A FEW years ago some engineers designed the sewage treatment plants in Karachi and Hyderabad, which over the years have all but failed, although the technology used, works successfully in the UK and other South Asian countries. Now, another group of engineers is proposing to use reed bed technology to clean the water quality of Nehr-i-Khayam.
For those unaware of how this technology functions, a reed bed works like constructed wetlands, in which the oxygen is drawn down to the roots of the plants (Phragmites australis is the most common species used) through leaves.
This is a relatively new technology, pioneered by Dr Kaethe Seidel. It is normally used in ‘water treatment’ as a polishing unit. The technology is used in outskirts where the water pollution is of less intensity. To date there is no consensus on the optimal design of reed bed systems, and there is restricted information on their long-term performance.
Since the technology is unaided, unlike sewage treatment plants employing aeration, reed bed is highly susceptible to toxic constituents in sewage. Karachi’s sewage contains toxic heavy metals and chemicals.
The performance is inconsistent as the treatment efficiencies vary in response to changing environmental conditions, including rainfall and drought. Reed bed technology cannot be relied upon if the effluent quality does not meet stringent discharge standards at all times.
Reed beds are sensitive to pesticides and ammonia. They require a minimum amount of water if they are to survive. They cannot withstand complete drying. Septic tanks, anaerobic ponds or rotating biological contactors, are provided as pre-treatment units, incurring additional expenditure.
A reed bed usually has an open water surface, which promotes mosquitoes breeding. For this reason, reed beds are not recommended as people living on both sides of Nehr-i-Khayam could face massive swarms of mosquitoes, every night.
For setting up a reed bed system, first a sand bed of six inches is laid. This is followed by the impermeable liner (a very costly item). Then a gravel layer of one foot depth of size 16 millimetres (mm) is laid. This is followed by another gravel layer of eight inches depth of size 4 mm. Finally, a sand bed of four inches depth is laid. These will have to be laid for a length of 2.5 km and a width of 120 feet, the length and width of Nehr-i-Khayam. One can imagine the cost involved!
Since Nehr-i-Khayam turns into a canal during heavy rains, the heavy water flow will simply wipe out the reed bed plants.
The engineers should reconsider the reed bed decision and instead use dependable, trusted and robust technology, normally used in aerobic sewage treatment plants.
Published in Dawn, February 25th, 2020