Measuring Gross Solids 1 Introduction

Because the bottle samplers only draw up relatively small samples though tubes only 10mm in diameter, they can give little information

about the larger objects in the flow. Such things, which may be

tissues, rags, faecal matter, plastics and so on, can be the cause of considerable aesthetic dissatisfaction if spilled to a watercourse, particularly if left strewn about on banks and low branches by receding water levels after a storm or settled in shoals on the river bed. In order to investigate whether the vortex is able to prevent such 'gross solids' spilling, a way of monitoring such matter was required.

In response to this need, the 'Gross Solids Monitor' (GSM) was

developed at the Water Research Centre. Various techniques were considered, such as using ultra-sonics or methods to take large physical samples, but a visual approach was eventually decided upon as the most appropriate, the others being thought either too expensive or impractical.

4.4.2 The Gross Solids Monitoring Equipment

The principle of the system was to record videos of samples of sewage flowing past a window, and to count the number of large objects visible.

Most of the equipment to achieve this was in the pit in the monitoring station (Fig.4.4, 4.5).

A large peristaltic pump was used to pull sewage up from the chamber through a 100mm diameter flexible hose, and pass it through

a steel tube with transparent sections top and bottom. It was

illuminated from below by a bank of near infra-red LEDs, and viewed

from above with a video camera sensitive to this radiation. The longer the wavelength the less it is scattered by particles in the sewage. However if the wavelength becomes too long the light

begins to be absorbed by the water. Near infra-red (880nm) was

considered the optimum wavelength to use. Any large objects in the flow appeared as dark shadows on the video image. By counting the

numbers of shadows it was hoped to get an estimate of the

quantities of large objects in the flow.

By opening and closing a pair of pneumatic valves in the sewer, sewage could be pumped either from the inlet channel or from the spilling effluent. Initially the sample was taken from near the bottom of the spill channel, but in April 1989 the inlet was moved to just inside the weir, after sediment on the floor of the channel was suspected of becoming re-entrained and biasing samples.

Originally the pipe drawing sewage from the inflow was cut off at an angle into the flow (Fig.4.6i) and that drawing from the spill was cut off horizontally just inside, and just below, the weir

(Fig.4.6iii). However there was uncertainty as to how representative

the samples would be, so after 8 months the inflow pipe inlet was replaced by a longer pipe with three entry points (Fig.4.6ii) in order to draw a mixed sample from the flow at different levels. It was also suspected that the horizontal end of the spill sample pipe may not be drawing samples in quite the same way as the angled inflow pipe, which may be biasing the results. In order to investigate this, a new inlet with an angled entry was fitted (Fig.4.6iv).

When the equipment was triggered, it began pumping sewage from the inlet at a fast rate. It allowed three minutes for the sewage to reach the viewing cell and the air pockets to be cleared before slowing down for one minute, to allow a better view of the sewage. Then the pneumatic valves were adjusted so as to suck from the

spill, and the cycle repeated, alternatively sampling from the

inlet and spill. During the slow periods one of two tones was recorded on the audio channel of the video tape to distinguish between images of inlet and spilling sewage.

The operation of the equipment was run by a microprocessor in the

the components on and off as necessary. It could detect a number of faults, such as problems with the pump or the pit filling up with sewage. In such cases it would close the system down and sound an alarm if necessary. If it detected a blockage in the pipe, it would reverse the pump in an attempt to dislodge it. If this was unsuccesful it would turn everything off. When testing this facility it was discovered that the perspex viewing tube originally installed wasn't strong enough, as it burst. It was been replaced with a steel tube with stronger windows, and pressure sensors were fitted which would cause the pump to be turned off it the pressure went too high.

4.4.3 Method of Counting Gross Solids

The GSM equipment was triggered by the Conquest when the swingmeters in the sewer indicated the vortex was spilling.

For each storm event the equipment produced a video tape with two minutes of useful film for every eight minutes of spill, the slow periods being identified by the tones on the audio channel.

Originally it was hoped to develop an automatic method of analysis

of these tapes. This involved playing the tape into a 'Sight Systems'

image processing system connected to a computer. The computer could use the tones on the audio channel to identify the parts of the tape to examine. Images from the tape were converted into digital format, in an array of 256 x 256 pixels, each of which had a brightness level

of 0 - 63. Software written by Foster-Finlay was used to attempt to

pick out and measure dark areas corresponding to objects in the sewage. Routines were available to capture images from the tape and

perform various manipulations, such as subtracting background

images, smoothing, threshholding to highlight areas darker than a given brightness and measuring routines to count the size and number of highlighted areas. These procedures were combined in various ways in order to find the best method of analysing the films.

Because of difficulties in getting the automatic system working satisfactorily, each tape was analysed by eye. This involved playing the sections of the tapes labelled with tones and counting the number of objects passing in the minute. Objects appearing larger than about 2-3mm were counted, and each section was examined several times to get an average. Those sections with more objects, or those that were

harder to count for some reason, were examined more times. The counts were put into the computer database program to be incorporated with the other data.

4.4.4 Assessing the Repeatability of Counting Objects by Eye

To have confidence in results produced by counting shadows on a minute of tape by eye, it was necessary to examine how repeatable the counts are. To test this two fellow research students watched several sections of film, noting the number of objects they counted. Each section was played several times and the mean result (excluding the occasional outliers) was taken. The results from different people were then compared.

4.5 Attempts to Examine the Gross Solids in the Sewage