DAILY PAPER REVIEW

1010_Pioneering demineralized and desalinated water cost reduction with innovative brackish...

 
 
ESEL Paper Review_20131010
By Hong Guo
Mail: hongguo@gist.ac.kr
Phone: (+82) (0)10 82276568
1, Title and Author
Title: Pioneering demineralized and desalinated water cost reduction with innovative brackish water RO membrane Technology
Journal: Desalination and Water treatment
Authors:
K.Majamaaa,*, A.Royb, J.Johnsonb, M. Peery b
a Dow Water &Process Solutions, Autovia Tarragona-Salou, s/n, 43006 Tarragona, Spain
B Dow Water&Solutions, 540 Dewey Hill Road, Edina, Mn 55439,USA
2. Summary of Paper
? This same feed spacer technology has also been leveraged to achieve active areas up to 500ft while maintaining pressure drop equivalent to today’s standard elements. These advances provide additional savings in energy and / or operational and capital costs of 10/16%. This paper describes the latest innovations in detail, and demonstrates the cost reduction impact in the price of demineralized and desalinated water.
3. Results
Feed spacer
? Feed spacers play a crucial role in the performance of the RO membrane module, impacting permeate qualify, energy consumption, and response to fouling.
? By far , the most common feed spacer configuration used in RO is the biplanar extruded net
? One of the early patents for making the net was obtained by Nalle, who described counter-rotating die for producing a continuous, cylindrical mesh that was slit to create a flat web.
? Most Ro feed spacers are made from polypropylene. Which offers the preferred combination of extrudability , low cost, and chemical inertness. Thicknesses between 0.6 and 0.9 mm are typical.
? Function of feed spacer: 1. it provides an open channel for the flowing feed water by maintaining separation between the membrane sheets. 2. It also promotes mixing and the movement of rejected substances away from the membrane surface.
? The spacer mixing effectiveness, or more precisely the mass transfer effectiveness, is expressed in terms of the concentration polarization of a given specie, usually a dissolved salt, which is partially or entirely rejected by the membrane.
? Polarization factor depends upon the local permeate flux, the mass diffusivity of the species of interest, the degree of rejection, and the extent of mass transfer.
? For the sodium chloride, conventional spacers and typical operating conditions provide average polarization factor in the range of 1.05~1.15. the osmotic barrier in many Ro application is therefore increased by 5-15% due to imperfect feed channel mixing. This increases direct energy consumption by about 2-4% in brackish water RO, and by about 4-8% in sea water desalination.
? Fouling mitigation may represent the most significant opportunity for operational savings through improved feed spacer design. However the magnitude of the potential improvement and the means by which spacers can reduce fouling through improved hydrodynamics are not yet well understood
? Examples of recent spacer research include investigations of bio fouling and particulate fouling . There appears to be less focus on the impact of spacers on other forms of fouling, such as colloidal and adsorptive organic fouling
The pressure drop tradeoff
? ? An unwanted byproduct of the mechanical support and mass transfer functions is feed channel pressure drop.
? Feed side pressure drop impacts system performance by reducing the trans-membrane pressure, and consequently the permeate production, in the downstream modules
? This under-utilization leads to over-utilization and increased rate of fouling in the upstream modules. These impacts are especially pronounced in low-energy brackish water application.
? Efforts to improve mass transfer through optimization of the biplanar extruded net and other configurations have not produced much change in commercial spacers, which remain similar to those used 20years
? Reasons:
1. The relatively small magnitude of the potential benefit associated with improved mass transfer compared to that achieved historically through improvements in membrane chemistry.
2. The mass transfer tied reduced polarization to increased pressure drop.
3. low cost of existing spacers. For low-energy brackish water, performance is improved by manipulating the spacer design to reduce pressure drop while minimizing the accompanying increase in polarization.
? 3D models of the extruded netting were used to predict the parametric impact of strand spacer, angle and thickness on pressure drop and mixing.
Lowe pressure drop spacers
? Optimized 34 mil spacer for 400 ft2 modules
? In high-fouling feed waters, thicker spacers have shown improved response to fouling by slowing the rate of pressure drop increase
? The optimized low pressure drop spacer showed lower initial pressure drop and a lower rate of pressure drop increase than either of the standard spacers. It also returned more closely to cleaning than did the standard spacers
? Low pressure drop 28-mil spacer for 440 ft2 modules
? For the majority of feed waters, the fouling potential is less severe and a thinner spacer may be used, bringing with it the savings of increased module active area.
