Focused Practice

I. Answer the following questions:

1. What do different regimes of turbulent combustion depend upon?

2. What are asymptotic methods used for?

3. Why is it convenient to treat turbulent premixed flames and turbulent diffusion flames separately?

4. What presentations may become appropriate in the future?

5. What combustion do burning velocities exist for?

6. When can asymptotic methods be fully exploited?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: The reaction sheet regimes.

Unit 15 Grammar: The verbs “to be”, “to have”. Modal Verbs Word List:

1. membrane surface	поверхность мембраны
2. adhesion	слипание, прилипание, сцепление
3. biofouling	биозагрязнение
4. hydrophobic material	гидрофобный материал
5. to exceed	превышать
6. at the expense of	за счёт
7. ubiquitons	вездесущий, повсеместный
8. nutrient concentration	концентрация питательных веществ
9. tolerance level	допустимый уровень
10. regardless of	независимо от, несмотря на

Membranes and Microorganisms

The treatment of water by membrane technology intrinsically implies the contact of very large quantities of water with the membrane surfaces. This water is not sterile. In drinking water, the numbers of cells actually present as demonstrated by microscopic quantification usually range between 10⁴ and 10⁶ cells/ml. These cells have a tendency to adhere to surfaces; in oligotrophic systems, this is considered a survival strategy. There is virtually no surface material which cannot be colonized, even under extreme conditions; regardless of hydrophobicity or hydrophilicity, smoothness or chemical composition - surface conditions and materials will simply select for colonizing species among the spectrum of organisms in a given water volume. In a membrane system, adhesion to the membrane surface is facilitated by the vertical transport vector which is given by the water flow through the membrane - this can be described metaphorically as “love at first sight”, because there will always be some organisms which prefer to settle on the given membrane material, be it hydrophobic or hydrophilic. Once the organisms colonize the surface, they will inevitably multiply and form biofilms. All membrane systems which are not operated under absolutely sterile conditions will carry biofilms.

Not all of the systems carrying biofilms suffer from bio-fouling – “biofouling” is an operational term, applied when the effects of biofilms exceed a certain threshold, or tolerance level, which is individually set for different systems.

In membrane systems, however, biofouling is the “Achilles heel” of the process, because all other fouling components, such as organic and inorganic dissolved substances and particles can mostly be removed by efficient pretreatment; however, microorganisms are particles which can multiply. Thus, if they are removed to 99.99 %, there are still enough cells left which will grow at the expense of biodegradable substances in the water. Microorganisms are ubiquitous in any technical system unless it is kept sterile by enormous and continuous effort. The biofouling potential is represented by the types of microorganisms and the nutrient concentration.