GB2332196A - Process and apparatus for treatment of waste - Google Patents

Abstract

A process and equipment for mixing the organic fraction of municipal solid waste with a digesting sewage sludge prior to passing the mixture into an anaerobic digester 15 comprises a vessel 1 having a first inlet 2 for addition of waste, a second inlet 3 in fluid connection with a source of digesting sludge, an outlet 5 for combined waste and digesting sludge and a settlement zone 4. Preferably the outlet for combined waste and sludge is in fluid communication with an anaerobic digestor. In use, a portion of the digestate from the anaerobic digestor is mixed with further waste and then passed through the settlement zone at a velocity sufficient to allow particles to settle.

Description

2332196 1 PROCESS AND APPARATUS FOR TREATMENT.

OF WASTE The present invention relates to a process for the treatment of waste materials, and to apparatus for carrying out the process.

Waste such as household waste or MSW (municipal solid waste) is most usually disposed of to landfill. For example, at present 90% of the UK's household waste is disposed of in this way. The London and South East Regional Planning Conference (SERPLAN) has calculated that no more than 15 years' supply of potential landfill void capacity exists in the region, this reducing to less than 10 years' supply for household waste.

Landfill void space is not being replenished at the rate of consumption owing to the lack of geologically suitable sites which can satisfy modern stringent requirements for containment and control of leachate and gas production. These criteria, together with land use planning policies, make new planning permissions for landfill difficult to achieve.

Not only is there often an adverse public perception of landfill, but it is also a government primary target to reduce the proportion of controlled waste going to landfill to 60% by the year 2005. This reinforces the government's 1990 target to recycle or compost 25% of household waste by the year 2000.

The government's sustainable waste management strategy identifies a wide range of options for waste minimisation, re-use or recycling, but accepts that for up to 75% of future household waste, energy recovery technologies are likely to represent the best practicable environmental option (BPEO).

2 The typical UK household waste contains approximately 55% by weight of biodegradable material., comprising putrescible kitchen waste (e.g. vegetable material) and waste paper (food wrappings, some newsprint).

Substantial efforts have been made by local authorities and commercial companies to recover and recycle metals, plastics, glass, fabrics and uncontaminated paper. Whether by kerbside collections or by "bring" systems these can make significant contributions to recycling targets.

However, the greatest proportion of the waste (which is also the most difficult to handle) cannot be recovered in the same way as the above materials. The two options most usually considered for this organic fraction of the waste are incineration or composting.

Incineration is undoubtedly an efficient means of recovering the greatest amount of energy from waste, but the practicability of utilising the ash as a recycled material has yet to be proved. Incinerators arc major strategic infrastructure projects of large capital investment. They often face significant barriers of acceptance from the general public which can result in opposition and resultant difficulty in obtaining planning permissions.

Composting is also an effective method of treating biodegradable material, but it has a number of disadvantages. As it is carried out in the solid phase, it is difficult to separate unwanted contaminants which are especially likely to be present in household waste (batteries., broken glass, plastics). Unless composting is very carefully controlled it can be odorous, and enclosing the process an be very expensive. Furthermore, no energy is produced in the process and therefore a significant sale price for the treated material may need to be achieved in order to render the process economic. The product quality can sometimes make this 3 a difficult task. The government's target for 40% of domestic properties with a garden to carry out home composting by the year 2000 could potentially make a significant contribution but may not be achieved in practice.

An alternative to incineration and composting is anaerobic digestion. Anaerobic digestion (AD) of M5W has been used in the past and it deals with the same part of the waste as composting does, but has a number of advantages. It takes place in enclosed vessels, so odour is minimised. It produces a fuel gas which can be readily used to generate electrical energy. This contributes very significantly to the economics. It can utilise spare capacity in existing treatment plants and therefore minimise the need for new construction.

However, use of AD technology for large scale MSW treatment has only been demonstrated to be economically viable in a limited number of situations, due to problems, for example, relating to the complexity of the raw waste.

It is an object of the present invention to overcome problems such as this.

