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OFF-GAS TESTING WITH MODULAR HOODS FOR AERATION EFFICIENCY TESTING, AND CARBON- AND ENERGY- FOOTPRINT MODELING

Wastewater aeration is the most energy-intensive unit operation in wastewater treatment, and that with largest margin for improvement. We are currently testing several air diffusion technologies in our aeration tank in both clean and process water. This dual experiment allows us to quantify the effects of wastewater contaminants on oxygen transfer and to include them in the carbon-footprint model currently being developed.

Four years ago, as part of the project A DIGITAL CONTROL SYSTEM FOR OPTIMAL OXYGEN TRANSFER EFFICIENCY sponsored by the California Energy Commission, we developed an automated off-gas analyzer and modular gas collection hoods equipped with dissolved oxygen probes mounted inside. Modular hoods can be used in single or multiple configurations, to study off-gas, aeration efficiency gradients, dissolved oxygen gradients, water velocity and mixing gradients. These are part of public domain and their specifications and drawings can be obtained by e-mailing us.

NITROUS OXIDE EMISSIONS IN WASTEWATER TREATMENT AND ITS CARBON-FOOTPRINT

This project aims at quantifying the nitrous oxide (N2O) emissions from wastewater treatment plants, and at calculating their contribution to process carbon footprint. Until recently, the N2O contribution to process emissions have been neglected throughout the wastewater research community (including our previous publications), because of the small amount of direct emissions and the measurement challenges. Despite the small amount of N2O emitted, its global warming potential is approximately 300 times that of CO2, thereby contributing with great effect to the total carbon-footprint calculation. We are able to directly measure N2O in samples collected in wastewater treatment plants in the liquid and gas phase, thus complementing our previous model for wastewater treatment process carbon-footprint calculation.

CARBON- AND ENERGY-FOOTPRINT OF BNR UPGRADES IN CALIFORNIA'S WASTEWATER TREATMENT PLANTS

Californians discharge each year approximately 3 trillion gallons of wastewater, which require approximately 2 billion kWh for treatment, according to the California Energy Commission. More stringent environmental regulations are currently requiring wastewater treatment plants in the West Coast to undergo process upgrade and include biological nutrient removal (BNR). If not removed in wastewater treatment plants, nutrients cause eutrophication in surface water bodies and severe damage to groundwater aquifers. In addition to the benefits of removing nutrients, BNR upgrades have the advantage of significant energy-savings. In our previous publications, we demonstrated that the energy-footprint of BNR processes is approximately 88% that of conventional wastewater treatment processes. In order to undergo BNR upgrade, wastewater treatment plants must face capital improvements in the form of structural and mechanical retrofit. These improvements require considerable amounts of construction and of mechanical parts, both associated with energy-intensive materials (i.e., cement and steel) and energy-intensive installation, with substantially negative effects on the energy-footprint of the upgrade's construction. In this project we compare the carbon- and energy- footprint of BNR structural improvements with the energy-savings of the process upgrade. We expect that after an initial period of negative balance, the long-term benefits of BNR upgrades will yield positive reduction of overall energy-footprint.

CARBON-FOOTPRINT ANALYSIS OF BIOSOLIDS GENERATED IN WASTEWATER TREATMENT

Biosolids are generated by digestion of sewage sludge and are posing a significant problem in large urban sprawls throughout the United States . This project is divided in two phases: I) Comparing the carbon-footprint of shipping biosolids to a point of disposal with the carbon-sequestration associated with biosolids disposal; II) Analysis of biosolids disposal options (i.e., shipment to landfill, or to land farm, or on-site incineration) and comparative carbon-footprint analysis. (Click on the image below to download our ppt presentation from CWEA 2009)

REMOVAL OF SELECTED PHARMACEUTICALS AND OTHER ENDOCRINE DISRUPTORS FROM DRINKING WATER AND ITS CARBON-FOOTPRINT

This project is a joint effort of our group, Bill Cooper's lab and Ken Ishida t Orange County Water District. Our group's involvement is centered on the calculation of carbon-footprint for each unit operation involved in the process (i.e., microfiltration, reverse osmosis, hydrogen peroxide addition and ultraviolet irradiation), and of the whole process. The goal to minimize process carbon footprint is being pursued by minimizing the carbon footprint function of the overall process instead of adding the minima of carbon footprints for each unit operation. This is a paradigm shift, as engineers have been operating processes by optimizing independently each operation for a very long time. For example, membrane filtration processes (such as microfiltration and reverse osmosis) have always been operated by monitoring the energy consumption for each individual operation and by attempting to minimize it. But since each unit operation has a different weight on the overall energy balance, it is possible to overburden an upstream operation if this results in a much more significant energy relief downstream.

CARBON FOOTPRINT ANALYSIS OF H2O2 STORAGE SCENARIOS IN WATER RECLAMATION PLANTS

Hydrogen peroxide is employed as disinfectant in water reclamation plants, but can rapidly decompose if not refrigerated, diminishing its effectiveness and requiring frequent delivery. We calculated the carbon footprint of hydrogen peroxide delivery and decay, and projected scenarios for footprint minimization. We calculated delivery frequency as function of the peroxide volume remaining in the storage tank. Peroxide storage footprint increases with delivery frequency and ambient temperature, when not refrigerated. Carbon footprint is minimized by on-site peroxide generation (i.e., no emissions for delivery). Less frequent re-stocking (i.e., when more air space is available in the storage tank), accelerates peroxide decomposition. Our experiments confirm that ambient temperature has a significant effect on decomposition rate. Since outside temperatures in most of the United States , in particular in California , are much higher than refrigeration temperatures, decomposition is inevitable. The energy required for refrigeration raises overall footprint but without the shortcomings of decomposition.

WASTEWATER TREATMENT AS A GLOBAL WARMING MITIGATION FACTOR

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. 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 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.

 

REAL-TIME AERATION EFFICIENCY MONITORING WITH THE OFF-GAS TECHNIQUE

We are currently developing an energy-efficiency monitoring device and a testing protocol for municipal wastewater treatment plants. This aims at minimizing the energy expenditure that treatment facilities have to face day by day. The potential savings are enormous, as quantified in one of our publications.

MATERIAL PROPERTIES CHARACTERIZATION FOR FINE-PORE POLYMERIC MEMBRANE DIFFUSERS

Fine pore diffusers utilized in wastewater aeration undergo foulign, scaling, and material changes. These affect bubble release and bubble geometry. In general, polimers experience orofice creep after time in operation, due to the tension applied by the air pressure inside the aerators onto the membranes. Dilated orifices release larger bubbles, which have lower mass transfer, and can lead to crack of the membrane, and sometimes to catastrophic membrane failure. Our ongoing research aims at quantifying the properties changes of diffusers harvested in the field, and at implementing an accelerated ageing protocol to simulate material changes in the laboratory.

CARBON FOOTPRINT OF WINERY WASTEWATER TREATMENT

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 in the biological treatment operations and showed significant margin for improvement.

 

HIGHLIGHTS


- UCI Urban Water Research Center Seminars
- UCLA Institute of the Environment Colloquia
- UCSD Environment and Sustainability Inititative
ENVIRONMENTAL PROCESS LABORATORY - 836 ENGINEERING TOWER - UNIVERSITY OF CALIFORNIA, IRVINE 92697-2175