ANAEROBIC PROCESS

Abstract: A two-step process for microbiological purification of contaminated water, comprising a first step using anaerobic conditions in a reactor (2) having a stationary bed (3) and a biofilm fixed thereon followed by conventional microbiological purification. In the first step the water is supplied at the upper end of the reactor (5) and is brought to trickle over the fixed biofilm without drenching the bed and while maintaning a continuous gasphase within the bed.

Description: The present invention relates to a process for the purification of contaminated water in a biological manner while using anaerobic conditions in a reactor having a stationary bed and biofilm fixed thereon. In biological purification methods two different processes are in principal used, namely aerobic process and anaerobic process. In using aerobic purification techniques two types of processes have hitherto dominated, namely the active sludge process and the biological bed. Not until recently one has started to utilize filler materials quenched in water in airiation basins. In many cases the purpose of such a procedure has been to maintain on the filler material slowly growing nitrification bacteria in the system. Within the anaerobic purification techniques the development has been different. The conventional digester was developed to the contact process, i.e. the anaerobic active sludge process. During the last ten to fifteen years anaerobic filters having fixed filler elements quenched in liquid have been developed and have entered into use. A different technique based on non-drenched filters or so called trickling filters is described in GB A 20 68 929. This patent indicates three different applications, namely: anaerobic decomposition of organic compounds with the formation of carbondioxide and other products; anaerobic decomposition of organic compounds with the formation of carbondioxide, methane and other products; and denitrification. The present invention utilizing anaerobic purification techniques aims at using the same type of filter as disclosed in the above-identified UK patent. It is directed to depoisoning by removal of sulphur compounds in a first step which is followed by a second step involving conventional microbiological purification. The present invention thus basically involves a process for the biological purification of contaminated water while using anaerobic conditions, said process taking place in a reactor having a staionary bed and a biofilm fixed thereto. In such process the contaminated water is supplied at the upper part of the reactor and is allowed to flow downwardly on the fixed biofilm without drenching the bed and while maintaining a continuous gas phase within the bed. A reactor of this type is in anglosaxon literature called "trickling or percolating filter". The expression thus means that the bed is not drenched in liquid but contains a continuous gas phase, whereas the liquid in the form of thin liquid layer flows on top of the biofilm present within the bed material. According to the present invention there is thus provided a two-step process for microbiological purification of contaminated water, said process comprising a first pretreating step using anaerobic conditions in a reactor having a stationary bed and a biofilm fixed thereon followed by a conventional second purification step, such as an anaerobic-aerobic process. Said process is characterized in that the contaminated water is pretreated in said first step by bacterial reduction of sulphur compounds to sulphides and removal of the resulting sulphides from the water. The bed material of the reactor can be constituted by some plastic or some inorganic material. As a plastic there may be mentioned polyesters or polyvinyl chlorides, whereas the inorganic material can be stone, glass, fired clay or other ceramic material. The process according to the invention can be performed in such a manner that treated water is allowed to recirculate through the reactor. Recirculation may also be used in relation to gases formed in the process, for example carbondioxide and methane gas. Also in later treatment steps gases formed can be circulated through the reactor. The biological reactor used in the process according to the invention thus comprises an upstanding container or tower, wherein solid fixed material is arranged and on which microorganisms are allowed to grow under the formation of a so called biofilm on the material. The contaminated water is then supplied at the top of the container and distributed over the solid material of the bed grown over by microorganisms. The reactor is sealed off from the surrounding air so that oxygen is excluded from the interior of the container, i.e. the biological reactions taking place in the reactor are of an anaerobic character. The surface of contact between the water flowing on the biofilm and the surrounding gas is very large in the reactor. The retention period will be relatively short in comparison with the case of using a drenched bed. The invention will in the following be further illustrated in connection with specific examples and with reference to the appended drawing, wherein the figure shows diagramatically an example of an embodiment of an anaerobically operating reactor. The reactor shown in the drawing and generally designated 1 contains a cylindric elongated container 2 containing a solid bed 3 divided up into three sections. Container 2 is provided with a liquid inlet 5 for the water to be purified passing over into a spray nozzle 7 for the