Liquid purification or separation – Processes – Treatment by living organism
Patent
1994-03-24
1996-06-11
Hruskoci, Peter A.
Liquid purification or separation
Processes
Treatment by living organism
210617, 210903, 210150, 210275, C02F 306, C02F 312
Patent
active
055252303
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process of and apparatus for treating a fluid, particularly for operating biological aerated flooded filters in the treatment of fluids, particularly domestic and industrial wastewaters, typically for the removal of suspended solids, biochemical oxygen demand, and ammoniacal nitrogen.
Traditionally wastewater treatment is accomplished by biological oxidation of organic and nitrogenous matter by bacteria and micro-organisms under aerobic conditions, suspended in a fluid as in the activated sludge process, or attached to inert support media as in a percolating filter. Although these treatment methods can be effective they require long retention times and the use of final settlement tanks to remove suspended biological solids prior to discharge of the treated effluent.
A proposed water and wastewater treatment apparatus is the biological aerated filter which comprises a bed of granular inert media, with the bacteria and micro-organisms (biomass) responsible for treatment attached to the media surface, in which the fluid to be treated flows constantly down or up through the bed, with the bed maintained in a submerged condition, with air, oxygen enriched air or other gases introduced at or near the base of the bed passing up through the bed to aerate the media and attached biological growth. The large surface area of the media promotes the growth of a high concentration of micro-organisms, giving much smaller aeration tank volumes than conventional treatment options. In addition treatment and solids removal are achieved in a single reactor eliminating the need for final settlement.
The degree of treatment achieved is dependent on the organic nitrogenous and hydraulic media loading rates. Typically the process would be designed on average applied rates with an appropriate "peak to average" constant to accommodate expected flow and load variations thus achieving the required degree of treatment at all times. However, the short retention time within the aerated falter and the plug flow characteristics of the fluid flow through the bed makes the process susceptible to variations in wastewater flow and strength, causing a drop in treatment efficiency and a potentially unacceptable increase in the concentration of pollutants in the effluent.
The need for improvement in river quality standards has also required a greater degree of purification in wastewater treatment processes in order to meet stricter limits on the concentration of, for example, ammoniacal nitrogen in the treated effluent.
In the case where the process is treating a wastewater to remove ammoniacal nitrogen, it having previously received treatment for the removal of the majority of the biochemical oxygen demand and suspended solids, the concentration of autotrophic nitrifying organism attached to the media at any point within the aerated bed will be dependent on the ammoniacal nitrogen substrate concentration and the temperature, providing the dissolved oxygen concentration does not limit growth.
Thus, for a given ammoniacal nitrogen and hydraulic loading rate a substrate concentration gradient will exist through the bed in the direction of wastewater flow and a similar concentration gradient of nitrifying organisms will exist. For example with the wastewater flowing down through the bed a higher concentration of micro-organisms responsible for treatment will exist at the top of the bed, decreasing towards the bottom of the bed as the substrate is removed.
It has been established that the growth rate of nitrifying organisms is slow. If the rate through the bed or the ammoniacal nitrogen loading rate exceeds the maximum nitrification rate, dependent on biomass concentration and time, the nitrifying organisms will not grow at a rate sufficient to cater for the increase in load and effluent quality will deteriorate.
Typically, to overcome this problem, the flow and load of wastewater to the plant must be balanced to equalise out these fluctuations or the depth of filter media is increased. These have the disadvantage of incurri
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Smith Alan J.
Wrigley David
Green Theodore M.
Hruskoci Peter A.
Thames Water Utilities Limited
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