Liquid-phase technologies involve the treatment of a wastewater stream to control the release of odour and corrosion-causing compounds from the stream. They involve the addition of a chemical to the wastewater to either control the formation of odorous compounds or react with those compounds once they are formed. Liquid-phase treatment is most commonly applied in wastewater collection systems, not treatment plants. It often is applied in a collection system for downstream control at the headworks of a treatment plant. Since liquid-phase treatment controls the odour compounds in the wastewater itself, it provides corrosion control in addition to odour control.
With liquid-phase treatment, hydrogen sulphide is prevented from escaping the liquid into the vapour, therefore it is not present to cause corrosion on process structures, pipe crowns, etc. Through proper application of a liquid-phase treatment in a collection system, multiple odour release points such as manholes, air relief valves and re-pump stations can be controlled through one chemical application point.
Iron salts such as Ferric Chloride are applied to wastewater to oxidize and/or precipitate dissolved sulphide. Ferrous salts precipitate sulphide as ferrous sulphide. Iron salt solutions are classified as hazardous compounds and often require double-wall tankage and piping systems.
Magnesium hydroxide (MHL) raises the wastewater pH to a level above 9.0 and thus prevents the transfer of hydrogen sulphide from liquid to gas phase. As the sewer odour and corrosion problems are directly related to the gas phase hydrogen sulphide concentration, the lower gas phase H2S concentration prevents the occurrence of the problems. MHL is a safer chemical than iron salts and therefore does not require the same standards for storage and dosing.