Separation and purification techniques for the recovery of added-value biocompounds from waste activated sludge. A review

• Waste activated sludge is a renewable and sustainable source of biomolecules . • Proteins, lipids, fatty acids, enzymes and humic acids can be recovered from sludge . • High protein recovery by precipitation using ultrasound and alkali treatments . • Polymeric membranes of 30–50 kDa were the most suitable to recover humic acids. • Two integrated methods for the complete recycling of activated sludge were proposed. The need of developing a new growth model based on circular economy has led to an increasing interest in the revalorization of urban and industrial wastewaters in order to use the resources efficiently. The most stablished way of valorising these residues implies the energy production in the form of biomethane. However, urban and industrial wastewaters can also be considered promising raw sources for the recovery of valuable chemical compounds. Especially, waste activated sludge from water treatment plants is a fantastic source of biomolecules such as lipids (triglycerides or fatty acids), proteins and enzymes, carbohydrates, and humic and fulvic acids. However, prior to the recovery of these biocompounds, sludge solubilization processes (thermal hydrolysis, sonication and acidification, among others) must be conducted, in order to break the cell walls and release the protoplasmic content into the liquid media, thus obtaining a matrix of high complexity, which condition the possible strategies to be applied. This review gathered and discussed in-depth the studies that deal with the recovery of valuable biocompounds from secondary waste activated sludge. Furthermore, other types of sludge comparable to the activated one, such as cell cultures and food-related sources, have been also discussed here, in order to be used as a starting point for further research on the valorisation of waste activated sludge.