Diaz-Elsayed, N., Xiaofan, X., Balaguer-Barbosa, M., Zhang, Q. (2017). An evaluation of the sustainability of onsite wastewater treatment systems for nutrient management. Water Research 121: 186-196. DOI: http://www.sciencedirect.com/science/article/pii/S0043135417303615
Where does your waste go?
Have you ever considered the fate of your waste? For many households in America, when water is rinsed down drains or flushed down toilets, it is sent to onsite wastewater treatment systems (OWTS), commonly referred to as septic systems. These OWTS process the wastewater, removing some harmful materials before the water enters the environment. OWTS are especially critical in coastal ecosystems because of the high levels of nutrients, such as phosphorous and nitrogen, in wastewater. In coastal waters, nitrogen is the primary factor that limits the growth of algae. So, when excess nitrogen enters the environment, a process known as eutrophication, algae can grow rapidly. Eutrophication boosts algae growth, and can lead to oxygen depletion when the algae dies and decomposes. Oxygen-depleted waters can lead to the deaths of fish, shellfish, mammals, and shore birds, which in turn can harm local economies by disrupting activities such as commercial fishing and beach tourism.
Wastewater and how we treat it
Many households rely on conventional OWTS, such as septic tanks, for wastewater treatment, which primarily facilitate the separation of solids from wastewater. However, in order to prevent nitrogen-rich wastewater from getting into coastal waters, advanced OWTS have been designed specifically for nitrogen removal. Advanced OWTS remove nitrogen by facilitating several processes of the nitrogen cycle in their various zones (i.e. compartments). The nitrogen cycle is a natural process that breaks down and removes nitrogen (check out this article for a brief overview of the nitrogen cycle). Some zones of the advanced OWTS facilitate the process of nitrification, which is the conversion of ammonium (NH4+) to nitrate (NO3–) while others facilitate denitrification. Denitrification is the final step of the nitrogen cycle; it removes nitrogen from the wastewater and releases it into the atmosphere as either nitrogen gas (N2) or nitrous oxide (N2O), preventing it from entering coastal waters.
How well do OWTS manage nutrients?
Researchers in Florida recently performed a study comparing the nitrogen-removal capabilities of conventional vs. advanced OWTS throughout the Tampa Bay area. Consistent with their designed intent, advanced OWTS removed approximately 35% more nitrogen from wastewater than the conventional systems. As their superior nutrient management capabilities would suggest, advanced systems also produced wastewater that resulted in less than 40% of the eutrophication caused by the wastewater processed by conventional systems.
This study evaluated the sustainability of conventional vs. advanced OWTS by comparing not only their impact on the environment, but also their economic impact on the population. Researchers found that the average costs associated with installing and maintaining advanced OWTS are higher than those of conventional OWTS. They also discovered that despite their lower contributions to eutrophication, conventional systems outperformed advanced systems in other environmental areas. For example, some advanced OWTS exert a much higher energy demand than conventional OWTS, suggesting that they may have a larger impact on fossil fuel usage and climate change. Furthermore, some of the materials used to build advanced OWTS are more toxic to people who must handle these materials during the building process.
Weighing the pros and cons
While there are benefits and drawbacks to both conventional and advanced OWTS, a global shift towards advanced systems is on the horizon. In many coastal neighborhoods, advanced systems are actually required in order to ensure the protection of the coastal ecosystem. This shift is due not only to an overarching goal to protect coastal waters, but also to the fact that many of the problems associated with advanced systems can be addressed. For example, a switch to safer, more energy-efficient materials and components could help make advanced systems more sustainable. As the pool of knowledge about these advanced systems continues to grow, OWTS designers and installers will be able to optimize advanced systems and help make wastewater safe for the population and the environment as a whole.