Stormwater

Stormwater is mostly rainwater running off impermeable surfaces in urban areas, including roofs, sidewalks, streets and parking lots. It is drained from urban areas by sewers or open channels to avoid local inundation. During this process, stormwater becomes polluted and its discharges into receiving waters cause environmental concerns. A schematics of runoff generation and pollution is shown in Fig. 3.5.

Fig. 3.5 Flows of water and pollutants in stormwater systems

Stormwater may be transported either by combined sewers, together with domestic and industrial wastewaters, or by separate sewers discharging to the nearest stream or lake. In combined sewers, high stormwater inflows exceed the pipe capacity and excess flows have to be diverted by flow regulators as combined sewer overflows (CSOs) to the nearest receiving waters. CSOs contain not only the stormwater, but also untreated wastewater and sewer sludge; their direct discharges into receiving waters cause serious pollution problems.

The stormwater contribution to the wet weather flow reaching the wastewater treatment plant also increases the concentrations of heavy metals and other contaminants in the WWTP effluent and in the sludge (biosolids). This is one of the main reasons why the use of sludge from European WWTPs as a fertiliser in agriculture

has been questioned. So, which sewer system is better – the separate system in which polluted stormwater may be discharged directly into the receiving water or the combined system, in which the stormwater is conveyed to the WWTP for treatment, but CSOs occur? There are no general answers to this question, as the definite comparisons of separate and combined systems performance depend on local conditions. The separate sewer system could be improved by implementing separate stormwater treatment plants, but that may be costly and inefficient in view of highly variable infrequent inflows with low concentrations of pollutants. Similarly, the combined systems can be improved by incorporation of CSO pollution abatement.

3.3.2.1 Stormwater characterisation

The literature on urban stormwater quality is very extensive. So far, more than 600 chemicals have been identified in stormwater and this list is growing. Makepeace et al.

(1995) identified about 140 important contaminants, which can be found in stormwater and would affect human health (i.e., mostly through contamination of drinking water supply) and aquatic life. This list contains solids, trace metals, chloride, nutrients (N and P), dissolved oxygen, pesticides, polycyclic aromatic hydrocarbons, and indicator bacteria. Typical concentrations of many such constituents were reported in the literature, mostly for developed countries. Summaries of data from two large databases appear in Table 3.4.

Stormwater quality data were also reported for less developed countries, but usually in small data sets. Examples of such data include those from Sao Paulo, Brazil (cited in Tucci, 2001), Johor, Malaysia (Yusop et al., 2004), Bandung, Indonesia (Notodarmojo et al., 2004) and Beijing, China (Che et al., 2004). In general, such data indicate significantly higher concentrations than those in Table 3.4, which may be caused by problems with infrastructure, such as cross-connections between storm and sanitary sewers. In any case, such data suggest that pollution loads conveyed by storm sewers in developing countries are larger than indicated by the literature data published for developed countries.

Table 3.4 Quality of urban runoff and combined sewer overflows: stormwater worldwide data (Duncan, 1999), U.S. NURP stormwater data (U.S. EPA, 1983) and European CSO data (Marsalek et al., 1993).

Urban stormwater

Elevated pollutant concentrations (compared to the data in Table 3.4) are observed during periods of snowmelt, when the pollutants accumulated in snowpacks are rapidly released and conveyed by storm sewers to the receiving waters (Viklander et al., 2003).

In older separate sewer systems, urban surface runoff is conveyed by storm sewers to the nearest receiving waters without any control or treatment. Only during the last 30 years, has stormwater management been introduced and practised by reducing runoff generation by allowing more rainwater to infiltrate into the ground, balancing runoff flows by storage and providing some form of runoff quality enhancement. Among the main pollutants of concern in stormwater, one could name suspended solids, nutrients (particularly P), heavy metals, hydrocarbons, and faecal bacteria.

3.3.2.2 Stormwater management

As a result of high discharges of stormwater, and pollutant concentrations and loads conveyed by stormwater, and their potential impacts on the environment, alternative techniques have been developed for stormwater management during the last several decades (Azzout et al., 1994; Baptista et al., 2005; Parkinson and Mark, 2005; Schueler, 1987; Urbonas, 1994), including the following:

x infiltration facilities, x ponds and wetlands, x swales and ditches,

x oil and sediment separators, and x real-time control operation systems.

Infiltration may be applied in the so-called percolation basins which are specially designed as underground gravel units. This technology has been used for a very long time particularly on a small scale in rural settlements. Only during the last few decades it has been further developed and used in urban areas on a larger scale. Stormwater infiltration helps keep the groundwater table at a natural level, which promotes good conditions for vegetation and a good microclimate. The construction costs of drainage systems with infiltration facilities are also cheaper than those of conventional systems.

Infiltration is also implemented on grass or other permeable surfaces, and in drainage swales and ditches. The use of this measure is steadily growing in many countries.

Ponds and wetlands have become in many countries a common and accepted means of attenuating drainage flows and treating stormwater by removal of suspended solids, heavy metals and, to some extent, nitrogen and phosphorus. The cost of construction and operation of such facilities is often low compared to the environmental benefits.

The sediments from ponds may contain high concentrations of heavy metals. However, ponds and wetlands should be considered as stormwater treatment facilities and not as natural water bodies, even if they often provide aesthetic values to the urban area.

Swales and ditches are applied commonly in the upstream reaches of drainage to control runoff flows and provide runoff quality enhancement. Flow control is obtained by stormwater infiltration into the ground, quality enhancement by filtration through the turf, solids deposition in low flow areas, and possible filtration through a soil layer.

Oil and sediment (grit) separators are used to treat heavily polluted stormwater from highways or truck service areas, or where polluted stormwater is discharged into sensitive receiving waters. The efficiency of these units in trapping oil, sediments and chemicals attached to the sediments is often poor, because of under-sized units or lack of flow-limiting devices preventing the washout of trapped materials.

Real-time control operation of sewer systems has been developed during the last two decades and implemented in some Canadian, European, Japanese, and U.S. cities. The applications are often in combined sewer systems and the purpose is mainly to reduce combined sewer overflows and/or overloading of wastewater treatment plants, by the maximum utilisation of the dynamic capacities of the system (Colas et al., 2004).

3.3.2.3 Special considerations for drainage in cold climate

In countries with a cold climate (i.e., occurrence of freezing temperatures over periods of several months) the precipitation falls as snow during a significant part of the year.

When the snow is cleared from streets in the cities, it is either brought to local snow dump sites, or to a central deposit site outside the city, or it is dumped into watercourses. When the snow melts, the meltwater runs off in the same way as stormwater. However, the impacts may be more severe due to the following facts:

x Snowmelt may generate high flows, causing surcharging of the sewer systems and possibly flooding in the receiving waters.

x The meltwater often has higher concentrations of heavy metals, sand and salt than stormwater (sand and salts being used for de-icing of urban roads and streets).

x The impacts of urban snowmelt on streams, lakes and ponds may be exacerbated by ice covers of such water bodies and densimetric stratification. High salt

concentrations and oxygen depletion were noted at a number of locations (Marsalek et al., 2003).

Various technologies have been adopted for treating urban meltwater. Examples are ponds, oil and grit separators, and infiltration facilities. They all have to be designed and operated with considerations of the special effects of temperature, ice and snow conditions, and the elevated pollutant concentrations (Viklander et al., 2003).