2. Wastewater and Stormwater
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
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
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
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.
Table 1: The waste management hierarchy
Step 1: Prevent or reduce waste generation
Step 2: Reduce the toxicity or negative impact of the waste
Step 3: Recycle waste in its current form
Step 4: Reuse waste after further processing
Step 5: Treat waste before disposal
Step 6: Dispose in an environmentally sound manner
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.