GB2431924A

GB2431924A - Sludge treatment

Info

Publication number: GB2431924A
Application number: GB0621587A
Authority: GB
Inventors: Manocher Asaadi; Philip Marsh
Original assignee: Thames Water Utilities Ltd
Current assignee: Thames Water Utilities Ltd
Priority date: 2005-10-28
Filing date: 2006-10-30
Publication date: 2007-05-09
Also published as: GB0522039D0; GB0621587D0

Abstract

The invention relates to a process and apparatus for treating liquid sewage sludge comprising adding dry lime to such sludge prior to dewatering, to provide an increased pH. Dry lime is added in the preferred embodiment either in the form of calcium oxide or calcium hydroxide, which is added directly to a contact zone or tank (2) defining two internal zones, namely a buffering zone (3) and a high intensity mixing zone (4) to which the dry lime is directly added, and from which sludge and lime can be recycled by a pump (13).

Description

I

Document: 1125479 M&C Folio: GBP93435

SLUDGE TREATMENT

The invention relates to sludge treatment, and particularly to the treatment of sewage sludge.

Sewage sludges are often treated when dewatered or partially dewatered by the addition of a slurry of lime quicklime in granular or powder form, to produce a more readily handleable product for application to land. The addition of slurry to sludge requires the initial production of the slurry, and a mechanical mixer to ensure thorough mixing of a sludge which has already gone through at least a partial dewatering process. This is time-consuming and expensive, and often does not produce a uniform "cake" if the initial mixing of the two slurries is not adequate.

It is an object of the invention to seek to mitigate these disadvantages.

According to one aspect of the invention there is provided a process of treating liquid sewage sludge, comprising adding dry lime to a sewage sludge prior to dewatenng thereof, whereby to provide an increased pH.

Thus, using the invention, dry lime, which could be in the form of either calcium oxide or calcium hydroxide, can be added directly to sludge to be treated before any dewatering stage, so enhancing the process.

The lime may comprise granular lime, e.g. as referred to above in the form of calcium oxide (quicklime) or hydrated lime in the form of calcium hydroxide.

The lime may suitably comprise a powder. This provides for ease of handling.

The sludge may comprise raw sewage sludge, or a digested sludge. Thus the process is generally applicable to the forms of sludge which fall to be treated.

The sludge may further be a settled or mechanically thickened sludge, whether it is a liquid raw sludge, biological waste activated or a digested sludge.

The process may be carried out in a mixing tank. This provides for a relatively simple yet efficient process, particularly when the mixing tank may comprise a plurality of zones into one of which the dry lime may be introduced into the sludge being treated.

Suitably there may be two zones, respectively comprising a buffering zone and a mixing zone, the latter of which may be adapted for high intensity mixing of lime and sludge.

This provides for an enhanced treatment process, particularly when the sludge may pass to the mixing zone directly, and from the buffering zone.

The direct passing of the sludge to the mixing zone may comprise pumping and dropping.

The lime may be added to the mixing zone by a rotary valve.

The treated sludge at least in part may be recycled to the buffering zone. This provides for a thorough treatment of the sludge.

The treated sludge may be recycled and passed to discharge by a mixing pump, preferably by ajet mixing pump.

The treated sludge may be subsequently subject to a dewatering operation.

According to a second aspect of the invention there is provided apparatus for carrying out a process as hereinbefore defined, comprising means to add dry lime to a sludge prior to dewatering.

The apparatus may comprise a tank, means to pass untreated liquid sludge thereto and means to add dry lime to the sludge.

The tank may comprise a plurality of treatment zones, preferably two zones which may comprise respectively a buffering zone and a mixing zone.

The mixing zone may be internally of the tank and defined by a weir and a boundary wall of the tank. This is a relatively simple yet efficient construction.

There may be rotary valve means for metering the lime accurately onto a screw auger for adding the dry lime. This provides a simple means of dosing the sewage sludge with lime.

The treated sludge pH, usually around 12, governs the amount of lime to be added. This is the means by which the rotary valve is adjusted/controlled.

There may be means to recycle at least part of the treated sludge to the tank. This can ensure a thorough treatment.

The means to recycle may comprise a mixing pump which may be adapted to recycle part of the treated sludge to the tank and to pass the remainder to or towards an outlet from the tank.

The mixing pump may comprise ajet mixing pump.

The apparatus may comprise a downstream dewatering station for the treated sludge.

The invention extends to sludge whenever treated respectively by a method or in apparatus as hereinbefore defined.

Apparatus for carrying out a method according to the invention is schematically illustrated in the accompanying drawing, which shows the addition of powdered lime to liquid sludge.

Referring to the drawing, there is shown apparatus I which has a contact zone or tank 2 defining two internal zones, namely a buffering zone 3 and a mixing zone 4 which in the embodiment acts as a high intensity mixing zone.

An internal overflow weir 5 defines the mixing zone 4, which is thus internally of the tank 2, though it could be separate therefrom. The tank 2 has a pH monitor and/or control means 6, an overflow 7, and a conical or pyramidal base 8 for collecting treated sludge from the buffering zone 3 and passing it via a discharge pump 9 to a downstream dewatering station (not shown) for dewatering the limed sludge.

Raw sludge, which can be either liquid raw or activated sewage sludge (settled or mechanically thickened) or digested sludge (thickened), is passed to the tank 2, in both zones, by a pump 11. A rotary valve attached to a lime silo and screw auger 12 feeds granular or powdered lime (quick lime), either calcium oxide or calcium hydroxide, directly to the high intensity mixing zone 4, before any dewatering of the sludge has taken place. The dry lime mixes with the sludge which is pumped and allowed to drop into the zone 4, and also sludge passes to the mixing zone 4 from the buffering zone 3 over the weir 5, as hereinafter described.

