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Explained: what is blackwater & greywater and a guide to its recycling

We produce both blackwater and greywater throughout the entire day. In both cases, it is wastewater that comes mainly from households. But do you know what their main difference is and how we can recycle them? If not, keep reading.

Let's start right away with the most important question. What are the differences between blackwater and greywater? They both come from households and denote wastewater, but they are not the same.

After all, even the names themselves give you a partial clue as to what it's all about.

Blackwater, also known as sewage water, is a mixture of water with excrement and/or urine. This means that blackwater flows into the WWTPs after you flush down the toilet.

To be specific, you may occasionally come across an assumption that blackwater is also the water that has been washed out of sewers after heavy rains.

However, the adjective “black” doesn't just refer to feces and the fact that it's darker to the eye.

Upon analysis, you would find that, when compared to greywater, blackwater contains far more contaminants, pathogens, and bacteria that can, in the worst-case scenario, spread unpleasant diseases.

Regardless of its color, blackwater is also known to be greasier than greywater.

Speaking of greywater, do you know how it is created? It also originates in the bathroom.

This is because greywater comes from sinks, showers, bathtubs, and washing machines. It is also produced when you run the water in the kitchen sink or turn on the dishwasher.

Greywater is much cleaner than blackwater. While blackwater contains a lot of solid particles floating around (toilet paper, excrement), with greywater it is not the case.

Instead, you'll find more chemicals that get into greywater because of washing powder, soaps, or detergents.

Treatment process

Reuse of water from the toilet or sink? Yes, it is possible. However, the treatment and recycling method differs for greywater and blackwater.

The reason is obvious - black water contains more pollutants and therefore requires more thorough treatment.

One of the specific common methods is aeration which consists of several steps. First, the solid particles are allowed to settle in the blackwater.

The next step is aeration which aids bacterial decomposition of organic matter. The last and crucial step is disinfection with chlorine.

This method is, naturally, one of many. But in general, blackwater should be treated biologically or chemically in a wastewater treatment plant and it should be disinfected at all times.

We also use biological treatment systems in Hydrotech’s wastewater treatment plants built as concrete, steel, or plastic tanks or containers.

Unlike blackwater, greywater is less demanding to treat. Why? As we mentioned earlier, it does not contain life-threatening pathogens. But that doesn't mean it doesn't require any treatment. On the contrary.

In this case, we also advise you to have a wastewater treatment plant designed to filter the water and remove all solids through anaerobic digestion.

Different form of pollution – different recycling

You know that blackwater and greywater differ in their composition and the way they are treated. And recycling is no different.

More precisely, after cleaning, each of them is used for a different purpose. Again, however, purified blackwater is at a disadvantage, because it can be used 'only' for watering plants and irrigation.

Greywater also serves this purpose, but it can also be used for building wetlands, cleaning public spaces, in car washes, or for flushing and cleaning toilets.

Under certain circumstances, greywater can also be used to heat water for bathing or washing clothes.

If you are not familiar with how and why greywater should be recycled, here is some final important information.

Each household of four produces around 360 liters of greywater per day (this accounts for up to 40% of household wastewater in total).

So, you can imagine the quantity of clean drinking water you could save if you used recycled water to wash your car or water your lawn and plants.

Many countries suffering from droughts and water shortages have already figured this out and are reusing greywater instead of discharging it into sewers.

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