GB2184110A

GB2184110A - Treatment of aqueous material

Info

Publication number: GB2184110A
Application number: GB08629150A
Authority: GB
Inventors: Frederick John Gould
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1985-12-11
Filing date: 1986-12-05
Publication date: 1987-06-17
Anticipated expiration: 2006-12-05
Also published as: GB8629150D0; GB2184110B; JP2567382B2; JPS62254897A; ZA868928B; GB8530548D0

Abstract

In an oxic biomass process oxygenation of an aqueous liquor is controlled by control unit 14 including electronic circuits 20 for computing the dissolved oxygen concentration from signals generated by a sensor 12 immersed in the liquor and for generating oxygenation control signals therefrom. The control unit 14 also includes a microprocessor 22 programmed to generate signals representative of the rate of decay of the dissolved oxygen concentration. If the rate of decay is less than a preselected value it indicates that the biomass has an undesirably low activity. and a signal is generated from the microprocessor 22 to actuate an alarm 30. <IMAGE>

Description

SPECIFICATION Treatment of aqueous material This invention relates to apparatus and method for the treatment of aqueous material which is intended for use in an oxic biomass process by which is meant a process in which an aqueous liquor containing biomass is oxygenated in a controlled manner, for example, by the method described in our UK patent specification No. 1 455 567, in order to provide the oxygen necessary for the respiration or metabolism of the biomass and typically also for the biochemical oxidation of oxidisable substances in the aqueous liquid. Examples of processes involving an active biomass include the fermentation of aqueous liquors to form a chemical or food product or the treatment of aqueous effluents such as sewage.

It is the typical practice in the art to maintain the dissolved oxygen content of the aqueous liquid at or above a chosen minimum.

Typically, an instrument for monitoring the dissolved oxygen content of the aqueous liquid is employed to monitor the dissolved oxygen content continuously and when the dissolved oxygen content falls to the chosen minimum, then oxygen supply means are actuated so as to effect the oxygenation of the liquid to bring its dissolved oxygen level up to a chosen maximum. When this maximum is reached, the dissolved oxygen monitoring means actuates a valve or other means to stop the oxygenation.

The dissolved oxygen is consumed quite rapidly by the biomass and thus the dissolved oxygen concentration begins to decay. It is desirable for operators of such plant to be able to guard against the possibility of the biomass becoming ineffective or dying. If the biomass does become ineffective or die, then natural aeration of the liquid will typically keep the dissolved oxygen level above the chosen minimum and therefore no demand for oxygen from the oxygen supply means is made. It is not however entirely satisafactory to use such an absence of oxygen demand to indicate the presence of a dead or ineffective biomass.

This is because it then typically becomes necessary to shut down the particular process and there may be a loss of production of many hours before a viable biomass is substituted for the dead one.

It is an aim of the present invention to provide an apparatus and method capable of solving or ameliorating the above described problem.

According to the present invention, there is provided apparatus for use in an oxic biomass process, comprising means for monitoring the dissolved oxygen concentration of aqueous liquor containing the biomass and undergoing oxygenation intermittently, means for computing the rate of decay of the dissolved oxygen concentration when said liquor is not undergaing oxygenation, and means for generating an alarm signal in the event that the rate of decay is less than a pre-selected value.

The apparatus according to the invention makes it possible to detect a decline in the activity of the biomass and to provide a signal that will bring this decline to the attention of the plant operator. The plant operator will then be able to investigate pauses of the decline and, if it is caused by a readily remediable cause, he may be able to take corrective action before the process is rendered inoperative.

The invention also provides a method of controlling an oxic biomass process, including the steps of monitoring the dissolved oxygen concentration of an aqueous liquor containing the biomass, intermittently oxygenating the liquor, computing the rate of decay of dissolved oxygen concentration, and generating an alarm signal in the event that the rate of decay is less than a precelected value.

The apparatus according to the invention may also include means such as a flow meter for monitoring the flow of aqueous liquor into a vessel in which said oxic biomass process is performed means for generating a signal representative of the flow, means for computing from the flow and the said rate of decay a parameter representative of the rate of consumption of oxygen per unit mass or volume of said incoming liquor, and means for displaying the value of said parameter. If the liquor in the vessel is well mixed, the value of said parameter approximates to the biochemical oxygen demand of the incoming liquor.

The accuracy of the approximation depends inter alia on the ratio of proportion of the overall consumption of oxygen in the process that is used in biochemical oxidation of chemical substances in the incoming liquor to the proportion that is respired or otherwise metabolised by the bacteria or other micro-orgasims in the bio-mass. The higher the ratio, the more accurate is the approximation. Typically, in sewage or effluent treatment, in the order of 5% by volume of the total oxygen consumption is respired or otherwise metabolised by the micro-organisms. If desired, the apparatus may be provided with means for reducing the computed value of said representation by a chosen amount so as to "correct" for the metabolised oxygen. If undissolved oxygen passes out of the liquor and escapes to the atmosphere, a correction for this factor may also be made.Conventional practice in the art is to measure biochemical oxygen demand by taking a sample of the liquor, and analysing parts of the sample before and after storage over a five day period. The apparatus according to the invention offers an improvement over this known practice in that it provides an instantaneous method of measuring biochemical oxygen demand.

The apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a plant for performing anoxic biomass process; Figure 2 is a block diagram illustrating the control unit shown in Fig. 1, and; Figure 3 is a schematic illustration of a typical printout from the chart recorder for the control unit shown in Fig. 2.

