All about soil bed sand and sand for reinforced/augmented infiltration

Sometimes you need to help the wastewater to infiltrate, sometimes you need to slow down the speed.
Then you can use soil bed sand as a reinforcement layer. Sometimes it is not possible to infiltrate and you need to construct a
soil bed with suitable material.

Guidance documents and factsheets on ground-based facilities often assume the use of natural gravel in the proportions 0-8 mm, known as soil bedding sand. However, natural gravel is difficult to obtain in many places.

A particle size analysis, sometimes called sieve analysis, gives a picture of the particle size distribution of a mixed material such as soil bedding sand or crushed material. The curve from a particle size analysis gives a particle size curve that should stay within the blue field in Figure 1 to be suitable for soil-based treatment.
There are currently no established limits for crushed material alone.

Is there suitable crushing material?
Find out if there is crushing material at nearby quarries that might be suitable.
Refer to Figure 1 in this info sheet to show the boundaries.

In many cases, the producer of crushed material needs to adapt his product range in order to produce crushed material suitable for land-based wastewater treatment plants.
This may involve mixing different types of fractions to obtain a suitable grain size distribution or treating the material
to remove the very smallest particles. For terrestrial treatment, you want to remove the really small particles that are smaller than 0.063 mm, known as fines.

Tents have different ways of ensuring the quality of their material and different conditions for producing suitable material.
This means that you as a contractor usually need to do your own tests on the crushed material to ensure the right quality.

Water permeability data?
Find out if the quarry can show the water permeability of the crushed material. The most common way is via a grain size curve or a water permeability measurement. You will probably need this information for the permit application and the inspection report as documentation of the finished plant.

If the quarry cannot demonstrate the water permeability of the crushed material, you can send samples for your own grain size analysis. The grain size curve should be within the limits indicated in Figure 1.

Often the quarry has other purposes for its quality control than whether the crushed material is suitable for wastewater treatment and their own quality control usually does not relate to delivered material.
In addition, the characteristics of the crushed material may differ between different batches and may differ in time between the time the control is carried out until it is sold or delivered.

Other factors affecting quality control are how the material is handled and stored. Any handling of material, such as moving and loading, creates a risk of segregation of the material (increased proportion of fine material) which can affect the infiltration capacity.

If the crushed material is stored over a long period of time, smaller particles may ‘stick’ to the larger particles and be more difficult to wash off. In the case of
long storage, the material can also become layered. This also applies to soil bedding sand.

Biomodules change the conditions for water permeability
The boundaries shown in Figure 1 apply to traditionally constructed ground-based installations.
Installations with biomodules work slightly differently. The instructions for these mention gravel fractions 2-4, 2-8 and similar. These fractions are likely to be more permeable than materials that stay within the grain size curve in Figure 1.

LTAR value can complement grain size analysis
If the material is outside the grain size limits as shown in Figure 1, you can measure the water permeability to see if the crushed material might still be suitable.

You measure the permeability of crushed materials in accordance with EN 12566-2 section B3.2.2 (SiS).
There are also test kits with instructions for obtaining the LTAR value. The technical report EN 12566-2, section C.2.2 (SiS), states that LTAR 50-150 can work.

However, the experts we have been in contact with for this information sheet believe that LTAR 30-75 can be a suitable range and a good recommendation for the LTAR value. High LTAR values can result in too high permeability and jeopardize infection control.
You always need to make your own assessment and ensure that the material is suitable based on grain size analysis or LTAR value. Sometimes both are needed.

Multiple measurements facilitate the assessment of the material
It is easy to get different LTAR values from the same batch of crushed material, even different values from the same sample.
It seems to be a bit more difficult to get reliable measurements from measurements on crushed material and there is a greater risk of the fine material being washed off after the first test, which affects the following tests.

Therefore, you should measure the permeability of several samples of the same material and measure several times in the same sample.
Multiple measurements facilitate the assessment of the material

Record the LTAR values for each sample so that it can serve as a basis in, for example, the permit application and inspection report.

Multiple samples and multiple measurements
Crushed material: take more than three samples from the material and perform the measurement 3-5 times in the same sample.
Soil bed sand: take at least three samples from the material and perform the measurement at least 3 times in the same sample.

In-depth study on the properties and treatment capacity of crushed material
Grain size distribution Crushed material 0-8 mm usually has a higher proportion of fine material with a grain size smaller than 0.063 mm compared to subsoil sand. Too large a proportion of such fine-grained material can lead to insufficient permeability and a risk of clogging in the filter material.

Cubized crushed material
Some quarries can process crushed material to grind and round off the edges of splintery grains, making the material less likely to pack together. This is called cubicizing and is used to adapt crushed material to be able to replace, among other things, soil bed sand in concrete production.
Cubicization has not been further evaluated in the studies on crushed material in wastewater treatment.
The intention, to make crushed material resemble soil bed sand, is obviously interesting to study further.

The treatment of infectious agents in crushed material
is considered equivalent to soil bedding sand in permanent housing. However, the start-up time for purification of infectious agents
seems to be longer for crushed material in traditional plants.

Phosphorus removal may be worse for crushed material
than for soil bed sand according to the studies done so far. Crushed materials can have different chemical properties and there are
uncertainties about how chemical binding of phosphorus works. If a land-based plant has supplementary phosphorus removal, crushed material can be expected to perform as well as soil bed sand.

The removal of nitrogen and organic matter
in crushed material is considered equivalent to soil bed sand.

Porosity
Refers to the volume of the material occupied by voids and can affect water permeability and the
purification efficiency of the material. Porosity is affected by the shape of the grains, the grain size distribution and especially the degree of compaction. The shape of the grains and the distribution of grain size in crushed materials often differ from natural gravel, which means that the porosity can be different.
It is particularly important that beds of crushed materials are not compacted too hard. The Swedish Environmental Protection Agency’s fact sheet 8147 generally describes how
the material should be handled in terms of compaction. If you follow these recommendations, any differences in the porosity of the crushed material are unlikely to affect the function in a decisive way.

More porous in loose compaction but less
porous in hard compaction In the crushed material, the grains become more chipped, elongated and angular, compared to the rounded
grains of a natural gravel. A crushed material can therefore be more porous than an equivalent fraction size of natural gravel in loose compaction.
If the material is compacted harder, crushed material can become less porous if the elongated grains end up lengthwise with each other.
This makes it more difficult to predict the porosity of the crushed material and thus to some extent its permeability.

Are there hazardous substances in the crushed material?
Both the municipal environmental office and you who intend to use crushed material for wastewater treatment should be aware if a local quarry
is associated with rocks containing hazardous substances. The quarry should have an idea of this, so check with them in the first
instance. The content of possible hazardous substances depends on the mineral in the rock being crushed.
Examples of substances to be aware of are sulphide minerals. Arsenic, lead, cadmium, copper, chromium, mercury, nickel, zinc and
others may be present but are rare. This issue is the same for natural sands whose chemical composition also varies with rock type.