Aeration and Stripping
Content Table
- Section A - Introduction
- Section B - Methods of Aeration
- Section C - Applications
- Related Articles
- References
- Links
Section A - Introduction
Aeration in the treatment of water is a process that is used for either of two purposes: either to add oxygen (or possibly another gas) to water (adsorption) or to remove an unwanted gas from water (desorption or stripping). It follows that aeration is most likely to be encountered in the treatment of ground waters. Aeration can be carried out in a variety of ways. The most appropriate method or methods depends much on the purpose.
Standard texts should be referred to (for the time being) for the theory of aeration and stripping (Refs?). The process depends on the rate of gas transfer across the air-water (or water-air) interface and this is governed by the principle of partial pressures (Link?) and relative solubility of the gas of interest in water. Additionally, solubility is dependent on water temperature. The relative effectiveness of the transfer mechanism is usually expressed in terms of the ‘mass transfer coefficient’ (Link?). The application of cascades and packing can be expressed in terms of ‘transfer units’ and the height of transfer units (HTUs) (Link?). In the simple removal of a gas from water by air (e.g. carbon dioxide), one transfer unit equates to about 60% removal.
A limitation to the application of aeration is governed by the removal of carbon dioxide from solution. For harder, more alkaline, waters excessive removal of carbon dioxide leads to precipitation of calcium carbonate that deposits as a hard scale which subsequently is costly to remove. Chemical treatments can be applied in industrial water treatment but not in potable water treatment to control the precipitation of carbonate.
Section B - Methods of Aeration
Aeration involves mass transfer (Link?) either of oxygen from air to solution in water (oxygenation) or of a contaminating gas dissolved in water to its dispersion in air (stripping/desorption). The process involves transfer of the target gas across the air-water (or water-air) interface. The effectiveness of the process depends on the area of interface available and the rate at which it is refreshed.
Sub-section B.1 - Cascade
A cascade can be as simple as water flowing over a weir (Image?). The effectiveness of such a simple cascade depends on height between the upper surface of the water flowing over the weir and the surface of the water it falls on to. Effectiveness also depends on how uniform the water flowing over the weir forms a curtain. The effectiveness of a cascade can be increased by flow over several weirs in series (Image?). It follows that effectiveness depends on overall height of the cascade and therefore this governs the operating (energy) cost of the process. Cascade aeration is most likely to be encountered in treatment of surface waters before application of initial disinfection or application of coagulation. However, as water flows through a treatment works it is repeatedly cascaded as it flows over weirs used for controlling water levels and flow rates, and sometimes to assist chemical dispersion. Overall, the accumulative aeration effect can be substantial and should not be neglected.
Sub-section B.2 - Spray
Spray can generate a reasonably large surface area for the water as the jets of spray break up into droplets (Image?). In principle, a spray can be no more effective than an efficient single step cascade.
Sub-section B.3 - Packed bed
Packed bed – n appropriate form of packing is used to increase the air-water interface. The packing has both a high specific surface area and voidage, and is contained in a suitable vessel (Image?). Effectiveness is dependent on the wetting efficiency. This is managed by how the water is distributed over the surface of the bed of packing. The main problem with packing is that it can be fouled by anything that might precipitate from solution, by particulate matter in the air or by biological growths. Increase in depth of packing leads to diminishing return in further adsorption/desorption so there is a practical and economic limit to depth of packing. A packed bed can be envisaged as an efficient multi-step but random cascade. Thus, like cascade aeration, operating (energy) cost is governed by height difference between release of the water for distribution over the packing and the top of the water collected in the base of the aerator (the ‘pond’).
Packing can be a relatively low-cost material such as coke that has a rough surface or be made from plastic or other relatively inert material (e.g. porcelain or stainless steel) to have a large proportion of voidage relative to surface area. Plastic is usually chosen because of its weight and robustness to being handled. Two of its simplest forms can be vertical sheets of corrugated plastic structured as modules, or small diameter plastic pipe cut into short lengths equivalent to the pipe diameter (Image?).
Sub-section B.4 - Forced draught
Forced draught – used to increase the rate of air flow and refresh the air-water interface. The air can be blown into the base of the aerator or sucked up through the aerator (Image?). The rate of flow of air is limited. If it becomes too great then its upflow competes with the downflow of water and can prevent the water percolating down uniformly. Likewise, if the water flow is too great even for natural upflow draught of air, the water does not percolate down through the bed fast enough and the packing can become flooded. The operating (energy) cost is the combination of the loss in head of the water passing through the aerator and the blowing (or sucking) of the air into and up through the aerator.
