GB2307234A - Monitoring oxygen use in biological filter - Google Patents

Abstract

Oxygen consumption, in an oxidising waste water filter 10 for example, is monitored by collecting gas as it leaves the waste filter and analysing the collected gas to determine the oxygen content. Preferably the filter comprises a base supporting a particulate media column 12, oxygen supply means 20, liquid supply means and oxygen analysing apparatus 13, the analysing apparatus consisting of a gas collector 14 located at the outlet of the media column and a gas analyzer 16 coupled to the collector. Preferably the gas analyzer is also coupled to control means 19 so that the oxygen supplied to the filter may be continuously adjusted in accordance with the determined oxygen content of the collected gas. The filter may be a biologically aerated filter, a submerged aerated filter both of which consist of particulate media columns or the filter may be used with an activated sludge system.

Description

A SYSTEM FOR CONTROLLING OXYGEN SUPPLY IN PROPORTION TO OXYGEN REOUIREMENT IN A FILTER The present invention relates to a method of monitoring the oxygen demand of an aeration filter, in particular, but not exclusively, a biological aerated filter for treating waste water.

The treatment of waste water and sewage has developed continuously since medieval times. Since the advent of municipal sewage systems, sewage plants have been built for the treatment of such waste on a regional basis. The early part of the 20th century saw the advent of trickling filters. Untreated liquid is spread evenly on the top of a bed of clinker, coke, broken stone, etc. As the liquid trickles through the bed oxidation of organic an nitrogenous material in the liquid occurs with the aid of bacteria, which exists on the surface of the media of the bed. Air trapped in the pockets of the bed is used as the source of oxygen. Solid debris thus formed which is separated from the liquid in settling tanks.

In the early part of the 20th century, it was realised that the efficiency of the process could be improved by pumping air through the waste water. The biomass is formed in the liquid media. This type of process is known as activated sludge plant and has the advantage over trickling filter that the amount of sewage treated per unit volume of the filter is increased due to the forced oxygen supply.

Many attempts have been made to further increase the efficiency of the system for a given footprint, that is the area of ground required by the process to treat a certain volume of sewage. This is especially important in existing sewage plants which have to increase their capacity to meet the demands for growing populations, etc., but cannot increase the area over which the plant extends. The filters have tended to become columns so that a greater filter volume is given by the height of the filter. It has also been found that the efficiency of the filter is often increased by the filter media being flooded, i.e. the waste liquid surrounds the media bed. Flow through the filter may be upwards or downwards. Air is pumped through the flooded filter media bed.When the media used is small (less than about 5 mm nominal diameter) solids are partially retained within the bed of the medium and have to be removed by backwashing. This type of filter is known as biologically aerated filter (BAF). Where the media is larger (more than about 20 mm nominal diameter) backwashing is not required, as the solids flow from the filter and must be separated in a settlement tank. This type of filter is known as a submerged aerated filter (SAF).

All the above mentioned types of filters are still in use depending upon the footprint available for the sewage plant and the loading of the plant. The first two types mentioned above (trickling filter, activated sludge plant) require a relatively large footprint. Whereas, BAF's and SAF's, require a small footprint, but have a higher energy requirement.

One of the major energy requirements of aerated sludge filters, BAF's and SAF's, is the cost of the pumping the air through the filter. Thus, from an economical viewpoint, the volume of air supplied to the filter must be kept to a minimum. However, there must always be enough oxygen fed to the filter to oxidise all the waste being passed through the filter. Otherwise, the effluent produced by the filter may still include oxidisable matter and thus the plant may fail to meet pollution control requirements.

In activated sludge plants, an oxygen probe is suspended in the liquid. The probe determines the amount of oxygen contained in the area of the liquid in which it is suspended. If a large excess of oxygen is determined, this is indicative of too much air being supplied to the process. If a very low oxygen content is found, this is indicative that not all the waste will be treated in the filter. If, however, a small excess of air is determined in the plant, this indicates the correct running of the plant. However, this type of measurement probe does not work as effectively in BAF's or SAF's as the amount of oxygen in the filter varies greatly (between 0 and 100%) from the top to bottom of the media column. Further, the rate at which the rate of change of oxygen content is not constant within such a media column as this depends on the biomass accumulated within the filter.Accordingly, it is more difficult to carefully control the oxygen supply to such a column filter.

The present invention seeks to provide a system by which the energy efficiency of an oxidising filter can be improved. In particular, but not exclusively, the system applies to filters such as BAF's and SAF's.

According to a first aspect of the present invention there is provided a method of monitoring the oxygen demand of an oxidising filter, including the steps of; (a) collecting gas emitted from the filter, (b) analysing the collected gas to determine the oxygen content.

