7.1.1. Filtration Process

In order to measure the solids that are suspended in water, we need to separate the solid particles from the liquid. The most common way to do this is through a process called filtration.

Filtration is a simple separation technique used to separate solids from liquids or gases. The key idea behind filtration is to pass the mixture through a material that can trap the solid particles while allowing the liquid or gas to pass through. In our case, we want to measure total suspended solids in water, which means we need to ensure that we effectively catch all the solid particles present in the sample.

Imagine you are trying to make clear lemonade from a mixture of juice and pulp. If you pour the mixture through a fine cloth, the cloth will allow the liquid lemonade to flow through while trapping the fruit pulp. This process is very similar to how filtration works in separating solid particles from water.

When we perform filtration, we set up a filter, such as filter paper, which is capable of physically trapping the solid particles.

Setting up a filtration system involves placing filter paper or another suitable material in a filter holder. When we pour the water sample through this filter, the liquid passes into a collection vessel below, while the solid particles get trapped on the surface of the filter. The filtered liquid that passes through is called the filtrate, and the solids that remain on the filter are referred to as the residue.

Think of using a coffee filter when making coffee. As hot water pours through the coffee grounds, the filter allows the liquid coffee to collect in the pot below while trapping the coffee grounds. Just like in our filtration process, we're separating two different materials.

After the filtration, we weigh the filter paper with the trapped solids to determine the mass of the total suspended solids (TSS).

To measure the total suspended solids, we need to conduct a gravimetric analysis. This involves weighing the filter paper before filtration (which gives us the mass of the empty filter) and then weighing it again after filtration to find the mass of the filter plus the solids. The difference in these weights provides the mass of the suspended solids that were present in the water sample.

Consider a chef using a fine sieve to sift flour. The weight of the sieve alone provides a benchmark. After sifting, the chef weighs the sieve with the flour to find out how much flour is left in the sieve. To find out how much flour passed through, the chef subtracts the weight of the empty sieve from this new weight.

It's important to note that the absence of visible solids in the filtrate doesn't mean there are no suspended solids. We need to consider the detection limit of our method.

When we obtain a measurement of total suspended solids, we need to understand what it means. If the measurement shows zero solids, it could indicate true absence or it might be that the mass of suspended solids is simply below the detection limit of our method, meaning our instruments are not capable of detecting such small amounts. Thus, we must understand the limitations of our measurement technique and be cautious while drawing conclusions from the results.

Think of a person trying to capture a shadow. If the shadow is too faint, it might not be visible to the naked eye, even though there is something present. This is similar to what can happen in our filtration process: just because we don't see solids doesn't mean they are not there; they might just be too small to measure accurately.

--