Evaluation of the low technology tubular digesters in the production of biogas from slaughterhouse wastewater treatment

The aim of this research is to identify the operational conditions and performance of low cost tubular digesters producing biogas from the slaughterhouse wastewater treatment. Three full scale sequenced tubular digesters (of more than 8 m³ each), operating in a slaughterhouse in Cochabamba (Bolivia), have been monitored during one year. The Organic Load Rate (OLR) varied from 0.04 to 1.13 kgSV/m³d (0.04–1.07 kgCOD/m³d), and the Hydraulic Retention Time (HRT) from 3.2 to 87.4 days. The passive solar heating design considered drove the slurry temperature to 27 °C, 8 °C more than mean ambient temperature. The first digester has been filled with soda bottle plastic rings in order to increase the internal effective surface and evaluate, obtaining that an increment in 67% of the internal effective surface, more than doubles the biogas production and enables higher OLRs. A Biogas Production Rate (BPR) peak of 0.2 m³/m³d was observed for OLR = 0.37 kgSV/m³d, HRT = 9.7d and Specific Biogas Production (SBP) = 0.55 m³/kgSV. COD removal achieves values above 70% from HRT>19 d. A comparison with other anaerobic digestion technologies demonstrates that low cost tubular digesters with biofilm carriers, are competitive for OLR<0.5 kg COD/m³d of biogas production, and for 0.25 kgCOD/m³d, when the focus is on COD removal.