distribution of supplied water over bed 3. The container is, furthermore, provided with an outlet 9 having a closure valve 11. In the area between bed sections 3 drainage tubes 13 are provided to enable sampling at different levels of the reactor. These drainage tubes 13 are each provided with two closure valves 15 enabling water sampling with lock under operation. The reactor is finally provided with a gas outlet 17 for discharge of generated gases. In the following two examples there is illustrated on the one hand total purification of sewage water, and on the other hand depoisoning of sulphur-containing water. EXAMPLE 1 A sewage water having a COD-concentration of 6 700 mg/l is anaerobically purified in a reactor working according to the principal of the present invention. The water flow is about 40m³/hour. The reactor contains as a packing material a PVC-plastic of the block type. (Biodek sold by AB Carl Munters, Sweden.) The bed height is 8 m and the cross sectional area 54 m² corresponding to a diameter of 8.3 m. The load on the bed is about 15 kg COD/m³ and day. The hydrolic load used is 1.5 m³/m² and hour and this value is based on water recirculated to 100%. The COD-reduction over the reactor used is about 75%, whereas the BOD-reduction is about 85%. In the purification methane gas is generated in an amount of somewhat more than 1500 Nm³ per day. EXAMPLE 2 A sewage water having a COD-concentration of 7 200 mg/l is depoisoned by treatment in a reactor working according to the principal on which the present invention is based. The sulphur contents in the water are about 1000 mg/l SO₄ calculated as sulphur. The flow is about 50 m³/hour. The packing material used is the same as in Example 1. The filter height is 8 m and the cross sectional area 31 m² corresponding to a diameter of 6.3 m. The load on the filter is about 35 kg COD/m³ and day. The hydraulic load used is about 1.6 m³/m² and hour. No recirculation is used. The COD-reduction in the treatment in the reactor is about 25 %, whereas the sulphur separation rate is about 75%. The methane production is low. The gas formed contains a high content of hydrogen sulphide and is washed in a separate washing step and recirculated over the reactor. The depoisoned sewage water is then subjected to conventional purification in an anaerobic-aerobic system. The use of a non-drenched anaerobic filter in accordance with the techniques of the present invention offers substantial advantages compared to the use of a conventional anaerobic filter or a conventional contact process. Among such advantages the following may be mentioned. The construction of the system means that the thickness of the biofilm can be hydraulically controlled. Moreover, increased reaction rate can be expected because of the very large contact surface obtained between gas and liquid and this in turn means that the concentration of the carbondioxide in the biofilm can be kept at a relatively low level. A high content of carbodioxide has been found to have an inhibiting influence on the metabolism of the methane bacteria. The system is simple in construction and involves substantial technological process advantages associated with processes utilizing biofilm. Moreover, the system can be used as a pretreatment step of a conventional system based on anaerobic-aerobic process. This pretreatment can be constituted by hydrolysis, acidification, removal of oxidating agents or removal or poisons, such as sulhpur-containing compounds. A process based on biofilm is normally substantially more resistant in relation to poisonous substances than systems using suspended cultures of bacteria. An ecologic succession in direction to the development of organisms capable of breaking down toxic substances is also easier obtained in a process based on biofilm where the through-flow is of the type plug flow. In connection with the use of the system as a pretreatment step the invention is particularly suitable to be used for reducing sulphur compounds to hydrogen sulphide and effective removal of same from the liquid to the gas phase, since the surface of contact between liquid and gas is quite large.

Claims: A two-step process for microbiological purification of contaminated water, comprising a first step using anaerobic conditions in a reactor having a stationary bed and a biofilm fixed thereon followed by conventional microbiological purification in for example an anaerobic-aerobic process , where in said first step the water is supplied at the upper end of the reactor and is brought to trickle over the fixed biofilm without drenching the bed and while maintaning a continuous gasphase within the bed, characterized in that the contaminated water is pretreated in said first step by bacterial reduction of sulphur compounds to sulphides and removal of the resulting sulphides from the water. A process according to claim 1 characterized in that the first purification step takes place in a bed of plastic or inorganic material. A process according to claim 2, characterized in that the plastic is a polyester or PVC. A process according to claim 2, characterized in that the inorganic material is glass or fired clay. A process according to any preceding claim, characterized in that the water is recirculated. A process according to any preceding claim, characterized in that gases generated in the reactor or in subsequent treatment steps are circulated through the reactor.

Inventors: SAERNER, ERIK, Särner, Erik
Applicants: AC BIOTECHNICS AB
Application year: 1987
Main classification: C02F3
IPC codes: C02F3