FR2518523A1 - Bacterial liquid treatment process - uses multistage impregnated porous beds spaced to allow gas absorption between stages - Google Patents

Abstract

A method for the biological treatment of liquids comprises the use of a series of beds of open porous materials, e.g. polyether foam, impregnated with bacteria, through which the liquid is allowed to trickle under gravity. A series of such slabs placed one above the other is used, with the vertical space between slabs sufficient to enable the liquid droplets to absorb air, or gas, to compensate for that consumed in the bacterial action in the slab above. Used for the conversion of ammoniacal liquors to nitrates using autotrophic bacteria.

Description

Method and device for bacterial treatment of a liquid. In ever more numerous fields of water use, it becomes necessary to carry out its purification both mechanical (elimination of suspended particles), and chemical (elimination or transformation of dissolved matter.

 In this context, biological filtration, or more precisely nitrification, is the biological oxidation of ammonia (NH4 +, NH3) to nitrite (N02-) then to nitrate (N03-) by autotrophic bacteria. It is preceded by the ammonification of nitrogenous organic compounds by heterotrophic bacteria.

 Fixed on a substrate, the bacteria come into contact with the dissolved metabolites (NH4 +, N02 and 02) either by immersion of the substrate in an ascending (up-flow) or descending (down-flow) flow, by trickling of the influent on the substrate (percolation, "trickling-filter"), by fluidization of the substrate in an upward flow (fluidized bed) or in a settling tank. It is the autotrophic bacteria which carry out the nitrification with the highest yield and especially nitrosomonas, responsible for the first stage (N-NH3- N-N02) and nitrobacter, active agent of the second phase (N-N02 - N-N03 ).

 Even when present in the liquid, bacteria exert their activity only when fixed on a substrate.

 Examination of the global equations of the metabolism of the strains used makes it possible to establish that the nitrification of a gram of N-NH3 consumes 4.25 g of O2, which explains why the lack of O2 for the respiration of bacteria can cause partial reversal of nitrification. In addition, 0.1425 g of H + ions (acidification) and 0.15 g of nitrosomonas cells, as well as 0.025 g of nitrobacter cells, are produced. All or part of the substrate can be natural or synthetic and have its own action on maintaining the physico-chemical properties of water (PPl, hardness, alkaline reserve, for example).

 The granulometry or porosity of the substrate is an important factor, a fine granulometry increases the surface available for the attachment of bacteria, but leads to an increased risk of clogging.

 Finally, certain characteristics of the water influence biological filtration. Aerobic bacteria of nitrification must have oxygen at will to develop their activity.

 Oxidation of NH3 releases H + ions and variations in PH affect the dissociation and harmfulness of ammonia and nitrites.

 The capacity of a filter being proportional to the surface available therefore to the volume of substrate, for a given filtration capacity, it has been necessary to provide relatively large thicknesses of substrate which can range up to several meters.

 However, we note that during its passage through the substrate, the first ten to fifteen centimeters are the seat of 60 of bacterial activity. Indeed1 beyond, the oxygen level decreases, therefore the bacterial activity and consequently, the capacity of nitrification. In addition, thick filters have serious drawbacks for their cleaning and maintenance, especially in the event of clogging, or the development of anaerobic metabolites which "kill" the filter (presence of toxic (NH4) 2S). In addition, since only the first ten to fifteen centimeters are really effective, the total efficiency of conventional filters of large thickness is relatively low. the treatment of a significant flow of water, quickly achieved in industrial # aquaculture, involves tanks with very large dimensions. This results in substantial civil works, resulting in capital immobilization, the depreciation of which is far from negligible on the cost price of the kilogram produced.

 The present invention intends to remedy these drawbacks by proposing a new type of biological filter and a new method for the bacterial treatment of aquaculture water.

This new filter is five to six times more effective per liter of substrate than that of filters known to date. In addition, its design makes it possible to considerably reduce the floor space required for a processing unit, and # # thereby, the resulting financial fixed assets. Finally, it allows for much easier maintenance, which does not require stopping the installation or costly additional provisions.

