Taking the "waste" out of "wastewater" for human water security and ecosystem sustainability

S.B. Grant, J.-D. Saphores, D.L. Feldman, A.J. hamilton, T.D. Flectcher, P.L.M. Cook, M. Stewardson, B.F. Sanders, L.A. Levin, R.F. Ambrose, A. Delectic, R. Brown, S.C. Jiang, D. Rosso, W.J. Cooper, and I. Marusic (2012)

Science 337(6095):681-686

ABSTRACT - Humans create vast quantities of wastewater through inefficiencies and poor management of water systems. The wasting of water poses sustainability challenges, depletes energy reserves, and undermines human water security and ecosystem health. Here we review emerging approaches for reusing wastewater and minimizing its generation. These complementary options make the most of scarce freshwater resources, serve the varying water needs of both developed and developing countries, and confer a variety of environmental benefits. Their widespread adoption will require changing how freshwater is sourced, used, managed, and priced.

Energy Footprint analysis of brackish groundwater desalination with zero liquid discharge in inland areas of the Arabian Peninsula

R. Sobhani, M. Abahusayn, C.J. Gabelich, and D. Rosso (2012)

Desalination 291(2):106-116

ABSTRACT - Semi-arid regions throughout the world face water scarcity and the need for more efficient and alternative sources of drinking water supply. Inland regions in the Arabian Peninsula have the alternate option of coastal seawater desalination and long-distance conveyance, often with lift to substantial elevation. In several aquifers of this region, naturally occurring radium in groundwater is above acceptable standards and must be reduced.
We analyzed the energy footprint of a modular process employing a combination of pellet reactor for radium and hardness minimization, reverse osmosis with intermediate precipitation, and concentrated brine crystallization to achieve high recovery with zero liquid discharge (ZLD). Pilot tests demonstrate technical viability of the selected processes to achieve high recovery, radium and hardness reduction, and over 95% salinity reduction with zero liquid discharge. The results indicate that the energy usage per unit volume of water produced from groundwater is consistently lower than coastal seawater desalination, regardless of the conveyance distance. The substantial reduction of energy, higher recovery and minimized residual discharge of this process are also beneficial to the environment when compared to conventional processes currently being used. The results may be applicable and beneficial to other regions with similar conditions.

Towards more accurate design and specification of aeration systems using on-site column testing

D. Rosso, L.-M. Jiang, D.M. Hayden, P. Pitt, C.S. Hocking, S. Murthy, and M.K. Stenstrom (2012)

Water Science & Technology 66(3):627-634

ABSTRACT - Fine-pore diffuser systems are selected for their potential energy efficiency, and during design their propensity for fouling and for an increase in pressure drop with time must be considered. Both fouling and pressure-drop increase cause an increase in blower power requirements. This paper presents a new approach to improve this design procedure, without altering the technical structure of the classical approach. While the administrative and bidding milestones are being carried out (i.e., in the first 6 months of the project milestones), an independent aeration team can test candidate diffusers suitable for design in an aeration column in situ. An extended fouling test in the plant's aeration tanks allows the collection of site-specific aeration performance data. These improve the accuracy of the design process, and limit the reliance on safety factors.

The relationship between mixed-liquor particle size and solids retention time in the activated sludge process

L. Chan, S.-Y. Leu, D. Rosso, and M.K. Stenstrom (2011)

Water Environment Research 83(12):2178-2186

ABSTRACT - Particle size distribution (PSD) analysis was used to evaluate the quality of mixed liquors collected from different activated sludge process modifications (i.e., conventional activated sludge, modified Ludzack-Ettinger, high-purity oxygen, step-anoxic, and oxidation ditch). An experiment protocol was developed to define the allowable sample holding time and provide representative and repeatable results. Samples of 26 treatment plants, with a total of 37 samples, were tested. A new indicator, called mean particle size (MPS), was introduced to describe the integrated mean particle size. The results of MPSs of three cut-off sizes (0.5 to 50, 100, and 200 um) showed that the average size of mixed-liquor biosolids increased with increasing solids retention time (SRT), and the number of particles in the sedimentation supernatant decreased with increasing SRT. Particle deflocculation occurred after excessive sample holding time, and analysis within 12 hours generally eliminated sample holding problems. The results provide a methodology using PSD for characterizing mixed-liquor biosolids.

