Knowledge Paper
The effective use of oxygen
in the biological treatment
of waste waters
By Peter A Barratt, Neil Hannay, Alain Michel and Ashley Smith
Summary
Oxygen has been widely used during the latter half of the twentieth century for the
biological treatment of waste waters, and yet its benefits compared with conventional
aeration are not always fully understood. The keys to the efficient use of oxygen are:
the design and operation of the waste water plant, the equipment used to transfer
gaseous oxygen to the liquid effluent in the treatment tank, and an intimate
understanding of the biological process, together with prior knowledge of how the
biomass reacts to perturbations.
© 1999 Air Products and Chemicals, Inc.
All rights reserved. No part of this publication may be reproduced without the prior permission of
Air Products.
Clockwise from top left:
Peter Barratt is responsible for Air Products’ global waste water treatment
technology portfolio. He has a degree in Microbiology, a PhD in Environmental
Biochemistry, and has worked on effluent treatment for well over 10 years.
Neil Hannay specialises in biological process engineering in Air Products’ European
R&D group. Neil has a degree in the biosciences and process experience from a major
UK water company, before joining Air Products in 1992.
Ashley Smith (d. 2002), a Chartered Engineer and original Engineering Design
Owner for Oxy-Dep,™ helped develop Air Products’ biological waste water treatment
technology. Ashley was involved in waste water treatment with oxygen for over
15 years.
Alain Michel is a senior waste water treatment engineer in Air Products waste water
team in France. Alain has installed numerous Oxy-Dep™ systems in the field and has
a wide experience in optimising biological treatment systems.
The effective use of oxygen in the biological treatment
of waste waters.
The biological treatment of waste waters includes a vast array of treatment methods
and equipment, developed over the years to treat a wide variety of wastes in the most
effective way in order to reach the desired level of treatment for the waste before it is
discharged.
Aerobic biological treatment
Biological treatment methods requiring oxygen (aerobic) can be broadly divided into
suspended growth and fixed film systems. Here, the active microorganisms
undertaking degradation of the waste are either present as freely suspended particles
in a stirred tank reactor (the activated sludge process), or as a biological film on a
fixed surface. Examples of the latter include Rotating Biological Contactors (RBCs) or
Trickling Filters.
The use of pure oxygen for biological waste water treatment has largely, although not
exclusively, been used for suspended growth processes, notably activated sludge; but
why have waste water treatment plant operators decided to use oxygen as opposed
to air to achieve the desired results?
High Purity Oxygen versus Air
There are a number of recognised advantages of using oxygen in the activated sludge
process, most of which are listed in Table 1, but do these always hold true, and why?
There are some properties of oxygen, whether supplied as high purity liquid oxygen or
as on-site produced oxygen (usually at purities <97%) which go towards explaining
some of these advantages.
1. Oxygen transfer rate
Solubility
High purity oxygen achieves faster oxygen mass transfer rates into water than air.
This is due to physico-chemical facts concerning the interaction of gases and liquids.
Henry’s Law says that the concentration of a gas dissolved in a liquid increases with
the pressure of the gas phase. At a given absolute pressure, the solubility of an
individual gas will increase with increasing concentration (partial pressure) of that
gas; thus for waste water:
equation 1 …….[O2](aq) = H. [O2](g)
where [O2](aq) is the oxygen concentration in the effluent, [O2](g) is the oxygen
concentration in the gas phase, and H is Henry’s constant, which is both specific to
the gas concerned and highly dependent upon temperature.
The saturation concentration of pure oxygen in water is 39.3mg/l at 25°C and 1
atmosphere, but according to the equation above this maximum solubility under the
stated conditions will decrease with decreasing oxygen concentration in the gas
phase. So, in air (21% oxygen by volume), maximum oxygen solubility becomes
8.25mg/l.
The difference between 39.3 and 8.25 is the fundamental reason why oxygen transfer
from gas to liquid is faster for pure oxygen than for air. These maximum solubilities
heavily influence the driving force for oxygen transfer rate into liquid. In just the same
way, carbon dioxide, with a relatively high aqueous solubility (H = 1450mg/l/atm @
25°C) can be dissolved at a faster rate than oxygen.
