Protein intercalated bentonite recovered using adsorption from stickwater

Abstract

Wastewater from the meat rendering industry is called stickwater and contains about 4 wt% protein and has a high biological oxygen demand (approximately 150,000 mg/l). Large volumes require treatment prior to disposal, leading to a potentially useful source of protein being lost. In this study the organic fraction of stickwater (largely protein) was recovered by adsorption onto bentonite followed by centrifugation. Adsorption behavior was examined for a range of pH and clay concentrations. Equilibrium behaviour was described using the Langmuir–Freundlich isotherm. 25% organics can be recovered from stickwater without clay at pH below 5. Organic matter recovery was between 75 and 90% at 1 g clay per 20 g solution. Recovery decreased with increasing pH above pH 7–25% at pH 12, but was independent of pH at higher clay concentrations. Bentonite basal spacing increased from 13 to 23.5 Å for all pHs showing that protein had intercalated between clay layers. At high pH, bentonite had a positive charge on its edges, giving an expanded structure with a low pellet density at 17% solids, whereas at pH below 5 pellet density was between 25 and 33% solids. This has implications for downstream processing because more water would need to be removed, increasing downstream processing costs. Using between 5 and 10 g clay/200 g solution, adsorption was sufficient to give greater than 60% organics recovery. Processing would therefore be a trade off between using as as little as possible clay to minimise cost and maximise percentage organics adsorbed per gram of clay, while maximising recovery and maintaining small process volumes.