Flocculants from wastewater

Fields such as water and wastewater treatment, dredging, mining, food processing, textile and paper making, petroleum and chemical industries face the challenge of particle removal from (waste) water. The removal of solids from these water streams is generally achieved via the coagulation/flocculation process, which is a simple and effective way to destabilise, agglomerate and remove particles from water and wastewater. Currently, this process is widely accomplished with the use of inorganic coagulants and fossil-based organic flocculants. Flocculants are particularly important because they are efficient at low dosages and able to form strong flocs. However, most synthetic flocculants biodegrade poorly and some of the degradation products/monomer residues are toxic, with acrylamide from polyacrylamide as a well-known example to have carcinogenic and neurotoxic effects. Besides, unreacted toxic chemicals used to synthesise the monomer units, such as formaldehyde, epichlorohydrin, and dimethylamine, have been found as sources of contaminants in treated water. Hence, the use of synthetic flocculants can hardly be considered a sustainable (waste) water treatment approach, especially in open systems such as in surface water treatment, dredging and mining.As an alternative, bioflocculants have gained increasing attention for water treatment since they are generally safe, biodegradable, and can effectively flocculate particles with performances sometimes comparable with synthetic flocculants. However, a class of promising bioflocculants yet to be fully explored are microbial extracellular polymeric substances (EPS). EPS are products of microbial biochemical secretions, comprising different macromolecules such as polysaccharides, proteins, lipids and humics substances. The high molecular weight of EPS coupled with their net negative charge make them anionic polyelectrolytes that can be applied in several fields such as for particle flocculation and heavy metal adsorption. Although EPS are already being produced from pure microbial strains (commercial exopolysaccharides such as alginate and xanthan gum), their high cost, due to the need for sterile conditions and expensive substrates such as glucose, limit their use to specialty applications such as food, feed, medicine and cosmetics.To produce cost-effective EPS that can be applied as flocculants, we established a mixed-culture approach that requires non-sterile cultures and feedstocks by using (industrial) wastewater as both carbon and nutrient source. Here, one uses the potential of cooperative growth and symbiotic relationships in mixed cultures of EPS-producing and non-EPS–producing bacterial strains as found in wastewater sludge. Moreover, this approach allows the combination of biological organic pollutant removal from wastewater with the production of EPS as a useful product. By converting most of the organics that would have been mineralised to EPS, wastewater treatment plants (WWTPs) can benefit from a reduced quantity of produced sludge, which is expensive to dispose of, and can also lower their CO2 footprint.The aim of this thesis was to valorise selected industrial wastewaters to extracellular polymeric substances (EPS) that can be employed as bioflocculants, heavy metal adsorbents and for other applications. To achieve this, it was important to study three aspects of the technology value chain: the production, characteristics and applications of EPS as flocculants and heavy metal adsorbents.EPS production was combined with fresh and saline wastewater treatment, using substrates such as glycerol, glucose, acetate and ethanol to mimic different industrial wastewaters. Under nitrogen-limited conditions, as much as 30 - 69 % wastewater-COD could be recovered as EPS-COD. Moreover, EPS produced under such nitrogen-limited conditions were mainly composed of polysaccharides, possessed higher molecular weights (1 - 4 MDa) and showed better flocculation performances than EPS produced from excess nitrogen. Highly charged anionic EPS (≥ 5 meq/g EPS) were also shown to have very high capacity to adsorb and recover heavy metals such as Cu, Pb, and Au.