MODELING OF AS-BIOFILM REACTORS

Abstract

Activated sludge-Biofilm (AS-Biofilm) processes incorporating biofilm carriers into conventional Activated sludge systems and thus accommodating both suspended and attached growth populations of organisms in the same unit are increasingly used in environmental biotechnology processes. This combination of suspended and fixed growth provides with the advantages of both the processes. In comparison to attached or suspended growth process, this system has many advantages like high efficiency for BOD removal and nitrification even at low temperature, short hydraulic retention time, high biomass concentration, small F/M ratio, small solid loading on settling tank, good sludge settling characteristics and low sludge production. In addition, this system has good resistance to hydraulic and organic shock loading. It is used for either upgrading the existing activated sludge plants or construction of new plants. Despite various advantages of the process, the adaptive behavior of the involved microorganisms imposes difficulties in terms of time-varying process parameters lo mathematically model them. The number of reactions and organism species that are involved in the system may be very large. An accurate description of such systems can therefore result in highly complex models, which may not be very useful from a practical, operational point of view. There is therefore a need of the hour to develop simple mathematical models for analysis and design of these systems. Such models need to take into account both substrate removal and nutrient metabolism, the essential characteristics of bioreactors. The thesis aims at filling this gap by developing mathematical models for the process and validating them with laboratory data. A steady-state mathematical model has been developed to describe the nitrification process in a activated sludge- biofilm reactor. The model developed uses the Monod's expressions for both the growths (suspended and attached) simultaneously and the Fick's diffusion law for biofilm in the basic equations for the aerobic hybrid reactor. In order to solve the developed model, modified expressions for the substrate flux proposed by Fouad and Bhargava (2005) have been used. The model developed has been validated with the experiments conducted on an aerobic hybrid system for nitrification. The model however can also be used for the simulation of carbon decomposition. It has also been validated with the COD data collected on the same reactor. It has become common practice in wastewater treatment technology to connect biological reactors in-series to minimize the land requirements, while fulfilling the effluent ii quality requirements. In the present study a mathematical model has been developed for a steady state multistage completely mixed AS-Biofilm reactor (hybrid reactor). A computationally efficient procedure for the optimum design of in-series AS-Biofilm reactors of equal volume, for a desired degree of treatment, has also been proposed. The model developed uses predefined effluent quality profile across the reactors in the system. Using a few reported sets of kinetic parameters, it is observed that the proposed algorithm can be efficaciously used for the design of multistage AS-Biofilm Reactors with minimal computation effort. The model results obtained for the three reactor system have been compared with the experimental results which conclude that this model can be successfullyadopted for the design of such systems. Experimental studies were carried out on the aerobic hybrid reactors to validate the above models as well as study their performance under various reactor configurations and operating conditions viz. loading rates, DO concentrations and type of media. These studies were carried out in three phases: i) the objective of first phase was to evaluate the in-series reactor performance at variable HRT [12, 10, 8 and 6 hr] and DO [2 to 3.5 and 3.5 to 5.5 mg/L] concentrations, ii) Second phase had been designed for comparative evaluation of fixed and movable media AS-Biofilm systems. Reactor configuration and operating conditions same as in Phase 1 but with different media has been used for this study and iii) Phase 3 experiments provided data for validation of the AS-Biofilm model developed and also the performance of the AS-Biofilm system under composite media. In addition to performance evaluation studies, hydrodynamics, particle size distribution, Single-Strand Conformation Polymorphism (SSCP) and Scanning electron micrograph studies were done on the system. Phase 1 experiments were carried out on a laboratory scale AS-Biofilm system consisting of three reactors fabricated with acrylic sheets each having equal volume. 22 numbers circular plastic nets (18 cm diameter) mounted on a circular, frame were used as Biofilm carrier. The nets occupied about 30% of the reactor volume. Synthetic wastewater used for the present study consisted of diluted molasses, urea, KH2PO4 and MgS04 in a mass ratio of COD/N/P =300/10/1. Phase 2 Experiments were conducted on two laboratory scale experimental reactors with same specifications as in Phase 1. Two types of biomass carriers were used in this phase. One reactor was operated with circular plastic plates (fixed media) and the other with foam cubes (movable media). Synthetic wastewater used for this study was same as that in Phase 1. During the experimentation, COD concentration in the influent and effluent, ammonical 111 nitrogen, nitrite nitrogen, nitrate nitrogen and biomass concentration (X) were analyzed according to standard methods. Phase 3 experiments were conducted on a 5 L double walled bio-reactor Three types of biomass carriers were used in the present phase i) Bioflow® 9 was used to study the performance of AS-Biofilm process and to extract the necessary data for model validation ii) to investigate the effect of composite media (Bioflow® 9, Flocor and Bio-ball) on the performance of the system. The substrate was prepared by diluting the vinasse with tap water and adding urea, KH2PO4 (source of nitrogen and phosphorous) and MgS04 resulting in a mass ratio of COD/N/P =300/ 10/1. During the study, Complete Analysis of Anion and Cation (By Ion Chromatograph), COD and volatile suspended solids were analyzed. In addition to this. Hydrodynamic studies [Phase3], Polymerase Chain Reaction -Single-Strand Conformation Polymorphism (PCR-SSCP) analysis [Phase3] and bacterial (suspended and attached) diversity in the reactor [Phasel], was carried out at intervals From the experiments on a three-reactors-in-series assembly it was observed that COD removal efficiency on an average decreased by about 10% while NfV-N removal efficiency decreased by about 12% when the DO concentration in the reactor varied from high to low range. Experimental results on two separate AS-biofilm systems with same configuration using fixed and movable media at variable HRTs revealed that 6 hour HRT was critical as nitrosomonas and nitrobacters got washed-out from biomass leading to increase in concentration of nitrite and ammonia in the reactors. However, less accumulation of ammonia was noticed in the reactor operated with movable media than that in the one operated with fixed media. Further, it is recommended that such hybrid systems must be designed with 6 - 8 h HRT. A study on the performance of the AS-Biofilm system at variable percentage of media showed that the treatment efficiency as well as shock absorbing characteristics improved with increasing biomedia volume in the reactor. However, the system when operated at OLRs beyond 6.5 kg/m3-d and high fillings, led to a stage where the sludge settling will degrade but when the OLRs are reduced, the system recovered to normalcy within a short time which indicates its excellent shock absorption capacity. Hydrodynamic studies on AS-Biofilm reactor in Phase 3 revealed that liquid mixing was good as the dispersion coefficient (D/uL = 1.85) value is relatively high. The concentrations were found to be homogeneously distributed within the reactor. RTD study IV demonstrated that the volume of liquid within the bioreactor did not change during the experiments. Scanning electron micrographs of suspended and attached biomass in AS-Biofilm system showed a dense and compact bacterial high cell density and optimal community structure that leads to the better results by greatly improving reactor performance and ability to withstand extreme environmental conditions. The comparison of Single-Strand Conformation Polymorphism (SSCP) results of fixed and suspended biomass reinforced the above statement. The present work shall be useful to the researchers and practitioners in design and operation of hybrid biological wastewater treatment systems.