Application of electrodialysis for nutrient recovery from Anaerobic digestate
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Date
2023-03-30Embargo Date
2025-03-30
Author
Meng, Jizhong
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Abstract
Electrodialysis (ED) is an electro-driven membrane-based technology, which can separate
ions from solutions with high efficiency. Recently, ED has been studied to recover nutrients,
i.e., ammonium (NH4
+) and phosphate (PO4
3-
), from anaerobic digestate, while ion-exchange
membrane (IEM) fouling is a big challenge. There are still considerable knowledge gaps of
membrane fouling during long-term ED application. The objectives of this PhD research
included: (1) evaluate the effects of the organic matter’s molecular size on the membrane
fouling, and investigate the membrane fouling behaviours during long-term operation; (2)
evaluate a novel strategy, electro-ion substitution modified ED (EIS-ED), in reducing
membrane fouling; and (3) integrate ED with anaerobic digestion (AD) for improvement of
the methane yield and in-situ NH4
+ recovery.
The results showed that the molecular size of organic matter in manure digestate significantly
impacted membrane fouling. Organic matter with a molecular size of <1 kDa led to severe
irreversible fouling. Although 1-10 kDa organic matter led to irreversible fouling, the fouling
area was close to the membrane surface, and there was no significant deterioration of
membrane characteristics. The organic matter with a size of >10 kDa was retained on the
membrane surface by steric hindrance and resulted in reversible fouling.
During the ED long-term operation, membrane fouling did not impact the membrane perm selectivity, and the membranes still showed high performance in desalination, after a
treatment loading of 5000 L/m2
. Both the heterogeneous and homogeneous anion exchange
membranes (AMs) showed a gradual deterioration in membrane resistance. Heterogeneous
cation-exchange membrane (CM) was more easily fouled by organic matter than
homogeneous CM. After the treatment loading of 5000 L/m2
, both heterogeneous and
homogeneous AM had a deterioration in membrane resistance and ion exchange capacity,
while CMs were more robust than AMs.
The proposed EIS-ED recovered over 90% of NH4
+ from the sludge centrate and mitigated
AM fouling significantly. Experimental data and modeling showed that the EIS-ED would
have a competitive power and energy efficiency over conventional ED as the concentration
polarization was eliminated. The energy consumption of EIS-ED was around 2.03 kWh/kg
NH4
+-N within 70% recovery rate, which was 14% lower than that of conventional ED.
With the integration of ED to AD process, compared to conventional AD, the methane yield
was greatly improved at the influent NH4
+-N of 5,000 mg/L and 10,000 mg/L
This PhD research contributes to an in-depth understanding of ED membrane fouling in the
ED application of nutrient recovery from digestate, and provides technical data for controlling
IEM fouling and broadening the ED application into the AD process.