GB2333467A

GB2333467A - Maintaining a solid/liquid suspension using selectively activated plural aerator/mixers

Info

Publication number: GB2333467A
Application number: GB9725261A
Authority: GB
Inventors: Michael John Beaumont
Original assignee: HYTECH WATER Ltd
Current assignee: HYTECH WATER Ltd
Priority date: 1997-11-29
Filing date: 1997-11-29
Publication date: 1999-07-28
Also published as: GB9725261D0

Abstract

A solid/liquid suspension (leachate) in a activated sludge process containment vessel 11 is maintained using a plurality of aerator/mixers (12, 13, 14) operable to cause one or more of the mixers to be de-energised while one or more of the others remain energised whereby to maintain the suspension and minimise energy consumption. Energisation of the mixers in controlled at 23 so that, for example, initially all the mixers are energised and then subsequently only one mixer is energised at a time.

Description

LEACHATE TREATMENT SYSTEM The present invention relates generally to a leachate treatment system, and particularly (but without prejudice to the generality thereof) to a system for use in the aeration and mixing of polluted water undergoing treatment by the activated sludge process.

Although the present invention has been devised particularly for use in the process defined above it is to be understood that it may find application in any system in which it is necessary to maintain a suspension of solids in particulate form in a liquid or liquids.

In the activated sludge process referred to above the polluted leachate is collected in a tank or container vessel, frequently referred to as a lagoon, in which active bacteria in the sludge acts to treat the pollutants in the leachate. For this to occur efficiently it is necessary for the leachate to be in suspension, for which purpose it is necessary to maintain the solids in suspension and to prevent settlement.

In known treatment systems, which typically operate on a 24 hour cycle, the maximum amount of mixing is first required in order to homogenise the settled sludge and the leachate. It is desirable for this to be achieved as quickly as possible. The containment vessel or lagoon is provided with a number of machines for causing agitation of the material within the tank for mixing purposes. Such mixers may be of one of a number of different designs, but typically they mix by mechanically circulating the contents of the lagoon using a pump or other type of mechanical mixer.

The mixing is accompanied by the introduction of air into the mixed liquor. This aeration effect is usually provided by the mixer/agitator. Such machines, referred to herein as mixer/aerators, are at first run simultaneously and at full power in order to mix the settled sludge and the leachate as quickly as possible.

However, once the settled sludge is fully suspended, the power requirement to maintain the sludge in suspension and aerate it is considerably reduced.

In a typical cycle a prior art system of this type operates at this step with all the mixer/aerators energised. This continues for a major part of the cycle time whereby to maintain the solids in suspension and allow the bacterial activity to progress. At the end of this initial time period the mixer/aerators are all deenergised and the mixture is allowed to settle for a further period. The sludge and the active bacteria then form a layer at the bottom of the containment vessel allowing the liquid to clarify. The treated leachate is then pumped out of the lagoon and the sludge is allowed a recovery period before fresh leachate is pumped into the lagoon for the treatment cycle to be repeated. In a 24 hour treatment cycle the aeration/mixing may typically take 18 hours, the settlement 2 hours, the pumping out of the treated leachate 3 hours and the quiescent or recovery period 1 hour.

One of the disadvantages of the conventional cycle discussed above is the considerable energy consumption required to maintain the solid material in suspension in the leachate for the bacteria to act effectively. The present invention is based on the realisation that the bacterial activity can take place satisfactorily without requiring the high levels of energy input previously employed in maintaining the suspension vigorously. The present invention is based on the realisation that the bacterial activity can take place providing the suspension is maintained by a reduced energy input without requiring a complete, full and vigorous mixing as has previously been undertaken.

According to one aspect of the present invention, therefore, a system for the treatment of a mixture of solids and liquid(s), such as the leachate from a polluted source, comprises a containment vessel, a plurality of mixers each operable to cause mixing of the solids component in at least part of the volume of the vessel, and control means operable to control one or more of the mixers to be de-energised while one or more of the others remain energised whereby to resist or prevent settlement of the solids, to encourage maintenance of the suspension and to aerate the lagoon contents.

The objective of the present invention is to maintain conditions for bacterial activity and prevent settlement of the activated sludge, whilst consuming as little energy as possible. Prior art systems which have maintained all mixers/aerators in an energised state for the entirety of the mixing/aeration step of the cycle have, it is believed, resulted in an excess energy consumption which is not necessary for the satisfactory performance of the bacterial treatment.

