Novel approaches of flocculant application in sewage treatment

Organic flocculants are typically only applied in the sludge line and sometimes in quaternary treatment of conventional sewage treatment plants (STPs) that aim for enhanced nutrient removal. However, with the ongoing societal changes directing towards a higher degree of circularity of resources and a higher degree of wastewater treatment demands, there is a need to re-asses the potentials that flocculants may offer in new wastewater treatment concepts. Our work investigated new possible applications of flocculants in an STP. Applying flocculants for chemically enhanced primary treatment (CEPT) increases the primary sludge production for biogas production, which may lead to a more positive energy balance of an STP. Results showed that 66% more influent COD could be used for biogas production via anaerobic digestion (AD), meanwhile the aerobic oxidation of this COD in the aeration tanks was prevented. However, removing the COD in CEPT with cationic flocculants led to a COD/N ratio of 3.75 g COD/g N in the water line, which is lower than the minimum ratio that is required for a conventional biological nutrient removal STP (BNR-STP). However, recently, novel N removal technologies have been introduced that function well at low COD/N ratios, such as N removal over nitrite, or that do not need any COD at all, such as the Anammox process in the waterline of an STP. With the application of these novel N removal techniques, CEPT with flocculants could be advantageous for the overall energy balance and space requirements of the future STP. Besides the STP energy balance, the application of cationic flocculants for CEPT also impacted the AD: the additional COD that was removed by CEPT was more readily biodegradable, leading to a 9% higher biomethane potential of the primary sludge. Also, in separate batch tests, it was found that flocculants decreased the viscosity of the sludge and, concomitantly, an increase in the hydrolysis rate up to 27% was observed. However, in contrast to the rate of digestion, the results showed that refractory polyacryl amide (PAM) flocculants, irreversibly bound the particles, and thus partially reduced the biomethane potential. Besides the energy aspects of an STP, also there are increasing challenges in the treatment of micro pollutants. STP effluents are one of the main sources of pharmaceuticals in the environment. Literature reports that a large part of the pharmaceuticals in an aquatic matrix, such as present in an STP, are sorbed to colloids. Since flocculation can remove colloids, flocculants in principle could be used to concentrate pharmaceuticals into a smaller sludge flow that subsequently could be treated more efficiently. The possibility of concentrating pharmaceuticals by flocculation in the primary settler was investigated. A jar test showed that pharmaceuticals were hardly removed from sewage with coagulation/flocculation. To investigate the discrepancy between reported colloidal sorption and the lack of removal when removing colloids, we tested a commonly applied experimental setup for determining the colloidal sorption of pharmaceuticals. Colloids were removed from a solution containing pharmaceuticals in two ways: by ultra-filtration (UF) and by flocculation. Both methods showed similar removal of colloids. However, during UF the observed retention of pharmaceutical was 93±4%. In contrast, when removing the colloids with flocculation, no pharmaceuticals removal was observed. These results strongly indicate that an analysis bias is introduced when using UF membranes in the determination of colloidal sorption of pharmaceuticals. Very likely, a direct retention of pharmaceuticals on the UF membrane occurred. Overall results of current work showed that pharmaceuticals hardly sorb to colloids and herewith the absence of removal of pharmaceuticals during coagulation/flocculation is explained. Therefore, flocculation does not seem to be a viable option for concentrating pharmaceuticals from sewage streams. As the cities grow, and the land becomes scarcer, there is an increasing requirement for compact STPs. To achieve this reduction in footprint, the digester volume may be decreased by uncoupling the solids retention time (SRT) from the hydraulic retention time (HRT). Separation of liquid and solids retention is a typical feature of an anaerobic membrane bioreactor (AnMBR) where a membrane keeps the solids inside the reactor, while the liquid can permeate. Membrane filtration of sludge will immediately result in the formation of a cake layer on top of the membrane’s surface. This cake layer or fouling layer forms an excellent barrier for solids and acts as a secondary membrane during the filtration process. Therefore, a simple woven cloth can also act as a support for this cake layer, avoiding the need for purchasing actual membranes, which would decrease the investment costs significantly. An AnMBR equipped with a woven cloth as filter medium is referred to as an anaerobic dynamic membrane bioreactor (AnDMBR).
Challenges in operating an An(D)MBR are the filterability and viscosity of the sludge, which limits the maximum SRT that can be achieved. Our results showed that flocculants reduced the viscosity and increased the filterability of the emulsion. Therefore, flocculants may play a positive role in the optimization of an AnDMBR. Our results showed that increased filterability was only obtained after adding a high concentration of flocculants. However, these high concentrations caused a significant decrease in biomethane potential of the sludge, as the VS destruction was lowered from 32% to 24% after adding the flocculants. In addition, a decrease in the mean particle size (d50) was observed from 58 µm to 32 µm. This was likely to be caused by refractory flocculants that shielded small particles which were turned refractory as well, a phenomenon that is described in literature as well. Likely, the accumulation of these small refractory particles affected the filterability of the sludge, which led to a doubling of the trans membrane pressure (TMP) from about 150 mbar to 300 mbar. Therefore, adding flocculants to an AnDBMR did not yield the benefits that were initially expected. A potential solution to prevent irreversible binding, and thus a decreased filterability, is to use biodegradable flocculants. Further research is needed to evaluate this possibility. As the disposal of sludge forms a large part of the operational costs of an STP, waste sludge reduction in an STP may have a big impact on the operational costs. A proven concept for waste sludge reduction in an STP, although not yet applied in practice, is predation of sludge by aquatic worms. In literature, several reactor configurations were studied in which secondary sludge is successfully predated by aquatic worms in lab-scale bioreactors. However, in contrast to AD, secondary sludge reduction by aquatic worms will cost energy for aeration, meanwhile the converted organic matter is not available anymore for energy recovery. Therefore, the ideal configuration would be to have AD followed by worm predation (WP) of the digested sludge, allowing for both energy recovery and a larger extent of sludge reduction. So far, this had not been considered a viable option, due to the low ammonia tolerance of aquatic worms and the high ammonium concentrations present in the anaerobic digester. However, by flocculating anaerobically digested sludge, the sludge solids can be easily separated from the ammonia-rich liquid creating the possibility of WP of digested sludge, reducing the amounts of solids that need to be disposed. Our results revealed an additional removal of 40% VS of the digested sludge in 12 days when applying worm predation. The solids remained well separated from the liquid, which facilitates further treatment. However, the cationic flocculants caused mortality of the used worms, due to toxicity. The assessed 4-day LD50 value was between 50 and 100 mg/L. For the possible full-scale application of WP for AD sludge degradation, a process could be designed with continuous worm addition. However, a non-toxic flocculant could also be the solution prevent mortality of the worms. Further research is needed to elucidate the best option and to validate the full-scale possibilities. The main barrier identified in this thesis for the application of flocculants in the current and future BNR-STP are i) the economic viability of flocculant applications other than the conventional applications ii) the refractory characteristics of flocculants and iii) the toxicity to aquatic worms. A solution for the above-mentioned challenges may lie in the production of polysaccharide bio-based flocculants such as alginate, chitosan, cellulose and starch. The performance of bio-based flocculants has been investigated for specific cases in numerous successful laboratory and full-scale studies. However, full-scale application is still marginal due to the high costs of production. A possible solution to make bio-based flocculants cost-effective is to find an organic waste source from which these polymers could be synthesized or extracted. However, thus far, there is only limited research in waste-based flocculants in STPs. Therefore, more research is needed in this field that could lead to new bio- and waste-based flocculants to be applied in sewage treatment.