Words on Biofilm

The past couple of weeks I've been on some business trips and haven't had time to write any posts. However, I did have time on a flight to read a technical paper that I want to discuss below (I also finished reading a book that I'm planning to review in an upcoming post).

"Biofilms, active substrata, and me" (2018) is a paper by Dr. Bruce Rittmann in Water Research. This journal now has a number of "... and me" papers, which seem to basically be review papers focused on the author's own research. The authors of these "... and me" papers are prominent researchers who have contributed greatly to the field; Dr. Rittmann was recognized with the Stockholm Water Prize this year.

I've previously posted about another "... and me" paper. Before I get into discussing the current one, I wanted to share a blog post that the author of that paper, Dr. Simon Judd, wrote recently that extends some of the points from there about bridging the gap between research and application in the wastewater treatment field. Here is a cliche he says should be avoided by future papers in this sector (but read the whole thing—disclosure: it favourably mentions the company I work for):

Cliché no. 2: Almost as infuriating is: ‘this [technology, technique …] offers much promise for wastewater treatment in the future’. This manages to tick three boxes at once in that it is (a) aspecific, (b) hackneyed, and (c) almost certainly untrue − unless the ‘promise’ is to go the way of most other bench-scale tests reported in the academic literature.

To start with, here is the opening of the abstract for Rittmann (2018):

Having worked with biofilms since the 1970s, I know that they are ubiquitous in nature, of great value in water technology, and scientifically fascinating. Biofilms are naturally able to remove BOD, transform N, generate methane, and biodegrade micropollutants. What I also discovered is that biofilms can do a lot more for us in terms of providing environmental services if we give them a bit of help. Here, I explore how we can use active substrata to enable our biofilm partners to provide particularly challenging environmental services. In particular, I delve into three examples in which an active substratum makes it possible for a biofilm to accomplish a task that otherwise seems impossible.

Biofilms are communities of microorganisms growing in a thin layer on a surface (their substratum). Apparently 90% of microorganisms in nature live this way, as opposed to freely floating around. They are held together—and protected—by extracellular polymeric substances (EPS), a kind of "glue" they excrete. Aside from the protection offered, biofilms have the advantage that different microorganisms can work together; chemical substances can diffuse to different depths, creating micro-niches along their gradients that are ideal for different types of microorganisms. In engineered systems, biofilms have the further advantage that they are less prone to wash-out than free-floating biomass:

a biofilm is an excellent means to retain microorganisms. For treatment technology, retention is of the utmost importance when critically important microorganisms are slow growers. ... e.g., methanogenic archaea, nitrifying bacteria, anammox bacteria, and dechlorinating bacteria.

This point is also made by Speece in his book Anaerobic Biotechnology and Odor/Corrosion Control for Municipalities and Industries:

One of the paramount issues in the design process of anaerobic reactors for a particular wastewater is the selection of the most appropriate method of immobilizing the biomass. The major advantage of anaerobic technology—low biomass yield—is also its major disadvantage when trying to increase the biomass inventory.
Retention of the biomass is indispensible for successfully maintaining the low synthesizing methanogens. (p. 307)

Active substrata*, the focus of this paper, are "surfaces that provide an essential service to the microorganisms". This service can be supplying, removing, or transforming a chemical species involved in the biochemical reaction carried out by the microorganism.

*To avoid confusion note that substrata and substrates are not used as synonyms. The former are the surfaces that biofims grow on and the latter are chemicals eaten or respired (the distinction blurs a bit when discussing single-celled organisms) in microbial metabolism.

The concept of "microbial infallibility" (mentioned in this post) implies that almost any substrate can be degraded by some kind of microorganism. Rittmann (2018) considers some examples where they need a boost.

