Quality Of Sewage

Household wastewater derives from a number of sources. Wastewater from the toilet is termed ‘blackwater’. Blackwater has a high content of solids and contributes a significant amount of nutrients (nitrogen, N and phosphorus, P). Blackwater could be additionally separated into faecal materials and urine. Every person on average excretes about 4 kg N and 0.4 kg P within urine, and 0.55 kg N and 0.18 kg P in faeces per year. Table 2 shows characteristics of human excreta and a comparison with nutrient contents of plant matter to indicate its value as a soil conditioner and fertiliser.

  Table 2* : Human Excreta – Per Capita Quantities and Their Resource Value

*Adopted from Strauss, M. (1996) Health (pathogen) considerations regarding the use of human waste within aquaculture. In: A. Schönborn and C, Recycling the Resource, J. Staudenmann,. Etnier (Eds.), Proceedings of the Second International Conference on Ecological Engineering to Wastewater Treatment, School of Engineering Wädenswil- Zürich, September 18-22, in year of 1995, Transtec Publications, Zurich, pp. 83-98.

Greywater consists of water from washing of clothes, from showering / bathing and from the kitchen. The latter might have a high content of solids and grease, and it’s depending on its intended reuse/treatment or disposal could be merged with toilet wastes and form the blackwater. greywater and blackwater both might hold human pathogens, by concentrations are commonly higher in blackwater.

The volume of wastewater and concentration of pollutants produced depend on the volume of water used and water conservation measures. The use of dry pit latrines and the practice of water conservation produce low volume and high concentration wastewater, although use of flushing toilets results in higher wastewater volumes and lower concentrations. 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 same manner.

Stormwater in a community settlement is produced from house roofs, paved fields and from roads during rainfall events. Further stormwater is produced from the catchment of a stream or river upstream of the community settlement. The amount of stormwater is, thus, assocaited to the amount of rainfall precipitation, and the nature of surfaces, along with impervious surfaces generating more run-off. In During a storm event the peakflow is higher and duration shorter along with an impervious surface, although the peakflow is lower and duration longer within a vegetated surface. Stormwater run-off might hold as much solids as household wastewater depending on the debris and pollutants within the path of the stormwater run-off, although in common the pollutant load of stormwater is lower than that of wastewater. Below Table 3 shows a comparison of classic urban stormwater sources and untreated sewage.

                   Table 3: Comparison of the Characteristics of Stormwater Sources and Untreated Sewage