liquid, a drop of about 3 µL dispensed with a micropipette, and an average reading for each specimen.

Data capture

Averages of repeated readings reported in tables: Table 2 in the 2022 paper and Table 5.2 in the thesis. The 2025 paper reports percent changes against an unused membrane.

Calibration

Not reported in the papers, which do not state the fitting method or temperature for the contact angle step.

Support Model

Email support from Droplet Lab. In March 2023 the lab was offered a free software update for its instrument.

Proof and validation: what the papers show, and what they do not

Test method

Sessile drop water contact angle with pure water as the probe liquid. The thesis states drops of about 3 µL from a micropipette, and the 2025 paper states 3 µL. Samples were reactor sludge (mixed liquor) and flat sheet PVDF microfiltration membranes with 0.1 µm pores, unused and after each run.

Sample size and operators

The 2025 study measured every mixed liquor and membrane sample in triplicate and reported averages. The 2022 paper reports mean ± spread for each loading phase but does not state n. The thesis reports ranges for each solids retention time.

Repeatability / reproducibility

Sludge spreads of ± 3.04° to ± 5.8° across the three loading phases (2022). In the 2025 study, the effect of solids retention time on membrane contact angle was statistically significant (p below 0.01).

Notes / assumptions

The 2025 paper gives membrane results as percent changes, not absolute angles, and does not report its mixed liquor values. The 2022 paper does not state the drop volume or the number of readings. No source states the fitting method or temperature for the contact angle step. A lower water contact angle means water spreads more easily on the surface. Contact angle is one of several fouling tests in these papers, not the main finding.

Our contact angle and pendant drop surface tension methods have been benchmarked against KRÜSS DSA100E reference measurements.

See scientific validation

Our instruments are referenced in peer reviewed journals, theses and conference publications.

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Outcomes to date

Measured Outcomes

24.6 ± 3.04°

Lowest sludge contact angle

Measured at the highest loading rate, 3.9 kg TSS/m³·d. The same phase had the heaviest fouling, with pressure across the membrane reaching 40.5 kPa in under 50 days. The paper reports both but does not link them.

20%, 24%, 39%

Lower membrane contact angle after each run

Compared with an unused membrane, after runs at 32, 45 and 55 day solids retention times. The effect of retention time was significant (p below 0.01).

49.3 to 62.4° vs 19.1 to 26.7°

Sludge at 32 vs 55 day retention

Thesis ranges show the sludge surface wetting more easily at the longest solids retention time.

Operational Outcomes

Fouling picture

Contact angle sits with zeta potential, particle size, SEM, FTIR and XPS in the group's account of residual fouling.

Cleaning insight

Full cleaning recovered 94.3%, 84.4% and 85.5% of flux after the three runs, showing residual fouling built up at longer retention times.

Biogas from mill sludge

The reactor produced biogas with 52 to 60% methane from pulp mill primary sludge in the 2022 study. This is a reactor result, not a Dropometer measurement.

What's next

Delivered

Sludge contact angle across solids retention times (PhD thesis, 2021)

Sludge contact angle across organic loading rates (Journal of Environmental Chemical Engineering, 2022)

Membrane and sludge contact angle in a residual fouling study (Separation and Purification Technology, 2025)

In Pilot

Invited in October 2026 to contribute drop images and metadata to Droplet Lab's open contact angle dataset

Why measure contact angle in membrane fouling studies?

Fouling depends on the sludge and the membrane surface, and no single test explains it. The group added water contact angle on sludge and membranes alongside zeta potential, particle size, SEM, FTIR and XPS.

What did contact angle show?

Sludge contact angle moved from 64.3° to 24.6° and back to 43.5° across three loading rates. Membrane water contact angle fell 20%, 24% and 39% after runs at 32, 45 and 55 day solids retention times.

What drop size did the lab use?

The PhD thesis and the 2025 paper describe water drops of about 3 µL, in triplicate in 2025. The instrument is a manual Dropometer M-3 with contact angle, surface tension and surface free energy, invoiced in December 2018.

Next Step

Measuring membranes, sludge or other wet, variable surfaces?

A water contact angle reading adds a direct surface number to fouling studies, next to charge, particle size and microscopy. The Dropometer measures contact angle, surface tension and surface free energy in one kit, with a drop shape method validated against a KRÜSS DSA100E in peer review. Setup takes about 2 minutes. Book a 15 minute demo and bring a sample.

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