December 7, 2022
Capillary Concrete uses a particular recipe resulting in concrete with water permeability, which can be used for several applications. One application is as a base for natural grass turfs, where grass is grown on top of a permeable concrete layer and a water basin, enabling a closed irrigation and fertilization system. This has been tried at social green areas, such as parks and fields, but could potentially be tried for sports turfs as well.
The soil and grass are irrigated and fertilized hydroponically through the permeable concrete, removing excess water and fertilizers that would otherwise not be absorbed by the soil but emitted to surrounding water streams and ground water. This reduces the water use by up to 85 % compared to a conventional turf. The purpose is for the hydroponically irrigated natural turf to replace synthetic turfs and conventional soil-grown turfs. Synthetic turfs are often constructed in parks and other social areas to reduce maintenance but come with other challenges such as spread of micro plastics. Conventional soil-grown turfs do not spread micro plastics but require significant maintenance to keep in shape as well as water and fertilizer use. With the hydroponics solution, maintenance and general need for care is reduced significantly. The purpose of this simplified LCA-analysis is to compare the capillary concrete hydroponics solution with a natural turf to two alternatives with respect to their carbon footprint:
The practical context is assumed to be a sports turf, where sports is practiced regularly requiring more maintenance than social park turfs. Since the hydroponics solution has not yet been implemented on a sports turf, the LCA is forward-looking and not based on actual project data. The analysis has not been third-party reviewed, and there are limitations to the data comparability as is described later. The results should be interpreted accordingly.
The method used is a simplified life cycle analysis (LCA), only focusing on the carbon footprint. The methodology involves mapping greenhouse gas emissions from all phases in the lifecycle; construction, use and end-of-life through gathering an inventory of materials and processes needed and conducting an impact assessment on the full inventory.
There is a general lack of comparable inventory data regarding synthetic and soil-grown turfs. An LCA study comparing synthetic and conventional soil-grown sports turfs in a Swiss setting has been used as main source for the inventory data (Itten et al, 2020).
The study was commissioned by the city of Zurich as basis for sports field strategy and follows ISO standards 14040 and 14044 and models conventional soil-grown and synthetic sports turfs in Switzerland. In the study, inventory data was gathered both for turf alternatives by the same methodology and assumptions.
To model the effects of the hydroponic system in this setting, we have replaced the conventional soil-grown turf substructure (mostly gravel and sand) as modelled in the study with the hydroponics solution and kept several other assumptions, including material transport distances and energy use for installation. This improves comparability.
In practice, the hydroponics base layer solution replaces a base layer and drainage system that are modelled in the study by Itten et al (2020) underneath the natural turf. All other activities (maintenance, renovations, end of life scenario etc) are adopted from the study with some slight modifications as to the maintenance need since maintenance is reduced with the capillary concrete solution.
There are only a few LCA studies analyzing synthetic turfs and soil-grown turfs together, which means that the available data is scarce. Some existing studies look at synthetic turfs separately, which are difficult to use as this requires estimating how soil-grown turfs would have been constructed and maintained instead of the specific synthetic turfs considered, in that location.
There are a number of uncertainties related to this approach as well, which are explored in the sensitivity analysis.
Three scenarios have been identified and modelled over 50 years, one for each solution:
The structure and the material volumes that are assumed for a synthetic and conventional soil-grown turfs (1 and 2) are adopted fully from the beforementioned study. For the hydroponically grown grass (3), the inventory for the substructure and maintenance need are based on data provided by Capillary Concrete LLC.
The starting point for the analysis is soil excavation to provide space to install the turf. This mainly requires fuel (diesel) for excavators and transports to remove the soil.
Above this, an elastic layer is placed with flexible polyurethane foam and synthetic rubber. The rubber in the elastic layer is assumed to be recycled from rubber tires, which is an assumption adopted from the background study.
On top, the synthetic turf is laid with synthetic grass made from plastic, together with an infill mix of silica sand and ground rubber. The infill is the material put between the synthetic plastic grass blades to fix the blades upright and improve flexibility. The infill rubber is assumed to be virgin rubber in this scenario but could also be recycled. The effects of using fully recycled or fully virgin rubber is shown in the sensitivity analysis.
Diesel use for equipment during construction and transport of material to site is also included in the scenario, adopted from the study by Itten et al (2020).
The synthetic turf is assumed to have a lifetime of 10 years, after which it is replaced. This applies to the top layer - the base layer and flexible layer are assumed to last for the entire modeling period. As the calculation period is 50 years, the synthetic turf requires 4 complete renovations during this period.
In addition to the complete replacement every 10 years; smaller, annual infill refills are assumed, where approximately 1,6 % of the total original rubber volume is added each year.
All disposed materials are either recycled, incinerated or landfilled depending on the conventional treatment for each material. This includes the substructure and base layer. The synthetic turf is assumed to be incinerated at end-of-life.
