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Water Quality: Greywater versus Tap water

Published on March 16th 2021

by Going.Local. All rights reserved

water quality
Water quality is an integral part of our lives. We are where we are today because society managed to build systems to store and purify water. As a gardener, we often hear about the importance of water quality, but what does this mean? Why is water hardness, pH and purity important to plants?

Why is water treated?

Stagnant water will recruit waterborne microbes over time (can be within 24-72 hours and is the reason you do not drink from a puddle). In other words, protozoa and bacteria will grow in the water. If you are sharp, you will note: “ But the water in a dam is stagnant and we use that for drinking water.” You would be correct. Water reservoirs can contain raw water with floating debris, clay and microbes (unless it is supplied by rivers or boreholes).
Did you know? The milky colour of some dams is due to suspended clay particles. Given enough time, the particles can settle out, but wind-based water circulation keeps them in suspension by stirring the water.
Some examples:


We are having so much rain in Pretoria. This was taken at Magnolia Dal. Its a park not a dam.


Water gushing into our Berg River dam, taken early this morning. Wonderful sight😀

The water will also react with the soil/rock as it passes through a landscape. This means under certain conditions minerals, such as calcium and magnesium, will dissolve in the water. Another example is how mountain water turns brown from humid/fulvic acids present in soil (both are known to stimulate plant growth). Before the water gets to you, it is treated to:
  1. kill microbial life (via chlorination or ozone)
  2. change the pH or taste, and
  3. make the water clear.
Did you know? The reason why mountain streams look like coca-cola (brown) is due to harmless fulvic/humic acids that are present in the soil.
water quality and plants
Some plants act like warning bells to tell you that all is not well with your water.

Water quality effects on plants

This topic is so vast that you can write a book on it. In short, if it affects your plant, then the plant will show signs of deterioration (yellowing/leaf drop). The things to look out for are pH, hardness and salts (e.g. chlorine).
  • How acid or alkaline your soil is will affect the ability of nutrients to dissolve in it. You can fertilise until you are blue in the face, but if your pH is wrong, the plant won’t be able to use it.
  • Micronutrients are less available at high pH (ex. 8), while phosphorous, calcium and magnesium are less available at lower pH (ex. 6).
  • Some plants like Ericas or blueberries will require acid soil. If you have alkaline (high pH) water, it will change the soil pH over time.
  • Hardness refers to the amount of calcium and magnesium in the water.
  • The practice of adding eggshell or oyster shell to the soil is a way of increasing calcium. This is unnecessary when your water already has a high concentration.
  • If you have hard water you might find white buildup in your kettle or borehole pump. It stays dissolved and precipitates when the temperature or pressure changes.
  • Water hardness can also be measured with a TDS meter.
  • Salts affect the way water moves. If you water your plant with seawater, the water will flow out of your plant, dehydrating it.
  • You can measure ionised salts and hardness with a TDS meter.
  • Note that your pH will affect the quantity of salts the water can hold.
  • Plants that are extremely sensitive to salts (including chloride/fluoride):
  • All orchids are affected to a degree.
Do not despair your local Department of Water and Sanitation (DWAF) facility will try to adhere to a ‘Target Water Quality Range’, which refers to a set of predetermined ranges for the above-listed parameters.
water quality crab
Animals trive in pure water, but will tend to leave or die off if contaminants enter the water.

So what about Greywater?

Many have turned to reuse their household water as water shortage becomes a reoccurring problem. Greywater can either be raw (untreated) or treated. The latter is when water facilities process the water to make it less harmful. Treated greywater can be used on golf estates and do not give off a smell. Untreated greywater is what most of us have.
Definition | Greywater refers to domestic wastewater (sink, shower, washing machine) that excludes sewage.
When diverted for garden use, greywater falls within wastewater regulations and are thus subject to several laws. It is good to familiarise yourself with these regulations as they put limitations on the amount you can store, how it is used and treated.
Greywater contains bacteria, soaps and salts that will all negatively affect plants unless diluted to a harmless level. In large quantities, it can affect an ecosystem and this is why it is normally funnelled to water treatment facilities. To learn more regarding the legalities of Greywater see this comprehensive report by Garden et al..
water quality plants water cress

Water purifying plants

In a natural setting, some species can clear water of toxic compounds. This water purification process, normally associated with wetlands, is in collaboration with bacteria in the sediments surrounding the banks, dead plant matter etc.
Researchers have identified the following plants that play an active role in purification:
These plants by no means replace conventional purification methods but are lovely proactive additions to help maintain water quality in your garden.

Take part in conserving this precious resource by sharing your tips below!

water quality river
Department of Water Affairs and Forestry, 1996. South African Water QualityGuidelines (second edition). Volume 1: Domestic Use.
Ncube, E. J., & Schutte, C. F. (2005). The occurrence of fluoride in South African groundwater: A water quality and health problem. Water SA, 31(1), 35-40.
Oberholster, P. J., & Ashton, P. J. (2008). State of the nation report: An overview of the current status of water quality and eutrophication in South African rivers and reservoirs. Parliamentary Grant Deliverable. Pretoria: Council for Scientific and Industrial Research (CSIR).
Rodda, N., Carden, K., Armitage, N., & Du Plessis, H. M. (2011). Development of guidance for sustainable irrigation use of greywater in gardens and small-scale agriculture in South Africa. Water SA, 37(5), 727-738.
Vymazal, J. (2013). Emergent plants used in free water surface constructed wetlands: a review. Ecological engineering, 61, 582-592.

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