Biofilm carrier migration model describes reactor performance

Biological and physical selectors for mobile biofilms, aerobic granules, and densified-biological flocs in continuously flowing wastewater treatment processes: A state-of-the-art review

There have been significant advances in the use of biological and physical selectors for the intensification of continuously flowing biological wastewater treatment (WWT) processes. Biological selection allows for the development of large biological aggregates (e.g., mobile biofilm, aerobic granules, and densified biological flocs). Physical selection controls the solids residence times of large biological aggregates and ordinary biological flocs, and is usually accomplished using screens or hydrocyclones. Large biological aggregates can facilitate different biological transformations in a single reactor and enhance liquid and solids separation. Continuous-flow WWT processes incorporating biological and physical selectors offer benefits that can include reduced footprint, lower costs, and improved WWT process performance. Thus, it is expected that both interest in and application of these processes will increase significantly in the future. This review provides a comprehensive summary of biological and physical selectors and their design and operation.

A mobile-organic biofilm process for wastewater treatment

The mobile-organic biofilm (MOB) process includes mobile biofilms and their retention screens with a bioreactor and liquid and solid separation. The MOB process is inexpensive and easy to integrate with wastewater treatment (WWT) processes, and it provides for high-rate WWT in biofilm or hybrid bioreactors. This paper describes three modes of MOB process operation. The first mode of operation, Mode I, has a mobile-biofilm reactor and a mobile-biofilm retention screen that is downstream of and external to a bioreactor and upstream of liquid and solid separation. Modes II and III have a hybrid (i.e., mobile biofilms and accumulated suspended biomass) bioreactor and liquid and solid separation. Mode II includes a mobile-biofilm retention screen that is downstream of and external to a hybrid bioreactor and upstream of liquid and solid separation. Mode III includes mobile-biofilm retention screening that is external to a hybrid bioreactor and liquid and solid separation, receives waste solids, and relies on environmental conditions and wastewater characteristics that are favorable for aerobic-granular sludge formation. This paper presents a mechanistic approach to design and evaluate MOB processes and describes MOB process: (1) modes of operation, (2) design and analysis methodology, (3) process and mechanical design criteria, (4) mathematical modeling, (5) design equations, and (6) mobile-biofilm settling characteristics and return. A mathematical model was applied to describe a fixed bioreactor volume and secondary-clarifier area with Modes I, II, and III. The mathematical modeling identified key differences between MOB process modes of operation, which are described in this paper. PRACTITIONER POINTS: MOB is a municipal and industrial wastewater treatment (WWT) process that reduces bioreactor and liquid and solids separation process volumes. It may operate with a mobile-biofilm reactor or a hybrid (mobile biofilms and suspended biomass) bioreactor. This paper provides a mechanistic basis for the selection and design of a MOB process mode of operation, and it describes MOB process modes of operation, design criteria, design equations, mathematical modeling, and mobile-biofilm settling characteristics. MOB integrated WWT plants exist at full scale and reliably meet their treatment objectives. The MOB process is an emerging environmental biotechnology for cost-effective WWT.

Modelling of aerobic granular sludge reactors: the importance of hydrodynamic regimes, selective sludge removal and gradients

Hydraulic selection is a key feature of aerobic granular sludge (AGS) systems but existing aerobic granular sludge (AGS) models neglect those mechanisms: gradients over reactor height (H_reactor), selective removal of slow settling sludge, etc. This study aimed at evaluating to what extent integration of those additional processes into AGS models is needed, i.e., at demonstrating that model predictions (biomass inventory, microbial activities and effluent quality) are affected by such additional model complexity. We therefore developed a new AGS model that includes key features of full-scale AGS systems: fill-draw operation, selective sludge removal, distinct settling models for flocs/granules. We then compared predictions of our model to those of a fully mixed AGS model. Our results demonstrate that hydraulic selection can be predicted with an assembly of four continuous stirred tank reactors in series together with a correction code for plug-flow. Concentration gradients over the reactor height during settling/plug-flow feeding strongly impact the predictions of aerobic granular sludge models in terms of microbial selection, microbial activities and ultimately effluent quality. Hydraulic selection is a key to predict selection of storing microorganisms (phosphorus-accumulating organisms (PAO) and glycogen-accumulating organisms (GAO)) and in turn effluent quality in terms of total phosphorus, and for predicting effluent solid concentration and dynamic during plug-flow feeding.

