ENVISIONing the LCA of a wastewater treatment plant

A Life Cycle Assessment (LCA) is a ‘cradle to grave’ assessment of the environmental impact that a product, process or set of activities has during its whole life (Bauman & Tillman 2004 and ISO 2006). The assessment includes direct environmental impacts from the use of the process and also its indirect impacts due to the manufacture and transportation of the required raw materials, chemicals and equipment. The LCA attempts to build a comprehensive, life cycle accounting of the resource use and global environmental impacts accumulated by the abstraction of raw materials from the environment all the way to the final emissions and disposal of materials back to the environment. It is a truly ‘holistic’ method for assessing environmental impacts.

There are several programs that can be used to carry out an LCA for a product or process, which are listed in Table 1 below.

Envision is a tool that uses an objective framework of criteria for the sustainable planning, design, construction and operation of infrastructure projects (ISI 2012). It can be used for such things as bridges, dams, water and wastewater treatment plants, and other civil infrastructure. It is not intended to be used for buildings and facilities that are more commonly assessed using programs such as LEED (Leadership in Energy and Environmental Design). Envision is a system with a broad perspective that recognizes that the sustainability of a project must be considered from the planning stage through the lifecycle of the project.

Envision consists of up to 60 credits categorized into 5 categories: Quality of Life; Leadership; Resource Allocation; Natural World; and Climate and Risk. LCA could be used to assist in assessing impacts across all of these categories, but is particularly useful for quantifying credits in the 2 categories of Resource Allocation and Climate and Risk. Whilst not mandating its use, Envision recognizes LCA as a useful tool that can be used to estimate the impacts of a project on the environment in general and also identifies specific credits for which an LCA could be used.

Table 2 presents a list of Envision credits that explicitly mention the use of LCA to assess sustainability improvements or require mass flows to be quantified, which can be effectively done using LCA.

In previous work (Tarallo et al. 2011), LCA was used for the Blue Plains Advance Wastewater Treatment Facility (AWTF) in Washington, D.C., USA as a tool of process selection to achieve lower effluent total nitrogen values in their two stage activated sludge system. Figure 1 presents the block diagram for the system used for the analysis.

In the original analysis two different scenarios were analysed for this 370 million gallons per day (MGD) nutrient removal facility. The baseline case (or the no-bioaugmentation alternative) sent the nitrified wasting activated sludge (NWAS) to the biosolids handling facilities for further treatment. Alternative 1 – the bioaugmentation alternative – recycled the NWAS from the second stage activated sludge tank back to the first stage for further nitrogen removal. The LCA boundary was set around the biological treatment processes.

Subsequent to the original study, a further, hypothetical, ‘Alternative 2’ was also analysed, wherein the AWTF was hypothesized to be energy neutral and the biogas produced at the plant is used for all the facility needs. In addition, the methanol added to the denitrification tank for the baseline case was also replaced by biogenic methanol produced from biofuel. This is an assumed scenario for this paper and was not considered during the original study phase for the facility.

SimaPro developed by PRé was used as the LCA tool while the sustainability guidelines developed by Institute for Sustainable Infrastructure (ISI) were used to develop the Envision points. The overall results from the LCA are presented in Figure 2, which shows the weighted impacts of using Alternative 1 over the baseline conditions within various categories.

The LCA outputs and the corresponding Envision scores for Alternative 1 are presented in Table 3, along with the hypothetical results for Alternative 2 in parentheses. The levels of achievement for various categories were developed using the percent improvements, with both alternatives over the baseline condition. Since the LCA categories corresponding to the Climate & Risk category of Envision (CR 1.2) have minimum impact on the total LCA (Figure 2), it was considered that the alternatives will have negligible impacts on the air quality. Therefore, ‘Conserving’ and ‘Restorative’ levels were assigned to Alternatives 1 and 2 respectively. The percent improvements of both the alternatives were used to determine the levels of achievement and finally the Envision scores. The table indicates that Alternatives 1 and 2 have respectively achieved 27% and 56% improvement over the baseline for these categories. Based on Envision recognition levels, this corresponds to a significant contribution toward a Bronze and Platinum award for the two cases if extrapolated to the total score, though the actual award level would be based on the overall percentage of points achieved for all categories, including those not directly measured using the LCA.

LCA is an holistic method for assessing environmental impacts. Envision is a broad-ranging tool that can be used to assess the sustainability of infrastructure projects. Coupling the two together provides a robust method not only to assess sustainability, but to investigate methods for improving the sustainability of a project across multiple objectives. An example is presented to show how this was carried out to show the positive environmental impact of bioaugmentation at a used water resource recovery facility, and how incorporation of biogenic energy and material sourcing could further improve sustainability.