Mass transfer rate
So, how does greater oxygen solubility help mass transfer in an activated sludge
process? Oxygen is of little or no use to a microorganism unless it is first dissolved in
water. The rate at which a gas passes from the gas phase into the liquid phase of an
activated sludge process is dictated by the following:
equation 2……dM/dt = kLa (CS - CAS)
where, dM/dt is the rate of gas mass transfer, kL is the liquid side gas mass transfer
coefficient, a is the interfacial surface area of the gas bubbles, CS is the maximum
solubility of the gas in the liquid (activated sludge liquors), and CAS the actual
concentration of the gas in the activated sludge liquors (measured as dissolved
oxygen)
Whilst oxygen can provide higher dissolved oxygen concentrations in the mixed
liquors than can air, there is no advantage in this alone (1). The important fact is that,
with CAS staying the same whether for air or oxygen (often in the range 1-4mg/l
dissolved oxygen), the CS - CAS term above is much larger for oxygen than for air, and
this has a marked effect on increasing the rate of oxygen transfer.
1 Faster rates of oxygen transfer
2 Smaller activated sludge reactor size
3 Greater flexibility for loading fluctuations
4 Lower emissions to atmosphere
5 Rapid response to shock loads
6 Low capital option for existing plant expansion
7 Improved sludge quality
8 Low mechanical wear
9 Low noise
10 Fast emergency response
Table 1:
Commonly cited advantages of oxygen compared with air for the aerobic biological
treatment of waste waters.
Bubble size
Equation 2 also implies that if oxygen passes into the waste water in the form of
many very fine bubbles as opposed to a fewer number of large bubbles, then, due to
the high surface area of oxygen bubbles in contact with the water the term a will be
much higher. Thus the rate of mass transfer will be even higher still. For this reason
devices which are used to dissolve oxygen efficiently in activated sludge plants
usually aim to produce fine bubbles. Air Products Oxy-Dep™ processes use equipment
which does this very effectively, although there are a number of devices which
attempt to do the same thing in different ways.
The following are some examples of devices used for the application of oxygen to the
activated sludge process.
Oxygen Dissolution Devices
Bubble diffusers
Gas is usually introduced into a liquid phase reaction as bubbles. This is the case for
most oxygen activated sludge processes. Bubble size has an effect on the rate at
which oxygen passes into the liquid phase, and it is here that it can then be used in
intracellular and extracellular biological processes requiring oxygen. Bubble diffusers
assist in the formation of a mass of oxygen bubbles within a given size range, by
pushing the gas, under pressure, through a solid medium perforated by pores. The
bubble size range is dependent upon the size of the pores through which the gas
passes. In this way systems are often referred to as either coarse or fine bubble
diffusers.
The medium used for the diffuser can be made of plastic (flexible or rigid) or ceramic
materials, and is usually installed on the bottom of the activated sludge reactor.
Bubbles formed at the surface of the diffuser rise through the tank and oxygen
transfer takes place at the bubble surface. In this way gentle mixing of the mixed
liquors in the tank takes place. Additional mixing may be required to keep the
biomass suspended. Additional mixing, and therefore power, can be added via low
energy turbine-type mixers.
In-pipe injection
Some simple oxygen biological treatment installations use direct injection of oxygen
into a moving liquid effluent stream within a pipe. The flow of liquid, if fast enough,
breaks up the incoming gas into bubbles and gives rise to a two-phase gas-in-liquid
flow regime along the pipe. At the end of the pipe the two-phase flow exits into the
treatment tank, where the bubbles have additional residence time for the oxygen to
dissolve in the liquid phase.
One classical use of this injection mode has been in the use of oxygen within rising
sewage mains, in order to avoid odours e.g. hydrogen sulphide formation, and
corrosion in the mains(2).
Floating mixers
There are a variety of oxygenation devices which broadly use the approach of surface
aerators. They float on the surface of the activated sludge liquors, and oxygen is
introduced near the liquid-gas interface. At the interface the oxygen is drawn down
into the mixed liquors, often through a submerged impeller-like device, where it forms
bubbles and performs some mixing(3).
These devices, whilst easy to install, may require anchoring to the sides of the basin,
and perform only localised mixing in the tank. Vertical passage of the oxygen bubbles
may also be limited such that the bottom of deeper basins may not be well mixed,
and oxygen bubbles have less residence time as they rise to the surface.
Air Products’ Oxy-Dep™ process equipment, whilst using a different means of fine
bubble formation and mixing, has been adapted to floating operation, especially in
large activated sludge lagoons, such as those used in the pulp and paper industry(4).
Sidestream injection (Oxy-Dep™)
In most parts of the world where oxygen is used in activated sludge plants to
accomodate BOD and COD loading rates, oxygen is added via a sidestream injection
system. The Oxy-Dep™ process uses an advanced type of sidestream injector to
deliver oxygen in the form of very fine bubbles to an activated sludge process.
