Greywater vs Black Water: Understanding the Difference

To manage water in modern buildings, it’s important to know where waste goes. Experts sort wastewater and stormwater into two main types based on where it comes from and how risky it is. Learning the difference between greywater blackwater, rainwater, and stormwater is the first step to reducing municipal water demand and reducing wastewater discharge and stormwater runoff.

Defining On-Site Water Sources

Greywater comes from baths, showers, laundry, and bathroom sinks and is considered relatively clean. It does not include water from toilets, urinals, or kitchen. Greywater is easier to treat and reuse mainly because it has a lower level of organic matter (BOD) and suspended solids (TSS). Likewise, greywater has a lower level of pathogens and requires lower log reduction for bacteria, viruses, and cysts to reach safe non-potable usage levels.

Safe Practices for Water Reuse

Reusing water can save money and protect an important resource. Greywater is the easiest type to recycle on-site. Treated greywater can be used for flushing toilet, spray irrigation, and cooling tower makeup, among other uses. Untreated, greywater can only be used in subsurface watering applications such as “laundry-to-landscape”.

Many commercial buildings have systems that collect and clean greywater. These systems use filters to take out debris and use methods like ultraviolet light to kill germs. Reusing black water is less common because it needs complicated treatment to make it safe. Many building owners choose greywater recycling since the rules are simpler with less capital cost for treatment. On the other hand, blackwater treatment does not require dual piping for collection and saves on this cost while providing a higher daily volume.

Rainwater and Storm Water Sources

Rainwater is another option for on-site water use. Water that falls on roofs is very clean and often tests as potable even before treatment. It is the cleanest type of stormwater. Other types of stormwater runs off parking lots, landscape (really any surface) picks up oil, trash, chemicals, and a high level of pathogens. As such, rainwater needs less treatment to reach any given water quality target, be it for non-potable or potable use.

You can treat collected rainwater, stormwater, condensate, and greywater together in the same treatment system, which simplifies treatment and lowers cost. It can be used for all the same non-potable uses as greywater alone.

Regulations and Standards

Plumbing codes and environmental regulations set the rules for treating wastewater, stormwater, and rainwater for non-potable use. The International Plumbing Code gives guidelines for building these systems. In many states, treated on-site water used for non-potable use must meet the NSF 350 standard, which makes sure the water is safe for use.

Building owners should check local laws before setting up a recycling system. Treatment systems must meet prescribed design or must have performance certifications such as NSF 350. Backflow prevention and pipe labeling are critical to assure there are no unsafe cross connections and accidental potable use. Engineers need to approve these plans to make sure the building meets health department requirements.

Start Your Water Management Plan

Managing water on-site means you depend less on city water and help the environment by reducing discharge and runoff. If you want to know more about setting up these systems, we’re here to help. Our team offers solutions for commercial greywater recycling and storm water management. Contact us to talk about your next project.