Statistical evaluation of photon count rate data for nanoscale particle measurement in wastewaters

J. Smeraldi, R. Ganesh, J. Safarik, and D. Rosso (2012)

Journal of Environmental Monitoring 14:79-84

ABSTRACT - The dynamic light scattering (DLS) technique can detect the concentration and size distribution of nanoscale particles in aqueous solutions by analyzing photon interactions. This study evaluated the applicability of using photon count rate data from DLS analyses for measuring levels of biogenic and manufactured nanoscale particles in wastewater. Statistical evaluations were performed using secondary wastewater effluent and a Malvern Zetasizer. Dynamic light scattering analyses were performed equally by two analysts over a period of two days using five dilutions and twelve replicates for each dilution. Linearity evaluation using the sixty sample analysis yielded a regression coefficient R2 = 0.959. The accuracy analysis for various dilutions indicated a recovery of 100 +/- 6%. Precision analyses indicated low variance coefficients for the impact of analysts, days, and within sample error. The variation by analysts was apparent only in the most diluted sample (intermediate precision 12%), where the photon count rate was close to the instrument detection limit. The variation for different days was apparent in the two most concentrated samples, which indicated that wastewater samples must be analyzed for nanoscale particle measurement within the same day of collection. Upon addition of 10 mg l-1 of nanosilica to wastewater effluent samples, the measured photon count rates were within 5% of the estimated values. The results indicated that photon count rate data can effectively complement various techniques currently available to detect nanoscale particles in wastewaters.

Oxygen transfer and uptake, nutrient removal, and energy footprint of parallel full-scale IFAS and activated sludge processes

D. Rosso, S.E. Lothman, M.K. Jeung, P. Pitt, W.J. Gellner, A.L. Stone, and D. Howard (2011)

Water Research 45(18):5987-5996

ABSTRACT - Integrated fixed-film activated sludge (IFAS) processes are becoming more popular for both secondary and sidestream treatment in wastewater facilities. These processes are a combination of biofilm reactors and activated sludge processes, achieved by introducing and retaining biofilm carrier media in activated sludge reactors. A full-scale train of three IFAS reactors equipped with AnoxKaldnes media and coarse-bubble aeration was tested using off-gas analysis. This was operated independently in parallel to an existing full-scale activated sludge process. Both processes achieved the same percent removal of COD and ammonia, despite the double oxygen demand on the IFAS reactors. In order to prevent kinetic limitations associated with DO diffusional gradients through the IFAS biofilm, this systems was operated at an elevated dissolved oxygen concentration, in line with the manufacturer's recommendation. Also, to avoid media coalescence on the reactor surface and promote biofilm contact with the substrate, high mixing requirements are specified. Therefore, the air flux in the IFAS reactors was much higher than that of the parallel activated sludge reactors. However, the standardized oxygen transfer efficiency in process water was almost same for both processes. In theory, when the oxygen transfer efficiency is the same, the air used per unit load removed should be the same. However, due to the high DO and mixing requirements, the IFAS reactors were characterized by elevated air flux and air use per unit load treated. This directly reflected in the relative energy footprint for aeration, which in this case was much higher for the IFAS system than activated sludge.

Effects of soluble and particulate substrate on the carbon and energy footprint of wastewater treatment processes

R. Gori, L.-M. Jiang, R. Sobhani, and D. Rosso (2011)

Water Research 45(18):5858-5872

ABSTRACT - Most wastewater treatment plants monitor routinely carbonaceous and nitrogenous load parameters in influent and effluent streams, and often in the intermediate steps. COD fractionation discriminates the selective removal of VSS components in different operations, allowing accurate quantification of the energy requirements and mass flows for secondary treatment, sludge digestion, and sedimentation. We analysed the different effects of COD fractions on carbon and energy footprint in a wastewater treatment plant with activated sludge in nutrient removal mode and anaerobic digestion of the sludge with biogas energy recovery. After presenting a simple rational procedure for COD and solids fractions quantification, we use our carbon and energy footprint models to quantify the effects of varying fractions on carbon equivalent flows, process energy demand and recovery. A full-scale real process was modelled with this procedure and the results are reported in terms of energy and carbon footprint. For a given process, the increase of the ratio sCOD/COD increases the energy demand on the aeration reactors, the associated CO2 direct emission from respiration, and the indirect emission for power generation. Even though it appears as if enhanced primary sedimentation is a carbon and energy footprint mitigation practice, care must be used since the nutrient removal process downstream may suffer from an excessive bCOD removal and an increased mean cell retention time for nutrient removal may be required.

