GB2376228A

GB2376228A - Sludge processor

Info

Publication number: GB2376228A
Application number: GB0113514A
Authority: GB
Inventors: Piers Benedict Clark
Original assignee: WS Atkins Consultants Ltd
Current assignee: WS Atkins Consultants Ltd
Priority date: 2001-06-04
Filing date: 2001-06-04
Publication date: 2002-12-11
Also published as: GB0113514D0

Abstract

The present invention concerns a sludge treatment tank 1 comprising first and second compartments 2 and 3; and means 4 for fluid flow regulation interconnecting the first and second compartments, said first compartment comprising an inlet 8 and mixing means 7 and said second compartment 3 comprising an outlet 9. Means 4 may be in the form of a baffle or wall. There may be agitation means 7 and heat exchanger included.

Description

Sludge Processor The present invention relates to the treatment of sludge.

In the UK, there are approximately 6,500 sewage treatment plants, 6,000 of which could be described as small and/or rural plants (i. e. serving less than 1000 people each).

Accordingly, it is estimated that less than 10% of the sewage produced by the UK's population is being treated in 80% of the UK's sewage treatment works.

Approximately half of the sewage produced in the UK is recycled to agriculture. This figure is expected to rise due to the implementation of the Urban Waste Water Treatment Directive which stipulates increased levels of secondary treatment (and thus increases the total sludge production).

Depending on the type of area to which the sludge is to be recycled (i. e landfill or agricultural land) the levels of Escherichia coli (E. coli) per g dry weight must be either 100,000 (conventional treated) or 1000 (advanced treated), respectively. In this regard, many different types of both aerobic and anaerobic treatment systems are used to digest and detoxify sludge and reduce the E. coli levels therein.

One anaerobic method is known as a two-phase digestion system.

This system consists of a highly loaded acid phase digester followed by a more lightly loaded"gas phase"digester.

One such anaerobic method is the"Anaerobic Stabilisation Thermophilic/Mesophilic"system which consists of a small thermophilic digester followed by a large mesophilic digester. The first stage provides an ideal environment for thermophilic

organisms and the second the ideal environment for mesophilic organisms. A typical design has a 2 to 5 day retention time in the first stage followed by a 10 to 15 day retention time

in the second stagc feed. A disadvantage of 2 phase systems (such as the Anaerobic Stabilisation Thermophilic/Mesophilic system) is that they require relatively long retention times in the initial reactor. This is a disadvantage since most 2 phase systems are retrofits in which a reactor tank is installed upfront of an existing larger sludge digestion reactor. If the additional reactor is relatively large then both the costs and the practicalities of fitting it onto one site can be prohibitive.

The main reason for installing a 2 phase system is to ensure that pathogen kill in the sludge is performed to a sufficient level (e. g conventional treatment 100, 000 E. coli per g dry weight). Traditionally, this is achieved by ensuring a minimum sludge holding time occurs inside the initial reactor (typically of between 1 to 4 hours). To achieve the minimum holding time the initial reactor is fed in batches with the period between batches equivalent to the minimum holding time.

On each feeding event some of the sludge inside the initial reactor is displaced and passes into the downstream larger digestion reactor. However, to ensure that the acidogenic bacterial microflora in the initial reactor are not displaced it is vital that the volume of each feeding event is only a small fraction of the total reactor volume (thus there will always be a large residual population of bacteria to re-seed the fresh incoming sludge). This results in relatively long retention times in the initial reactor of between 2-5 days, even though the minimum holding time is often between 1 to 5

hours.

An object of the present invention is to provide a sludge treatment anaerobic digestion method which seeks to alleviate the above problems.

According to a first aspect of the invention there is provided a sludge treatment tank comprising first and second compartments; and means for fluid flow regulation interconnecting the first and second compartments, said first compartment comprising an inlet and mixing means and said second compartment comprising an outlet.

The tank of the present invention allows the initial anaerobic degradation phase of sludges to occur in a small, continuously fed tank. In this connection, either new purpose built tanks can be produced or tanks already used in anaerobic digestion can be modified. As there are two separate compartments in the tank through which the sludge must pass the likelihood of contamination of the treated sludge with raw or partially treated sludge is reduced.

The sludge continuously flows through the tank thereby reducing the overall retention time.

In preferred embodiments the means for fluid flow regulation is a baffle.

The baffle controls the flow of the sludge from the first compartment to the second compartment such that significant partial digestion of the sludge and pathogen kill occurs before transfer into the second compartment.

Preferably the first and/or second compartments comprise means for adjusting their internal environment. Conveniently such a means for adjusting the internal environment is a heat

excli-ianger. Mlc exchanger. The heat exchanger ensures that the tank or compartments therein are kept at the optimum temperature for digestion of the sludge and/or pathogen kill.

