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.[O 2 ] (aq) = H. [O 2 ] (g) where [O 2 ] (aq) is the oxygen concentration in the effluent, [O 2 ] (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 = k L a (C S - C AS ) where, dm/dt is the rate of gas mass transfer, k L is the liquid side gas mass transfer coefficient, a is the interfacial surface area of the gas bubbles, C S is the maximum solubility of the gas in the liquid (activated sludge liquors), and C AS 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 C AS staying the same whether for air or oxygen (often in the range 1-4mg/l dissolved oxygen), the C S - C AS 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 100kgO 2 /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 kgo 2 /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 5-6kgO 2. Aerators, depending on type, use 1kWh to transfer 0.7-1.5kgO 2 (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. Oxy- Dep 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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