Such mixer/aerator devices may be spaced around the perimeter of an open containment vessel at fixed locations. Alternatively, however, the mixer/aerator devices may be allowed to float on the surface of the mixture or be submerged below the surface. They need not necessarily be constrained to remain in a fixed location.

The control means preferably include a softwareimplemented control programme and, moreover, it is preferable that the program allows for operator intervention to allow different operating requirements to be taken into account.

The system of the invention preferably also includes means for enabling selective energisation of individual mixers or aerators by a supervising operator. Likewise, should circumstances permit, the operator may also preferably have the available option of de-energising selected mixers or aerators.

The control system of the present invention preferably energises the mixers and/or aerators in a predetermined sequence as appropriate to the conditions and/or configuration of the containment vessel and there are preferably provided means for varying the order in which the mixers and/or aerators are energised or de-energised in dependence on operating requirements and/or the configuration of the containment vessel. In a preferred embodiment, therefore, after the programme has activated all mixer/aerator units to create a suspension of the activated sludge and the freshly introduced leachate some of the mixer/aerator units are de-energised for a period leaving only one or at least less than the full complement of mixer/aerators in operation.

After a certain time period, determined by the operating requirements and/or configuration of the system, the energised mixer/aerator is de-energised and one or more of the previously de-energised mixer/aerator units is or are then energised. This cyclic rotation of energisation of the mixer/aerator units continues until the end of the period required for the bacteria in the activated sludge to complete their treatment. Thereafter all the units are de-energised to allow settlement of the sludge for clarification of the leachate prior to removal in a manner similar to that of the conventional process.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a settlement lagoon having three mixer/aerator units and controlled by a system according to the present invention; Figure 2 is a cross-section through the settlement lagoon of Figure 1; and Figure 3 is a flow chart of the software-implemented process of the invention.

Referring now to the drawings there is schematically shown a settlement tank or lagoon generally indicated 11.

Such containment vessels are typically of the order of 20 metres in diameter and in the region of 5 metres deep.

Around the perimeter of the containment vessel 11 are three mixer/aerator units 12, 13, 14 the structure and function of which will be described in more detail below.

A feed pipe 15 delivers leachate to the containment vessel 11 and a discharge pipe 16 carries away the clarified leachate, driven by a pump 17. From the pump 17 the effluent is discharged to a balancing tank (not shown) for a further step in the treatment process which is not relevant to the present consideration.

As can be seen in Figures 1 and 2 each mixer/aerator has two discharge nozzles, a, b and a suction pipe c. In fact the aerator mixers each comprise a submersible pump having an inlet for the liquid (not shown), and two outlet nozzles identified a, b. A suction inlet identified c extends to the atmosphere and leads to a point within the pump at which a depression exists when the pump is in operation (for example with a construction such as an ejector pump) so that, in operation, the solids/liquid mixture is positively drawn through the pump and expelled through the nozzles a, b whilst the ejector effect draws air in through the inlet c to be mixed with the solids/liquid mixture for aeration purposes.

When air is delivered to each of the mixer/aerator units 12, 13, 14 are in operation circulation currents generally represented by the arrows 18, 19 of Figure 2, are set up within the containment vessel or lagoon 11.

Each of the mixer/aerator units 12, 13, 14 is connected by a respective control line 20, 21, 22 to a central control unit 23 having an operator interface 24. The control unit 23 is programmed to deliver control signals on lines 20, 21, 22 to the mixer aerator units 12, 13, 14 in accordance with a programme determined in dependence on the nature of the pollutants, the physical properties of the leachate and the dimensions and configuration of the tank 11. This involves a preliminary energisation period with all three mixers working together (for example 1 hour), followed by a low-energy regime in which only one mixer is energised at a time whilst the other two are de-energised. After a predetermined period (for example M hour) the energised mixer is de-energised and the next one in sequence is energised so that the whole of the material in the tank is maintained in suspension with the minimum energy input from the mixer/aerator units.

Figure 3 is a flow chart of the process. In this drawing the main steps in the process are illustrated: other parts of the overall program have been omitted for clarity. This programme is run on a central processor which has input information on the individual components of the system, such as mixer/aerators, operations console, energy source etc so that it can make decisions on the basis of the step reached in the programme and the status of the input signals. At the main control console the operator may first determine the time desired for all three aerators 12, 13, 14 to operate simultaneously in order to provide an initial mixing of the solids in the liquid so as to ensure that the solids are fully suspended at the start of the cycle. This initial time period is identified in Figure 3 as the "preset time".

