Sustainable alternatives to natural gravel for infiltration facilities

Background

Natural gravel is used as a filter material in ground-based facilities for wastewater treatment. However, natural gravel is a limited resource with great importance for society's drinking water supply. It is therefore important to find alternative filter materials that are functional and long-term sustainable.

This project aims to investigate which materials have the highest potential as sustainable alternatives to natural gravel for ground-based wastewater treatment. The evaluation takes into account the purification effect of alternative materials on important parameters such as BOD, phosphorus, nitrogen, infectious agents and potentially harmful substances such as pharmaceuticals and microplastics, but the evaluation also takes into account socio-economic and environmental sustainability factors. Availability of, and risks with, different filter materials are also important factors that will be investigated.

This study is based on existing literature and reports and already developed evaluation methodology as well as the existing knowledge and experience that can be obtained from experts within the project group, industry, authorities, academia and institutes. Experience and input will be gathered via meetings and workshops with designated experts. In order to ensure that both the work and final results are highly relevant and are directly useful in the development of policy and industry standards, there is a continuous exchange of knowledge with authorities and other stakeholders during the course of the project.

Literature searches

We conducted literature searches on Scopus and Web of Science to try to find all possible scientific literature relating to functional aspects of potential alternative filter materials. The search returned 1529 potentially relevant articles. These were reviewed and sorted into 36 different types of potential filter media. A selection of the identified materials can be found in the image above.

First sorting based on found experts

Experts were found for 16 of the potential alternative filter media types (See image marked first evaluation). Based on that, we organized a series of workshops and interviews with the experts to more deeply evaluate the potential of each media to function as a viable alternative to natural gravel. The experts had access to a lot of knowledge and test results that were not published in scientific literature. Data and information are collected and evaluated against predetermined criteria for functionality.

Second evaluation

Research group together with the project's reference group went through the 16 alternative materials and sorted according to a number of selection criteria to choose the 6-8 of highest relevance for deeper study on. The criteria for selection were, among other things, number of articles found in the scientific literature, known purification effect, risk of leakage of unwanted substances, degradability/durability, geographical availability and potential transport distances, amount of available material and potential climate impact of using the materials.

Based on this selection, the project developed 7 possible options:

• Moraine
• Crushed rock
• Washed excavation masses
• Crushed concrete
• Forest by-products
• Biochar
• Shredded tires

Next step

The next step in the project is to compile all the data and knowledge about these options regarding purification efficiency, mechanisms involved and the properties of the materials including identifying potential inherent risks associated with the different filter materials and taking into account socio-economic and environmental sustainability aspects. After that, we will perform a Multi Criteria Analysis (MCA) to be able to select the most relevant and up-to-date options, as well as identify any knowledge gaps that exist before recommending use to replace natural gravel for ground-based infiltration facilities.