Sub-section B.5 - Bubbling
Bubbling is the principal method of aeration of activated sludge in sewage treatment. The bubbles need to be as small as possible in order to generate as large air-water interface as possible. It can be an appropriate method in some instances in water treatment. It is a method used in ozonation (Link?).
Bubble curtains of air are used in large and deep raw water reservoirs to overcome and prevent thermal stratification and associated problems of iron and manganese. In this case, the inertia of the rising bubbles generates circulation of the water body so that oxygenated water in the upper level of the reservoir is exchanged with anaerobic water in the lower level. Oxygenation of the water otherwise takes place at the top surface of the water with the free air (Link?).
Generation of small bubbles is required for successful dissolved air flotation (Link?). In this case, the bubbles are required to agglomerate with floc and are not intended to assist mass transfer between air and water, although this will happen.
Section C - Applications
Sub-section C.1 - Oxygenation
Water drawn from low level in reservoirs can have low concentration of dissolved oxygen. Therefore it is not uncommon for such raw water to first be aerated by a multi-step cascade (fountain). Water drawn from some reservoirs can have iron and manganese in solution because of oxygen depletion and therefore the aeration helps with their removal. However, it is not uncommon for groundwater to be depleted of oxygen. Such anaerobic ground waters depending on the degree of oxygen depletion (redox) (Link?) can have iron, plus possibly manganese or hydrogen sulphide and even perhaps arsenic in solution.
i) Iron – in solution is in ferrous , [Fe2+], state. Aeration provides oxygen to convert the iron to ferric , [Fe3+], state and thence precipitate as hydroxide species. Oxidation and precipitation can be relatively rapid and is catalysed by presence of already precipitated iron or is facilitated by presence of oxidising bacteria. The oxidation is affected by temperature and pH (Link?). Precipitated iron is usually easily removed by filtration.
ii) Manganese – in solution is in manganous , [Mn2+], state. Aeration provides oxygen to convert the manganese to the insoluble manganese dioxide, MnO2. As with iron, oxidation and precipitation can be relatively rapid, if the pH is appropriate (Link?), when it is catalysed by presence of already precipitated iron and manganese or is facilitated by presence of oxidising bacteria. The precipitated manganese is usually easily removed by filtration, especially when iron is present, otherwise it might irreversibly coat the sand which then has to be periodically replaced.
iii) Arsenic – Arsenic is generally a groundwater problem existing mainly as inorganic forms of As(III) (arsenite) and As(V) (arsenate) species. As(III) is the more toxic and soluble and oxidation is required to convert As(III) to As(V). Subsequent removal of As(V) is achieved by precipitation onto or with iron, Fe(III), and their subsequent removal by filtration. The efficacy of this approach is dependent on the amount of iron present as well as redox and pH. Alternatively, arsenic can be removed effectively by anion exchange (Link?), activated alumina adsorption, and adsorption onto pelleted iron oxide.
Sub-section C.2 - Desorption (Air Stripping)
i) Carbon dioxide – Some ground waters can have quite substantial concentrations of carbon dioxide and be relatively acidic. Carbon dioxide is relatively easy removed from water by aeration. The alternative is to chemically neutralise, or use a combination of the two. It is important that aeration is not allowed to remove carbon dioxide to the extent carbonate is precipitated. This applies also to aeration for desorption of other gases or solvents.
ii) Hydrogen sulphide – hydrogen sulphide in ground waters arises from the reduction of sulphates. The gas has an unpleasant smell and is toxic in small concentrations (Link?). Aeration will remove most hydrogen sulphide easily, but consideration will need to be given to possible treatment of the exit air to remove the hydrogen sulphide..
iii) Radon – ground or spring water arising from catchments with granite strata can be rich in radon (a source of radioactivity (Link?)). Radon is relatively inert and the simplest method for its removal is aeration.
iv) Organic solvents – some ground waters have become contaminated by anthropogenic activity with solvents. Their removal can require large air volumes (using forced draught) and therefore aeration may be limited by carbonate precipitation. In which case, adequate removal may require filtration through granular activated carbon. Even if aeration is used, the exit air must have the solvent removed from it to comply with environmental legislation.
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References
Hand D.W., Hokanson D.R. and Crittenden J.C., Gas-Liquid Processes: Principles and Applications, Chapt.6 in Water Quality & Treatment, 6th Edtn., AWWA & McGrawHill. 2010
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Links
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