With the power saving opinion selected, the software described by the flowchart acts to: - run all three aerators together for the initial part of the cycle - the "preset time"; - after the expiry of the preset time, to run each aerator one by one in rotation for the "aeration time", with a 10 second overlap at each aerator changeover point; - at the mid-point of the aeration period to run all three aerators together for an operator-preset time of X minutes to restore dissolved oxygen levels if necessary; and - at the expiry of the X minutes, to return to running each aerator in rotation until the end of the aeration period.

To achieve this the process begins at "start" and the first step in the process is to determine that the aerators 12, 13, 14 are all "OK to run". In this context the term "OK to run" means that the individual aerator is turned on and there is no signal indicating a fault identifying that aerator as faulty. At step 1 of the process the determination of whether the aerators are "OK to run"includes a check that the central processor is at the appropriate part of the cycle, that the float switches are all in the correct positions, that mains power is available etc. If the response to the "OK to run" query is negative the system cycles back to make the same interrogation until the signals are all in an affirmative state at which point it moves on to step 2 in which it determines whether the pre-set time (as defined above) has passed since the start of the aeration cycle.

If the answer to this interrogation is negative the system passes to step 3 in which the three aerators 12, 13, 14 are all operated simultaneously in their normal mode of operation and the system returns to step 1 to interrogate the sensors for confirmation that all systems are in a normal state. This loop continues until the preset time elapses such that, at step 2, the affirmative response is received to the interrogation, at which point the system moves on to step 4 to determine whether the operator has selected to operate the system with full energisation or in the power saving mode. If the response to this interrogation is negative the system passes back to step 3 to operate the aerators 12, 13, 14 simultaneously full time and the interrogations of steps 1, 2 and 4 are repeated. If, on the other hand, the response at step 3 is affirmative, that is having determined that the system is in the power saving mode at step 4 the system passes on to step 5 to determine whether the relevant aerator timer has reached the end of its individual aerator time. If the response to this interrogation is in the negative the system passes on to step 6 where it is determined whether the aerator in operation is within 10 seconds of the end of the aerator time. If the response is negative the system passes to step 8 at which it checks if the aerator pointer is at number '0'. The significance of this will be explained below. If the response is affirmative, it passes to step 7 which commences a block of control steps to ensure that a subsequent aerator is turned on just before the energised aerator is turned off.

At step 7 the system first determines whether the aerator pointer is at number 0. In this context it should be explained that the central processor assigns values 0, 1 and to the aerators 12, 13, 14 respectively. This is achieved by a so-called aerator pointer which comprises a register in the central processor which is incremented by a control input from the system. If the aerator pointer is at number 0, such that the response to interrogation is affirmative, the system moves on from step 7 to step 9 to check whether aerator 13 is "OK to run" as hereinbefore defined. Assuming that all conditions are appropriate and the response is affirmative the processor then moves on to step 11 to run aerator 13.

At this step aerator 12 is still running since the previous response to interrogation at step 8 was affirmative. Aerator 12 is still "OK to run" so the interrogation at step 10 is affirmative and aerator 12 therefore continues to run at step 14. This continues for the 10 seconds until the end of the aerator time.

Then, the process passes from step 5 to step 3 at which the aerator timer is rest. On passing to step 10, however, the response to the query "is aerator 12 OK to run" will be negative since the aerator time has now reached the preset time and the process moves on from step 10 to step 15 at which the aerator pointer is incremented by 1.

From step 11, with aerator 13 now operating alone, and aerators 12 and 14 de-energised the process recycles to step 6 until reaching the period 10 seconds before the end of the new aerator time at which the response to interrogation at step 6 becomes "yes" and the process moves on to steps 7 and 8. Now, however, the aerator pointer is at number 1 so the response at steps 7 and 8 is negative with the result that the process moves on to steps 16 and 17 at which the responses are both affirmative so aerator 14 is now energised while aerator 13 continues to be energised. The process recycles to step 5 until the aerator timer expires at which point the response becomes affirmative and the system returns to step 15 to increment the aerator pointer by 1 to now read 2.

