GB2419877A

GB2419877A - Equipment for treatment of sludge by ultrasonic radiation and anaerobic digestion

Info

Publication number: GB2419877A
Application number: GB0424663A
Authority: GB
Inventors: Paul Stephen Maryan
Original assignee: AEA Technology PLC; Accentus Medical PLC
Current assignee: Accentus Medical PLC
Priority date: 2004-11-09
Filing date: 2004-11-09
Publication date: 2006-05-10
Also published as: GB0424663D0

Abstract

Equipment for treating sludge created by an aerobic waste treatment plant, the equipment comprises an ultrasonic irradiation duct through which the sludge is flowing. The duct comprises a cylindrical pipe with a multiplicity of ultrasonic transducers attached to the outside of the pipe in an array of separate transducers, that extends both circumferentially and longitudinally. Each transducer is connected to a signal generator so that the transducer radiates no more than 3 W/cm<2>. The equipment further comprises an anaerobic digester with an outlet for biogas. Sludge, that has passed through the duct, is supplied to the digester, and energy is obtained from the biogas in a conversion step, e.g by using an electricity generator driven by an engine for which the biogas is the fuel. The irradiation breaks up cell walls and reduce the bulk of the sludge, and useful electrical energy is produced. Heat can be transferred from the conversion step to the sludge, that is caused to flow through the duct, and so enhancing the effectiveness of the ultrasonic irradiation. The equipment is e.g. a module, incorporating a support frame, the module being of convenient size for transport.

Description

Sludge Treatment The invention relates to equipment for treating a sludge

waste stream created by an aerobic waste treatment plant.

Waste streams from factories such as food processing plants may have a high biological (or chemical) oxygen demand due to suspended or dissolved organic material.

Such waste streams can be treated using an aerobic waste treatment plant in which the waste stream is contacted by suitable microorganisms in the presence of oxygen, so that the organic material is broken down with the formation of carbon dioxide and water. This process is widely used, and is very effective at destroying the organic waste products. However the aerobic treatment process will also create a sludge of excess bacteria, and this must also be disposed of.

According to the present invention there is provided equipment for treating sludge created by an aerobic waste treatment plant, the equipment comprising means to cause the sludge to flow through a treatment duct, the treatment duct comprising a cylindrical pipe with a multiplicity of ultrasonic transducers attached to the outside of the pipe in an array of separate transducers that extends both circumferentially and longitudinally, each transducer being connected to a signal generator so that the transducer radiates no more than 3 W/cm2, and an anaerobic digester to which sludge that has passed through the treatment duct is supplied, the anaerobic digester having an outlet for biogas, and energy conversion means for obtaining energy from the biogas.

Preferably the pipe is of a diameter at least 0.l0m, and preferably the transducers are sufficiently close together and the number of transducers is sufficiently high that the power dissipation within the treatment duct is between 25 and 150 W/litre.

In relation to the treatment duct, the values of power given here are those of the electrical power delivered to the transducers, as this is relatively easy to determine. Such an ultrasonic irradiation pipe is described in Wa 00/35579. With such a pipe there is little or no cavitation at the surface of the wall, so that cavitation shielding does not prevent propagation of the ultrasound into the pipe contents, and there is no erosion of the pipe wall. Preferably the ultrasound is simultaneously supplied by all the transducers coupled to the wall of a pipe carrying the mixture, the sludge flowing at such a rate that it is insonated for less than 2 s, although the transducers might instead be subdivided into a plurality of groups (for example two or three groups) activated in succession.

The energy conversion means may comprise a boiler, in which the biogas is burnt to produce heat, typically in the form of hot air or hot water for circulation around a heating system. More preferably the energy conversion means incorporates an electricity generator driven by a motor for which the biogas is the fuel. For example the motor may be a gas turbine. This has the advantage that the electricity can be used for a wide range of different purposes, and may indeed be supplied to the local distribution network. Preferably the sludge that is subjected to ultrasonic irradiation is at an elevated temperature, most preferably between 55 C and 75 C, and heat from the energy conversion means is used to maintain the sludge at this elevated temperature.

Alternatively the energy conversion means may comprise chemical processing plant for converting the biogas into a liquid fuel. This may for example incorporate a compact catalytic reforming unit (such as a steam reforming unit) for producing a syngas of hydrogen and carbon monoxide; and a compact catalytic Fischer- Tropsch unit for generating hydrocarbons from this syngas.

The sludge treating equipment is preferably provided as a treatment module, in which the equipment is supported by a support frame, and the treatment module is preferably small enough that it can be readily transported, for example being no larger than a standard ISO container.

The present invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawing, which shows a flow diagram of equipment for treating sludge.

Referring to the drawing, a food processing factory generates an aqueous waste stream 10 with a high chemical oxygen demand; before discharge into a public waste water system this waste stream is passed through an aerobic treatment unit 12, so that the resulting aqueous waste stream 14 is sufficiently clean for discharge. This aerobic treatment unit 12 also generates sludge 16 consisting primarily of excess microorganisms. Such microorganisms are resistant to biodegradation, primarily because of the chemical constitution of the cell walls.

This sludge 16 is supplied through a pipe 20 to a sludge treating module 22 (outlined by a broken line), the dimensions of this module 22 being smaller than those of a 20 foot ISO container. The module 22 incorporates a pump 23 to pump the sludge to an insulated holding tank 24. A pump 26 circulates sludge from the tank 24 through an ultrasonic treatment unit 30 (shown in greater detail), and back to the tank 24. An outlet pipe 27 carries sludge from the tank 24 to an anaerobic digester 34 to generate biogas. This biogas is supplied as fuel to an engine 36 driving a generator 38. Coolant from the engine 36 is recirculated through a duct 28 that extends into the holding tank 24, so as to transfer heat to the sludge.