? The optimized spacer reduced the feed ?side pressure drop by approximately 60% bin both the first and second stages. This improved the productivity of the downstream modules and lowered the required system feed pressure.
? The net applied pressure in the first stage was reduced by 7% with no change in first stage permeate quantity or quality. The reduction was sustained for the duration of the trial.
? New 23-mil spacer for 500ft2 modules
? The pressure drop performance of this module: feed- side pressure drop is plotted against feed flow rate. The pressure drop of the 500 ft2 modules is slightly lower than that of its 440ft2 counterpart. This is required in view of the higher feed flow rate anticipated in 500ft2 systems running at the same flux and overall recovery as similarly designed systems that use 400ft2 or 440ft2 modules
4. Contribution:
? Breakthrough RO membrane chemistry and innovative materials of construction have been developed to achieve highest possible Nacl rejection while maintaining maximum element productivity.
? Novel brackish water RO membrane element delivers to end users state of the art salt rejection (99.7%) at 30% lower energy consumption compared to current high rejection BWRO membranes. Combined with new low differential pressure feed spacers , end users can enjoy additionally up to 15% savings in direct RO energy reductions via the lowest feed ?side pressure drop available in the market.
? The feed spacer modification additionally enables a new 500 ft2 active are 8 inch BWRO element, which is the highest active product in the market.
?
This research studies the behavior of activated sludge and the obtained profiles are very appropriate date for future studies.
5. Contact (Mail address): kmajamaa@dow.com
? In the model only the aerobic respiration rates with intermittent aeration are introduced. The anoxic rate is simplified as being a fraction of the aerobic rate therefore a reduction factor , the same for AOB and NOB is used.
? The net growth rate of autotrophic bacteria is significant for the calculation of the minimum sludge retention time with regard to the nitrification capacity. Series of dedicated batch tests were made to assess net growth rates and their temperature and to illustrate the differences between AOB and NOB
? In this study of the calibration was made according to ta process of manual trial and error . The generation and use of statistical indicators such as correlation coefficient, average error, root mean square error or modelling efficiency was of little practical help and expert knowledge and visual fitness of the modeled curved were rather sued for the ultimate parameter calibration
? The selection of sensitive parameters for the calibration was made according to indications from earlier studies and an experience based approach.
Model validation
? The aeration control strategy of period B had a similar and even stronger effect on nitrite production than in period A. Constant aeration in Bio3 induces deficient aeration in Bio2, which leads to incomplete nitrification and high nitrite values in the effluent.
? Furthermore, since the maximum net growth rate of AOB is slightly higher than that of NOB above 20 c the NOB get progressively washed out during period B and nitrite started to accumulate in Bio3
? Heterotrophic denitrification is a complex combination of parallel and sequential processes carried out by different groups of heterotrophic microorganisms. The suggested two-step denitrification model with only one heterotrophic microorganism is therefore a strong simplification, similar to the simplified ASM3 assumption having first storage of substrate and then growth on storage product instead of parallel processes including direct growth on substrate. This limits the application of the extended model to the simulation of nitrite accumulation in conventional municipal wastewater treatment and the tested pre-denitrification flow schemes mostly used in Europe.
? For other flow schemes, as intermittent aeration and SBR the validity of the model parameters has to be verified with corresponding data.
? The presented model cannot be applied to side stream processes with high ammonium inlet concentrations because the accumulation of ammonium oxidizers and heterotrophic nitrite reducers due to inhibition of nitrite oxidizers cannot be accurately modeled.
4. Contribution:
? The presented two-step nitrification/denitrification model with one heterotrophic microorganism is a strong simplification of reality and limits the application of the extended model to the simulation of nitrite accumulation in municipal wastewater treatment.
? The model was able to reasonably describe the dynamics of the investigated pre-denitrification full-scale plant with a standard set of kinetic and stoichiometric parameters. For other flow schemes, as intermittent aeration and SBR the validity of the model parameters has to be verified with corresponding data
 
5. Contact (Mail address): leiv.rieger@gci.ulaval.ca

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