Stop and wonder

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

Discuss in groups

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

Water-readings

The wastewater treatment process

“Black water” is the wastewater from the toilet bowls. The municipal wastewater generated from the houses, offices, schools, public buildings, stores, etc. of a city is usually lead to to the wastewater treatment plant through the sewage system.

There wastewater undergoes certain processes:

1. Screening: The sewage is led to tanks with screen nets or with large vertical grates (like huge combs). These devices hold large solid waste material (e.g. pieces of plastic, wood, cloth, etc).
2. Primary settlement: After screening, the sewage is lead to these tanks and remains still for some time, to allow the heavier particles to settle in the bottom of the tank, as sludge.
3. Secondary or biological treatment: The overflowing liquid from the primary settlement tanks moves on to the aeration tanks, where biological treatment is taking place. Wastewater contains both microorganisms and organic matter (carbon). These microorganisms with the help of the 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 join with each other, creating flocks.
4. Secondary settlement: The effluent is led to tanks (see photo above) and the remaining solids – sludge together with flocks of microorganisms- 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 liquid is free of organic substances.
5. Disinfection: To make sure no microorganisms are left in the recovered water, a disinfection is usually done through adding chlorine.

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.

DID YOU KNOW THAT ?

1. Municipal wastewater treatment is mandatory for EU countries and many Mediterranean countries. What about your country?
2. Large industries and enterprises (e.g. hotels) are in many countries obliged to process their own wastewater.
3. There is a Mediterranean country that uses  80% of the recovered wastewater for irrigation in agriculture. Which one?
   We give you an hint: while this country is almost arid, it has developed such a technology that it gradually turns the desert into arable land; it actually exports a lot of its produce 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.

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