Sustainable Water Treatment Solution for Railway Maintenance Operation

At a train maintenance depot in southern Sweden, process water is generated from train washing and sanitation operations, including removal of graffiti and contamination from operational incidents. These activities result in wastewater containing elevated levels of heavy metals such as copper, zinc, chromium and nickel as well as high concentrations of oil and organic contaminants.

The facility previously operated an existing treatment system; however, the final treatment stage was insufficient to consistently meet local discharge requirements. Pilot testing confirmed that key parameters, particularly copper and zinc, exceeded the allowable limits defined by ABVA (General Provisions for the Use of Public Water and Sewer Systems).

In addition to achieving compliance, the solution needed to ensure stable operation under varying wastewater compositions, including complex mixtures of oil, particles, and dissolved contaminants. The system also had to be integrated into an existing installation while maintaining continuous operation during the upgrade process.

To address these challenges, the existing chemical-based treatment stage was replaced with a custom-designed SRE system. The solution consists of two electrocoagulation reactors combined with an integrated OptiFloc™ flotation unit, enabling efficient separation of contaminants from the water. Wastewater is collected and treated in a multi-step process including filtration, electrocoagulation, flocculation, and flotation.

During electrocoagulation, metal ions are released into the water through an applied electric current, destabilizing emulsified oils and dissolved contaminants. This promotes rapid coagulation and formation of flocs. These flocs are subsequently separated in the flotation unit, supported by microbubbles generated during the electrochemical process.

Compared to conventional chemical treatment, the electrocoagulation process reduces the need for chemical handling and dosing, while providing a more controlled and efficient removal of contaminants. To ensure optimal removal of metals such as zinc and copper, controlled pH adjustment was incorporated into the system. This is a critical factor for achieving effective precipitation of dissolved metals and ensuring compliance with discharge requirements.

The project included dismantling of existing equipment, installation and commissioning of the new system, as well as a temporary SRE unit to maintain compliant operation during the transition period.

SRE is based on electrocoagulation. Direct current releases iron or aluminum ions into the water, causing contaminants to coagulate in a manner similar to traditional chemical coagulation with polymer addition. The benefit of electrocoagulation compared to chemical coagulation is that the need for neutralization is reduced and it does not add any chloride ions to the solution. Uniform release of coagulants makes the process fast and efficient.

This results in a more environmentally friendly alternative compared to conventional chemical coagulation. As the wastewater passes through an electric field, surface charges on oil droplets and colloidal particles are neutralized, destabilizing emulsions and improving separation. During the process, hydrogen gas is generated, supporting flotation and separation of oil and other contaminants.