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The major principle for treatment of urban and highway runoff in wet detention ponds is sedimentation of suspended particles. However, several other processes occur to an unknown extend; i.e., plant uptake of dissolved pollutants, sorption of dissolved matter and colloids to surfaces, as well as flocculation of fine particles and colloids. The latter processes act on the dissolved and colloidal fraction of the stormwater pollutants, which are the most mobile in the aquatic environment and consequently possess the highest risk of causing adverse effects.

Each demonstration facility incorporates the unit operations of sedimentation, filtration and treatment by aquatic plants as mentioned above. In addition, the treatment will be extended with one of the following technologies:

The image below illustrates the different treatment technologies. You can click on the image to read more.

Treatment of stormwater runoff by sedimentation relies on the fact that a significant part of the pollutants occurring in stormwater is associated with particles that can be settled out of the water column. In order to achieve good treatment efficiency, the suspended particles must therefore be able to settle out of the water column during the dry periods separating the rain events. Accordingly, the treatment efficiency is largely determined by the settling velocity of the particles and the hydraulic retention time of the pond. During large rain events, the runoff water flows through the pond resulting in low treatment efficiencies.

The concentration of particulate matter in stormwater runoff is generally low. The distance between stormwater particles is therefore large, and settling in the pond occurs as free, gravitational settling. Under such conditions, the settling velocity of the particles is primarily governed by their shape, density and size. However, the settling velocity in actual systems is also affected by the hydraulic conditions governed wind and flow.

Aquatic plants
Emergent aquatic plants are often integrated in the design of wet detention ponds, but also where plants have not been actively included in the design, emergent as well as submerged plants tend to colonize a pond. The aquatic plants fulfill numerous purposes with respect to pollutant removal, and their combined effect is predominantly beneficial. In order to achieve good treatment efficiency, aquatic plants should therefore be actively included in the design of wet detention ponds.

The picture below illustrates a shallow region of a wet detention pond that has been colonized by emergent aquatic plants.

Both submerged and emergent aquatic plants contribute to an enhanced treatment, as they are able to take up dissolved pollutants as well as provide fixed surfaces for adsorption of fine particulate (colloidal) material. In addition, the aquatic plants can be actively integrated into the design of the pond thereby achieving an overall impression of the wet detention ponds of a natural aquatic habitat with a diverse flora and fauna.

Filtration of the effluent from a wet detention pond in a fixed media filter - such as a sand filter - is an efficient method for retaining particles. During the filtration process, particles are deposited on the surface of the filter creating a filter cake. This filter cake will typically have a much lower hydraulic conductivity than the filter medium and therefore controls the overall hydraulic capacity of the filter.

Sorption to iron-enriched bottom soil
Investigations of the phosphorus cycle in shallow lakes have shown that high concentrations of iron in the bottom soil can efficiently control the concentration of dissolved phosphorus in the water column. The main mechanism responsible for these observations is adsorption of phosphate ions onto precipitated iron (oxy)hydroxides (primarily FeOOH and Fe(OH)3) as illustrated in the image below.

By deliberately blending e.g. ferric hydroxide into the bottom soil of wet detention ponds, the retention of phosphorus can be significantly enhanced. The retention of heavy metals may also be significantly enhanced, as several of these are strongly associated with iron (oxy)hydroxides.

It is important that only ferric iron, which is stable under oxic conditions, will efficiently control the phosphorus concentration. If anaerobic (no dissolved oxygen present) conditions occur, the iron (oxy)hydroxides may be destabilized thereby releasing sorbed phosphorus into the water column. It is therefore important to maintain aerobic conditions throughout the water column and the upper sediment layers. This is achieved by designing the pond with a low water depth resulting in a large surface area through which oxygen is adorbed (aeration) per volume. I.e., a deep pond is much more likely to become anaerobic than a shallow pond.

Coagulation/flocculation by aluminum addition
The addition of aluminum salts has been practiced for restoration of eutrophic lakes in terms of phosphorous removal from the water column and immobilization of phosphorous in the lake sediments. Also for ponds, the addition of aluminium has been found effective.

Addition of aluminum salts to the bulk water produces aluminum hydroxide (Al(OH)3), which is highly insoluble and therefore precipitates forming relatively large settlable flocs. Similar to the iron (oxy)hydroxide, the aluminum hydroxide flocs have a high sorption capacity for both phosphate and heavy metals. However, the aluminum flocs are not destabilized during anaerobic conditions, which is the case for iron (oxy)hydroxides. However, high pH values (> 8.5-9) will increase the solubility of the aluminum hydroxides, which thereby releases the sorbed pollutants into the bulk water. Such conditions are likely to occur if the primary production becomes excessive e.g., during algae bloom. The reason for this is the plants uptake of dissolved carbon dioxide during growth, which increases the pH.

Fixed media sorption
Various pollutants may effectively be removed by adsorption in a fixed media filter. Which pollutants are retained by such a filter depends largely on the chemical composition and structure of the sorption media. For example, materials containing calcite (CaCO3) or dolomite (CaMg(CO3)2) like marble, limestone, dolomite rock and different types of shells from marine organisms have proven efficient in removing especially phosphorus. Similarly, various organic materials and materials containing iron or aluminum oxides provide efficient absorption of heavy metals.

The selection of the best-suited filter media depends on several parameters such as the sorption capacity and the rate with which pollutants are adsorbed. The first parameter determines the expected lifetime of the filter and the latter determines the necessary contact time between the filter medium and the water. Possible detrimental effects resulting from the filtration process should also be considered. An example could be the pH increase due to filtration through a limestone filter.

Prior to filtration in a fixed media filter, the concentration of particles in the runoff water should be reduced e.g. by sand filtration. Hereby, the risk of clogging the fixed media filter will be reduced and its expected lifetime extended.

The image below shows shells from marine organisms. This type of media is used in the facility in Odense.

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