Outlet Process Streams
Water electrolyzers with aqueous process streams, such as proton exchange membrane (PEM), anion exchange membrane (AEM), and alkaline electrolyte (AE) electrolyzers, produce aqueous waste streams that contain dissolved hydrogen or oxygen gas. Both hydrogen and oxygen dissolve in water-based fluids in proportion to gas process pressure. Examples of electrolyzer waste streams with dissolved gases include:
- Water condensed and periodically drained from wet waste gas vent systems
- Water periodically drained from product gas dryers
- Water or electrolyte drained from systems during maintenance
When a system releases these waste streams, dissolved gases escape the solution. While initial volumes of gas may seem small, these streams should be disposed with great care to prevent the released hydrogen or oxygen from collecting in enclosed drains or areas. Over time, these gases can accumulate and present a hazard if not properly vented and diluted.
Best practices when draining gas dryer condensate from any electrolyzer, as well as water waste streams from PEM and AEM electrolyzers, include the following:
- Use an open drain that allows de-gassing into a well-ventilated space away from ignition sources
- Use dedicated drains; avoid connecting to other water drains where escaping hydrogen gas could accumulate or migrate to unintended areas
For example, an explosion incident in 2018 was the result of improper connection of an electrolyzer hydrogen gas dryer water drain into an enclosed common facility condensate drain system. The explosion blew apart the electrolyzer enclosure and the facility drain system, and caused serious injury to personnel. 
It is important that users are aware of the outlet stream compositions during all use scenarios and that installation locations for vent outlets are considered carefully. Remember that during startup and shutdown, air will also be in the vent lines, which results in short-duration flammable mixtures in the hydrogen vent. Guidance provided in H2Tools best practices for venting is applicable to electrolyzer hydrogen gas vents.
Waste caustic electrolyte from AE systems is typically drummed and disposed in accordance with hazardous waste regulations. Best practice includes a period to release de-gassing hydrogen to a safe location where it can be diluted and exercising caution when managing waste drum bungs, including operating in a well-ventilated space and use of non-sparking tools.
 In physical chemistry, Henry's law states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid. For example, a liter of water at 20 °C and one atmosphere can hold about 15 cm3 of hydrogen, but the same liter of water at a typical water electrolyzer pressure of 30 bar and 20 °C holds 23 times more, about 349 cm3)
CGA G5.5, Hydrogen Vent Systems
CHS Webinar: Safety of Water Electrolysis
CSA/ANSI 22.2 No. 22734 (to be published in 2023)
EIGA Doc 154 09 E, Safe Location of Oxygen and Inert Gas Vents
EIGA Doc 211 17, Hydrogen Vent Systems for Customer Applications
IEC 61010-1, Safety requirements for electrical equipment for measurement, control, and laboratory use - Part 1: General requirements
ISO 22734, Hydrogen generators using water electrolysis — Industrial, commercial, and residential applications (current version 2019)
ISO/TR 22734-2, Hydrogen Generators Using Water Electrolysis – Part 2: Testing guidance for performing electricity grid service (to be published in 2023)
NFPA 2, Hydrogen Technologies Code