Nitrous Oxide Emissions from Wastewater Treatment and Water Reclamation Plants in Southern California

A. Townsend-Small, D.E. Pataki, L.Y. Tseng, C.-Y. Tsai, and D. Rosso (2011)

Journal of Environmental Quality 40:1542-1550

ABSTRACT - Nitrous oxide (N2O) is a long-lived and potent greenhouse gasproduced during microbial nitrification and denitrification.In developed countries, centralized water reclamation plantsoften use these processes for N removal before effluent is usedfor irrigation or discharged to surface water, thus making thistreatment a potentially large source of N2O in urban areas. In thearid but densely populated southwestern United States, waterreclamation for irrigation is an important alternative to long-distance water importation. We measured N2O concentrationsand fluxes from several wastewater treatment processes in urbansouthern California. We found that N removal during waterreclamation may lead to in situ N2O emission rates that arethree or more times greater than traditional treatment processes(C oxidation only). In the water reclamation plants tested, N2Oproduction was a greater percentage of total N removed (1.2%) than traditional treatment processes (C oxidation only) (0.4%).We also measured stable isotope ratios (d15N and d18O) ofemitted N2O and found distinct d15N signatures of N2O fromdenitrification (0.0 ± 4.0‰) and nitrification reactors (-24.5 ± 2.2‰), respectively. These isotope data confirm that bothnitrification and denitrification contribute to N2O emissionswithin the same treatment plant. Our estimates indicate thatN2O emissions from biological N removal for water reclamationmay be several orders of magnitude greater than N2O emissionsfrom agricultural activities in highly urbanized southernCalifornia. Our results suggest that wastewater treatment thatincludes biological nitrogen removal can significantly increaseurban N2O emissions.

Molecular characterization of effluent organic matter identified by ultrahigh resolution mass spectrometry

M. Gonsior, M. Zwartjesa, W.J. Cooper, W. Song, K.P. Ishida, L.Y. Tsengd, M.K. Jeung, D. Rosso, N. Hertkorne, and P. Schmitt-Koppline (2011)

Water Research 45(9): 2943-2953

ABSTRACT - Effluent dissolved organic matter (EfOM) collected from the secondary-treated wastewater of the Orange County Sanitation District (OCSD) located in Fountain Valley, California, USA was compared to natural organic matter collected from the Suwannee River (SRNOM), Florida using ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Furthermore, the two different treatment processes at OCSD, activated sludge and trickling filter, were separately investigated. The blend of these two effluents was further evaluated after it had passed through the microfiltration process of the Advanced Water Purification Facility (AWPF) at Orange County Water District (OCWD). EfOM contained 872 different m/z peaks that were unambiguously assigned to exact molecular formulae containing a single sulfur atom and carbon, hydrogen and oxygen atoms (CHOS formulae). In contrast, the SRNOM sample only contained 152 CHOS formulae. The trend in CHO molecular compositions was opposite with 2500 CHO formulae assigned for SRNOM but only about 1000 for EfOM. The CHOS-derived mass peaks with highest abundances in EfOM could be attributed to surfactants such as linear alkyl benzene sulfonates (LAS), their co-products dialkyl tetralin sulfonates (DATS) and their biodegraded metabolites such as sulfophenyl carboxylic acids (SPC). The differences between the treatments were found minor with greater differences between sampling dates than treatment methods used.

