Liquid purification or separation – Processes – Treatment by living organism
Patent
1993-10-18
1995-12-05
Wyse, Thomas
Liquid purification or separation
Processes
Treatment by living organism
210794, 210150, 210220, 210274, C02F 320, C02F 330, B01J 802, B01D 3906
Patent
active
054726107
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for blowing a gas into a granular filter bed of a type where the filter medium moves downwards against the flow of a suspension that is to be treated in the filter bed. The invention is particularly suitable for blowing air into a filter bed intended for the nitrification of wastewater.
In recent times there has been an increased need of nitrogen reduction in wastewater, not in the least due to governmental edicts. In Sweden, a nitrogen reduction of at least 50% will be demanded in 1992 for certain sensitive parts of its coastline, and the prerequisites for a 75% reduction are to be investigated. Nitrogen reduction is usually biologic, and in two steps. In the first step, ammonium compounds are oxidized to nitrates, i.e. nitrification, and in the second step nitrates are reduced to nitrogen gas, i.e. denitrification. The processes are usually carried out under agitation in sludge basins.
Greater interest has been shown in recent times for biological reduction using granular filter beds, for nitrification as well as denitrification. Great advantages may be obtained by biological purification in filter beds, as compared with the basin processes, there being, inter alia, no need for a number of sedimentation basins for separating and recirculating sludge. However, there are difficulties in the use of filter beds for nitrogen reduction, and one such difficulty will be dealt with below.
The first nitrogen reduction step, i.e. nitrification, takes place according to the general formula: N+0.941 H.sub.2 O+0.98 NO.sup.-.sub.3 +1.98 H.sup.+.
The ammonium compounds are reduced by oxygen to nitrates. The term 0.021 C.sub.5 H.sub.7 O.sub.2 N corresponds to the production of new bacteria. The filter bed used may be a continuously operating sand filter, where the sand at the bottom of the bed is removed for washing and returned to the top of the bed. The wastewater that is to be treated is supplied at the bottom of the bed, and the treated water, i.e. the clear solution, is taken off from the top, or above the bed. Air is used as a source of oxygen, and is supplied at the bottom region of the bed via a number of horizontally disposed pipes having a large number of holes with a diameter of less than 1 mm. A sand filter operating continuously in this way is described, e.g. in the French patent specification No. 2 623 794.
The bacteria attach themselves to the grains of sand. If washing the sand is not performed too intensively, a bacterius strain remains after washing sufficiently for the nitrification. The strain grows in size as the sand in the filter bed moves downwards. Bacteria, which in such a case can form long threads, also attach themselves to the air inletpipes, causing the small air holes to become blocked. This causes the air supply to be cut off locally, or to decrease in magnitude. Another common cause of blocking up these holes is the deposition of lime. The difficulties caused by blockage of the air supply holes have been found impossible to overcome, except by dismantling the pipes after they have been in service for some time in order to clean them, which is expensive and troublesome.
The present invention relates to a new method and new apparatus for supplying gas into a granular filter bed of the kind mentioned in the introduction. In accordance with the invention, the gas is conveyed out into the filter bed by being continuously distributed along at least one elongated edge, flows round this edge and upwards within the filter bed, whereby the inflow being such that a substantially uniform distribution of the gas through the filter bed is created.
With the inflow of the gas in accordance with the invention, the ability of the bacteria to block the gas supply is simply and effectively inhibited. In addition, it is essential for obtaining effective utilisation of the invention that substantially uniform distribution of the gas blown into the bed is obtained, e.g. for nitrification there is desired as uniform as possible oxygenation of the bed.
It is
REFERENCES:
patent: 4683062 (1987-07-01), Krovak et al.
Nordic Water Products AB
Wyse Thomas
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