Please note that this article was previously published in H2O’s Water matters
Summary:
In certain types of (industrial) wastewater and under certain process conditions, micro-organisms can convert a considerable proportion (50-60%) of organic contaminants in wastewater into polymers. These polymers have a high molecular weight and high charge density, which means they are suitable as anionic flocculants and as adsorbents for (heavy) metals. The key benefits: the contaminants in the wastewater are converted into a valuable product that is more environmentally-friendly than standard synthetic variants, and at the same time yields considerable purification cost savings. The knife thus cuts both way.
Introduction:
Large quantities of synthetic petroleum-based polymers (flocculants) are used every year in the treatment of surface and wastewater, and for the thickening of all kinds of slurries, which is expensive and environmentally unfriendly. Is there a cheaper and cleaner alternative?
Justification:
Flocculants are used on a large scale for the aggregation of particles such as clay particles or organic matter, in order to promote the separation of these particles by means of sedimentation, flotation or membrane filtration. Examples include the treatment of surface water or as an aid in dredging activities. In many instances, anionic polymers are used as flocculants. Cationic polymers are frequently used to further thicken and dewater slurries such as sewage sludge from sewage plants.
Synthetic, petroleum-based polymers are generally used as flocculants, such as polyacrylamides and polyethyleneimines for example. The global market for this type of polymers amounts to 6 billion euro per annum, of which around half are anionic and half are cationic flocculants. Synthetic flocculants have a number of disadvantages associated with their production and application, including a high CO2 footprint and high costs. In addition, the toxicity of the degradation products/monomers and of the chemicals (such as formaldehyde) used in the production process, which may still be present in the product, also plays a role (Lee et al., 2014). This latter may prevent the reuse of the treated water or of the separated particle concentrate. These disadvantages explain the increasing interest in ‘green’ and biodegradable flocculants such as chitosan, cationic starch, polymers produced by plants, tannins, etc. But there are disadvantaged associated with this category of flocculants too: (1) there is limited availability of the raw materials, or these compete with food production, or (2) energy-intensive chemical modification is required, or (3) the end product is very expensive in comparison with synthetic flocculants.
Flocculants from industrial wastewater
Reason enough to start research into the production of biodegradable flocculants – from wastewater (figure 1). Because under the right conditions, micro-organisms are able to convert the organic matter in certain types of wastewater into large volumes of (extracellular) (bio)polymers, rather than breaking it down into H2O and CO2. Because these polymers generally have a high molecular weight and a high charge density, they should in principle be suitable for use as a flocculant. Wastewater is a still untapped source for these ‘green’ flocculants. Industrial wastewater is more suitable for use than domestic wastewater, for one because it generally has a less complex composition, and because the processing conditions are easier to control.
The principle was first tested with (simulated) wastewater from biodiesel production, which contains glycerol and ethanol as the main organic substances. This demonstrated that a broad range of process conditions is possible, but that the COD/N ratio and the sludge age in particular play an important role in stimulating the micro-organisms to produce sufficient quantities of the right polymers (Ajao et al., 2019). (COD = chemical oxygen demand, a commonly used measure of the amount of degradable (oxidisable) organic material in water; N = nitrogen, necessary for the growth of micro-organisms)
As an example: given a sludge age of just three days and a COD/N ratio of 100:1, no less than 50 to 60% of the COD in the wastewater was converted into extracellular polymers. These polymers mainly comprised polysaccharides with negatively charged carboxyl groups, a high average molecular weight (1-2 MDa) and a high charge density (3-5 meq g‑1 at pH 7). It is interesting, and of great significance, that these polymer properties can be controlled on the basis of the COD/N ratio of the wastewater, the sludge age and the type of organic matter, i.e. the type of wastewater used as the raw material.
Figure 1. Production of extracellular polymers from wastewater to replace synthetic flocculants
A lot of oxygen and energy (aeration) are required for the degradation (oxidation) of the COD. Because a high fraction of the COD is converted into polymers, considerable savings are possible (approx. 30-40%). The amount of biomass produced also falls significantly (approx. 40-50%). So the knife cuts both ways: the contaminants in the wastewater are converted into a valuable end product, and considerable costs and energy savings can be achieved at the same time. Other types of industrial wastewater than biodiesel wastewater can also be used, provided the wastewater meets the following criteria: (1) it contains dissolved and readily biodegradable substrate (COD) and (2) the COD/N ratio in the wastewater can be controlled. Further research is required, but it is already clear that the properties of the polymer produced (chemical composition, molecular weight and charge) are highly dependent on the type of organic substrate in the wastewater.