1.3 SCENARIO 2: CONVENTIONAL SOIL-GROWN TURF WITH A DRAINAGE LAYER
The conventional soil-grown turf starts with excavation just like the synthetic turf. The substructure consists of a gravel base layer and drainage system with plastic tubes. Above this, a drainage layer of sand and gravel is laid, and the natural turf after that. There is no concrete or asphalt used.
The natural turf is assumed to arrive pre-grown for approximately 12 months. During its assumed lifetime of 15 years, it is irrigated and fertilized regularly. When renovated, the turf layer is removed and exchanged for a new, 12-month pre-grown turf.
There are several annual maintenance activities included, such as fertilization, lawn mowing, removing leaves, top dressing, pest management et cetera. We refer to the study by Itten et al (2020) for the complete list, which has been adopted here in its entirety.
At the end of the analysis period, all materials are disposed of and either recycled, incinerated or landfilled depending on the material.
The natural grass sequesters carbon as it grows, some of which is released back to the atmosphere when the lawn is mowed and the grass is left to molder or is incinerated, and some of which is stored in the soil over time. Whether this is a permanent net sink of carbon is dependent on the turf management and end of life scenario. If the soil is excavated and moved at end of life, much of the carbon stored in the soil will be oxidized and released back to the atmosphere.
Although uncertain, we include an assumption of annual sequestration to indicate the order of magnitude from this effect and display the results both including and excluding sequestration. This is not included in the study by Itten et al (2020) but is added to show the potential from sequestration rather than asserting that it is certain. An assumption of annual sequestration of 100 gC per square meter is assumed, based on estimates from Braun and Bremer (2019). This is equal to sequestration of approximately 0,37 kg CO2 per square meter and year.
The permeable concrete solution and the basin underneath replaces the base layer and drainage layer described for the synthetic turf and conventional soil-grown turf. The same emissions from excavation are assumed for scenario 3 as for scenarios 1 and 2.
The solution consists of an impermeable liner that is placed on the soil, over which drainage gravel is placed together with drainpipes. On top of this lay permeable concrete and sand, on which the natural turf is placed. The hydroponically grown turf consists of less organic material (soil) than a conventional natural turf as the nutrients and water are applied hydroponically, which reduces the need for soil.
The materials needed for the hydroponics solution are based on an actual application of the solution in Helsingborg, Sweden, including the annual use of water and fertilizers. The structure would be the same regardless of the turf purpose, so it is believed that this data set is representative for a sports turf setting as well. The energy use for installation, transport distances for the materials and disposal scenario are all adopted from the study by Itten et al (2020), even though the hydroponics solution was never implemented there. This was done to simulate the same geographical boundaries for the two systems, i.e., installed in the same location, to improve the comparison. The same background data has been used in both scenarios for the same reason, i.e., the emissions arising from production of concrete. The annual maintenance activities needed for the capillary concrete solution have been adapted from the Swiss study based on empirical data. The adaptations made are as follows.
The turf is assumed to have a lifetime of 25 years, and thus requires 1 full turf renovation. The substructure is not renovated, only the turf. The reason for the longer lifetime compared to the conventional soil-grown turf is that the conventional soil quality deteriorates faster over time. The hydroponic feeding of nutrients and water does not deteriorate as fast. After 50 years, the turf, including the whole substructure, is fully disposed of just as in all other scenarios. The same assumptions regarding sequestration are assumed in scenario 3 as in scenario 2.
Figure 2 and table 1 show the contribution to climate change from each scenario. The hydroponics solution has a carbon footprint estimated to around 65 kg CO2e/square meter turf when including sequestration (84 kg CO2e/square meter excluding sequestration) compared to around 341 kg CO2e/square meter for synthetic turf over 50 years, and approximately 76 kg/square meter conventional soil-grown turf with a drainage layer including sequestration (94 kg CO2e/m2 without sequestration).
The impact from the substructure (all equipment below the actual turf) is similar for the capillary hydroponics solution compared to the artificial turf scenario. Both solutions use significant concrete volumes which is carbon intensive, in comparison to the conventional soil-grown turf solution which does not require any concrete, asphalt or similar in the substructure – only gravel and sand – giving that scenario a relatively low substructure footprint. The production of the synthetic turf is associated with a much larger carbon footprint than a soil-grown turf as production of plastic and rubber is energy intensive and produce significant emissions. It should be mentioned that the emission factors used are average values for a European setting, which can vary between producers and thus both increase of decrease the impact. The carbon footprint from the full installation of the systems (substructure and turf) and production of all the materials is highest for the synthetic turf, followed by the hydroponically grown turf and the conventional soil-grown turf solutions respectively.
The footprint from annual maintenance is significant in all scenarios. The annual maintenance for the synthetic turf consists of refill of rubber, as well as several other activities mostly requiring fuel. In both scenarios containing a natural turf, the maintenance consists of irrigation, fertilizer use as well as fuel for all other activities mentioned in chapter 1.4. As the maintenance need is reduced with the hydroponics system, the carbon footprint from annual maintenance is lower compared to the conventional soil-grown turf.
