GB2025922A - Process for biodegradable effluent treatment - Google Patents

Abstract

Biodegradable effluent is anaeobically treated to produce a methane-containing gas, and the resultant liquor is catalytically oxidized.

Description

SPECIFICATION Process for treating biodegradable effluents This invention relates to a process for treating biodegradable effluents e.g. pig slurry, cow manure, distillery wastes, starch wastes and abbatoir wastes. Such effluents may occur in places where an excess of effluent is produced of the current requirements of use of the said effluents for various purposes, or where, because of constraints applied to the effluent source, it is not convenient for these to be discharged by their current methods. Such effluents may currently be used for land fertilisation purposes, including places where there are restrictions on the use of raw or partially treated effluents as fertilisers, or for the use of feeding to glasshouse crops, for feeding to animals, or may be discharged either directly or indirectly into public sewers to a watercourse, or to a landfill site.The effluent used in the process can be produced purposefully in order to be able to recover useful by-products therefrom by the process of the invention. Thus, for example, at a central treatment works, several effluents could be mixed to provide a suitable biodegradable effluent mixture for the process.

It is an object of a preferred embodiment of the invention to provide a process wherein a biodegradable effluent of suitable characteristics can be economically treated to produce re-cyclable water, a fertiliser base having negligible srnell, and a methane-containing gas.

According to the present invention there is provided a process for treating biodegradable effluent, including the steps of anaerobically treating the effluent so as to reduce the BOD thereof and to poduce a methane-containing gas, collecting the methane-containing gas, and subsequently ozonising or otherwise catalytically oxidising, the anaerobically treated liquor. It may be possible in certain circumstances to ozonise or otherwise catalytically oxidise the liquor immediately after the anaerobic treatment step, but because of the relative cost of ozonisation or other catalytic oxidation, it may be advantageous to separate certain materials, for example, suspended solids and/or emulsions, out of the liquor, and/or to treat the liquor in order to be able to separate out coagulatable solids before the ozonisation or other catalytic oxidation step.Such methods would include, for example, gravity settling of suspended solids in a clarifier, and/or filtration in a filter press and/or chemical treatment to separate out coagulatable solids, such methods being well known in the art.

Most advantageously, the anaerobically treated effluent, after a solids removal step, is chemically treated to coagulate coagulatable soluble matter, and a flocculant is added to flocculate the coagulated solids and any other fine suspended solids.

Following coagulation-flocculation, the resulting solids can be beneficially removed from the liquor. Suitable methods are by sedimentation, flotation, filtration, centrifugation, or a combination of methods may be used. A particularly preferred method is electroflotation.

The solids removed, whether from the solids removal step, if any, immediately after the anaerobic treatment step, and/or those obtained from the coagulation, flocculation, sedimentation, flotation, filtration or centrifugation steps are advantageously further dewatered to yield a cake of solids. A particularly suitable machine for this is a filter press, alternatively a pressure filter belt press could be used. In some cases, it is possible to omit the above-mentioned clarification and/or coagulation-flocculation step(s), in which cases the anaerobically treated liquor may be dewatered directly in a filter press, or similarly suitable machine. The solids resulting from the above procedures, whether present as a slurry or as a cake may be useful, depending upon the type of effluent, as a fertiliser, a fertiliser base, a soil-conditioner, or as an animal feed.The solids may also be dewatered further, if desired, by drying, e.g. thermal drying, such as in a steam heated rotary drum dryer, spray drying, or freeze drying. Alternatively, or additionally, the solids may be burned in a boiler or an incinerator, either alone or in conjunction with a suitable fuel.

In either case the resulting ash may still be suitable as a fertiliser. The ash may also have use as a chemical feedstock.

Following chemical treatment, and before any separation stage, for example electro-flotation, it may be advantageous, if lime has been added to the liquor, to recover ammonia from the liquor stream. This operation can be performed in a gas stripper in a manner known per se.

Such treatments can reduce the quantity of ozonisable or oxidisable material and consequently the cost of ozonisation or oxidation would also be reduced.

It is preferred for the liquor which has been subjected to the ozonising or catalytic oxidation step to be filtered, for example by using a mixed media filter.

In the process of the present invention part of the collected methane-containing gas produced may be beneficially used as an energy source to heat or cool the effluent being anaerobically treated. This may be necessary depending upon the ambient temperature conditions and whether there is another suitable energy source.

Advantageously, collected methane-containing gas is used to power an electricity generator set.

The electricity thus produced is preferably used to power the equipment described above e.g., the ozone generator.

Such an electricity generator will normally be a dual fuel generator which can be powered either by the methane-containing gas or by another fuel e.g., fuel oil.

