Wastewater Recycling

Abstract

Clean, safe drinking water is an essential requirement for all life, but not all people around the world have regular access to this important resource. Nearly 70% of the world is covered by water, but only 2.5% of that is available for human consumption. Rising temperatures and contamination of fresh water sources reduce this portion of available water even further.

Need for Water

The Food and Agriculture Organization of the United Nations (FAO) defines water scarcity as an insufficient supply of water to satisfy total demand after all feasible options to enhance supply and manage demand have been implemented.13 Climate factors, insufficient water resources, a lack of poorly maintained infrastructure, and/or chemical/biological contamination of existing sources can exacerbate these water shortages. Water stress is the lack of water available to supply human demand.

Water scarcity is comprised of two elements: absolute and economic water scarcity. Absolute water scarcity is the physical lack of available water in an area, while economic scarcity is the inability to manage the available water in an area.

The water stress index is often used to delineate the levels of physical water shortage. Water availability of 1700 m3/person/year or less is considered water stress, fewer than 1,000 m3/person/year is water scarcity, and anything less than 500 m3/person/year is categorized as absolute water scarcity.

According to the United Nations, water consumption has been growing twice as fast as the population for the last century. Water use is predicted to increase by 50 percent between 2007 and 2025 in developing countries and 18 percent in developed ones, with much of the increased use in the poorest countries and with an increase of people moving from rural areas to cities.11 This growth puts more strain on municipal water resources. Cities already tend to use more water per capita; mostly due to the presence of industries, universities, medical facilities, and retail businesses that people come to the city to use. In addition, the relationship between water and energy production is extremely close and co-dependent. It takes a massive amount of water to generate energy, and to process water that can be utilized by humans for drinking and other purposes.9 An increasing number of regions, particularly urban areas, are finding it more difficult to provide adequate volumes of clean water to the local populations.1 This problem is only expected to get worse in the future. Urban centers are increasingly diverting water from rural and farming communities. Industrial farmers sometimes move into an area and deplete an entire community’s water resources, as happened in the Sulphur Springs Valley of Arizona in 2014.6 Rising temperatures, commonly attributed to climate-change, increase water demand even further. Warmer temperatures result in greater energy use for things like air conditioning and greater water use for irrigation and consumption.9 Warmer air holds more water, and warmer temperatures speed the rate of evaporation trapping the water in the atmosphere.

From 2015 to 2018, Cape Town, South Africa went through a severe drought and nearly became the world’s first major city to run out of water.8 Reservoir levels fell from 97% in 2014 to 38% in 2017, at the beginning of what was forecasted to be a long, hot summer. Municipal authorities told residents to reduce their water consumption significantly. For suburban households, that meant decreasing usage from the pre-drought amount of around 200 liters per person per day to 50 liters per person per day.12 Fortunately, the rains returned in 2018 and the city’s reservoirs refilled. Authorities are now much more cognizant of the importance of water planning and the conservation of water resources, allowing them to strategize for future water shortages.

A 2012 United Nations Education, Scientific and Cultural Organization (UNESCO) report stated that, By 2025, 1.8 billion people are expected to be living in countries or regions with absolute water scarcity, and two-thirds of the world population could be under water stress conditions.5 Almost inevitably, these water shortages lead to conflicts. Ethiopia is currently building the Grand Ethiopian Renaissance Dam on the Blue Nile. The dam will impound 10 million cubic meters of water and provide Ethiopia with about 6,000 megawatts of electricity. It will also cut off a major tributary of the Nile River, which has long been the lifeblood of Egypt. The Blue Nile River is the Nile’s largest tributary and supplies about 85% of the water entering Egypt. This is a huge problem because Egypt receives less rainfall than any of the 186 nations surveyed by the FAO16, and as CNN reports, With its population predicted to reach 120 million by 2030, Egypt is on track to hit the threshold for ‘absolute water scarcity’ And that’s without factoring in any complications caused by the dam.7 This has caused increasing tensions between the two nations, the resolution of which has yet to be determined.

