Dissolved gas separation for engineered anaerobic wastewater systems

Abstract

Dissolved gases produced within engineered anaerobic processes subsequently create a fugitive emission which can have financial, environmental and health and safety implications. Whilst desorption technology has been used to control dissolved gases in the drinking water sector, there is considerably less understanding of its deployment in wastewater for which there are numerous existing and emerging challenges. This review therefore focuses on existing and proposed technological approaches to gas desorption in engineered anaerobic wastewater processes, with specific emphasis on technology compatibility and downstream gas phase management. Simplified engineered solutions such as diffused aeration and multi-tray aerators appear robust solutions for implementation into wastewater. However, these processes are characterised by a low mass transfer coefficient and require high gas to liquid ratios (G/L) to achieve reasonable separation, which suggests their suitability is limited to small scale applications, in which gas recovery is not a priority. Packed columns and membrane contactors afford process intensification through increasing interfacial area which favours large scale applications; although both will require prefiltration technology to obviate media clogging. Vacuum or steam is the preferred driving force for separation when gas recovery is sought, while sweep-gas is energetically favoured. Sweep-gas has been used for gas recovery by operating at G/L toward the equilibrium value, which somewhat constrains mass transfer. Process selection must therefore be weighted on whole life cost, but will also be dependent upon process scale, financial (e.g. incentivisation) and non-financial (e.g. carbon) instruments, which are strongly influenced by regional policy.

Membrane-based technologies for post-treatment of anaerobic effluents

“…column, or diffused aeration have been applied for the recovery/ removal of dissolved CH 4 from the anaerobic effluents. 12 A critical drawback of these techniques is that the stripping gas (usually air), which directly contacts the anaerobic effluents, can cause foaming and flooding. To prevent such operational problems, the use of gas/liquid "membrane contactor" process was proposed.…”

“…To prevent such operational problems, the use of gas/liquid "membrane contactor" process was proposed. 12 The gas/liquid membrane contacting process uses a stripping gas (or vacuum) to establish a mass transfer driving force between the gas and liquid phases, which are isolated using polymeric membranes as phase barriers. 12,23 To reduce the mass transfer resistance, membranes are commonly designed to have high porosity and low wettability.…”

“…5,9 Second, a significant amount of produced methane (CH 4 ) remains dissolved in the effluents. [10][11][12] Typically, over 60-70% of the organic carbons in wastewater is converted into CH 4 and carbon dioxide (CO 2 ), which leads to the formation of biogas generally consisting of~55-60% CH 4 and~40-45% CO 2 ( Fig. 1).…”

“…11,13,14 From the economic standpoint, discharging these effluents means a loss of valuable energy resource, which can otherwise be recovered and combined with the biogas stream for increasing the overall energy efficiency of the wastewater treatment processes. [10][11][12] Notably, the wastewater industry also faces intense financial pressures and remains partially dependent on government compensations and public funds to run the anaerobic wastewater treatment plants. 4,15 Besides, being a potent greenhouse gas, releasing undue CH 4 to the environment exacerbates the global warming problem.…”

Gas–liquid mass transfer and bubble size distribution in a multi‐Cyclone separator

“…Lower amounts of sweeping gas imply a greater concentration of the dissolved gas collected at the gas outlet. This is found to be conducive for optimum utilization of the separated gas …”