A modern wastewater treatment facility with a setting sun in the background

When it comes down to water usage and conservation, there’s no shortage of terminology. Blackwater, greywater, and recycled water are all types of wastewater that have a secondary use after their primary purpose has been fulfilled. 

This article will take a closer look at the differences between grey, black, and recycled water.

Greywater is used water that comes from showers, kitchens, and other household uses. Blackwater contains human excrement, such as water from a toilet. Recycled water is wastewater that has been filtered to remove organic and chemical substances and is safe for secondary use.

Now we’ll look more in-depth at the distinctions between these types of recycled water. Let’s start from the beginning. 

Differences Between Greywater, Blackwater, and Recycled Water

In a typical scenario, purified water is pumped by the municipality into homes for consumption. After consumption, the water loses its purity, gets contaminated, and becomes wastewater.

The following table summarizes the differences between greywater, blackwater, and recycled water.

CharacteristicGreywaterBlackwaterRecycled water
SourceShowers, bathtubs, washing machines, and sinksToiletsFactories, raw sewage, and stormwater runoff
CompositionContains remainders of bath, cosmetic and cleaning products, grease, hair, and dead skinContains human excrement, sanitary towels, and rubber condomsContains herbicides and personal care products
Usage after treatmentIrrigationNon-drinking purposesDrinking and household use such as laundry and showering
Level of contaminationFew contaminantsHigh level of contaminantsFew contaminants

Let’s break down these differences between grey, black and recycled water in more detail.

Wastewater Source

Wastewater can be classified depending on its source or previous use. For example, greywater, blackwater, and recycled water all come from different sources, which we discuss below.

What Is Greywater?

Greywater is used water that comes from the washing machine, sink, dishwasher, bathtub, or shower. It can be used for irrigating plants, flushing toilets, or even as a drinking water source when filtered thoroughly. 

A jar of dehumidified and greywater side-by-side on a table
A greywater sample on the left. Courtesy of Wikipedia.

Greywater contains several kinds of contaminants, including:

  • Hair
  • Soap and other household chemicals
  • Dead skin
  • Grease
  • Salts
  • Industrial chemicals

These contaminants are relatively easy to extract with a greywater treatment system and natural filters such as activated charcoal powder. Greywater is vital in mobile homes and areas where water is scarce. Compared to blackwater, greywater is a much safer option for landscaping applications.

What Is Blackwater?

Blackwater is the water you flush down the toilet. It has higher levels of contamination than greywater because it’s been in direct contact with hazardous human waste. 

Some contaminants in blackwater are:

  • Feces
  • Urine
  • Plastic waste
  • Rubber condoms
  • Grease runoff
  • Sanitary towels
  • Bacteria

The primary contaminants of blackwater are feces and urine. Therefore, it’s not suitable for any reuse before treatment. In addition, stringent local and state health regulations are often set when reusing blackwater since untreated human waste risks both human health and the environment.

The fact that you cannot reuse blackwater has more to do with its properties than its appearance; while it may not look particularly wholesome, there are several effective ways to use it after treatment without posing a risk to humans or the environment.

What Is Recycled Water?

Recycled water is produced when wastewater from factories, raw sewage, and stormwater runoff is filtered and cleaned to a high standard.

Unlike grey and blackwater, recycled water goes through a carefully monitored treatment process that removes impurities. Then, you can use it for drinking, irrigation, or industrial applications. After treatment, recycled water is said to be recycled because it’s been used before.

An outstretched arm pouring a glass of water on to one of two plants placed next to each other

Despite efforts to safeguard recycled water’s purity, some substances of concern have been speculated to exist in recycled water. 

These contaminants include:

  • Water disinfection by-products (N-Nitrosodimethylamine)
  • Herbicides
  • Personal care substances, e.g., galaxolide 
  • Phthalates

Municipals use a multi-stage water treatment process that removes as many impurities as possible. Once the water has been purified, it’s distributed throughout local areas via a network of underground pipes for consumption.

