10 Advancements in Environmental Engineering

Green communities like West Village at UC Davis in California seem like an ultra-modern concept, but mankind has been finding ways to work with the environment for centuries. ©Billy Hustace/Corbis

Environmental engineering is something that you can get a degree in these days, but the field is one that existed long before it had a name, begun at the dawn of civilization when we started modifying our environment to meet our needs. It involves applying science and engineering practices to how we utilize and impact our natural resources. Modern environmental engineers work on solutions to issues like pollution reduction and cleanup, energy consumption and emissions, land erosion, water treatment and waste management in an effort to properly manage and maintain the quality of our soil, water and air. They strive to keep everyone healthier and happier by helping us live off the land more efficiently and less destructively.

Environmental engineers are perhaps unsung heroes who have helped make the modern world what it is today, replete with relatively safe food and water, breathable air, largely plague-free living environments and energy-efficient fuel consumption to help power pretty much everything we do. The human population is around 7 billion and counting. The field will only increase in importance as that number grows.

There have already been some major innovations that helped get most of us to this point alive and well. Read on to find out what sorts of things these stewards of the earth have provided for us in the past, and are working on for the future.

Thames Water Utilities sewer workers inspect a sewer beneath the streets of London. © Richard Baker/In Pictures/Corbis

We have long desired to live in an environment free of human waste, initially because of the foul smell, and later, once we made the connection, to prevent serious and deadly outbreaks of disease. Sewer systems fit the bill by transporting large amounts of human excrement away from populated areas, and they have been evolving for thousands of years.

Between 2000 and 4000 B.C.E., the Mesopotamian Empire (modern-day Iraq), Mohenjo-Daro (modern-day Pakistan), Egypt, the island of Crete and the Orkney Islands in Scotland already had drainage systems -- and, in some cases, indoor sanitation facilities. By a few hundred years B.C.E., the Greeks had sewer systems that transported rain and wastewater to collection basins that irrigated and fertilized fields. The ancient Romans had underground sewers that fed into the Tiber River.

There was a lot of trial and error through the years, with disease outbreaks pointing out the need to keep sewer outlets away from drinking water. Over time, we also learned of the need to maintain the sewers, and the manhole was born (or re-invented, as we'll see later). Most were also constructed to be periodically flushed out with tidewater or rainwater.

From ancient times to just a few decades ago, sewers mainly transported raw waste directly to rivers, oceans or other large bodies of water. Modern sewer systems are more complex, leading to sewage treatment plants where the water is treated via filtration and addition of various chemicals to disinfect and remove contaminants before it's returned to nature. And no doubt they will continue to evolve.

Ruins of the Claudian Aqueduct, built in 313 B.C.E., near Rome, Italy. © Bettmann/CORBIS

We need water to live, so it's no coincidence that many ancient civilizations sprang up around natural water sources. But the ancient Greeks and Romans found a way to thwart, or at least divert, nature with the invention of aqueducts. Aqueducts were used to transport large amounts of water from one place to another, sometimes over as far as 60 miles (96.6 kilometers). They used the force of gravity to move water downhill via manmade conduits constructed at a steadily falling incline.

The aqueducts were mainly made of materials like concrete, cement, brick and stone. They would often originate at springs in hilly areas, but damns and reservoirs were also built to feed them from rivers or streams. When we think of aqueducts, the arcades, or aboveground stone bridges supported by arches, spring to mind. But the aqueducts were also made up of shorter walls, covered ground-level trenches, underground tunnels and pipes to facilitate the water's travel across a wide variety of landscapes.

An aqueduct's destination was a distribution tank called a castellum, which was usually at a high point in the city. It sent water to smaller castella, from which it flowed via masonry conduits or pipes to feed fountains, baths, public drinking basins and sometimes even private residences.

