METHOD AND APPARATUS FOR SEPARATING GASEOUS BUBBLES AND SAND FRO M A LIQUID FLOW

Abstract: A filtrate which has passed a filter bed (2) of granular filter medium intended for treating a suspension or a liquid solution can contain air bubbles and granular filter medium as a result of a gas, preferably air, being brought into contact with the filter medium. Separation of air bubbles and filter medium now takes place by the filtrate before being taken away from the filtering process, being caused to flow substantially downwards in a special path (12a; 15) at a rate sufficiently low for gas bubbles to rise upwards and is subsequently taken substantially upwards at a rate sufficiently low for the granular filter medium to be separated by precipitation.

Description: METHOD AND APPARATUS FOR SEPARATING GASEOUS BUBBLES AND SAND FROM A LIQUID FLO The present invention relates to a method and apparatus for preventing undesired sand spoilage from a filter bed in which air or other gas is used in contact with the filter medium. Th invention is particularly usable in a continuously operating nitrification reactor for biological nitrogen reduction of a suspension or liquid solution. From the Swedish patent specification 396 552 it is known to perform filtration of a suspension or an emulsion in a filter bed of particulate filter medium by allowing the suspension to flow upwards through the filter bed simultaneously as the filter medium flows downwards through a filtration tank, wherein filtration takes place. Supply of suspension takes place in a lower part of the filter bed and the filtered liqui phase is taken from a zone above the filter bed. The polluted filter medium close to the bottom of the filtration tank is taken out for conveyance to a washing apparatus above the filter bed and after washing is returned to the upper surface of the filter bed. Such conveyance of the filter medium takes place most simply using a so-called mammoth pump, which works with such as air as a conveying medium. It is also known to use a continuously operating granular filter bed for biological nitrogen reduction of a suspension o liquid solution, i.e. nitrification and denitrification. Bacterial strains suitable for the purpose are used in this case, and these strains are established in the filter bed. In the nitrification process, primarily ammonium compounds are oxidized to nitrates with the aid of oxygen, usually coming from air. Sand of a suitable grain size is preferably used as filter medium. When the filter is used as a nitrification reactor, a relatively large air stream is blown into the lower part of the filter bed. However, the stream must be restricted to the order of magnitude which does not cause fluidizing of the filter bed. The uppermost layer of the filter sand is even so inevitably whirled up by air bubbles, causing the filtrate above the filter bed to be contaminated by sand. This whirling sand mixed with air bubbles causes some difficulty. In the first place the sand is entrained at the outlet for the filtrate, and in the second place air and sand penetrate from below into the washing apparatus for the sand, in the case where the washing apparatus is sunken into the filtrate zone. The washing function is thus blocked. The present invention has the intention of remedying these difficulties. In accordance with the invention, separation of air and sand from the liquid stream is achieved at the places where the difficulties are to be found, and this is done in the following manner. First of all the mixture of air and sand in the liquid is permitted to flow downwards at a rate sufficiently low for the majority of the air bubbles to be separated by rising upwards against the flow. The liquid is subsequently allowed to flow upwards at a rate sufficiently low for the sand not to be entrained in the upward flow but to separate downwards to the bed. The invention is characterised in accordance with the accompanying claims. The accompanying figure schematically illustrates an example of how an apparatus in accordance with the invention can be formed. The figure illustrates the upper part of the filter tank 1 containing a filter bed 2 of a granular filter medium, e.g. sand. A suspension or a liquid solution flows continuously through this filter bed from below and upwards for treatment. The ready-treated filtrate is collected in zone 3 above the filter bed. A pipe 4 extends through the filter bed from the lower part thereof close to the bottom and upwards such as to constitute a conveying means, e.g. a mammoth pump, for polluted sand to a washing apparatus 5. The pipe 4 opens out above the washing apparatus which comprises a washing path 6 between the pipe 4 and a pipe concentric and external thereto. The sand moves downwards in this washing path and encounters an upward flow of the filtrate. The washing liquid polluted in this way is taken out via an overflow to the discharge or outlet 7. The filtrate is taken out at another overflow to the outlet 8. These outlets, which are suitably regύlatable, determine the level 9 of the washing liquid, and the level 10 of the filtrat in the filter tank. The difference in level determines the amount of flow of washing liquid upwards in the washing path. When the sand bed is used as a nitrification reactor, air is blown in from below to the bed, and this air rises up through the bed together with the suspension or solution which is to b treated. The filtrate and air bubbles leave the sand bed at it upper surface. In this situation a sand layer several centimeters thick will be fluidized, and sand together with ai bubbles will whirl about in practically the whole of the filtrate volume, i.e. zone 3. As previously mentioned, air bubbles could then penetrate into the washing path from below and block the washing process. In addition, sand would accompany the filtrate at the outlet 8. In accordance with the invention, the filtrate containing air bubbles and sand is first caused to flow downwards for separating the air bubbles, and subsequently to flow upwards for separating sand. In the washing apparatus this can be achieved by means of a horizontal, preferably circular plate 11 under the washing apparatus 5 and at a distance from it such that the downwardly falling sand from the washing path can for a sand cone having a natural angle of repose on the plate. A cylinder 12 is arranged concentrically round the washing path, and extends almost down to the sand cone on the plate 11. The liquid which is to constitute washing liquid in the washing path is thus compelled to flow downwards into the space or under the chamber 12a between the cylinder 12 and the washing apparatus, arrow A. This space is given dimensions sufficiently large so that a flow rate is obtained which is low enough for the air bubbles to depart upwards in time before the liquid flow changes direction for entry into the washing path. A method in accordance with the invention can also be provided at the filtrate outlet. Two mutually inclined plates 13,14 and having an angle to the horizontal plane which is greater than the angle of repose for the sand are required for this purpose. The first, inner plate 13 extends from a level immediately below the filtrate level 10 and down to the outer plate 14, but leaves a gap between the plates. The filtrate will now flow in on the upper side of the plate 13, arrow B, and is urged downwards by the filter tank shell towards the plate 14, which is inclined in the opposite direction and up towards the outlet. The quieter flow conditions which occur here in the space 15 above the plate 13 allow departure of the air bubbles upwards. In the following outlet chamber 16 the sand grains are separated by precipitation. They collect on the plate 14 and move back towards the filter bed through the gap between both plates. This gap is dimensioned sufficiently large to pass a sand layer through, but not to include any air bubbles in the other direction. However, should some air bubbles come through the gap even so, they can rise up to the surface via the space 15 without disturbing sand separation in the outlet chamber 16. In certain cases it may be necessary to decrease turbulence in spaces 15 and 16. This may be done with the aid of an insertion of a plurality of plates parallel to the plate 13 and/or plate 14. The centrifugal force which can occur when the water changes its flow direction from downwards to upwards is utilized here. Although the invention has only been described as applied to a continuously working nitrification reactor, it can also be used in a stationary filter bed and in other connections having similar problems.

