Ultraviolet (UV) disinfection is now a
standard feature in most wastewater
treatment systems. UV has also been
embraced by the drinking water
community as an effective barrier
against chlorine-tolerant organisms
such as cryptosporidium and Giardia.
The technology is widely favoured
in wastewater treatment due to its
non-chemical nature, the fact that no
subsequent de-chlorination process is
required, and its ability to be unselective
in disinfection performance.
Consulting engineers sometimes
overlook the recent progress in
UV technology and the impact that
hydraulics can have on UV system
performance by continuing to place
UV lamps in open channels. A more
efficient approach is to contain the
waste stream in a pipe and disinfect the
fluid in a closed vessel.
UV light works by causing
permanent damage to the DNA found
in all living species. Once the DNA
becomes damaged, the organism
is unable to carry out the routine
cell functions of respiration, the
assimilation of food, and replication.
Once the cell is rendered non-viable,
the organism quickly dies. The
difference in UV system efficiency
from the various UV manufactures was
made transparent with the advent of
UV system validation using bioassay
techniques. The techniques have been
well established for drinking water
and are being investigated for use in
wastewater.
A bioassay involves the introduction
of non-pathogenic organisms (bio-
dosimeter) into the fluid stream before
the UV system. The entire procedure
is
performed
under
controlled
conditions and system variables: flow,
transmittance, power loads, and lamp
intensity are carefully recorded as
samples are taken before and after the
UV system.
Once the sample data is returned
from the analysing laboratory, the
actual system’s ability to disinfect can
be compared to the manufacturer’s
estimation. Of course, such bioassays
should be carried out under the
inspection of a credible third party.
As bioassay validations became
the standard, design engineers started
to notice how water hydraulics play
a vital and often overlooked role in
system performance. In essence, if a
UV system design allows short circuits
Transforming water, enriching life
An open channel system.
or poor turbulence, then the water will
receive differing degrees of UV dose.
In extreme cases, the water can short-
circuit straight through a UV system,
rendering it inefficient.
Most UV systems need to cope with
a variety of flow rates, and usually an
operating flow range is considered
when designing the UV system. A
persuasive case can be made to put the
UV system for wastewater disinfection
into a closed pipe to ensure optimised
hydraulics and to prevent the operator
from the exposure to the wastewater
and the UV light.
Dead zones or spaces can be
formed within the channel, which leads
to short circuiting and untreated water.
Erratic or reduced inactivation
performance caused by poor hydraulics
can create density currents that cause
incoming wastewater to flow along
the top or bottom of the lamp banks,
resulting in short circuits and poor
disinfection. Flow straighteners can
introduce new problems.
It’s not unusual for a submerged
perforated diffuser to have an open
area of less than 20% of the cross-
sectional area of the open channel —
head-loss and overflow problems can
then exist.
Under-sized channel width and
depth can create very high velocities
and can reduce the residence time
required for adequate UV dose delivery.
Large open water sur faces can lead to
fly and mosquito nuisance and cause
corrosion of electrical components due
to the elevated humidity. Level control
is vital, but fragile. The level of the
fluid in the channel must be carefully
controlled.
Advantages of closed
vessel UV treatment
• Savings: The reduced number
of lamps, quartz, and reduced
footprint of closed vessel design,
will considerably reduce the
capital expenditure cost of a
project. The ‘end-feed’ closed
vessel chamber design removes
the requirement for large civil
structures, while the high output
800W Amalgam UV lamps provide
a significantly increased treatment
capacity, with energy efficiency
and reduced consumable
components.
• O p e r a t i o n a n d m a i n t e n a n c e
advantage: The ETS-UVTM designs
offer the highest UV output with
the fewest number of lamps, in
the smallest footprint currently
available in the UV market.
Operational costs, including power,
lamps, sleeves, power supplies,
and maintenance, can be 15–20%
less when compared to traditional
open channel systems. u
Water Sewage & Effluent July/August 2017
5