What are the differences between blackwater and greywater? And why is it important to separate them?
From most residential and commercial buildings, outgoing water lines head straight to a wastewater treatment system, be it a septic tank or sewer. In some situations, outgoing water lines are plumbed for blackwater and others for greywater. What’s the difference? And why keep them separated?
The Differences
The main difference between blackwater and greywater is whether waste and potential pathogens are in the mix.
Blackwater comes from your toilets and kitchen sink. It contains urine, feces, food particles, and remnants of leftovers, says Angela Bounds, wastewater industry professional and co-host of Orenco Systems’ “Make It Clear” podcast.
“Blackwater would also come from a sink with a garbage disposal,” she says, “and your dishwasher, because it could expel food particles.”
Greywater comes from your other plumbing fixtures that don’t normally expel pathogens, like sinks and showers, as well as your washing machine. “We actually have more sources for greywater,” notes Shawn Rapp, training specialist and co-host of “Make It Clear.”
The Treatment Train
Because of their different constituents, blackwater and greywater have different treatment requirements.
Blackwater goes through the traditional wastewater treatment train – a sequential, multistage process where effluent passes through preliminary, primary, secondary, tertiary, and other treatment technologies to remove pollutants, pathogens, and nutrients before it is released into the environment or reused.
Greywater requires mechanical filtration to remove hair and other solids, sedimentation to settle out any contaminants like soap, and biological treatment for microscopic filtration, followed by disinfection via ultraviolet light, chlorine, or some other means.
“You’re going to filter out large particles or impurities in the greywater using a physical filter,” Angela explains. “You also need sedimentation time to allow the settling out of particles, as well as biological treatment where naturally occurring bacteria decompose any organic matter.”
Depending on state regulations, it can then be reused for sub- or surface irrigation, toilet flushing, commercial vehicle washing, and more.
The Separation
Why would property owners want to separate their blackwater and greywater? Three reasons: water conservation, water health, and soil health.
Consider the environmental impact when deciding whether to have both black-water and grey-water egresses, Angela says. Plumbing a separate line for greywater means less effluent overall is going into the treatment system. Plus, rather than using fresh water to irrigate a lawn or flush toilets, the system uses treated effluent, resulting in water conservation.
“Separating greywater is a way to reduce the demand on freshwater resources,” she says. “If you don’t have to use fresh water to water your garden or flush your toilets, you’re saving gallons upon gallons of water by using greywater.”
Water and soil health also benefit from separation of black and greywater. By removing the greywater from the treatment train, the drainfield will be able to absorb more black-water effluent without showing damage to the soil.
“Your soil can only take so much,” Angela says. “So, the more that you can protect that soil and not put contaminants into it, the more longevity you’re going to get out of it.”
Something to Consider?
Greywater separation and reuse isn’t for everyone, but it is a way to protect the world’s water – the groundwater, surface water, and waterways – while reaping the benefits of reduced septic system load, healthier gardens, and gallons of water saved.
“The whole reason Orenco exists is to protect groundwater and waterways,” Angela says. “It’s critical to dispose of blackwater appropriately, and the reason is to prevent water pollution.”
Still curious about blackwater and greywater? Listen in to Orenco’s “Make It Clear” podcast, “Talking About Greywater and Blackwater” or “Resource Conservation: Reusing Wastewater.” Find it on Orenco.com/resources/podcasts or from your favorite streaming provider.