December 6, 2013 by Heather Wiatrowski
In Part 1 of this series, I briefly discussed sampling at MASSTC. In Part 3, I’ll be writing about when septic systems go bad, and in Part 4, I’ll be writing about why we know so little about them.
Twenty two percent of US households, and most of the toilets installed in the developing world, utilize Onsite Wastewater Treatment Systems (OWTS) to remediate household wastewater. Yet we know almost nothing about the microbial ecology of OWTS. In this post, I’m going to give a summary of what is currently known about the microbiology of OWTS.
Why Use an OWTS?
Shouldn’t we replace these all with municipal wastewater treatment plants? Not so fast. OWTSs are used to treat household water in areas where a municipal plant isn’t cost effective. They are relatively inexpensive, and, after installation, they typically deliver years of problem-free service without needing maintenance. The tank needs to be pumped, but depending on usage, that might only need to be done every five years or so. While they can and do contaminate groundwater, they are best utilized in areas with low population density.
Important Processes in Wastewater Treatment
The household wastewater is first collected in the tank, where solids settle out. We presume it’s partially digested there by fermenters, but hey, we just don’t know, but I’ll get back to the state of our knowledge later. After that, it travels through a series of pipes into the leach field, and from there percolates into the soil.
At the interface of the soil and the pipes, a black, slick biofilm called the “biomat” forms. The biomat is important because we think these organisms are responsible for degrading organic carbon…. but they can build up rapidly, and the biomat has low hydraulic conductivity. If a biomat overgrows, the septic system will clog and won’t function properly.
After the biomat, the leachate flows by gravity into the soil, where nitrification occurs. We can take a few guesses on processes and organisms involved in nitrification based on what we know about nitrification in soils and in wastewater treatment plants.
In a municipal wastewater treatment plant, one of the first steps in treating wastewater is nitrification, or ammonia oxidation. This is a process where ammonia is used as an electron donor, and oxygen is used as an electron acceptor. Since the nitrification tanks have to be kept aerated, wastewater treatment plants spend vast amounts of energy making that happen, both by pumping in air and stirring the sewage.
We used to think we knew everything about nitrification in wastewater treatment plants, but that’s not even the case. If you remember your own micro class, you probably remember learning about nitrification by Nitrosomonas and Nitrospira in wastewater, and they are certainly there…. but that information comes from enrichment cultures and predates the revolution in culture-independent microbiology. It also predates the discovery of ammonia oxidation by Archaea, and now that researchers have gone looking ammonia oxidizing archaea in wastewater treatment plants, they’ve found them (1).
In OWTS, nitrification occurs in the leach field, but is it Nitrosomonas and Nitrospira? Or it could be ammonia oxidizing Archaea, because those are abundant in soils (2) except for when they’re not (3). One thing George Heufelder at MASSTC told me is that he frequently observes robust nitrification in his systems when the pH is too low for Nitrosomonas and Nitrospira… which leads me to think that the Archaea might be dominant (4).
There’s yet another process that can remediate ammonia in wastewater, and that’s anammox, or anaerobic ammonia oxidation. This is a fascinating process carried out by bacteria in the phylum Planctomyces. A handful of municipal plants are running using the anammox process (5), and we think in at least one instance, scientists have observed anammox in an OWTS (6).
In municipal wastewater treatment, following nitrification, the wastewater moves to an anaerobic denitrification tank. Denitrification may or may not occur in a leach field.
State of knowledge about OWTS microbial ecology:
These systems have been left behind the culture independent revolution in microbiology. Here’s the sum total of all the data we have on OWTS: A 16S clone library (~400 amplicons) from an OWTS was published in 2009 (7) . A study by Robertson and colleagues, published in 2011, identified anammox as a dominant process in a septate plume by sequencing 18 DGGE bands (6). Jose Amador’s group at the University of Rhode Island constructed 16S clone libraries of 100 amplicons on OWTS mesocosms, published in 2013 (8) . Alex Vickers, one of my undergraduate students, constructed a 16S clone library from MASSTC leachfields of approximately 200 amplicons. No previous study investigates Archaeal community composition, and to the best of our knowledge, nobody has probed an OWTS for functional genes. That’s not a lot of information!
Next blog post is going to be about what happens when OWTSs fail, and how we might be able to improve OWTS with a bit more knowledge.
1. Sauder LA, Peterse F, Schouten S, Neufeld JD. 2012. Environmental Microbiology 14: 2589-600
2. Leininger S, Urich T, Schloter M, Schwark L, Qi J, et al. 2006. Nature 442: 806-9
3. Di H, Cameron K, Shen JP, Winefield C, O’Callaghan M, et al. 2009. Nature Geoscience 2: 621-4
4. Yao H, Gao Y, Nicol GW, Campbell CD, Prosser JI, et al. 2011. Applied and environmental microbiology 77: 4618-25
5. Kartal B, Kuenen J, Van Loosdrecht M. 2010. Science 328: 702-3
6. Robertson WD, Moore TA, Spoelstra J, Li L, Elgood RJ, et al. 2011. Ground Water: no-no
7. Tomaras J, Sahl JW, Siegrist RL, Spear JR. 2009. Appl. Environ. Microbiol. 75: 3348-51
8. Atoyan JA, Staroscik AM, Nelson DR, Patenaude EL, Potts DA, Amador JA. 2013. Water 5: 505-24