Low-voltage electrochemical pretreatment for enhancing sludge stabilization

Abstract

Stabilization of waste activated sludge (WAS) is one of the major challenges in wastewater treatment plants worldwide. To achieve effective WAS stabilization, pretreatment techniques are generally applied for solubilizing WAS prior to anaerobic digestion (AD). However, existing pretreatment methods (e.g., focused-pulsed voltage (>1 kV), thermal, mechanical and chemical, etc.) are restricted by drawbacks such as having a large footprint, requiring intensive chemical addition, and high energy consumption. This research proposes a low voltage (≤15 V) and short time (≤30 mins) electrochemical PreTreatment (EPT) method to achieve sludge stabilization including i) dewaterability enhancement, ii) pathogen disinfection, iii) odor control, and iv) carbon recovery. A voltage supply at 0-15 V for 12 mins with graphite fiber electrodes was used for sludge pretreatment. The mechanism underlying EPT was analyzed based on the changes in the sludge physicochemical and biological characteristics, including particle size, zeta potential, hydrophobicity, dewaterability, pathogen content, fluorescent distribution of live/dead cells and biodegradability, etc. EPT (≥ 8 V) can effectively disintegrate sludge flocs and destroy cell membranes, resulting in the releases of intracellular and extracellular substances (e.g., protein and polysaccharides). With the release of interstitial water and cytolysis after EPT, ~40% of sludge dewaterability enhancement and nearly 5 log₁₀ removal of the indicator pathogens (E. coli, Salmonella spp. and Streptococcus faecalis) were achieved. The sludge biodegradability was evaluated using biochemical sulfide potential (BSP) and biochemical methane potential (BMP) tests. Interestingly, EPT at 10 V or higher dramatically suppressed sulfide production in the BSP tests, and selectively inhibited biogas (CH₄ and CO₂) generation in the BMP tests. Based on the findings of the sulfide suppression in the BSP tests, we hypothesize that EPT can be used for sludge sulfide control. EPT operated at 12 V for 12 mins eliminated 99% of dissolved sulfide and 100% of gaseous H₂S_(g) in the anaerobic sludge digester. In comparison, the dissolved sulfide reached 104 ± 1 mg S/L in the control test. A sulfur mass balance analysis showed that 90% of the produced sulfide was removed via metal precipitation. The metal distribution results confirmed that metals (i.e., Fe, Mn, and Ni) in the sludge became soluble after EPT and were released from their residual and organically bound fractions. EPT at 15 V solubilized around 73, 92, and 72% of Fe, Mn, and Ni, respectively, and these metals precipitated the sulfide that was produced from the biological sulfate reduction. Mechanistic analysis indicated that EPT disrupted the metal-binding functional groups. Specifically, a reduction of 17% in the C=O functional groups in the sludge was found, which can be associated with the metal release. The impact of oxidants (e.g., chlorine) generated from EPT on sulfide oxidation was minimal. Inspired by the reduction of biogas production with EPT in the BMP tests, the inhibition mechanism and the potential of using EPT to produce volatile fatty acids (VFAs) were further explored. In a semi-continuous anaerobic sludge digestor (1 L), biogas containing 61% of CH₄ (95±5 mL CH₄/g VS_added) was stably produced without EPT. However, after EPT at 10 V for 30 mins, the biogas gradually decreased to 2~5 mL CH₄/g VS_added, with the accumulation of VFAs up to 100±5 mg C/g VS_added. The results of batch tests shown that the electrode material in EPT can alter the products from the anaerobic sludge digestion. EPT with titanium electrodes (Ti-EPT) enhanced CH₄ production by ~20%, compared with the control. However, EPT with carbon and graphite electrodes, named as carbon-EPT and graphite-EPT, both selectively inhibited methanogenesis, thereby producing VFAs. Carbon- and graphite-EPT showed a biocidal effect on the methanogens (mainly Methanolinea and Methanothrix), while graphite-EPT dramatically down-regulated the expression of a core enzyme in methanogenesis, i.e., heterodisulfide reductase. More importantly, this study is the first time to demonstrate that acetate can be the dominant product (~80% of total VFA) of conventional sludge digestion, enabling potential VFA reuse in PHA biosynthesis and biofuel. The outcomes of this research are expected to enhance sludge stabilization and promote sludge resource recovery.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology