Ultramarine Magazine Issue 64 JuneJuly 2017

44 http://www.ultramarinemagazine.co.uk

Definitive Guide to:

UV & OZONE

Text and images by

Dave Wolfenden

Ultraviolet sterilisation and ozone are two useful tools for the

marine aquarist for disease management and for managing

water quality respectively. Here, we’ll look at how each of these

approaches can be employed, and how to avoid the main pitfalls.

There are a number of misconceptions surrounding the use of UV

and ozone, and we’ll also be taking a look at these along the way.

Ultraviolet (UV) light is used

for its germicidal properties

in the aquarium. Specifically,

UV radiation of the short wave

kind (known as UV-C) is used.

Special lamps are needed for

this, with the most efficient

wavelength for being 253.7nm.

The aquarium’s water needs

to pass over the lamp (which

is usually covered by a special

quartz sleeve to allow maxi-

mum UV penetration into the

water, normal glass absorbing

a significant proportion of the

required radiation) into a void

space which must be of the cor-

rect dimensions; the flow rate

is also crucial to ensure that the

unit functions effectively. The

effect of UV-C radiation on mi-

croorganisms is to disrupt their

genetic material – this prevents

them from reproducing. Differ-

ent pathogens require different

doses of UVC to disrupt their

DNA or RNA.

The dosage of UV-C radiation

required to destroy pathogens

is usually expressed as micro-

watts per centimetre squared

of bulb surface area per second

(μW/cm2/s), which is dictated

by the strength of the bulb

and the exposure time. Larger

organisms are afforded greater

protection from UV radiation

by their cellular material than

smaller organisms, so larger

pathogens tend to require

higher doses of UV-C. Protozo-

an parasites for example, tend

to need more intense radiation

than bacteria, which are much

smaller.

For example, most non-spore

forming bacteria only require

an exposure of around 10,000

μW/cm2/s to render them

unable to reproduce. Crypto-

caryon irritans, the protozoan

responsible for causing white

spot, requires much greater ex-

posure – the figures cited vary,

but you’re looking in the region

of 300,000 μW/cm2/s or more.

You could actually work out the

exposure of any model of UV

steriliser at a given flow rate if

you wanted to, but in practical

terms what this means is that

the manufacturer’s recommen-

dations for each unit should

be followed, and selecting a

model which is underpowered

is a false economy. Each unit

will come with instructions

detailing the flow rate required,

which determines the ‘dwell

time’ – the amount of time the

water spends within the unit’s

void space. Too short a dwell

time, and the free-living patho-

gens pass through without be-

ing effectively zapped; too long

a dwell time, and it may blitz