Cultivation of low-temperature (15°C), anaerobic, wastewater treatment granules

Low-Temperature Removal of Refractory Organic Pollutants by Electrochemical Oxidation: Role of Interfacial Joule Heating Effect

Low temperature presents a challenge to wastewater treatment in the winters of cold regions. In the electrochemical oxidation (EO) process, the interfacial Joule heating (IJH) effect results in interfacial temperature higher than that of bulk electrolytes, which would alleviate the negative impact of low water temperature on organic oxidation occurring within the boundary layer of the anode. This study investigated the electrochemical oxidation of the representative recalcitrant organic pollutant, i.e., phenol, p-chlorophenol (p-CP), and 2,4-dichlorophenoxyacetic acid (2,4-D) on titanium suboxide (TiSO) anode at a low water temperature (8.5 ± 1 °C). At a low current density of 2 mA cm^-2, the IJH effect was insignificant and thus had a slight impact on interfacial temperature, leading to a low-efficiency and incomplete organic removal via direct electron transfer (DET) oxidation. Increasing the current density to 20 mA cm^-2 promoted the working up of the IJH effect and thus resulted in a dramatic increase in the interfacial temperature from 8.1 to 38.7 °C. This almost eliminated the negative impact of low temperature on the abatement of organic pollutants as though the low temperature of the bulk solution did not interact with interfacial reactions at all. This was indicated by the oxidation rates of 0.158 min^-1 (phenol), 0.084 min^-1 (p-CP), and 0.070 min^-1 (2.4-D) at a temperature of 8.5 ± 1 °C, the values being almost comparable to that obtained at room temperature (23.5 ± 1 °C). Both theoretical and experimental results demonstrated that the extent to which the low- and room-temperature cases deviated from each other was positively correlated with the activation energy of organic pollutants when reacting with ^•OH. The improvement of organic oxidation at low temperature should result from the compensation of the IJH effect, giving rise to higher ^•OH reactivity, more activated organic molecules, and enhanced mass transfer. This study may prompt new possibilities to develop an IJH effect-based electrochemical manner for decentralized water decontamination in cold regions.

Microbial Community Redundancy and Resilience Underpins High-Rate Anaerobic Treatment of Dairy-Processing Wastewater at Ambient Temperatures

High-rate anaerobic digestion (AD) is a reliable, efficient process to treat wastewaters and is often operated at temperatures exceeding 30°C, involving energy consumption of biogas in temperate regions, where wastewaters are often discharged at variable temperatures generally below 20°C. High-rate ambient temperature AD, without temperature control, is an economically attractive alternative that has been proven to be feasible at laboratory-scale. In this study, an ambient temperature pilot scale anaerobic reactor (2 m³) was employed to treat real dairy wastewater in situ at a milk processing plant, at organic loading rates of 1.3 ± 0.6 to 10.6 ± 3.7 kg COD/m³/day and hydraulic retention times (HRT) ranging from 36 to 6 h. Consistent high levels of COD removal efficiencies, ranging from 50 to 70% for total COD removal and 70 to 84% for soluble COD removal, were achieved during the trial. Within the reactor biomass, stable active archaeal populations were observed, consisting mainly of Methanothrix (previously Methanosaeta) species, which represented up to 47% of the relative abundant active species in the reactor. The decrease in HRT, combined with increases in the loading rate had a clear effect on shaping the structure and composition of the bacterial fraction of the microbial community, however, without affecting reactor performance. On the other hand, perturbances in influent pH had a strong impact, especially when pH went higher than 8.5, inducing shifts in the microbial community composition and, in some cases, affecting negatively the performance of the reactor in terms of COD removal and biogas methane content. For example, the main pH shock led to a drop in the methane content to 15%, COD removals decreased to 0%, while the archaeal population decreased to ~11% both at DNA and cDNA levels. Functional redundancy in the microbial community underpinned stable reactor performance and rapid reactor recovery after perturbations.