Abstract
In view of the economic reality of developing countries, it will not be possible to build all the necessary wastewater treatment plants (WWTP) needed to control the pollution of their rivers in the next 20 years. Therefore, low-cost alternative technologies must be developed to restore the water quality of polluted rivers. It is well-known that the self-purification cycle in nature uses several biotic and abiotic processes to restore polluted water to its former pristine quality. This cycle has been surpassed in many rivers due to continuous discharges of wastewater into them. A low-cost structure that will enhance the water quality in small polluted rivers is proposed and can be constructed in situ, based on three conditions: Disruption of plug flow, flow velocity and support material for bacterial growth. The design of the experimental stage of this structure can control slope, water flow, length, support material and the number of locks. Two 175m-long experimental models were constructed; both models were filled with crushed, washed and screened 10 to 12 mm diameter river stone. A mixture of primary and secondary effluents from a WWTP was used to test the models, with a chemical oxygen demand of COD ≈50 and 100 mg•l-1 respectively. For a uniform 0.5% slope, the maximum flows achieved were 27 and 30 l•min-1 with and without locks. The system worked efficiently breaking the plug flow, mixing the water flow and allowing stable aerobic microbial communities of 5.58 and 8.86 log UFC•g-1, and COD reductions ranging from 90.27 to 555.2 mg•min-1 depending on the pollutant concentration. Key words: freshwater contaminants, self-purification, alternative technology, microbial communities Water SA Vol.31(1) 2005: 101-106
Highlights
The natural process of self-purification of rivers has allowed the transport and transformation of most of the anthropogenic wastes discharged into them
The results reported as chemical oxygen demand (COD) removal efficiency were established by the differences between the inflow and outflow COD concentrations in each structure
When the activated sludge from the wastewater treatment plants (WWTP) was introduced into the structure, and uniformly dispersed into the system without the inferior locks, the sludge that flowed settled in the bottom of the second half in each section
Summary
The natural process of self-purification of rivers has allowed the transport and transformation of most of the anthropogenic wastes discharged into them. Even though with available technology for wastewater treatment water quality can be restored totally before it is discharged into rivers or creeks, this technology is not generally employed in most of the so-called “developing countries” and the “less developed countries”. This will last for at least 10 or 20 more years due to the economic, political and social conditions of these countries (Palupi et al, 1995; Australia and World Affairs 1995; WHO-UNICEF, 2000; Josephson, 2001). 159 m3·s-1 of industrial wastewater are produced and only 15% undergo biological treatment (SanchezSantillan et al, 2002)
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