# Analysis of the Landfill Leachate Treatment System Using Arima Models: A Case Study in a Megacity
## 3 Results and Discussion  
### Untreated Leachate  
A global comparative analysis with mature landfills [4; 40; 41; 42; 43; 44; 45; 46] showed that, on average, the BOD\({}_{5}\) (32.8-19,402 mg/L) and COD (2198-32,358 mg/L) concentrations in the study landfill were within the same range of variation (Table 1). However, concentrations of NH\({}_{4}\), Cd, Hg, and Pb were, on average, 3.22, 1.21, 6.26, and 4.13 times higher compared to reference landfills, respectively. Indeed, the results suggested that these high concentrations of heavy metals (HMs) in leachate were possibly associated with the type of MSW disposed. This could also imply limitations on the access control of other types of MSW to the facility under study (e.g., industrial waste). The comparative analysis was performed with respect to mature landfills due to the analysis period considered in this study (13 years). However, the average BOD\({}_{5}\)/COD ratio (0.548) of the leachate could correspond to a young landfill [47]. [PERSON] et al. [48] reported that concentrations of HMs and organic matter indicators were determining factors in the selection of leachate treatment technology, so the concentration of these parameters affected their removal efficiency. The results showed significant correlations between climate variables and chemical parameters of untreated leachate. The climatic variable that showed the best correlations was temperature. Specifically, temperature showed negative correlations between'medium' and 'considerable' with BOD\({}_{5}\) (r = \(-\)0.709, \(p\)-value \(<\) 0.001) and COD (r = \(-\)0.709, \(p\)-value \(<\) 0.001). The findings suggested that, during periods of temperature increase, concentrations of BOD\({}_{5}\) and COD in untreated leachate tended to decrease. Namely, during these periods of temperature increase, a lower volume of leachate was possibly generated, which allowed an increase in the hydraulic retention time (HRT) in the storage and homogenization ponds before its treatment. Moreover, increasing the HRT in the ponds possibly also increased the sedimentation of solids contained in the leachate. All of the above possibly resulted in a reduction in BOD\({}_{5}\) and COD concentrations at the entrance of the leachate treatment plant. [PERSON] et al. [49] reported a similar trend in storage and homogenization ponds prior to leachate treatment. However, in the absence of storage and homogenization ponds, [PERSON] et al. [48] reported that in the rainy periods the BOD\({}_{5}\) and COD concentrations in untreated leachate tended to decrease (i.e., by dilution) compared to dry periods or temperature increases. An analysis with [PERSON]'s coefficient showed that among the HMs, Cu, and Pb had the best correlations in untreated leachate (see Figure 2, and Table S1 in the supplementary information). These HMs had significant correlations between'medium' and'very strong' with Hg (r \(\geq\) 0.940; \(p\)-values \(<\) 0.001), Cd (r \(\geq\) 0.790; \(p\)-values \(<\) 0.001), and Co (r \(\geq\) 0.763; \(p\)-values \(<\) 0.001). These HMs also had correlations between 'weak' and'medium' with Zn (r \(\geq\) 0.306; \(p\)-values \(<\) 0.001) and Cr (r \(\geq\) 0.171; \(p\)-values \(<\) 0.001). In other words, this group of HMs (Cu, Pb, Hg, Cd, Co, Zn, and Cr) possibly had affinity in the origin, or were generated from the same type of MSW disposed in the landfill. Other HMs did not have significant correlations with Cu and Pb, but did have significant correlations with HMs from the previous group. This was the case with Ni, which showed significant correlations between 'weak' and'medium' with Zn (r = 0.421; \(p\)-values \(<\) 0.001) and Cd (r = 0.408; \(p\)-values \(<\) 0.001). In this study, the previous correlations initially suggested Cu and Pb as the possible indicators of HMs contained in the untreated leachate.