Assessment of characteristics and treatment processes of wastewater from slaughterhouses in the state of Minas Gerais, Brazil

The state of Minas Gerais is one of Brazil's largest animal protein producers, and its slaughterhouses generate highly polluting wastewater, which needs to be treated for discharge or reuse. As a novelty, this review article focused on assessing the characteristics and methods to treat wastewater from slaughterhouses in the state of Minas Gerais, and verifying its compliance with environmental regulatory agencies. The aim was to present data that helps to better manage this residue in other Brazilian states and countries. By analyzing the literature data, it was found that raw slaughterhouse wastewater (SWW) showed a high concentration of organic matter. For most SWW, the BOD5/COD ratio was above 0.4, which implies that it can be treated biologically. Generally, treated wastewater was in accordance with legal discharge standards, considering COD and BOD5 removals above 70% and 75%, respectively. It was found that wastewater treatment plants (WWTPs) consisted of some type of pretreatment (screens, grease traps) to remove coarse solids and fatty material, eventually followed by a flotation step and finally by biological processes, mostly anaerobic and/or aerated (or facultative) ponds. However, the absence of an aerobic process at the end of the treatment in some WWTPs, in addition to a system allowing better removal of biological flocs, might be the reason for ammoniacal nitrogen and suspended solids values being above the allowed maximum in treated wastewater, respectively. Besides the discharge into water bodies, it was verified that fertigation using treated SWW is very common in the state of Minas Gerais.