Wastewater Composition

Greywater contains soap, toothpaste, laundry detergents, hair, body oils, and chemicals from soaps, shampoos, and cosmetics. Laundry detergents high in sodium and phosphorus are the most significant pollutants in greywater. 

Blackwater comes from toilets and contains human excrement and used toilet paper. 

For the most part, the recycling process removes the impurities in recycled water. Recycled water contains more dissolved salts and nutrients than undissolved matter. Homeowners can use this water for various non-internal consumption uses, such as landscape irrigation.

Usage After Treatment

Depending on the level of contamination, wastewater can be used in various ways. For starters, greywater can water plants during the dry seasons. Plants can easily filter out the soap and other contaminants in the water.

Blackwater contains solid waste that must be filtered out before the water can be treated and reused. This waste is not readily soluble and can negatively impact the environment and the health of those who may come into contact with it.

Recycled water is safe for domestic use, such as laundry, dishwashing, and showers. Recycled water is filtered and treated to get rid of waste and contaminants. Different states have various guidelines on how to handle wastewater. 

A plate floats vertically atop water

Level of Contamination

Greywater contains fewer contaminants than blackwater, making it easier to treat and reuse. However, in some state codes, kitchen water is classified as blackwater because it may contain disease-causing microorganisms.

Blackwater has high levels of contaminants as it contains feces and urine. Therefore, treating and purifying blackwater requires the proper equipment and handling. 

Recycled water is clean but may contain tiny amounts of contaminants or none, depending on the treatment process. This water may even be safe to drink after thorough purification.

Treatment Processes for Black, Grey, and Recycled Water

Reusing untreated wastewater can result in health complications and environmental pollution. There are different ways to purify wastewater to ensure it is safe for use.

Ariel view of a various pools in a blackwater treatment facility

Blackwater Treatment Process

Wastewater containing fecal matter and urine flows to a tank where it can settle for 24 hours. During this time, a colony of bacteria feeds on the solid particles and breaks them down. 

After settling, the wastewater flows into a three-chamber tank.

The first chamber is the aeration tank. Here, air and water are added to assist the bacteria feeding on the tank’s sludge.

The wastewater then flows to the second chamber, where the heavy sludge settles at the bottom. The third chamber is the irrigation chamber, where the water is treated with chlorine. 

You can reuse treated water for non-drinking purposes.

Recycled Water Treatment Process

Recycled water contains chemical and biological contaminants and is treated in different stages.

Pre-Treatment Stage

In this stage, sewage water flows through a vertical bar screen that removes large solid particles. Next, the water flows through a grit tank, where sand particles and pebbles settle at the bottom. 

Next comes a settling tank where more particles settle at the bottom to form a sludge. Finally, fine scum particles float to the top. Once the scum has been removed, the water is clarified.

Secondary Treatment Stage

In this stage, the clarified water flows through an aeration tank where aerobic bacteria break down biological matter. The water then flows to a clarifier, where the activated sludge settles—both sludges from the settling tank and clarifier flow to where anaerobic bacteria decompose the sludge to produce biogas.

In the final tertiary treatment process, the water is treated with chlorine and then piped, ready for consumption.

Greywater Treatment Process

Greywater can be filtered using a simple homemade filtration system.

Diagram of a DIY Greywater System
This DIY greywater system uses wood and gravel as a filtration medium. Courtesy of Climate Biz.

A basic homemade water purification system is made up of the following filter layers: 

  • A top layer consisting of humus-rich topsoil sits on a fine sand layer. 
  • The fine sand sits on a layer of course sand. 
  • A pea-shingle layer rests at the system’s bottom for efficient drainage. 

Treatment facilities can also use membrane bioreactors (MBR) and ultrafiltration technology to purify greywater. Membrane bioreactors combine biological treatment and physical membrane barriers to make the filtration process more efficient. 

Ultrafiltration removes microorganisms, although it doesn’t remove salts or fluoride dissolved in the water.


Wastewater is a resource that we must manage effectively to avoid creating new problems while solving old ones. No one knows for sure what the future holds, but one thing remains clear—wastewater will become even more critical as the human population grows. 


Leave a Reply

Your email address will not be published. Required fields are marked *