HEALTH AND SANITATION
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lower organic content, although the pathogen and chemical
load varies depending on the source. to this is that the volume of treated effluent being
discharged from sewage works will be reduced.
Most guidelines (including these) exclude the dark grey
water / Class III fraction (kitchen wastewater) from grey
water reuse schemes, because it can be highly polluted
with organic material, fats, oils etc. Significant amounts
— 50 to 75% of water used in a household — may be
grey water, which could potentially be harvested and
reused in a productive manner rather than being directed
to a municipal sewer. It is estimated that the reuse of
grey water for toilet flushing and garden irrigation could
reduce household potable water use by up to 50%. Since grey water may contain nitrogen, phosphorous and
potassium, it can be a potential source of nutrients for
plant growth. The use of grey water in urban and peri-
urban agriculture has in fact been shown to reduce the
agricultural requirement for application of plant nutrients,
and thus contribute to food security, particularly in poor
settlements, if cognisance is taken of the potential health
risks involved. A recent study has shown that when
grey water diluted by 50% with fresh water is used for
irrigation purposes, better, more marketable crops in
terms of yield and aesthetic appeal are produced.
Depending on the source of the grey water — i.e.
whether it comes from the laundry or bath — it may be
associated with different health risks. All water emanating
from toilets (including bidets) is faecally contaminated
and is considered to be black water; i.e. sewage. Certain
classes of grey water, in certain households, would be
better referred to as black water, for example Class II grey
water should be treated as black water if a household is
using washable nappies. In the same vein, water used for
washing babies should not be used as grey water.
WHY HARVEST GREY WATER?
As outlined in the Introduction, South Africa faces a
range of challenges with respect to water management,
specifically related to issues of water scarcity. By 2030,
parts of the country are predicted to be severely impacted
as demand exceeds available supply. Additionally, 2015
was the driest year on record in South Africa, leaving many
towns with extremely compromised water supply systems
whilst also limiting food production across the country. This
is placing pressure on the already fragile economy.
POTENTIAL IMPACTS OF GREY WATER HARVESTING
Pathogens in grey water can potentially cause diseases
through direct contact or through the consumption of
irrigated produce. Aside from these health risks, there are
several potential negative impacts associated with the
use of grey water:
•
•
•
•
To avert a future water crisis, the country needs to prevent
water wastage and ensure more efficient use. Within urban
areas, individuals and municipalities need to find ways to
adapt to, and mitigate the threats from, water insecurity
resulting from, inter alia: droughts, climate change and
increasing water demand (driven by population growth and
rising standards of living). One such measure is to harvest
and reuse grey water in a ‘fit-for-purpose manner’.
Improving water security is one reason for harvesting
grey water; another, equally important reason is
the use of grey water as a resource. Economic and
environmental benefits are derived from the use of grey
water as a productive water source within households,
as well as the reduction in wastewater loading (both
in terms of quantity and quality) achieved at sewage
treatment works — resulting in reduced pumping and
treatment costs. An added environmental benefit related
Reduced crop yield as a result of salinity, nitrogen
overload, specific ion effects (e.g. boron, sodium
and chlorides), high pH levels, and soil clogging
effects from surfactants, oils and grease,
phosphates and sodium, as well as lint, hair, etc. if
the grey water is unfiltered.
Soil degradation due to high sodium (sodicity), salinity
and other substances — in saline soil, high sodium
levels replace magnesium and calcium necessary for
plant growth and absorption of water, particularly in
sandy soils.
Groundwater contamination as a result of movement
Environmental hazards
of the grey water through the soil.
associated with grey water
Reduced flows and higher solids content causing
used for irrigation purposes.
blockages in sewerage systems.
PARAMETER EXAMPLES OF POSSIBLE IMPACT
pH Corrosion of equipment; damage to biota; changes in
biochemical processes
Electrical conductivity Reduction of plant productivity; possible changes in soil
properties
Chloride Accumulation in soil may adversely affect plants
Sodium adsorption ratio Exacerbation of soil erosion; c hanging soil hydraulic
properties; reduction of plant growth
Boron Accumulation in soil may be toxic to plants
Phosphorous Eutrophication if excess concentrations reach surface
water; bio-clogging of equipment
Surfactants Accumulation may change soil hydraulic conductivity;
plant toxicity
Oil and grease Accumulation may change soil hydraulic conductivity
Xenobiotic compounds Toxicity to biota
Source: Maimon et al., 2010
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July 2018 Volume 24 I Number 5