According to the invention there is provided a process for treating waste, comprising the step of mixing the waste with digesting sewage sludge.

The sewage sludge may originate from a sewage treatment works, and the process may include the step of passing the mixture to an anaerobic digester.

It is preferred that the process includes the step of continuously recycling a proportion of the digestate from the anaerobic digester for mixing with further waste.

The process may include the steps of mixing the waste and the digestate in a 4 vessel including a settlement zone, and passing the mixture through the settlement zone at a velocity sufficient to allow settlement out of particles by gravity.

The mixture may be passed through a substantially vertically disposed tube of the vessel which contains the settlement zone.

The waste and digestate may be mixed by mixing means before passing to the settlement zone, and the waste and digestate may be mixed by pump means, which may be one or more macerating pump.

The process may include the step of passing the mixture through a screen to remove floating particulate matter.

The process may also include the step of pulverising the waste prior to combining it with the sludge.

According to a second aspect of the invention there is provided apparatus for treating organic waste comprising a vessel having a first inlet for addition of waste, a second inlet in fluid communication with a source of digesting sludge, an outlet for combined waste and sludge, and a settlement zone.

It is preferred that the outlet for combined waste and sludge is in fluid communication with an anaerobic digester, and it is further preferred that the second inlet is in fluid communication with an anaerobic digester.

It is most preferred that the outlet and second inlet are in fluid communication with the same anaerobic digester.

The outlet and second inlet may be in fluid communication with an anaerobic digester of a sewage treatment works.

The vessel may include a settlement zone, which may be included in a substantially vertically disposed tube which feeds onto the outlet.

The vessel may include a frusto-conical base, and there may be a second outlet at a lower, in use, part of the frusto-conical base. The second outlet may include a grit cell. There may be gas inlet means adjacent a lower, in use, part of the vessel.

According to a third aspect of the invention, there is provided a sewage treatment works, including apparatus as hereinbefore defined.

The invention will further be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a diagram showing the typical composition of household waste; Fig. 2 is a schematic illustration of apparatus and a process according to the invention; and Fig. 3 is a graph showing trial results of a demonstration of the invention and a control reactor.

Referring to Fig. 2, the invention consists of a process and equipment to incorporate the organic fraction of municipal (domestic) solid waste into a digesting sewage sludge matrix in order to allow it to be anaerobically digested to yield a fuel gas and a stabilised product for recycling to agriculture or other 6 beneficial use.

As illustrated in Fig. 2, the apparatus comprises a vessel 1, known as a slurrying tank, having a first inlet 2 for addition of waste, a second inlet 3 in fluid connection with a source of digesting sludge 15, an outlet 5 for combined waste and digesting sludge and a settlement zone 4.

The first inlet 2 is cited in a low stack 2a at the top of the vessel 1, and is led by a conveyor 2b. The stack allows odours to be vented to an odour filter (not shown). The stack can be provided with an access door 2c.

The vessel 1 has a frusto-conical lower section 13, the sides of which feed into an outlet 6 for materials such as grit which are denser than the liquid in the vessel. The outlet 6 is provided with a grit cell 7 in which the denser materials accumulate under gravity. The grit cell 7 includes upper and lower valves 7a and 7b. Grit can be discharged to suitable removal apparatus such as a spiral classifier 7c.

The vessel 1 can be made from any suitable material such as for example a metal and the general dimensions are mainly dictated by the required rate of solids processing. In this embodiment the vessel is about 8m high and about 6m in diameter. The frusto-conical lower section is about 2m in height, and the grit cell 7 is about 1m in diameter and Im in height. The vessel is suitably situated with a ground clearance of about 2.2m from the bottom of the grit cell 7 to allow easy access thereto for grit removal.

A ring of gas-sparge nozzles 8 is provided in the lower section 13 through which compressed air or steam can be injected.

7 The vessel 1 is provided with an outlet 9 through which material is pumped to macerator or chopper pumps (not shown) which act like large food blenders, incorporating the waste into the sewage sludge, when it is passed back into the vessel via an inlet 10.

