Methods Manual for Salt Lake Studies/Suspended solids

Authors: PSJ Coleman,

Suspended solids dried at 103-105^oC

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. The main issue that makes this process tricky in salt lakes is the presence of salts in the water. If these are not rinsed completely out of the filter, the dried salts will be recorded as part of the mass of suspended solids. Very dense brines do not rinse out of the filter easily, and so a predilution step is advisable.

Apparatus required in addition to analytical scales

Induction stirrer,
glass-fibre filters (GD 120, to retain particles >2.7micron)
sartorius filtration support set or similar,
beakers,
drying oven,
dessicator,
wide bore pipettes,
deionised water,
vacumn apparatus,
aluminium weighing pans or pieces of aluminium foil.

Method

Turn on the analytical scales (0.1mg) and allow to equilibrate. Weighing too soon is inaccurate. Allow twenty minutes for equilibration.

Weigh filter and weighing pan, and record weight. Apply filter to sartorius unit, wrinkled side up. Be aware that the filter may not seat properly, so seat filter by wetting it with a little distilled water first.

Start vacumn.

Where samples contain high levels of salts, they should be diluted to seawater equivalence PRIOR to filtering. Very saline water is viscous and will neither filter easily, nor rinse from the filter. For example, waste brine from solar salterns needs diluting 1:10 with deionised water before it is of a similar salinity (and viscosity) to seawater.

The predilution prevents the measurement of filter-entrapped salts as 'solids'. It is necessary to record the dilution amount so that the final calculation can be corrected.

Continuously stir sample while pipetting a measured volume onto centre of filter. After filtration, wash sample through with five 10mL volumes of distilled water.

Remove filter carefully from the filter support 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.

Calculation

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

Things to be aware of with this method

Weighing while the sample is hot gives an unstable reading. Use the dessicator to cool the sample to the same temperature as the balance.

Sometimes your sample volume may be too large and your filter may clog up. Start again with a smaller volume. Ideally you would use sufficient sample to produce between 10mg and 200mg of dried solids. Use 'typical' values from the list below to decide what volume is required.

If too small a volume is used, you will not gain an representative measure.

List of typical suspended solids loads found in various waters:

Marine waters 5-80mg/L, typ 25, so a litre is a reasonable sample
Salt lake waters 0-80mg/L, needs diluting, filters don't deal with huge volumes so between 750mL and a litre is a reasonable sample
River waters 0-200mg/L, headwaters frequently <10, Australian lowland rivers may be 200, so sample a litre of very clear headwaters, ranging down to about 100mL for very turbid lower Murray water
Stormwaters 80-100mg/L, so sample between 200mL and a litre
Raw sewage 200-350mg/L, so sample between 100mL and 500mL
Treated sewage 100-200mg/L, so sample between 200mL and 750mL