The Changing and Distribution Laws of Oxygen Transfer Efficiency in the Full-Scale IFAS Process

The integrated fixed-film activated sludge (IFAS) process has been widely used in the upgrading of wastewater treatment plants (WWTPs). The oxygen transfer efficiency (αOTE) is of great significance to the design and operation of the IFAS process. The carrier filling ratio (CFR) and aeration type are two critical factors affecting αOTE and standard oxygen transfer efficiency (αSOTE). However, the distribution and changing laws of αOTE and αSOTE in the full-scale IFAS process areunclear. To optimize the operation of a WWTP and to improve the αOTE of the aeration systems, several off-gas tests were conducted under different aeration types and different CFRs. The results show that for the aerobic tank investigated (the ratio of length and width was 8:1), the αOTE and the αSOTE of the middle of the aeration systems were higher than those of the other two sides. However, the reason for the low αOTE at the beginning and the end of the tank may be different. Coarse-bubble aeration systems had a lower αOTE and almost the same oxygenation capacity (αSOTE) as the fine-bubble aeration systems under constant CFR (43%). The average αSOTE (18.7–28.9%) of the hybrid aeration systems increased with increasing CFR (7.7–57.7%), and different locations exhibited different degrees of change. The results reveal the distribution and changing law of the αOTE of aeration systems in the IFAS process, and attention should be paid to the improvement of the OTE of the plug-flow IFAS process.

Removal of pharmaceuticals in pre-denitrifying MBBR - Influence of organic substrate availability in single- and three-stage configurations

Due to the limited efficiency of conventional biological treatment, innovative solutions are being explored to improve the removal of trace organic chemicals in wastewater. Controlling biomass exposure to growth substrate represents an appealing option for process optimization, as substrate availability likely impacts microbial activity, hence organic trace chemical removal. This study investigated the elimination of pharmaceuticals in pre-denitrifying moving bed biofilm reactors (MBBRs), where biofilm exposure to different organic substrate loading and composition was controlled by reactor staging. A three-stage MBBR and a single-stage reference MBBR (with the same operating volume and filling ratio) were operated under continuous-flow conditions (18 months). Two sets of batch experiments (day 100 and 471) were performed to quantify and compare pharmaceutical removal and denitrification kinetics in the different MBBRs. Experimental results revealed the possible influence of retransformation (e.g., from conjugated metabolites) and enantioselectivity on the removal of selected pharmaceuticals. In the second set of experiments, specific trends in denitrification and biotransformation kinetics were observed, with highest and lowest rates/rate constants in the first (S1) and the last (S3) staged sub-reactors, respectively. These observations were confirmed by removal efficiency data obtained during continuous-flow operation, with limited removal (<10%) of recalcitrant pharmaceuticals and highest removal in S1 within the three-stage MBBR. Notably, biotransformation rate constants obtained for non-recalcitrant pharmaceuticals correlated with mean specific denitrification rates, maximum specific growth rates and observed growth yield values. Overall, these findings suggest that: (i) the long-term exposure to tiered substrate accessibility in the three-stage configuration shaped the denitrification and biotransformation capacity of biofilms, with significant reduction under substrate limitation; (ii) biotransformation of pharmaceuticals may have occurred as a result of cometabolism by heterotrophic denitrifying bacteria.

From biofilm ecology to reactors: a focused review

Biofilms are complex biostructures that appear on all surfaces that are regularly in contact with water. They are structurally complex, dynamic systems with attributes of primordial multicellular organisms and multifaceted ecosystems. The presence of biofilms may have a negative impact on the performance of various systems, but they can also be used beneficially for the treatment of water (defined herein as potable water, municipal and industrial wastewater, fresh/brackish/salt water bodies, groundwater) as well as in water stream-based biological resource recovery systems. This review addresses the following three topics: (1) biofilm ecology, (2) biofilm reactor technology and design, and (3) biofilm modeling. In so doing, it addresses the processes occurring in the biofilm, and how these affect and are affected by the broader biofilm system. The symphonic application of a suite of biological methods has led to significant advances in the understanding of biofilm ecology. New metabolic pathways, such as anaerobic ammonium oxidation (anammox) or complete ammonium oxidation (comammox) were first observed in biofilm reactors. The functions, properties, and constituents of the biofilm extracellular polymeric substance matrix are somewhat known, but their exact composition and role in the microbial conversion kinetics and biochemical transformations are still to be resolved. Biofilm grown microorganisms may contribute to increased metabolism of micro-pollutants. Several types of biofilm reactors have been used for water treatment, with current focus on moving bed biofilm reactors, integrated fixed-film activated sludge, membrane-supported biofilm reactors, and granular sludge processes. The control and/or beneficial use of biofilms in membrane processes is advancing. Biofilm models have become essential tools for fundamental biofilm research and biofilm reactor engineering and design. At the same time, the divergence between biofilm modeling and biofilm reactor modeling approaches is recognized.