GB2311990A

GB2311990A - Controlling rate of aeration in sewage sludge treatment

Info

Publication number: GB2311990A
Application number: GB9607742A
Authority: GB
Inventors: David Anthony Stafford
Original assignee: Enviro Control Ltd
Current assignee: Enviro Control Ltd
Priority date: 1996-04-13
Filing date: 1996-04-13
Publication date: 1997-10-15
Anticipated expiration: 2016-04-13
Also published as: GB2311990B; GB9607742D0

Abstract

The treatment of sewage sludge comprises feeding the sludge into an aerobic reaction vessel 10 via a sludge inlet pipe 12, mixing the sludge in the reaction vessel 10 with air drawn or forced into the sludge through an air inlet pipe 17, measuring the organic content of the sludge fed into the reaction vessel, and varying the quantity of air drawn or forced into the sludge according the volume and organic content of the sludge fed into the reaction vessel. The sludge may be mixed with air by pumping the sludge through a venturi mixer 11 which draws air into the sludge and the quantity of air drawn into the sludge may be varied by varying the flow rate at which the sludge is pumped by recycle pump 18 through the venturi mixer. Preferably the temperature of the sludge in the reaction vessel is also monitored such that, if the temperature falls, the quantity of sludge fed into the reactor and/or air fed into the sludge is increased.

Description

A Method Of And Apparatus For Treating Sewage Sludge This invention relates to a method of and apparatus for treating sewage sludge.

The enforcement of the EC directive and US EPA guidelines on the disposal of sewage sludges and their use in agriculture has brought about changes in sludge disposal practice as well as a need to study the options available and their costeffectiveness. Treatment is required to reduce significantly the fermentability of the sludge as well as its health hazard due to the presence of pathogens.

According to one aspect of the invention there is provided a method of treating sewage sludge comprising the steps of: (a) feeding sludge to be treated into an aerobic reaction vessel, (b) mixing the sludge in the reaction vessel with air drawn or forced into the sludge, (c) measuring the organic content of the sludge fed into the reaction vessel, and (d) varying the quantity of air drawn or forced into the sludge according to the volume and organic content of the sludge fed into the reaction vessel.

Preferably, the sludge is mixed with air by pumping the sludge through a venturi mixer which draws air into the sludge and the quantity of air drawn into the sludge is varied by varying the flow rate at which sludge is pumped through the venturi mixer.

Preferably, the method further comprises the steps of monitoring the temperature of the sludge in the reaction vessel and increasing the quantity of sludge fed into the reaction vessel over a given time period and/or increasing the quantity of air drawn or forced into the sludge over a given time period if said temperature falls below a predetermined value.

According to another aspect of the invention there is provided apparatus for treating sewage sludge, comprising an aerobic reaction vessel, means for feeding sludge to be treated into the reaction vessel, means for drawing or forcing air into the sludge, means for mixing the sludge with the air, means for measuring the organic content of the sludge fed into the reaction vessel and means for varying the quantity of air drawn or forced into the sludge over a given time period according to the volume and organic content of the sludge fed into the reaction vessel.

According to yet another aspect of the invention, there is provided apparatus for treating sewage sludge, comprising an aerobic reaction vessel, means for feeding sludge to be treated into the reaction vessel, a venturi mixer for mixing sludge in the reaction vessel with air, means for pumping sludge in the reaction vessel through the venturi mixer, means for measuring the organic content of the sludge fed into the reaction vessel and means for varying the flow rate at which the sludge is pumped through the venturi mixer according to the volume and organic content of the sludge fed into the reaction vessel.

Preferably, the apparatus also includes means for monitoring the temperature of sludge in the reaction vessel and means for increasing the quantity of sludge fed into the reaction vessel over a given time period and/or increasing the flow rate at which the sludge is pumped through the venturi mixer if said temperature falls below a predetermined value.

It is thus possible to provide a single stage unit in which the quantity of air drawn into the sludge is matched to the volume and strength of the sludge fed into the reaction vessel. It is also possible to maintain the temperature of the sludge in the reaction vessel at an appropriate temperature for thermophylic aerobic digestion to take place. This temperature is usually maintained at between 550C and 700C.

