Stop and wonder

Scientists describe the different “types” of water, depending on their origin and uses, giving them different “colors”.

Discuss in your groups the following:

1. Have you ever heard of “black water”? What do you think it is?
2. What is the difference between “blackwater” and “grey water”?

Water-readings

The wastewater treatment

“Black water” water is the wastewater from the toilet bowls. The wastewater of a city generated from the houses, offices, schools, public buildings, stores, etc. ends up in the sewage system and through its network, to the wastewater treatment plant. There wastewater undergoes certain processes:

1. Screening: The sewage is led to open tanks that are penetrated vertically by large grates, imagine them as huge combs. These hold large solid debris (e.g. pieces of plastic, wood, cloth, etc).
2. Primary settlement: After screening, the sewage flows at a slow rate to allow grit- the heavier materials-  to settle in the bottom of the tank, as sludge.
3. Secondary or biological treatment: The overflowing liquid from the primary settlement moves on to the biological treatment reservoirs (aeration tanks) where intensive aeration is taking place. Wastewater contains both microorganisms and organic matter (carbon). These microorganisms using oxygen, grow rapidly, multiply and feed on the organic substances. So, thanks to the microorganisms, the effluent is cleansed. As they grow , microorganisms tend to joint with each other.
4. Secondary settlement: The effluent is led to tanks for the remaining solids – sludge together with flocks of microorganisms- to settle down. In fact, a part of them is sent back to the aeration tank to “help” microorganisms to consume the organic matter. The secondary settling tank is also open and slowly filled up so that it overflows. The overflowing fluid is free of organic substances.
5. Disinfection: To make sure no microorganisms left in the recovered water, we add chlorine, which destroys them.

The “recovered” water, meaning the water resulting from the wastewater treatment is usually led to a water body (sea or river) without causing risks of contamination or pollution. However, this practice is a big waste, since this water is suitable for various uses, after some further treatment (polishing) in order to meet certain quality criteria. For example, it can be used to water the parks of a city, or some crops, as well as for cooling purposes in factories. In addition, the sludge produced during the wastewater treatment can be used as a fertilizer or fuel.

Do you know…

1. … that wastewater treatment is mandatory for all municipalities in many countries of the Mediterranean? What about your country?
2. … that many large industries and enterprises (e.g. hotels) throughout the Mediterranean are obliged to process their wastewater at their own treatment facilities?
3.  … which Mediterranean country uses 80% of the recovered wastewater for irrigation in agriculture? We give you an hint: while this country is almost arid, it has developed such water recovery technology that gradually turns the desert into arable land; it actually exports a lot of vegetables to Europe and elsewhere!

Group activity

Visit to the wastewater treatment plant

Organise a visit to the wastewater treatment plant of your area and collect information about the processes included.

Before your visit:

Observe on the map the area where the wastewater plant is located. Why do you think the particular location was chosen?

During your visit:

1. Keep notes  and take photos of all the stages.
2. Do you find any differences between this plant and the general one described in the Water Readings? Is there any step ‘missing’ or ‘new’?
3. Ask the staff where the recovered water ends up e.g. to the sea or a river  or is it used for a certain purpose (if yes, which one)?
4. Try to collect information about the sludge produced as well, if it is used somehow.

Way forward

Dive in…

Rainwater harvesting   

Do you know about grey water?   

Desalination

if you want know about other non-conventional water resources.

Go back to the homepage!