TREATMENT OF

Wastewater

Wastewater is used water. It includes substances such as human waste, food scraps, oils, soaps and chemicals. In homes, this includes water from sinks, showers, bathtubs, toilets, washing machines and dishwashers. Businesses and industries also contribute their share of used water that must be cleaned.

  • We consider wastewater treatment as a water use because it is so interconnected with the other uses of water. Much of the water used by homes, industries, and businesses must be treated before it is released back to the environment.

    If the term "wastewater treatment" is confusing to you, you might think of it as "sewage treatment." Nature has an amazing ability to cope with small amounts of water wastes and pollution, but it would be overwhelmed if we didn't treat the billions of gallons of wastewater and sewage produced every day before releasing it back to the environment. Treatment plants reduce pollutants in wastewater to a level nature can handle.

    Wastewater also includes storm runoff. Although some people assume that the rain that runs down the street during a storm is fairly clean, it isn't. Harmful substances that wash off roads, parking lots, and rooftops can harm our rivers and lakes.

  • It's a matter of caring for our environment and for our own health. There are a lot of good reasons why keeping our water clean is an important priority:

    FISHERIES: Clean water is critical to plants and animals that live in water. This is important to the fishing industry, sport fishing enthusiasts, and future generations.

    WILDLIFE HABITATS: Our rivers and ocean waters teem with life that depends on shoreline, beaches and marshes. They are critical habitats for hundreds of species of fish and other aquatic life. Migratory water birds use the areas for resting and feeding.

    RECREATION AND QUALITY OF LIFE: Water is a great playground  for us all. The scenic and recreational values of our waters are reasons many people choose to live where they do. Visitors are drawn to water activities such as swimming, fishing, boating and picnicking.

    HEALTH CONCERNS: If it is not properly cleaned, water can carry disease. Since we live, work and play so close to water, harmful bacteria have to be removed to make water safe.

  • If wastewater is not properly treated, then the environment and human health can be negatively impacted. These impacts can include harm to fish and wildlife populations, oxygen depletion, beach closures and other restrictions on recreational water use, restrictions on fish and shellfish harvesting and contamination of drinking water. Environment Canada provides some examples of pollutants that can be found in wastewater and the potentially harmful effects these substances can have on ecosystems and human health:

    • Decaying organic matter and debris can use up the dissolved oxygen in a lake so fish and other aquatic biota cannot survive;

    • Excessive nutrients, such as phosphorus and nitrogen (including ammonia), can cause eutrophication, or over-fertilization of receiving waters, which can be toxic to aquatic organisms, promote excessive plant growth, reduce available oxygen, harm spawning grounds, alter habitat and lead to a decline in certain species;

    • Chlorine compounds and inorganic chloramines can be toxic to aquatic invertebrates, algae and fish;

    • Bacteria, viruses and disease-causing pathogens can pollute beaches and contaminate shellfish populations, leading to restrictions on human recreation, drinking water consumption and shellfish consumption;

    • Metals, such as mercury, lead, cadmium, chromium and arsenic can have acute and chronic toxic effects on species.

    • Other substances such as some pharmaceutical and personal care products, primarily entering the environment in wastewater effluents, may also pose threats to human health, aquatic life and wildlife.

  • The major aim of wastewater treatment is to remove as much of the suspended solids as possible before the remaining water, called effluent, is discharged back to the environment. As solid material decays, it uses up oxygen, which is needed by the plants and animals living in the water.

    "Primary treatment" removes about 60 percent of suspended solids from wastewater. This treatment also involves aerating (stirring up) the wastewater, to put oxygen back in. Secondary treatment removes more than 90 percent of suspended solids.

RENEWABLE

Natural Gas

  • Renewable natural gas (RNG) is a pipeline-quality gas that is fully interchangeable with conventional natural gas and thus can be used in natural gas vehicles. RNG is essentially biogas (the gaseous product of the decomposition of organic matter) that has been processed to purity standards. Like conventional natural gas, RNG can be used as a transportation fuel in the form of compressed natural gas (CNG) or liquefied natural gas (LNG). RNG qualifies as an advanced biofuel under the Renewable Fuel Standard.

    Biomethane, which is another term for this purified pipeline-quality fuel, refers to biogas that has also been cleaned and conditioned to remove or reduce non-methane elements. Biogas is produced from various biomass sources through a biochemical process, such as anaerobic digestion, or through thermochemical means, such as gasification. With minor cleanup, biogas can be used to generate electricity and heat and is used as a replacement for traditional natural gas to generate combined electricity and heating for power plants—not in vehicle applications.