Wastewater is an Untapped Resource

With populations growing, usable sources diminishing, and conflicts rising, people are looking for solutions to the water scarcity problem. Because there are many different reasons that an area might be struggling with water scarcity, finding one solution for every shortage is difficult. However, analyzing situations where there is a singular reason for the scarcity may offer insight. For instance, there are sometimes water scarcity problems in areas with otherwise plentiful water resources. Water scarcity is often a contamination problem, where there is plentiful available water that is not fit for consumption. Preventing or remediating this contamination is a viable option to increase water supplies. One effective solution to water availability and contamination may be to recycle wastewater.

With minimal treatment, wastewater can be repurposed for irrigation. This can reduce farming’s dependence on water resources in an area, freeing up available water for human consumption. With a little more treatment, effluent from a wastewater treatment facility can be released back into the environment to replenish depleted aquifers. The wastewater will travel through ground surface, filtering most contaminants naturally. Many times, the effluent from a properly functioning wastewater treatment plant meets or exceeds the drinking water quality standards provided by federal and state regulations. While wastewater recycling can help to reduce water scarcity, there are significant concerns with using this process for direct human consumption.

Wastewater Recycling Challenges

The major challenge of wastewater recycling for direct human consumption the wide range of contaminants, and bacteria and viruses specific to wastewater [1]. The current federal and state regulations for drinking water quality are tailored for surface water and groundwater treatment, not recycled wastewater. Drinking water treatment facilities do not often test for many contaminants found in wastewater, and these contaminants may not be regulated by any government entity.

If wastewater is not recycled using reliable methods the finished water will not meet the water quality targets, potentially causing harm to the public. The contaminants found in wastewater that may pose a health risk include bacteria, viral pathogens, and pharmaceuticals. Water treatment facilities are equipped to handle the treatment of bacterium and viral pathogens, but may not have the ability to mitigate issues cause by pharmaceutical contamination.

Pharmaceuticals are chemicals, which can be either synthetic of natural, that are found in pharmacy products such as prescription medicines, and over-the-counter drugs. These chemicals are discharged into wastewater at a nearly continuous rate due to contamination by urine and feces of patients, and by improper drug disposal. Most pharmaceuticals are not fully absorbed by the patient, and some of that medication is released through their wastes into the sewer system. Improper drug disposal commonly occurs when a person discards their unwanted medication or illicit drugs into a toilet. Pharmaceuticals in wastewater can vary greatly from location to location. There is still some debate on what amount of exposure is safe. However, according to the World Health Organization, current observations suggest that is very unlikely that exposure to very low levels of pharmaceuticals in drinking-water would result in appreciable adverse risks to human health, as concentrations of pharmaceuticals detected in drinking-water (typically in the nanogram per litre range) are several orders of magnitude (typically more, and often much more, than 1000-fold) lower than the minimum therapeutic dose [5]. Therefore, studies show that if water is treated in a way that reduces the amounts of pharmaceuticals to minimal levels, then they should not pose a substantial risk to public health.

Many water and wastewater treatment plants disinfect with chlorine. Certain medications can react with chlorine, rendering them even more toxic to humans than the medication itself. More research is needed on the best way to chemically disinfect recycled wastewater, or another method must be used to remove pathogens.

While water treatment facilities are equipped to treat bacteria and viruses, there is concern that the concentrated amounts of fecally transmitted pathogens found in wastewater could negatively impact human health. Water that does not meet quality guidelines may contain bacteria that can result in the spread of diseases such as cholera and typhoid. Cholera is an infectious disease caused by the bacterium vibrio cholera. It is often spread by the consumption of food or water that is contaminated by feces, and results in severe watery diarrhea which leads to dehydration. Cholera can result in death if not treated.[2] Typhoid is caused by bacteria and can be spread through wastewater if improperly treated. Common typhoid fever symptoms include high fever, weakness, stomach pain, headache and loss of appetite. Less common typhoid fever symptoms can include constipation, rash, internal bleeding, and in rare cases, death.

