UV disinfection in shrimp
hatcheries
From: Juan Fernando Martinez postlarmar@epm.net.co
To: shrimp@yahoogroups.com
Sent: 5 March 2003
QUESTION:
I am buying UV systems for my shrimp hatchery, but I
have no clear idea on the recommended UV dosage required to kill vibrios and
most pathogens that affect larvae.
Juan Martinez
e-mail: postlarmar@epm.net.co
***************
COMMENTS 1:
Generally, many of the UV’s on the market are rated
at a given flow and this is sometimes misleading since the transmission
coefficient of UV through seawater is different than for freshwater. Many
units are based upon a freshwater dosage rating and that is a radiation
level of about 16,000 micro-watt second/cm².
Seawater dosage is established (on average) at around
30,000 mico-watt second/cm², or more. There are two ways of killing a bird
here. First, you get a unit with the correct seawater dosage at the desired
flow rate, or, second, you increase the dwell time of flow such that the
freshwater dosage makes up for the difference (this is the same as
over-sizing the freshwater unit).
Now, the radiation rate of a new lamp is different
than for an aged lamp. Most are rated for about 7500-8000 hours of
continuous use. The curve of effective radiation drops off from a new lamp
and plateaus out after around 6000 hours and could reach 50% of the starting
dosage of a new lamp at the “end of bulb” mark (8000 hours).
So, while the lamp may be still lighted, your actual
effectiveness may not be the same as the unit ratings. This is also why some
units are equipped not only with a lamp indicator light, but also an hour
meter for tracking the use life of the lamps. Prudent management tells you
to change the lamps as you approach the 7500-8000 hour mark. I have supplied
customers with bulb changes after 3-4 years of continuous use and they say
that they did not think of this factor since the lamps were still
functional.
So while you can use a fresh water rating to select
your UV filter, the formula is not simply to double the flow rate capacity
of the freshwater rating to be equivalent to the seawater units, but you
must also take into consideration the lamp effectiveness curve. In reality,
if you use a freshwater unit and wish to be safe, I would get a unit rated
at three times the flow rate to be equivalent to the seawater unit “end of
bulb life” rating.
This is also assuming that the bulb is running at the
right temperature since if you are using colder water, the design of the
unit should isolate the lamp from the water. This is normally accounted for
by the units that are offered with a quartz sleeve in which the lamp is
inside of the quartz tube. That air space between the quartz tube and the
lamp allows for the lamp to reach the correct operating temperatures to
properly emit the right wave length for UV. Such system are known as “lamp
in water system”.
The other option is where the water to be treated
flows inside a tube and the lamps are surrounding the tube. This is known as
“water in tube” systems and usually the tube material is AFP or Teflon.
These are much easier to maintain since you just flow mild acedic acid to
clean the inside contact surfaces of the AFP tube. With the units using
quartz sleeves, you have to take the sleeves out and scrub them off
periodically. Manual handling inevitably leads to breakage.
Vibrio is very sensitive to UV and is probably why
you don't have this in outside exposed tanks to the sun. We don't have
the specific research done with vibrio yet but it is somewhat closely
related to Pseudomonas and those dosage levels span between 6000 to 17,000
micro-watt sec./cm2. Again, there is a danger in making this
type of
comparison since this would just be a guideline, sensitivity or not.
As a standard protocol, in most hatcheries using UV, the idea is not only to
check one category of bacteria but to see if you can eliminate as many as
possible good or bad (and molds, fungii, spores, protozoa, yeast, and some
viruses, etc.), since UV (or ozone for that matter) is non-selective for
most sensitivities up to the dosage rating at any given flow rate. If
you need the good bacteria, then you selectively add them back in if
possible.
The organisms or pathogens requiring very high dosage are the mold
spores, Bacillus subtilis spores, Clostridium tetani, Sarcina lutea,
Penicillium family (mold), Aspergillus family (mold), Rhisopus, Paramecium,
nematode eggs, and viruses, in which the lower threshold starts at
mid-20,000 and goes well over 300,000 micro-watt sec/cm2 Note,
however, that most of the mentioned organisms are not a threat to shrimp
larval culture per se. The standard kill rate at which these dosages
are computed is generally 99.99% kill rate such that there is effective
prevention of colony formation. At levels over 50,000 micro-watt
sec/cm2, it may be more cost effective to start looking at ozone.
Leland
Aquafauna Bio-Marine, Inc.
e-mail: lelandlai@aquafauna.com