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Figure 2 illustrates the pH variation with time for each of the substrates. Peat demonstrated the highest potential to lower system pH, which started above 10.5 initially and was reduced to below 8 after two HRT cycles. After one HRT cycle (2.5 days), the pH in the peat reactor dropped from 9.87 to 8.40, and dropped further to 7.76 and remained at that level until the end of the experiment. The reactors to which gravel, organic mulch, and topsoil were applied exhibited average effluent pH levels of 9.21, 9.12, and 9.27, respectively, throughout the experiment. These results demonstrated that pH levels below 9.50 could be consistently achieved within two HRT cycles, which would allow for the polished effluent to be discharged to the receiving environment. The result from the ANOVA analysis rejected the null hypothesis (p < 0.0001) that the mean effluent pH of all substrates were all equal. However, the ANOVA analysis can only indicate whether there is a significant difference between the mean effluent pH levels within the four groups. It cannot identify which groups have statistically significant differences. Therefore, post-hoc pairwise comparison analyses (Tukey pairwise comparison test, Fisher test, and Bonferroni test) were performed to identify the statistically significant difference between groups. The three tests uniformly demonstrated that the pH attenuation performance of peat was significantly different from the other substrates, while there was no statistical difference among gravel, organic mulch, and topsoil, as shown in Table 4.
Table 4

Statistical analysis comparing pH attenuation ability of different substrates. (Significant statistical difference (p < 0.05) is indicated in bold numbers)

 PeatGravelOrganic mulchTopsoil
Peat N/A < 0.0001 < 0.0001 < 0.0001 
Gravel < 0.0001 N/A 0.999 0.981 
Organic mulch < 0.0001 0.999 N/A 0.933 
Topsoil < 0.0001 0.981 0.933 N/A 
 PeatGravelOrganic mulchTopsoil
Peat N/A < 0.0001 < 0.0001 < 0.0001 
Gravel < 0.0001 N/A 0.999 0.981 
Organic mulch < 0.0001 0.999 N/A 0.933 
Topsoil < 0.0001 0.981 0.933 N/A 
Figure 2

Trends in pH levels with different substrates over time.

Figure 2

Trends in pH levels with different substrates over time.

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