GB2454507A

GB2454507A - Biological waste treatment

Info

Publication number: GB2454507A
Application number: GB0722034A
Authority: GB
Inventors: Jake Christopher Spooner
Original assignee: ECO SOLIDS INTERNAT Ltd
Current assignee: ECO SOLIDS INTERNAT Ltd
Priority date: 2007-11-09
Filing date: 2007-11-09
Publication date: 2009-05-13
Also published as: EP2227443A2; CN102123958A; GB0722034D0; WO2009060201A3; WO2009060201A2

Abstract

An apparatus for treating biological waste sludge comprises a primary sludge flow path 21, a mixer, a primary pump 24 for driving the sludge along the flow path, a primary reaction chamber 22 in the flow path downstream of the mixer, a recycle flow path 29 for diverting a proportion of the sludge from the flow path back to the mixer, and a secondary pump 28 downstream of the divergence for controlling the proportion of treated sludge to be recycled and discharged. A method for treating a sludge of biological solids comprises mixing the sludge with an oxide-containing chemical, feeding the sludge under pressure through a primary reaction chamber, recycling a portion of the treated sludge back to the mixing step and discharging a portion of the treated sludge, wherein the ratio of sludge being recycled to sludge and oxide-containing chemical being introduced to the mixer is greater than 1:1.

Description

1 2454507

APPARATUS AND METHOD FOR TREATING WASTE

The present invention relates to an apparatus and method for treating waste. More particularly, the present application relates to an apparatus and method for the treatment of biological waste sludges by mixing oxide containing chemicals with the sludge for the purpose of sterilising and/or decontaminating the sludge and, further, recycling a proportion of the treated sludge in order to increase the efficiency and effectiveness of the process.

Legislation in the EU, the USA and elsewhere sets standards for the constituents of treated sewage sludge with respect to the amount of pathogens (e.g. faecal coliforms) and volatile solids. If these standards are not achieved, then the routes by which sewage sludge may be disposed may be complicated and costly.

In the USA, these standards are set out in sections 32 and 33 of US Environmental Protection Agency (EPA) 40 CFR Part 503. In Europe, the nature of such products is governed by regulation EU 86/278. Treated sludge meeting these regulations is referred to as a Class A / Enhanced Quality (EQ) product.

In order to simplify the disposal of sludges (e.g. by spreading on agricultural land), it is desirable to achieve these standards by the use of a suitable treatment process. It is also desirable that this process is efficient and cheap as possible in terms of capital expenditure required and ongoing operating expenditure.

From US Patent 5868942, it is known to mix biological sludges with calcium oxide (CaO, quicklime), ammonia and carbon dioxide in order to elevate the temperature of the sludge (due to a chemical reaction between the sludge and the calcium oxide) and to pressurise *...

the sludge. After a suitable residence time in a reaction chamber in these conditions, a * sludge meeting the aforementioned standards can be obtained.

****** * * * ****** * Subsequent to the filing of the above patent, it has been recognised that the above process ** can be optimised by recycling a proportion of the treated sludge. This has been achieved : 35 by diverting a proportion of the outflow from the reaction chamber by using a valve or fixed orifice plate to restrict the discharge pipe. However, the present applicant has recognised that use of an orifice plate still fails to provide an optimal sludge treatment process, since the recycle rate of 0.3-0.8:1 of treated to untreated sludge is the maximum that can realistically be achieved without the risk that blockages may occur.

The present invention seeks to overcome, or at least mitigate the problems of the prior art.

Accordingly, one aspect of the present invention provides apparatus for treating a sludge of biological solids, the apparatus comprising: a primary sludge flow path; a mixer for mixing the sludge with an oxide containing chemical; a primary pump for driving the sludge along the primary sludge flow path; a primary reaction chamber in the primary sludge flow path downstream of the mixer for reacting the mixture arid enabling the temperature thereof to be elevated; a recycle flow path divergent from the primary flow path downstream of the primary reaction chamber for diverting a proportion of the sludge from the primary flow path back to the mixer; and a secondary pump downstream of the divergence for controlling the proportion of treated sludge to be recycled and the proportion of treated sludge to be discharged.

