Dr. Fadhl Ali Al-Nozaily recently earned his PhD by developing a water-purifying system that uses duckweed, which is the green scum covering many of Holland’s brooks and ponds.
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In Yemen, wastewater is ten times more polluted than in the Netherlands. ”There’s precious little water in Yemen; therefore, the discharge water is extremely polluted. The Dutch use much more water, so, consequently, the pollution is more dissipated,” says Al-Nozaily, a Yemeni sanitary engineer.
Most existing waste-stabilisation ponds in Yemen are poorly performing because of high organic loading rates and the presence of high sulphate and ammonia levels. This situation offers a favourable environment for developing purple bacteria that causes a pink colour in the water and causes high turbidity levels. Because sunlight can’t reach the algae, they die.
But even if the algae and bacteria were working well together, it still causes problems. The effluvia, which is the water discharged from the water-purifying system, contains high algae concentrations. This water pollution causes problems for the surface water and the irrigation system where the effluvia is released. Extremely polluted waste water is, therefore, a recurrent problem.
”The ponds where the water is purified work better if there’s duckweed growing there. We’ve known that for a long time. This phenomenon is also known in other warm countries. In Bangladesh, there’s a full-scale water-purifying system that works fine,” exclaims Al-Nozaily. How to optimally design such a system for Yemen was unknown. Al-Nozaily spent years studying how duckweed does the trick. Based on his findings, he graduated from the TU, in conjunction with IHE, on January 10th.
Too cold
The principle is simple: ”Duckweed needs nitrogen and phosphor to grow, and it gets it from its roots in the waste water,” Al-Nozaily explains. The nitrogen is transformed into ammonia, which is then absorbed by the roots. Phosphorus, which in water always appears as phosphate, can be absorbed directly.
In fact, the duckweed takes over the algae’s role; but the duckweed does a better job. ”The duckweed absorbs phosphate much more efficiently. And more suspended solids are removed from the water by bacteria located near the roots.” The duckweed mat, which is but a few millimetres thick, blocks sunlight from entering the water beneath it. Harmful algae and phototrophic bacteria, which extract energy from sunlight, don’t have a chance.
Al-Nozaily tested four different species of duckweed and came to the conclusion that the most common type, Lemna gibba, worked best in Yemen. During the experiments he carried out in the IHE laboratory, in Delft, and outdoors in Yemen, Al-Nozaily really got to know the weed. Al-Nozaily: ”Judging by the duckweed’s appearance, you can tell the condition of the water. If the sewagewater isn’t contaminated with a high load of nutrients like phosphate and nitrogen, then the duckweed must work harder to get enough of these nutrients. To do this, they grow longer roots.”
Duckweed is sensitive to temperature and light. Daylight appears to be sufficient, but it mustn’t be too cold. For this reason, the method isn’t really suitable for the Netherlands.
One of Al-Nozaily’s remarkable discoveries concerns water depth:
”The Duckweed mat is only one duckweed thick. But the depth of the water underneath the mat has no effect on the removal of chemical compounds, because the bacteria in the water must do the trick. I get the same results with a half meter or with one and a half meters of water.”
Buckets
To properly purify the water, it’s necessary to frequently harvest the duckweed, because it needs room to grow. The harvested duckweed, however, can be put to good use as fodder, because it’s rich in proteins
Yemeni wastewater is too polluted to confront the duckweed with directly. The duckweed would immediately die because of the high, toxic concentrations of ammonia in the water. However, it’s not possible to simply dilute the wastewater with clean water, because there’s no available water to so with in Yemen. To decrease the ammonia concentrations to an acceptable level, then, purified wastewater is recirculated in order to dilute the water. To determine which recirculation method worked best, Al-Nozaily used buckets to test the various dilution combinations.
In practice, it would be wise to put the traditional purifying systems in front of the duckweed-covered sewage ponds. ”Both have benefits. The algae grow quickly and can survive under more polluted conditions. And duckweed absorbs more nitrogen and phosphate.”
Al-Nozaily hasn’t done any full scale tests yet, ”because to grow the required amount of duckweed takes too long.” He is starting to grow the duckweed now, however, so in a few years, the first full-scale duckweed-covered sewage pond can be built.
