23.2.1 Stormwater Quality in Urban Areas

The stormwater quality of industrial areas is highly dependent on the activities at the industrial site and measures taken to prevent emissions. Not many measurements are published since companies are cautious about generating bad publicity. Basic knowledge of stormwater quality from surfaces in commercial and residential areas can give a quick insight regarding substances and concentrations that can be found in stormwater. In paragraph 23.2.2, special attention to concentrations from industrial areas is given, but must be regarded as a rough indication since the stormwater quality is highly dependent on the specific at the industrial site.

The quality and characteristics of stormwater can strongly differ per country, location and even between and during stormwater events [1]. International data of stormwater quality from USA, Australia, and Europe is given in Table 23.1.

To give an indication of the content of databases, a short description of the Dutch stormwater quality database is given, one of the largest databases in the world. For the Dutch situation, research monitoring data was collected over a 15-year period (the earliest measurement in the database is from 1999) from more than 60 municipalities and over 200 locations throughout the country. The total number of individual events included in the database now is 8,300. The national database of all collected stormwater monitoring data allows for a scientific analysis of the data and information and recommendations for improving the quality monitoring. Each data set has gone through a quality assurance/quality control review based on reasonableness of data, extreme values, relationships among parameters, sampling methods and a review of the analytical methods [3].

Most data on the characterization of stormwater quality (contaminants concentration, particle size distribution of suspended sediment, fraction bound to suspended solids) was found by sampling stormwater during rainfall. Most of the samples were analyzed in certified laboratories according to standard methods and standard quality control/assurance procedures.

Preferably data from well-described stormwater research sites were used for the database (peer reviewed journals). In addition the following information was entered: Aim of the research, site descriptions (state, municipality, land use components), and sampling information (date, season, sampling method, sample type) with links to the original research reports and articles.

23.2.2 Industrial Stormwater Quality

Figure 23.1 shows stormwater concentrations from several industrial sites in the Netherlands. In this research, categories were separately analyzed according to national guidelines on environmental management. The categories are determined by emission of particles, odor, noise and possible risk for the surrounding area. Category 1 includes office buildings, Category 2 are areas with car showrooms, parking areas, garages. Category 3 includes industries dealing with construction and demolition waste or galvanizing. Categories 4, 5 and 6 are the most dangerous Categories with steel factories or oil refineries. Of course the data show a lot of variability within the different industries, but it was concluded that the categories could be a first indication of the stormwater quality from industrial sites. On most parameters, categories 1 and 2 are cleaner than the Category 3 and Categories 4, 5 and 6. Average concentrations of heavy metals such as nickel (Ni) and copper (Cu), and nutrients (Nkj) exceed the MAC (Maximum Acceptable Level) concentration and therefore treatment of the stormwater is advised.

23.2.3 Fraction of Pollutants Attached to Particles

Treatability of stormwater runoff by sedimentation depends on the degree of pollutants bound to particles. Distribution between dissolved and particle-bound pollution load can be determined by comparing the total concentration in samples with the filtered sample (0.45 µm).

Figure 23.2 shows the average values of pollutants bound to suspended solids in stormwater from roofs and roads in residential areas. The plus and minus give the range of the data values, which indicate a large variability in the ability of pollutants to bind to suspended solids. The dot gives the typical average value found throughout the world, which was taken from comparable international studies [10].

From Figure 23.2, the pollutant behavior can be derived. Nutrients are less bound to particles than most of the heavy metals and PAH and, therefore, harder to retain than other contaminants. Within a certain pollutant group, such as metals, the individual pollutants have their own specific behavior. The average Dutch research results are similar to the average from international data [3]. Figure 23.2 also gives an indication of the maximum removal efficiency rate that can be achieved by using settlement devices. To get a detailed insight of the removal efficiency, knowledge of particle size distribution of suspended sediment in stormwater is required, in order to find out which particles can be captured by settlement facilities.

As heavy metals are bound in the order of 65% (lead up to 90%), a higher removal rate with settlement basins (removal only of suspended solids and not solved pollutants) should not be expected, but is rarely determined in the field. When an 80% removal rate is needed to achieve the WFD goal for copper, then it is unlikely that this quality standard will be achieved with sedimentation basins only since copper is bound for 65% (average) to suspended solids. Therefore, additional purification systems to filtration are needed, such as adsorption or phytoremediation of pollutants in sustainable urban drainage systems such as constructed wetlands. Information on the particle size distribution is also an important part for a detailed insight into treatability of stormwater.

23.2.3.1 Particle Size Distribution

To obtain detailed information on the achievements of settlement and filtering of sustainable urban drainage systems, an examination of particle size distribution is advised. Measurements at several locations around the world were taken to determine the particle size distribution. The results are given in Figure 23.3. The particle size distribution highly varies with each different stormwater drainage location. To get a general idea; roughly half of the mass consists of particles smaller than 90 µm. These fine particles will hardly be removed by settlement facilities [11] and need filter or adsorption systems such as constructed wetlands.

