This week, I have unfortunately been feeling a little under the weather with a self-diagnosed common cold. The seemingly massive quantities of DayQuil, tea, and Kleenexes I have consumed over the past few days have begged the question: what are the environmental impacts of the common cold? To begin to answer that question, I will examine the impacts of tea.
Previous research by Nigel Melican suggests that tea’s carbon footprint can vary from 200 g CO2 to -6 g CO2 per cup of tea, depending on how it is grown, processed, shipped, packaged, brewed, and discarded. I will not go as in depth in my analysis here as Melican did in his, but I hope to examine at least a few key components of the process.
Since I discovered that mint tea with honey works wonders on a sore throat, I’ve been slowing sipping the beverage from a Keurig Stainless Steel Travel Mug (14 oz) for the past few days. The mug keeps my tea warm for several hours, so I estimate I’ve had about 2 full mugs worth of tea each day for the past four days, or 8 mugs (112 oz) total. To examine the environmental impacts of tea, consider each of the following sections separately: cleaning and transporting the water, heating the water, manufacturing and transporting the tea bag and honey, disposing of the tea bag, and washing my Keurig Travel Mug.
Water delivery: 1.6 g CO2eq/14 oz
Moving and treating water in our municipal supply system certainly requires energy, so there is embedded carbon associated with the energy required to deliver water to campus for my tea. Note that the energy intensity of these processes range considerably based on the systems used and their efficiencies. I am not yet familiar with the water treatment system in Houston, so I will use a worst case scenario (high end estimates) for combined water supply and conveyance, treatment, and distribution: 31,200 kWh/MG. I won’t directly include the energy associated with wastewater collection, treatment, and discharge, which would add another 5,000 kWh/MG to the total energy intensity, because I am unsure of what percentage of the tea will actually pass as wastewater. Overestimating energy intensity on the delivery side will likely account for this regardless.
With these assumptions, 14 oz of water requires 0.0034 kWh of electricity to deliver the water. At 0.47 kg CO2eq/kWh consumed, this is 1.6 g CO2eq.
Heating the water: 14 g CO2eq/14 oz
For the most part, I get most of the hot water for my tea from the hot water tap on the side of the coffee maker in the servery, a Bunn Model ITCB-DV. According to the installation manual for the machine, the recommended water temperature at sea level is 200°F (93.3°C) – just below the boiling point of water. Also, the manual specifies to connect the brewer to a cold water system. For now, I’ll assume that the water comes in at about 60°F (15.6°C). This is somewhat based on an anecdote that cold tap water in Milwaukee is 49.6°F (9.8°C), and Houston is generally warmer than Milwaukee, and the maximum temperature considered safe in Denmark is 77°F (25°C).
The coffee maker uses energy in two main ways: heating water to the appropriate temperature, and keeping water hot in the storage tank (offsetting losses from conduction through the walls of the tank). To calculate the energy required to heat the water for my tea, I’ll assume that enough people use the hot water that it doesn’t sit in the holding tank for an extended period of time. In this scenario, most of the energy required to deliver hot water to my cup comes from heating the water from supply temperature to 200°F. From Q=mc(T2-T1), heating 14 oz (m=237 g) of water (c=4.184 J/g/°C) from T1=15.6°C to T2=93.3°C requires 77 kJ of energy, or 0.0214 kWh of electricity. This is equivalent to 195 Wh/gal.
The brewer in the servery is not Energy-Start Certified, as far as I could tell. However, for reference, Energy Star requires that heavily used commercial coffee makers consume at most 280 Wh/gal while brewing (and thus heating water). If the servery brewer did consume energy at 280 Wh/gal, it would be about 70% efficient.
Since I don’t have specific information on the efficiency of the brewer, I will assume that it does consume energy at the maximum Energy Star rate: 280 Wh/gal. If electricity in ERCOT has a weighted average carbon footprint of 0.47 kg CO2eq/kWh consumed, then one 14 oz serving of hot water for tea is equivalent to 14 g CO2eq.
Tea bag and honey: 18.3 g CO2eq/14 oz serving
Based on this life cycle analysis for honey, I estimate that the honey I add to my tea has a carbon intensity of about 0.7 kg CO2eq/kg honey, including production, processing, transportation, packaging, etc. Assuming I add about 9 g of honey to one 14 oz serving of tea, this contributes 6.3 g CO2eq to the carbon footprint.
The largest component of the impacts here are actually the teabag; the servery provides tea in tea bags, which has ten times the footprint of loose leaf tea. Most of the difference comes from the elaborate packaging associated with tea bags. For a very rough estimation, consider this estimate that the carbon footprint of plastic is 6 kg CO2eq/kg plastic. A tea bag with 2 g of plastic (a rough estimate) would then have 12 g CO2eq of embedded carbon.
The total footprint for the tea bag and honey is 18.3 g CO2eq/14 oz serving. For this analysis, I’ll assume that the footprint of producing the tea itself if fairly small, and I will look further into this assumption at a later date.
Waste disposal: 1.65 g CO2eq/14 oz serving
Unfortunately, I have no way of composting at my university, so I tend to throw my used tea bags in the garbage. Emissions resulting from landfilling the tea bag are complicated, encompassing methane emissions from anaerobic decomposition, CO2 emissions from transportation and landfilling equipment, biogenic carbon stored in the landfill, and CO2 emissions avoided by implementing landfill gas-to-energy projects. When all of these factors are combined, food waste has a net emission factor of 0.71 MTCO2eq/short ton food waste, and most plastics have 0.04 MTCO2eq/short ton plastic. Estimating 2 g of tea and 2 g of plastics in each tea bag gives a total of 1.65 g CO2eq per tea bag due to landfilling.
Washing the Keurig Travel Mug: 147 g CO2eq/14 oz
Since I don’t have a dishwasher, I was my mug by hand; per the instructions from Keurig, I try to wash it after every use. Estimating that I use about 1 gallon of water per wash, with the energy and carbon intensity estimates used earlier, this gallon represents about 147 g CO2eq.
Here are the totals in summary:
- Water delivery: 1.6 g CO2eq/14 oz serving
- Water heating: 14 g CO2eq/14 oz serving
- Tea bag and honey: 12 g CO2eq/14 oz serving
- Waste disposal: 1.65 g CO2eq/14 oz serving
- Mug washing: 147 g CO2eq/14 oz serving
- TOTAL: 182 g CO2eq/14 oz serving
The total carbon footprint of each mug of tea I drank this week was about 182 g CO2eq, which is within the range suggested by Nigel Melican. Mug washing is clearly the largest contributor to the carbon impacts (as illustrated in the graph below), suggesting that perhaps I should be more efficient with my water use when washing my Keurig mug.