    To fuel vehicles, biogas must be processed to a higher purity standard. This process is called conditioning or upgrading, and involves the removal of water, carbon dioxide, hydrogen sulfide, and other trace elements. The resulting RNG, or biomethane, has a higher content of methane than raw biogas, which makes it comparable to conventional natural gas and thus a suitable energy source in applications that require pipeline-quality gas, such as vehicle applications.

    For a comprehensive list of projects that are upgrading gas for pipeline injection or use as vehicle fuel, see the Renewable Natural Gas Database developed and maintained by Argonne National Laboratory.

  • Landfills are designated locations for disposal of waste collected from residential, industrial, and commercial entities. Landfills are the third-largest source of human-related methane emissions in the United States, according to the U.S. Environmental Protection Agency(PDF) (EPA). Biogas from landfills is also called landfill gas (LFG), as the digestion process takes place in the ground rather than in an anaerobic digester. As of July 2023, there were 532 operational LFG projects in the United States, according to the EPA. However, most of these projects use biogas to produce electricity rather than power natural gas vehicles.

    Learn about these LFG alternative fuel transportation projects:

  • Biogas recovery systems at livestock operations can be used to produce RNG. Animal manure is collected and delivered to an anaerobic digester to stabilize and optimize methane production. The resulting biogas can be processed into RNG and used to fuel natural gas vehicles or produce electricity.

    As of 2023, there were about 470 anaerobic digester systems operating at commercial livestock farms in the United States. Most of these facilities use biogas for electricity generation. A few farms are using biogas to produce transportation fuel, including Calgren Dairy Fuels in California and Fair Oaks Farms in Indiana. EPA's AgSTAR database provides more information about the use of such systems in the United States.

  • Biogas can be produced by digesting the solids removed in the wastewater treatment process. According to EPA estimates, this biogas potential(PDF) is about 1 cubic foot of digester gas per 100 gallons of wastewater. Energy generated at U.S. wastewater treatment plants (WWTPs) could potentially meet 12% of the national electricity demand, according to a study released(PDF) by the National Association of Clean Water Agencies, the Water Environment Research Foundation, and the Water Environment Federation. This could spur some production of RNG for vehicle use as well.

    There are more than 16,000 WWTPs in the United States, but only about 1,200 have anaerobic digesters and of those, 860 have the equipment to use their biogas on site. The City of Longmont Wastewater Treatment Plant in Colorado is an example of a plant that uses biogas to produce RNG for use in vehicles.

  • Other sources of biogas include organic waste from industrial, institutional, and commercial entities, such as food manufacturing and wholesalers, supermarkets, restaurants, hospitals, and educational facilities. Learn about the City of Perris, California, biodigester, that produces enough RNG to fuel their fleet of 900 vehicles.

    Biogas can also be produced from lignocellulosic material (such as crop residues, woody biomass, and dedicated energy crops) via thermochemical conversions, co-digestion, and dry fermentation. These technologies are underway in Europe, with limited applications in the United States.

THE PROCESS OF

Anaerobic Digestion

  • Anaerobic digestion is a process through which bacteria break down organic matter—such as animal manure, wastewater biosolids, and food wastes—in the absence of oxygen. Anaerobic digestion for biogas production takes place in a sealed vessel called a reactor, which is designed and constructed in various shapes and sizes specific to the site and feedstock conditions (learn more about AD system design and technology). These reactors contain complex microbial communities that break down (or digest) the waste and produce resultant biogas and digestate (the solid and liquid material end-products of the AD process) which is discharged from the digester.

    Multiple organic materials can be combined in one digester, a practice called co-digestion. Co-digested materials include manure; food waste (i.e., processing, distribution and consumer generated materials); energy crops; crop residues; and fats, oils, and greases (FOG) from restaurant grease traps, and many other sources. Co-digestion can increase biogas production from low-yielding or difficult-to-digest organic waste.

    The following figure illustrates the flow of feedstocks through the AD system to produce biogas and digestate.

  • Anaerobic digestion produces two valuable outputs: biogas and digestate.

    Biogas

    Biogas is composed of methane (CH4), which is the primary component of natural gas, at a relatively high percentage (50 to 75 percent), carbon dioxide (CO2), hydrogen sulfide (H2S), water vapor, and trace amounts of other gases. The energy in biogas can be used like natural gas to provide heat, generate electricity, and power cooling systems, among other uses. Biogas can also be purified by removing the inert or low-value constituents (CO2, water, H2S, etc.) to generate renewable natural gas (RNG). This can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or processed further to generate alternative transportation fuel, energy products, or other advanced biochemicals and bioproducts.