Evaluation of Nanocopper Removal and Toxicity in Municipal Wastewaters

R. Ganesh, J. Smeraldi, T. Hosseini, L. Khatib, B.H. Olson, and D. Rosso (2010)

Environmental Science & Technology 44(20): 7808-7813

ABSTRACT - Bench scale studies were performed to evaluate removal and toxicity of copper nanoparticles (CuNPs) and copper ions in activated sludge biomass. The data indicated that, under the test conditions, copper nanoparticles were removed more effectively (95%) than copper ions (30-70%) from the wastewater. Mechanisms of CuNP removal were further investigated by equilibrating CuNP and copper ion in activated sludge filtrate (0.45 um). The predominant mechanisms of copper removal appear to be aggregation and settling (CuNP) or precipitation (copper ion) rather than biosorption. Most probable number (MPN) test data indicated that addition of 10 mg/L of copper ion was toxic to both coliform and ammonia oxidizing bacteria in the wastewater while no inhibitory effects were observed with the addition of the same amount of copper nanoparticles. Respirometry data indicated a 55% decrease in respiration rate when 10 mg/L ionic copper was added. However, no significant decrease in respiration rate was observed in the presence of copper nanoparticles. The toxicity of copper to activated sludge microorganisms appears to be a function of the concentration and characteristics of copper remaining in solution/suspension.

Carbon footprint of aerobic biological treatment of winery wastewater

D. Rosso and D. Bolzonella (2009)

Water Science and Technology 60(5): 1185-1189

ABSTRACT - The carbon associated with wastewater and its treatment accounts for approximately 6% of the global carbon balance. Within the wastewater treatment industry, winery wastewater has a minor contribution, although it can have a major impact on wine-producing regions. Typically, winery wastewater is treated by biological processes, such as the activated sludge process. Biomass produced during treatment is usually disposed of directly, i.e. without digestion or other anaerobic processes.
We applied our previously published model for carbon-footprint calculation to the areas worldwide producing yearly more than 106 m3 of wine (i.e., France, Italy, Spain, California, Argentina, Australia, China, and South Africa). Datasets on wine production from the Food and Agriculture Organisation were processed and wastewater flow rates calculated with assumptions based on our previous experience. Results show that the wine production, hence the calculated wastewater flow, is reported as fairly constant in the period 2005-2007. Nevertheless, treatment process efficiency and energy-conservation may play a significant role on the overall carbon-footprint. We performed a sensitivity analysis on the efficiency of the aeration process (aSOTE per unit depth, or aSOTE/Z) in the biological treatment operations and showed significant margin for improvement. Our results show that the carbon-footprint reduction via aeration efficiency improvement is in the range of 8.1 to 12.3%.

Temperature modelling and prediction for activated sludge systems

S. Lippi, D. Rosso, C. Lubello, R. Canziani and M. K. Stenstrom (2009)

Water Science and Technology 59(1): 125-131

Temperature is an important factor affecting biomass activity, which is critical to maintain
efficient biological wastewater treatment, and also physiochemical properties of mixed liquor as
dissolved oxygen saturation and settling velocity. Controlling temperature is not normally possible
for treatment systems but incorporating factors impacting temperature in the design process,
such as aeration system, surface to volume ratio, and tank geometry can reduce the range of
temperature extremes and improve the overall process performance. Determining how much
these design or up-grade options affect the tank temperature requires a temperature model that
can be used with existing design methodologies. This paper presents a new steady state
temperature model developed by incorporating the best aspects of previously published models,
introducing new functions for selected heat exchange paths and improving the method for
predicting the effects of covering aeration tanks. Numerical improvements with embedded
reference data provide simpler formulation, faster execution, easier sensitivity analyses,
using an ordinary spreadsheet. The paper presents several cases to validate the model.

Real-Time Efficiency Monitoring for Wastewater Aeration Systems

Shao-Yuan Leu, Diego Rosso, Pan Jiang, Lory E. Larson, Michael K. Stenstrom (2009)

Water Practice and Technology 30(3)

Aeration is the most energy intensive unit operation in municipal wastewater treatment. To improve oxygen transfer rate, fine-pore diffusers have been wildly applied in aeration practice. However, during operation, this type of diffuser
suffers from fouling and scaling problems, which cause a rapid decline in aeration performance and significant increase in energy consumption. Diffusers must be cleaned periodically to reduce energy costs. The cleaning frequency of diffusers is site-specific, and can be evaluated by oxygen transfer efficiency (OTE) measurements over time in operation. Off-gas testing is the only technique that directly measures oxygen transfer efficiency in real-time. This paper presents a time-series of off-gas measurements which demonstrate the value of implementing energy-conservation practices. Our results include the real-time prediction of plant load and alpha factors from off-gas testing, as well as the quantification of the increased energy costs caused by fouling. Our off-gas analyzer can be used to develop an aeration efficiency monitoring protocols, and an aeration feedback control system for blowers.

