Microbial electrolysis cell (MEC), one of the bioelectrochemical system, is a recently developed technology that can recover energy in the form of biogas such as hydrogen or methane. Exoelectrogenic bacteria decompose and oxidize organic matter to release protons, electrons and carbon dioxide at the anode, electrons move to external circuits, protons move to the cathode through the internal solution and be reduced electrochemically or biologically producing biogas such as hydrogen and methane at the cathode. In the research of anaerobic digestion (AD) and MEC, hydrolysis is commonly reported as a rate-limiting step because of the slow decomposition rate due to the low surface area of large organic molecules. Various studies are being conducted to reduce the hydrolysis time, one of which is sludge pretreatment. The purpose of sludge pretreatment is to forcibly destroy large organic molecular flocs and cell walls to shorten the hydrolysis time, which is the rate-limiting step, and in this process, the organic acids and SCOD in the cell walls are used for biogas production. This study describes the effect of the sludge solubilization according to alkaline (pH 7~11) and ultrasonic pretreatment (40~120 mins) time and the effect of pretreatments and applied voltages in the MEC operation.
The indicators such as organic matter removal, methane production, microbial taxonomy analysis are used as evaluation index. In the sludge solubilization test, degree of solubilization was low at pH 7 (35.8 %) and 8 (39.4 %) at 120 minutes of ultrasonic time, but the highest degree of solubilization was showed at 84.2 % at pH 11.
When comparing AD and MEC, organic removal rate of TS was 10.7~68.0%, COD was 3.1~86.2%, and methane production was 40.4~113.7% higher. When comparing MEC operation performance by applied voltage, at applying 0.6 V, TS increased by 27~32%, COD increased by 14~62%, and methane production increased by -7~10%. When comparing the operating performance of MEC using ultrasonic and alkali-ultrasonic combined pretreated sludge, TS was 45.3% for both ultrasonic and alkaline ultrasonic pretreatment under the condition of applying 0.3 V, COD was 52.6% and methane production was 6086 mL for alkali-ultrasonic combined pretreatmet, showing the highest removal rate and production. When 0.6 V was applied, the ultrasonic pretreatment showed the highest removal rates of 59.9 and 65.9% respectively for both TS and COD, and the highest methane production was for alkali-ultrasonic combined pretreatment at 5660 mL. In the modified Gompertz plot, were decreased when 0.6 V was applied (0.6~1.3 days) compared to 0.3 V (1.6~2.4 days). were increased when the pretreated sludge were used, which is thought to be due to the accumulation of acetic acid due to the increase in the sludge solubilization rate due to the pretreatment.
At the bacterial community at phylum level, Proteobacteria decreased compared to MECs without pretreatment under all applied voltage conditions, and Firmicutes, on the contrary, increased compared to MECs without pretreatment under all applied voltage conditions. At the archaeal community at genus level, Methanothrix, known as one of the acetoclastic methanogens increased during pretreatment when 0.3 V was applied, but decreased under the pretreatment condition when 0.6 V was applied. Methanoculleus, known as one of the hydrogenotrophic methanogens, decreased during pretreatment when 0.3 V was applied, contrary to Methanothrix, but increased when 0.6 V was applied.