Air/Water Oxygen Transfer in a Biological Aerated Filter

The oxygen-transfer characteristics of an upflow biological aerated filter filled with angular clay media were determined over a wide range of gas and liquid flow rates. Liquid-side, oxygen-transfer coefficients (KLa) were measured using a nitrogen gas stripping method under abiotic conditions and were found to increase as both gas and liquid superficial velocity increases, with values ranging from 12 to 110?h?1 based on empty bed volume. The effect of gas and liquid velocity, wastewater to clean water ratio, and temperature dependence was correlated to within ±20% of the experimental KLa value. Stagnant gas holdup is roughly double in wastewater compared to clean water, but the dynamic gas holdup is the same. The oxygen-transfer coefficient is directly proportional to the dynamic gas holdup. Stagnant gas holdup does not influence the rate of oxygen transfer. The results suggest that dynamic gas holdup largely determines the specific interfacial area (a), whereas the interstitial liquid velocity largely controls the oxygen-transfer coefficient (KL).     

Oxygen Transfer in High-Speed Surface Aeration Tank for Wastewater Treatment: Full-Scale Test and Numerical Modeling

Oxygen transfer is one of the key processes in the bioreactor. Herein a computational fluid dynamics model for the oxygen transfer in high-speed surface aeration tank has been developed and validated through a full-scale aeration test. The test results indicate that the oxygen transfer mainly comes from the spray water in air and that the gas entrainment by the plunging of spray water and the surface reaeration in the aeration tank contribute little to the total oxygen transfer in high-speed surface aerator. A simple method was proposed to measure the oxygen transfer rate for high-speed surface aerator.     

Modeling and Control of Oxygen Transfer in High Purity Oxygen Activated Sludge Process

A dynamic mathematical model for the high purity oxygen activated sludge process, which incorporates structured biomass, gas–liquid interactions and control systems, was developed. The model was calibrated using pilot plant data associated with the development of the West Point Treatment Plant near Seattle, Wash. The calibrated model was used to simulate oxygen transfer rates for various operating conditions. Simulations showed that an optimal control system can reduce aerator power by 33% as compared to a conventional design, and reduce average oxygen feed gas by as much as 18%. Vent gas purity control dramatically reduced the peak aerator horsepower required to maintain set point dissolved oxygen concentration during high loadings. Step feed operation reduced the stag-to-stage variation in aerator horsepower and also reduced the required peak power. Predicted power savings for a 605,000?m3/day plant were $500,000 per year at current power costs.     

Modeling VOC Emissions in the High-Purity Oxygen Activated Sludge Process

Mass balances for volatile organic compounds (VOCs) were added to a structured mathematical model of the high-purity oxygen activated sludge (HPO-AS) process. The model was sized to correspond to two large existing HPO-AS treatment plants. The stripping of ten different VOCs was modeled and compared to stripping from conventional air activated sludge process. The results show that the covered aeration tanks can reduce stripping by more than 90%, depending on the specific VOC. If biodegradation is considered, the HPO-AS process degrades more than the conventional process due to the higher liquid-phase concentrations that result because of reduced stripping. The increase in biodegradation depends on the VOCs degradability but should increase to nearly 100% for highly volatile but biodegradable VOCs.     

Anaerobic Filter Treatment of Municipal Wastewater: Biosolids Behavior

Three 12.5?L upflow anaerobic filter (AF) reactors, with ceramic saddle, plastic ring, and crushed stone packing, were employed to evaluate the treatment of raw municipal wastewater under a wide range of hydraulic and organic loadings and operating conditions. Emphasis is placed in this paper on column profile sampling, draining, and biomass evaluation studies conducted to ascertain the functioning of the reactors and accumulation of biosolids. The concentration of organics and solids within the AF gradually increased, starting at the low end and progressing to higher elevations. The peak chemical oxygen demand value measured within the reactors was 11,500?mg/L and was noted in the stone-packed unit at 20?cm after 1,009?days, and the stabilization of suspended material was greater in lower column sections. Packing material morphology influenced the removal by draining of entrapped biomass, and the volume of drainage recovered was substantially smaller than available void space. Biosolids retained in the saddle-packed unit after 34?months totaled 33?kg/1,000?m3 wastewater treated, and no signs of clogging were observed in any of the reactors.     

