Environmental Engineering Reference
In-Depth Information
CHAPTER 10
Irrigation with ponds effluent - impacts on
soils and groundwater
R. Hranova & W. Gwenzi
ABSTRACT: Waste stabilization ponds - the source of effluents to be
reused beneficially for irrigation purposes, have been discussed together
with specific guidelines and criteria regulating the practice of wastewater
irrigation. The impacts of long-term irrigation with ponds effluent on soils
and ground water quality have been assessed by means of a specific case
study of pasture irrigation at Imbwa farm in Chitungwiza, Zimbabwe
during the period 2000-2001. Results show that the quality of naturally
acidic soils has been improved in terms of increase of the pH and the
essential nutrients content. The metals content of soils were far below the
recommended maximum values. Adverse impacts on ground water were
found with respect to metals - Cd, Zn and specifically Cr, which showed
high background values increased by the irrigation practice. High
contamination with respect to ammonia, and pronounced pollution with
respect to EC, nitrate, phosphate and FC was found as well.
Recommendations to improve the present practice have been made.
1 INTRODUCTION
Wastewater stabilization pond systems (WSPS) are a reliable treatment technology,
widely applied in countries with temperate climates and where land is available. At low
construction and maintenance costs and with no need of expert labor for their operation,
they achieve considerable treatment efficiency regarding organic material and pathogens.
For these reasons, they are a preferred option of sewage treatment in the Southern
African region. In Zimbabwe, from a total number of 139 off site wastewater treatment
plants, 110 are WSPS. The treated effluent from these plants is reused for irrigation or
disposed on land, as the regulatory instruments do not allow their discharge into surface
water bodies. WSPS are mainly applied to serve small towns and rural settlements, and in
some cases, combined domestic and industrial wastewater is treated. However, it has
been reported (Mtetwa 1998) that the vast majority of the plants do not meet the design
treatment efficiency mainly due to operational problems and overloading.
WSPS effluent contains a relatively high concentration of nutrients and organic
material, it is safer microbiologically, compared to conventional and BNR processes, and
is easy to pump due to relatively low suspended solids concentrations. Therefore, it
should be viewed as a valuable source of water for irrigation purposes, especially in cases
where the origin of the wastewater is purely domestic. In other cases, where combined
 
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