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2. Wastewater and stormwater characteristics (Topic a)

Household wastewater derives from a number of sources (Figure 2.1). Wastewater from the toilet is termed 'blackwater'. It has a high content of solids and contributes a significant amount of nutrients (nitrogen, N and phosphorus, P). Blackwater can be further separated into faecal materials and urine. Each person on average excretes about 4 kg N and 0.4 kg P in urine, and 0.55 kg N and 0.18 kg P in faeces per year. In Sweden it has been estimated that the nutrient value of urine from the total population is equivalent to 15 - 20 % of chemical fertiliser use in 1993 (Esrey et al., 1998). Table 2.1 shows characteristics of human excreta and a comparison with nutrient contents of plant matter to indicate its value as a soil conditioner and fertiliser.

Figure 2.1: Sources of household wastewater, showing wastewater from toilet, kitchen, bathroom, laundry and others

Greywater consists of water from washing of clothes, from bathing/showering and from the kitchen. The latter may have a high content of solids and grease, and depending on its intended reuse/treatment or disposal can be combined with toilet wastes and form the blackwater. Both greywater and blackwater may contain human pathogens, though concentrations are generally higher in blackwater.

The volume of wastewater and concentration of pollutants produced depend on the method of anal cleaning, volume of water used and water conservation measures. Dry anal cleaning results in higher solids and fibre content. The use of dry pit latrines and the practice of water conservation produce low volume and high concentration wastewater, while use of flushing toilets results in higher wastewater volumes and lower concentrations. The characteristics of wastewater in the Regions are described in the Regional Overviews under Topic a.

The flow of wastewater is generally variable with peak flows coinciding with high household activities in the morning and evening, while in the night minimal flow occurs. Pollutant loads vary in a similar manner.

Stormwater in a community settlement is produced from house roofs, paved areas and from roads during rainfall events. In addition stormwater is produced from the catchment of a stream or river upstream of the community settlement. The amount of stormwater is therefore related to the amount of rainfall precipitation, and the nature of surfaces, with impervious surfaces producing more run-off. During a storm event the peakflow is higher and duration shorter with an impervious surface, while the peakflow is lower and duration longer with a vegetated surface (Figure 2.2).

Stormwater run-off may contain as much solids as household wastewater depending on the debris and pollutants in the path of the stormwater run-off, although in general the pollutant load of stormwater is lower than that of wastewater. Table 2.2 shows a comparison of urban stormwater sources and untreated sewage in North America.

Figure 2.2: Rainfall runoff relationship showing two different surfaces (impervious and natural) (lager image)

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