Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Pump and Dump

Posted by sogasex on March 28, 2008

By Dale Hubbard, Oregon State University

There are approximately 60 people, including the crew and the science party, aboard the Ronald H. Brown. The good folks in the galley serve three meals each day, there are snacks available after hours, and there’s a seemingly unlimited supply of things to drink. This is a very good thing (it certainly beats going hungry and being dehydrated), yet it poses a quandary: once we’ve metabolized all of that food & drink, what to do with all of our waste?

There are basically two types of sewage generated aboard the vessel: grey water, which originates from sinks, showers, laundry, and the dishwasher; and black water, which originates from the toilets. (The engines also generate wastewater of a more industrial nature, e.g. oily waste, but this is contained separately and later pumped ashore when the ship is in port, as oily waste is illegal to discharge at sea.) Aboard the Ronald H. Brown gray water and black water are commingled and contained within a holding tank of approximately 5000 gallons capacity. Sewage aboard the Ronald H. Brown is not treated—it is mechanically ground up en route to the tank, then the contents are pumped overboard. Once the sewage tank has accumulated 4400 gallons, it’s time to break off from our study site and make a run for it.

Since our project involves continuously observing a relatively small patch of water in order to collect time-series measurements, we must move at least 3 nautical miles off-site before the ship evacuates the contents of its sewage tank. Sewage contains an assortment of compounds that serve as phytoplankton nutrients, so dumping the holding tank inside of our study area would drastically alter the biological and chemical processes within. Therefore, at least twice each day we must leave the patch to undertake what many aboard refer to as a “Pump and Dump.”

Unfortunately, several times during the course of a Pump & Dump, the ship’s underway seawater line entrained some of the sewage. This caused caused a great deal of excitement in the lab, as steaming through this particular hydrographic feature generates some of the most dramatic measurements observed during the cruise. This secondary “patch” is exemplified by elevated pCO2 and nitrate (both byproducts of the degradation of metabolic waste) and elevated temperature (see pictures below).

On at least one occasion we’ve been able to resolve a sewage signal in the data from our underway transmissometer, an instrument which essentially measures water clarity (or lack thereof) by passing a beam of light through a sample stream of water. Eeeewwww…

In order to pick these Pump and Dump events out when we’re processing the data long after the cruise is over, it’s important that we remember to log them carefully.

fig-1.png

Effect of “Pump & Dump” on pCO2 and temperature. Pump and dump signal, from approximately 12:00-12:30, manifested in approximately 20 ppm increase in pCO2 (white trace) and approximately 0.5o C increase in surface seawater temperature (red). “H2O” parameter is water vapor measured inside of shipboard pCO2 equilibrator and also reflects higher surface seawater temperature. The surface salinity values are highly variable (and rather irrelevant) because it had been raining.

fig-2.png

Effect of “Pump & Dump” on NO3 concentration. “Amplitude” values represent voltage generated by photodiode detector (note scale is reversed). The ~0.1 V decrease between approximately 12:00 -12:30 represents an approximately 3mM increase in NO3. The variation in amplitude at approximately 13:30 is a standard sequence—a blank, 20 mM, and 9.5 mM KNO3 solutions.

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