Carbon and Wastewater Treatment
- September 1, 2013
- Posted by: Eric & Patti Wahlberg
- Category: From the Mind of Eric Wahlberg
I am sitting here between my grapefruit tree and new raised-bed garden, built this spring and filled with compost I made, now teaming with tomatoes, fava beans, and corn all swaying in the cadence of the evening breeze. Jack London is the author of one of my favorite quotes:
“I am a farmer. It is because I am a farmer that I live in the country. I am that sort of farmer who after delving in all the books to satisfy his quest for economic wisdom, returns to the soil as the source and foundation of all economics.”
“What do gardening and farming have to do with wastewater treatment?” you are undoubtedly asking. Answer: the carbon cycle.
Producers, consumers, decomposers: we learned their roles in the carbon cycle in high school biology. Beer in hand, I sit here and contemplate the carbon cycle a lot. I have this theory—no doubt long ago proven, or not, unbeknownst to me—that soil microorganisms, bacteria mostly (like activated sludge mixed liquor), have a symbiotic relationship with the plants above, whose roots push through the soil on a relentless uptake of the nutrients necessary for the plants’ growth. But one nutrient those roots don’t search for is carbon. While soil microorganisms need organic carbon, which they mostly receive from falling leaves or the plants’ death, the plants above don’t need organic carbon because they fix inorganic carbon, CO2, from the atmosphere through photosynthesis. Central to my theory is that because the soil microorganisms need the organic carbon from the plants, they actually assist the plants by making the nutrients the plants need more readily available to the roots. If a soil bacterium could think, its thought process might go something like this: “If I get the plant all the nutrients it needs, then it will produce more organic carbon that will ultimately be delivered to me.” A variant of this hypothesis is the microbial loop hypothesis that states the plant’s roots release a carbon-rich exudate that bacteria thrive on in return for mineralizing soil organic nitrogen, a process which makes the nitrogen more available to the plant.
The staggering amount of carbon that cycles through the biosphere may qualify it as the most important of all nutrients. And operators have been removing this most important of nutrients from sewage since wastewater treatment began. We should give ourselves more credit for doing so.
We use BOD as an indirect measure of the amount of organics in a sample. With nitrification inhibitor, BOD becomes “carbonaceous BOD.” We use COD as an indirect measure of the amount of organics in a sample, although not all COD is biodegradable. The volatile solids that are burned away in a 550-degree-Centigrade furnace, whether total, suspended, or dissolved, are referred to as “organic solids.”
We all need to remember that whenever we refer to “organic” anything in wastewater treatment, we’re referring to organic carbon. It’s funny how we can make things so difficult for ourselves sometimes. For example, across a primary clarifier we talk about TSS removal (because primary clarifiers are settleable TSS removal devices) and BOD removal (because the BOD that continues on to the secondary system is expensive to remove) but not, specifically, about VSS or VS removal. This is strange to me because we express the organic loading to digesters in terms of pounds of VS per ft3 of digester volume, and we talk about the “percent VS reduction” that occurs in digesters, and digester gas production is expressed in units of ft3 of gas produced (made up of carbon-containing methane, CH4, and CO2) per pound of VS destroyed, and then we talk about the organic content of biosolids benefiting the tilth of the land to which they are applied. Just know: it’s all about the carbon.
As operators, we are masters of the carbon cycle, which may not be the universe, but it is certainly Earthly.