Rome's first aqueduct was constructed in 312 B.C.E. By the time of the construction of the Aqua Traiana by the Emperor Trajan around 109 C.E., the Roman aqueducts brought hundreds of millions of gallons of water into the city daily. These waterways allowed Roman cities to support much larger populations than they would have been able to with natural water sources alone.

Biofiltration Systems
Biofiltration pond near Mombasa, Kenya. Nile cabbage in the pond removes impurities from the water so it can be used as a fish farm. ©Chinch Gryniewicz; Ecoscene/CORBIS

Biofiltration is the process of passing air or water through a porous, moist material containing microorganisms in order to remove odors and contaminants. The contaminants are degraded to basic compounds like water or carbon dioxide, along with other benign biomass products, all as byproducts of the microbes' metabolic processes. Biofiltration systems are used to treat wastewater and industrial gaseous emissions, as well as emissions from composting operations, among other applications. They have been used since the 1950s for removing noxious odors, but are now seeing widespread use for removal of industrial contaminants as well.

Different strains of bacteria, along with moisture, pH and temperature control, can be used to effectively degrade various target contaminants. Unlike traditional filters, biofilters destroy harmful substances rather than just filtering them out, but they can only work with biodegradable contaminants. Biofiltration is mainly used to destroy toxic emissions like fuel-generated hydrocarbons and certain types of volatile organic compounds (VOCs).

VOCs are created and released during production of a wide variety of products that contain organic chemicals, including paints, cleaning supplies, cosmetics and fuels. They are technically carbon compounds that react with oxygen-containing molecules in the atmosphere when exposed to sunlight, leading to the formation of ozone containing smog.

The West Village of the UC Davis campus the largest zero net energy (ZNE) community in the U.S. -- it generates as much energy as it consumes, and uses bioswales to capture rainwater. ©Billy Hustace/Corbis

Bioswales are patches of vegetation made up of grass, flowers, trees or other plants that absorb storm water runoff, helping to degrade or remove pollutants before it flows untreated into any nearby bodies of water, or into sewer systems. Bioswales can be used to form channels that direct the flow of and filter the water, or they can be placed in strips (sometimes called biofiltration strips or filter strips) to catch water that flows over in thin sheets from paved areas. Some bioswales also include other mechanisms to further direct and filter runoff, such as under-drains and infiltration trenches.

Bioswales remove contaminants like heavy metals, oil, grease and sediment from runoff. They also cool water that has heated up while traveling across pavement before it reaches natural bodies of water, where warmer water could harm wildlife. They can be used in parking lots in place of storm drains, and, in urban areas that don't have a lot of plant cover, they can help prevent sewers from overflowing due to too much rainfall going directly down the drain.

The vegetation will vary by region, and unfortunately, bioswales are not ideal for arid climates. But in places that can support them, bioswales can do a lot of good. They also look like little landscaped parks in some cases, which are more aesthetically pleasing than concrete drainage structures. Bioswales can even end up sheltering small forms of wildlife like butterflies and birds. They're a win-win for nature.

Hybrid Vehicles
Toyota’s Prius was the first hybrid car commercially available. Here, Toyota shows off the Aqua -- a hybrid gasoline-electric subcompact hatchback -- at the company's show room in Tokyo on May 8, 2013. © KAZUHIRO NOGI/AFP/Getty Images

Hybrid cars were invented far earlier than most of us imagine. In the late 19th and early 20th centuries, they competed alongside gas, electric and even steam-powered cars for dominance. Of course, gas-only vehicles won the day. But as issues of fuel efficiency and emissions became increasingly important, hybrids reemerged. Newer hybrid prototypes were developed starting in the 1970s, but most never made it to market. The first commercially available hybrid was the Toyota Prius, introduced in Japan in 1997 and in the U.S. in 2001. Many more have since come out.

We're referring here to hybrid-electric vehicles (HEVs) that use combustion engines and electric motors (also called motor generators) in conjunction to yield better gas mileage than standard cars.