23.2.4 Removal Efficiency

The removal efficiency of constructed wetlands for stormwater derived from existing monitoring results differ from study to study, but are mostly within the ranges of international literature. Not many monitoring results of wetlands at specific industrial areas are available, but the removal efficiency is expected to be within this range as from residential and commercial surfaces.

If we compare Figure 23.4 with Figure 23.2 (distribution of pollutants in Dutch stormwater), we can see a correlation between the removal efficiency and the amount of bound particles. Nutrients are less bound to particles and have lower removal efficiency than heavy metals. The removal efficiency of constructed wetlands derived from existing monitoring results differ from study to study, but are mostly within the ranges of international literature.

23.3.1 Amsterdam Westergasfabriekterrein

In this paragraph, the horizontal wetland in Amsterdam in Westergasfabriekpark, namely, a closed water system in a park is discussed, this being one of the most polluted industrial sites of the Netherlands.

At the end of the 19^th century, the Imperial Continental Gas Association (ICGA) built a gas factory complex in Amsterdam. By the time the factory shut down, the site was heavily polluted, making it difficult to find a new purpose for the area. The redevelopment of the site demanded an integral approach and much care was given to the (polluted) water system. The water system in this transformation from a “polluted no-go area” to a multifunctional park needed to be self-sufficient, in order to prevent spreading of polluted (ground-)water to the areas around it. In order to accomplish this goal, the water is stored and purified in a water system with constructed wetland. A small area is used as a bathing area, which is filled with drinking water [11]. The constructed wetland area is one of the key elements of the water system to purify the water in the park that is intensively used for recreation. The Westergasfabriek is regarded as a model for redevelopment, far beyond the Netherlands' borders (Figure 23.7).

23.3.2 Constructed Wetland Oostzaan: Multifunctional High Removal Efficiency

In the industrial site Brombach in Oostzaan, a horizontal wetland is implemented. Unique is the end filtration step with Lava and Olivine (Mg,Fe₂SiO₄) before the water is discharged to the surface water. Doubt about the effluent quality from this industrial site was the main reason for the implementation of this high removal efficiency wetland (Table 23.2). The wetland is created as a multifunctional wetland to increase the value of the district for ecology and recreation, next to water storage and water quality improvement, and risk management of possible calamities at industrial sites (Figure 23.8).

23.3.3 Constructed Wetland Hoogeveen, Oude Diep

Stormwater from an industrial area is collected and treated in the horizontal flow constructed wetland. The stormwater is treated alongside surface water from an agricultural area. The total wetland area is 60,000 m² (Figure 23.9).

The residence time of the water in the filter is not exactly known, but is estimated to be around 20 days. The main goal of the filter is the treatment of storm water and sewerage overflows to reduce the pollution of the surface water “Het Oude Diep”. From 2011 to 2016 the filter was monitored. Every month samples were taken and analyzed. The results are shown in Table 23.3. Note that horizontal flow constructed wetlands can show negative removal efficiencies due to: measurement method (grab samples), emissions from top soil layer or vegetation, feces from birds or other wildlife etc.

The resulting removal efficiencies of compartments 1 and 2 are shown in Figure 23.11.

The results show that the Hoogeveen filter mainly removes metals (aluminum, cobalt, copper, iron, nickel, lead, vanadium and zinc). The removal of nitrogen and phosphorus are low to negligible, but it has to be kept in mind that the sewerage overflows are not monitored effectively. The results are in general in line with international research results on removal efficiencies. As shown in Figure 23.4, nutrients are less bound to particles and in most cases have a lower removal efficiency than heavy metals. Note that the removal efficiency of constructed wetlands derived from existing monitoring results differ from study to study, but are mostly within the ranges of internationa lliterature.

23.3.4 Cost

The average monetary cost of implementation of vertical flow drainage wetlands are usually in the order of 50–100 €/m² field area. A large part of the costs is used for the filling of the filter (1 m³/m²) and earth movement. Surface flow wetlands usually require much lower costs of implementation. Usually the costs are 10–20 €/m² field area. The costs of the reeds are an important variable: 15 €/field area. The construction costs of several wetlands are described in Figure 23.10.

The average cost of implementation of the vertical flow wetlands was in the order of 55 €/field area, in contrast to the cost of the surface flow wetlands (15 €/field area).