Preferably the inlet is positioned in the first compartment at the base of the tank. Accordingly, the sludge has to travel from the base of the tank upwards before it flows into the second compartment.

I Conveniently the outlet is positioned in the second compartment at the base of the tank. Accordingly, the partially treated sludge from the first compartment has to travel down through the second compartment before flowing through the outlet and out of the tank.

In preferred embodiments the relative volumes of the first and second compartments are adjusted to meet desired flow requirements and holding times.

Preferably the volume of the first compartment is larger than the volume of the second compartment. Accordingly, this allows any microbiological digestion and pathogen kill which is occurring in the first compartment to take, if necessary, longer than the digestion occurring in the second compartment. The relative sizes of the compartments can thus be altered depending upon the required holding time at each stage of the process.

In preferred embodiments the first and second compartments are formed by a pair of concentric tubular members, with the inner

member being shorter than the outer member. The volumes of the compartments thus formed can be varied by increasing or decreasing the height of the inner member.

According to a further aspect of the invention there is provided a method for the treatment of sludge using a tank as defined above, the method comprising: a) introducing into the first compartment, which has the preferred growing conditions for a first microbiological flora, sludge through the inlet opening;

I b) mixing said sludge with the mixing means for a first holding time; c) allowing sludge to flow into the second compartment, which has the preferred growing conditions for a second microbiological flora, at a first rate; d) allowing sludge to flow from the second compartment through the outlet at a second rate such that it is held in the second compartment for a second holding time.

In the first compartment hydrolysis and acetogenesis will occur such that volatile fatty acids will be produced. When the sludge flows into the second compartment the volatile fatty acids and the temperature of the sludge result in the E. coli and other pathogens in the sludge being killed (known as pathogen kill). Accordingly, it is important to keep the environment in the first compartment at optimum conditions such that sufficient acetogenesis will occur.

Preferably the minimum first holding time is substantially

between three and eight hours. More preferably the minimum first holding time is three hours.

In preferred embodiments the minimum second holding time is substantially between one and two hours. Ideally the minimum second holding time is one hour.

In this connection, if the first and second rates are equal and the holding time in the first compartment (i. e the time it takes for the fluid to flow from the inlet through the first compartment and enter the second compartment) is set at three hours and the total volume of the tank is x the volume of the second compartment should be 1/4x, if the holding time in the second compartment (i. e the time it takes for fluid to flow from the flow regulation means to the outlet) is set at one hour.

Conveniently the temperature of the tank is substantially 30 to 800C. Preferably the temperature of the tank is substantially 55 C. 55 C is the optimum temperature for the microbiological flora in the tank.

In preferred embodiments the first microbiological flora comprises microorganisms which participate in hydrolysis and/or acetogenesis reactions. Preferably the second microbiological flora comprises microorganisms which participate in acetogenesis reactions. In this connection the first and second microbiological flora can be substantially similar.

The microorganisms can be from the thermophillic acetogens family. Such microorganisms can survive in the aforementioned temperatures and work optimally at a pH of substantially 5 to

7. As described above, as the microbiological flora digest the sludge the levels of volatile fatty acids increase. As the volatile fatty acids are toxic to the pathogens in the sludge (inc E. coli) the levels thereof will be reduced (i. e the E. coli levels of the processed sludge will be within the aforementioned acceptable conventional or advanced levels depending upon the length of treatment within the tank). The processed sludge then is transported to, for example, a main digester where the solid particles therein are further digested.

Preferably the first and second rates are equal.

I The invention will now be described, by way of illustration only, with reference to the following example and the accompanying figures.

Figure 1 shows a vertical cross-section of a tank according to the present invention; and Figure 2 shows a open-plan view of the tank of Figure 1 from above.

Figure 1 shows a sludge treatment tank 1 with a circular base 5 connected to upstanding tubular wall 6. The tank is covered by a lid 10. The tank has first and second compartments 2 and 3, which are separated by a baffle 4. In this connection, the first compartment 2 is formed between the upstanding tubular side wall 6 and the baffle 4 and the second compartment 3 is formed by the baffle 4. The height of the baffle 4 is dependent on the height of the tank 1 and the rate of flow that is required between the first and second compartments.

As can be seen from Figure 2, in the present embodiment three equidistantly spaced mixing means 7 are provided in the first compartment. However, any number of mixing means can be

-----. :.-)-. j JHJV < = : UL. An inlet pipe 8, with an opening 11 is provided adjacent the base 5 through the side wall 6 into the first compartment.

