# Analysis of the Landfill Leachate Treatment System Using Arima Models: A Case Study in a Megacity
## 3 Results and Discussion
### Treated leachate  
The comparative analysis with the Colombian guideline for wastewater discharge showed that, on average, COD was the only parameter that did not comply. This parameter showed concentrations 1.26 times higher than the Colombian limit (Table 1). However, there were periods in which other chemical parameters of the treated leachate also exceeded the established limits. The concentrations of Pb, Cr, Ni, Hg, and COD showed excesses during 6.25%, 27.8%, 11.8%, 0.69%, and 38.2% of the study time, respectively. Compared to the Spanish guideline, the results showed greater excesses. For example, COD concentrations exceeded the Spanish limit by an average of 100 times. The concentrations of Pb, Cr, Hg, BOD\({}_{5}\), and COD showed excesses during 100%, 100%, 100%, 95.5%, and 94.2% of the study time, respectively. Indeed, in the last two years of study, a significant reduction in concentrations of BOD\({}_{5}\) and COD was observed, possibly associated with landfill stabilization. On average, this reduction was 2.27 and 6.28 times, respectively. [PERSON] and [PERSON] [53] reported a similar trend during the stabilization of a MSW landfill. The results showed significant correlations between climate variables and chemical parameters of treated leachate. The climate variable that showed the best correlations was temperature (Table S3). The temperature showed positive correlations between weak and medium with Co (r = 0.473; \(p\)-value \(<\) 0.001), As (r = 0.470; \(p\)-value \(<\) 0.001), Cr (r = 0.427; \(p\)-value \(<\) 0.001), Hg (r = 0.421; \(p\)-value \(<\) 0.001), Fe (r = 0.378; \(p\)-value \(<\) 0.001), and Ni (r = 0.232; \(p\)-value = 0.004). Moreover, temperature also showed a positive correlation between medium and considerable with Cu (r = 0.648; \(p\)-value \(<\) 0.001). In the treated leachate, findings showed better correlations between temperature and HM concentrations earlier than with COD. This trend was opposite compared to untreated leachate. In fact, HM concentrations in the treated leachate depended mainly on the treatment plant operation, rather than on the climate conditions. [PERSON] and [PERSON] [54] reported the influence of climate factors on the leachate quality generated in a MSW landfill, which could also condition the functioning of their leachate treatment plant. An analysis between HMs with Pearson's coefficient showed that Fe associated the best correlations in treated leachate (Figure 4 and Table S3). This HM showed positive correlations between'medium' and 'considerable' with Ni (r = 0.511, \(p\)-value \(<\) 0.001) and Cr (r = 0.629, \(p\)-value \(<\) 0.001). This HM also showed positive correlations between 'weak' and'medium' with As, Zn, Cu, Cd, Hg, Pb, and Co (\(p\)-values \(<\) 0.001). Indeed, the previous correlations were conditioned by the type and efficiency of the leachate treatment plant under study. These correlations initially suggested Fe as the possible indicator of HMs during the operation of the leachate treatment plant under study. The Fe concentration in the treated leachate also showed positive correlations between 'weak' and'medium' with COD (r = 0.279, \(p\)-value < 0.001) and BOD\({}_{5}\) (r = 0.426, \(p\)-value < 0.001) concentrations. Thus, in treated leachate, there was probably similar behaviour between Fe and BOD\({}_{5}\) concentrations. A principal component analysis allowed visualization of a first component of four, consisting of the following parameters and elements (determining < 0.001): Fe, Ni, Cd, Pb, Zn, BOD\({}_{5}\), As, and COD (variance = 36.6%). In this way, Ni was also selected as a possible operation indicator of the leachate treatment plant under study. [PERSON] et al. [55] also used Fe and Ni as operating indicators of a leachate treatment plant for HM removal. Figure 3b shows the indicator HMs of the treated leachate on a diagram of two principal components, for better visualization. It was also observed that in each quadrant of the diagram was located a HM indicator (Fe and Ni).