In the centre of the vessel, a stilling tube 11 is provided, which feeds, via a pipe 12 and a trough 12a to the main outlet 5.

The settlement zone 4 is contained within the stilling tube 11, the diameter of which, together with the sludge recirculation rate defines the upward flow velocity to allow settlement of grit etc. In the illustrated embodiment the diameter of the stilling tube 11 is about 1.3m and the pipe 12 and trough 12a are both about 0.5m in diameter.

A valve 12c which can be motorised is provided to control flow through pipe 12 to the main outlet 5. The outlet 5 is in fluid connection with a circuit which eventually feeds back into the vessel by way of a fine screen 14, and a sewage digester 15. The fine screen has a mesh size of about 6mm. The sewage digester 15 is of a known type suitable for anaerobic digestion, such as is commonly used in municipal sewage treatment works.

In use, waste to be treated can be sujected to a preliminary pulverisation into fines. This can be achieved using known equipment such as for example a Dano (TM) rotary drum (not shown). Fig. 1 shows the typical composition of household waste. The feed material may be for example, source-segregated organic household waste or mechanically pre- sorted whole household waste. In either case, non-biodegradable contaminants are expected, typically, broken glass, grit, metal fragments and plastics, and therefore the fines may contain not only food, plant and paper waste, but also grit, glass, metal and plastic 8 fragments.

The fines product is then added to the vessel 1 via inlet 2 where it is mixed with digesting sludge from the digester 15 which is fed to the vessel 1 via inlet 3 by for example a pump or by gravity flow. The vessel 1 may contain or, as illustrated, may be in fluid connection with, pump macerators (not shown) via outlet 9 and inlet 10, which act like food blenders to slurry the waste into the sludge.

The mixture, in which unwanted materials are now suspended, passes at low upward velocity through the settlement zone 4 in the stilling tube 11, where grit substances denser than the liquid such as metal and glass fall to the base of the vessel I under gravity and can be removed via the outlet 6.

The ring of gas-sparge nozzles 8 on the sloping floor of the vessel 1 can receive a pulsed flow of air to mobilise settled grit and move it towards the outlet 6. Alternatively, steam can be injected via the nozzles 8 to carry out the dual function of heat input and grit mobilisation. Periodically, the upper valve 7a which is normally open, can be closed, and the lower valve 7b can be opened to allow discharge of the grit into the spiral classifier 7c below for removal.

From the stilling tube 11, the mixture of waste and sludge passes via pipe 12 and trough 12a to the main outlet and then passes through a fine screen 14 in order to remove plastics and any other floating materials. These rejects can be washed and compacted before disposal. About 35% of the whole input waste remains by this stage.

Light materials., such as plastics, float on the surface of the liquid in the vessel 1 and periodically, the motorised valve 12c in pipe 12 can be closed to allow the 9 liquid level in the vessel 1 to rise until the liquid starts to [low over the edges of trough 12a in the manner of a weir. The level continues to rise until the head of flow over the weir is sufficient to discharge sludge at an equivalent rate to the inflow at port 3. In this way floating material is flushed from the vessel annulus into the discharge trough 12a leading to the screen 14. This discharge method may be operated for a short period of, for example one or two minutes per hour only in order to prevent the possibility of freshly introduced waste short circulting and overflowing the trough 12a.

The outlet of channel 5 is dimensioned so that there is insufficient restriction to fluid flow to allow the level in the tank to rise to the top. However, in the event of a partial or complete blockage of the outlet from the channel 5, a detector 16 is provided to detect unusually high liquid levels in the tank, and the flow of sludge to inlet port 3 can then be cut automatically by a suitable valve and control arrangement.

The slurried waste together with its carrier sludge is then fed to the anaerobic digester 15 by way of pipe 12 and outlet 5 through, for example. suitable pipework.

