5. What factors have to be considered when designing a UV system?
FLOW AND TURBULENCE
Before any system can be designed it is vital to ascertain the amount of water that is to be treated and irradiated on within a certain time frame. This defines the flow rate, expressed in litres per hour (l/h) or in cubic meters per hour (m³/h) for higher flow rates. This flow rate must be determined for the actual peak demand, which means the highest possible litres per hour to be radiated during any 24-hour period.
To ensure that all microorganisms receive the correct UV intensity for the necessary length of time and thus reliable disinfection taking place, the turbulence within the reactor must be optimized. This will prevent the microorganisms from short circuiting the required flow path. It is therefore crucial that the correct dimensions and hydraulic characteristics are incorporated into the reactor at design stage, otherwise optimum turbulence may not be achieved at minimum demand levels.
TYPE AND CONCENTRATION OF MICROORGANISMS
A bacteriological examination of the water to be treated must be performed to detect sewage pollution of the water. As a general rule, water contains many types of relatively harmless saprophytes, which normally inhabit the soil, air and water. However, both ground and surface water may be contaminated by domestic sewage, which may contain human pathogens, such as Typhoid, Dysentery, Hepatitis, Diarrhea and Cholera. These viruses or bacteria are considered to be of faecal origin.
Contrary to common belief, no examinations are made to detect specific pathogenic microorganisms. The routine bacteriological analysis is based on the approximate determination of the total numbers of organisms present, plus the presence or absence of the more common organisms of intestinal or sewage origin.
The most common are the bacteria of the coliform group and they are therefore singled out to serve as index organisms of faecal pollution. The prevalent species are the various strains of Escherichia coli, followed by Enterobacter aerogenes.
The advantage of testing for these organisms rather than for specific pathogens is that the coliform organisms are constantly present in both healthy and diseased humans in large numbers, billions are excreted daily by the average person.
It is estimated that for every Typhoid bacillus or other pathogens e.g. viruses of Polio or Hepatitis in polluted water supplies, there are usually millions of coliform organisms, especially E. coli. Another advantage is that the coliform group survives for longer in an aquatic environment than do most other intestinal pathogens.
In addition, the presence of these organisms is easily detected using a relatively quick method, in contrast to the more tedious, time consuming identification of a specific pathogen. For complete termination, different pathogens need specific UV doses. In general, a determination is made of the UV dose required to terminate E. coli. This is the basis for all other system-design calculations.
LEVEL OF DISINFECTION REQUIRED
All applications will not require complete disinfection. In some instances partial reduction may be adequate i.e. when treating certain types of process, irrigation or closed recycling water systems.
INFLUENCE OF WATER TEMPERATURE
For low-pressure lamp systems the temperature of the water to be treated should fall into the range of 5°C to 30°C with optimum efficiency at 19°C.
TRANSMISSION POTENTIAL OF WATER (Transmissivity)
The ultimate amount of UV light that will pass through a volume of water depends on several factors, which relate to the physical properties of both the UV radiation and the water.
- The UV intensity at the lamp surface effects how far into the volume of water the radiation will be able to travel.
- Due to the inclusions of both organic and inorganic substances either suspended or dissolved in the water the UV light will either be absorbed or deflected by the inclusions. This creates shadow areas within the reactor because of the rectilinear nature of the UV light's propagation. This must be corrected by providing the proper turbulence (see again: Flow and Turbulence).
Factors that lower the transmissivity of water:
SUSPENDED SOLIDS
They effect UV transmission by blocking, scattering and absorbing UV light and may also shield or carry within them some microorganisms, which need to be treated. Suspended solids increase the number of surviving microorganisms.
Therefore they have to be limited by filtration, coagulating or settling prior to the UV disinfection of the water.
EXAMPLE:
| SUSPENDED SOLIDS |
EFFLUENT WATER |
INFLUENT WATER |
| mg/l |
microorganisms/ml |
survivors/ml |
| 0-5 |
10 000 |
<10 |
| 5-10 |
10 000 |
<100 |
| 10-20 |
10 000 |
10-1 000 |
| 20-100 |
10 000 |
100-5 000 |
COLOUR
The discolouration of water is in most instances caused by the presence of minute particles of organic matter. Surface water usually contains complex organic molecules known as Humic and Fluvic acids, which shows as a brownish discolouration. These molecules absorb UV light. Measuring devices available read the rate of absorption of visible light, meaning that water that appears very clear might still have a high UV light absorption potential.
Organic matter should be limited by activated Carbon filtration.
TURBIDITY
Due to the suspension of finely divided matter, turbidity can be seen as a lack of clarity or brilliance in water. Turbidity is the measure of light transmission through a sample. It must not be confused with colour, since water might have a dark colour but still be clear and not turbid. Clay, silt, fine organic and inorganic matter as well as microscopic organisms may all cause turbidity. Although closely related with suspended solids, which is the absolute quantity of matter in a sample, turbidity is not equivalent to the amount of suspended matter. If necessary, it might be limited by activated Carbon or ultra filtration.
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