The system then continues with aerator 14 energised and aerators 12 and 13 de-energised until 10 seconds before the end of the pre-set time for this aerator. At this point the response to interrogation at step 6 becomes affirmative and the system runs through steps 8 and 16 with negative responses to reach step 22 at which the response is affirmative. Likewise the process passes through steps 7 and 17 to reach step 23 where, again, with the aerator pointer at number 2 the response is affirmative and aerator 14 therefore continues to run whilst aerator 12 is energised. This continues for the remainder of the 10 second period until the response at step 5 becomes affirmative to indicate that the aerator timer has reached the time out at which point the process returns again to step 15 to add 1 to the aerator pointer now making it 3. From step 15 there is a sub-routine at step 18 checking whether the aerator pointer is greater than 2. At this point, having been incremented to 3 it is greater than 2 and with an affirmative response the system passes to step 29 which sets the aerator pointer back to 0 so that for the subsequent interrogations at steps 7 and 8 the response will be affirmative. In this way the system continually cycles through the three individual aerators.

The separate part of the process then determines at the end of the individual aerator maintenance routine that all aerators should be de-energised and an input to the system at step 1 results in the response becoming negative, that is indicating that the aerators are not "OK to run". A separate timer then allows the system to remain quiescent, effectively cycling only in step 1, until the solids have settled at which point the pump 17 is energised to withdraw the clarified liquid through outlet pipe 16. After this a new charge is introduced through inlet pipe 15 and the whole cycle is repeated.

In the middle of the aeration using individual aerators, for a time period of X minutes which can be selected by the operator in dependence on requirements, the programme will activate all three aerators together upon reaching step 30 which sets a window of common operation for all three aerators, after which the system returns to its cyclic energisation of each aerator individually.

Claims

CLAIMS 1. A system for the treatment of a mixture of solids and at least one liquid, such as the leachate from a polluted source, comprising a containment vessel, a plurality of mixers operable to cause mixing of the solids component in at least part of the volume of the vessel, and control means operable to cause one or more of the mixers to be de-energised while one or more of the others remain energised whereby to resist or prevent settlement of the solids and encourage maintenance of a suspension.
2. A system as claimed in Claim 1, in which the mixers also act as aerators whereby to encourage aerobic biological activity in the mixture.
3. A system as claimed in Claim 1 or Claim 2, in which the mixers or mixer/aerator devices are spaced around the perimeter of the containment vessel at fixed locations.
4. A system as claimed in Claim 3, in which the containment vessel is an open vessel.
5. A system as claimed in any of Claims 1 to 4, in which the control means include a software-implemented control programme.
6. A system as claimed in any preceding claim, further including means for enabling selective energisation of individual mixers or mixer/aerators by a supervising operator.
7. A system as claimed in any preceding claim, in which there are provided means for varying the order in which the mixers and/or mixer/aerators are energised or deenergised in dependence on operating requirements and/or the configuration of the containment vessel.
8. A system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings
9. A method of treating a mixture of solids and at least one liquid, such as a polluted leachate, in which the mixture is held for a period of time in a containment vessel and the solids component in at least part of the containment vessel is maintained in suspension by operation of one or more of a plurality of mixer devices while others of the said plurality are de-energised by control means acting selectively to energise and deenergise mixers in a sequence such as to maintain the suspension with a minimal energy input.
10. A method as claimed in Claim 9, in which the mixture of solids and liquid is also aerated by one or more of the mixers when energised.
11. A method as claimed in Claim 9 or Claim 10, in which the sequence of energisation and/or de-energisation of the mixers or mixer/aerators is predetermined by the said control means.
12. A method as claimed in Claim 9 or Claim 10, in which the sequence of energisation and/or de-energisation of the mixers or mixer/aerators is determined on the basis of determinable parameter of the mixture.
13. A method as claimed in Claim 12, including the step of determining the degree of settlement of the solids component of the mixture at different regions of the container whereby to determine the sequence of energisation and de-energisation of the mixers or mixer/aerators.
14. A method as claimed in any of Claims 9 to 13, further including the steps of conducting preliminary tests on the settlement rate of the solids in the said at least one liquid and basing the control sequence of energisation and de-energisation of the mixers and/or mixer/aerator devices on the results of the said preliminary tests.
15. A method of treating a mixture of solids and at least one liquid substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
16. A system for the aeration and mixing of polluted water undergoing treatment by the activated sludge process in a containment tank or lagoon having a plurality of aerator/mixer devices operable to disturb the polluted water to maintain the solids content in suspension, in which there are provided control means determining a programme of control operation whereby selectively to activate the aerator/mixer devices individually in performing the treatment process.