Reuse of lime sludge from immediate one-step lime precipitation process as a coagulant (aid) in slaughterhouse wastewater treatment.

ABSTRACT

The circularity of wastewater treatment subproducts is on the worldwide agenda. In this way, this work aims to evaluate alternatives for the reuse of sludge from slaughterhouse wastewater treatment. Wetted sludges produced in the immediate one-step lime precipitation process were applied directly or first calcined, as a coagulant or coagulant aid, in the absence or presence of Ca(OH)2, to slaughterhouse wastewaters with different characteristics. For the best sludge reuse, successive reuses of the sludge were carried out and the characteristics of treated slaughterhouse wastewater were evaluated after each reuse. Results showed a great similarity between slaughterhouse and treated slaughterhouse wastewaters using wetted and calcined sludges as a coagulant for highly contaminated slaughterhouse wastewater. In addition, a great similarity was also observed between the calcined and the wetted sludges, both as a coagulant aid, for all the slaughterhouse wastewaters tested. However, the latter consumed more hydrated lime, more volume of sludge sedimented, and higher concentrations phosphorus and organic matter in the treated wastewater. Calcined sludge as a coagulant aid guaranteed the best slaughterhouse wastewater quality for almost of the tested paramenters (≥94% for absorbances at 254 nm and 410 nm, E. coli, turbidity, and phosphorus, chemical oxygen demand between 3 and 91%, and total Kjeldahl nitrogen between 3 and 62%) independently of the wastewater characteristics. Calcined sludge as a coagulant aid can be three times reused for the tested parameters and slaughterhouse wastewater characteristics without significantly decreasing the quality. The successive sludge reused saves the hydrated lime dose applied (up to 28.4%) and the sedimented sludge volume (up to 24.7%), and can be a solution to stabilize sludge due to the pH increase (sludge pH = 12).