A second aspect of the present invention provides a method for treating a sludge of biological solids comprising the steps of: a) mixing the sludge with an oxide containing chemical; S. :.::: b) feeding the sludge under pressure through a primary reaction chamber such *** that a chemical reaction occurs to elevate the temperature of the sludge; S.....

* c) recycling a portion of the treated sludge back to the mixing step; and **... * .

d) discharging a portion of the treated sludge, *...** * S wherein the ratio of sludge being recycled to sludge and oxide containing chemical being introduced at step a) is greater than 1:1.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings in which: FIGURE 1 is a schematic diagram of treatment apparatus according to an embodiment of the present invention; FIGURE 2 is a graph showing the relationship between recycle rate and residual quicklime content in the apparatus; FIGURE 3 is a graph illustrating the effect of recycle rate on temperature; and FIGURE 4 is a graph illustrating the effect of recycle rate on temperature when a specific residence time at a specific temperature is required.

With reference to Figure 1, apparatus 10 according to the present invention comprises a an infeed hopper 12, into which sludge 14 is introduced by a conveyor 16. The sludge 14 is preferably de-watered to a certain extent, it may have a water content of between 2% and 99%, more typically 50% to 93% by weight. De-watenng may be achieved by use of a centrifuge (not shown) or any other suitable known method. The sludge may be wastewater sludge that has been output from a wastewater treatment plant, e.g. surplus activated sludge (SAS) (sometimes referred to as waste activated sludge in the USA), municipal (primary) sludge, agricultural wastes, biological industrial wastes or the like.

Calcium oxide 18 (CaO, quicklime) is also introduced to the hopper 12 from a silo 20.

The proportion of quicklime 18 to sludge 14 is determined by the amount of quicklime required to achieve a raise in temperature of the sludge to a predetermined temperature of 50°C to 120°C, preferably 68°C to 70°C for obtaining a class A sludge. If a class B standard sludge is required (i.e. one with a lower level of pathogen kill), a temperature in S...

excess of 25°C may be sufficient however. The quicklime 18 typically contains approximately I l4OkJ/kg of energy and the specific heat capacity of solids and liquids in the sludge is approximately 1.26J/g/K and 4.19J/g/K respectively. Therefore it is possible to estimate the amount of quicklime 18 required to achieve a desired temperature (also factoring in heat losses through the walls of the reaction chamber). Thus, for a typical de-watered sludge, it can be approximated that 22kg of quicklime 18 is required to raise the temperature of 100kg of sludge 14 by 10°C.

The reaction of the quicklime 18 with the remaining water in the dc-watered sludge 14 is a typical first order chemical reaction, which can be represented as: Ca = Caoe where Ca is the unreacted quicklime content, CaO is the original quicklime dose, k is a reaction co-efficient, and t is time.

This equation can be used to approximate the unreacted quicklime 18 content of the sludge with time after the quicklime and sludge are mixed together. The actual reaction rate will vary slightly with the type of sludge, the type of quicklime 18 used and the mixing process. However, it provides a useful illustration.

To ensure effective mixing of the sludge 14 and quicklime 18, and to further ensure a correct reaction pressure and residence time, a primary pump 24 is used to simultaneously feed the quicklime 18 and sludge 14 along an enclosed primary flow path 21 into the primary reaction chamber 22 and to cause the sludge and quicklime to mix, thus causing the two materials to react.

The pump 24 is preferably a positive displacement pump, more preferably a progressive motive mixer such as a screw-type pump, progressive cavity pump or reciprocating pump which mixes the materials when contained therein, but provides a plug flow condition within the primary reaction chamber 22 downstream of the primary pump.

The reaction causes the temperature of the sludge to raise to between 50°C and 80°C (for class A treated sludge to be obtained) and the pH of the material to rise to between 11 and 12. Inside the primary reaction chamber 22, the sludge is pressurised to in excess of lOOkPa, typically between 200 to 500kPa. Since the reactor is pressurised, substantially *...

