Separation into groups were those used for bod table

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Unformatted text preview: udy pub- trations of the six constituents. The loading conditions and lished by von Sperling & Oliveira (). separation into groups were those used for BOD (Table 1 – under, usual and overloaded groups in terms of specific Primary facultative ponds parameters; Table 2 – groups 1 and 2 in terms of both flow-rate and MI). The final results of the statistical tests Figure 5 presents the relationship between effluent BOD con- are summarized in Tables 7 and 8, which present only centration and BOD removal efficiency and the following 46 S. C. Oliveira & M. von Sperling Figure 4 | | Performance of wastewater treatment technologies Journal of Water, Sanitation and Hygiene for Development | 01.1 | 2011 Comparison between mean removal efficiencies and expected values according to the literature. variables: surface loading rate (Ls), hydraulic retention time in terms of BOD and COD, but also in terms of nitrogen and (HRT), influent flow and monitoring index (MI). The loading coliforms: organic loading influences dissolved oxygen con- conditions (under, usual or overload) for each pond, operating centrations and pH levels in the ponds, which are directly as a primary facultative, are also shown in the figure. linked to the main N and FC removal mechanisms. The stat- It is seen that, as expected, when the ponds operated under istical tests indicated that the flow (indicator of plant relative overloading conditions (high Ls or low HRT), there was a ten- size) and monitoring index (possible surrogate for operational dency to an increased effluent BOD concentration, with involvement) had little influence on the effluent concen- results confirmed by the statistical tests (Table 7). However, trations, but more influence on removal efficiencies. in some cases very high loading rate values did not seem to have caused a significant deterioration in the effluent quality. Secondary facultative pond The concentration values were, in general, above the expected ranges reported in the literature. BOD removal efficiencies Figure 6 presents the plots of Ls, HRT, influent flow and MI were not significantly influenced by the loading conditions. for the secondary facultative ponds, and the results show The mean and median effluent concentrations of BOD, that the applied surface organic rate and resulting HRT COD, TN and FC from those ponds operating at organic over- did not influence substantially the performance of the sec- loading were significantly higher than those operating at usual ondary FPs. Differently from what could be expected, or underloading conditions. This is a coherent result, not only there were cases of very poor effluent quality with 47 S. C. Oliveira & M. von Sperling | Performance of wastewater treatment technologies Journal of Water, Sanitation and Hygiene for Development | 01.1 | 2011 underloading conditions and also good effluent quality with were no statistically significant differences in terms of Lv, overloading conditions. Although the poor performance of considering all constituents analysed. Most of the ponds some overloaded ponds seemed to have been influenced operated under hydraulic underload, that is, with hydraulic by this condition, there were cases of ponds operating retention times longer than the usual, an...
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