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Room to breathe
oxygen for wastewater treatment
New process technology means that oxygen for wastewater treatment is
more affordable than ever. Dr Peter Barratt of Air Products explains how
to breathe new life into biological effluent treatment processes.
There’s no denying it: aerobic processes will be the mainstay of
biological wastewater treatment in refineries—and most other industrial
plants—for years to come. And as discharge consents continue to tighten,
and volumes of wastewater continue to rise, many operators will have to
improve the performance of their treatment processes.
The trouble is that no-one wants to pay for process improvements that
contribute nothing to refinery throughput or profit. As a result,
upgrades and revamps are the commonest ways to boost wastewater
treatment performance, and even these must be done at minimum cost. One
traditional way to upgrade a struggling wastewater treatment plant is to
supplement atmospheric air with pure oxygen. This generally works well,
but at a cost that some operators find hard to justify. Liquid oxygen
imported by tanker means high operating costs, while the alternative of
on-site oxygen generation usually needs considerable investment. So it’s
good news to hear that advances in technology have cut the cost of
oxygen-based wastewater treatment for some applications. This article
describes a system that combines low-cost, reliable oxygen generation
with high-performance gas injection. The technology can be used for new
installations as well as retrofits, and satisfied customers attest to
its performance and cost-effectiveness.
All aerobic plants need oxygen
Aerobic biological wastewater treatment processes depend on a supply
of oxygen. This is usually done by adding air, using either simple
bubble diffusion or more intensive jet or surface aeration. In the case
of a typical activated sludge plant, aeration of the mixed liquors is
the standard technique. Atmospheric air is perceived as being cheap, so
it is an ideal solution as long as the plant can cope with the
prevailing conditions. However, the supply of oxygen is generally the
factor that limits the performance of the plant. If process conditions
change, the microbial population may become starved of oxygen and plant
performance can suffer, with traumatic consequences for plant operators
as well as the environment. The commonest causes of insufficient oxygen
are increases in the plant’s duty: higher throughput (m3/day), higher
COD or BOD (kg/m3), or more stringent standards for the treated effluent
(lower allowable COD or BOD). Old plants can also suffer from reduced
efficiency as equipment wears out or fails altogether. In many cases two
or more of these factors apply simultaneously.
Advantages of pure oxygen
Air-based plants struggling to cope with allowable discharge limits
have for many years used pure oxygen to supplement or replace air.
Atmospheric air contains 21% oxygen, so pure oxygen provides an O2
partial pressure that is nearly five times greater. This means five
times the driving force for oxygen dissolution—more than enough to
ensure that oxygen transfer rates need no longer be the factor that
limits plant performance. Another advantage of pure oxygen is that it
requires much smaller gas volumes than when using air. The nitrogen and
other gases that make up 79% of atmospheric air consume blower power and
create foam and odour problems, while contributing nothing to the
process of oxygenation. A well-designed oxygen injection system creates
few or no bubbles at the liquid surface, because all the oxygen
dissolves before it reaches the surface. With air, large volumes of
nitrogen bubbles at the surface can create foam and strip volatile
substances, including smells, into the atmosphere. Oxygen typically
improves floc formation, yielding smaller, denser flocs that improve
sludge dewatering, and helps avoid the growth of filamentous bacteria.
Process flexibility and response to load changes are also improved by
the use of oxygen. Today, many biological wastewater treatment plants
use supplemental oxygen delivered by tanker, stored on site in a
dedicated tank, and vaporised as required. Liquid oxygen is readily
available and easy to store, but it is a relatively high purity product.
An alternative is to generate the oxygen on site, using pressure swing
absorption (PSA), vacuum swing absorption (VSA) or even cryogenic air
separation. For sites with large (>10 t/d) and consistent oxygen
requirements, these technologies can provide oxygen at lower variable
cost than liquid oxygen delivered to site on demand. Such plants can be
expensive to build, however, and operators end up paying for the capital
costs, whether they rent the oxygen generator or buy it outright. Oxygen
generators also bring their own maintenance requirements, of course.
A new way to generate oxygen
Engineers at industrial gases company Air Products decided that the
wastewater treatment market needed oxygen technology that was comparable
in cost to conventional aeration, and could be used for both large and
small plants. The answer lay in the combination of two inventions. The
first is a highly-simplified VSA technology for generating oxygen on
site that combines low cost with high reliability. The new technology is
commercially-proven, and available in modules with oxygen generation
rates in the range 200–900 kg/day. Larger requirements are met by using
several modules in parallel. The second part of the process is a unique
low-energy propeller mixer that combines liquid mixing and oxygen
dissolution, with considerably lower energy consumption than the venturi
systems commonly used for oxygen injection. The combination of oxygen
generator and mixer-oxygenator is sold under the name OXY-DEPVSA. Two
pieces of clever engineering design underlie the simplicity, and hence
the low cost, of the OXY-DEP VSA system. All forms of absorption are
essentially batch processes, so conventional PSA or VSA plants use two
absorption beds to provide quasi-continuous operation; at any time, one
bed is on-line while the other is being regenerated. Air Products’
engineers realised that for an application such as wastewater treatment,
where high oxygen purity is not critical, the second bed could be
replaced by a small oxygen buffer tank, as long as the
absorption/regeneration cycle is sufficiently short. The second piece of
inspired thinking was to base the plant around a positive-displacement
blower, which is much cheaper than the compressor normally used by both
VSA and PSA plants. By operating the absorption step of the VSA cycle at
around 1.5 bara and the regeneration step at around 0.5 bara, a single
blower can fulfil both duties.