25 all the heat generated by the chemical reactions is used to elevate the temperature of the sludge mixture (no heat is lost to vaporisation). This increases the thermal efficiency of * the process and results in a lower requirement for quicklime 18 than in other processes that ***.** * operate at ambient pressure. ** S.

* At a pH of 10.5, and at the elevated temperature mentioned above, ammonia previously S.....

* 30 held within the sludge 14 is released from the sludge, further raising the pH to approximately 13.3. This high pH, coupled with the elevated temperature, serves to further enhance the pathogen destruction.

The primary reaction chamber 22 is pipe designed to have appropriate dimensions that enable a minimum residence time therein to be achieved, so that the required removal of pathogens is achieved. The primary reaction chamber 22 is also heavily insulated in order to minimise heat loss and to further maximise the efficiency of the reaction.

Downstream of the primary reaction chamber 22, the flow of treated sludge is split into two. A proportion of the treated sludge 25 is discharged along the primary flow path 21 into a collection hopper 26, or a load area of a vehicle (not shown) for onward disposal.

The remainder is diverted along an enclosed recycle flow path 29.

The amount of treated sludge 25 discharged is regulated by a secondary pump 28 and any material not being discharged is recycled via the recycle flow path 29 and deposited back into the infeed hopper 12.

The secondary pump 28 is also preferably a positive displacement pump such as a screw-type pump, progressive cavity pump or reciprocating pump.

It will be appreciated that the throughput of secondary pump 28 compared to the primary pump 24 dictates the throughput of sludge and quicklime 18 through the primary reaction chamber 22, as well as the proportion of treated sludge 25 deposited into the discharge hopper 26 compared to the proportion recycled back into the infeed hopper 12. The rate of infeed of untreated sludge 14 and quicklime 18 compared to the rate of infeed of treated sludge 25 into the infeed hopper 12 dictates the recycle rate of the process.

In preferred embodiments a suitable control system is provided for the apparatus to ensure the correct rate of operation of the primary and secondary pumps 24, 28, the rate of irifeed ne.. of quicklime from the silo 20, and the rate of infeed of untreated sludge that will result in *

25 treated sludge meeting the desired regulation, whilst ensuring economic operation of the * process. Temperature sensors (not shown) and/or pump speed data may be used for this S..... * .</p>..DTD: <p>* purpose.

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As discussed above, known processes do not employ any recycling, or recycling is limited to a ratio of less than around 0.8:1 due to the risk of blocking caused by diverting a proportion of the discharge flow merely by using a prior art orifice plate or valve. The applicant has recognised that significant improvements in efficiency due to increasing the recycle rate to a ratio of 1:1 or more (that is 1 part recycled treated sludge to 1 part untreated sludge and quicklime being introduced into the primary reaction chamber 22) can be achieved by the use of a secondary pump 28 to control recycling in conjunction with the primary pump 24. Advantageously, use of a pump overcomes the risk of blocking. Furthermore, it enables the recycle ratio to be adjusted "on the fly".

It has been found in trial work that a recycle rate of in excess of 1:1, preferably 1.5 to 4:1 more preferably 2 to 3:1 represents the best trade off between improving the efficiency of the process (and thereby minimising the amount of quicklime required) versus increasing the operating cost (primarily due to an increased requirement for energy caused by the higher amount of material being pumped). Residence times in the range of 30 to 90 minutes have similarly been found to be optimal in most circumstances.

Examples illustrating the increases in efficiency using particular operating parameters can be seen in Figures 2 and 3. Figure 2 shows that over a fixed residence time in the primary reactor of 60 minutes, increasing the amount of recycling reduces the amount of unreacted quicklime 18 at the output end of the reaction chamber, thus making more efficient use of the quicklime that is introduced, and enabling a reduced amount of quicklime to achieve the required reaction temperature and pH.

Figure 3 shows the effect of increasing the recycle rate on the temperature of the sludge at different locations along the length of the primary reaction chamber 22. Since the recycled sludge has a higher temperature than the untreated sludge 16 at the input end of the reaction chamber, higher temperatures are achieved along the length of the reaction chamber and therefore the reaction will occur more efficiently along a greater proportion of the primary reaction chamber. Furthermore, this increase in temperature is particularly :..:: beneficial when regulations stipulate that a minimum sludge temperature needs to be maintained for a minimum period of time. At the time of writing, this is the case with relevant US EPA regulations.