The anaerobic digestion process takes place in the digester 15 which is a fully enclosed reactor of known type. Most existing sludge digesters are between 2500 and 5000 cubic metres in capacity. Research has shown that the organic loading on these digesters can be doubled with no loss of treatment efficiency, in fact, the specific performance increases with the additional solids load. For example, laboratory trials demonstrated a 150% increase in gas production from a 100% increase in organic loading.

The digester 15 is fed with raw sewage sludge from a source such as a municipal sewage treatment works, as well as the sludge and waste mixture from the vessel 1. The feed to the digester may be for example on the basis of one period per day (e.g. one hour) or at short intervals, such as six minutes per hour for ten hours. The contents of' the digester are either continuously or discontinuously mixed by mechanical stirrer, by gas bubbles injected near the base of the digester or by recirculated pumped flow. A proportion of the digestate from the digester 15 is fed back by pumps or gravity feed into the vessel I for mixing with further organic waste added via inlet 2 and thus it will be appreciated that the process can be operated as a continuous treatment process, with recirculation of digestate being controlled to match the rate of fresh waste input into the vessel 1.

Because MSW organics have a much higher dry solids content than liquid sewage sludge, the increase in hydraulic load to the digester is only 12 15% for a doubled organic load.

As the waste is broken down by the treatment bacteria it produces a fuel gas containing about 60% methane. This is used in on-site generators to produce electrical energy. As a guide, an installation treating 50,000 tonnes per annurn of MSW organics would generate approximately an additional 2MW of energy.

The digestion process is continuous and the waste remains in the digester for an average of about 21 days. After this, the stabilised digested product is removed and stored for a further 14 days to allow cooling and further maturation.

Following this it is usually dewatered in mechanical presses to give a solid cake. After a further storage period on site (dependent on weather conditions and time of year) the biosolids product is delivered to farms for stockpiling or direct application to land. The material is an excellent soil conditioner and fertiliser 11 containing significant concentrations of nitrogen, phosphorous and potassium, essential nutrients for crop growth.

Bench scale research trials were carried out using a simulated MSW mixture consisting of kitchen waste, paper and green waste.

The test digesters were fed with a mixture of simulated MSW and sewage sludge, the control digesters were fed with sludge alone.

The results are shown in Table 1. Digester organic loadings could be doubled compared with sewage sludge alone and gas production was 250% of that from sludge alone. The loading rate was achievable because MSW organic fraction has a high solids content (145%) and does not place a major additional volumetric load on the digesters.

Table 1. Summarv of Bench Scale Results Sludge + MSW Sludge (Control) Digester Organic Loading (kg/m'/d) 3.4 1.6 Gas Production (M3,/M3 /d) 2.5 1.0 Methane (%) 58 A full scale plant trial was also initiated. The reactor was an on-line sludge digester of 3400 m' capacity,which also treated some 120m'/day of sewage sludge. The results are shown in Fig. 3.

12 From the above it will be apparent that the process and apparatus of the invention allow for ready separation of unwanted contaminant materials by dispersing and suspending the waste in the digesting sewage sludge matrix, then allowing gravity settlement of the heavier than water materials by passing the material through the stilling tube 11 at low upward velocity. Before returning the waste material to the digester 15, the (low passes through the screen 14, which removes lighter than water materials down to a 6mm profile. This effectively removes plastics.

The process uses a re-circulating flow of digesting sewage sludge rather than a combined raw sludge plus solid-waste one-way feed for the following reason. The co-digestion process allows for the solid loading of the digester to be doubled in comparison with sewage sludge alone. This produces more than double the usual amount of gas. Raw sewage sludge is typically fed to digesters at 511r dry solid content. If the solid waste were incorporated with the raw sludge prior to feeding the digester, the feed material would be at 10% dry solids. At this thickness, the material would be very difficult to pump and mix, and its high viscosity would make gravity separation of contaminant materials ineffective.

The process uses the fact that digestion reduces both the solid content of the sludge and its viscosity. The rate of flow of the digester contents through the slurry tank can be adjusted to allow only a small increase in dry solid content after the incorporation of the solid waste, thus allowing the gravity separation process to work effectively.