Combined Anaerobic/Aerobic Secondary Municipal Wastewater Treatment: Pilot-Plant Demonstration of the UASB/Aerobic Solids Contact System

Anaerobic pretreatment followed by aerobic posttreatment of municipal wastewater is being used more frequently. Recent investigations in this field using an anaerobic fluidized bed reactor/aerobic solids contact combination demonstrated the technical feasibility of this process. The investigation presented herein describes the use of a combined upflow anaerobic sludge bed (UASB)/aerobic solids contact system for the treatment of municipal wastewater and attempts to demonstrate the technical feasibility of using the UASB process as both a pretreatment unit and a waste activated sludge digestion system. The results indicate that the UASB reactor has a total chemical oxygen demand removal efficiency of 34%, and a total suspended solids removal efficiency of about 36%. Of the solids removed by the unit, 33% were degraded by the action of microorganisms, and 4.6% accumulated in the reactor. This low solids accumulation rate allowed operating the UASB reactor for three months without sludge wasting. The long solids retention time in this unit is comparable to the one normally used in conventional sludge digestion units, thus allowing the stabilization of the waste activated sludge returned to the UASB reactor. Particle flocculation was very poor in the UASB reactor, and therefore, it required postaeration periods of at least 100?min to proceed successfully in the aerobic unit. Polymer generation, which is necessary for efficient biological flocculation, was practically nonexistent in the anaerobic unit; therefore, it was necessary to maintain dissolved oxygen levels greater than 1.5?mg/L in the aerobic solids contact chamber for polymer generation to proceed at optimum levels. Once these conditions were attained, the quality of the settled solids contact chamber effluent always met the 30?mg BOD/L, 30?mg SS/L secondary effluent guidelines.     

Treatment of High-Strength Pharmaceutical Wastewater and Removal of Antibiotics in Anaerobic and Aerobic Biological Treatment Processes

Anaerobic and aerobic treatment of high-strength pharmaceutical wastewater was evaluated in this study. A batch test was performed to study the biodegradability of the wastewater, and the result indicated that a combination anaerobic-aerobic treatment system was effective in removing organic matter from the high-strength pharmaceutical wastewater. Based on the batch test, a pilot-scale system composed of an anaerobic baffled reactor followed by a biofilm airlift suspension reactor was designed. At a stable operational period, effluent chemical oxygen demand (COD) from the anaerobic baffled reactor ranged from 1,432 to 2,397?mg/L at a hydraulic retention time (HRT) of 1.25 day, and 979 to 1,749?mg/L at an HRT of 2.5 day, respectively, when influent COD ranged from 9,736 to 19,862?mg/L. As a result, effluent COD of the biofilm airlift suspension reactor varied between 256 and 355?mg/L at HRTs of from 5.0 to 12.5 h. The antibiotics ampicillin and aureomycin, with influent concentrations of 3.2 and 1.0?mg/L, respectively, could be partially degraded in the anaerobic baffled reactor: ampicillin and aureomycin removal efficiencies were 16.4 and 25.9% with an HRT of 1.25 day, and 42.1 and 31.3% with HRT of 2.5 day, respectively. Although effective in COD removal, the biofilm airlift suspension reactor did not display significant antibiotic removal, and the removal efficiencies of the two antibiotics were less than 10%.     

Modeling of Cadmium Removal from Domestic Wastewater in Constructed Wetlands Using STELLA Simulation Program

A mathematical model was developed to simulate cadmium removal from wastewater in free water surface (FWS) and subsurface flow (SF) constructed wetlands using STELLA simulation program. The model simulated the accumulation of cadmium in soil (Cds), uptake by plants (Cdp), and residual concentration in effluent (Cdww_eff). The model was calibrated using one-half of the experimental data for the adjustment of the coefficients and the remaining data for model verification. The comparison of simulated and experimental values of Cds, Cdp, and Cdww_eff showed good agreement. The results indicated that the developed mathematical model could be useful for predicting the fate of cadmium when treating domestic effluents in constructed wetlands.     