You still have to fill them up with gasoline, but the electric motor leads to gains in fuel efficiency by allowing the combustion engine to shut down while idling via automatic start/shutoff. It also provides additional power while the car is accelerating or going uphill through electric motor drive/assist, enabling installation of a smaller, more efficient gas engine. Some hybrids use regenerative braking. While the motor is applying resistance to the drive train and slowing the car, energy from the wheel is turning the motor and generating electricity, which is stored in the metal hydride (NiMH) battery for later use. Some of the more expensive hybrids can also operate in electric-only mode for a few miles, although others will shut down if they have no gas.

Depending upon make and model, hybrid-electric cars can get far better gas mileage than comparatively sized traditional vehicles.

LEED, BREEAM, Green Star and Other Certification Programs
Hearst Tower, the first occupied green commercial building with LEED certification. © Ramin Talaie/Corbis

Buildings are going certifiably green. As we've become more conscious of the effect our buildings have on the environment and on us directly, organizations have developed voluntary methods of rating the environmental impact and efficiency of buildings, homes and other similar structures. These include the Building Research Establishment Environmental Assessment Method (BREEAM) and Leadership in Energy and Environmental Design (LEED). BREEAM was started in 1990 by the BRE Trust and has been the dominant assessment standard in the U.K. LEED is a U.S. standard created by the U.S. Green Building Council in 1998. BREEAM and LEED are the most commonly used methods worldwide at the moment, but others are springing up, like Green Star -- created by the Green Building Council of Australia (GBCA) in 2003 -- as well as CASBEE in Japan and Estidama in Abu Dhabi.

Assessments take place both during design and after completion. Existing structures or commercial interior spaces can also be rated. The standards can be tailored to different regions or construction types, and buildings are rated on various things, including energy efficiency, water efficiency, land use, pollution, waste and indoor environmental quality.

The existence of such assessment entities helps to bring environmentally friendly construction and operational practices into the mainstream, which is especially important since buildings apparently contribute more than 20 percent of greenhouse gas emissions in some areas [source: HVN Plus]. Going green can also cut down on energy, water and other costs and improve the health of people working in the structures. As an added bonus, good ratings might qualify a building for tax rebates and other monetary incentives, and may increase property and rental values.

Ecosan Systems
Use of a composting toilet is demonstrated at a yoga retreat in Goa, India in February 2012. Pots with material to cover waste and aid in decomposition are kept next to the latrine. © EyesWideOpen/Getty Images

Ecosan (ecological sanitation) systems include various designs of environmentally friendly toilets or latrines that generally require little or no water, while isolating waste in a way that prevents odor and disease. In many cases, the resulting waste can even be composted and used as fertilizer or fuel. Some designs immediately separate the urine and feces (urine diversion systems). Some require covering the waste with sawdust, lye, sand or other material to eliminate odor, remove moisture and assist with decomposition for disposal or composting. Such systems are ideal for places where water is scarce, since they usually require no connection to a plumbing or sewer system.

One brand -- EcoSan -- was introduced in 2000. It's a stand-alone toilet; lifting the lid causes waste to make its way through a coiled conveyor over 25 or so days, all the while evaporating and venting the liquid waste and breaking down the solid waste using biological processes. Dry, odorless matter only 5 to 10 percent of its original mass is eventually deposited into a receptacle for removal and repurposing.

An ecosan toilet described by Unicef India is similar to a large outhouse with a concrete bunker underneath each toilet. The floor-level toilets have separate holes for liquids (which are diverted to pots outside) and solids, plus a cleansing water basin and a hole for users to drop a handful of lime, sawdust, ash or something similar after depositing solid waste to help with decomposition, moisture reduction and odor control.

There are other ecosan toilet construction methods and products that vary in price, functionality and complexity.

Ultraviolet Germicidal Irradiation
The NEWater plant in Singapore, pictured here, uses ultraviolet radiation to disinfect water. © ROSLAN RAHMAN/AFP/Getty Images

Ultraviolet germicidal irradiation (UVGI) rids water, air and surfaces of harmful microorganisms such as viruses and bacteria. Sunlight does this naturally to some extent. We know that UV light damages our skin and eyes; it also kills or inactivates some microorganisms.