The high rate of re-circulating digesting sludge flow improves the overall mixing of the sludge digester and allows the input of additional heat energy, either by steam injection, or by using hot water heat exchangers in the returning screened 13 sludge flow. The source of hot water can be the cooling flow from CHP generators producing electricity from the additional fuel gas.

The high sludge [low rate and the thicker equilibrium sludge in the digester also maintains a greater degree of suspension of residual sewage grit in the sludge tlow. This is then removed in the settlement process within the slurry tank 1, preventing build-up and loss of digester capacity. The process therefore enhances the overall performance of typical municipal digesters by improving heating, mixing and grit removal.

The process and apparatus of the invention thus; a) use the concept of re-circulating flow as a carrier and separation medium for municipal organic waste; b) enable the use of gravity and screening processes for separation of unwanted contaminant materials; c) allow effective blending of organic material into digesting sewage sludge; d) allow cleansing of unwanted materials from the slurry tank by periodic overtlow-flushing and grit-dumping; c 1) allow simple input of beneficial heat energy by use of steam injection or in-line heat exchangers; f) improve the mixing of typical municipal anaerobic digesters, enhance their digestion performance and maintain effective capacity.

14

Claims (25)

1. A process for treating waste, comprising the step of mixing the waste with digesting sludge.

2 A process according to Claim 1, wherein the sludge originates from a sewage treatment works.

3. A process according to Claim 1 or Claim 2, including the step of passing the mixture to an anaerobic digester.

4. A process according to claim 3, including the step of continuously recycling a proportion of' the digestate from the anaerobic digester for mixing with further waste.

5. A process according to any preceding Claim, including the steps of mixing the waste and the digestate in a vessel including a settlement zone, and passing the mixture through the settlement zone at a velocity sufficient to allow settlement out of particles by gravity.

6. A process according to Claim 5, wherein the mixture is passed through a substantially vertically disposed tube of the vessel which contains the settlement zone.

7. A process according to any preceding claim, wherein the waste and digestate is mixed by mixing means before passing to the settlement zone.

8. A process according to Claim 7, wherein the waste and digestate is mixed by pump means.

9. A process according to Claim 8, wherein the waste and digestate is mixed with one or more macerating pump.

10. A process according to any preceding claim, including the step of passing the mixture through a screen to remove floating particulate matter.

11. A process according to any preceding claim, including the step of pulverising the waste prior to combining it with the sludge.

12. A process substantially as hereinbefore described, with reference to the accompanying drawings.

13. Apparatus for treating waste, comprising a vessel having a first inlet for addition of waste, a second inlet in fluid communication with a source of digesting sludge, an outlet for combined waste and sludge, and a settlement zone.

14. Apparatus according to Claim 13, wherein the outlet for combined waste and sludge is in fluid communication with an anaerobic digester.

15.

Apparatus according to Claim 14, wherein the second inlet is in fluid communication with an anaerobic digester.

16. Apparatus according to Claim 15, wherein the outlet and second inlet are in fluid communication with the same anaerobic digester.

17. Apparatus according to Claim 16, wherein the outlet and second inlet are 16 in fluid communication with an anaerobic digester of a sewage treatment works.

18. Apparatus according to any of Claims 13 to 17, wherein the vessel includes a settlement zone.

19. Apparatus according to Claim 18, wherein the vessel includes a substantially vertically disposed tube containing the settlement zone, which feeds onto the outlet.

20. Apparatus according to any of Claims 13 to 19.. wherein the vessel includes a frusto-conical base.

21. Apparatus according to Claim -20, including a second outlet at a lower, in use, part of' the frusto-conical base.

Apparatus according to Claim 21, wherein the second outlet includes a grit cell.

23. Apparatus according to any of Claims 13 to 22, including gas inlet means adjacent a lower, in use, part of the vessel.

24. Apparatus substantially as hereinbefore described, with reference to the accompanying drawings.

25. A sewage treatment works, including apparatus according to any of Clairns 13 to 24.