water conservation

Greywater Recycling Basics

What it is, benefits, and system overview

What is greywater?

Greywater is the wastewater produced in households from everyday activities–except for toilet use, which is known as blackwater, aka sewage. It typically makes up 75–90% of domestic wastewater flow. Compared to blackwater or untreated municipal wastewater, greywater usually contains lower levels of microbial contamination, organics, and nutrients.

Greywater is often divided into three categories:

  • Dark greywater – Wastewater from kitchen sinks, dishwashers, utility sinks, and laundry loads used for heavily soiled items (like diapers). Dark greywater has more chemicals, fats, oils, grease, and pathogens, and in some jurisdictions it may be classified as blackwater due to its higher contamination risk.
  • Light greywater – Wastewater from bathroom sinks, showers, bathtubs, and washing machines. This type generally contains fewer contaminants.
  • Clearwater – Solid-free, relatively clean water that is gathered from heating and cooling systems, such as the ‘sweat’ from swamp coolers and air conditioners, and refrigerator compressor drip.

Greywater vs blackwater

The primary way greywater differs from blackwater is that it does not contain urine or fecal material and associated pathogens, which require more intensive treatment before being considered safe.

However, if not handled properly, greywater can quickly become as unsafe as blackwater. It decomposes at a much faster rate than blackwater, and if stored for more than 24 hours, the bacteria present consume all oxygen, leading to anaerobic conditions, where it becomes more like blackwater–foul-smelling and a health hazard.

What is greywater recycling?

Greywater recycling involves onsite capture and physical, chemical, or biological treatment of greywater for reuse in various applications such as landscape irrigation, car washing, flushing toilets, or laundry, thereby decreasing freshwater consumption. When a composting toilet is used, all household wastewater becomes greywater, making the reuse of 100% of wastewater possible.

Some greywater recycling systems can even harness the heat and gas emissions from greywater and convert it to energy.

Why recycle greywater?

Greywater recycling is an excellent strategy to mitigate overall water use. Managing water sustainably is vital to ensure continual access to safe water while addressing challenges posed by drought, climate change, urbanization, and population growth.

Greywater recycling systems are very efficient as they allow for the collection, treatment, discharge, and reuse of wastewater at the site of production and use, eliminating the need for additional energy to transport and treat water offsite. Production is also relatively predictable, in contrast with the variability of methods like rainwater collection. Greywater recycling offers numerous environmental and economic benefits.

Environmental benefits

  • Water conservation – Greywater recycling leads to a decrease in fresh water usage, helping to alleviate stress on watershed sources. Household greywater reuse for toilet flushing could save up to 30% of water usage, and in office buildings, this jumps to 60%. Reusing water for landscape irrigation can yield 40% savings.
  • Decreased potable water usage – Especially for irrigation, greywater reuse can significantly decrease the amount of treated water used in households.
  • System energy & cost savings – For municipal systems, decreased input leads to reduced energy and costs for wastewater transport and treatment. In the United States, water and wastewater treatment processes account for 35% of municipal energy use (4% of global energy consumption), and water transportation alone is estimated to require an average of four times the energy required for the actual water treatment. In the U.S., the total operation and maintenance costs of water waste treatment plants account for about 3% of electricity use, and some estimates calculate the sector’s greenhouse gas emissions to be 3% of global totals.
  • Decreased eutrophication – Wastewater contains high amounts of organic materials, which leads to excess nutrients in bodies of water. This creates ideal conditions for algae growth, which consumes oxygen in the water, leading to adverse conditions for aquatic life.
  • Potential to combat water contamination – 80% of the world’s wastewater is released untreated. In areas without adequate infrastructure, greywater recycling could be a solution to protect soil and water from harmful microbes and heavy metals. Developed areas, including the U.S. and the UK, are not immune to pollution from raw sewage, due to aging infrastructure and improper practices.

Household & economic benefits

  • Monthly savings – Less water usage = decreased utility bill.
  • Reduced strain on septic system – Since greywater makes up the majority of household wastewater, diverting it from septic systems extends their life and capacity, again saving money.
  • Develop otherwise unsuitable real estate – A greywater recycling system, along with the use of composting toilets, can enable the development of property that is unsuitable for a septic system. This is especially valuable in undeveloped parts of the world, where around 4.5 billion people still lack access to safely managed sanitation.
  • Groundwater Recharge – Greywater recycling for irrigation replenishes groundwater, helping the natural hydrologic cycle to keep functioning.
  • Drought resilience – Irrigating with greywater can support plant growth in areas that might otherwise not have enough water.
  • Maintain soil fertility – The nutrients in greywater that might otherwise contribute to eutrophication instead serve as fertilizer, promoting plant growth.
  • Enhance water quality – The quality of groundwater and surface waters are much better preserved by the natural purification processes the greywater undergoes in the top layers of the soil than by any engineered water treatment.
  • Personal satisfaction & responsibility – Taking ownership for reducing personal environmental impact can offer a sense of fulfillment.

Is Greywater Safe to Use?

The safety of greywater depends on chemical and microbial content, in addition to proper management.

What to Avoid

When recycling greywater, it is especially important to consider the ingredients and products that you are pouring down the drain. Many chemicals are associated with health risks, are not biodegradable, or contribute to ecological damage. Frankly, greywater systems or not, these chemicals still end up in groundwater, soil, and larger bodies of water, whether recycled or sent to a wastewater treatment facility, so they should be avoided in general.

There are too many commonly used chemicals to name here, so refer to Toxic Ingredients to Avoid in Everyday Products to learn which ones you want to steer clear of for your own health and that of the environment. To find clean alternatives to the products that contain concerning ingredients, make sure to visit:

Pathogen concerns and how they’re managed

Concerning microbial contamination, pathogen levels in greywater are generally reported to be considerably lower than in combined municipal wastewater and are typically similar to those present in secondary treated municipal wastewater (see Table 3) [40,41]. However, even this level of microbial contamination still entails significant public health risks while reusing untreated greywater 

Greywater may be treated with a variety of physical, chemical, and biological treatment processes. The most commonly used systems incorporate membrane bioreactors (MBRs), constructed wetlands, media filtration (sand, activated carbon), disinfection methods (UV, chlorine, ozone), and advanced oxidation processes.

Greywater recycling systems overview

Indoor greywater systems

  • Short description: How greywater can be reused for toilets, laundry, or other in-home applications.
  • Coming soon: a full guide to indoor greywater systems.

Outdoor greywater irrigation

  • Short description: How household greywater can safely irrigate gardens and landscapes.
  • Coming Soon: a detailed guide to greywater irrigation systems.

Regulations and legal considerations

  • Local Building Codes & Restrictions
  • Permits and Safety Standards
  • Resources for Checking Regulations

Choosing the right greywater system

Greywater recycling systems are best installed in new construction projects because retrofitting these systems into existing homes or buildings is a significant challenge, requiring major plumbing modifications. That being said, it is possible if desired.

Start by identifying the simplest wastewater source to divert. For many, this may be the washing machine. A laundry-to-landscape system is one of the simplest greywater reuse methods. Another option for landscape irrigation may come from a bathroom shower and/or sink.

  • Factors to Consider (climate, household size, goals)
  • Costs and Maintenance Needs
  • When to Consult a Professional

Frequently Asked Questions

Can you drink greywater?

No! Untreated greywater is not safe to drink; even with treatment, greywater should not be consumed.

Sources:

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