Infrastructure can be expensive, not only fiscally but also environmentally. Buildings, the most visible examples, account for nearly half of all US energy consumption in their construction and ongoing usage. Consider a less obvious but still essential infrastructure component: the ubiquitous urban sewer access point. Our life-cycle assessments (LCAs) show the dramatically large life-cycle carbon footprint savings that could be achieved by choosing plastic maintenance shafts over traditional concrete manholes.
Systems modeled in this study
The following figures illustrate the supply chains modeled in our LCAs of the Poo Pit™ plastic maintenance shaft from Quickstream Solutions North America and the equivalent concrete manhole. The LCA system boundary is cradle-to-grave in both cases.
The Poo Pit™ is manufactured using HDPE plastic sourced from China that is then shipped to Indiana for injection molding and sub-assembly, as shown in Figure 1. Final assembly with additional PVC pipe fittings and a rubber gasket occurs in Windsor, Ontario, the company’s headquarters. The sourcing of these smaller components was unknown, so for now we make a reasonable placeholder distance (500 km) in line with other non-imported components. Although the location of install sites will vary, we set a moderate distance (200 km) from the headquarters as representative. While at 22 Kg the Poo Pit™ is sufficiently light to be carried by hand, excavators and dump trucks are needed to remove debris, haul it away, and then truck in the backfill, resulting significant usage of diesel fuel. When it is time to remove the shaft at its end of life, we assume the disposal site is a short (50 km) distance away.
Figure 2 captures an equivalent concrete manhole. The steel rebar comes from China, but the ingredients to make concrete are sourced far more locally. To keep the comparison consistent with the Poo Pit™ we utilize identical sourcing for the PVC fittings and gasket, as well as the distances to the install and disposal sites. Given that this product is substantially heavier, at over 5 tonnes, and displaces more excavation debris, more fuel is used at the installation site. Later, we will consider the energy used for wastewater treatment.
LCA tool and LCI database
The three life-cycle impact categories CarbonScope quantifies are embodied carbon (Kg CO2e), embodied energy (MJ) and embodied water (L), but to keep the comparisons simple, we report only embodied carbon. Figure 3 breaks down the embodied carbon by category and shows that the majority of the 130 Kg of embodied carbon associated with a Poo Pit™ is attributed to the energy used at installation. The next largest impact comes from the material inflows, which are mostly (80%) due to the HDPE plastic body, so the placeholder data for the other components is justifiable. Transport and processing energy have only a small part of the total share of the embodied carbon.
At 1790 Kg CO2e, the concrete equivalent has over thirteen times the carbon footprint of a Poo Pit™. Although nearly four times as much diesel fuel is burned during installation, installation energy is only the second largest impact. Figure 4 shows that material inflows have the greatest share of the embodied carbon. In particular, Portland cement is carbon-intensive and contributes half the total carbon footprint, and the steel rebar contributes 15%. The other ingredients contribute little themselves except that their massive weight leads to greater transportation emissions: Hauling 5 tonnes even short distances with efficient transport modes still takes a toll.
The vast embodied carbon difference between these two shafts is further magnified when one considers their lifespans. Plastic is more durable than concrete and Poo Pits™ are rated to last 100 years, twice the lifespan of their concrete equivalents. A more appropriate comparison of embodied carbon would double the carbon footprint of the concrete equivalent, and Table 1 shows that with this accounting a Poo Pit™ has less than 4% the embodied carbon of the concrete equivalent.
Furthermore, plastic’s impermeability provides another benefit over concrete: no infiltration of rainwater runoff. Rainwater seeps into concrete manholes, up to a rate of 0.8 liters per second, and must then be processed at a wastewater treatment center, requiring energy (0.00032 kWh per liter). The increased inflow associated with building out new waste management infrastructure can often restrict their development. The amount of rainwater that will need to be treated over 100 years would vary with the climate and topology of an install site.
We thus consider two scenarios in Table 1 that vary by an order of magnitude: a wet one where one of ten days has intensive rainwater infiltration, and a much dryer one where only one out of 100 days experiences such infiltration. In either scenario the treatment of the wastewater becomes the dominant contributor of carbon emissions. So once rainwater infiltration is considered a Poo Pit™ will have between 0.73% to 2.60% the carbon footprint of their concrete equivalent.
The analysis assumes the installation occurs in Canada, where the electric grid is less carbon-intensive than many countries. Most install sites in the United States would result in even a larger carbon footprint for concrete manholes and higher emission savings for Poo Pit maintenance shafts.
Plastic’s inherent durability creates problems when used in single-use disposable packaging but benefits infrastructure projects, and its impermeability prevents rainwater infiltration. The long life of infrastructure means that even small usage factors add up over time. For buildings, major energy savings come from modest tweaks in the design to better utilize natural light or ventilation. For waste management, the unwanted infiltration of rainwater that occurs with concrete may seem like small drops in a bucket, but over 100 years those drops add up.
A comparison of the two sewer access systems via CarbonScope shows that installing a Poo Pit™ shaft instead of its concrete counterpart will reduce the carbon footprint by an immense factor: 38 to 137 times. There are about 20 million manholes in the US alone, many of which are in need of rehabilitation or replacement. If just half of them are replaced with plastic maintenance shafts, we could potentially save 150 to 540 million metric tonnes of CO2e over a 100-year period.
Real environmental benefits can indeed be realized with a long term, systematic approach to designing infrastructure projects. Using LCAs as a decision-making tool in infrastructure development is both easy and practical as we have shown here.