Enhanced Start-Up of Upflow Anaerobic Filters by Pulsation

In this work, the effect of the pulsing flow on the start-up efficiency and hydraulics of upflow anaerobic filters (UAF) has been assessed. With this aim, a reactor operated in pulsing form (P) was compared with a conventional nonpulsed UAF reactor (NP). Although the operation of the two reactors was similar, the pulsed UAF attained a higher efficiency (92 versus 85%) at a higher organic loading rate (15 versus 11?kg chemical oxygen demand/m3?day), after a start-up period of 140 days. The hydraulic behavior, determined from residence time distribution curves, of the two reactors showed that the useful liquid volume in the pulsed reactor was 10% higher than in the NP reactor. Further, the useful volume of the pulsed UAF was higher (76%) than in reactor NP (58%), which indicates the efficiency of the pulsing device for degasification.     

Contrasting the Benefits of Primary Clarification versus Prefermentation in Activated Sludge Biological Nutrient Removal Systems

The potential benefits prefermentation can provide to biological nutrient removal are measured and compared to the costs of excess oxygen consumption and sludge production incurred by an activated sludge system that utilizes prefermentation, instead of primary clarification. Prefermentation was found to produce superior performance in regards to enhanced biological phosphorus removal. A lower soluble orthophosphorus effluent value [3.2?mg/L for the prefermented activated sludge (PAS) train versus 4.6?mg/L for the control train with primary clarification (PCAS)] and a higher percent phosphorus (% P) content of the biomass (9.0% for the PAS train versus 7.8% for the PCAS train) were both found to be statistically significant (P values of 4.26×10?5 and 0.0082, respectively). In addition statistically significant improvements in denitrification rates and reduced observed yields were observed due to prefermentation. However statistically significant increases in solids inventory and in particular oxygen uptake rates offset these improvements. Waste activated sludge production was slightly higher in the PAS train but was not found to be statistically significant.     

Oxygen Transfer Model for a Flow-Through Hollow-Fiber Membrane Biofilm Reactor

A mechanistic oxygen transfer model was developed and applied to a flow-through hollow-fiber membrane-aerated biofilm reactor. Model results are compared to conventional clean water test results as well as performance data obtained when an actively nitrifying biofilm was present on the fibers. With the biofilm present, oxygen transfer efficiencies between 30 and 55% were calculated from the measured data including the outlet gas oxygen concentration, ammonia consumption stoichiometry, and oxidized nitrogen production stoichiometry, all of which were in reasonable agreement. The mechanistic model overpredicted the oxygen transfer by a factor of 1.3 relative to the result calculated from the outlet gas oxygen concentration, which was considered the most accurate of the measured benchmarks. A mass transfer coefficient derived from the clean water testing with oxygen sensors at the membrane-liquid interface was the most accurate of the predictive models (overpredicted by a factor of 1.1) while a coefficient determined by measuring bulk liquid dissolved oxygen underpredicted the oxygen transfer by a factor of 3. The mechanistic model was found to be an adequate tool for design because it used the published diffusion and partition coefficients rather than requiring small-scale testing to determine the system-specific mass transfer coefficients.     

Formation of Aerobic Granules with Domestic Sewage

The use of aerobic granules in wastewater treatment can reduce the land area that is needed for the treatment of sewage. Until now granulation has been mainly studied using artificial wastewater. Studying the possibility of forming aerobic granules on domestic sewage in a sequencing batch reactor was a logical step in the scaling-up process and development of this technology. Therefore, aerobic granulation was studied using presettled sewage as influent. After 20?days of operation at high chemical oxygen demand (COD) loading heterogeneous aerobic granular structures were observed, with a sludge volume index after 10?min settling of 38?mL?g?1 and an average diameter of 1.1?mm. Applying a high COD load was found to be a critical factor for the formation of aerobic granules on this type of influent. Therefore short cycle times and concentrated wastewater are preferred to form granules in a sequencing batch reactor when low strength wastewater is used. The nutrient removal was not optimized in this study.     