23.3.5 Choosing Best Location(s) of Wetlands on Industrial Areas

Since the stormwater of industrial areas can be polluted, constructed wetlands or other purification methods are implemented as end-of-the-pipe measures at drainage outlets. Surface flow of stormwater on the surfaces of the industrial sites should also be taken into account: at times of intensive rainfall (which will become more frequent due to climate change) or calamity situations during firefighting. These polluted surface stormwater flows can potentially be an environmental disaster to soil, groundwater and or surface water. In order to map these storm water flows, a Digital Elevation Model (DEM) or quick scan can be performed to select the right location(s) for a purification method such as constructed wetlands (Figure 23.12).

Most quick scans such as CLOUDS (Calamity Levels Of Urban Drainage systems) are based on only the following readily available data [15]:

• Accurate DEM (Digital Elevation Model) with four points per square meter and a vertical accuracy of several centimeters; this provides an insight in the surface elevation.
• GIS-map with houses, streets and waterways.

The resulting maps show the expected water depths for cloudbursts and the main stream lines of the above groundwater flow during intensive rainfalls or firefighting.

23.4.1 Innovative Determination of Long-Term Hydraulic Capacity of Wetlands

The hydraulic capacity and clogging of wetlands in the long-term are hardly monitored. A new innovating monitoring method to determine the hydraulic capacity of wetlands is full-scale testing. In such a test, a small constructed wetland or bioswales is filled with a certain amount of water, where the detention time is measured. A review of horizontal wetlands and wet bioswales that have been functioning in the Netherlands for more than 10 years showed that the hydraulic discharge and infiltration capacity can still be sufficient without maintenance (Figure 23.13). Most of the small horizontal wetlands and wet bioswales will treat the water within 1 day, emptying the storage capacity for a new stormwater event. In some cases, the construction is altered to induce the detention time when higher removal efficiency was needed.

23.4.2 Innovating Monitoring of Removal Efficiency and Eco-Scan

When the removal efficiency needs to be improved, more knowledge-based systems are needed. In order to gain a better understanding of spatial issues in constructed wetlands (e.g., thickness of deposits and water quality parameters), an innovative monitoring tool can be applied to determine the water quality and, in addition, the ecological state of the wetlands. The tool is a semi-autonomous underwater drone (Figure 23.14). The drone is equipped with sensors for pressure (depth), temperature, conductivity, nitrate, ammonium, dissolved oxygen and turbidity. In addition to the data from the sensors, the drone can also collect video images, which are used for eco-scans. The ecoscans show detailed information on the development of organisms and vegetation in time from which positive and negative developments may be derived in order to optimize the constructed wetland. The 3D detailed information of water quality, instead of a single grab sample, shows an in-depth investigation of the wetland and possible improvement areas in the wetland.

The underwater drone proved to be a cost-effective tool and gave a quick insight into the spatial variation of selected performance parameters. As a side effect, the drone provides video footage of the underwater ecology and biodiversity. These drones can be navigated to areas within the constructed wetland that are usually omitted in monitoring, thus extending the knowledge on the wetland.

23.5.1 Conclusions

Regarding the characteristics of stormwater quality and aims for removal efficiency for achieving water quality goals in Europe, purification methods based on only settling as the primarily treatment process, will not be able to achieve the required removal efficiencies. An additional stormwater treatment step with filtration or adsorption will be necessary such as that provided by constructed wetlands. The removal efficiency of constructed wetlands derived from existing monitoring results differs from study to study, but is mostly within the ranges of the international literature. Nearly all the results come from input–output studies.

The average monetary cost of implementation of vertical drainage wetlands were in the order of 55 €/field area in contrast to the cost of the horizontal wetlands (15 €/field area).

The hydraulic capacity and removal efficiency are in most cases not monitored because of the lack of budget. Cost-effective innovative monitoring can be acquired. Two examples are regarded in this chapter. Aquatic drones can be a cost-effective solution providing spatial variation of quality and video footage of biodiversity. The hydraulic capacity of small-scale constructed wetlands and bioswales can be monitored by full-scale testing. In the research it was found that there was a wide variation of the infiltration capacities, mostly leading to a detention time of less than 2 days.

23.5.2 Recommendations

• In most cases the constructed wetlands were shown or perceived to be effective. Long-term performance, however, remains an issue. New guidelines should be set up for the design, implementation and maintenance for Dutch wetlands.
• DEM models are advised that show the main stream lines of the above groundwater flow during intensive rainfalls or firefighting in order to choose wisely the location(s) of new constructed wetlands.
• Good examples of BMPs are needed for international knowledge exchange (see BMPs of constructed wetlands on industrial areas in this chapter).
• Use of interactive communication tools is recommended to show the benefits (and not highlighting only the risks) of constructed wetlands and to obtain more engagement from stakeholders (Figure 23.15).
• Data and videos from constructed wetlands are available in an online tool (www.climatescan.nl). This tool is available for all, and everybody is encouraged to add functioning SUDs to this public database.