Conclusion
Based on the results of the BOD-DO model it can be concluded that the river has a good assimilation capacity in terms of dilution flow, even during the low flow season. This is reflected in the small immediate changes in BOD and DO concentrations in the river after the discharge of the wastewater. Besides this, the BOD and DO concentrations within the river reaches were proved not to vary widely, meaning that the local effects in terms of these two parameters can be considered almost negligible. However, on a larger scale, i.e., downstream of the municipality, the results showed some significant effects and differences among seasons, namely very high oxygen deficits and longer self-purification periods during the high water season, compared to no critical oxygen deficits and a faster self-purification process during the low water season. The main reasons for these differences were the higher background content of organic matter in the river during the high water season, and the favorable conditions for faster degradation and reaeration during the low water season.
Regarding the limitations of the BOD-DO model it can be said that the results obtained for the river many kilometers downstream of the Puerto Berrío, show a more or less hypothetical scenario in that the model assumes constant hydraulic and bathymetric characteristics of the river along the distance, when in reality these characteristics are changing.
The transversal mixing model fits well the real conditions of the river stretch under study and allows for an easier analysis of the local effects of the wastewater discharge in terms of pollutant plume distribution. The river width, plotted in the x-axis, gives a very good idea of the pollutant dispersion across the river, and the distance of the reach downstream of the discharge can be adjusted to the real one, offering also the possibility of a segmented analysis of the distribution curves. The results of this model showed differences among reaches, being the effects in reach 1 negligible due to very low resulting concentrations and in reach 2 more significant due to higher concentrations. For the latter, it was seen that the river geometry, combined with the location of the pipe outlet and the wastewater flow and concentration of pollutant, result in a dispersion pattern that always takes place in the first half across the river. Considering that the river related activities usually take place near the riverbank, the mixing of the wastewater in this zone is not desired.
The low water season was seen to be the less favorable for the mixing conditions of the wastewater in reach 2, not only because it results in higher concentrations of pollutant near the riverbank, but also because the water level in not high enough to cover the pipe outlets, entailing health and aesthetic problems. However, this should not be considered a low mixing capacity of the river, but more a bad design of the pipe outlet, whose relocation 5 meters across the river was proved to improve the situation significantly, even when the concentration of pollutant in the wastewater remains the same.
Thanks to the analysis of the water quality index, a broader assessment of the effects of the wastewater discharges could be done. The results obtained made clear that the same wastewater discharges have effects of different magnitude depending on the seasonal water quality variations of the receiving water body. It can be concluded that the better the water quality, the higher the assimilation capacity of the river.
Putting the results together it can be said that for the case of Puerto Berrío there is no season where the conditions care for a wastewater discharge completely free of adverse effects. During the high water season, when there is enough flow for dilution and immediate mixing, the water quality is bad, accentuating the effects of the wastewater discharges, what goes against the aim of recovering or maintaining an acceptable water quality of the water body. On the other hand, during the low water season the river has a more or less good water quality, but the water level is too low to care for an immediate mixing of the wastewater under the current conditions of the pipe outlets.
Besides the non-favorable conditions mentioned above, the wastewater discharges should be avoided considering the stipulations of the decree 1594, according to which, all municipalities must treat their wastewater to remove 80% of the BOD and 50% of the suspended solids, among other requirements. If the recently built wastewater treatment plant “Lagunas” started functioning in the near future, these requirements would be fulfilled for 80% of the municipal wastewater. For the remaining 20%, the current sewer system needs to be upgraded and improved in the first place, so that a higher coverage can be guaranteed and then a treatment can be provided. If the municipality cannot meet the needs of the population today, the wastewater problems will continue growing as the population increases.