In Mexico, a total of 838 slaughterhouses have been officially recorded, collectively producing large volumes of wastewater resulting from the slaughter and processing of an average of 20,771 animals per month across bovine, porcine, ovine, caprine, equine, and poultry species. This wastewater is characterized by high pollutant loads, including chemical oxygen demand (COD), total suspended solids (TSS), fats, and nutrients such as phosphorus and nitrogen. The direct discharge of untreated slaughterhouse wastewater (SWW) into surface water bodies has led to eutrophication, ecosystem degradation, and serious public health risks due to the proliferation of pathogenic microorganisms. To address this issue, biological treatment processes have gained prominence, leveraging the metabolic activity of microorganisms to break down organic pollutants while generating biogas as a form of bioenergy. It is estimated that slaughterhouse waste can yield up to 8.654 million m3 of biogas annually, reducing both carbon footprint and energy demand. Furthermore, non-hazardous organic byproducts such as fats and proteins can be converted into nitrogen-rich organic fertilizers, enhancing agricultural productivity through improved nutrient availability and pesticide degradation. This chapter focuses on biological treatment strategies for SWW, emphasizing sustainable, circular-economy frameworks through the integration of process-treatment-byproduct-application pathways.