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* When the treated sludge 25 is discharged from the primary sludge path 21, the pressure is :. 30 relieved and inter a/ia, steam, ammonia, amines and volatile solids are vaporised.

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The apparatus 10 further comprises a vapour collection line 32 from the discharge hopper 26, and a second vapour collection line 34 from infeed hopper 12. The vapour is collected into a barometric water condenser 36, which also has a water infeed 38. The vapours dissolve into the water which is output via liquid discharge line 40 from which it may be collected for use as a fertiliser, or returned to the head of the wastewater treatment works.

The barometric water condenser also has a treated air discharge 42.

The loss of these vapours from the treated sludge 25 causes a pH drop to approximately 11 to 12 and the percentage of dry solids to increase. The pH can be further reduced by blending it with suiphates, or any other suitable material (including organic material) having a pH of less than 5. The loss of ammonia from the treated sludge reduces its nitrogen content, and therefore permits a higher application rate of the treated sludge to land.

In some embodiments it may be desirable to improve the "handleability" and "stackability" of the treated sludge. In these embodiments a relatively small amount of pozzolanic type material is also be added to the sludge prior to treatment to improve its final texture. A suitable pozzolanic material is, for example, fly ash.

The treated sludge is therefore highly stable, low in odour and low in pathogens and therefore does not require silo storage. This results in additional savings for the overall cost of the apparatus being achievable.

The present invention will now be illustrated with some specific examples.

Example 1 Treating sludge at 70°C for 30 minutes Holding sludge at 70°C for 30 minutes is a standard specification to meet US EPA regulations to provide a Class AJEQ biosolid (US EPA 40 CFR Part 503 1994). Sludge (15% dry solids) and quicklime 18 rates for different recycle rates are shown in Table 1,

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.i.. 25 and the temperature along the reactor is shown in Figure 4. These figures clearly show the reduction in quicklime required to achieve this requirement. The amount of quicklime required is halved with recycling. Also, the temperature profile is more consistent along S.....

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Table 1: Comparison of lime dose reduction as a function of recycle rate for treating sludge at 70 Cfor 30 minutes

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Recycle rate 0 1:1 2:1 3:1 Sludge (kg/br) 2268 2268 2268 2268 Quicklime (kglhr) 794 445 376 363 Quicklime dose (%) 35.0% 19.6% 16.6% 16.0% Quicklime reduction from no recycling -44.0% 52.6% 54.3% Note: Sludge assumed to be 15% dry solids Example 2 Treating sludge at 55°C for 30 minutes Studies in the USA have shown that high levels of pathogen kill can be achieved by treating sludge at 55°C for 30 minutes. This relies on the ammonia gas that is released in the process acting as a disinfectant.

An increase in temperature in conjunction with the presence of ammonia serves to increase the vapour pressure of the ammonia gas. The vapour pressure of ammonia at 50°C is approximately five times that at ambient temperatures, which increases the diffusion of the ammonia into the sludge, therefore allowing greater contact with pathogens. The increased pressure and presence of ammonia serves to improve the effectiveness of the present process compared to open systems.

Therefore, this temperature and duration may provide a sufficient pathogen reduction to meet relevant compliance requirements, i.e. such as the "Safe Sludge Matrix" in the UK, which governs the application of sewage sludge onto agricultural land. Sludge and quicklime rates are shown in Table 2 for different recycle rates. These figures clearly : show the reduction in quicklime required to achieve this requirement. The amount of quicklime required is reduced by nearly 40% with recycling. As before the temperature profile is much more consistent along the length of the reactor. This offers significant e further savings in operating costs where appropriate over the full treatment as shown in

Example 1.

