Suspended solids is widely used as an indicator of water purity, particularly in discharge consent.

When testing by the full gravimetric method, a balance, oven and desiccator are required to thoroughly dry the isolated solids until a constant weight is found. (For more details on this, see our previous post about the difference between gravimetric testing and other methods.)

It may be tempting to use a moisture balance to speed up this process, but this may not be suitable for the job.

We have taken a look at when a moisture balance can be used, and why it may not be right for this type of analysis.

What is a moisture balance?

Moisture balances combine a balance or weighing scale with a heating element, and are used to determine the water content of solids.

They are extensively used in manufacturing for quality control of food, seeds, medical creams, paper or other products, to ensure each batch has the same % of water present.

The heating and re-weighing is all conducted in one unit, so for basic % water analysis of solids they are an ideal all-in-one unit.

Can they be used for suspended solids testing?

As part of the gravimetric analysis of suspended solids, the collected sediments have to be dried thoroughly. This ensures that only the weight of the solids themselves, not any water, is reported.

A moisture balance is not usually suitable for this part of the test, this is for several reasons;

• The readability of moisture balances is not high enough
  • Since the amount of suspended solids is usually very small, a 4 decimal place balance analytical balance (i.e. can read to 0.0001g) is required to detect the tiny differences between batches. Moisture balances are designed for quality control in manufacturing, so they will usually read to 2 or 3 decimal places. This is an ideal level of accuracy for quality control, but is not sufficient for the very small differences found in suspended solids testing.
• The prolonged heating required will put stress on the moisture balance
  • Following a full gravimetric method of suspended solids testing you would leave the sediment drying in an oven for hours. Moisture balances are simply not designed for heating for such long periods of time as this.
• It is not specified in official methods
  • The Environment Agency Blue Book (aka the 1980 SCA Blue Book) details preferred test methods for environmental reporting. Many sites are measuring suspended solids for reporting to the environment agency, and so should use the higher accuracy method of a 4 decimal place balance, desiccator and oven. See our previous post for a full list of the equipment needed.

Is a moisture balance ever suitable for suspended solid testing?

The only type of suspended solids testing where a moisture balance may be suitable is in cases where the suspended solid amount is very high – e.g. in sludge.

This will mean that the overall amount weighed is higher, so the weighing accuracy is of lesser importance.

Even in these cases it is recommended that you liaise with your Environment Agency officer to see if they will accept this method.

For more information on suspended solids testing, or moisture balance applications, follow the links above or contact us;