Liquid purification or separation – Processes – Treatment by living organism
Patent
1996-12-02
1998-09-01
Wyse, Thomas G.
Liquid purification or separation
Processes
Treatment by living organism
210630, 210807, 210151, 210284, C02F 304, C02F 306
Patent
active
058007091
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of construction and operation of Biological Aerated Filters, usually flooded, in the treatment of fluids, particularly domestic and industrial waste-waters, typically for the removal of suspended solids, biochemical oxygen demand (BOD), ammoniacal nitrogen and total nitrogen.
The biological aerated flooded filter typically comprises a submerged bed of inert packing on which the micro-organisms responsible for treatment are attached. The waste-water to be treated flows up or down through the packed bed and air, oxygen enriched air, oxygen, or other gases are introduced at or near the base of the bed. As the treatment proceeds, the packed bed becomes blocked with the growth of micro-organisms and the capture of suspended solids causing a resistance to the fluid flow. When this resistance to flow, or head loss, reaches a predetermined level or at a preset time, the filter must be washed. The filter is washed using a combination of air, to scour the media, and water to carry away the surplus biomass and captured solids. The rate of headloss development and efficiency of filter washing are critical to the operation and performance of biological aerated flooded filters.
Typically, suspended solids, biochemical oxygen demand and ammoniacal nitrogen are removed in the aerated filter. The process of ammonia removal produces the nitrate ion which is discharged in the effluent. Nitrate is nutrient for biological growth and problems with eutrophication have led to a requirement for nitrate removal in the process. If the treated effluent is recycled to an un-aerated section of the packed bed the bacteria responsible for the removal of BOD will utilise the oxygen bound up in the nitrate ion, releasing nitrogen gas into the atmosphere and reducing the level of nitrate discharged in the final effluent. This process is known as denitrification and is typically carried out in a separate treatment vessel.
In a prior biological aerated flooded filter, there is a packed bed comprising small granular media sized typically between 2 and 6 mm, the depth of the packed media bed being typically 1.5 to 2.5 meters. As described, air or other gas in introduced through coarse sparge pipes or diffusers at or near the base of the filter. The transfer efficiency of oxygen or other gases from the gas to the liquid phase is dependent on, amongst other things, the depth of water above the aeration device. Oxygen transfer efficiency can be improved by increasing the depth of the submerged packed bed, however this has the disadvantage of increasing the hydraulic loading rate, in terms of waste-water flow per unit surface area, increasing the rate of headloss development, and increasing backwash air and water flow rates needed to clean the deeper filter bed. This means that a granular media biological aerated flooded filter with a bed depth greater than typically 2.5 m, will require backwashing more often, with a higher energy and cost requirement per wash.
The headloss across the packed bed is dependent on the depth of the packed media bed, the size and angularity of the individual media particles, the waste-water flow and applied pollution load, the biodegradability of the pollution load and the growth rate of the attached biomass. If the packed bed comprises media grains of too small a size the headloss development during operation will be excessive and frequent washing of the bed will be required. Similarly, if the media has been sized correctly and excessively high flows are experienced the packed bed can be subject to premature headloss development causing the bed to block.
An advantage of small granular media as described is seen in the large surface area available for the growth of the bacteria and micro-organisms responsible for treatment. This leads to high concentrations of attached biomass and hence much smaller reactor volumes can be used. In addition physical and biological filtration are accomplished within the packed media bed resulting in treatment and solids removal in a single treatment vesse
REFERENCES:
patent: 4925552 (1990-05-01), Bateson et al.
patent: 5422020 (1995-06-01), Tylmann et al.
patent: 5525230 (1996-06-01), Wrigley et al.
patent: 5573671 (1996-11-01), Klein
patent: 5618430 (1997-04-01), Fuchs
Thames Water Utilities Limited
Wyse Thomas G.
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