Hydrodynamic cavitation (HC) was examined as a post-biological polishing step for cattle-slaughterhouse wastewater at Villanueva, Santander, Colombia. Five field batches (270 L each), collected on different days and at different times of day during a three-month campaign, were recirculated independently for 60 min through a fin-type restriction. Samples collected before treatment and after 30 and 60 min were analyzed in triplicate for two coliform assays, an aerobic-mesophile count, and a combined mold/yeast count. The loop operated at mean inlet pressure and flow of 680.3 ± 2.9 kPa and 5.975 ± 0.178 L s⁻¹, respectively. By 60 min, the mean bulk-temperature increase was 19.5 ± 1.345 °C and the calculated circulation was 79.68 ± 9.22 tank-volume cycles. Pressure and flow showed no significant time effect. Thirty-minute mean reductions ranged from 26.09% (molds and yeasts) to 98.311% (total coliforms). At 60 min, no colony was recovered for either coliform assay or for aerobic mesophiles under the specified culture procedure; molds and yeasts decreased by 95.0 ± 7.071%. Those zero-colony observations denote assay-bounded non-detection and are not evidence of sterility. The five trials show a reproducible direction of microbial change in this high-strength secondary effluent. Since electricity use, cavitation number, and post-treatment regrowth were not measured, energetic or sustainability performance cannot be inferred.