Recommendations for crushed rock as filter material in soil beds

Information sheet – Recommendations for crushed rock as filter material in soil beds

1 Background

This information sheet has been produced with funds from the Swedish Agency for Marine and Water Management, grant 1:12 measures for the marine and aquatic environment. The information sheet’s recommendations apply primarily to the use of crushed rock as filter material in soil beds for wastewater treatment. The recommendations can also be followed when crushed rock is used as a reinforcement layer in infiltration facilities,
, however, note the exception addressed in section 5 “Recommendation – Organic matter, bacteria and phosphorus”.

Content/restrictions

Annex 1 provides the background and explanation of the study and recommendations, as well as a discussion of nitrogen purification, recycling and BET surface area. Annex 2 presents comments on the fact sheet contributed by a number of contractors and manufacturers that could not be investigated within the fact sheet study.

1.1 Purpose/objective

Natural gravel is a finite resource that is important for water supply. Sweden’s environmental quality objectives include switching from natural gravel to crushed rock where possible (“Good quality groundwater”).
The purpose of the information sheet is to simplify and ensure the quality of the use of crushed rock material in connection with the construction of land-based sewage facilities so that crushed rock can replace natural gravel in an appropriate manner in the long term.

The aim is for the fact sheet to outline technical recommendations for the use of crushed rock materials in land-based sewerage systems and the expected differences compared to natural gravel-based systems.

The information sheet is intended to serve as a tool that environmental inspectors can use when providing advice and assessing permits, and as a guide for the rock materials industry and contractors building soil beds.

1.3 Glossary of terms

Designation & Description

Crushed rock Mineral particles produced by crushing rock.

BET surface area Specific surface area is a measure of the size of the total surface of the particles (grains). BET analysis is a method for estimating the specific surface area. BET surface area
means the specific surface area estimated by BET analysis.

Fine material Particles passing through a 0.063 mm sieve.

Hydraulic load A measure of how much water is added to a soil/filter material (or other porous medium) per unit time. Expressed in mm/d or l/(m2
∙d).

Hydraulic conductivity Hydraulic conductivity is a measure of how much water a soil/filter material (or other porous medium) lets through per unit time.

Infiltration facility Facility where wastewater is treated through the soil layer. The groundwater is the receiving water.

Grain size curve A curve showing the percentage by weight of a material made up of grains up to a certain grain size.

Soil-based treatment system A facility where wastewater is treated through soil materials (collective name for soil beds and infiltration facilities).

Soil bed Also called filter bed. Facility where wastewater is treated by a limited volume of soil material. The soil bed should have a well-defined outlet to surface water.

Natural gravel Mineral particles sorted and rounded by nature. The term ‘natural gravel’ includes not only the gravel group of grains but also other groups of grains such as
e.g. sand and stone.

Porosity Measure of the volume of the material occupied by voids.

Washing (Here) To remove particles with a diameter smaller than 0.063 mm.

Water permeability Measure of how quickly a material lets water through.

Annex 3. Methods for measuring water permeability – compilation and evaluation

This annex contains a review of the advantages and disadvantages of different methods for measuring water permeability. The methods examined are:
– Permeameter test
– in 40 mm diameter pipes
– in 100 mm diameter pipes
– Permeability measurement in test pit
– Measurement by color of leachate
– Detection by color or tracers
– Ring infiltrometer (single ring and double ring)

Conclusions (read the full report under the files and media tab above the text)

In order to assess a batch of crushed rock, the quarry is recommended to carry out repeated measurements using the permeameter method with 40 mm diameter tubes (see description above). Alternatively, 100 mm tubes can be used.
In practice, it should be possible to start carrying out these measurements immediately.
In order to be able to assess the water permeability of the crushed rock material under the conditions prevailing in the soil bed (porosity, etc.), it would also be interesting to carry out field measurements of water permeability in constructed soil beds in investigations and research projects.
Double ring infiltrometer tests are recommended in the first instance. Secondly, the constant head well permeameter
test is recommended. However, in practice, more investigation of the methods and current limits will be required before these methods can be applied.