GB2203543A

GB2203543A - Respirometer

Info

Publication number: GB2203543A
Application number: GB08808094A
Authority: GB
Inventors: Anthony Brian Oaten
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1987-04-08
Filing date: 1988-04-07
Publication date: 1988-10-19
Also published as: GB8808094D0; GB8708378D0

Abstract

The respirometer apparatus comprises a sample chamber (1) adapted to entrap a sample of liquid to be analysed. Aeration means (11, 16) for introducing oxygen or air to the chamber are provided, as in an escape port (12) to relieve the excess oxygen or air in the chamber to a predetermined level. The chamber includes a probe (8) to determine the dissolved oxygen content of the trapped sample. The apparatus may be automatically controlled to release the entrapped sample after a predetermined period of time and later to entrap a fresh sample, and so on. The apparatus may be used in a method of measuring biological activity of a liquid. <IMAGE>

Description

RESPIROMETER The present invention relates to respirometers and also to a method of measuring biological activity of waste and other water'using a respirometer. The invention is particularly, but not exclusively, useful in determining the purity of water or the effluent quality of a plant.

The effluent quality of a plant is usually measured in terms of ammoniacal nitrogen, suspended solids and the five day biological oxygen demand (BOD) test. These all exhibit an oxygen demand. The most common is the BOD test but it cannot be used fo; waste water treatment control because of the five day delay. The method of the present invention allows the final BOD value to be predicted from short term measurements of respiration rate of the bacteria present in the waste water. The respiration rate can be determined by measuring the dissolved oxygen (DO) content of a sample over a period of time and calculating the rate at which it decreases. The waste matter in the effluent comprises the food for the bacterial culture, and thus the more waste matter, the more oxygen is consumed.

Respirometers are known which are adapted to trap a sample, measure the DO level in percentage saturation and release the sample, all under manual control. However, there are a number of disadvantages with respirometers of this type.

In order to give accurate reproducible results, each sample taken should start its test under the same conditions as all the others. This is not always the case with existing respirometers since the initial DO content may vary quite considerably between each sample.

Furthermore, existing respirometers require manual operation which is not always convenient, especially over more lengthy testing periods.

It is an object of the present invention to provide a respirometer and method of measuring biological activity which overcomes the above disadvantages.

According to a first aspect of the present invention, there is provided a respirometer apparatus comprising a sample chamber adapted to entrap a sample of liquid to be analysed, aeration means for introducing oxygen or air to the chamber, escape means to relieve the excess oxygen or air in the chamber to a predetermined level, and probe means to determine the dissolved oxygen content of the trapped sample.

Preferably the apparatus comprises automatic control means to cause release of the entrapped sample after a predetermined period of time and later to cause entrapment of a fresh sample, and so on.

Measurements may be taken at intervals throughout the predetermined period.

A stirrer, preferably magnetic, will preferably be provided in the chamber. The dissolved oxygen content results may be fed to computing means, and biological activity, e.g. BOD, computed therefrom.

According to a second aspect of the present invention, there is provided a method of measuring biological activity of a liquid comprising the steps of entrapping a sample of said liquid of predetermined volume, aerating said sample to a predetermined dissolved oxygen content, and measuring the dissolved oxygen content of the sample at intervals or continuously over a predetermined period of time or predetermined range of DO content, and computing from the measurements a required parameter of biological activity, e.g. BOD.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 is a cross-section through a respirometer embodying the invention showing the chamber closed; FIGURE 2 is a cross-section of the respirometer of Figure 1 showing the chamber open; and FIGURE 3 is a schematic of the pneumatic systems used with the respirometer.

Referring now to Figure 1, there is shown a respirometer for use under the surface of the liquid being tested. It comprises a sample chamber 1 bounded by a cylindrical wall 2 and end walls 3 and 4. The chamber 1 is connected to a piston 5 of a pneumatic piston and cylinder 6 arrangement in such a manner that the chamber may be moved to the left (as shown in Figures 1 and 2) under the influence of the piston and cylinder. Moving the chamber allows the sample trapped in the chamber to escape and eventually be replaced by a fresh sample. Figure 2 shows the respirometer with the chamber open.

In end wall 3 of the chamber is provided a probe wiper 7 which is adapted to wipe a probe 8 as the chamber opens and closes.

In the opposite end wall 4 is provided a stirrer 9 operated by a stirrer motor 10.