Flocculation tests with clay suspensions:
The flocculants extracted from wastewater were tested on clay suspensions. These contain naturally charged particles that repel each other and therefore form a sediment very slowly, if at all. Figure 2A shows an example of the flocculation activity (increased clarity) on a 5 g L‑1 kaolinite suspension as a function of the flocculant dose. In this instance, the flocculants had been produced from biodiesel wastewater with a sludge age of three days with two COD/N ratios, namely 100:1 and 20:1. Even at a very low dose of 0.1 mg flocculant g‑1 clay, the COD/N 100:1 flocculant achieved an activity of more than 90%, meaning it was not inferior to synthetic flocculants. At higher doses, the flocculation activity declined slightly.
Suspensions of montmorillonite yielded similar, and sometimes even better, results. This is a different clay type comprising even smaller particles (0.1‑1 µm) than kaolinite (1‑10 µm). The COD/N 20:1 flocculant had rather less of an effect, which can be attributed to the somewhat lower fraction of polysaccharides in the polymers. Tests with clay suspensions and flocculants from various types of wastewater further demonstrated:
- Flocculants produced from saline wastewater show better flocculation activity in saline conditions than in fresh water conditions, and vice versa;
- In comparison with synthetic flocculants, a broader range of doses with good flocculation activity is possible before destabilisation of the suspension occurs.
Experiments were also carried out to look at sedimentation at very higclay concentrations, which are more representative of dredging activities. Figure 2B shows that at doses of 0.1‑0.3 mg flocculant g‑1 clay, the effect on the sedimentation of a 200 g L‑1 suspension was considerable, and a much lower volume of sediment was achieved.
Figure 2. Flocculation activity in kaolinite suspension (5 g L-1) at different doses of polymer produced from biodiesel wastewater with COD/N 100:1 and COD/N 20:1 (A) and the effect of COD/N 100:1 flocculant on sedimentation in 200 g L-1 kaolinite suspension (B).
Various applications:
Apart from the flocculation tests with (clean) clay suspensions described in this article, trials are ongoing with genuine surface water, one of the aims being to see whether and how it might be possible to remove particle-bound phosphate, and to improve the microfiltration and ultrafiltration of surface water.
An entirely different application is for the removal and recovery of heavy metals by means of adsorption. In column experiments in which the (anionic) flocculants were immobilised on a carrier material, very large amounts of, for example, copper (562 mg g‑1) and lead (1204 mg g‑1) were successfully adsorbed (Ajao et al., 2020); the remaining concentrations remained below the detection limit. Of note is that these adsorption capacities are far higher than those of commercial ion exchangers. The columns could then be regenerated, whereby the metals were recovered and the columns could be reused.
Cationic flocculants for dewatering sewage sludge:
The flocculants produced from wastewater are anionic, and thus in principle not suitable for dewatering slurries such as (municipal) sewage sludge. In most instances, a cationic polymer is needed. Therefore, a number of exploratory trials have been carried out whereby the polymers from biodiesel wastewater were made cationic with the help of a mild chemical process (a reaction with glycidyltrimethylammonium chloride (GTMAC) in the presence of NaOH). The initial experiments with algae and bacteria suspensions have yielded hopeful results. It is not yet known whether the cationic variant is also suitable for dewatering sewage sludge in terms of effectiveness, environmental impact and costs, but the next logical would be to investigate this more closely. It can be calculated that the 3,700 tonnes of synthetic flocculants used annually for dewatering municipal sewage sludge could easily be replaced by flocculants produced from Dutch biodiesel wastewater. That would be a perfect, financially attractive contribution to the circular economy, and thus merits further investigation.
Acknowledgements:
The authors would like to thank the members of the ‘Natural Flocculants’ Theme of Wetsus, European centre of excellence for sustainable water technology, for the discussions and the financial support.
References:
- Ajao, V., Millah, S., Gagliano, M. C., Bruning, H., Rijnaarts, H., & Temmink, H. (2019). Valorization of glycerol/ethanol-rich wastewater to bioflocculants: recovery, properties, and performance. Journal of hazardous materials, 375, 273-280.
- Ajao, V., Nam, K., Chatzopoulos, P., Spruijt, E., Bruning, H., Rijnaarts, H., & Temmink, H. (2020). Regeneration and reuse of microbial extracellular polymers immobilised on a bed column for heavy metal recovery. Water Research, 115472.
- Lee, C. S., Robinson, J., & Chong, M. F. (2014). A review on application of flocculants in wastewater treatment. Process Safety and Environmental Protection, 92(6), 489-508.
