# Analysis of the Landfill Leachate Treatment System Using Arima Models: A Case Study in a Megacity
## 4 Conclusions  
The findings of this study allow us to visualize the following conclusions:  
* The ARIMA results confirm that the concentrations of HMs, BOD\({}_{5}\), and COD in untreated leachate do not follow the same annual cycles observed for the MSW quantity disposed in the landfill. This difference is possibly associated with the leachate HRT in the conduction and pre-treatment system. ARIMA analysis suggests an HRT of up to one month (AR = 1) for HMs identified as indicators of untreated leachate (Cu, Pb, and Zn). As expected, there is also no seasonal component for ARIMA models of the HMs identified as indicators of treated leachate (Fe and Ni). Therefore, there is no transfer in time of the effect, which allows scheduling the operation of the treatment system under study;
* The findings suggest that Cd is the HM with the largest concentration variations in untreated leachate during the study period (MA = 11). This HM shows variations over periods of 11 consecutive months. Differences in the MA term of the developed models suggest that Cd and Co are the most difficult HMs to homogenize in pre-treatment ponds;
* The removal efficiency of indicator HMs of the treatment plant operation (Fe and Ni) is probably conditioned by processes carried out over a period of one month (AR = 1). The high input concentration of these indicator HMs may prevent changing their ARIMA temporal structure during leachate treatment. This is reflected in the low removal efficiencies for all HMs under study (average = 26.1%);
* The results show that during the treatment plant operation it is more difficult to control fluctuations in COD and BOD\({}_{5}\) concentration (MA between 2-4), compared to fluctuations in HM concentration (MA between 0-2);
* Finally, this study will be useful for deepening knowledge regarding the use of statistical models during the operation of leachate treatment systems in developing countries. This research will also be relevant for the public and private companies responsible for optimally scheduling the operation of these treatment systems.  
**Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app11156988/s1](https://www.mdpi.com/article/10.3390/app11156988/s1)]([https://www.mdpi.com/article/10.3390/app11156988/s1](https://www.mdpi.com/article/10.3390/app11156988/s1)), Figure S1: Scatter plots for the observed and simulated (ARIMA) values of Fe and Ni concentrations in untreated and treated leachate during the study period, Table S1: Pearson's correlations between chemical characteristics of untreated leachate during the study period, Table S2: Parameter estimates for the ARIMA model terms selected for the variables considered in the untreated leachate, Table S3: Pearson's correlations between chemical characteristics of treated leachate during the study period, Table S4: Parameter estimates for the ARIMA model terms selected for the variables considered in the treated leachate.  
**Author Contributions:** Conceptualization, H.A.R.-Q., C.A.Z.-M., and D.A.Z.-A.; methodology, H.A.R.-Q. and C.A.Z.-M.; validation and formal analysis, D.A.Z.-A., H.A.R.-Q. and C.A.Z.-M.; resources, D.A.Z.-A., H.A.R.-Q., and C.A.Z.-M.; writing--original draft preparation, H.A.R.-Q. and C.A.Z.-M.; writing--review and editing, D.A.Z.-A. and H.A.R.-Q.; funding acquisition, D.A.Z.-A. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Centro de Investigaciones-Escuela de Ingenieros Militares (ESING), budget item A-02-02-02-008-001, Ejercito Nacional de Colombia.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.