    Digestate

    Digestate is the residual material left after the digestion process. It is composed of liquid and solid portions. These are often separated and handled independently, as each have value that can be realized with varying degrees of post processing.

    With appropriate treatment, both the solid and liquid portions of digestate can be used in many beneficial applications, such as animal bedding (solids), nutrient-rich fertilizer (liquids and solids), a foundation material for bio-based products (e.g., bioplastics), organic-rich compost (solids), and/or simply as soil amendment (solids), the latter of which may include the farm spreading the digestate on the field as fertilizer. Digestate products can be a source of revenue or cost savings, and are often pursued to increase the financial and net-environmental benefit of an AD/biogas project.

UNDERSTANDING

​Fats, Oils

& Greases

  • Some of the most environmentally friendly biodiesel feedstocks are used cooking oil and waste grease. According to the EPA’s Renewable Fuel Standards Program Regulatory Impact Analysis, released in February 2010, biodiesel produced from waste grease results in an 86% reduction in greenhouse gases, compared to petro-diesel.

    There are two general categories of waste oil and grease. The cleanest and easiest to work with is used cooking oil, also called waste vegetable oil or used fryer oil. This is edible oil that has been used multiple times in a deep-fat fryer. The second type is called “trap grease.” Restaurants are required to install grease traps in their drains to prevent fats and oils that go down the drain from entering sewer pipes. The grease in these traps can be collected to make biodiesel.

    While oilseed feedstocks originate in rural areas, used oil and grease are urban resources. The processing costs for waste oil and grease are higher per gallon than the processing costs for virgin vegetable oil. However, the cost of the feedstock is generally low and sometimes free.

    The main challenge to biodiesel production from used oils and greases is the high percentage of free fatty acids (FFAs) in the feedstock. Fats and oils are composed of triglycerides – three fatty acid molecules attached to a glycerol molecule. In used oils and greases, some of the triglycerides have broken down so that the fatty acids are separated from the glycerol molecule. These are called free fatty acids. These free fatty acids tend to react with the alkali catalyst in biodiesel production to form soap instead of biodiesel. This reduces the level of free catalyst and thus reduces the speed of the transesterification reaction. Soap formation tends to inhibit the separation of the ester from the glycerin and slow down the reaction. In addition, because the soap must be removed and discarded, more soap formation means less biodiesel.

    Used grease with less than 15% FFAs is called “yellow grease.” Used grease with more than 15% FFAs is called “brown grease.”

  • Used cooking oil is not really a waste product. It is collected by rendering companies, purified, and used for animal feed supplements and other products as well as biodiesel.

    A 1998 study by the National Renewable Energy Laboratory of 30 metropolitan areas found that about 9 pounds of used cooking oil are generated per person per year (Wiltsee, 1998, p. 2). Given a U.S. population of 308 million, this translates into about 2.7 billion pounds of used cooking oil per year. It is not known how much of this is currently being used to make biodiesel. However, because there is already a market for used cooking oil, and because the supply is limited, diverting more used cooking oil to biodiesel would probably cause the price of this feedstock to rise.

  • The term “trap grease” technically refers only to kitchen waste, not septic grease. However, sometimes trap grease is cross-contaminated with septic grease.

    Trap grease is often considered waste. It is generally disposed of at wastewater treatment plants and landfills, although sometimes it is collected by rendering companies to be purified and sold for nonedible applications such as burner fuel.

    A 1998 study by the National Renewable Energy Laboratory of 30 metropolitan areas found that about 13 pounds of trap grease are generated per person per year (Wiltsee, 1998, p. 2). Given a U.S. population of 308 million, this translates into about 4 billion pounds of trap grease per year. Because it takes about 7.5 pounds of trap grease to make 1 gallon of biodiesel, this works out to about 500 million gallons of biodiesel per year that could potentially be made from trap grease. To put this in perspective, according to the National Biodiesel Board, in 2008 the United States produced about 700 million gallons of biodiesel from all feedstocks.

    Challenges to using trap grease for biodiesel production include foul odor, up to 98% FFA, contamination with food and trash, heavy emulsification, cold flow issues (trap grease is solid at room temperature), and water contamination. In addition, the trap grease collection system is disorganized and fragmented (Haas, 2010).

    Despite these technical and infrastructure challenges, at least one firm, BlackGold Biofuels, has developed technology for the successful production of high-quality biodiesel from trap grease.