Waterborne diseases can also be caused by viral pathogens that can be found in wastewater. The World Health Organization has classifies adenovirus, astrovirus, hepatitis A and E viruses, rotavirus, norovirus, and polioviruses as moderate to high health significance include. The majority of these viral pathogens have low infectious doses, meaning small amounts of virus can cause large outbreaks. These viruses often cause relatively harmless gastroenteritis; however, they can also lead to severe illnesses, such as encephalitis, meningitis, myocarditis, cancer, and hepatitis. Rotavirus alone results in over a half million deaths globally each year. These viruses have a higher impact on developing countries due to their lack of clean water, access to vaccines, and available healthcare. The spread of these waterborne virus-based diseases can be fought with vaccines, but the most effective solution is to reduce human exposure to them by properly treating water[4].

The concern with wastewater recycling is that the current water treatment systems do not account for such concentrated levels of bacteria and viruses, and they may not be equipped to reduce them to a level safe for consumption.

Reverse Osmosis

Conventional wastewater treatment plants can be very effective at removing some of the contaminants addressed above, including most viruses, bacteria, and some pharmaceuticals. Wastewater treatment plants may not be equipped to reduce the high bacterium and virus levels found in wastewater low enough for human consumption. Their effectiveness in removing pharmaceuticals is varying, and highly dependent on the type of drugs found in the wastewater. Replacing conventional treatment systems with reverse osmosis to recycle municipal wastewater into drinking water is the only known method to remove 99% or more of targeted pharmaceutical contaminants[4]. Reverse osmosis membranes can remove bacteria and viruses larger than 0.0001 microns.

Osmosis is a natural process involving two solutions, separated by a semi-permeable membrane. A solution consists of a solvent and a solute. The solvent is typically water, made up of relatively small molecules. The solute is anything that is not water in the solution, and is generally considered to be the contaminant in water treatment. The larger solute molecules are unable to cross the membrane, while the smaller solvent molecules are able to move freely from one side of the membrane to the other. If the solute concentration is higher on one side of the membrane than the other side, then the solvent tends to migrate towards the higher concentration side. This occurs until both sides have the same concentration of solute and solvent.

In reverse osmosis, high pressures are applied to the membranes to overcome the natural tendency for the solvent to migrate to the higher concentration side. This process leaves the high concentration side of the membrane to increase its concentration of solute, while decreasing its concentration of solvent. The majority of the solvent (water) is pushed through to one side of the membrane, leaving a very concentrated solution on the other side. In the case of reverse osmosis for drinking water, fresh water (permeate) is forced through the membranes and into a drinking supply system, and the concentrate is disposed of as waste.

The majority of municipal reverse osmosis systems are designed to remove salt from brackish water (water with a concentration of salt in between fresh water and seawater) or seawater, in a process called desalination. This allows water treatment plants near the coasts to make seawater available for human consumption, significantly reducing reliance on groundwater aquifers. This process can also be used to remove dissolved contaminants, such as pharmaceuticals from wastewater. Reverse osmosis is exceptionally effective at removing bacteria and viruses, as well.

There are limitations to using reverse osmosis for the treatment of municipal wastewater to reuse as potable water. The first and foremost obstacle is the energy required to operate a reverse osmosis system. A typical seawater reverse osmosis treatment plant requires 3-10kWh of electric energy to produce 1m3 of permeate[1]. However, brackish water reverse osmosis systems require significantly less energy, 0.5-3kWh, to produce 1m3 of permeate[2]. The level of total dissolved solids (TDS) found in seawater is substantially higher than that of recycled wastewater, which can be considered brackish water for the purpose of reverse osmosis treatment. The levels of pharmaceutical contaminants found in recycled wastewater are much lower than the levels of salt found in seawater, making it possible to use brackish water membranes instead of seawater membranes. Brackish water reverse osmosis membranes are less expensive and work at a higher flow rate than seawater membranes.

In the United States, most municipalities have an existing wastewater treatment plant within city or county limits. Reverse osmosis can be added as a tertiary treatment to existing wastewater treatment plants. The reverse osmosis systems will require high pressure pumps and significant infrastructure updates to transport the finished water to the distribution system. The American Society of Civil Engineers rated the current US infrastructure system with a D, making it unlikely that wastewater recycling with reverse osmosis is a practical option. It is possible that some US cities or states will utilize this emerging technology to mitigate future water crises, but this method will likely be limited to new wastewater treatment design. The most economical use of this technology will be in developing countries, where new water distribution systems and wastewater treatment plants are being constructed.

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