The renovation stage carries a significant footprint for the synthetic turf solution as the turf is replaced four times during the 50-year period, giving rise to emissions from production of more rubber and plastic, and disposing of the old turf. The disposal stage also results in significant emissions from the synthetic turf, much larger than for the conventional soil-grown turf. This is primarily caused by the plastic being incinerated, which is energy intensive.
The analysis shows that under the conditions assumed from the study by Itten et al (2020), the hydroponics solution carries a significantly lower carbon footprint over 50 years compared to a synthetic turf and a slightly lower footprint compared to a conventional soil-grown turf. There are a number of assumptions and circumstances that have been adopted from the study by Itten et al (2020) worth noting. In table 2, we analyze a number of the assumptions made to see the impact on the carbon footprint. One such circumstance is the fact that the infill rubber in the turf is assumed to be virgin rubber. It is possible to use recycled rubber for the infill, which changes the carbon footprint. Another such circumstance is the lifetime of the synthetic turf, which is assumed to be 10 years but may last longer (different figures are reported in the literature)). In table 2, we analyze the effect of the lifetime being 15 years for the synthetic turf, requiring three full renovations rather than four.
It is clear that the results are sensitive to the assumptions made. By using only recycled rubber and assuming a lifetime of 15 years instead of 10, the carbon footprint per square meter is reduced by approximately 120 kg CO2e, or by about 36 %. The technical assumptions and application of the turf are thus important, and the carbon footprint comparison will vary along with it. In general, however, it can be concluded that the hydroponics solution seems to carry a lower carbon footprint than the synthetic turf solution under the assumptions adopted in this analysis, even with favorable conditions for the synthetic turf as analyzed in table 2. Compared to a conventional soil-grown turf with a drainage layer underneath consisting of gravel and sand, the carbon footprint is marginally higher. Furthermore, this analysis has not considered the available use time for each turf solution. An artificial turf is assumed to be able to handle as much as twice as many effective use hours per year compared to a properly drained natural turf. For this reason, another relevant metric for future analyses would be to look at the carbon footprint per effective use hour, which would relate the benefits (use hours) to the costs (carbon footprint).
The calculations are based on the study by Itten et al (2020) previously mentioned as well as input data received from Capillary Concrete for the hydroponics solution as implemented in Helsingborg. Emission factors have been retrieved from the Ecoinvent 3 database (Wernet, et al., 2016) for the hydroponics solution, which is the same database used in in the background report, see more in the references chapter. Some comments about the comparison below:
The analysis provides indications that a hydroponics-based turf is favorable to a synthetic turf with respect to the contribution to climate change, and equivalent or slightly unfavorable compared to a conventional soil-grown turf. It is important to note that climate change is only one of many environmental aspects, each important to the function and well-being of ecosystems and human beings. The production of the synthetic turfs and rubber are energy and resource intensive processes and cause significant emissions upstream. On a general level, as a synthetic turf can be replaced by a natural turf without losses in quality or need for more space, replacing the synthetic materials with natural, organic materials that grow from natural processes such as photosynthesis is positive as it reduces stress on our ecosystems and frees up resources and energy to be put to use elsewhere. On a more specific level, microplastics which are released from the synthetic grass have been repeatedly found in animals both at land and at sea, which is a growing concern although the specific health effects are not entirely clear as of today1. With rising concentrations, however, it is likely that problems will arise, if not else by purely mechanical means such as disturbing (blocking) the digestion and nutritional habits of animals. Concerning the comparison to a conventional soil-grown turf – the lower water and fertilizer use with the hydroponics solution are two highly significant benefits that reduces the stress on the ground and drinking water supply and eutrophication respectively. These environmental concerns are not captured in the carbon footprint comparison.
It is important to stress that this is a simplified, forward looking LCA-study not based on primary data. The hydroponics solution has been hypothetically inserted into another system as modelled by Itten et al (2020), which brings along with is many uncertainties and the results should be regarded with that in mind. In essence, we cannot confidently draw comparative conclusions about the environmental impact based on this approach, but the results indicate that these is potential in the hydroponics solution that warrant further analysis, preferably when primary data that are comparable can be gathered from all alternatives that are up for comparison.
Braun, R, C. and Bremer, D, J. 2019. Carbon Sequestration in Zoysiagrass Turf under Different Irrigation and Fertilization Management Regimes. Agrosystems, Geosciences & Environment, 2: 1-8 180060. https://doi.org/10.2134/age2018.12.0060
Itten, R., Glauser, L and Stucki, M (2020). Ökobilanzierung von Rasensportfeldern: Natur- Kunststoff- und hybridrasen der Stadt Zürich im Vergleich für Grün Stadt Zürich. Institut für Umwelt und Natürliche Ressourcen
Wernet, G., Bauer, C., Steubing, B., Reinhard, J., Moreno-Ruiz, E., and Weidema, B., 2016. The ecoinvent database version 3 (part I): overview and methodology. The International Journal of Life Cycle Assessment, [online] 21(9), pp.1218–1230. Available at: <http://link.springer.com/10.1007/s11367-016-1087-8> [Accessed 10 12 2021].