Thirty-seven years ago, when Cleveland’s Cayuhoga River caught fire, the race was on to clean up the nation’s waters. Engineers assured the country they could clean things up, provided there was enough money for construction. Almost no one was paying any attention to the relationship between kilowatts of electricity and biochemical oxygen demand (BOD) or nitrogen removal.
When electricity cost only $0.03 per kWh and atmospheric carbon was something required by plants, the last thing anyone thought about was carbon emissions or energy needs. So blowing bubbles in water at prodigious rates via extended aeration and activated sludge was all the rage.
It wasn’t until the oil embargoes in 1979 that we recognized energy wouldn’t be cheap forever. In October 1981, the U.S. Environment Protection Agency (EPA) published the Process Design Manual for Land Treatment of Municipal Wastewater (EPA 625-1-81-013). Hidden away in this manual was a very revealing table (see below) that compared the energy demands of various wastewater treatment systems. (Until about 2004, this information remained largely untapped by popular engineering textbooks on wastewater.)
This table points out three key concepts to today’s engineers:
· Some technologies are more energy-efficient than others, and the tradeoff for less energy generally is more land.
· Carbon footprint relates directly to energy requirements.
· Different effluent standards apply when land-applying than when discharging to a stream.
The EPA table estimates the total annual energy demand for a typical 3785-m3/d (1-mgd) municipal treatment plant using various wastewater treatment systems. For example, an activated sludge treatment system meeting an advanced wastewater treatment (AWT) standard would use 3.809 million kWh/yr. The table also shows that a slow-rate ridge-and-furrow facultative pond achieves similar treatment objectives using only 181,000 kWh/yr. The tradeoff is that the facultative pond would require more land than the AWT plant.
Still, the energy savings are great, especially considering that on top of the energy requirements, transmission losses increase the amount of energy that must be produced to power the equipment by 9.5%. This brings the total energy needed for the AWT system to 4.17 million kWh/yr.
Coupling this amount of electricity used with the average value of 0.61 kg (1.34 lb) of carbon dioxide emissions per kilowatt of electricity generated — based on the July 2000 report Carbon Dioxide Emissions From the Generation of Electric Power in the United States issued by the U.S. Energy Information Administration — yields 2.54 million kg of carbon dioxide emissions each year from the energy needed to move the water around the plant and blow bubbles to remove BOD and oxidize nitrogen.
While the treatment causes the release of carbon into the atmosphere through energy requirements, it also removes some carbon from the system by oxidizing BOD. So, how efficient is the system at removing carbon?
According to molecular weight, about 30% of the carbon dioxide emissions are carbon; this means the AWT plant’s carbon emissions from energy requirements are about 760,000 kg/yr.
On the carbon-sequestering side, assuming influent BOD is 220 mg/L and effluent BOD is 10 mg/L, the system removes 210 mg/L, which converts to 795 kg/d, or 290,120 kg/yr. Assuming that about 50% of BOD is carbon, the actual carbon removed by wastewater treatment is 145,000 kg/yr.
So, for every unit of carbon removed from the water, the energy use puts 5.24 times as much carbon into the atmosphere at the power plant.
This disturbing fact is exacerbated when one examines the fate of the removed BOD. About 30% to 40% of the BOD-based carbon removed from the wastewater is oxidized to carbon dioxide in the aeration tank and released to the atmosphere. So, that amount — about 50,750 kg/yr — should be added to the carbon emissions side of the balance.
Now, the total carbon balance looks like this: (760,000 kg/yr carbon emissions from power + 50,750 kg/yr carbon emissions from BOD removal) ÷ (145,000 kg/yr carbon removed through treatment) = 5.59. The plant emitted 5.59 times as much carbon to the atmosphere as it removed from the wastewater.
On the bright side, improvements in aeration efficiencies, pump motors, and process controls during the 28 years since the EPA table was written have improved the ratio calculated above. However, soil-based systems, such as constructed treatment wetlands and direct land application systems, still require as little as one-fifth the energy of activated sludge and membrane bioreactor systems.
Using the same assumptions as above, a slow-rate ridge-and-furrow facultative pond requires 198,000 kWh/yr, including transmission losses. So, the energy needs require 121,000 kg/yr of carbon dioxide emissions, which contain 36,200 kg/yr of carbon emissions. The treatment system, however, still sequesters 152,000 kg/yr of carbon.
The ratio of carbon emission due to power needs to carbon removed through treatment equals 0.24, which means that four times more carbon is sequestered by treatment than is needed for energy production.
Soil-based systems are readily amenable to distributed systems. They can and should be designed to be much less operation-intensive through use of fixed-film reactors with effluent drip dispersal. These treatment–drip systems produce high-quality effluents that can be reused onsite to promote grass and tree growth.
The merits of soil-based systems include the following:
- Land application of wastewater is a low-energy option.
- Soil-based treatment systems turn carbon and phosphorus into resources, rather than pollutants.
- Soil-based systems avoid treating U.S. waters as waste receptors with defined pollutant-carrying capacity (total maximum daily load limits treat carbon and nutrients as pollutants, not resources.)
An interesting possibility grows out of the land treatment option: If the nutrients in wastewater are considered a resource, it is possible to design a carbon-sequestering system. Wetlands, prairies, and woodlands serve as carbon sinks, and the nitrogen and phosphorus can be used to increase plant growth, which may reduce atmospheric carbon, as well as greatly reduce the amount of energy required to treat the wastewater.