UVGI systems use concentrated UV light to do so in a controlled manner, emitting shortwave ultraviolet-B and ultraviolet-C radiation at certain wavelengths, namely in the germicidal range between 200 and 320 nanometers -- often via a low-pressure mercury lamp. The UV light damages the cells or DNA of the affected microorganisms, killing them or rendering them unable to replicate. UV light in the higher 320 to 400 nanometer range is not effective against germs.

UVGI has been incorporated into ventilation ducts, heating and air conditioning systems and air disinfection units. It has also been used on entire rooms, preferably while they are unoccupied or everyone is in protective gear. Some systems emit UV light in near-ceiling areas to disinfect the air above peoples' heads in conjunction with vertical airflow mechanisms. High-efficiency particulate air (HEPA) filters or other types of filtration can be used alongside UVGI to remove other contaminants that UV won't kill.

Heavy research on UVGI was done from the 1930s through the 1970s in hospitals and schools, but despite its demonstrated efficacy, UVGI was mostly abandoned, in part due to breakthroughs in immunization, antibiotics advancements and safety concerns about UV radiation.

The increasing prevalence of antibiotic-resistant germs (including drug-resistant strains of tuberculosis) and fear of bioterrorism has renewed interest in UVGI. It's most commonly accepted for water disinfection, but air and surface disinfection uses continue to gain ground. In 2003, the Centers for Disease Control (CDC) sanctioned its use in hospitals in conjunction with air cleaning systems to help control the spread of TB.

This farm in Kenya keeps trees growing alongside crops to maintain an agriculturally productive landscape. ©Wendy Stone/Corbis

Agroforestry is the simultaneous management of trees and shrubs with crops and/or livestock for more efficient, integrated and environmentally sustainable land use. Applied properly, it increases product diversity, agricultural production and soil and water quality and decreases erosion, pollution and susceptibility to harsh weather conditions. It can also be used to shelter wildlife, protect watersheds and manage carbon emissions more effectively. All of these can add up to greater income for farmers and a better environment.

Various agroforestry methods can be employed depending upon the available land and resources. One is alley cropping -- growing crops alongside rows of trees like oak, ash, walnut, pecan or other nut trees. The crops and nuts can be harvested and sold while the trees mature and continue to produce nuts. Another is forest farming, using canopies of trees to provide the right level of shade for crops like ferns, mushrooms and ginseng. These can also be sold before the trees are ready for harvesting. A third is the creation of riparian forest buffers -- groups of trees, shrubs and grasses are planted as a buffer to prevent pollution and erosion of banks and waterways. Similarly, trees and shrubs can be planted in configurations called windbreaks that shield crops from wind damage and erosion and protect animals from harm. Windbreaks can increase bee pollination and manage the spread of snow over crops or roads. Another agroforestry method is silvopasture, using trees to shelter livestock and the grasses and other plants they eat. In all cases, crops, animals and trees symbiotically coexist together, and the farmer can concentrate on harvesting whatever is ready at the time.

In some countries, governmental policies stifle these practices, partially because of disconnects between the agencies that deal with the different items involved. But there's increasing attention being given to agroforestry as a sustainable farming method. In the U.S., the 1990 Farm Bill led to the creation of the USDA National Agroforestry Center.

High-altitude Wind Energy from Kites
In this photo of Makani’s test of Wing 7 in Alameda, California in late 2011, you can see the energy-generating turbines mounted on the structure. ©Makani Power, A. Dunlap, 2011

When we think of harnessing the power of wind to provide electricity, most of us probably think of windmills. Very few think kites. But a San Francisco-area start-up founded in 2006 called Makani Power has been working on using kite-like wind turbines attached to tethers to generate wind power at high altitudes, where there are stronger and steadier winds than we have at ground level. Makani means wind in Hawaiian, incidentally.