Finally, there is a non-return valve air inlet 11 to the chamber 1 and an escape port 12 to allow escape of excess air or oxygen. A length of tube (not shown), possibly 12 inches (305 mm) in length and of internal diameter 2 inch (12.5 mm) is attached to the escape port 12. As can be seen from Figure 3, air is fed via a valve 14 and a flow regulator 13 to the non-return valve 11 to aerate the contents of the chamber to a predetermined level of dissolved oxygen. Once this has been accomplished, the escape port 12 ensures that a predetermined pressure is not exceeded, and also serves to vent excess oxygen or air, thereby ensuring that the chamber is full of liquid to be tested. At the end of the aeration period the tube attached to the escape port 12 will contain a volume of aerated sample displaced by the volume of air in the sample chamber.As this air is used up during the measurement period, the aerated liquid in the tube may be drawn into the sample chamber through port 12. This prevents unaerated liquid from contaminating the sample.

The piston 5 and cylinder 6 arrangement is operated by air pressure via valve 15. The three air lines, two for the piston and cylinder and one feed to the chamber are taken together to the submerged unit in a quad co-axial arrangement also including the stirrer motor power cable.

In order to operate the respirometer, the sequence of events is that the probe is firstly wiped by closing and opening the chamber under the influence of piston and cylinder 5 and 6; the chamber is closed to trap the sample; the sample is aerated by stirring with magnetic stirrer 9 and pumping air or oxygen into the chamber via non-return valve 11 until a desired dissolved oxygen high point is reached; and measurements are taken by the probe 8 over a period of time until the dissolved oxygen low point is reached, at which point the chamber is opened to release the sample. The valve 11 is connected to the chamber by pipes 16 which are preferably disposed to allow inlet of air in such a pattern at one end of the chamber that an even level of aeration is achieved. The respirometer is operated under control of a micro-computer system, to which results can also be fed.A number of parameters can be entered into the system, as follows.

The valve 11 should preferably allow a constant flow of air, irrespective of the depth at which the respirometer is acting.

The dissolved oxygen high-point is the percentage saturation of the sample at which aeration of the sample stops and logging of the DO level starts.

The dissolved oxygen low-point is the level in terms of percentage saturation at which logging of the DO level stops and the trapped sample is released.

A sample period can be set, which is the time between trapping each sample which may range between 0.1 and 48 hours. The apparatus can also be set to take a number of samples, by which is meant the total number of samples which are to be trapped within the total operating time of the package. A delay time may also be set, which is the time delay before the first sample is trapped.

Finally, a log period can be set, by which is meant the time between data logs during measurement of the DO level of the sample. This can vary between 0.1 and 60 seconds.

The logged data can be fed to a micro-processor and used to compute the BOD and/or biological activity measurements. Use of the respirometer allows these measurements to be taken over a much shorter period of time and with greater accuracy than hitherto has been possible.

The preferred probe is an LTH probe although others of suitable geometry and performance may be used if so desired. The computer results may be inspected on site or transferred to a central monitoring station which may have control of a number of such respirometers and possibly other monitoring equipment.

Claims

CLAIMS:

1. A respirometer apparatus comprising a sample chamber adapted to entrap a sample of liquid to be analysed, aeration means for introducing oxygen or air to the chamber, escape means to relieve the excess oxygen or air in the chamber to a predetermined level, and probe means to determine the dissolved oxygen content of the trapped sample.

2. An apparatus as claimed in claim 1, further comprising automatic control means to cause release of the entrapped sample after a predetermined period of time and later to cause entrapment of a fresh sample, and so on.

3. An apparatus as claimed in claim 2, wherein measurements are taken at intervals throughout the predetermined period.

4. An apparatus as claimed in any one of the preceding claims, further comprising a stirrer provided in the chamber.

5. An apparatus as claimed in claim 4, wherein the stirrer is a magnetic stirrer.

6. An apparatus as claimed in any one of the preceding claims, further comprising computing means to which the dissolved oxygen content results are fed and adapted to compute therefrom biological activity, e.g. BOD.

7. A respirometer apparatus substantially as described herein with reference to the accompanying drawings.

8. A method of measuring biological activity of a liquid comprising the steps of entrapping a sample of said liquid of predetermined volume, aerating said sample to a predetermined dissolved oxygen content, and measuring the dissolved oxygen content of the sample at intervals or continuously over a predetermined period of time or predetermined range of DO content, and computing from the measurements a required parameter of biological activity, e.g. BOD.

9. A method oftmeasuring biological activity of a liquid substantially as described herein.