Distributed Simulation with Cellular Automata Using the Multispin Coding Technique

Cellular automata (CA) are discrete dynamical mathematical systems that have been used for modeling many physical and engineering systems. Usually they are used as an alternative to more complex models based on partial differential equations. CA models can be implemented efficiently in parallel hardware due to the properties of locality, uniformity, and spatial regularity of the rules that govern their behavior. At the same time, CA simulations make intensive use of memory and processing time. For a widespread use of these models, implementations able to run in short periods of time without requiring specialized hardware are needed. This work presents a generic library able to simulate efficiently CA models, using the multispin coding technique and parallel processing on a network of conventional personal computers. Results of three different models tested show that it is possible to obtain a linear speedup. The observed speedup improves as the domain size increases or the simulated models become more complex, as in the case of a heterogeneous biofilm model used for designing wastewater treatment systems.     

Engineering Aspects of the Integration of Chemical and Biological Oxidation: Simple Mechanistic Models for the Oxidation Treatment

Oxidation processes can oxidize biorecalcitrant compounds into biodegradable intermediates, which in turn can be treated less expensively by a subsequent biological process. To design such a two-step (chemical+biological) process to treat poorly characterized wastewaters, it is useful to model the time evolution of characteristic global variables, chemical oxygen demand (COD) and biochemical oxygen demand (BOD), in order to develop a useful treatment strategy based upon these classical variables. We consider two simple model reaction networks, requiring three- and five-rate constants, respectively. The first model, proposed recently, involves conversion of a nonbiodegradable species, C, into a single biodegradable intermediate S. Here, biodegradable compounds are immediate kinetic products of oxidation. In general, it is not probable that a single recalcitrant compound undergoes a single-step reaction to CO2. However, when working with complex undefined wastewaters streams, single-step reactions may be used to simplify. The second new model corresponds to a lag time in BOD formation due to the necessity of multiple partial oxidations to reach a first biodegradable intermediate. The second network includes two intermediates, I and S, which are, respectively, nonbiodegradable and biodegradable. We then compare model behavior with an unfortunately sparse literature on BOD and COD values versus time in chemical reactors, and demonstrate the convenience of the first model, and the occasional necessity of the second, which reflects the presence of early intermediates which are nonbiodegradable.     

Anaerobic Treatment of Synthetic Textile Wastewater Containing a Reactive Azo Dye

In this study, anaerobic treatment of synthetic textile wastewater containing a reactive azo dye, namely, Remazol Brilliant Violet 5R, was investigated. A fluidized bed reactor (FBR) was used in the study. Before the operation period, start-up of the FBR was completed in 128 days with an immobilized microorganism level of 0.069 g volatile suspended solids per g support material (pumice). Anaerobic treatment of synthetic textile wastewater revealed that 300 mg/L dye was removed in the FBR system. Chemical oxygen demand (COD) and color reduction in the system were approximately 60 and 94%, respectively. Under anaerobic conditions, formation of two sulfonated aromatic amines (SAAs) was detected due to anaerobic reduction of the dye. The SAAs were not degraded under anaerobic conditions. In addition to the anaerobic treatment, the effectiveness of aerobic treatment was investigated in order to further reduce the COD after the anaerobic treatment.     

Performance Assessment of Hydraulic Efficiency Indexes

The analysis of residence time distribution functions originating from tracer studies is one of the main tools for the assessment of hydraulic performance in water and wastewater treatment units. In order to simplify the analysis, hydraulic indexes extracted from these functions are normally used. In general, these indexes are divided into two categories: short circuit and mixing indicators. However, some indexes may be related to more than one physical phenomenon (i.e., short circuit, mixing, recirculation, dead zones), leading to erroneous interpretation. In this work their capability to evaluate short-circuit and mixing levels in water and wastewater treatment units is assessed. Among the indexes analyzed, t10, which is the time necessary for 10% of the tracer to leave the unit, is recommended as a short-circuit indicator and the dispersion index (σ2) and the Morril index as mixing indicators, depending on the mixing level.     

