Suspended solids

This is a simple but important measurement of the amount of silt in suspension in the water. The pollution control authorities often restrict the amount of suspended solids that can be present in a discharge. Suspended solids can be a particular problem resulting from quarries, building sites, new roads and ploughed land. Excessive amounts reduce the penetration of light into the water and this in turn can restrict the photosynthesis of aquatic plants. The suspended silt eventually sinks to the river bed and too much silt will clog the gaps between stones and will thus change the habitat for invertebrates and other water creatures.

Apparatus required:

Oven regulated to about 105°C Water pump to provide suction 1 litre filter flask Hartley funnel (see Figure 40) Measuring cylinder

Glass-fibre filter paper (preferably grade C) of a size compatible with the filter funnel A pair of tweezers Desiccator

An accurate balance capable of measuring to 0.1 mg


A clean glass-fibre filter paper is labelled with a number written in pencil and placed in the oven at 105°C for at least an hour. Next it is put into the desiccator to cool and then weighed accurately (weight A). The paper should be picked up with tweezers to avoid it being handled and picking up sweat from fingers which will change its weight. The dried filter paper is then put into the Hartley funnel on top of the perforated disc.

Figure 40. Hartley funnel and filter paper

Figure 40. Hartley funnel and filter paper

A pre-determined volume of one of the collected samples is mixed well and measured into the measuring cylinder. The water pump (or vacuum pump) is started and the sample then poured carefully into the funnel. When all the water has passed through the filter paper, the filter pad is rinsed with a little distilled water, taken off the filter funnel and placed back in the oven for at least two hours. After cooling in the desiccator the filter paper is re-weighed: this is weight B.

The amount of suspended solids, = (B - A) x 106 expressed as mg/l (ppm) volume of sample

The preferred volume of sample to take for this test depends on the amount of suspended solids present. If the water appears very clean, then a larger volume is required for filtering in order to obtain enough solids on the filter paper to make a weighable amount. By contrast, for a very turbid sample, only a small volume is required, otherwise it will take a long time to pass through the filter paper. As a rough guide, the following volumes are suggested for different types of samples:


Suggested volume (ml)

Clean river water Dirty river water





Clean sewage effluent Poor sewage effluent

Untreated sewage Turbid, silty water

Some laboratories will not have an analytical balance of the accuracy and sensitivity required (measurable to 0.1 mg). In this case, an indication of the suspended solids can be obtained by measuring the turbidity of the sample using a simple measuring technique.

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