  • When a feedstock contains less than 3% or 4% FFA, most people just add extra catalyst, let the FFAs convert to soap, and then remove the soap. From 3% or 4%, up to 10% or 15% FFAs, a common approach is to use vacuum distillation to remove the FFAs from the oil. Then the oil can be processed normally, and the FFAs can be sold as animal feed or esterified separately.

    However, used cooking oil and trap grease can contain much more than 15% FFAs. These feedstocks need additional processing before they can undergo traditional alkali-catalyzed transesterification.

  • One way to deal with a high percentage of FFAs is to use an acid catalyst such as sulfuric or hydrochloric acid to convert the FFAs to esters, followed by an alkali catalyst to convert the triglycerides to esters. One problem with this approach is that the conversion of FFAs to esters causes water formation, which can cause soaps to form during the alkali-catalyzed process.

    However, this problem can be overcome by using an acid pre-treatment process to reduce the FFAs of the oil or grease. An acid catalyst and alcohol are added and reacted, the mixture is allowed to reach equilibrium, and the methanol, water, and acid portion that separates is removed. Then, if necessary, more acid and alcohol are added, and the process is repeated until the FFA level is less than 1%. After this pre-treatment process, the reaction is continued with alkaline-catalyzed transesterification. Using this pre-treatment process results in fuel that meets ASTM specifications for acid value. For more information, see Biodiesel Production from Oils and Fats with High Free Fatty Acids.

    While the acid pre-treatment is effective in reducing FFAs, the acid can cause damage to metal tanks. Use tanks coated with teflon or plastic or invest in super-alloy tanks.

    Another disadvantage of the acid pre-treatment is that extra methanol is required because methanol is added with the acid and then removed with the water/acid layer. This methanol becomes contaminated with water. To recover and reuse the methanol, a distillation tower is required to separate the water from the methanol.

    This technique was used to set up a pilot plant in Nevada, Iowa, to process high FFA feedstock into biodiesel. For more information, see A Pilot Plant to Produce Biodiesel from High Free Fatty Acid Feedstocks.

  • An alternative process called “glycerolysis” can be used with feedstocks containing more than 10% FFAs. This involves adding glycerin at 400°F and letting it react with the FFAs to form monoglycerides, a glycerol molecule to which one free fatty acid has been joined. These monoglycerides can then be processed using a standard alkaline catalyst transesterification process.

    Waste glycerin from biodiesel processing can be used in this process.

    Glycerolysis can be expensive because of the high heat involved, which requires a high-pressure boiler and trained boiler operator. Also, vacuum must be applied while heating to remove water that is formed during the reaction.

    Another disadvantage is that the glycerin will also react with the triglycerides in the oil to convert some of them to monoglycerides. While this does not negatively impact the reaction, it means that more glycerin is required for the process, and therefore more glycerin must be removed at the end of the transesterification.

  • The use of solid acid catalysts is a relatively new technology in biodiesel production. Solid acid catalysts are packed in a canister. As a mixture of oil and alcohol flows through this canister, the FFAs are converted to esters. The advantage of this process is that the acid does not contaminate the oil and therefore does not need to be removed at the end.

    However, a disadvantage is that contaminants in the oil such as water and phosphorus can foul the catalyst and prevent it from reacting with the FFAs. One solution is to install a guard column in front of the acid catalyst canister to remove contaminants from the oil.

    Even with a solid catalyst, water is formed by the esterification reaction and will contaminate the excess methanol that is used to force the chemical equilibrium to give a complete reaction. To be economically viable and environmentally sustainable, this methanol must be recycled, which requires a distillation process to remove water.

  • Another option is currently under development for processing high free fatty acid feedstocks. This option involves performing the reaction under supercritical conditions (275° to 325°C and high pressure). At high temperature and pressure, the reaction does not require a catalyst, so soap formation is not a problem. Water also does not appear to inhibit the reaction. Both free fatty acids and triglycerides react easily, so there is no need to separate these materials before processing. In fact, even very low quality feedstocks can be processed successfully.

    However, the high reaction pressure requires heavy-duty reaction vessels. The reaction conditions are so extreme that many side reactions can occur which produce undesired products. The formation of these non-ester compounds means that the final product will probably need to be distilled to meet ASTM quality requirements. In addition, another drawback is the extra energy needed to achieve and maintain the high temperature.

    In spite of these concerns, this method is of interest because it allows processors to make use of low-cost feedstocks such as trap grease.

The most environmentally friendly biodiesel feedstocks are used grease and oil. This article discussed how to turn waste oil and grease from restaurants into energy.