The tethers can reach up to 2,000 feet (609.6 meters) above ground, and they're both the suspension method and the method for transmitting power back to the base. The kites themselves are around one hundred feet long and made of carbon fiber. They have four propellers and incorporate sensors and GPS units on the wings that transmit data that can be used to optimize their flight. They actually fly in loops rather than hover. And they are light enough to maintain altitude in winds slower than 15 miles per hour (MPH).

The turbines reportedly have the potential to generate twice as much power, perhaps even more, at half the cost of modern ground-level wind turbines. The costs are competitive with that of coal burning, and take up less space than other power generation methods.

The kites -- still a few years away from commercial availability -- are likely to be used along shorelines, or in the ocean attached to buoys. Makani Power has received funding from Google and the Advanced Research Projects Agency for the Department of Energy (ARPA-E), and it is slated to be acquired by Google X, the laboratory working on projects like Google Glass and self-driving cars.


Tons of Mardi Gras Beads Clog NOLA's Storm Drains

Tons of Mardi Gras Beads Clog NOLA's Storm Drains

City workers have pulled 46 tons of the colorful beads from New Orleans' clogged catch basins. HowStuffWorks looks at how they're fixing the problem.

Author's Note: 10 Advancements in Environmental Engineering

As an inhabitant of this planet, I'm very interested in what we can do to properly use and conserve our natural resources. Partially because it's the right thing to do, and partially because I like living and breathing. I also prefer my food, air and water uncontaminated by disease and pollutants. I love having clean running water that comes straight into my house and working bathroom facilities free of noxious odors.

This is all pretty obvious stuff, but how often do we think about how our current hygienic state of being was attained? I only gave it scant thought before researching this article. I am thankful for all of our modern sanitary conveniences and the scientists and engineers past and present who have made them possible. Let's stay cholera-free, people!

