UV-Technology > UV-reactors for UV- disinfection > Theoretical Background of UV-Disinfection

Theoretical background of UV-disinfection reactors

Up-front to technical description of UV-Reactors for UV-Disinfection, the physical relations of UV-disinfection shall be explained briefly:

During a broad R&D project of the German government a high number of germs (Bacteria, Vires, Yeast etcetera) and their deactivation by UV-light of the wave length 254 nm (main emission of a Mercury low presser lamp) was investigated. One focuses on the incident UV-dose in dependence of the deactivation rate. This work leads to fixing the UV-dose of 400 J/m² for drinking water disinfection, because this dose showed a reliable reduction of germs of at least more than 99,99% for the relevant types of bacteria by UV-disinfection.

: UV-spectra and germ deactivation curve.

UV-reactors have to be able to penetrate water with a given minimum dose. The absorption of UV-light in water, the geometry of the irradiation chamber, the residence time in the irradiated area amd the residence time distribution of the water are important parameters.
The law of Lambert-Beer describes the reduction of intensity of the UV-light passing through the irradiated zoone:

I₀ is the initial intensity [in W] and I is the Intensity at the location x.
SAC (Spectral Absorption Coefficient ) is an optical property of the liquid with the dimension [1/length]. For x₁ this means:

The irradiance (Dosis) is a physical function of the Intensity (Intensität), the residence time (Zeit) and the irradiated area (Fläche):

Relation of SAC and transmission at different path length for uv-light for sterilization (wavelength 254 nm)
SpectralAbsorptionCoefficient	UV-Transmission (path length)
SAC in 1/m	T (1cm) in %	T (5cm) in %	T (10cm) in %
0,5	99	95	90
1	98	90	82
2	95	79	63
5	89	56	31
10	79	30	9
15	70	17	3
20	62	9	1
25	56	6	0,5
30	50	3	0,1

The lyout of uv-reactors is the important base of the design. The residence time in a conventional uv-reactor is always very complex. For this reason an exact calculation of the dose is not possible for conventional uv-reactors. This is the reason why a.c.k. uses uv-reactors with the rotational flow only. A central quartz tube separats the lamp and the water, and in the ring space around this tube (and the horizontal axis) rotates the liquid (see Fig. below).
This design can be calculated exactly.

Visualisation of the rotational flow typical for the uv-disinfection reactors of a.c.k. The narrow residence time behaviour of the uv-reactor leads to a defined and calculable flow.

: Rotational flow of UV-reactor MicroUV

The advantages of the rotational flow are:

The rotational flow allows a simple calculation of he important parameters. This enables a.c.k. as the only supplier to realize a customer fit design.
The residence time behaviour is very narrow. All volume segments are irradiated with exactly the same dose.
The rotating liquid is abrasive to the central quartz sleeve, to prevent settlements.
The high turbulence leads to a very good mixing and improves an effective disinfection.
In the a.c.k. - reactor system the integral intensity of all lamps is monitored. This means, that the corresponding signal represents the actual state of uv-emission perfectly (Fig. 1).

The integral monitoring of all -lamps in one reactor leads to a reliable signal of the uv-dose inside the a.c.k. - MicroUV® - reactor.

: MicroUV-Reactor design

Poor reactor design of conventional UV-reactors with essential disadvantages

Reactors constructed in a way shown below, should not be used, as they show the following disadvantages:

The way A shows a too short residence time, and the disinfection is not good enough. On the way B the residence time is too long, and energy and lamp life time is wasted.
A by-pass flow can be observed and has to be compensated by higher -power. This caused higher running costs by additional energy consumption and additional spare lamps.
Furthermore particles sediment in the conventional reactors on the lamps, what leads to complex and sensible mechanical cleaning equipment.
Another disadvantage of the conventional -reactors is the limited function of the uv-sensors. Typically the signal of only 1 lamp is measured as a reference. This can lead to a number of defaults: e.g. al lamps are old, and only the measured is new; in this case the dose at the sensor is significantly higher than the real value inside the reactor. An unsatisfactory state of seeming safety.