The addition of the dry lime and mixing it with the sludge partially dewaters the liquid sludge in that whilst no water is removed, the lime reacts chemically with the water in the sludge and increases the solids content and pH. Thus a treated product is produced, which passes from the zone 4 to a mixing pump 13 such as a jet mixing pump, which recycles the lime/sludge mixture to the buffering zone 3 and to the discharge pump 9.

Thus there is a thorough mixing of the lime and sludge, that which falls to the base 8 of the tank 2 being drawn off from the base S by the discharge pump 9.

There is an overflow 14 from the zone 4, which can pass lime and sludge back to the zone 4 and/or to the buffering zone 3, and/or pass it to waste.

All the sludge treated with lime is passed to the downstream dewatering station.

In a typical reaction in the process CaO + water (in sludge)-+Ca(OH)2 + heat. This reaction also results in a high pH. Hydrated lime could also be used to produce a high pH i.e. CA(OH)2 + water (in sludge) does not undergo a reaction as such, but produces a high pH as previously mentioned.

The apparatus and process herembefore described with reference to the drawings can be modified. Thus due to more stringent criteria for nitrogen levels in receiving land, sludge application rates have to be reduced i Nitrate Vulnerable Zones (NVZ). The process developed can be used to strip off ainonia in the contact zone or tank 2 of the apparatus. This is done by means of intensive air stripping of ammonia at elevated pH.

Under the elevated pH condition, sludge solids containing organic nitrogen are broken down and converted to soluble ammonia. Introduction of air through nozzles positioned in the bottom of the tank 2 remove free ammonia, thereby reducing the overall concentration of nitrogen in sludge. Ammonia laden air is then treated by an appropriate method of odour abatement. This process will enable more of sludge to be recycled to agricultural land.

There is thus provided a process using dry lime to treat raw sewage sludge or digested sludge to increase its pH prior to any dewatering step. A high pH as provided by the process is required to kill pathogenic microorganisms and make the product fit for recycling to land, such as agricultural land.

Claims

CLAIMS

1. A process of treating liquid sewage sludge, comprising adding dry lime to a sewage sludge prior to dewatering thereof, whereby to provide an increased pH.

2. A process according to Claim 1, the lime comprising granular lime.

3. A method according to Claim I or Claim 2, the lime comprising a powder.

4. A process according to any preceding claim, the sludge comprising raw sewage sludge and activated sludge.

5. A process according to any of Claims 1 to 3, the sludge comprising digested sludge.

6. A process according to Claim 4 or Claim 5, the sludge being a settled sludge.

7. A process according to any preceding claim, carried out in a mixing tank.

8. A process according to Claim 7, the mixing tank comprising a plurality of zones into one of which the dry lime is introduced into the sludge being treated.

9. A process according to Claim 8, comprising two zones, respectively comprising a buffering zone and a mixing zone.

10. A process according to Claim 9, the mixing zone being adapted for high intensity mixing of lime and sludge.

11. A process according to Claim 10, sludge passing to the mixing zone directly, and from the buffering zone.

12. A process according to Claim II, the direct passing of the sludge to the mixing zone comprising pumping and dropping.

13. A process according to any of Claims 10 to 12, the lime being added to the mixing zone by a rotary valve.

14. A process according to any of Claims 9 to 13, the treated sludge at least in part being recycled to the buffering zone.

15. A process according to Claim 14, the treated sludge being recycled and passed to discharge by a mixing pump.

16. A process according to Claim 15, the mixing pump comprising ajet mixing pump.

17. A process according to any preceding claim, the treated sludge being subsequently subject to a dewatering operation.

18. A process according to any preceding claim, comprising the step of stripping off ammonia in the mixing tank.

19. A process according to Claim 18, the said step comprising intensive stripping of ammonia by air at a high pH.

20. A process for treating liquid sewage sludge, substantially as hereinbefore described with reference to the accompanying drawing.

21. Apparatus for carrying out a process according to any one of Claims 1 to 20, comprising means to add dry lime to a sludge prior to dewatering.

22. Apparatus according to Claim 21, comprising a tank, means to pass untreated liquid sludge thereto and means to add dry lime to the sludge.

23. Apparatus according to Claim 22, the tank comprising a plurality of treatment zones.

24. Apparatus according to Claim 23, there being two zones comprising respectively a buffering zone and a mixing zone.

25. Apparatus according to Claim 24, the mixing zone being internally of the tank and defined by a weir and a boundary wall of the tank.

26. Apparatus according to any of Claims 23 to 25, comprising rotary valve means for adding the dry lime.

27. Apparatus according to Claim 26, comprising means to recycle at least part of the treated sludge to the tank.

28. Apparatus according to Claim 27, the means to recycle comprising a mixing pump which is adapted to recycle part of the treated sludge to the tank and to pass the remainder to or towards an outlet from the tank.

29. Apparatus according to Claim 28, the mixing pump comprising ajet mixing pump.

30. Apparatus according to any of Claims 21 to 29, comprising a downstream dewatering station for the treated sludge.

31. Apparatus according to Claim 21, there being a mixing zone and a buffering zone, the buffering zone being internally of a tank and the mixing zone being separate from the tank.

32. Apparatus according to any preceding claim, including means to pass high intensity air into the apparatus for stripping of anunonia from the sludge.

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

34. Sludge whenever treated by a method according to any of Claims I to 20, or apparatus according to any of Claims 21 to 33.