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The ultimate home grey water treatment

Grey water sign with tapThe re-use of grey water (from your laundry, bath and kitchen sink) for garden purposes is a worthwhile consideration. As the amount of water available to us shrinks in the face of our expanding population, 're-use' is fast becoming a necessity. Click on the Re-use of grey water for the processes involved.

In order to re-use grey water we need to clean it to regain (as near as practically possible) its original composition and natural quality.

How to do it

Grey water has to undergo two key measures before it is ready for re-use, namely:

  1. Removal of fats and contaminants; and
  2. Replacement of dissolved oxygen, much of which has been driven out by heating.

Grey Water System

Typical grey water installation

For more information, advice,  and to order please contact:

Tel: 021 786 1601

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For ultimate success BIO-SYSTEMS suggests these measures be included in the following protocol:

1.    Pre-treat kitchen wastewater. Removal of fats and contaminants

Preparation and wash up for an average three-course meal generates 50-65g per person of fat that literally goes 'down the drain'. Kitchen wastewater should pass through an efficient Under Sink Particle Interceptor (USPI) – a small stainless steel box – to strain out all food scraps and permit emulsified oils to float to the surface for easy daily removal.

USPIs (also called 'fat traps') come in two stock sizes:

  • Model 500 is ideal for home and small retail/bar use. It measures 500mm long x 300mm wide and 400mm deep. It has a single basket that collects solids and must be emptied every day.
  • Model 700 is for larger homes, restaurants and supermarkets etc. It is 700mm long x 400mm wide x 400mm deep and has twin baskets with staged perforations.

For rural systems that discharge to a septic tank, these interceptors are essential.

By simply dosing the sink waste every second evening with BIO-SYSTEMS FogFree  and using BIO-SYSTEMS HSDG (as directed) to clean inside the basket and interceptor lid, your grease trap will never smell – neither will any of the drain points. During busy family visits or for commercial installations, a 25g soluble pouch of BIO-SYSTEMS FogFree powder in the interceptor once a week will help to ensure your wastewater is much cleaner and easier to re-use.

Under sink particle interceptor   Under sink particle interceptor

2.    Divert wastewater

Diverting grey water from going into the underground outfall sewer can be a problem. No two houses are the same. However, in fifty percent of cases it is a relatively easy task that can be attempted by the average 'handy' homeowner.

Double storey home uses are usually pretty straight forward, but where bathrooms are below natural ground level, problems may be encountered. One possible solution is to open up the line and form a wet sump (a small plastic tank or a brick box) fitted with a small electric pump that will fill with water and can then be pumped out automatically. Most older suburban houses are fitted with three pot grease gulleys that receive separately via 40mm pipes from the bath, basin and the shower. It is usually quite simple to redirect this water into a new, larger diameter pipe that will run to your treatment/storage tank.

A good tip is to fit an old stocking over the drain outlet/s from the shower, bath and basin just before they discharge into the 'gulley'. This will catch any stray solids (hair, food scraps etc.) and can be replaced every month or when they get greasy.

3.    Install treatment/storage tank

Ideally, storage capacity for a minimum of four days' wastewater is required. This means a 1,000 litre tank (or larger if you have the room) per resident. For ease of management, the tank - which can be made of concrete, polypropylene or a plastic lined depression in the ground - should be accessible over its entire surface to enable the periodic removal of scum and cleaning etc. The top of the tank is ideal at waist height for ease of maintenance, but you need a snuggly fitting lid to keep the leaves, insects and mosquitoes out. This said, cylindrical 'roto-moulded' tanks standing on one end are more convenient for the suburban dweller. They are cheaper, take up less floor space and are more readily available via hardware outlets. They can be disguised easily using trellis, fence staves, creepers etc.

The larger the tank the better. A pair or group of gravity flow interlinked smaller tanks can also be used especially on larger properties where you need more capacity – here you could run rain water off the gutters into the system too.


For larger smallholding setups it is best for the tank to be split into two chambers by an 'underpass' baffle that reaches 150mm short of the floor (for more information Contact BIO-SYSTEMS SA).

4.    Mount AirMix Aerator Injector. Replace dissolved oxygen

Inside the treatment tank, running on either a submersible or a surface-mounted 'well point' pump (while only a 0.375-0.50kW pump may be required, it is often easier and cheaper to obtain a more popular 1hp unit as these are usually standard stock items and you can gain from the economy of scale), the AirMix Aerator unit circulates the water over a pressure gradient that draws in atmospheric air infusing it into the effluent in micro-fine bubbles that stimulate the latent and seeded bacteria in our Grey Water Additive (GWA) product to proliferate. Airmix Aerators are supplied by Wassertec Ozone Systems 021 7882343 /

Drip irrigationThese bacteria are there already, but in insufficient quantities so you will need to add a level teaspoon of BIO-SYSTEMS GWA mix per 500 litres occupied volume every week. This will help remove the COD (emulsified fatty substances) by metabolization and thus prevent anaerobic conditions, controlling the formation of odour. The result is re-usable water that is relatively 'clean' and does not smell.

Fast fact:
The added bonus is that the AirMix Aerator agitates the water surface, preventing mosquito larvae from developing.

For more information on how to use grey water in your garden click on Drip irrigation.

We Do Not Install Grey Water Systems.

When considering the installation of a grey water system, it must be understood that there is no 'one size fits all'.

This is because:
  • No two properties are the same;
  • No two gardens are the same;
  • Some houses are on sloping ground;
  • Some houses are on flat ground; and
  • Many other reasons along the above lines.

To overcome these problems, we offer the following solutions:

For the DIY homeowner an explanatory Installation Manual, which is comprehensive and explains the principles that can then be interpreted to your specific conditions and limitations. This is sent via email in pdf format.

The Manual sets out the principles and method in simple terms and is intended as a guide for the average handyman/homeowner. You will need to buy your own pump, tanks and piping.

For a complete list of BIO-SYSTEMS products click on our Product table.

Click here to go back to Re-use of grey water.

Go to Septic Tanks and Soakaways and Pit latrines for more on the domestic disposal system.
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