Thermophylic aerobic digestion of sewage sludges has been shown to be a practical proposition as well as a robust system, and good pathogen kill and high organic removal from the contained solids will occur if the temperature can be controlled consistently.

For the effective treatment of sludge using thermophylic aerobic digestion, primary and secondary activated sludges (including extended aerobic sludges) need a minimum exposure of about 4 hours at about 550C and a mean retention period of at least 7 days aerobic digestion. This complies with Directive Article 6 (EU) which requires that sludge will be treated before being used in agriculture.

In an aerobic biochemical process the oxidation of organic material is associated with a release of heat energy. In aerobic digestion the sludge temperature will rise due to the microbial activity within the reactor, given sufficient aeration and presence of oxidisable organic carbon. At about 430C the thermophylic microbes take over and the sludge changes its characteristics. In a well balanced system the removal of between 60-80 % of the volatile solids is achieved and a thermal balance usually obtains at between 550C and 700C.

One of the major advantages of the system is that evaporation losses account for a significant reduction in volume of sludge to be discharged and this contributes to its cost-effectiveness.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawing which is a schematic view of one embodiment of apparatus for treating sewage sludge according to the invention.

Referring to the drawing, the apparatus shown therein comprises an insulated aerobic reaction vessel 10, a venturi mixer 11, a sludge inlet pipe 12, a feed pump 13, a liquid outlet 14 and an air outlet 15.

The venturi mixer 11 comprises a recycle tube 16 connecting the lower end of the vessel 10 to the upper end of the vessel 10, an air inlet pipe 17 connected to the recycle tube 16 intermediate the ends thereof and a recycle pump 18 for recycling sludge through the recycle tube 16 and past the inner end of the air inlet tube 17.

The sludge inlet pipe 12 is connected to the recycle tube 16 at the lower end thereof and upstream of the recycle pump 18.

Two temperature sensors 19, 20 are mounted in the reaction vessel 10. The temperature sensor 19 is mounted at a position above the sensor 20 so that the temperature of the sludge at two levels in the reaction vessel 10 can be monitored.

A syphon break tube 21 is connected to the liquid outlet 14.

In operation, sludge to be treated is fed into the aerobic reaction vessel 10 through the sludge inlet pipe 12 by the feed pump 13 and sludge is recycled in the aerobic reaction vessel 10 through the venturi mixer 11 by the recycle pump 18.

As sludge is pumped through the venturi mixer 11 by the pump 18, air is drawn into the sludge from the air inlet tube 17. The quantity of air drawn into the reaction vessel 10 in a given period of time can be varied by varying the flow rate at which the sludge is pumped through the venturi mixer 11.

Because the sludges to be treated may vary as to their organic content (as measured by their contained volatile solids or Biological Oxygen Demand), it becomes important to control the rate of oxidation by means of knowing the incoming organic loading of the incoming sludges.

As the organic loading (measured in terms of kg of Volatile Solids per M3 of reactor per day for example) increases, the rate of oxidation and mixing can be increased to provide sufficient aeration capacity for the microbes providing the metabolic heat. This process can be effected automatically by means of an automated BOD sensor and used to control the flow rate at which the sludge is pumped through the venturi mixer 11. Thus, the quantity of air drawn into the sludge can be matched to the volume and strength of sludge fed to the reaction vessel 10 from knowledge of the volume of sludge fed into the vessel 10 by the feed pump 13 and from the organic content of that sludge.

In order to ensure that thermophylic aerobic digestion takes place, the sludge in the aerobic reaction vessel 10 must be maintained above a predetermined temperature and is usually maintained at between 550C and 70 C. The temperature sensors 19 and 20 monitor the temperature of sludge in the reaction vessel 10 and if this temperature falls below a predetermined value, the quantity of sludge fed into the reaction vessel 10 over a given time period is increased to increase the oxidisable organic carbon in the reaction vessel 10 and/or the flow rate at which the sludge is pumped through the venturi mixer 11 is increased to increase the quantity of air in the reaction vessel 10. The quantity of sludge fed into the reaction vessel can be increased either by increasing the frequency at which the feed pump 13 is operated or by increasing the duration of feed.