SewerSnort: A Drifting Sensor for In-situ Sewer Gas Monitoring

Jihyoung Kim, Jung Soo Lim, Jonathan Friedman, Uichin Lee, Luiz Vieira, Diego Rosso, Mario Gerla, Mani B. Srivastava


Biochemical activities in sewer pipes generate various volatile substances that lead to several serious problems such as malodor complaints and lawsuits, concrete and metal corrosion, increased operational costs, and health risks. Frequent inspections are critical to maintain sewer health, yet are extremely expensive given the extent of the sewer system and the "unfriendliness" of the environment. In this paper we propose SewerSnort, a low cost, unmanned, fully automated in-sewer gas
monitoring system. A sensor float is introduced at the upstream station and drifts to the end pumping station, collecting location tagged gas measurements. The retrieved SewerSnort provides an accurate gas exposure profile to be used for preventive maintenance and/or repair. The key innovations of SewerSnort are the fully automated, end-to-end monitoring solution and the low energy self localizing strategy. From the implementation standpoint, the key enablers are the float mechanical design that fits the sewer constraints and the embedded sensor design that matches the float form factor and complies with the tight energy constraints. Experiments based on a dry land emulator demonstrate
the feasibility of the SewerSnort concept, in particular, the localization technique and the embedded sensor design.

The carbon-sequestration potential of municipal wastewater treatment

D. Rosso and M.K. Stenstrom (2008)

Chemosphere 70(8): 1468-1475

ABSTRACT - The lack of proper wastewater treatment results in production of CO2 and CH4 without the opportunity for carbon sequestration and energy recovery, with deleterious effects for global warming. Without extending wastewater treatment to all urban areas worldwide, CO2 and CH4 emissions associated with wastewater discharges could reach the equivalent of 1.91x105 tCO2 d-1 in 2025, with even more dramatic impact in the short-term. The carbon sequestration benefits of wastewater treatment have enormous potential, which adds an energy conservation incentive to upgrading existing facilities to complete wastewater treatment. The potential greenhouse gases discharges which can be converted to a net equivalent CO2 credit can be as large as 1.21x104 t CO2 d-1 by 2025. Biomass sequestration and biogas conversion energy recovery are the two main strategies for carbon sequestration and emission offset, respectively. The greatest potential for improvement is outside Europe and North America, which have largely completed treatment plant construction. Europe and North America can partially offset their CO2 emissions and receive benefits through the carbon emission trading system, as established by the Kyoto protocol, by extending existing technologies or subsidizing wastewater treatment plant construction in urban areas lacking treatment.
This strategy can help mitigate global warming, in addition to providing a sustainable solution for extending the health, environmental, and humanitarian benefits of proper sanitation.

Fine-pore aeration diffusers: accelerated membrane ageing studies

A. Kaliman, D. Rosso, S.-Y. Leu, M.K. Stenstrom (2008)

Water Research 42(1-2): 467-475

ABSTRACT – Polymeric membranes are widely used in aeration systems for biological treatment. These membranes may degrade over time and are sensitive to fouling and scaling. Membrane degradation is reflected in a decline in operating performance and higher headloss, resulting in increased energy costs. Mechanical property parameters, such as membrane hardness, Young’s modulus, and orifice creep, were used to characterise the performance of membranes over time in operation and to predict their failure. Used diffusers from municipal wastewater treatment plants were collected and tested for efficiency and headloss, and then dissected to facilitate measurements of Young’s modulus, hardness, and orifice creep. Higher degree of membrane fouling corresponded consistently with larger orifice creep. A lab-scale membrane ageing simulation was performed with polyurethane and four different ethylene-propylene-diene (EPDM) membrane diffusers by subjecting them to chemical ageing cycles and periodic testing. The results confirmed full scale plant results and showed the superiority of orifice creep over Young’s modulus and hardness in predicting diffuser deterioration.