Dr. Fadhl Ali Al-Nozaily recently earned his PhD by developing a water-purifying system that uses duckweed, which is the green scum covering many of Holland’s brooks and ponds.
In Yemen, wastewater is ten times more polluted than in the Netherlands. ”There’s precious little water in Yemen; therefore, the discharge water is extremely polluted. The Dutch use much more water, so, consequently, the pollution is more dissipated,” says Al-Nozaily, a Yemeni sanitary engineer.
Most existing waste-stabilisation ponds in Yemen are poorly performing because of high organic loading rates and the presence of high sulphate and ammonia levels. This situation offers a favourable environment for developing purple bacteria that causes a pink colour in the water and causes high turbidity levels. Because sunlight can’t reach the algae, they die.
But even if the algae and bacteria were working well together, it still causes problems. The effluvia, which is the water discharged from the water-purifying system, contains high algae concentrations. This water pollution causes problems for the surface water and the irrigation system where the effluvia is released. Extremely polluted waste water is, therefore, a recurrent problem.
”The ponds where the water is purified work better if there’s duckweed growing there. We’ve known that for a long time. This phenomenon is also known in other warm countries. In Bangladesh, there’s a full-scale water-purifying system that works fine,” exclaims Al-Nozaily. How to optimally design such a system for Yemen was unknown. Al-Nozaily spent years studying how duckweed does the trick. Based on his findings, he graduated from the TU, in conjunction with IHE, on January 10th.
Too cold
The principle is simple: ”Duckweed needs nitrogen and phosphor to grow, and it gets it from its roots in the waste water,” Al-Nozaily explains. The nitrogen is transformed into ammonia, which is then absorbed by the roots. Phosphorus, which in water always appears as phosphate, can be absorbed directly.
In fact, the duckweed takes over the algae’s role; but the duckweed does a better job. ”The duckweed absorbs phosphate much more efficiently. And more suspended solids are removed from the water by bacteria located near the roots.” The duckweed mat, which is but a few millimetres thick, blocks sunlight from entering the water beneath it. Harmful algae and phototrophic bacteria, which extract energy from sunlight, don’t have a chance.
Al-Nozaily tested four different species of duckweed and came to the conclusion that the most common type, Lemna gibba, worked best in Yemen. During the experiments he carried out in the IHE laboratory, in Delft, and outdoors in Yemen, Al-Nozaily really got to know the weed. Al-Nozaily: ”Judging by the duckweed’s appearance, you can tell the condition of the water. If the sewagewater isn’t contaminated with a high load of nutrients like phosphate and nitrogen, then the duckweed must work harder to get enough of these nutrients. To do this, they grow longer roots.”
Duckweed is sensitive to temperature and light. Daylight appears to be sufficient, but it mustn’t be too cold. For this reason, the method isn’t really suitable for the Netherlands.
One of Al-Nozaily’s remarkable discoveries concerns water depth:
”The Duckweed mat is only one duckweed thick. But the depth of the water underneath the mat has no effect on the removal of chemical compounds, because the bacteria in the water must do the trick. I get the same results with a half meter or with one and a half meters of water.”
Buckets
To properly purify the water, it’s necessary to frequently harvest the duckweed, because it needs room to grow. The harvested duckweed, however, can be put to good use as fodder, because it’s rich in proteins
Yemeni wastewater is too polluted to confront the duckweed with directly. The duckweed would immediately die because of the high, toxic concentrations of ammonia in the water. However, it’s not possible to simply dilute the wastewater with clean water, because there’s no available water to so with in Yemen. To decrease the ammonia concentrations to an acceptable level, then, purified wastewater is recirculated in order to dilute the water. To determine which recirculation method worked best, Al-Nozaily used buckets to test the various dilution combinations.
In practice, it would be wise to put the traditional purifying systems in front of the duckweed-covered sewage ponds. ”Both have benefits. The algae grow quickly and can survive under more polluted conditions. And duckweed absorbs more nitrogen and phosphate.”
Al-Nozaily hasn’t done any full scale tests yet, ”because to grow the required amount of duckweed takes too long.” He is starting to grow the duckweed now, however, so in a few years, the first full-scale duckweed-covered sewage pond can be built.
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