Difference Between Blackwater and Greywater

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. 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.

Blackwater vs Greywater 

The main 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.

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Blackwater is the water that comes from the bathrooms and toilets or drainage systems. this water is highly contaminated and contains urinal substances band fecal matter. Blackwater contains a high amount of bacterial pathogens that are responsible for spreading diseases through the fecal-oral route which can be very harmful to human beings.

Greywater generally refers to the wastewater that comes from a domestic household, or offices, or any kind of stream which does not contain any disease-spreading pathogens that can tend to harm humans. Greywater is easier to treat and handle than black water as it does not contain any harmful pathogens and can be reused again.

Comparison Table Between Blackwater and Greywater 

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 fecal-oral route. This wastewater contains impurities. Like urine substances, feces, and flush water from toilets. They are considered to be different from greywater that is obtained from kitchen tools or appliances, bathtubs, sinks apart except for toilets.

Disease-spreading pathogens are present in black water, that are 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 it break down the organic bacterial matter for making it easy to decompose that matter 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 knew as graywater 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 fecal substances that can lead them to be contaminated. All the streams that contain wastewater can be considered greywater except for the water that comes 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 the washing of the clothes and cleaning of 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 instead of that, use natural biological agents.  
  • 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 waters.
  5. For using Blackwater, it needs waters chemicals first whereas greywater can be reused without treating with chemicals.


In conclusion, Blackwater and greywater, both are wastewaters. Blackwater is from toilets and is highly contaminated, while greywater comes from kitchen sinks, bathtubs, washing machines, etc., and is less contaminated. Though blackwater is highly contaminated, it can still be treated with chemical disinfectant to prevent its harmful effects and reuse as a fertilizer for plants.

While greywater can be used for watering the plants without any treatment. Blackwater contains pathogens that carry harmful diseases that can be fatal whereas greywater does not contain such harmful pathogens. Therefore, black water is considered to be more of a danger to human beings than greywater.


  1. https://ascelibrary.org/doi/abs/10.1061/JEEGAV.0001177
  2. https://iwaponline.com/wst/article-abstract/64/12/2417/15498
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