| Abstract: |
Rehabilitation and restoration of stream water has been an important area of researchand investigation over the past decade. This is due to the fact that streams have beenreceiving massive quantities of untreated wastewater and pollutants being disposed by man.These have degraded the quality of water leading to deterioration in the levels of DO leadingto the death of aquatic organisms and the spreading of disease. Several attempts have beendirected towards developing simple economic effective techniques that could be used toenhance the self purification process. One of the methods that has shown promising resultsamong the different structures implemented (such as weirs, spillways, gated conduitoutlets, ... etc), in improving aeration efficiency and consequently the levels of dissolvedoxygen (DO) in surface water and wastewater is the use of stepped cascade. Stepped cascadehas proven to be a multi-purpose structure as it facilitates the transfer of air molecules intosurface water to increase the concentration of DO, remove chemicals as chlorine and volatileorganic carbon components as methane in wastewater treatment plants, volatize chlorine andeliminate offensive odor in drinking water. They have been used to enhance aeration in thePetit-Saut Dam in French Guyana and in the Calumet waterway system in the Chicago River.A step cascade has also been built in the upgrading of a WWTP located in Milton City inFlorida to post aerate and de-chlorinate the effluent treated wastewater.The purpose of this research was to study the effect of cascade height, flowrate andpollutant load on enhancing the level of aeration efficiency and consequently the amount ofdissolved oxygen in stream water. This has been accomplished through three phases. Phase Iincluded a set of experiments performed in the lab in which wastewater was made to fall overdifferent heights (from 0.0 to 100 cm @ 10 cm interval and from 0.0 to 75 cm @ 15 cminterval). Phase 11 included the construction of a channel in which a stepped cascade structurewas installed. Different heights (45 cm, 60 cm, & 75 cm), different flowrates ((0.018 m3/s (66m3/hr), 0.015 m3/s (53 m3/hr), 0.011 m3/s (41 m3/hr), 0.0086 m3/s (31 m3/hr), and 0.0058 m3/s(21 m3/hr) and different concentrations of COD that ranged from 150 mg/l to 400 mg/l wereexamined. Phase III included the use of a computer model, Qual2k, to simulate the modelbuilt in the field with same boundary conditions.There results have shown the direct effect of the cascade height, the amount of flowand the concentration of COD on the levels of DO. For the experimental tests performed inthe lab, the average increase in DO was 2.1 mg/l over the 1.0 meter height. The percentage of
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