Ponds and settling basins are common treatment systems for road runoff in Sweden. The treatment occurs primarily by sedimentation and pollutants are entrained in the sludge. This performs satisfactory for larger particles that will settle during the targeted residence times, but not for smaller particle fractions. These fractions tend to stay in suspension as the drag forces acting upon them are stronger than the gravitational force. From a water quality perspective, these smaller fractions are important to remove, as they present a much larger surface area for pollutants to adsorb onto.
Coagulation/flocculation is a treatment method that increases the particle’s settling potential by counteracting the natural forces stopping particle aggregation. This increases the removal efficiency and gives rise to precipitation reactions that may reduce the amount of dissolved organic material and metals.
Electrocoagulation is primarily used today as a treatment for industrial waters and is a development of coagulation where instead of adding chemicals an equivalent reaction is attained using electrodes in an electrolysis cell. There exists a number of interesting benefits with the use of electrocoagulation, such as a lower energy demand, a more stable sludge, a higher removal efficiency due to redox reactions, and emulsion breaking properties.