21st Century Sustainability Metrics: Assessing the Environmental Impacts of Wastewater Treatment
Last Modified: Nov 19, 2009
- Katya Bilyk, P.E., Joe Rohrbacher, P.E., Joyeeta Banerjee, P.E., Brian Henn, P.E. - Hazen and Sawyer
- Sandeep Mehrotra, P.E., Paul Pitt, P.E., PhD, Norman Bradley, P.E. - Hazen and Sawyer
- Bob Dodson
Lifecycle Assessment (LCA), often referred to as “cradle-to-grave” analysis, evaluates the environmental impact of a given product or process throughout its lifespan. As it relates to wastewater treatment, LCA can provide a comprehensive picture of environmental impacts such as energy use, carbon footprint, greenhouse gas emissions, and related costs for a variety of material selection and facility design alternatives. With increased attention on energy independence and efficiency, global warming, and sustainability, municipalities are likely to have an interest and desire to take stock of their facilities and to the extent possible, reduce their environmental impacts.
PRé Consultants of the Netherlands have developed impact modeling software, SimaPro 7.0, to perform LCAs. SimaPro is a robust product that leads the LCA market. It allows for accurate management of a project’s inputs, uses, emissions and waste material; and it offers a vast database of environmental impacts associated with thousands of industrial processes and products. For example, the components associated with producing an aluminum cover for a preliminary treatment channel are summarized in Figure 1.
The purpose of this study was to (1) quantify the environmental impact of a wastewater treatment plant by unit process using Simapro 7.0, and (2) assess the applicability of this tool to the water industry. As a result of this exercise, defendable and quantifiable recommendations about energy savings and sustainability could be provided to municipalities.
SimaPro 7.0 was used to inventory all of the resources associated with construction and operation of a 20 million gallon per day (mgd) wastewater treatment plant. The South Durham Water Reclamation Facility, located in Durham, North Carolina, was used as the basis of this analysis. This plant was chosen because its size, site layout, and nutrient limits (total nitrogen limit of 5.5 mg/L, total phosphorus limit of 0.5 mg/L) were representative of other nutrient removal facilities. A process flow diagram of the SDWRF is shown in Figure 2.
The following unit processes were cataloged in the model: influent pumping, preliminary treatment, primary treatment, activated sludge facilities, tertiary treatment, UV disinfection, anaerobic digestion, solids thickening and dewatering, odor control, pipes, and miscellaneous facilities.
Simapro quantifies environmental impacts in several ways. For example, the contribution of each unit process to an environmental impact category, expressed as a percentage of the total impact by that category, is shown in Figure 3. In this assessment, pipes at the plant account for approximately 55 percent of the mineral extraction impact associated with treating wastewater. Environmental impacts can also be displayed in a normalized fashion; for a specific category such as greenhouse gas emissions by unit process in units of mass of carbon dioxide emitted (carbon footprinting); or using one of several available total impact scoring methodologies.
A summary of environmental impacts by unit process, using Impact 2002+ methodology to weight each impact category and develop a total environmental impact score, is shown in Figure 4. This analysis concludes that the activated sludge process and anaerobic digestion have the largest overall environmental impact. Traditional energy audits of a treatment plant have typically focused on these two processes for their significant energy demands. Energy demands would likely be encompassed by the global warming and non-renewable energy categories in Figure 4. However, several other unit processes such as primary and preliminary treatment contribute nearly as much to global warming as anaerobic digestion. Modifications to these processes may present opportunities for significant cost savings as well.
Simapro has potential to serve utilities, consultants, and regulators in the water industry in many ways. For example, the regulatory sector could use this tool to approach environmental regulation more holistically by quantitatively evaluating the environmental benefits of marginal decreases in nutrient loads to receiving streams in the context of the additional impacts of construction and operation of these facilities. LCA and Simapro can also be utilized for optimum replacement of aging infrastructure, and identifying better alternatives to existing technology.
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