An integrated approach to waste stabilization pond design in Marrakech

Abstract

Most wastewater treatment programs in Morocco are directed towards reuse of the effluent in irrigation. A ten-year running of an experimental two-stabilization-pond system in Marrakech showed that despite retention times around 50 days, the quality of the effluent corresponded to a restrictive irrigation reuse (B category according to the WHO guidelines). This quality can be achieved even with BOD loading reaching 780-1000 kg BOD / ha.d in one anoxic stabilization pond with a depth of 2.3 m and a retention time of 9.5 days. This primary anoxic pond did not show signs of organic matter over-loading with an active algae population, a pH all the time higher than 7 and a nematode egg concentration lower than 1 egg/L. The performance of such a system in terms of organic matter removal can be evaluated using a linear relation of the same type as the one proposed by Mac Gary and Pescod but with lower removal efficiency. However, nitrogen levels were still high with a concentration above FAO irrigation standards and can lead to groundwater quality degradation. We proposed an integrated approach to stabilization pond design that can be based on the required effluent quality to protect both public health (WHO guidelines) and the receiving environment. Groundwater vulnerability maps should be drawn and evaluated for nitrogen and other pollutant tolerance before any wastewater treatment system design. Models for nitrogen removal in stabilization ponds should be combined with those already in use for organic matter and pathogenic organisms removal to allow design of systems that are aesthetically attractive, sanitarily safe and harmless to the receiving environment.