Evaluation of cleaning conditions for polymeric ultrafiltration membranes after black liquor filtration

Abstract

This thesis investigated the chemical cleaning of a polymeric ultrafiltration membrane after
black liquor filtration. The main objective was to evaluate how cleaning temperature, cleaning-
agent concentration, and cleaning time affected flux recovery after cleaning.
Black liquor filtration caused a gradual decline in permeate flux during the 25 h fouling
experiment, confirming that fouling developed on the membrane. After fouling, eight cleaning
experiments were performed using a full factorial Design of Experiments approach. The fouling
factor was not identical in all experiments, which means that the initial degree of fouling varied
between membrane samples and should be considered when interpreting the cleaning results.
The cleaning experiments showed that the highest flux recovery was obtained using Ultrasil
110 at 50 °C and 1 wt.%. The highest measured recovery in the experimental design was
achieved after 30 min of cleaning, while the model suggested 60 min as the most favourable
condition within the investigated range. Therefore, the validation experiment was performed at
50 °C, 1 wt.% Ultrasil 110, and 60 min. The validation experiment resulted in a flux recovery
of 76.43%, which was slightly higher than the best result obtained in the original experimental
design. The model should be treated as a screening tool rather than a strong predictive model
because of the limited dataset and the absence of statistically significant effects.
Membrane characterisation was performed using scanning electron microscopy with energy-
dispersive X-ray spectroscopy (SEM–EDS), atomic force microscopy (AFM), and Brunauer–
Emmett–Teller (BET) nitrogen adsorption–desorption analysis. SEM–EDS and AFM showed
that black liquor fouling caused local surface deposits and increased surface heterogeneity.
After cleaning, the surface appeared partly restored, although some deposits and irregularities
remained. BET analysis showed only small differences between the conditioned, fouled, and
cleaned samples, suggesting that major changes in the nitrogen-accessible pore structure were
not detected.
Overall, the results showed that cleaning temperature and cleaning-agent concentration were
the most important parameters within the tested range, while cleaning time had a weaker effect.
Chemical cleaning improved membrane performance after black liquor fouling, but additional
experiments with repetitions and more factor levels would be needed to confirm the trends and
improve reliability of the model.

Popular Abstract

Why is membrane cleaning important in the pulp and paper industry?
The pulp and paper industry generates large process streams containing organic and inorganic
compounds. One example is black liquor, a by-product formed during wood processing. Instead
of treating such streams only as waste, membrane filtration can help recover valuable
compounds and produce cleaner water streams for reuse.
However, membrane fouling is one of the main challenges in this process. During filtration,
particles and organic compounds can accumulate on the membrane surface or inside the pores.
This reduces flux and limits long-term membrane operation. Therefore, effective cleaning is
needed to restore membrane performance.
What was investigated in this thesis?
This thesis investigated the chemical cleaning of a polymeric ultrafiltration membrane after
black liquor filtration. The membrane was first fouled with black liquor, and then different
cleaning conditions were tested to see how well the membrane performance could be recovered.
Three cleaning parameters were studied: cleaning temperature, cleaning solution concentration,
and cleaning time. Eight cleaning experiments were performed using a Design of Experiments
approach, followed by one validation experiment under the condition suggested by the model.
The membrane was also analysed before fouling, after fouling, and after cleaning using surface
characterisation methods. These analyses were used to evaluate visible and measurable changes
on the membrane surface and structure.
What were the main findings?
Black liquor filtration caused a gradual decrease in membrane flux, confirming that fouling
developed during the experiment. Chemical cleaning recovered part of the membrane
performance, but the recovery depended on the cleaning conditions used.
Among the tested conditions, the best recovery was obtained at the highest tested temperature
and cleaning-agent concentration, corresponding to 50 °C and 1 wt.% Ultrasil 110. Cleaning
time had a smaller effect within the tested range. The validation experiment gave a slightly
higher flux recovery than the best result from the original experimental design, but the model
should be interpreted carefully because only a limited number of experiments was performed.
Surface analysis showed local deposits on the fouled membrane. After cleaning, part of this
material was removed, although the membrane surface was not fully restored to its initial
condition. Overall, the study showed that chemical cleaning can improve membrane
performance after black liquor fouling, but more experiments are needed to confirm the
observed trends.

• author: Svyrhun, Kateryna
• supervisor: Mariona Battestini Vives
• organization: Chemical Engineering (M.Sc.Eng.)
• course: METM01 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: membrane cleaning, ultrafiltration, black liquor, membrane fouling, polymeric membrane, flux recovery, design of experiments, membrane technology
• language: English
• id: 9247100
• date added to LUP: 2026-08-10 09:56:05
• date last changed: 2026-08-10 09:56:05