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  • BBC. "Exploring Rome's 'sacred sewers.'" December 7, 2012. (June 8, 2013) http://www.bbc.co.uk/religion/0/20627618
  • Berg, Phil. "What to Do When Your Hybrid Car's Battery Dies." Popular Mechanics. August 9, 2011. (June 16, 2013) http://www.popularmechanics.com/cars/how-to/repair/what-to-do-when-your-hybrid-cars-battery-dies
  • Berman, Brad. "History of Hybrid Vehicles." Hybrid Cars. June 14, 2011. (June 16, 2013) http://www.hybridcars.com/history-of-hybrid-vehicles/
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  • BREEAM. "What is BREEAM?" (June 16, 2013) http://www.breeam.org/about.jsp?id=66
  • Bureau of Labor Statistics. "Occupational Outlook Handbook - Environmental Engineers." (June 8, 2013) http://www.bls.gov/ooh/architecture-and-engineering/environmental-engineers.htm
  • California Department of Transportation. "Biofiltration Strips." (June 14, 2013) http://www.dot.ca.gov/hq/LandArch/ec/stormwater/biofiltration_strips.htm
  • California Department of Transportation. "Biofiltration Swales." (June 14, 2013) http://www.dot.ca.gov/hq/LandArch/ec/stormwater/biofiltration_swales.htm
  • California Department of Transportation. "Bio Filtration Swales and Strips." January 5, 2012. (June 14, 2013) http://www.dot.ca.gov/hq/env/stormwater/ongoing/pilot_studies/bmps/details/bs_strips/
  • Center for Agroforestry. "Agroforestry Forest Farming." University of Missouri. (June 16, 2013) http://www.centerforagroforestry.org/
  • Clancy, Heather. "6 trends that will drive electric vehicle adoption in 2013." ZDNet. December 27, 2012. (June 16, 2013) http://www.zdnet.com/6-trends-that-will-drive-electric-vehicle-adoption-in-2013-7000009207/
  • CNN Money. "Best Jobs in America - Environmental Engineer." (June 8, 2013) http://money.cnn.com/magazines/moneymag/bestjobs/2010/snapshots/5.html
  • Devinny, Joseph S., Marc A. Deshusses and Todd S. Webster. "Biofiltration for Air Pollution Control." CRC Press. 1999. (June 14, 2013) http://books.google.com/books?id=NGluHrlVV_IC&pg=PA2&lpg=PA2&dq=biofiltration+history&source=bl&ots=XE9XJ-dXu7&sig=MraC34nnCLyAsK91c7nTq4wY_LE&hl=en&sa=X&ei=Jg68Ud31NYSk9ATu3IDoDw&ved=0CDsQ6AEwAw#v=onepage&q=biofiltration%20history&f=false
  • Ecologist. "The power of poo." February 2005, Volume 35, Issue 1, Page 9. (June 11, 2013)
  • EcoSan. "The EcoSan Waterless Dry Sanitation Toilet Introduction." (June 11, 2013) http://www.ecosan.co.za/introduction.html
  • EcoSan. "EcoSan Waterless Toilet - Product Information." (June 11, 2013) http://www.ecosan.co.za/product_info.html
  • Edmunds, Dan. "What Is a Hybrid Car? How Do Hybrids Work?" Edmunds. April 29, 2009. (June 14, 2013) http://www.edmunds.com/fuel-economy/what-is-a-hybrid-car-how-do-hybrids-work.html
  • Edwards, Lin. "Ancient sewer excavation sheds light on the Roman diet." Phys.org. June 17, 2011. (June 11, 2013) http://phys.org/news/2011-06-ancient-sewer-excavation-roman-diet.html
  • Engineering for Change. "Closed Waterless Toilet Systems." (June 11, 2013) https://www.engineeringforchange.org/solution/library/view/detail/Sanitation/S00100
  • Environmental Protection Agency. "An Introduction to Indoor Air Quality (IAQ) - Volatile Organic Compounds (VOCs)." (June 16, 2013) http://www.epa.gov/iaq/voc.html
  • Environmental Protection Agency. "Volatile Organic Compounds (VOCs) - Technical Overview." (June 16, 2013) http://www.epa.gov/iaq/voc2.html
  • Federal Remediation Technologies Roundtable. "Air Emissions/Off-Gas Treatment - Biofiltration." (June 16, 2013) http://www.frtr.gov/matrix2/section4/4-55.html
  • Fehrenbacher, Katie. "Google X is acquiring high altitude wind startup Makani Power." Gigaom. May 22, 2013. (June 14, 2013) http://gigaom.com/2013/05/22/google-x-is-acquiring-high-altitude-wind-startup-makani-power/
