Wetlands shy appearance in a developing country

In the mid-nineties of the last century, Wetlands spreads secretly the southern shore of the Mediterranean, when the decisions-makers only paid attention to the classical waste water treatment plant like activated sludge, physicochemical treatment. Classic water treatment systems have caused catastrophic environmental damage in developing countries, where technicians face many difficulties in exploiting and maintaining them. Although maintenance of wetland is less complex, it still faces a lot of neglect in many developing countries. Perhaps this is what made her “wetlands” choose irregular migration, this time to the southern bank of the Mediterranean.

By 1996, a water leak appears through a hole in a concrete wall in El Omran secondary school located in Tunis city. Over time the amplitude of the leak began to increase, cracks appeared in several places and the environmental framework inside the school was strongly affected. The clear color of the water leak betrayed everyone. Everyone had thought that the leak was due to a fault in the internal potable water network.  Under this assumption, water leaks detection missions have followed one another, but without results. The file is closed and the building containing the leak was also closed. The file was transferred to the Ministry of Equipment, which, after several studies, blocked around 250000 USD as a budget for the renovation of the school's internal drinking water distribution network.

In the overtime, after the wave of water savings that invaded Tunisia in 2003, the Ministry of Education decided to initiate a mission of school's water system diagnosis.   In this context I started a mission of water systems diagnosis of El Omrane School in 2006. Unfortunately, new leak detection techniques, such as monitoring minimum night flow using a data logger, have failed to identify the origins of the water leak. As much, we were led to carry out a physico-chemical analysis of the water leak, whatever that could help us to dismantle this enigma. Pleasantly surprised when the leak water sample was taken, its bottling exposed its true face, a yellowish face - The face of a womanizer who plays the role of a lover, sorry this is the writing fantasy late at night when I'm drunk- I mean, it smelled cloudy and yellow in color, as if it was waste water just exited from the water treatment plant.

 In a place where everyone mistakes everybody, even water has learned to cheat and has disguised itself.

Our first reaction was to disguise the analyze, we did not ask for the TDS, or hardness but we asked the laboratory to determine the COD and BOD5.

This time, assumptions reconciled with the results, the COD and BOD5 of the water leak were successively 116 and 53 (mg / l). It is not a leak on the drinking water distribution network but earlier a leak or leaks on the wastewater collection network. Wastewater leaves toilets with a COD between 800 and 1000 and a BOD5 between 400 and 600 (mg / l), it is treated by a spontaneous buried wetlands and then evacuated by a hole in a concrete wall with a COD and BOD5 successively 116 and 50 (mg / l), an efficiency superior than 85%. Would it has been “wetland” so efficient if we have planned it before? If you want to know the answer, join us , continued ....

After the wave of Arab Spring that violently hit the region in 2011, everyone decided to appear in public, even wetlands left the secret work, but her appearance was shy.

In 2013, an association active in the environmental field consulted me after having installed, in a school for children with abilities, an autonomous sanitation system using micro-wetlands. Indeed, the gray water (that which comes from the shower and the sinks) will be treated by a planted vertical filter. It is a filter bed made up by a three layers (a sand filtration layer, a gravel transition layer and a draining layer of coarse gravel) on which are planted macrophytes. It is an effective individual sanitation process, if well designed, that simulates nature. This system is set up to treat gray water discharged by a single household with a treatment capacity of around 500 (Liters / day). The water system for this purification process was designed by an electrician. Electricians are like computer scientists, they believe they have solutions to all problems, they can replace everyone, even farmers. As originally designed, gray water will be pumped by a horizontal multistage pump (Q: 15 to 30 (L / min)) from a manhole to a planted vertical filter. But, how are we going to control the starting and stopping of the pump? The designer has associated the pump with a timer (Figure 1).

Figure 1: Installation before internention

The pump will run for 15 minutes every two hours. Then, three scenarios will be possible:

·        Senario1: The pump will be run when there is no waste water, so it will suck in air which will interfere with its operation,

·        Senario 2: Overflow at the manhole during the evacuation of wastewater when the pump is stopped.  

·        Senario 3: the most favorable senario by coincidence, the pump starts when the wastewater is discharged, which will be pumped with a flow rate that varies between 15 to 25 liters per minute. This flow rate is much higher than the infiltration capacity of the planted filter, so there will be an overflow at the level of the planted filter (Figure 1).

This ineffective water circulation system is the result of a bad choice of pump and poor regulation. Who thinks a professional is expensive has no idea what it can cost an incompetent. Below we will explain how we will modify the water system network to improve the performance of this purification process (Figure 2).

The horizontal multistage pump will be replaced by an immersed pump with integrated contactor float: the pump starts spontaneously when the water level rises in the manhole and it stops when the level drops, everything is automatically done by to the integrated contactor float, thus, there will be no more overflow at the manhole and the water user can evacuate the water when he wants. But how are we going to control the flow that feeds the planted filter to prevent overflow on the one hand and ensure perfect filtration on the other? For this, the filter must be supplied with a flow rate less than 2 (liters / minute), this is the equivalent of a small water leak flow in a tap. This can be done by installing a buffer tank (Figure 2), a mediator between the pump and the planted filter. 

Figure 2: Installation after intervention

A tank is more than equipment; it is a machine that remains more efficient until today than a pump. A semi-open valve can put a pump out of its operating range, but it cannot do anything for a tank. A tank can provide a flow rate that varies between zero and infinity, something that can only be done by a whole range of pumps associated with several other pieces of equipment: a pressure maintenance tank, electrical cabinet, pressure switch, contactor, pressure gauge, etc.). Furthermore, the cost of maintaining a tank is negligible compared to the cost of maintaining pumping stations. Figure 2 only shows a design of an efficient water system circulation for treating wastewater with a planted filter. There is no doubt that you implicitly knew the answer that I asked before: Would wetland be so efficient if we plane it before? But the question that will remain with no answer is how we spent the sums allocated to repair the leaks that I talked about above.

Nejeh Bouguerra / DROMADAIRE

?Water management consultant

Email: [email protected]

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