In recent years, there have been increasing concerns regarding the carbon footprint of municipal infrastructures. Carbon footprint is a measure of the environmental impact in terms of greenhouse gas (GHG) production in units of carbon dioxide. Although the rise of GHGs is a concern due to its contribution to climate change, other types of impacts can also be included for a more holistic assessment.
Life Cycle Assessment (LCA) is a tool that can effectively be used to investigate the impact of a product in a holistic manner. LCA includes the extraction of raw materials, processing, manufacturing, distribution, use, reuse, maintenance and disposal processes.
The environmental burdens that are usually evaluated include the use of land, energy, water, and other materials and the release of substances to the air, water, and soil.
Comparing Disinfection Alternatives
Wastewater treatment systems are designed to minimize the environmental impacts of discharging treated wastewater into aquatic or terrestrial ecosystems. Disinfection is one of the most important steps of the wastewater treatment process since it prevents the spread of waterborne diseases to downstream users and the environment.
This fact sheet describes a study of specific environmental impacts caused by the lifecycle of three wastewater disinfection systems:
- Chlorine gas
- Sodium hypochlorite
- Ultraviolet (UV) disinfection.
Results are summarized and allow the comparison of these disinfection systems in terms of the effects on human health and the environment throughout three stages of their life cycle: civil works, operation and disposal.
The majority of the data collected for this study was from the National Renewable Energy Laboratory and US Environmental Protection Agency.
To compare different disinfection technologies, the functional unit has been established as 20 years of operation. The specifications and assumptions for the disinfection systems of this study can be seen in Table 1.
Table 1. Specifications for LCA Example
| Peak Flow | 30 million gallons per day (4,732 m3/hr) |
| Average Flow | 15 MGD (2,366 m3/hr) |
| UVT | 65% (minimum) |
| Disinfection Limit | 200 Fecal Coliform/100ml |
The electrical composition in a region can significantly alter the environmental impact of a disinfection system. For the purpose of this study, California’s electrical composition will be used as seen in Figure 1.
Civil Works
Typical materials used in the construction of a disinfection facility are concrete, steel and wood. Concrete production is considered to have a high environmental impact because of its various constituents.
There are many different types of concrete depending on the manufacturer, desired quality and strength. For the purpose of this study, it has been assumed that generic 100% Portland concrete is used for the civil works of the disinfection systems (National Institute of Standards and Technology, 2007).
Operation
The environmental impact of the operation of a disinfection system has been measured in terms of resource consumption and waste production over a 20-year period.
The following processes have been included for the analysis:
- Production of chlorine gas and sodium hypochlorite
- Salt mining and purification
- Manufacturing of the UV lamps
- Transportation of chlorine gas, sodium hypochlorite and sulfur dioxide
- Transportation of the UV lamps
- Energy consumption at the disinfection site
Disposal
The values of solid waste for the civil works and operation phases included the disposal of concrete and UV lamp components at the end of the life cycle.
Impact Categories
The impact assessment methodology was based on midpoint characterization. The impact categories taken into consideration are summarized in Table 2. Characterization and normalization factors were obtained from the U.S. EPA Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI).
Table 2. Impact Categories for Life Cycle Assessment
| Impact Category | Characterization (US EPA) | Examples of Data |
|---|---|---|
| Ozone Depletion | kg CFC-11-eq. | CFCs, HCFCs, Halons, CH3Br |
| Global Warming | kg of CO2 eq. | CO2, NO2, CH4, CFCs. |
| Acidification | H + moles eq. | SOx, NOx, HCL, HF, NH4 |
| Eutrophication | kg of N eq. | PO4, NO, NO2, NH4 |
| Eco-Toxicity | kg 2, 4-dioxane-eq. | Toxic chemicals to rodents |
| Human Health Non-Cancer | kg toluene-eq. | Toxic chemicals to humans |
| Human Health Cancer | kg benzene-eq. | Cancer causing chemicals |
| Resource Depletion | kg of antimony eq. | Minerals and fuel used |
| Land Use (Solid Waste) | lbs. | Landfills |
In this study, the data was normalized relative to the environmental impacts caused by a served population of 50,000 people. This demonstrates the contribution from each of the disinfection systems in each environmental impact category as a fraction of the total emissions in that category.
Normalization gives a dimensionless percentage for each impact category, which allows the results to be aggregated as seen in Figure 2 (De Hass, 2008).
UV – The Green Solution
This LCA concluded that UV has the least impact compared to chlorine gas and sodium hypochlorite. The total environmental impact of UV is mostly affected by the electrical grid composition. This makes UV disinfection the most versatile option since its environmental impact is directly related to the electrical energy source.
As sources of electricity become more “green”, UV will inherently have less environmental impact. UV can also be used as a public relations tool to educate residents about the importance of protecting the environment. As a result, many communities have implemented UV disinfection as a safe, non-chemical disinfection technology to protect their water resources and to accomplish long-term sustainability goals.
References:
De Hass D, Foley J and Barr K. Greenhouse inventories from WWTP’s – The trade off with nutrient removal. Sustainability conference, Maryland, 2008. Water Environment Federation.
Environmental Protection Agency (US EPA). Life Cycle Assessment: Principles and Practice. Scientific Applications International Corporation (SAIC). National Risk Management Research Laboratory. USA. 2006.
Environmental Protection Agency (US EPA). The emissions and generation resource integrated database for 2006 (eGRID2006). Office of Atmospheric Programs, Climate Protection Partnerships Division. April 2007.
National Institute of Standards and Technology. Building for Environmental and Economic Sustainability. US Department of Commerce, 2007
National Renewable Energy Laboratory (NREL). Chlorine/Caustic Soda Production, 2007. U.S. LCI Database Project. U.S. Department of Energy. www.nrel.gov.