Dimensioning of a natural ponds as wastewater treatment plant for the city of Ouazzane
Abstract
Morocco has a mobilizable water heritage of 21 billion m3 including 16 billion m3 of surface water and 5 billion m 3 of groundwater. Unfortunately due to the exponential population growth and economic and social development, these water resources cannot meet the growing demand of the agriculture, industry and drinking water sectors.
In addition, more than 546 million m3 of raw wastewater is discharged annually into the receiving environment and only a tiny proportion is reused in the irrigation of about 7000 ha. Most of the wastewater discharged is responsible for the pollution of the environment.
To seek new water resources while curbing pollution, Morocco has undertaken a gigantic project of treatment of its wastewater. The city of Ouazzane, like other urban agglomerations, must choose an adequate system for the purification of its wastewater discharges.
The present work proposes the design and sizing of a natural lagoon WWTP based on demographic data, pollutant load, wastewater flow and availability of land for the city of Ouazzane.
The project proposes a pretreatment using a vertical manual screens of 36 bars of 10 mm diameter and spaced 12 mm and a Grit/Grease Separator of 5.6x1.8 m in size. Primary treatment with a circular primary decanter 16 m in diameter, 3 m deep and 602.88 m in volume. Four anaerobic basins 4 m deep, 8204 m3 in volume and 58.02 x 35.35 m in size. Also four Optional ponds 2m deep, 41008m3 deep and 107 x 192m in size. Finally, four maturation ponds of 1 m deep and 6400 m3 of volume each.
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Introduction
Our study focuses on the choice of the appropriate purification system to treat wastewaters from the city of Ouazzane. Unfortunately, several constraints will have an influence on the choice of the most suitable method for the habitat of Ouazzane. Among these, we can note: the high altitudes, a rugged topography and difficult accesses; irregularity of use; Difficult energy supply; weakness of economic and financial resources [1].
So after the examination of the studied treatment variants (Activated sludge, Lagoon, Bacterial bed, Reed plant filters ...) our choice was based on the techniques presenting a good robustness of treatment, a level of technicality easy to implement and reasonable investment and operating costs. As a result, the process that will be used for the purification of the effluent volumes to be treated is a natural lagoon.
In agreement with ONEP, we included a pretreatment, primary treatment (anaerobic lagoons) and secondary treatment (optional lagoons) and we added a tertiary treatment (ripening lagoons). In what follows in the present study, the objective was to calculate or deduce the volume load, the pollutant load, the surface load, the flow rates of the raw sewage and the dimensions of the different lagoons, the time of sewage stays, the total surface area of the WWTP.
Conclusion
When using an extensive process such as natural lagooning, the quantities of sludge produced are less than in the case of the use of an intensive process. However, sludge is not removed continuously but periodically. This is why we need to build important infrastructures for the evacuation of sludge. These infrastructures are not used regularly but must be maintained, they occupy space and require investments.
Natural lagooning requires the installation of very important infrastructures as well as the reasonable investment cost. Nevertheless it requires little maintenance and few qualified personnel compared to the bacterial bed and activated sludge. This process gives a very good robustness and reliability of treatment in addition it presents olfactory nuisance (smell), considerably in summer because of the presence of the wind Chergui having the direction of the EST towards the West).
According to the multi-criteria comparison of the available variants, it seems that lagooning is the most appropriate method for the treatment of wastewater from the city of Ouazzane.