Broadly, there are two types of Oxy-Dep™ installation but both use the sidestream
system; these are skid-mounted systems and fixed installations.
Skid-mounted Oxy-Dep™
Figure 1 shows an Oxy-Dep™ skid designed to deliver 100kgO2/h into the process.
The device is simple in operation, robust in structure and the effects are quite
dramatic. The Oxy-Dep™ skid is lowered into the activated sludge basin and rests on
the bottom. Oxy-Dep™skids have been adapted to rest on purpose built plinths where
the bottom of a basin may be sensitive to damage e.g. a membrane-lined lagoon.
Once installed, with power to the submersible pump and an oxygen line from the
oxygen source, mixed liquors from the basin are pumped into the “sidestream”
through the centrifugal pump and through a specially designed venturi. At the throat
of the venturi, oxygen gas is pushed into the vane of liquid passing through the
venturi constriction and oxygen is transferred into the moving liquid stream. The
liquid-gas mixture is then returned to the bulk mixed liquors via a manifold with a
number of nozzles. The pressure drop across the nozzles assists further in breaking
up the large number of oxygen bubbles into an even larger number of micro bubbles.
These bubbles are ejected into the basin through a secondary ejector at the end of
each nozzle, which helps to direct liquid-liquid mixing just after the nozzle, thereby
dissipating oxygen whilst optimising mixing energy requirements in the bulk
mixed liquors.
Fixed Oxy-Dep™
In fixed Oxy-Dep™ installations an externally mounted pump is used instead of a
submerged type. Otherwise the design criteria are the same. Compared with the skidmounted Oxy-Dep™, the external pumps are easier to service, and there is more
flexibility over where the nozzles can be positioned in the basin. Often, whereas skids
have been used to retrofit existing air activated sludge basins, fixed installations are
usually installed where a new plant is being built, or where a permanent oxygen
system is required.
Oxy-Dep™ VSA
A unique oxygenation system incorporating simple on-site oxygen generation in
small, modular units each delivering from 260 to 875 kgO2/day, and submerged lowenergy propellor mixers. Air Products’ novel single valve, single separation bed
technology for air-oxygen separation delivers the process benefits of oxygen at the
cost of aeration. An Oxy-Dep™ VSA package can be installed within two hours on
site, and requires only a power supply.
Figure 1: A skid-mounted Oxy-Dep™ unit
Performance
Amongst plant operators there are some commonly asked questions regarding the
use of high purity oxygen and Oxy-Dep™ for activated sludge waste water treatment,
especially where air has been used as the sole source of oxygen before. Some of
these questions, and the answers, are as follows.
Will mixing be as good?
Yes. Oxy-Dep™ processes are designed to impart generous mixing to the activated
sludge basin at reasonable power consumption. The more homogeneous mix of gas
bubbles in the activated sludge, and the mixing provided by the pump at the nozzles
ensures more even and controlled mixing than in many air-fed systems.
Figure 2 shows output from a Computational Fluid Dynamics model, designed by Air
Products using Fluent software. It illustrates the modelling of a typical waste water
treatment basin containing an Oxy-Dep™ skid, and shows a liquid velocity contour
map through the basin. This type of model can be run for any waste water treatment
basin, even large lagoons, with uneven base and sides, and the position of the
nozzles altered to optimise the velocity profile of the basin, and so reduce the
opportunity for areas with poor mixing to occur.
Scale, m/s
Without Baffle
Figure 2: CFD velocity profile of an activated sludge reactor showing the profile of liquid
movement.
How efficient is the oxygen transfer?
Good provided certain key criteria are met. In an ideal situation 100% of the oxygen
delivered into the activated sludge will be used by the biomass, and none will escape
at the surface. More usually, in an optimised process, Oxygen Transfer Efficiency
(OTE) should be >90%. However, in order to approach 100% OTE the following are
required:
• equipment producing fine bubbles and good mixing
• >3m depth above the point where oxygen is introduced
• a high concentration of active aerobic biomass
• adequate residence time
• α and β factors(6) approaching unity.
These criteria amongst others suggest that the only accurate way to assess OTE is by
direct measurement. Estimates can be made by measuring COD removed across the
plant and oxygen delivered, over a defined period of time. However, in an established
activated sludge, neither COD nor BOD are likely to be removed in a 1:1 ratio with
oxygen. Oxygen mass balance across the plant, using off-gas analysis of oxygen at
the surface, has been used to give an estimate of OTE(5), although this is not easy to
measure accurately.
