On behalf of Tyréns AB, Swedish Hydro Solutions was tasked with proposing suitable purification options for the water generated during the cleaning process of the Swedish Transport Administration's tunnels. The assignment included collecting water samples during an ongoing wash of Gnistängstunneln in Gothenburg and conducting bench trials to explore appropriate treatment techniques for the wash water.

A site visit to Gnistängstunneln took place on June 27, 2016. During the visit, the site and the equipment used in the washing process were also examined. The group had the opportunity to discuss and ask questions to the personnel involved in tunnel washing. Water samples were taken directly from the suction truck collecting water during the washing process after a few hours of operation. The on-site personnel assessed that the sample taken was representative of the type of water generated in the washing process.

The collected water samples were then taken to the Swedish Hydro Solutions laboratory, where they were studied to investigate how the wash water could be treated.

Prerequisites

During tunnel washing, wash water is generated. The amount of water created varies for different tunnels. The main factors determining the amount of water created include the tunnel's size and the type of wash performed. Approximate amounts of water from tunnel washes in the Gothenburg area can vary between 40 – 180 cubic meters per wash. Various pollutants, including fine particles, heavy metals, nutrients, oils, fats, and other organic pollutants, end up in the wash water.

Another factor in tunnel washing is the detergents used during the washing process. These chemicals can have an environmental impact and may also affect the water treatment process. When the contaminated wash water is discharged, it can cause damage to the surrounding flora and fauna, including fish deaths and other ecological issues. In light of this, authorities impose requirements for discharged water to meet certain criteria.

The city of Gothenburg has compiled these, along with guidelines, in a document: Environmental Management Guidelines for the Discharge of Polluted Water to Receptors and Stormwater (City of Gothenburg, 2013).

Treatment alternatives

The concentration of suspended fine material in tunnel wash water is normally very high. As a first step to prevent the discharge of these fine particles to the receptor, the water is currently directed to collection basins for natural sedimentation. According to information from Tyréns AB, this sedimentation, however, does not significantly improve water quality. Consequently, the majority of fine particles, pollutants bound to fine particles, and dissolved pollutants are further released to the receptor, the Göta River, if the wash water is not collected or purified.

To meet discharge requirements, it will likely be necessary to implement a method to increase the sedimentation rate. This can be achieved through chemical precipitation and flocculation, where chemicals are added to the water to aggregate smaller particles into larger ones, making them heavier and settling faster. Filtration can be an effective method to further improve water quality.

However, it is usually very ineffective to rely on filtration without first removing the bulk of fine particles with the preceding flocculation and sedimentation step. It is generally challenging to achieve sufficient purification efficiency, and the filters quickly become saturated with particles, leading to high costs for filter replacement and a heavy workload for operation and maintenance.

Flocculation

The alternatives for chemical precipitation/flocculation and sedimentation available in the Swedish market today require high competence and complex equipment for accurate dosing/application. They also pose risks of negative environmental impact and health hazards for personnel handling them. For the current project, it is proposed instead to use a method with passive dosing of chitosan.

The chitosan technology was originally developed in the state of Washington in the USA, where it has been shown to be very effective over more than 20 years. It has been used in thousands of water treatment projects.

Swedish Hydro Solutions has developed the technology further, resulting in lower costs, simpler operation, and better reliability. The technology is designed to meet regulatory requirements and handle the high flow rates that can occur during water treatment from construction and construction projects, as well as from industrial stormwater and process water.

Swedish Hydro Solutions' specially formulated chitosan-based flocculating agents are made from a naturally occurring substance extracted from crab shells. The substance is completely biodegradable and is a highly potent flocculating agent. In the USA, water treatment with chitosan is classified as "Best Management Practices" (BMP) for stormwater treatment. BMPs are methods approved by the state government to meet the federal Clean Water Act (State of Washington, 2012).

The treatment system works by pumping contaminated water into a dosing unit. In the unit, the water flows over "bio-socks" containing Swedish Hydro Solutions' specially formulated chitosan. This is passively dosed into the water. The bio-socks have a specially designed fabric that allows controlled dosing of the right amount of chitosan. Further adjustments to dosing can also be made in the purification unit. Each sock weighs 0.5 kg and can purify up to 200 m³ of water (depending on dosing) before it needs to be replaced.

After the water has passed through the bio-socks, it enters a sedimentation system where fine particles flocculate, and sedimentation occurs.

BioMedia® Filtration

Swedish Hydro Solutions' BioMedia® filter is a biological material made from by-products of the forestry industry (tree and brush residues) and has very high treatment efficiency for dissolved pollutants such as heavy metals and organic pollutants.

Results

A significant visual improvement in water quality was observed for each stage of the treatment process, see Figure 1 below. The untreated water had an almost black color and a high amount of suspended fine particles.

The turbidity for the water sample was measured at 402 NTU. After chitosan treatment and sedimentation, a large amount of settled particles could be observed, and the sample had changed color to a more yellowish hue. Turbidity had been reduced to 51 NTU. After BioMedia filtration, no visible particle content remained, and the water was completely clear. Turbidity in the filtered water was 0.9 NTU.