 For this purpose, the invention therefore has for first object, a device for biological filtration of a liquid, constituted by a substrate allowing the fixing of at least one colony of bacteria suitable for degrading the matters dissolved in said liquid which crosses the percolation filter.

 According to one of the main characteristics of the invention, said substrate is divided into a plurality of superimposed layers, each of which has a thickness corresponding to a bacterial activity consuming a partial amount of gas dissolved in the liquid passing through it, so that the concentration in gas in the liquid remains close to saturation and is separated from the adjacent layers by a layer of gas whose thickness is determined so as to allow the liquid passing through this layer of gas by percolation, a contact time with said gas, sufficient so that a quantity of this gas, at least equal to that consumed by the bacterial activity in the layer of substrate previously traversed, is dissolved in the liquid.

 In a typical embodiment; each layer of substrate is between 10 and 20 cm thick and is individually removable. The layers are separated by a space of 10 centimeters of gas. The substrate is a flexible open pore crosslinked polyether foam at 30 PPI.

 It will be advantageous to provide on the upper layer, the installation of a removable mechanical filter. It is also advisable to provide under the lower layer, the addition of one or more layers of different natural or synthetic substrates, chosen for their ability to reconstitute the physico-chemical characteristics of the treated water (PH, hardness, alkaline reserve, temperature, etc ...).

 It can be seen that the implementation of such a device is actually a biological or biochemical treatment process, a second object of the present invention which is characterized by the fact that a divided flow of the liquid is caused through a alternating plurality of layers of porous substrate for fixing bacteria suitable for degrading the matters dissolved in the liquid and of gaseous layers containing for each of them, the chemical element consumed during the bacterial activity sitting in the layer of substrate located immediately upstream with respect to the direction of flow of the liquid.

 The division of the flow separ percolation through synthetic or natural substrates of controlled and crosslinked porosity.

 The invention will be better understood during the description given below by way of indicative and non-limiting example, which will make it possible to identify the advantages and the secondary characteristics thereof.

Reference will be made to the accompanying drawings in # squels
FIG. 1 is a schematic sectional view of a filter according to the invention,
- Figure 2 is the schematic plan view of a filter bank according to the invention.

 Referring to these figures, it can be seen that a filter 1 according to the invention consists of a plurality of superimposed layers 2 of substrate for fixing a colony of bacteria suitable for the degradation of materials dissolved in the liquid to be purify. Each of these layers 2 is constituted by a foam mattress of a crosslinked plastic material with adequate porosity and suitable vacuum coefficient, arranged on a support 3 in the form of a box spring. Each of these bed bases removably rests on transverse support bars 4, themselves maintained at a fixed vertical spacing determined from each other by a support structure 5. The support 4 and the structure 5 can be of any kind suitable supporting fixedly or removably, or even allowing adjustment of the vertical spacing of the fixing substrates.

 Under this stack of "mattresses" 4, a basin 6 for collecting the liquid, provided in known manner, has been provided with means for extracting this liquid, or even recycling.

 These means, not shown, belong to the known filtration technique.

It is not shown either in this figure, the layer or layers or additional substrate, located on said mattresses or interposed between them, capable of due to their synthetic or natural composition, to reconstitute the physico-chemical characteristics of the water. treated (PH, hardness ...)
Finally, above said mattresses, devices 7 have been installed, for rain distribution of the liquid to be treated. These devices can be of the ramps, turnstiles or any other type, so as to uniformly distribute the fluid to be treated on the first tray (mattress). Thus, the flow of liquid to be filtered is divided and circulates by percolation through layers 2 and intermediate gas spaces for reconstitution. Note that in FIG. 1, there are 6 layers of substrate 2,
This figure illustrates a basic half-module of the filter according to the invention, comprising six series of layers superimposed between two support structures. Depending on the needs to be treated, complete modules can be used comprising twelve layers of substrate in an appropriate number.