Energy-Saving Benefits of Denitrification

D. Rosso and M.K. Stenstrom (2007)

Environmental Engineer 43(3): 29-38

ABSTRACT - Nitrogen removal in wastewater treatment can be achieved by introducing anoxic zones in biological reactors within an activated sludge process, operating at medium or long mean cell retention time (MCRT). Anoxic zones at the head of the process also function as biological selectors and provide benefits in addition to nitrogen removal, including improved stability by avoiding filamentous bulking, enhanced removal of many recalcitrant pollutants and reduced energy consumption due to the oxygen credit and higher oxygen transfer efficiency. Improved oxygen transfer occurs because the readily biodegradable organic compounds are used by the denitrifiers for nitrate reduction. These organic compounds, which are surfactants, would otherwise reduce oxygen transfer efficiency, increasing plant operating costs. We tested 22 treatment plants which included either conventional, nitrifying-only, or nitrifying-denitrifying (NDN) operations. Off-gas tests confirm that oxygen transfer efficiency for NDN operations is higher. Our economic analyses show that NDN operation can have the lowest aeration costs, contrary to long-standing beliefs. The net operating costs can be lower than conventional, short MCRT operation and are always lower than nitrifying-only operation. However, depending on the local plant situation, expansion of aeration volume and/or clarifier area might be necessary, and the operating savings could be offset by debt service on plant expansion.

Surfactant Effects on Alpha Factors in Full-Scale Wastewater Aeration Systems

D. Rosso, Lory E. Larson, M.K. Stenstrom (2006)

Water Science and Technology 54(10): 143-153

ABSTRACT - Aeration is an essential process in the majority of wastewater treatment processes, and accounts for the largest fraction of plant energy costs. Aeration systems can achieve wastewater oxygenation by shearing the surface (surface aerators) or releasing bubbles at the bottom of the tank (coarse- or fine-bubble aerators). Surfactants accumulate on gas-liquid interfaces and reduce mass transfer rates. This reduction in general is larger for fine-bubble aerators. This study was conducted to evaluate mass transfer effects on the characterization and specification of aeration systems in clean and process water conditions. Tests at different interfacial turbulence regimes were analysed, showing higher gas transfer depression for lower turbulence regimes. Higher turbulence regimes can offset contamination effects, at the expense of operating efficiency. This phenomenon is characteristic of surface aerators and coarse bubble diffusers and is here discussed. The results explain the variability of a factors measured at small scale, due to uncontrolled energy density. Results are also reported in dimensionless empirical correlations that describe mass transfer as a function of physiochemical and geometrical characteristics of the aeration process.

Economic Implications of Fine-Pore Diffuser Aging

D. Rosso, M.K. Stenstrom (2006)

Water Environment Research 78: 810-815

ABSTRACT - Aerobic biological treatment, such as the activated sludge process, is commonly used for municipal wastewater treatment. Fine-pore diffusers have virtually replaced coarse-bubble diffusers in such operations, but most of the plant’s energy expenditure is still consumed by aeration. Therefore, the performance of diffusers will critically affect plant economics. This paper analyzes and quantifies the consequences of aging processes on fine-pore diffusers. Datasets from 94 field measurements were analyzed and showed a clear pattern of performance decline with time in operation. Efficiency declines rapidly during the first 24 months of operation when the rate of decline decreases and efficiency stabilizes at a low value. For example, cost analysis scenarios were performed using the measured rate of decline in diffuser performance. The analyses include loss of transfer efficiency and elevated headloss, which both increase operating cost. Cleaning the diffusers within 12 months of operation is generally economically favorable, restores efficiency, and reduces power overhead. Periodic cleaning prolongs the economically viable lifespan for the aeration system.

