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2. Wastewater and Stormwater Characteristics Household wastewater derives from a number of sources (Figure 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 was equivalent to 15–20 % of chemical fertiliser use in 1993 (Esrey et al., 1998). This represents a considerable potential resource that is generally underutilised.
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 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 as well as the nature of the surface. Vegetated surfaces slow the rate of runoff to stormwater and also allow rainfall to penetrate the soil whereas impervious surfaces do not and therefore produce more run-off. During a storm event, the peakflow of stormwater is higher and duration shorter with an impervious surface, while the peakflow is lower and duration longer with a vegetated surface. 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. The environmental impact of wastewater and stormwater can be substantial. Solids in both wastewater and stormwater form sediments and can eventually clog drains, streams and rivers. Grease particles form scum and are aesthetically undesirable. The nutrients N and P cause eutrophication of water bodies, with lakes and slow moving waters affected to a greater degree than faster flowing waters. In the former the algae that are fertilised by the nutrients, settle as sediment when they decay. The sediment acts as a store of nutrients and regularly releases the nutrients to the water column, thus the cycle of bloom and decay of the algae is intensified. In the early stages of eutrophication, aquatic life is made more abundant because fish, for example, graze on the algae. With too high a concentration of algae, the decaying algae contribute to the biological oxygen demand (BOD) and the water is deoxygenated. Thus wastewater, which has been treated to reduce BOD but still high in nutrients, can still have a significant impact on the receiving water. In addition, some algae produce toxins that can even be fatal to humans and animals. Eutrophic water adds to the cost of water treatment, when the water is used for drinking purposes. Other pollutants in wastewater and stormwater are heavy metals and possible toxic and household hazardous substances. Heavy metals include copper, zinc, cadmium, nickel, chromium and lead. The content and concentration are dependent on the pipe materials employed to convey drinking water, household-cleaning agents used, and for stormwater, the type of materials used for roofing and guttering. In high enough concentrations these heavy metals are toxic to bacteria, plants and animals, and to people. Toxic materials may also be disposed with household wastewater. These could be medicines, pesticides and herbicides that are no longer used, as well as excess solvents, paints and other household chemicals. These substances can corrode sewer pipes and seriously affect the operation of treatment plants. They will also limit the potential of water reuse, and therefore should not be disposed with household wastewater. Spills of chemicals, particulates from motor vehicle exhausts and deposition of atmospheric pollutants can similarly contaminate stormwater. These pollutants will affect downstream receiving waters, and treatment systems if the stormwater is treated. Wastewater and contaminated stormwater can contaminate groundwater. This is through infiltration of the wastewater or stormwater through the soil to an unconfined groundwater aquifer. Soil can filter some pollutants, but soluble pollutants (e.g. nutrients and heavy metals) and very small particles (e.g. viruses) travel with the water to the groundwater aquifer. 2.1 Integrated Waste Management Integrated Waste Management, as the name suggests, refers to the practice of considering wastewater, stormwater and solid waste management as inextricably linked. This is in contrast to the practice of viewing each waste stream as independent and separate from the others. Critical to wastewater and stormwater management are solid wastes and wastewater produced by industry. In many instances these may not differ in characteristics from domestic wastes, consisting primarily of biodegradable organic substances. Industry, however, produces numerous types of wastes that may be toxic to the bacteria that are utilised to treat domestic wastewater. The practice in many communities is for industrial wastes to be disposed with domestic wastes and this often leads to problems. One principle that logically emerges from adopting an integrated approach to waste management is that different types of waste should not be mixed. Solid wastes should not be dumped into stormwater drains, but should be collected, recycled, reused, or treated and disposed separately. Dumping of solid wastes in stormwater drains will not only restrict the flow of stormwater, but also contaminate stormwater. Treatment of the stormwater will involve separating the solids and other contaminants from the water. Similarly industrial wastes should be treated separately, and industrial wastewater should be pre-treated if they are to be discharged to the sewer.
A useful tool that can help towards achieving integrated waste management is the waste management hierarchy. It has been used to direct waste management towards achieving environmentally sound practice. The waste management hierarchy in its most general form is shown in Table 1. In using this tool for waste management we systematically go down the list to see if step 1 (Prevent or reduce waste generation) can be implemented, before considering the next step (2) and so on. Only when steps (1) to (5) have been fully considered that we consider disposal of the waste (step 6). 2.2 Sustainable versus Unsustainable Wastewater and Stormwater Management In nature, waste materials are produced by living organisms (plants, animals and people). These wastes include faecal materials, leaf litter, food wastes and dead biomass. Yet streams and rivers flowing through a pristine forest, or freshwater lakes in a forest, have generally an excellent water quality. There are natural processes that purify the naturally produced wastes and provide a basis for determining environmentally sustainable management practices for wastewater and stormwater. Discharge of wastewater and stormwater into an environment exceeding the natural purification capacity of that environment will result in the accumulation of organic materials (carbon), nitrogen, phosphorus or other pollutants that cannot be absorbed by the ecosystem constituting the receiving environment. Accumulation of organic materials will result in a high oxygen demand that cannot be met by oxygen transfer from the atmosphere and anaerobic conditions result.
In figure 2, the nitrogen and phosphorus in wastewater are discharged to a river resulting in their accumulation in the river, leading to eutrophication. The nitrogen and phosphorus in the wastewater come from food consumed by people. To grow this food fertilisers containing nitrogen and phosphorus are required. These are manufactured chemically from atmospheric nitrogen and from phosphate rock. The flow of materials (N & P) is one way from the atmosphere for N and from the phosphate rock mine for P into the river. There is depletion of a resource (mined phosphate rock) and accumulation and pollution in the river. This practice is unlikely to be sustainable in the long term, because phosphate rock deposits will be exhausted and pollution of the river by N and P needs further treatment of the wastewater.
In a sustainable wastewater management system, nutrients in the wastewater are reused to grow food. In this way there is not the need to use as much chemical fertiliser and at the same time, there much less discharge of nutrients to the river. The problem of resource depletion and pollution of the river is overcome by closing the material cycles. Figure 3 also emphasises the need to treat industrial wastewaters containing toxic substances separately, and not to mix industrial wastewaters with domestic wastewater. In addition stormwater should be separately collected and treated and infiltrated locally.
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