Abstract

The rapid increase of greenhouse gases (GHG) in the atmosphere generates several consequences; one of them is global warming. One of the most obvious effects of global warming is the increase in temperatures around the world and changes in climate. Several sources which produce carbon dioxide, methane and, nitrous oxide at a high level are recognized. Some of these sources are transportation, industrial sources, chemical production, petroleum production, agriculture, and wastewater treatment plants (WWTPs). WWTPs are recognized as one of the more extensive sources of GHG emissions such as CO2, CH4, and N2O. Due to biological microbial anaerobic and aerobic respiration processes
Thus, conversion of CO2, CH4, and N2O generated in the WWTPs into added-value products without the utilization of fossil fuels is so important to control global warming.
The main objective of this study is the production of added-value products from biogas and simultaneously mitigating the emission of GHG. For this reason, a novel electrochemical device has been proposed to convert biogas to the fuel and simultaneously produce the valuable by-products (such as struvite, fungicide) and remove impurities from the effluent used as an electrolyte. The investigations were conducted in the lab and a wastewater treatment plant in three phases and several stages. In phase 1, a novel electrochemical device was designed based on sustainable development principles. Phase 2 focused on the generation of the optimal operation conditions for an electrochemical device to be able to convert CO2, CH4, the mixture of CO2, and CH4 into fuel and valuable by-products. The influence of gas input time, gas flowrate, the configuration of the electrochemical device, temperature, and electrical potential on by-product production were investigated. The removal of N2O and its impact on the properties (the WWTP effluent used as an electrolyte) were also studied.
In Phase 3, the possibility of using real effluent as an electrolyte was investigated at a WWTP. The study focused on the conversion of biogas to the fuel and production of some by-products such as struvite from contaminated effluent.
The study described the impact of gas input duration, its flowrate, temperature fluctuation, electrical potential, and device configuration on methanol production.
The results showed that methanol is the main product of interest. Its production from pure biogas reached 84 ppm, while from real biogas it was 2.8 times less (30 ppm) at a lower temperature.
The result showed the maximum methanol production from real biogas was about 30 ppm at a temperature between 13 and 18 ◦c, 80.5 V cathodic potential, 30-minutes gas input time, and 10 cm distance between electrodes.
Besides the device was able to produce copper (II) hydroxide and copper (II) sulphate, respectively, in ambient temperature using only 8V/cm.
Application of on-site WWTP effluent as an electrolyte, permitted efficient removal of nitrate (88%), phosphate (98%), and sulphate (79%). Hence, this system besides depletion of GHG, production of methanol, struvite, and fungicide can improve the quality of effluent and better protect water resources. Furthermore, considering the circular economy, produced methanol at the WWTP on-site, can be returned to the influent as an extra carbon source for the nitrification process and decrease N2O generation.
Finally, an analytical model was used to investigate the effect of operational parameters on the methanol production efficiency. For this purpose, the Modelica V.17 based EMTtype simulation tool has been used.
The model demonstrates how different gas input durations, and the faradaic efficiencies of different products influence methanol production. Modeling shows good accordance with experimental results, particularly during the initial supply of gas, later differences could reach 25% - 31%.
This research results show the capability of the proposed device to convert biogas to methanol based on sustainable development principles. The electrochemical device can be introduced to a municipal wastewater treatment plant in order to mitigate greenhouse gases and reuse effluent as an electrolyte, but also it can be used in many industrial sectors.