TEST METHOD: Suspended solids dried at 103-105oC

Revised: 30 January 2008 (addition of filter size and more advice on filtration volumes)

OVERVIEW

Suspended solids are an important indicator of water quality. A well mixed sample is filtered through a weighed standard glass-fibre filter and the retained residue dried and reweighed.

TASK SAFETY REQUIREMENTS

Lab coat and safety shoes are mandatory. Use appropriate PPE.

SPECIFIC JOB STEPS HAZARDS WITHIN THIS STEP HAZARD CONTROL
Turn on the analytical scales (0.1mg) and allow to equilibrate. Weighing too soon is inaccurate. Allow twenty minutes for equilibration.
Apparatus required in addition to analytical scales:
  • Induction stirrer,
  • glass-fibre filters (GD 120, to retain particles >2.7micron)
  • sartorius filtration set,
  • beakers,
  • drying oven,
  • dessicator,
  • wide bore pipettes,
  • deionised water,
  • vacumn apparatus,
  • aluminium weighing pans or pieces of aluminium foil.
   
Weigh filter and weighing pan, and record weight. Apply filter to sartorius unit, wrinkled side up. Start vacumn. Filter may not seat properly Seat filter by wetting it with a little distilled water first.
Continuously stir sample while pipetting a measured volume onto centre of filter. After filtration, wash sample through with five 10mL volumes of distilled water (3 volumes if sample is 'fresh' water).

Too small a volume is used to gain an accurate measure.

Sometimes your volume may be too large and your filter may clog up.

Sufficient sample to produce between 10mg and 200mg of solids should be used. Use 'typical' values from Table 1 below to decide what volume is required.

Start again with a smaller volume

Where samples contain high levels of salts, they should be diluted to seawater equivalence PRIOR to sampling. For example, bitterns needs diluting at least 1:10.

Very saline water is viscous and will not filter or rinse.

The predilution prevents the measurement of salts as 'solids'. It is necessary to record the dilution amount so that the final calculation can be corrected.
Remove filter carefully from Sartorius unit and transfer to the weighed pan. Dry for at least one hour at 103-105oC, cool in a dessicator to balance temperature and weigh. Repeat the drying/weighing cycle until the weight change is less than 0.5mg. Weighing while the sample is hot gives an unstable reading. Use the dessicator to cool the sample to the same temperature as the balance.
Calculate as follows:
mg total suspended solids/L = [(A-B)x1000] / sample volume in mL
where
A = weight of filter + dried residue in mg
B = weight of filter in mg

Table 1 - Typical suspended solids loads for common waters:

Marine waters
5-80mg/L, typ 25, so filter a litre
River waters
0-200mg/L, headwaters frequently <10, Lower Murray may be 200, so filter a litre of very clear headwaters, ranging down to about 100mL for very turbid lower Murray water
Stormwaters
80-100mg/L, so filter between 200mL and a litre
Raw sewage
200-350mg/L, so filter between 100mL and 500mL
Treated sewage
100-200mg/L, so filter between 200mL and 750mL

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