Mixed Liquor Recirculation Research Articles

Performance analysis of anoxic selector in upgrading activated sludge process in tropical climate

The paper describes and analyses the performance of anoxic selectors in upgrading activated sludge process in a municipal wastewater treatment plant under tropical climate, where poor sludge settleability is a problem due to elevated temperature. Site monitoring and laboratory experiment were conducted to study the denitrification, sludge settleability, kinetics, mass balance, pH and alkalinity variation in the system. The sludge settleability measured in Sludge Volume Index (SVI) was observed to improve with the increasing degree of denitrification in the anoxic selector. Under well-developed stable state, an average SVI value of 136 ml/g was achieved, which was significantly lower than the value of 250 ml/g before the application of anoxic selector. The specific reaction rates for denitrification and nitrification at 30 degrees C were 0.06 mg NO3-N/mg MLSS day and 0.08 mg NH4-N/mg MLSS day, respectively. The overall efficiencies of nitrification and denitrification were 86 percent and 55.4 percent, respectively, with an alkalinity recovery ratio of 15.4 percent. 32 percent of total COD removed was used up as electron donor in the denitrification process. However, due to absence of the internal Mixed Liquor Recirculation (MLR), a higher degree of denitrification occurred in the secondary sedimentation tank than in the anoxic zone. Issues for further studies are also discussed.

Effect of pump shear on the performance of a crossflow membrane bioreactor

The influences of pumping shear on the performance of a crossflow membrane bioreactor (MBR) were investigated. To compare the intensity of pumping shear, two types of pumps (a centrifugal pump and a rotary one) were used in turn to recirculate mixed liquor. Rotary pump system imposed much stronger shear to microbial floc than centrifugal one and resulted in severe floc breakage. Colloidal particles and organics were liberated from microbial floc by shear and caused rapid loss of membrane permeability by the formation of dense cake layers on the surface of membrane. Recirculation of mixed liquor with a rotary pump gradually increased the soluble COD in the bioreactor and deteriorated microbial activity. After 7 days’ operation, specific oxygen uptake rate (SOUR) of microorganisms in rotary pump system reduced to 78% of initial condition. With a centrifugal pump, however, buildup of soluble COD was not observed and change in microbial activity was negligible. Sludge yield in MBR process was lower than that (0.4–0.5 g MLVSS gCOD −1) reported in a conventional activated sludge process: 0.3 g MLVSS gCOD −1 for the centrifugal pump system and 0.2 g MLVSS gCOD −1 for the rotary pump system.

Nitrogen removal in a partial nitrification/complete denitrification process

Using computer modeling and experimentation with a bench‐scale prototype, a novel process for partial nitrification/complete denitrification (PN/CD) to achieve nitrogen removal equivalent to that obtained with the Ludzack‐Ettinger (LE) process is demonstrated. Based on simulation results, the PN/CD process reduces aeration energy costs by up to 16% and energy requirements for recirculation and recycle pumping by up to 28% compared to the LE process. Furthermore, the total reactor volume of the PN/CD process is 26 to 44% less than that required by the LE process for equivalent nitrogen removal. The bench‐scale PN/CD process was operated to confirm the feasibility of the proposed approach. The wastewater feed to this system was settled domestic wastewater supplemented with ammonia and acetate. The experimental PN/CD process met a 12‐mg/L discharge limit on total nitrogen. Relative concentrations of ammonia and nitrate in the effluent were successfully controlled by varying the rate of mixed liquor recirculation. Process performance was not sensitive to dissolved oxygen concentrations of 1 to 3 mg/L in the aerated reactors.

Operational Strategies for Predenitrification Process

A detailed mathematical model of the predenitrification process was employed to study the effects of key operational variables on nitrogen removal and secondary clarification. Effects of influent flow rate and solids residence time on thickening‐imposed capacity were elucidated. The dependence of optimal settings of dissolved oxygen concentration, anoxic volume fraction, and mixed liquor recirculation ratio on solids retention time, influent flow rate, influent wastewater characteristics, and temperature was determined. The influence of clarifier bio‐reactions was also examined.

Biological process design and pilot testing for a carbon oxidation, nitrification, and denitrification system

AbstractIn response to a new NPDES permit incorporating Organic Chemical, Plastic, and Synthetic Fiber (OCPSF) effluent limits for BOD (carbonaceous and nitrogenous), TSS, and priority pollutants, a treatability study was conducted to establish design criteria for a new process wastewater treatment plant for DSM Chemicals North America, Inc. The permit also requires partial removal of total nitrogen. Based on the discharge criteria, a two‐stage biological process configuration was selected. The configuration consisted of an anoxic followed by an aerobic reactor with mixed liquor recirculation. To establish design criteria, a two‐phase study was conducted. The first phase involved determining wastewater characteristics and kinetic and stoichiometric coefficients. These values were used as input to “Simulation of Single‐Sludge Processes” (SSSP), a computer model that was used to evaluate potential process configurations. In the second phase, a pilot‐scale system was operated to verify performance. With a volume distribution of 40 percent anoxic and 60 percent aerobic and a recirculation ratio of 2, BOD, COD, and total nitrogen removal efficiencies of 99, 88, and 80 percent were obtained, respectively. Approximately 84 percent of the biodegradable COD was removed through denitrification.