An outlet pipe 9 with an opening 12 is provided in the base 9 in the second compartment 3.

In use, raw sludge (not shown) flows into the first compartment 2 through the inlet pipe 8 (the flow of the sludge is shown by the arrows). The conditions provided in the first compartment encourage the growth of microorganisms which participate in hydrolysis and acetogenesis reactions (i. e high temperatures and low pH). Over a set period of time the raw sludge is at least partly digested by the microorganisms, first by hydrolysis and thereafter by acetogenesis and some pathogen kill occurs. The flow into the first compartment is such that a major portion of the sludge is kept within the first compartment for 3 hours. Thereafter the partially treated sludge flows over the baffle 4 and into the second compartment.

No mixing means are provided in the second compartment and therefore the compartment acts as a quiescent tank.

Accordingly, the partially treated sludge slowly flows towards the outlet opening 12, over a set period in this case arranged to be 1 hour. During this time the partially treated sludge is further digested by the microorganisms in the second compartment which are participating in acetogenesis to produce partially digested sludge and further pathogen kill occurs such that an acceptable pathogen level is obtained. In this

connection, the conditions in the second compartment are at substantially high temperature and low pH which encourages the growth of microorganisms which participate in acetogenesis.

The partially digested sludge with an E. coli level within the allowed levels flows from the outlet opening through the outlet pipe. The partially digested sludge is then further digested in, for example, a main digester before being recycled to agricultural land or landfill sites. The main digester is not required to lower the E. coli levels any further.

It will be understood that the embodiment illustrated shows one application of the invention only for the purposes of illustration. In practice the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.

The outlet opening could be part of a pipe which is directed through the first compartment and the side wall.

The mixing means could be provided at different heights to ensure adequate mixing of the sludge. The mixing means could be top mounted. As an alternative to the mixing means shown in the example, an external recycle loop can be provided.

The relative volumes of the first and second compartments can be chosen depending upon the required holding times in each compartment.

The two compartments need not be concentric. In other words the second compartment could be positioned off centre to the first compartment.

The horizontal cross-section of the first and second compartments need not be circular and need not be identical.

In this connection, for example, compartments with hexagonal or square horizontal cross-sections could be used.

Claims

Claims 1. A sludge treatment tank comprising first and second compartments ; and means for fluid flow regulation interconnecting the first and second compartments, said first compartment comprising an inlet and mixing means and said second compartment comprising an outlet.
2. A sludge treatment tank according to claim 1 wherein the means for fluid flow regulation is a baffle.
3. A sludge treatment tank according to either claim 1 or 2 wherein the first and/or second compartments comprise means for adjusting their internal environment.
4. A sludge treatment tank according to claim 3 wherein the means for adjusting the internal environment is a heat exchanger.
5. A sludge treatment tank according to any preceding claim wherein the inlet is positioned in the first compartment at the base of the tank.
6. A sludge treatment tank according to any preceding claim wherein the outlet is positioned in the second compartment at the base of the tank.
7. A sludge treatment tank according to any preceding claim wherein the first and second compartments are formed by a pair of concentric tubular members.
8. A sludge treatment tank according to any preceding claim

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wherein the relative volumes of the first and second compartments can be adjusted to meet the flow requirements between the two compartments.
9. A sludge treatment tank according to claim 8 wherein the volume of the first compartment is larger than the volume of the second compartment.
10. A method for the treatment of sludge using a tank as claimed in any of claims 1 to 9, the method comprising: a) introducing the first compartment, which has the preferred growing conditions for a first microbiological flora, sludge through the inlet opening; b) mixing said sludge with the mixing means for a first holding time; c) allowing sludge to flow into the second compartment, which has the preferred growing conditions for a second microbiological flora, at a first rate; d) allowing sludge to flow from the second compartment through the outlet at a second rate such that it is held in the second compartment for a second holding time.
11. A method according to either claim 10 wherein the minimum first holding time is substantially between three and eight hours.
12. A method according to claim 10 or 11 wherein the minimum second holding time is substantially between one and two hours.

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13. A method according to any of claims 10 to 12 wherein the temperature of the tank is substantially 30 to 80 C.
14. A method according to claim 13 wherein the temperature of the tank is substantially 55 C.
15. A method according to any of claims 10 to 14 wherein the first microbiological flora comprises microorganisms which participate in hydrolysis reactions.
16. A method according to any of claims 10 to 15 wherein the first microbiological flora comprises microorganisms which participate in acetogenesis reactions.
17. A method according to any of claims 10 to 16 wherein the second microbiological flora comprises microorganisms which participate in acetogenesis reactions.
18. A method according to any of claims 10 to 17 wherein the first and second rates are equal.