4.3 Stormwater Treatment
4.3.4 Specific considerations during design and for construction
The storage capacity of the wetland should be determined by hydraulic modelling, based on the maximum of tolerated overflows in a given time. This gives the storm event to be stored and treated, e.g., the monthly, annual or decennial event, the stochastic occurrence of events and their intensity, the runoff patterns generated by these events and the throttled outflow of the wetlands. The treatment capacity needs to be adapted to the pollution loads and runoff patterns specific to the catchment, considering first flush effects.
In some cases, an additional storage volume can be provided for water which does not have to undergo full treatment, and in all cases, at some point excess water has to be evacuated by overflows. The design should, however, always ensure that the most polluted part of the runoff (often, but not always the first flush) is properly treated.
Conditions for nitrogen transformation in FWS wetlands are more effective when the permanent water level is shallow enough (approx. 0.3 m depth) to allow sufficient oxygen exchange. Floating Treatment Wetlands can be used in zones with higher depths, e.g., to retrofit ponds or in concrete tanks, or when space is limited. It is, however, important that the design favours hydraulic conditions without shortcuts.
Local plant species with extensive root growth into the water column should be used which remove fine particles and dissolved substances by sorption on biofilm forming on the roots.
Bigger,“end of the pipe”VF wetlands for stormwater runoff treatment can be designed like those for combined sewer overflow treatment (Chapter 4.4), as shown in Figure 4.1 below, although removal of
Figure 4.1 Cross section of VF wetland with storage volume on top of the filter surface and throttled outflow, as used for stormwater and CSO storage and treatment.
filtered sediments can play a minor role due to lower organic loading. Storage volumes on top of the filter level can be designed between 0.3 m and 1.0 m or even higher for less frequent immersions (once per month or less). Bioretention filters usually have a shallow freeboard of less than 0.4 m.
Recommended filtration velocities compatible with a good treatment efficiency can be up to 5×10−5m/s, which means throttling the outflow at 0.05 L/(s · m2) (Grotehusmannet al., 2016b; Molle et al., 2013). They should be 1×10−5m/s if pathogen removal is required, but placing an UV-lamp for pathogen removal at the outflow of the VF wetland is often preferred instead of the slower filtration velocity.
Recommended filter material for VF wetlands treating stormwater runoff is fine to coarse sand (d10
between 0.2 and 0.5 mm). Finer sand is more efficient, especially for ammonia removal, but coarse sand is less prone to clogging. For bioretention filters, not throttling the outflow, a sandy loam is the recommended filter material (e.g., Woods-Ballard et al., 2015). Over time, a secondary filter layer forms on top of the surface layer from the retained solids which provides additional sorption capacity, and which will increase the filtering efficiency. Phosphorous removal can be enhanced by reactive media, but it has to be considered that the reactive media will be saturated at some point and the efficiency of P-abatement will, therefore, decrease over time, limiting the lifespan of the reactive media. In Germany, it is considered that the addition of a few percent of iron hydroxide to the mass of the filter material can allow for a lifespan of 50 years (Grotehusmann et al., 2016b).
As most of the treatment efficiency is based on filtration and sorption on fixed biofilms, the depth of the filter material is of lesser importance, and a depth of 30 cm of sand layer can be considered satisfactory in most cases (Molleet al., 2013). A depth of 0.5 m to 0.75 m is recommended in Germany (Grotehusmann et al., 2016b). Deeper filters can have a higher adsorption capacity for ammonia and, if reactive filter material is used, for phosphorous.
Generally, the required filter area is between 0.5 and 2% of the impervious catchment area for bigger VF wetlands with sandy filter material and a throttled outflow, 4–8% for FWS wetlands, and up to 6% for bioretention filters. Too frequent flooding of the filter surface and/or too long periods to drain down the filter after a rain event can result in a lack of oxygen for the aerobic degradation of the pollutant load during the dry period, resulting in a reduced treatment efficiency, and, more importantly, possible clogging of the filter. Hence, dimensioning of the filter can be based:
• On the annual load of fine solids: Grotehusmannet al.(2016b) recommend a maximum annual load of 7 kg/m2fine solids (,0.063 mm)/(m2· yr);
• On the time the filter needs to drain after the storm event (24–48 h; see Grotehusmannet al., 2016b;
Molleet al., 2013);
• On the cumulative annual load which is used in older German guidelines, such as DWA-M 178 (2005), which recommends dimensioning VF wetland for stormwater runoff on the basis of a cumulative hydraulic load of 40–50 m (=m3/(m2· yr)) and a maximum of 70 m/yr. However, Grotehusmannet al.(2016b) only recommend having a minimum filter surface of 100 m2per ha of active catchment area if the annual rainfall exceeds 1000 mm.
In climates with frequent rainfall, it should be considered to divide the filter surface into two parts, which would be used alternately on a weekly basis for the more frequent, but less important rain events. In case of the less frequent but important rain events, the design should allow the entire filter surface and the entire storage volume to be used.
In climates with long dry periods, the treatment design needs to be functional even after extensive phases without rainfall. This can be partly overcome by a saturated layer in the lower parts of the filter, which provides a hydraulic reserve for the plants. In that case, intermediate passive ventilation is required
above the saturated layer to allow for gas exchange when the surface of the filter becomes quickly flooded.
However, the biofilm in the unsaturated layer degrades during long dry phases, thus reducing the treatment efficiency for dissolved pollution.
As in the case of CSO systems, plant species used for VF wetlands must be able to cope with low nutrient supply and long-lasting phases without loading, followed by hydraulic shock loading.