Wastewater characterization of swine and its treatment by different processes in Mexico

Many wastewater treatment systems in piggeries have low removal efficiencies due to the processes installed do not take in account the high variation of pollutants concentration in the effluents of different productive stages (maternity, nursery, accretion, and mixed effluents). This work presents a research about the variation in pollutants concentration in 14 effluents of pig farms in Mexico and the removal efficiency reached in five different full scale treatment processes applied at industrial scale. Comparing the pollutants concentration in effluents from a same productive stage, e.g., accretion, the pollutants concentration increase with the farm size as follows: in small farms (until 2500 pigs) the chemical oxygen demand (COD) was between 3478 and 9300 mg/L; in middle size farms (2500 to 7999 pigs) the COD was between 19 344 and 38 544 mg/L and in big farms (≥ 8000 pigs) the COD was between 34 310 and 40 498 mg/L. This increase in concentration is due to the fact that water is used more efficiently in big farms. On the other side, in farms with the same size, it was also observed a difference in the pollutants concentration, depending on the productive stage. For example, in middle size farms, the COD concentration increased as follows: maternity (3500 mg COD/L); mixed effluents (between 19 365 and 25 205 mg COD/L); nursery (37 498 mg COD/L) and accretion (between 19 334 and 38 544 mg COD/L). In relation with the full scale treatment systems evaluated it was determined that the one with the best pollutants removal efficiency was the anaerobic digester of liquid and solids (dalys) operated with a hydraulic residence time (HRT) ≤ 60 d. Treating two accretion effluents, the following efficiencies were achieved: TSS ≥ 92.5 %, COD ≥ 97 %, BOD₅ ≥ 96 %, TP ≥ 66 % and 2 log units of fecal coliforms (FC). However, even though the high removal efficiencies achieved in the dalys, the treated effluent quality is not enough, neither to discharge it directly in water bodies nor to reuse it in agricultural irrigation, therefore it is necessary to continue the treatment for removal of nitrogen, TSS, FC and COD in an aerobic polishing system. An interesting option is an aerobic filtration (percolation) system over organic materials. Which is operated at low filtration rate and require a low operation and maintenance cost; being successfully operated by the pig farm workers themselves.