Effects of interfacial surfactant contamination on bubble gas transfer

D. Rosso, D.L. Huo, M.K. Stenstrom (2006)

Chemical Engineering Science 61: 5500-5514

ABSTRACT - Surface active agents depress gas transfer at gas–liquid interfaces. They are present as measurable trace contaminants at all environmental and at most industrial interfaces. An experimental apparatus to concurrently measure dynamic surface tension and mass transfer was constructed and tested for single-bubble and multi-bubble experiments. In this work, the parameters describing time-dependent bubble surface contamination were characterized. The application of a Ward–Tordai transient model and of a Langmuir saturation model showed that for fine-bubbles in low molecular weight surfactant solutions the interfacial surfactant accumulation equilibrates before bubble detachment. This is reflected in the bubbles behaving as solid-spheres, which is shown in our dimensionless results. For a given contamination, interfaces with higher renewal rates have higher mass transfer. At higher renewal rates, the variation due to different contamination is smaller than the variation at lower renewal rates, concluding that higher interfacial flow regimes can offset contamination. Our experimental evidence shows a gas transfer reduction of 30–70% of pure water values in surfactant solutions, which confirms full-scale field measurements. Results are consistent with expectations and correct previous Frössling-like dimensionless correlations for pure water systems. Our results offer a tool for mass transfer prediction from flow regime and surfactant properties.

Surfactant effects on aplha-factors in aeration systems

D. Rosso, M.K. Stenstrom (2006)

Water Research 40(7): 1397-1404

ABSTRACT - Aeration in wastewater treatment processes accounts for the largest fraction of plant energy costs. Aeration systems function by shearing the surface (surface aerators) or releasing bubbles at the bottom of the tank (coarse- or fine-bubble aerators). Surfactant accumulation on gas-liquid interfaces reduces mass transfer rates, and this reduction in general is larger for fine-bubble aerators. This study evaluates mass transfer effects on the characterization and specification of aeration systems in clean and process water conditions. Tests at different interfacial turbulence regimes show higher gas transfer depression for lower turbulence regimes. Contamination effects can be offset at the expense of operating efficiency, which is characteristic of surface aerators and coarse bubble diffusers. Results describe the variability of a factors measured at small scale, due to uncontrolled energy density. Results are also reported in dimensionless empirical correlations describing mass transfer as a function of physiochemical and geometrical characteristics of the aeration process.

Comparative economic analysis of the impacts of mean cell retention time and denitrification on aeration systems

D. Rosso, M.K. Stenstrom (2005)

Water Research 39: 3773-3780

ABSTRACT - Biological nutrient removal is practiced in various modifications of the activated sludge process (ASP) throughout the world. This paper compares conventional, nitrifying-only and combined nitrifying/denitrifying (NDN) processes. The authors performed 113 oxygen transfer efficiency measurements with the off-gas method over 20 years. This dataset was analysed and used to perform an economic analysis for three example scenarios, one for each layout (conventional, nitrifying-only and NDN). Field oxygen transfer efficiency and relevant plant operative costs and credits were considered (i.e., aeration cost, sludge disposal cost, methane production credit). The conclusion is that NDN operations always have lower aeration costs, and generally have the lowest combined operating cost. Reduced aeration costs result because of improved aeration efficiency at higher mean cell retention times and the use of nitrate as an electron acceptor. The improved aeration efficiency overcomes the increased oxygen required at higher cell retention time due to cell decay.

Fifteen years of off-gas transfer efficiency measurements on fine pore aerators: key role of sludge age and normalized air flux

D. Rosso, R. Iranpour, M.K. Stenstrom (2005)

Water Environment Research 77(3): 266-273

ABSTRACT - Fine pore diffusers, often called fine bubble diffusers, have nearly replaced coarse bubble diffusers in municipal wastewater treatment over the past 20 years. The rapid increases in energy costs which began in the 1970's created financial incentives to upgrade to this more expensive and maintenance intensive method of aeration. Fine pore diffusers have the added benefit of reducing VOC stripping and reduced aeration heat loss. This paper summarizes 15 years of oxygen transfer efficiency measurements using the off-gas technique. Efficiencies are shown for different types of diffusers at various tank geometries (depth, diffuser size and number), air flow rates, and mean cell retention times (MCRT or sludge age). Normalizing the air flow rates per unit of depth and diffusing area, efficiencies measured in different plants can be compared. The results show that aeration efficiencies are logarithmically related to the ratio between MCRT and the normalized air flux, with transfer rates suppressed by low MCRT or high normalized air flux systems. There is no evidence for different a factors among the different types of fine bubble diffuser types.

