Electrocoagulation-electroflotation for primary treatment of animal rendering wastewater to enable recovery of fats

Abstract

Rendering wastewater (RW) contains high levels of fats from the rendering process that recovers fats and proteins from waste animal fatty tissue. Primary treatment to remove floatable fats is necessary to avoid issues for downstream RW treatment. Recovery of waste fats that can be sent back to the rendering process can be an additional gain from RW primary treatment. Dissolved air flotation (DAF) with the addition of polymer coagulants is commonly used for primary treatment of RW, but this has two disadvantages: polymer coagulants are undesirable contaminants in reused fats and the fat quality is degraded through oxidation by pressurized air during DAF treatment. In this work, RW was treated by electrocoagulation-electroflotation (EC-EF) in a novel horizontally-placed electrode pair design as an alternative to DAF. EC-EF performed better in COD removal with increasing current densities. In situ flotation within the electrochemical cell from hydrogen gas bubbles produced was able to concentrate and preserve long chain fatty acids (LCFA). Treatment at higher current densities within the electrochemical cell generated surplus coagulants and allowed further coagulation in a separate following step. Longer mixing duration in the post-EC-EF treatment step enhanced overall COD removal. The overall COD removal for EC-EF with applied current densities of 2 mA/cm2 reached 69.5 ± 4.8% after 15 min of mixing at 100 rpm followed by 15 min of sedimentation. EC-EF thus provides competitive treatment performance and recovery of well-preserved rendering products.