  • Ferris, David. "A Wind Power Innovator Dies Too Young." Forbes. November 28, 2012. (June 16, 2013) http://www.forbes.com/sites/davidferris/2012/11/28/a-wind-power-innovator-dies-too-young/
  • Food and Agriculture Organization of the United Nations. "About agroforestry." (June 16, 2013) http://www.fao.org/forestry/81630/en/
  • Food and Agriculture Organization of the United Nations. "Agroforestry." (June 16, 2013) http://www.fao.org/forestry/9469/en/
  • Gaylord, Chris. "Hybrid cars 101: How long should batteries last?" Christian Science Monitor. March 6, 2012. (June 16, 2013) http://www.csmonitor.com/Innovation/2012/0306/Hybrid-cars-101-How-long-should-batteries-last
  • Green Star. "About." (June 16, 2013) http://www.gbca.org.au/about/
  • Griffith, Saul. "High-altitude wind energy from kites!" TEDEducation. February 22, 2013. (June 14, 2013) http://www.youtube.com/watch?v=fC_y1u9jQ2w
  • H&V News. "BREEAM, LEED and Green Star agree on approach." May 27, 2010. (June 14, 2013) http://www.hvnplus.co.uk/breeam-leed-and-green-star-agree-on-approach/3101246.article
  • Hindu. "Ecosan toilet ideal alternative." July 2, 2005. (June 11, 2013) http://www.hindu.com/2005/07/02/stories/2005070216680300.htm
  • Kowalski, Wladyslaw. "Ultraviolet Germicidal Irradiation Handbook: UVGI for Air and Surface Disinfection." Springer. 2009. (June 14, 2013) http://books.google.com/books?id=ReqUM_XNGjoC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
  • Leafe, David. "Move over, Caligula! Book reveals the story of cross-dressing boy emperor Elagabalus, whose savagery and sexual hedonism were unparalleled." Daily Mail. December 28, 2011. (June 8, 2013) http://www.dailymail.co.uk/news/article-2079169/A-book-Roman-Elagabalus-reveals-story-cross-dressing-boy-emperor.html
  • LEED. (June 16, 2013) http://www.usgbc.org/leed/
  • LEED. "Why LEED?" (June 16, 2013) http://www.usgbc.org/leed/why-leed
  • Mohammed T., S. Vigneswaran and J. Kandasamy. "Biofiltration as pre-treatment to water harvesting and recycling." Water Science and Technology. Water Science & Technology. May 15, 2011, Volume 63, Issue 10, Pages 2097-2105. (June 14, 2013)
  • North Carolina State University. "Department of Biological and Agricultural Engineering - Environmental Engineering Programs." (June 11, 2013) http://www.bae.ncsu.edu/academic/environmental-engineering.php
  • Nova. "Watering Ancient Rome." February 22, 2000. (June 16, 2013) http://www.pbs.org/wgbh/nova/ancient/roman-aqueducts.htmlnova
  • Nova Online. "Roman Aqueduct Manual." (June 16, 2013) http://www.pbs.org/wgbh/nova/lostempires/roman/manual.html
  • Our Soil. "What We Do - Toilets." (June 11, 2013) http://www.oursoil.org/what-we-do/toilets/
  • Our Soil. "What We Do - Toilets - Models." (June 11, 2013) http://www.oursoil.org/what-we-do/toilets/models/
  • Owen, James. "Sacks of Human Waste Reveal Secrets of Ancient Rome." National Geographic News. June 23, 2011. (June 8, 2013) http://news.nationalgeographic.com/news/2011/06/110623-ancient-rome-human-waste-herculaneum-science-diet-excrement-italy/
  • Papadopoulos, A.M. and E. Giama. "Rating systems for counting buildings' environmental performance." International Journal of Sustainable Energy. March 2009, Volume 28, Issue 1-3, Pages 29-43. (June 14, 2013)
  • Parker, James. "BREEAM or LEED - strengths and weaknesses of the two main environmental assessment methods." BSRIA. February 2009. (June 14, 2013) http://www.bsria.co.uk/news/article/breeam-or-leed/
  • Pearson, Andy. "Essential guides: BREEAM, LEED, Green Star & Estidama." (June 14, 2013) http://www.building.co.uk/buildings/technical/essential-guides-breeam-leed-green-star-and-estidama/5002213.article
  • Pennsylvania State University Department of Architectural Engineering. "Ultraviolet Germicidal Irradiation." (June 16, 2013) http://www.engr.psu.edu/iec/abe/control/ultraviolet.asp
