The Potential of Anaerobic Digestion combined with Dissolved Air Flotation (AD-DAF) for Wastewater Treatment

In the context of a worldwide scenario characterized by a progressively expanding human population, the combining effects of climate change, escalating water stress, and the degradation of freshwater resources, water reclamation has emerged as a viable solution to alleviate the critical issue of water scarcity. Several streams around the world are subjected to a wide range of pollutants concentration and water-born pathogens, like antibiotic-resistant bacteria (ARB), due to human activity. The latter can be considered as a global emerging threat, due to its potential to deteriorate the human health system. Thus, adequate treatment of these polluted streams is needed to overcome water scarcity. While anaerobic membrane bioreactors (AnMBR) systems are a promising anaerobic digestion (AD) technology to treat municipal and concentrated wastewater, the application of membranes to separate solids from the bioreactor broth also has considerable constraints. An alternative physical separation method could be used to overcome the AnMBR limitations. Replacing the membrane unit of an AnMBR with a dissolved air flotation (DAF) system, and returning the flotation layer to the anaerobic reactor, may ensure high total suspended solids (TSS) retention while overcoming the membrane limitations. However, the oxygen-saturated flotation layer and the overall introduction of oxygen into the reactor through the DAF may negatively impact the anaerobic conversion process. This dissertation investigates the potential to use an AD coupled with a DAF system (AD-DAF) as a pre-treatment technology, specifically for the treatment of drain- and wastewater that mimics the ever-changing conditions of the Barapullah drain in New Delhi. Since testing an AD-DAF system on a laboratory-scale is not practically feasible, due to the constraints in downscaling a DAF unit, the implications of coupling these two technologies were assessed in two different systems: a column bench-scale DAF unit, and a lab-scale micro-aerated anaerobic membrane bioreactor (MA-AnMBR). To begin with, a data-driven experimental DAF model was developed to predict TSS removal. Input values for the experimental model were particle and bubble characteristics. The experimental model outcomes were verified in a bench-scale column DAF and two full-scale DAF systems. Results showed a predicted TSS removal aligned with the measured one of Delft canal water, anaerobic sludge, and DAF2 influents, 68 ± 1% vs. 66-96%, 77 ± 3% vs. 68-92%, and 98 ± 1% vs. 96± 1%, respectively.Afterwards, the bench-scale DAF was used to investigate the removal of suspended solids under four different influent conditions and seven DAF independent control variables (influent TSS, pH, temperature, DAF particles residence time, white water pressure, coagulants and flocculants concentration and mixing time). The influents simulated the Barapullah drain conditions under 1) dry and 2) monsoon times, and 3) close or 4) far from the pollution source. The results obtained indicated that TSS removal efficiency on the bench-scale DAF unit could mimic a full-scale system and that a DAF can remove over 90% of TSS for the four different tested influents. On the other hand, the effect of the performance variables altered depending on the influent type, with pressure showing a positive influence on the separation efficiency.Secondly, to assess the effect of coupling the DAF system with AD, a lab-scale AnMBR system was subjected to an oxygen load similar to the one used on a DAF unit. The effects of the oxygen load were compared to a fully anaerobic system, and the MA-AnMBR performance was assessed, for removal of organic matter, biogas production, nutrient concentration, operation and maintenance, and removal of two antibiotics sulfamethoxazole, SMX, and trimethoprim, TMP). Results showed a slight significant increase in COD removal, from 98.2 to 98.5%, and an increase of 35% in the ammonium concentration in the MA-AnMBR permeate, which indicated improved hydrolysis. Furthermore, biogas production decreased by 27%, but methane concentration on both MA-AnMBR and AnMBR was high (85%). Micro-aeration of the AnMBR had no negative effect in the removal of the tested antibiotics, which have a preferred anaerobic degradation pathway. TMP was rapidly adsorbed onto the sludge biomass and then degraded due to the long solids’ retention time (27 days). SMX adsorption was minimal, but the system hydraulic retention time of 2.6 days allowed its biodegradation. The addition of SMX and TMP led to an increase in the relative abundance of all studied anti-microbial resistant genes (ARGs) ( sul1, sul2, and dfrA1) and one mobile genetic element (intI1) in the MA-AnMBR sludge. Furthermore, the presence of antibiotic-resistant bacteria and antibiotic-resistance genes in the reactor permeate indicated that further treatment was needed. The outcomes obtained in this dissertation showed that an AD-DAF system has the potential to effectively remove total suspended solids under different influent conditions, and that the added oxygen load could improve hydrolysis with minimal impacts on the anaerobic conversion processes.