A dual fuel powered generator is preferred because of external source of fuel many be required when starting up the process and may also be required if the biodegradable effluent being treated does not have a suitable BOD to enable a sufficiently high rate of methane-containing gas production to permit this to be used as the sole energy source to ensure that the anaerobic treatment is maintained at a suitable temperature and that sufficient electricity is available to power the other steps in the process. In this respect, it is preferred for the biodegradable effluent being treated to have a typical BOD of the order of 5000 mg/litre upwards. In certain circumstances lower BODs could be used, this depending on the specific local conditions.The methane-containing gas may also be used to effect mixing of the effluent during the anaerobic treatment step, to provide hot water for various purposes by burning in a boiler, and to provide a source of direct heat, for example, in the solids dryer mentioned above.

Alternatively, or additionally, the methanecontaining gas may be used, either directly or indirectly after separation of its two main components (carbon dioxide and methane), as a chemical feedstock.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic flow diagram illustrating one embodiment of a process for treating organic effluent according to the present invention.

Referring now to the drawing,-raw organic effluent is fed via a line 10 tQ a fine screen 1 where large solids (having a size of 2mm or over) are removed. The removed solids are passed along a line 12 into a sludge holding tank 13 equipped with a motorised stirrer 14. The effluent from which the large solids have been removed is then passed into an anaerobic digesting tank 14 maintained under mesophilic-temperature conditions e.g. 350C. the average residence time of the effluent in the tank 14 is normally 10 to 20 days. During anaerobic digestion, methane and carbon dioxide (30-6040:70 vol ratio) are evolved and collect in the tank 14 above the liquid level therein. Some of this methane-containing gas is passed along a line 15 via a control valve 16 to a dual fuel electricity generator 17.The remainder of the methane-containing gas evolved during the digestion process burned in a boiler B and the hot water generated is utilized via a heat exchanger E for maintaining the tank 14 at the required temperature. Anaerobically treated effluent from the tank 14 is passed via a line 18 into a clarifier 22 where a solids-liquid separation takes place. The resultant separated solids are removed and passed via a line 23 to the sludge holding tank 13. Supernatent liquor from the clarifier 22 is passed via a line 26 into a balance tank 27 which is used to provide a balanced flow of partially treated effluent to the next stage of the process. The level of liquid in the balance tank 27 controls the operation of pumps 28 disposed downstream of the balance tank 27.From the tank 27, the effluent is pumped by the pumps 28 via a line 29 to the first pair of reaction tanks 30 and 31 which are serially connected via an overflow weir.

The reaction tank 30 is supplied with one chemical reagent from a reagent holding tank 32 whilst the other reaction tank 31 is supplied with another reagent from a reagent holding tank 33. In the reaction tanks 30 and 31, the effluent is treated with ferrous sulphate, ferric sulphate, ferric chloride or aluminium sulphate and with lime or caustic soda, respectively. Alternatively, the lime or caustic soda may be added first. This chemical treatment serves to coagulate any remaining coagulatable matter in the effluent. From the tank 31 the chemically treated effluent is passed via a line 34 to the mixing box 35 in which it is mixed with a flocculant (e.g. a polyelectrolyte, such as polyacrylamide) supplied from a storage tank 36.

The resultant mixture flows from the box 35 to an electroflotation unit 36 where any floating solids are removed by a skimmer band 37 and any solids which fall to the bottom of the unit 36 are removed via a line 38 and passed to the sludge holding tank 13. Solids removed by the skimmer band 37 are discharged and carried to the sludge holding tank 13 via a line 39. Liquor from which the flocculated and coagulated solids have been removed is passed via a line 40 to another balance tank 41. The level in the balance tank 41 serves to control operation of pumps 42 disposed downstream of the tank 41. Liquor from the balance tank 41 is passed through an aspirator 43 whereat it is mixed with ozone or ionised air (ozone-containing air) before being passed into a catalytic oxidation reactor 44. An ozone generator 45 is used to supply ozone or ozone-containing air to the aspirator 43.From the reactor 44, the liquor is passed into a second catalytic oxidation reaction 46 via a line 47 and a further aspirator 48 whereat the effluent is mixed with further ozone or ozonecontaining air from the ozone generator 45. The reactors 44 and 46 are vented to atmosphere via a line 49. After catalytic oxidation, the effluent is passed to a mixed media filter 50 containing sand and activated carbon to remove any solids which may have been formed by virtue of the catalytic oxidation process. A portion of the effluent liquor from the catalytic oxidation reactor 46 is passed back via a line 51 and pumps 52 to the balance tank 41 to ensure that the flow conditions through the reactors 44 and 46 are maintained at the optimum level. From the filter 50, the majority of the liquor is passed along a line 53 to a monitor tank 54 whereat the pH and the flow rate of the liquor are monitored. From the monitor tank 54, the effluent may then pass via a line 55 to be re-used as drinking water since it is clear and sterile. A portion of the liquor from the filter 50 is supplied back to the sludge holding tank 13 to impart a suitable liquid consistency to the sludge therein to enable it to be pumped via a line 56 by ram pumps 57 to a filter press 58. The resultant press cake is collected and may be bagged. The liquor removed in the filter press 58 is passed via a line 59 back to the reaction tank 30. The dual fuel electricity generator 17 serves to supply the ozone generator 45 and also serves to power the stirrers in tanks 13, 30, 31, 32, 33 and 24 as well as powering the pumps and the electroflotation unit 36 including the skimmer band 37. With a suitably high BOD level in the feed effluent the process can be virtually completely self-contained.However, for starting up purposes and for use where the BOD level in the effluent is not sufficient, the dual fuel electricity generator 17 may be supplied with fuel oil via a line 60.

