Blackwater vs Greywater: Difference and Comparison

Water is an important resource without which we won’t be able to live. We use water every day, and most of it is stored as wastewater.


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Wastewater is divided into two parts: blackwater and greywater. Even though they are wastewater, they must be treated in different ways because of their levels of contamination.

Key Takeaways

  1. Blackwater is wastewater from toilets and contains human waste and pathogens, while greywater is wastewater from sources like sinks, showers, and washing machines.
  2. Blackwater must be treated before it can be reused or discharged into the environment, while greywater can be treated and reused for irrigation or other non-potable purposes.
  3. Blackwater is more dangerous and requires more complex treatment processes than greywater.

Blackwater vs Greywater 

The difference between blackwater and greywater is that blackwater is the wastewater that comes from the toilets, while greywater is the water that comes from washbasins, washing machines, bathtubs, and dishwashers. Blackwater is more contaminated than greywater, with more bacteria. And if freshwater is stored for more than a day, it can convert into black water.

Blackwater vs Greywater

Blackwater is the water that comes from the bathrooms and toilets or drainage systems. This water is highly contaminated and contains urinal substances band faecal matter.

Blackwater contains many bacterial pathogens responsible for spreading diseases through the faecal-oral route, which can be very harmful to human beings.

Greywater generally refers to the wastewater from a domestic household, offices, or any stream that does not contain any disease-spreading pathogens that can harm humans.

Greywater is easier to treat and handle than black water as it contains no harmful pathogens and can be reused.

Comparison Table

Parameters of ComparisonBlackwaterGreywater
Level of contamination Blackwater is highly contaminated wastewater.
Greywater is less contaminated than blackwater.
Obtained fromobtained from toilets and bathrooms.It comes from domestic household chores like cleaning or washing etc.
Usage Cannot be used again.Can be reused.
Pathogenscontain pathogens that can be harmful to humans.Does contains a less or neglectable amount of pathogens that cannot cause harm.
How to treatBlackwater can be treated by using disinfectant or chemical treatments.Greywater does not need any treatment and can be directly reused in toilets to flush or to water the plants in the garden.

What is Blackwater? 

Blackwater is the wastewater that is obtained from bathrooms and toilets, which has disease-spreading pathogens. That spread through the faecal-oral route.

This wastewater contains impurities like urine substances, faeces, and flush water from toilets.

They are considered different from greywater obtained from kitchen tools or appliances, bathtubs, and sinks apart except for toilets.

Disease-spreading pathogens are present in black water that is needed to be decomposed before letting this wastewater out in the environment.

Blackwater is also referred to the sewage water and brown water as they may contain an equal percentage of harmful pathogens in them as black water.

For treating blackwater wastewater, it needs to be treated with high chemicals containing disinfectant or biological disinfectant treatment as it is a highly contaminated liquid that can’t be allowed to be let out into the environment as it is.

In Australia, the aerated system is the most common treatment and reusing system for wastewater. The aerated system includes the following steps

  • Wastewater is aerated to help break down the organic bacterial matter, making it easy to decompose it quickly. 
  • Chlorine pellets are used as a chemical disinfectant as it destroys harmful substances making the water a little safer for the environment.

What is Greywater?

Greywater, known as grey water in the United States, is referred to as the water that is wasted and comes from domestic households or corporate working places like offices

and which does not contain any faecal substances that can lead them to be contaminated.

All the streams containing wastewater can be considered greywater except for the water from the toilets.

The waste from the washbasins, shorts, sinks, clothes washing machines, and dishwashing machines is considered to be the source of greywater.

As greywater contains less percentage of harmful pathogens than blackwater, they are a lot safer to use and a lot easier to be treated for reuse for other purposes

like toilet flushing, watering the plants or crops irrigation, and many other uses which do not require the drinking of that wastewater but they do contain some pathogens

due to washing the clothes and cleaning anal areas during bath.

The reuse of greywater in all these activities has proven to be beneficial for the water supply system and the wastewater system as by reusing it, the demand for freshwater is reduced,

which also leads to less percentage of wastewater being sent to the wastewater system for treatment.

Though greywater contains less harmful substances, there are still some ways to simplify its treatment process, and they are as follows:

  • Use less harmful chemicals for cleaning or washing clothes and dishes, and use natural biological agents instead of that.  
  • Placing a strainer on the sink in the kitchen so that the waste is left in the strainer and only the water gets out without any waste substances in it. 

Main Differences Between Blackwater and Greater

  1. Blackwater is the wastewater that is obtained from toilets and bathrooms, while greywater comes from streams of houses, offices, etc.
  2. Greywater is less contaminated than Blackwater.
  3. Blackwater contains disease-causing pathogens that can be dangerous for humans, whereas Greywater does not contain such harmful pathogens.
  4. Blackwater can only be reused as fertilizer, while greywater can be used for watering plants or flushing water.
  5. Blackwater needs water chemicals first, whereas greywater can be reused without treating it with chemicals.
Difference Between Blackwater and Greywater

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