How will solids management be affected?
Oxygen can show benefits. Whilst, in a conventional gravity sludge settlement system,
clarifier size limits hydraulic flow and sludge recycle in a highly loaded plant, oxygen
has been shown to assist sludge management. In many cases oxygen will improve
biological floc formation in a heavily loaded plant, by making oxygen freely available
to each floc. Smaller, denser flocs in Oxy-Dep™ processes often show enhanced
dewatering, and lower waste sludge solids. At a site using the Oxy-Dep™ process to
digest sludge, field trials showed that the digested sludge was dewatered to 24%
solids as opposed to 16% solids for a parallel aerated sludge digester, using the same
dewatering device.
Oxygen has also been shown to assist in avoiding the development of filamentous
bulking (due to filamentous bacteria) in activated sludge(2).
What is the real benefit of oxygen over air?
Table 1 highlighted the recognised benefits of oxygen activated sludge over
conventional aeration. Today, the high efficiency of gas generation and gas-to-liquid
transfer equipment means oxygenation running costs do compete with air, but the
real benefits of oxygen lie in the rapid and sustainable process acclimatisation which
oxygen offers for effluents which are inconsistent both in load and in their chemistry.
Aside from the process advantages of oxygen activated sludge processes, and the
added comfort for plant operators that this gives compared with aerated systems,
running costs for oxygen are not high. Aeration running costs comprise the power
applied to run blowers, surface aerator motors etc, whereas oxygen costs comprise
the cost of the gas plus the power used by the dissolution device i.e. liquid pumping
for Oxy-Dep™. Oxygen costs vary according to geographical availability and mode of
supply, but power costs for oxygenation are uniformly low.
The Oxy-Dep™ process uses approximately 1kWh to transfer 5-6kgO2. Aerators,
depending on type, use 1kWh to transfer 0.7 - 1.5kgO2(5) and Oxy-Dep™ VSA
transfers oxygen at a specific power well within this range.
If I use oxygen, where should I get it from?
As a gas and process provider, Air Products produces its industrial gases in the most
economic way, and allies them with the most effective process equipment, and
process know-how. In this way Oxy-Dep™ processes may use liquid oxygen as the
gas supply, or gaseous oxygen from one of a number of lower pressure gaseous
sources. The latter is likely to be a Vacuum Swing Absorption (VSA) process, where
oxygen and nitrogen from the air are separated by changing the pressures in a vessel
containing an adsorbent medium with different selectivities for the two gases. OxyDep™ VSA makes full use of this technology.
Process equipment differs depending on the site, the oxygen source and the process
requirements, and this combination will dictate the most effective solution.
The future of oxygen-based processes
Oxygen processes for the biological treatment of waste water are established
technology, with recognised benefits, but current Oxy-Dep™ processes are not the
final word. Air Products continues to make advances in the field by improving oxygen
generation equipment performance, increasing plant performance through biological
process understanding and optimised equipment, and pushing into completely new
technologies for the effective biological treatment of waste waters and sludges. Some
of the references Air Products now have demonstrate the biological treatment of
waste waters never before considered treatable by these means, and at loading rates
hitherto unknown.
References
1. G F G Clough. Wastewater Treatment. 1979. In Developments in Environmental
Control and Public Health - 1. Editor A Porteous. pp1-28.
2. A G Boon, C F Skellett, S Newcombe, J G Jones and C F Forster. 1977. The Use of
Oxygen to Treat Sewage in a Rising Main. Water Pollution Control, Vol. 76. 98-112.
3. T A Badar. 1986. Oxygen injection - an alternative effluent treatment. Tappi
Journal, Vol 69, No. 10. October 1986. 82-85.
4. J Pinto and R Leite. Yield increase, control and automation of biological waste
water treatment stations by the oxygen uptake rate method. In Press.
5. G T Daigger and J A Buttz. 1998. Upgrading Wastewater Treatment Plants. Water
Quality Management Library, Vol. 2. Technomic Publishing Co. Inc. 243 pages.
6. G Tchobanoglous and F L Burton. 1991. Wastewater Engineering - Treatment,
Disposal and Reuse. McGraw-Hill, Inc. Third edition. p286.
7. Air Products case study RSAG. Available from Air Products.
8. P Rüütel, S-Y Lee, P Barratt and V White. 1998. Efficient use of Ozone with the
Chemox™-SR reactor. Air Products Knowledge Paper No. 2.
Available from Air Products.
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