In the above-described process, the fine screen 11 may be replaced with a coarser screen as it may be advantageous to keep a iarger proportion of solids in the effluent in order to increase the efficiency of the process and thereby the methane-containing gas producing potential thereof. The fine screen may be of the brush-type with perforated plate media or it may be of socalled Hydrasieve Gravity Screen whose structure is not unlike that of a conventional side hill screen but which functions in a different manner. The Hydrasieve Gravity Screen achieves high fluid removal by hydraulic shear using the Coander effect of a multiplicity of specially designed Vsection transverse wires.

In the reaction tank 30, a pH control may be effected in order to keep the pH at approximately 8 whilst, in the reaction tank 31, the pH is maintained at about 11 or vice versa.

The digestion tank 14, in this embodiment may be fitted with a system sold by Simon-Hartley Limited and known as the "Heatamix" system. This system provides the necessary heating and mixing of the anaerobically digesting effluent and features a heat exchanger fitted externally of the digestion tank. A similarly suitable system is the Jones and Attwood Limited "Burper mixer" and features a heat exchanger fitted internally of the digestion tank.

The ozone-generator employed is, in this embodiment a corona-discharge, air-cooled ozone generator supplied by Union Carbide. In the above-described embodiment, the effluent being treated is passed serially through the reactors 44 and 46. However, it is within the scope of the present invention to connect these in parallel rather than serially. In each reactor 44, 46 the effluent mixed with the ozone or ozone-containing air is passed through a fixed bed upon which is supported a catalyst of one or more high valency metallic oxides (e.g. MnO2) and/or carbon. The temperature within each reaction chamber 44, 46 is at ambient temperature. As a result of the catalytic oxidation process, the organic compounds in the effluent are oxidised to carbon dioxide and water, the carbon dioxide escaping from the reactors 44 and 46 via the line 49 and being discharged to atmosphere. The typical residence time of the effluent in each reactor is 10 minutes and the concentration of ozone in the effluent fed to each reactor 44, 46 is 1~3% by volume.

In an alternative embodiment, each reactor 44, 46 takes the form of a fluidized bed reactor rather than a fixed bed reactor. Suspended solids can pass through a fluidized bed reactor better than through a fixed bed reactor where there is a risk that fine solids are deposited and tend to blind the fixed bed.

Claims (13)

1. A process for treating biodegradable effluent, including the steps of anaerobically treating the effluent so as to reduce the BOD thereof and to produce a methane-containing gas, collecting the methane-containing gas, and subsequently catalytically oxidizing the anaerobically treated liquor.

2. A process as claimed in claim 1, including the step of removing solids from anaerobically treated effluent before the catalytic oxidation step.

3. A process as claimed in claim 2, wherein the solids removal step is effected by filtering using a filter press to produce a solids cake.

4. A process as claimed in claim 2, wherein the solids removal step comprises passing the effluent through a clarifier to remove solids, then passing the solids in a filter press to remove any remaining liquor and to produce a filter cake, and adding the liquor removed from the filter press to the main body of effluent.

5. A process as claimed in claim 4, wherein the anaerobically treated effluent after clarification is chemically treated to coagulate any coagulatable material and then subjected to a coagulated material removal step before being catalytically oxidised.

6. A process as claimed in claim 5, wherein the coagulat#ed material removal step is effected by electro-flotation.

7. A process as claimed in claim 3, wherein the effluent after passing the filter is subjected to chemical treatment to coagulate any coagulatable material and then subjected to a coagulated material removal step before being catalytically oxidized.

8. A process as claimed in claim 7, wherein the coagulated material removal step is effected by electro-flotation.

9. A process as claimed in claim 1, wherein the methane-containing gas is used as an energy source in the process.

10. A process as claimed in claim 9, wherein the methane-containing gas is used to maintain the effluent being anaerobically treated at the desired temperature.

11. A process as claimed in claim 9, wherein the methane-containing gas is used for electricity generation.

12. A process as claimed in claim 1, wherein the catalytic oxidation step is performed in a liquid phase ozonisation reactor.

13. A process for treating biodegradable effluent substantially as hereinbefore described with reference to the accompanying drawings.