According to a second aspect of the present invention there is provided a waste liquid filter, comprising a vessel with a base arranged to support a particular media column in the vessel, oxygen supply means and liquid supply means each being arranged to supply, respectively, oxygen and liquid to flow along the column, and further including a gas collector arranged to collect gas from the region of the outlet side of the media column, coupled to a gas analyser, adapted to determine the oxygen content of gas collected by the collector.

The first and second aspects of the present invention, both have the advantage that the oxygen content is taken from gas emitted from the filter, rather than from within the media column itself. In this way, the reading is not dependent upon the biomass accumulated in any particular region of the filter.

Preferably, the method further includes the step of; (c) adjusting the oxygen supply to the filter based on the oxygen content determined in step (b).

Preferably, the filter according to the second aspect of the invention has the analyser coupled to control means arranged to control the oxygen supply means.

In the method of the invention, the adjusting steps (c) may be conducted continuously or a predetermined interval.

Advantageously, the filter is a biological aerated filter or a submerged aerated filter or a filter where the oxygen content in the filter media varies. However, the method is applicable for use on activated sludge plants and the invention is accordingly also drawn to this type of plant.

The present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a filter system with the oxygen analysing apparatus according to the present invention.

Figure 1 shows schematically a waste water filter (10) such as a biological aerated filter (BAF), a submerged aerated filter (SAF). The filter includes a column (12) of filter media such as sand or gravel.

Waste water is pumped into the base of the filter (feed for treatment) the air supply is also fed in to the base of the filter. The media column is supported on a base (not shown) such as that described in US Patent No.

4,923,606, the contents of which are hereby incorporated by reference. The air is supplied from a blower (20) the amount of air fed into the reactor is controlled by the control means (19).

The oxygen analysing apparatus (13) comprises a gas collector (14) which is arranged to collect gas emitted from the top of the filter media column (12). In the illustrated embodiment, the collected air is dried in dryer (15) but in some cases the dryer (15) may not be required.

The dried collected gas is then passed into an oxygen analyser (16) which analyses the oxygen content of the collected gas. A signal is generated by the oxygen analyser (16) which is transmitted to the control means (19) for regulating the oxygen supply to the filter.

For example, the analyser (16) analyses the oxygen content and outputs this information as electrical signal, which will directly vary the speed of the oxygen blowers (20), i.e. as the oxygen content of the collected gas increases the blower (20) speed will decrease and conversely as the oxygen content of the collected decreases the oxidation blower (20) speed will increase.

Alternatively, the data on the oxygen content may be stored in the control means (19) for a period. Then, at predetermined intervals, the average oxygen content for the period is determined and at these predetermined intervals the speed of the blowers (20) is varied. Another alternative would have the analyser (16) only analyses the collected gas at predetermined intervals.

The oxygen analysing apparatus (13) may be retro fitted to an existing filter.

Claims (13)

CLAIMS:

1. A method of monitoring the oxygen demand of an oxidising filter, including the steps of: (a) collecting gas emitted from the filter, (b) analysing the collected gas to determine the oxygen content.

2. The method according to claim 1, further including the step: (c) adjusting the oxygen supply to the filter based on the oxygen content determined in step (b).

3. The method according to claim 1 or claim 2, wherein the filter is a Biological Aerated Filter or a Submerged Aerated Filter.

4. A waste liquid filter comprising a vessel with a base arranged to support a particulate media column in the vessel, oxygen supply means and liquid supply means each being arranged to supply, respectively, oxygen and liquid to flow along the column, and further including a gas collector arranged to collect gas from the region of the outlet side of the media column, coupled to a gas analyser adapted to determine the oxygen content of gas collected by the collector.

5. The filter according to claim 4, wherein the analyser is coupled to control means arranged to control the oxygen supply means.

6. The filter according to claim 1 or claim 2, wherein the filter is a Biological Aerated Filter BAF or a Submerged Aerated Filter SAF.

7. A control apparatus for controlling the oxygen supply from an oxygen supply means to a waste liquid oxidising filter, comprising a gas collector for collecting gas emitted from the filter fluidly connected to an oxygen analyser for determining the oxygen content of the collected gas, and a control means adapted to vary the supply of oxygen in accordance with the determined oxygen content of the collected gas.

8. The control apparatus according to claim 7, wherein the control means is adapted to vary the oxygen supply continuously.

9. Use of the control apparatus according to claim 7 or claim 8, to control a waste liquid oxidising filter, preferably a Biological Aerated Filter or a Submerged Aerated Filter.

10. A method of monitoring the oxygen demand of an oxidising filter as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

11. A waste liquid filter as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

12. A control apparatus as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

13. Use of a control apparatus as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.