In practice, the feed pump 13 will be operated intermittently, e.g. for about 10 minutes every three hours, and during operation of the feed pump 13, the recycle pump 18 can be switched off. This enables the treated sludge to be discharged from the vessel 10 before the new sludge is mixed by the venturi mixer 11 with the contents of the vessel 10. In this case, the incoming sludge will be fed into the lower end of the vessel 10.

Alternatively, the recycle pump 18 can remain on while the feed pump 13 is operating. In this case, some of the sludge will be fed into the lower end of the vessel 10 passing through the lower end of the recycle tube 16 against the flow of sludge being recycled through the recycle tube 16 and some of the sludge will be carried round with the recycled sludge and will be fed into the upper end of the reaction vessel 10.

The rate at which sludge is pumped through the venturi mixer 11 can also be increased by increasing the speed of the recycle pump 18 if the differential temperature sensed by the temperature sensors 19 and 20 exceeds a certain value as this will indicate insufficient mixing of the contents of the vessel 10.

The embodiment described above is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the present invention. For example, instead of drawing air into the sludge by pumping the sludge through a venturi mixer, the air could be forced into the sludge by a blower, which typically forces air into the lower end of the reaction vessel. In this case, some additional means for mixing the air and sludge will be required. This mixing could be achieved by recycling the sludge through a recycle tube, similar to the tube 16, but in this case no air inlet pipe will be connected to the recycle tube.

Claims

1. A method of treating sewage sludge comprising the steps of: (a) feeding sludge to be treated into an aerobic reaction vessel, (b) mixing the sludge in the reaction vessel with air drawn or forced into the sludge, (c) measuring the organic content of the sludge fed into the reaction vessel, and (d) varying the quantity of air drawn or forced into the sludge according to the volume and organic content of the sludge fed into the reaction vessel.

2. A method as claimed in claim 1, wherein the sludge is mixed with air by pumping the sludge through a venturi mixer which draws air into the sludge and wherein the quantity of air drawn into the sludge is varied by varying the flow rate at which the sludge is pumped through the venturi mixer.

3. A method as claimed in claim 1 or claim 2, wherein the method further comprises the steps of monitoring the temperature of the sludge in the reaction vessel and increasing the quantity of sludge fed into the reaction vessel over a given time period and/or increasing the quantity of air drawn or forced into the sludge over a given time period if said temperature falls below a predetermined value.

4. A method of treating sewage sludge, substantially as hereinbefore described with reference to the accompanying drawing.

5. Apparatus for treating sewage sludge, comprising an aerobic reaction vessel, means for feeding sludge to be treated into the reaction vessel, means for drawing or forcing air into the sludge, means for mixing the sludge with the air, means for measuring the organic content of the sludge fed into the reaction vessel and means for varying the quantity of air drawn or forced into the sludge over a given time period according to the volume and organic content of the sludge fed into the reaction vessel.

6. Apparatus for treating sewage sludge, comprising an aerobic reaction vessel, means for feeding sludge to be treated into the reaction vessel, a venturi mixer for mixing sludge in the reaction vessel with air, means for pumping sludge in the reaction vessel through the venturi mixer, means for measuring the organic content of the sludge fed into the reaction vessel and means for varying the flow rate at which the sludge is pumped through the venturi mixer according to the volume and organic content of the sludge fed into the reaction vessel.

7. Apparatus as claimed in claim 6, further including means for monitoring the temperature of sludge in the reaction vessel and means for increasing the quantity of sludge fed into the reaction vessel over a given time period and/or increasing the flow rate at which the sludge is pumped through the venturi mixer if said temperature falls below a predetermined value.

8. Apparatus for treating sewage sludge, substantially as hereinbefore described with reference to the accompanying drawing.