By Sean Murnan
Eutrophication has become a major issue across many areas of the US and other countries, due to increased levels of nutrients entering water bodies from various sources. Eutrophication occurs when nutrients such as nitrogen and phosphorus build up in bodies of water, essentially acting as fertilizers for algae and aquatic plants. The algae and plants then grow more rapidly than the local habitat can handle, causing the death of aquatic life as they fight for the limited supply of oxygen (hypoxia). The damage caused by these issues effect not only the ecosystem, but the communities and businesses that rely on these water bodies for a source of income. A study conducted by Kentucky State University researchers in 2008 quantified the economic losses associated with freshwater sources in the US. They found that between the loss of recreational water usage and waterfront real estate, and spending on the recovery of endangered species and drinking water, the annual losses were approximately $2.2 billion .
As concern for the damage caused by eutrophication has grown, wastewater treatment facilities have become more involved in trying to reduce the amount of nutrients they are discharging to the environment. Municipal and industrial wastewater streams can contain high levels of nitrogen and phosphorus. Hence, many facilities now have limits for these two nutrients on their discharge permits. The traditional treatment method for removing nitrogen and phosphorus is known as Biological Nutrient Removal (BNR). There are many different designs and configurations for these systems, but they all use the same principles to remove the nutrients from the wastewater: various combinations of aerobic, anaerobic, and anoxic conditions to sustain the microorganisms that will facilitate the nitrification/denitrification processes alongside phosphorus degradation and removal . These systems typically reduce Total Nitrogen (TN) levels to 2-10 mg/L and Total Phosphorus (TP) levels to 0.1-1 mg/L .
There has been some advancement in the development of sustainable nutrient removal technologies. One of these is the Anammox process, which is short for Anaerobic Ammonium Oxidation. The anammox bacteria convert ammonium and nitrite, two major sources of nitrogen in wastewater, directly into nitrogen gas and water instead of having to go through the nitrification/denitrification process . This reduces operating cost for the nutrient removal system, as well as the energy consumed and greenhouse gases emitted by the process. The anammox system can also be beneficial to other treatment processes in plants as well. The reduced carbon-to-nitrogen ratio required for the anammox bacteria means that plants can remove more carbon material with an enhanced primary treatment system without worrying about killing their biological process, reducing operational costs and energy consumption. The anammox bacteria also grow much slower than conventional microorganisms used in wastewater treatment, leading to smaller sludge volumes and costs associated with sludge handling.
Another technology being utilized for nutrient removal is Reverse Osmosis (RO) membranes. This technology started off as a means for removing salt from seawater in desalination plants, but has since found its way into many industrial applications. One of these applications is wastewater treatment, where the RO membranes can be used to remove a wide array of contaminants. The membranes work by passing pressurized water over a semi-permeable membrane with a pore size of 0.001-0.0001 microns. The contaminants in the wastewater will be trapped on the pressurized side of the membrane, while the purified water is allowed to pass through and be discharged. Even in high strength industrial wastewater (TN & TP > 300 mg/L), membranes have been shown to remove TN down to <1.5 mg/L and TP down past detectable limits (0.02 mg/L). This ensures operators will reach any discharge levels required by permits, and gives them full control over their nutrient streams. These systems have the added benefit of removing the biology from the process completely, greatly reducing the material and energy requirements for operations.
Both of these technologies are making headway here in the US, and will hopefully have a large impact on the municipal wastewater treatment infrastructure soon. In my next post I will discuss the beneficial uses of the nutrients captured from wastewater sources.
|||W. K. Dodds, “Eutrophication of U.S. Freshwaters: Analysis of Potential Economic Damages,” Environmental Science & Technology, vol. 43, no. 1, pp. 12-19, 2009.|
|||S. Jeyanayagam, “True Confessions of the Biological Nutrient Removal Process,” Florida Water Resources Journal, pp. 37-46, January 2005.|
|||United States Environmental Protection Agency, “Biological Nutrient Removal Processes and Costs,” Washington, DC, 2007.|
|||B. Kartal et al, “How to make a living from anaerobic ammonium oxidation,” FEMS Microbiology Reviews, no. 37, p. 428–461, 2013.|