 The complete installation consists of several complete modules arranged side by side, the assembly being supplied by the devices 7 extended over all of the modules, while a basin 6 is arranged under all of the above modules. Figure 2 illustrates schematically in a plan view, such an arrangement. We find in this figure, the elements already described with the same references.

 It can be seen from the above that the construction of a filtration installation in accordance with the invention is very simple, therefore of extremely low cost.

 In addition, it allows you to be very scalable. It is indeed possible to have on a basin 6 provided for a given maximum capacity, a number of modules adapted to a given flow rate, certainly lower than this capacity and capable of being modified as a function of the variation in these needs.

 It is prudent, for health reasons, to plan several units of five or ten complete modules, rather than one very large unit. The collection basins and their feeding and recovery system, not described here and being well known in the art, will be kept separate and will feed different breeding units so as to avoid the spread of possible epizootics . In addition, the maintenance of this filter is extremely easy. Indeed, the mattresses 2 and their box spring are very easily # accessible, removable.

We can therefore easily remove them and clean them outside the filter. Thus, each of the layers can be successively removed at regular time intervals and replaced with a clean layer. The layer 2a which has just been cleaned can advantageously be stored on a support Sa, above the bottom of the basin, but spaced from it to allow the circulation of water, and repopulate with bacteria while awaiting replacement. of the next layer. The filter can thus operate continuously with reduced reserve equipment,
The filter will be started by seeding the appropriate bacteria, which bacteria will be nourished and multiplied by providing them by addition, according to an appropriate method, so as not to exceed the concentration-rate of auto-inhibition, the metabolites (NH3 N02), in the percolation liquid. By checking that NH3 and 102 are well transformed, the quantity of added metabolites is gradually increased, until the desired treatment capacity of the filter is obtained. The population being then constituted, it can be given to treat the water of a fish farm or treatment plant to be treated. In the case of significant variations in loads in the metabolites to be treated, it may be advantageous to compensate for a reduction in these by a corresponding addition of metabolites, so as to keep the bacterial population alive.

 Finally, it will be interesting to provide the upper mattress 2 of each module, with a mechanical filter 8, intended to retain the particles in suspension in the liquid # e. This filter, which may consist of a sheet of permeable nonwoven fiber, will be easily removed for frequent washing.

 In addition to these construction advantages, the modular filter according to the invention is of great interest from the point of view of efficiency. Indeed, we realized in conventional filters, that more than 60 of the bacterial activity had their seat in the first ten to twenty centimeters of thickness of the substrate encountered by the liquid.

 Beyond this, this activity decreases by the lack of oxidizer, that is to say of gas dissolved in the liquid and consumed by the bacteria in their degradation activity. The filter of the invention taking this observation into account, only uses layers of substrate, thin enough for the concentration of oxidant dissolved at the outlet of the substrate, remaining close to saturation, thus allowing the development of optimal bacterial activity. Then, between each layer, a space is filled with ambient gas of thickness (E) which the liquid crosses in a very divided flow (due to its admission by percolation). This division promotes the exchange between the liquid and the gas and the reconstitution of the saturation rate of oxidant in the liquid.

 Thus, if this gas comprises the gaseous element consumed in dissolved form by the bacteria of the previous layer, the liquid can be recharged in this element during its passage through the gaseous layer. The thickness (E) of the layer will be determined as a function of the exchange time necessary for the liquid to dissolve an amount of gas, at least equal to that consumed in the previous layer. The liquid arriving in the next layer, ensures the optimal conditions so that in this layer, the bacterial activity is again as intense as possible. Ii follows a considerable increase in the efficiency of the installation which saves a significant space compared to known filter constructions. Indeed, for the same substrate, the invention provides a yield six times higher in terms of g N-IH3 transformed per liter of substrate and per day compared to known filters (1.6 to 1.8 g per liter and per day against 0.3g per day and per liter maximum) In other words, to obtain identical results, the invention is approximately thirteen times less bulky than a known filter (for example with upward flow with as substrate log rings 3. Economically speaking, the cost of civil engineering is fifteen to twenty times lower and the cost of the substrate five times lower.