Table 2: Comparison of quicklime dose reduction as a function of recycle rate for treating sludge at 55°C for 30 minutes Recycle rate 0 1:1 2:1 3:1 Sludge (kglhr) 2268 2268 2268 2268 Quicklime (kg/hr) 222 154 141 136 Quicklime dose (%) 9.8% 6.8% 6.2% 6.0% Quicklime reduction from no recycling -30.6% 36.7% 38.8% Note: Sludge assumed to be 25% dry solids It will be appreciated that numerous changes may be made within the scope of the present invention. For example, potassium oxide, potassium hydroxide, sodium hydroxide, aluminium silicate, iron oxide or calcium hydroxide may be used singly, or in combination, instead of quicklime. The recycled treated sludge may be reintroduced directly into the primary reactor rather than via the infeed hopper 12. The secondary pump 28 may be provided in the recycle path rather than in the primary path of the sludge.

A secondary recycle reaction chamber may optionally be provided in the recycle flow path 29. * *** S.. * .

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Claims

1. Apparatus for treating a sludge of biological solids, the apparatus comprising: a primary sludge flow path; a mixer for mixing the sludge with an oxide containing chemical; a primary pump for driving the sludge along the primary sludge flow path; a primary reaction chamber in the primary sludge flow path downstream of the mixer for reacting the mixture and enabling the temperature thereof to be elevated; a recycle flow path divergent from the primary flow path downstream of the primary reaction chamber for diverting a proportion of the sludge from the primary flow path back to the mixer; and a secondary pump downstream of the divergence for controlling the proportion of treated sludge to be recycled and the proportion of treated sludge to be discharged.

2. Apparatus according to claim 1, wherein the secondary pump is located in the primary flow path.

3. Apparatus according to claim 1, wherein the secondary pump is located in the recycle flow path.

4. Apparatus according to any preceding claim, wherein the primary pump also functions as the mixer.

5. Apparatus according to any preceding claim, wherein the primary pump is a positive displacement pump, preferably a screw-type pump, progressive cavity pump or reciprocating pump. *... * .

6. Apparatus according to any preceding claim, wherein the secondary pump is a positive displacement pump, preferably a screw-type pump, progressive cavity pump or reciprocating pump. ** S. S. * * S

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7. Apparatus according to any preceding claim, wherein the primary reaction chamber is configured to cause a plug flow of sludge therethrough.

8. Apparatus according to any preceding claim, wherein the primary and secondary pump and geometry of the primary sludge path are configured to generate a sludge pressure of greater than 1 OOkPa, preferably 200-500 kPa in the primary reaction chamber.

9. Apparatus according to any preceding claim, wherein the recycle path comprises a recycle reaction chamber.

10. A method for treating a sludge of biological solids comprising the steps of: a) mixing the sludge with an oxide containing chemical; b) feeding the sludge under pressure through a primary reaction chamber such that a chemical reaction occurs to elevate the temperature of the sludge; c) recycling a portion of the treated sludge back to the mixing step; and d) discharging a portion of the treated sludge, wherein the ratio of sludge being recycled to sludge and oxide containing chemical being introduced at step a) is greater than 1:1.

11. A method according to claim 10, wherein the ratio is in the range of 1.5:1 to 4:1.

12. A method according to claim 11, wherein the ratio is in the range of 2:1 to 3:1.

13. A method according to any one of claims 10 to 12, wherein a pump downstream of the primary reaction chamber controls the ratio of sludge being recycled to sludge and oxide containing chemical being introduced.

14. A method according to claim 13, wherein a further pump upstream of the primary *.SS. . reaction chamber acts to control the ratio of sludge being recycled to sludge and oxide containing chemical being introduced in conjunction with the downstream pump. S... * .

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15. A method according to any one of claims 10 to 14, wherein the pressure at step b) is greater than lOOkPa, preferably in the range of 200-500kPa. ***.* * S

16. A method according to any one of claims 10 to 15, wherein the temperature of the sludge at step b) reaches 25°C -120°C, preferably 50°C -80°C.

17. A method according to any one of claims 10 to 16, wherein the residence time in the primary reaction chamber at step b) is in the range of 30 minutes to 90 minutes. S. * S * S.. S... * . S...

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