Claims: CLAIMS 1. Method of separating air bubbles and granular filter medium from a filtrate, which has passed a filter bed formed by the filter medium intended for treatment of a suspension or liquid solution, in this treatment a gas, preferably air, being used in contact with the filter medium, characterized in that before being taken away from the filtering process the filtrate is caused to flow substantially downwards in a special path at a rate sufficiently low for air bubbles to separate and move upwards, the filtrate being subsequently led substantially upwards at a rate sufficiently low for the granular filter medium to separate by movement downwards. 2. Method as claimed in claim 1, wherein the filter medium from a bed is continually taken out in its most polluted state and is cleaned by the filtrate for subsequent refluxing to the bed, characterized in that the downward flow of filtrate takes place downwards through the zone of filtrate towards the inlet of the apparatus, in which the filter medium is cleaned by the filtrate, and in this apparatus is given the low upwardly moving velocity. 3. Method as claimed in claim 1, and particularly intended for operation of a nitrification reactor, where air is blown into the granular filter medium, characterized in that the downward flow of the filtrate takes place in a separate path immediately before the outlet for the filtrate from the filtering process, subsequent to which the filtrate is caused to change direction for flowing upwards towards the outlet. 4. Method as claimed in claim 1, characterized in that both the method according to claim 2 and that of claim 3 are carried out si ultaneously. 5. Apparatus for separating air bubbles and granular filter medium from a filtrate in accordance with claim 1, the filtrat having passed through a filter bed formed by the filter medium intended for treatment of a suspension or liquid solution, there being included in the apparatus a container for the filter bed with a filtrate zone above it having an outlet for the filtrate, a gas, preferably air, being used in contact wit the filter medium during the filtering process, characterized in that a first chamber (12a; 15) with an inlet placed high an an outlet placed low for the filtrate is located in the zone for the filtrate, said first chamber having flow communication from the outlet and to a second chamber (6; 16) via an inlet placed low in this chamber, the outlet for the filtrate from the second chamber being placed no higher than the level of th zone with the filtrate. 6. Apparatus as claimed in claim 5, constituted as a continuously operating filter and thus including a washing apparatus for the filter medium disposed at the level of the filtrate zone, to which washing apparatus the filtrate is supplied at the lower part of said apparatus, characterized in that the first chamber (12a) is juxtaposed the washing apparatus (6) and in that the second chamber comprises the washing apparatus, there being a reception plate (11) for receiving granular filter medium precipitated from the second chamber (6) under the inlet of the second chamber which is als the washing apparatus. 7. Apparatus as claimed in claim 5, characterized in that both the first chamber (15) and the second chamber (16) are dispose at a level with the surface of the filtrate in the container and adjacent the outlet (8) of the filtrate from the container (1) , and such that the second chamber (16) , appart from a flow connection to the first chamber (15) placed low, has an outlet opening placed lower for granular filter medium precipitating from the filtrate. 8. Apparatus as claimed in claim 7, characterized in that the chambers (15,16) are formed by a trough-like container comprising two mutually opposing and mutually inclined, downwardly sloping walls (13,14) which leave a slot-like opening at the bottom and an approximately vertical screen extending towards the bottom but leaving an opening along it for the formation of a flow passage between the chambers, the container also being provided with two end walls, the outlet from the second chamber (16) being level with the surface of the filtrate zone.

Inventors: HAGROT MAGNUS, JOENSSON KURT, LENVIK LENNART, HAGROT, MAGNUS, JOENSSON, KURT, LENVIK, LENNART
Applicants: NORDIC WATER PROD AB, NORDIC WATER PRODUCTS AB
Application year: 1992
Main classification: B01D24
IPC codes: B01D24, B01D36, C02F3