 In the case shown, the gas in question is air because generally, water treatment involves oxygen-consuming bacteria (aerobic). However, the invention can easily be implemented in a different gaseous atmosphere adapted to the specificity of the colonies of bacteria used and to the nature of the filtered liquid. For this, it is sufficient to enclose the filter modules in an enclosure filled with this atmosphere (possible application: elimination of traces of hydrocarbon resulting from accidental pollution).

 It should be noted that the whole of said filter can be enclosed in this sealed or # opaque enclosure so as to take account of the etiology of the bacteria used if they have a photophobic behavior, for example.

 It can be seen that the invention can be generalized into a biological or biochemical treatment method according to which a liquid in divided flow is passed through a plurality of thin layers of substrate for fixing one or more colonies of bacteria, alternating with gaseous layers containing the element or elements consumed by the bacterial activity of the previous layers in order to recharge the liquid by dissolution before it reaches the next substrate layer.

 The invention is not limited to the description which has just been given, but on the contrary covers all the variants which may be made to it, without departing from its scope or from its spirit. It finds an interesting application in the field of water treatment and more particularly of recycling water from an aquaculture installation. Thus, in miniaturized form, the invention makes it possible to completely renew the philosophy of the treatment of the waters of aquariums of sea water or fresh water, taking up in a clearly # more effective form the problems of biological filtration. and maintaining the quality of the environment.

 An important variant of the invention is therefore represented by the use of alternating layers of substrate and gas, preceded by a mechanical filtration sheet and followed by one or more layers of various materials intended to maintain the desired parameters. for the life of cultivated aquatic species.

Claims (9)

R E V E N D I C A T I O N S 1. A device for biological filtration of a liquid, consisting of a substrate for fixing at least one colony of bacteria suitable for degrading the materials dissolved in said liquid which passes through the filter by percolation, characterized in that said substrate is divided into a plurality of superimposed layers (2), each of which has a thickness corresponding to a partial consumption of the gas dissolved at saturation in the liquid passing through it and is separated from the adjacent layers by a layer of gas the thickness (E) of which is determined by so as to allow the liquid passing through this layer of gas by percolation, a contact time with said gas sufficient for a quantity of this gas, at least equal to that consumed by the bacterial activity in the layer of substrate (2 ) previously crossed, or dissolved in the liquid. 2. Device according to claim 1, characterized in that each substrate layer is of a thickness between 10 and 20 centimeters while the gas layers have a thickness of approximately 10 centimeters. 3. Device according to any one of the preceding claims, characterized in that the substrate is a cross-linked synthetic material with open pores of dimensions appropriate to the hydraulic head and to the desired bacterial activity. 4. Device according to claim 3, characterized in that the substrate is a cross-linked polyether foam with open pores at 30 PPI. 5. Device according to any one of the preceding claims, characterized in that each substrate layer is individually removable. 6. Device according to any one of the preceding claims, characterized in that the upper layer (2) of substrate is covered on its upper part with a removable mechanical filter (8). 7. Device according to any one of the preceding claims, characterized in that under the lower substrate layer is one or more layers of natural or synthetic substrate, intended to restore the physico-chemical properties of the liquid (PH, hardness, reserve alkaline ...) possibly altered by bacterial action.  8. A method of biological filtration of a liquid, characterized in that a divided flow of the liquid is caused through an alternating plurality of layers of a porous substrate for fixing bacteria suitable for degrading the materials dissolved in the liquid and gaseous layers (E) containing for each of them, the chemical element consumed during the bacterial activity sitting in the layer of substrate located immediately upstream with respect to the direction of the flow of liquid. 9. Method according to claim 8, characterized in that the division of the flow is carried out by percolation or # spraying.