The pipe 20 may be of nominally six inch (150 mm) diameter, and the ultrasonic treatment unit 30 incorporates a stainless-steel duct 40 of the same internal diameter. The ultrasonic unit 30 also includes ten transducer modules 42 in a regular array attached to the outside of the duct 40. Each transducer module 42 comprises a 50 W piezoelectric transducer 44 which resonates at 20 kHz, attached to a conically flared aluminium coupling block 46 by which it is connected to the duct wall, the wider end of each block 46 being of diameter 63 mm. The transducer modules 42 are arranged in two circumferential rings each of five modules 42, the centres of the coupling blocks 46 being about 105 mm apart around the circumference, and about 114 mm apart in the longitudinal direction. The modules 42 and the duct are enclosed within a housing 48. A signal generator drives all the transducer modules 42.

In operation, the sludge in the holding tank 24 is held at a temperature of about 70 C, by virtue of heat transfer from the coolant in the duct 28. This hot sludge is recirculated through the ultrasonic treatment unit 30 on a continuous basis. With this ultrasonic unit 30 the power intensity is only about 1.6 W/cm2, and is such that cavitation does not occur at the surface of the wall, so erosion of the surface does not occur. Nevertheless the power density is sufficient to ensure cavitation in the sludge at this elevated temperature. The volume of sludge which is subjected to insonation is about 5 1, so the power density is about 100 W/litre. (The power density can be adjusted by adjusting the power supplied to the transducer modules 42, but is usually between 40 and 120 W/litre.) The power deposition in the sludge may be increased by providing more transducer modules 42 on the outside of the duct 40 - for example the duct 40 might instead have four closely spaced rings each of eight modules 42. This would increase the power density to about 350 W/litre.

Alternatively the sludge recirculation loop might comprise a number of these treatment units 30 arranged in series, or a longer duct with rings of transducers along a greater length. Using a duct of a smaller diameter, such as 100mm, would enable a greater power density to be achieved, and the power density might be as much as 500 W/litre.

The effect of this ultrasonic treatment is to break down the cell walls and membranes within the sludge, so that the compounds within the cell walls and the cell contents are released into the water. Intact cell walls are resistant to biodegradation; so this treatment considerably increases the proportion of the organic substances that can readily be biodegraded, increasing the biogas production from the biodigester 34, and reducing the quantity of residual solid matter that remains in suspension. The flow rate into the bioreactor 34 is such that the temperature there is suitable for mesphilic microorganisms, preferably between 25 and 40 , such as 35 C; the bioreactor 34 is not thermally insulated, so it can lose heat to the surroundings.

The flow rate through the ultrasonic treatment unit 30, and so through the duct 40, should be such that the sludge is insonated for a period between 1 s and 10 S1 for example about 3 s. A larger quantity of sludge can be treated (per unit time), by using a longer irradiation duct of the same diameter, with more circumferential rings of five modules 42 each, the rings being spaced apart by 114 mm centre to centre in the longitudinal direction, as described in relation to the drawing. For example, using a duct with twenty such circumferential rings of five modules 42, and so with an insonation volume about ten times that of the duct shown in the drawing, the same insonation time can be achieved with a ten times increase in flow rate.

The bioreactor 34 has a capacity of 25 m3, so that with a flow rate of about 17 litres/mm through the inlet pipe 20, and (at steady-state) the same flow rate into the bioreactor 34, the residence time of the sludge is about one day. The overall result is that the sludge 16 is primarily converted into water and biogas in the bioreactor 34, so that the remaining treated sludge emerging from the bioreactor 34 is considerably reduced in quantity. The sludge treating module 22 is of a convenient size for transport, and all the component parts are held together by a framework (not shown) so that the module can be installed adjacent to the aerobic waste treatment unit 12, only requiring electrical connections to provide power (for example for the pumps 23 and 26 and the signal generator 50), a connection to the mains for the output electricity, and pipes to take the outflowing water and treated sludge from the bioreactor. If the treatment capacity of one module 22 is not sufficient for the aerobic treatment unit 12, it will be appreciated that additional such modules 22 may be provided.

Claims

Claims 1. Equipment for treating sludge created by an aerobic waste

treatment plant, the equipment comprising means to cause the sludge to flow through a treatment duct, the treatment duct comprising a cylindrical pipe with a multiplicity of ultrasonic transducers attached to the outside of the pipe in an array of separate transducers that extends both circumferentially and longitudinally, each transducer being connected to a signal generator so that the transducer radiates no more than 3 W/cm2, and an anaerobic digester to which sludge that has passed through the treatment duct is supplied, the anaerobic digester having an outlet for biogas, and energy conversion means for obtaining energy from the biogas.
2. Equipment as claimed in claim 1 wherein the energy conversion means incorporates an electricity generator driven by an engine for which the biogas is the fuel.
3. Equipment as claimed in claim 1 or claim 2 wherein heat transfer means are provided to transfer heat from the energy conversion means to the sludge that is caused to flow through the treatment duct.
4. Equipment as claimed in claim 3 wherein the heat transfer means is arranged to maintain the sludge at a temperature between 55 and 75 C.
5. Equipment as claimed in any one of the preceding claims also incorporating a holding tank for the sludge, and means to circulate sludge from the holding tank through the treatment duct.
6. Equipment as claimed in any one of the preceding claims also incorporating a support frame to support all the components of the equipment as a treatment module, the equipment being of a convenient size for transport.
7. Equipment for treating sludge created by an aerobic waste treatment plant substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

15885 MdH P.T. Mansfield

Chartered Patent Agent Agent for the Applicants