Aeration of large-scale municipal wastewater treatment plants: state of the art

D. Rosso, L.E. Larson, M.K. Stenstrom (2008)

Water Science and Technology 57(7): 973-978

ABSTRACT - Aeration is the most energy intensive operation in wastewater treatment, amounting to 45–75% of plant energy costs. Fine-pore diffusers are today almost ubiquitous in municipal wastewater aeration, due to their advantageous aeration efficiency (mass of oxygen transferred per unit energy required). Nevertheless, older municipal treatment facilities and many industrial treatment plants are still equipped with coarse-bubble or surface aerators. Fine-pore diffusers are subject to two major disadvantages: a) fouling, if not cleaned periodically; b) decrease in oxygen transfer efficiency caused by dissolved surfactants. Coarse-bubble and surface aerators are typically not subject to the traditional problems affecting fine-pore diffusers. Nonetheless, they achieve oxygen transfer at the expense of increased energy intensity. The increased biomass concentration associated with high mean cell retention time (MCRT) operations has a beneficial effect on aeration. Nutrient-removing selectors are able to further increase aeration efficiency, as they sorb and utilize the readily available substrate which otherwise would accumulate at bubble surfaces and dramatically decrease aeration efficiency. We summarise here our 30-year long experience in aeration research, and results obtained with clean- and process-water tests are used to show the beneficial effects of high MCRT operations, the beneficial effect of selectors, and the decline of aeration efficiency due to dissolved surfactants.

Membrane properties change in fine-pore aeration diffusers: Full-scale variations of transfer efficiency and headloss

D. Rosso, J.A. Libra, W. Wiehe, M.K. Stenstrom (2008)

Water Research 42: 2640-2648

ABSTRACT - Fine-pore diffusers are the most common aeration system in municipal wastewater treatment. Punched polymeric membranes are often used in fine-pore aeration due to their advantageous initial performance. These membranes are subject to fouling and scaling, resulting in increased headloss and reduced oxygen transfer efficiency, both contributing to increased plant energy costs. This paper describes and discusses the change in material properties for polymeric fine-pore diffusers, comparing new and used membranes. Three different diffuser technologies were tested and sample diffusers from two wastewater treatment facilities were analysed. The polymeric membranes analysed in this paper were composed of ethylene–propylene-diene monomer (EPDM), polyurethane, and silicon. Transfer efficiency is usually lower with longer times in operation, as older, dilated orifices produce larger bubbles, which are unfavourable to mass transfer. At the same time, headloss increases with time in operation, since membranes increase in rigidity and hardness, and fouling and scaling phenomena occur at the orifice opening. Change in polymer properties and laboratory test results correlate with the decrease in oxygen transfer efficiency.

Oxygen Transfer in a Full-Depth Biological Aerated Filter

M.K. Stenstrom, D. Rosso, H. Melcer, R. Appleton, V. Occiano, A. Langworthy, P. Wong (2008)

Water Environment Research 80(7): 663-671

ABSTRACT - The City of San Diego, California, evaluated the performance capabilities of biological aerated filters (BAFs) at the Point Loma Wastewater Treatment Plant. The City conducted a 1-year pilot-plant evaluation of BAF technology supplied by two BAF manufacturers. This paper reports on the first independent oxygen-transfer test of BAFs at full depth using the offgas method. The tests showed process-water oxygentransfer efficiencies of 1.6 to 5.8%/m (0.5 to 1.8%/ft) and 3.9 to 7.9%/m (1.2 to 2.4%/ft) for the two different pilot plants, at their nominal design conditions. Mass balances using chemical oxygen demand and dissolved organic carbon corroborated the transfer rates. Rates are higher than expected from fine-pore diffusers for similar process conditions and depths and clean-water conditions for the same column and are mostly attributed to extended bubble retention time resulting from interactions with the media and biofilm.


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