  • Reed, Nicholas G. "The History of Ultraviolet Germicidal Irradiation for Air Disinfection." NCBI. January/February 2010, Volume 125, Issue 1, Pages 15-27. (June 14, 2013) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2789813/
  • Richard, Tom. "Odor Treatment - Biofiltration." Cornell Waste Management Institute. (June 15, 2013) http://compost.css.cornell.edu/odors/odortreat.html
  • Roberts, Jeff John. "Google's X factor: 'Captain of Moonshots' describes secret lab." Gigaom. March 13, 2013. (June 16, 2013) http://gigaom.com/2013/03/13/googles-x-factor-captain-of-moonshots-describes-secret-lab/
  • Rocher, Vincent, Catherine Paffoni, Alexandre GonÇalves, Sabrina Guérin, Sam Azimi, Johnny Gasperi, Régis Moilleron and André Pauss. "Municipal wastewater treatment by biofiltration: comparisons of various treatment layouts. Part 1: assessment of carbon and nitrogen removal." Water Science & Technology. May 1, 2012, Volume 65, Issue 9, Pages 1705-1712. (June 14, 2013)
  • Rosenblum, Dan. "The bioswales of New York: A city plan to make more tree-stands and less sewage runoff." Capital New York. March 13, 2012. (June 14, 2013) http://www.capitalnewyork.com/article/politics/2012/03/5327266/bioswales-new-york-city-plan-make-more-tree-stands-and-less-sewage-
  • Schladweiler, Jon C. "Tracking Down the Roots of Our Sanitary Sewers." Sewerhistory.org. January 20, 2011. (June 14, 2013) http://www.sewerhistory.org/chronos/roots.htm
  • Schwartz, Robert. "The Sewers of Paris: A Brief History." Mount Holyoke College. (June 14, 2013) https://www.mtholyoke.edu/courses/rschwart/hist255-s01/mapping-paris/Paris_Sewers_Page.html
  • Shapley, Dan. "5 Air Pollution Facts and Myths." Daily Green. July 12, 2011. (June 17, 2013) http://www.thedailygreen.com/environmental-news/latest/ozone-air-pollution-smog-0706
  • Squires, Nick. "Ancient Rome sewer tunnels 'in danger of collapsing.'" Telegraph. November 14, 2012. (June 8, 2013) http://www.telegraph.co.uk/news/worldnews/europe/italy/9677683/Ancient-Rome-sewer-tunnels-in-danger-of-collapsing.html
  • Surfer Today. "Futuristic kite turbines generate high-altitude wind power." February 20, 2012. (June 14, 2013) http://www.surfertoday.com/kiteboarding/6963-futuristic-kite-turbines-generate-high-altitude-wind-power
  • Taylor, Rabun. "How a Roman Aqueduct Works." Archaeology. March/April 2012, Volume 65, Number 2. (June 11, 2013) http://archive.archaeology.org/1203/features/how_a_roman_aqueduct_works.html
  • Taylor, Rabun. "Rome's Lost Aqueduct." Archaeology. March/April 2012, Volume 65, Number 2. (June 11, 2013) http://archive.archaeology.org/1203/features/rome_aqua_traiana_aqueduct_carestia.html
  • Unicef, India. "Constructing an ecosan toilet -- A film from UNICEF." August 19, 2009. (June 11, 2013) http://www.youtube.com/watch?v=YV-1To9DkJQ
  • University of Tennessee, Knoxville. "What are Environmental Engineers?" (June 8, 2013) http://www.engr.utk.edu/civil/about/envirengineer.php
  • Upper Des Plaines River Ecosystem Partnership. "Bioswales." (June 14, 2013) http://www.upperdesplainesriver.org/bioswales.htm
  • U.S. Department of Energy and U.S. Environmental Protection Agency. "How Hybrids Work." (June 14, 2013) http://www.fueleconomy.gov/feg/hybridtech.shtml
  • USDA National Agroforestry Center. (June 16, 2013) http://nac.unl.edu/
  • USDA National Agroforestry Center. "Working Trees." (June 16, 2013) http://nac.unl.edu/Working_Trees/index.htm
  • U.S. Department of Energy and U.S. Environmental Protection Agency. "Compare Hybrids Side-by-Side." (June 16, 2013) http://www.fueleconomy.gov/feg/hybrids.jsp
  • Wagner, Eric. "High-Altitude Wind Power." Conservation Magazine. (June 14, 2013) http://www.conservationmagazine.org/2012/12/high-altitude-wind-power/