Modeling Nitrogen Transport in Duckweed Pond for Secondary Treatment of Swine Wastewater

A mathematical model was developed to describe nitrogen transport in duckweed-covered static ponds for nutrient recovery from swine lagoon water. A finite difference technique was used to solve the partial differential equations describing the ammonia transport and concentration in the pond. The key parameters in the model include the diffusion coefficient of ammonium in the medium (D) and kinetic constant of nitrogen uptake by duckweed (k). Using one order of magnitude parameter variations, the simulations showed that the model was clearly much more sensitive to D than to k, indicating the process of nitrogen removal in a static pond by duckweed is diffusion limited. Laboratory testing was conducted with Spirodela punctata 7776, a duckweed strain, to calibrate the model. The calibration of the model with experimental data yielded a new ammonium transport coefficient (T) that is 85 times of D value. Model results showed good agreement with depth-wise experimental ammonium concentration and the model also demonstrates that intermittent mixing every 3 h can enhance ammonium uptake. Additionally, an apparent drop in pH near the duckweed mat at the surface was observed that may explain low rates of ammonia emission from duckweed ponds.     

Statistical Modeling to Forecast Odor Levels of Biosolids Applied to Reuse Sites

Proper use or disposal of wastewater solids is an important responsibility of wastewater treatment plants. At present, there are several options for wastewater solids, including agriculture, forestry, and mine reclamation reuse; production of marketable products such as compost and dried pellets; and disposal in landfills and incinerators. Land application of biosolids products is beneficial as part of recycling efforts on local farms, forests, tree farms, and mines and has gained greater acceptance of late. Coupled with these beneficial aspects are odors, which must be managed relative to the receiving populations. In this paper we present several statistical models that predict biosolids odor levels based on processing and management variables as well as ambient conditions. Such models are useful to managers at advanced wastewater treatment plants in helping them to better forecast the biosolids odors and minimize the “odor footprint,” thus making these biosolids products better received.     

Modeling a Retention Treatment Basin for CSO

Combined sewer overflows (CSOs) result in hazardous and unsightly contamination of receiving waters, particularly swimming areas. The removal of suspended solids and associated biological oxygen demand (BOD) can accelerate the recovery following a CSO event. This paper presents a numerical model to simulate the solids removal efficiency of a retention treatment basin (RTB) that utilizes polymers to improve the flocculation and settling rates for the suspended solids. The model includes settleable, nonsettleable, and floatable solids. The sludge is treated as a non-Newtonian fluid. Discrete, zone, and compression settling/floatation regimes are included. In-tank flocculation and a storage zone for sludge flushing are also included in the model. The model was calibrated and validated with data from a RTB pilot plant, and was applied to evaluate preliminary designs for a prototype RTB for the City of Windsor. The calibrated model showed that the optimum location of the target baffle was approximately 30% of the distance to the scum baffle. For design flows of 20?m/h and run durations of up to 2?h, it was found that the removal was insensitive to slopes from ?1 to ?3% and depths greater than 2.5?m (L/H = 10). The simulations indicate that 70 to 78% of solids removal can be achieved at surface overflow rates up to 25?m/h.     

Modeling Urban Storm-Water Quality Treatment: Model Development and Application to a Surface Sand Filter

A mathematical and statistical model for simulating contaminant removal from a surface sand filter is reported. The model was based on a mass balance equation and an advection-dispersion transport model. The unknown parameters of the model were the deposition rate and the hydrodynamic dispersion. Changes in space were allowed within the filter media depth and time variability of flow and influent contaminant concentration were taken into account. System field monitoring was performed between 2004 and 2006. A total of 17 storms were selected for the study. Runoff constituent analyses included: total suspended solids, total petroleum hydrocarbons-diesel range hydrocarbons, and zinc. The objective was to explore the capabilities of a two parameter model for predicting effluent contaminant concentrations. Optimized model parameter values were calculated on a storm by storm basis. Thereafter, a gamma distribution was fitted to the optimized values. A Monte Carlo simulation was performed to explore the predicting capabilities of the model by using two storms left for validation. Results of the validation phase show an acceptable performance of the model since, in general, estimated effluent concentrations fell within the uncertainty limits.