Referring to Fig. 1 of the drawings, an oxic biomass plant 2 has an inlet 4 for aqueous material having a biochemical oxygen demand, and an outlet 5 for treated effluent. In addition, there is a recycle conduit 6 having an inlet and an outlet communicating with the aqueous liquor in the plant 2. A pipe 8 connected to a source of oxygen (not shown) terminates in the conduit 6. The pipe has an on/off valve 10 located therein. The recycle conduit 6 and the oxygen pipe 8 may be arranged and operated so as to oxygenate the aqueous liquor in the plant 2 in the manner described and claimed in our UK patent specification 1 455 567.

A dissolved oxygen sensor 12, of conventional kind, is located under the surface of the aqueous liquor in the vessel 2. It is adapted to generate electrical signals and to pass them to a control means 14. The control means 14 is able to generate signals in response to the sensed dissolved oxygen to open and close the valve 10 and thus respectively start and end oxygenation of liquid passing through the recycle conduit 6.

If desired, the apparatus shown in Fig. 1 may also include a flow meter (not shown) in the inlet 4 to measure the rate of flow of aqueous liquor into the plant 2 and to generate a signal to the control means 14.

One example of a control means 14 for use in apparatus according to the invention is shown in more detail in Fig. 2 of the drawings. The control unit includes a number of different functions. First, it has electronic circuits 20 for computing the dissolved oxygen concentration from the signal given by the sensor 12 and for generating control signals to the valve 10 as described above. The control unit secondly includes a micro-processor 22 programmed to generate in response to dissolved oxygen signal from the circuits 20 a signal representative of the change in dissolved oxygen concentration, say every second or fraction of a second.

This signal being representative of the activity of the bacteria in the biomass. The signal generated by the micro-processor 22 is converted by electronic circuits 24 into chosen units (e.g. mg l-1 hr-1) suitable for display on a digital indicator 26. The control unit is typically also provided with a chart recorder 28 on which both the dissolved oxygen concentration and the respiration rate may be displayed.

The control unit shown in Fig. 2 also has an alarm 30 (associated with the micro-processor 22) for generating an alarm signal in the event that during periods in which no oxygenation is preferred the rate of decay of the dissolved oxygen falls below a preselected value. The alarm 30 may be of any conventional kind in such instruments.

In Fig. 3 there is shown schematically a typical print-out from the chart recorder 28 of the control unit shown in Fig. 2. The curve A is shown in Fig. 3 represents the variations in dissolved oxygen concentration of the aqueous liquor in the plant shown in Fig. 1. The minima of the curve A represent those times at which oxygen supply through the pipe 8 shown in Fig. 1 is initiated and the maxima those times at which the oxygen supply from the pipe 8 is stopped. The minima thus represent the minimum dissolved oxygen concentration and the maxima the maximum dissolved oxygen concentration. It will be seen that the dissolved oxygen concentration decays relatively rapidly (though not as rapidly as it is built up again) once the oxygen supply is stopped. This indicates a healthy rate of activity (and hence respiration) for the bacteria in the biomass, the slope of the curve at any instant being proportional to the rate of biochemical oxygen consumption (sometimes referred to as the respiration rate). The curve B indicates the respiration rate during periods in which oxygen is not supplied to the plant 2.

Should the respiration rate fall to below a chosen value, indicating that the biomass is not functioning as intended, the alarm 30 is actuated by virtue of a signal from the microprocessor 22.

Claims

1. Apparatus for use in an oxic biomass process, including means for monitoring the dissolved oxygen concentration of aqueous liquor containing the biomass and undergoing oxygenation intermittently, means for computing the rate of decay of the dissolved oxygen concentration when said liquor is not undergoing oxygenation, and means for generating an alarm signal in the event that the rate of decay is less than a pre-selected value.

2. Apparatus as claimed in claim 1, in which the apparatus additionally includes means for oxygenating the liquor in response to signals from the dissolved oxygen sensor, the arrangement being such that in operation supply of the oxygen is initiated when the dissolved oxygen concentration falls to a minimum and is terminated when the dissolved oxygen reaches a maximum, said means for computing the rate of decay of the dissolved oxygen being adapted to compute the said rate of decay in the periods when oxygen is not supplied.

3. Apparatus as claimed in claim 1 or claim 2, in which the said computing means comprises electronic circuits for computing the dissolved oxygen concentration from signal generated by said monitoring means, in operation, and a microprocessor programmed to generate, in response to signals from said electronic circuit, signals representative of the change in dissolved oxygen concentration.

4. Apparatus as claimed in claim 3, in which said microprocessor is adapted to generate said representative signals at least every second.

5. Apparatus as claimed in any one of the preceding claims, additionally including means for monitoring the flow of aqueous liquor into a vessel in which said oxic biomass is performed, means for generating a signal represenative of the flow, means for computing from the flow and the said rate of decay a parameter representative of the rate of consumption of oxygen per unit mass or volume of said incoming liquor, and means for displaying the value of said parameter.

6. Apparatus for monitoring the activity of a biomass substantially as herein -described with reference to Figs. 1 and 2 of the accompanying drawings.

7. A method of controlling an oxic biomass process, including the steps of monitoring the dissolved oxygen concentration of an aqueous liquor containing the biomass intermittently oxygenating the liquor, computing the rate of decay of dissolved oxygen concentration when said liquor is not undergoing oxygenation and generating an alarm signal in theevent that the rate of decay is less than a preselected value.

8. A method of controlling an oxic biomass process, substantially as described herein with reference to the accompanying drawings.