Inside OXY-DEP VSA
Indeed, the modular oxygen generator used in the OXY-DEP VSA process
is not much more complicated than the blower itself (Figure 1). During
the absorption part of the cycle, the four-way valve V-1 directs air
through an inlet filter to the inlet of the blower, while the compressed
air leaving the blower is routed to the vessel containing the absorption
bed. With most of the nitrogen removed by the absorber, the remaining
gas—containing about 95% oxygen—leaves the vessel via a non-return valve
and is fed to the mixer-oxygenator, filling the oxygen buffer tank on
the way. After a few minutes the timer-controlled four-way valve changes
position so that the inlet of the blower is connected to the absorber
vessel. As the pressure in the absorber falls, nitrogen is drawn out of
the bed and discharged to atmosphere. Towards the end of the cycle, the
spring-loaded valve CV-2 opens to allow a little oxygen from the buffer
tank to flush the remaining nitrogen from the absorber. The rest of the
buffer tank’s capacity, meanwhile, keeps the process supplied with
oxygen while the absorber is off-line. Once the absorber is regenerated,
the whole cycle starts again. Because the system works at low pressures,
it is much more energy-efficient than PSA systems, in which air is
pressurised to typically 8 bar before most of it is rejected to the
atmosphere. If oxygen is needed at a higher pressure than the blower can
supply, for instance to operate in a very deep basin, it is simple to
add a small supplementary oxygen compressor.
Mixing and dissolving with low power
Connected to the oxygen generator via a simple plastic pipe is the
other half of the OXY-DEP VSA process: a submerged mixer-oxygenator
mounted in the wastewater treatment tank to be oxygenated (Figure 2).
This unit is essentially a propeller mixer, with oxygen injected into
the low-pressure zone just behind the propeller. The oxygen is sucked
into the propeller zone, where the rotating blades create high shear
that breaks the gas up into small bubbles. The fast-moving jet of water
leaving the propeller zone carries these bubbles round the tank and
ensures excellent mixing of both dissolved and dissolving oxygen. The
mixer-oxygenator is mounted on a vertical mast at the side of the
wastewater treatment basin. This allows its position to be adjusted
easily for best mixing and oxygen distribution, and for occasional
maintenance the unit can be simply winched up by hand and swung out of
the tank. This ease of fitting, operation and maintenance fits in very
well with that of the VSA unit. As a guide, an OXY-DEP VSA™ unit
generating 200 kg/day of oxygen can remove around 250 kg/day of BOD in a
conventional activated sludge process, and oxygen-enhanced treatment
systems can handle around 10 kg COD/day per cubic metre of basin volume
if the process conditions are optimised. OXY-DEP VSA units are slightly
less energy-efficient than large multi-bed VSA systems, but then they
are very much cheaper. Power consumption of the OXY-DEP VSA unit under
real conditions is about 1kWh/kg O2 which is comparable to figures
quoted for air-based systems (in dirty water), which usually state
optimistic figures from tests performed on clean, cold water. This
includes the power consumption of the mixer-dissolver: 0.2 kWh/kg O2 in
small or highly-loaded basins and rather more for typical applications.
As long as power is available on site, once the OXY-DEP VSA equipment
has been delivered it can be installed and running within two hours.
Showing its worth in practice
Among the satisfied customers for OXY-DEP™ VSA technology is a
leading European aerospace company. Many applications of oxygen in
wastewater treatment are designed to improve COD removal across the
board, but this customer had a more specific requirement: limiting
discharges of ammonia. Their biological wastewater treatment plant at a
large production facility in the north of England receives wastewater
from across the site. Since the site occupies a relatively remote
setting alongside a major river, it warrants its own wastewater
treatment plant. This was built during the 1990s, since when it has
generally performed well, keeping discharges to the river within consent
levels set by the Environment Agency. The wastewater treatment plant is
currently operated by a third party waste management contractor, which
employs another company, WEBS Ltd., to provide process monitoring and
support. During the early summer of 2002 the discharge limits for
ammonia were tightened. This meant that during periods of heavy load, or
in hot weather, the treatment plant would struggle to meet the new
ammonia consent. In its capacity as process consultant, WEBS advised
that the plant’s oxygenation capacity needed to be increased. Additional
oxygen would increase the degree of biological nitrification and so
allow the plant to meet the new ammonia consent. Air Products was asked
to provide options for increasing the oxygenation by up to 250 kg/day.
Following these discussions it was decided to install an OXY-DEP VSA
system to make up the oxygen shortfall. The plant has now run with
assistance from oxygen for more than 12 months, and has maintained lower
ammonia discharge levels than the aeration system alone could achieve.
The system has met all the expectations of the parties involved with
providing effluent treatment at the site. A representative from the
customer project team said: “Without the Air Products system we would
have struggled to meet our discharge limits with respect to ammonia
levels, and may have been in serious trouble. In the time the VSA unit
has been on site it has been invaluable in maintaining our good
environmental record”. <<
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