The first example is a membrane biofilm reactor (MBfR) that supplies hydrogen gas (H2) directly to a biofilm growing on a membrane. This process is similar to a membrane aerated biofilm reactor (MABR) except for the gas supplied through the membrane. The purpose of supplying hydrogen instead of air is to assist the biofilm in degrading oxidized contaminants. Traditionally, reduced contaminants have gotten more attention in environmental biotechnology. Reduced contaminants are known as electron donors (ED) since they have surplus electrons that they need to give up to be stabilized; biochemical oxygen demand (BOD) falls into this category. For biochemical reactions to take place, electron donors must be paired with electron acceptors (EA), or oxidized compounds—in the vast majority of cases, oxygen is supplied via aeration to stabilize reduced contaminants. However, for oxidized contaminants (e.g. nitrate or perchlorate), an ED must be supplied instead:

When an oxidized contaminant is to be a respiratory electron acceptor, its reduction becomes possible only when the bacteria oxidize an electron-donor substrate. The electron-donor substrate has to be supplied as part of the treatment technology.

The conventional example of this is adding methanol for denitrification (i.e. reducing nitrate), but the MBfR represents a novel approach. Rittmann describes how H2 meets the criteria of a good electron donor: bioavailable, relatively inexpensive, and able to be dosed accurately (since residual ED = added BOD). To get these advantages in spite of its low solubility, the MBfR uses just-in-time delivery by diffusion through a membrane that supports a biofilm on its wetted side.

The second example is a microbial electrochemical cell (MxC). While the first example used a membrane as the active substratum, this process uses an anode in that role. The anode acts as a "solid-phase electron acceptor"; rather than an oxidized chemical participating in the biochemical reaction, an electrical circuit is used as a direct electron sink.

Anode-respiring bacteria can act as living catalysts at the anode of an electrochemical cell ... Anode-respiring bacteria (ARB) are able to oxidize an electron-donor substrate and respire the electrons to an conductive solid.

This requires extra-cellular electron transfer (also discussed in a previous post), which can occur via direct contact, soluble electron shuttles, or a conductive biofilm matrix. The third one is the focus in Rittmann (2018) due to the biofilm angle.

Microbial electrochemical cells are a generalization of fuel cells. Each configuration generally has the same process occurring at the anode (i.e. reducing BOD) which makes ARB very relevant to study. Various beneficial reactions can be driven at the cathode, so they are a flexible and promising platform.

The third example in Rittmann (2018) is intimately coupled photobiocatalysis (ICPB). In this example, the active substratum is a porous media with a light-stimulated coating. When UV light hits this coating, it generates hydroxyl radicals, which are strongly oxidizing and can break down compounds that are otherwise refractory/recalcitrant. Processes that generate hydroxyl radicals are known as Advanced Oxidation Processes (AOP). ICPB is a hybrid between AOPs and biological processes: bacteria are sheltered in the pores of the media—protected from the UV light—to further degrade the compounds that the hydroxyl radicals have started to break down. By applying a "small degree of advanced oxidation that makes recalcitrant organic pollutants readily biodegradable", it combines the strengths of AOPs and biological processes to get decent removals without outrageous costs.

In summary, the active substrata in the three examples discussed were membranes that H2 can diffuse through, solid anodes, and porous media with a light-activated coating. The one thing I felt this paper was lacking was a more explicit statement of the stage of development for each example. The author said that MBfR technology has been applied commercially and ICPB has been demonstrated (for MxC technology I think it varies widely based on the cathode configuration), but I'd like to know quantitatively the largest scale each one has been used on.

I also recently listened to a podcast interview (29 min.) with Dr. Rittmann that touched on the same topics as the paper (my recollection is that all three examples from the paper were at least mentioned). Here are some notes I took from that:

  • Environmental biotechnology is defined (by him) as partnering with microorganisms (to do something like removing pollution or generating a resource).
  • Normally it involves dealing with microbial communities rather than a single species.
  • It is a "voluntary" activity so we have to keep the microorganisms involved "happy".
  • He points out that most pollutants are resources that are in the wrong place or wrong form.
  • BOD (biochemical oxygen demand) is food (i.e. energy) for a lot of microorganisms. So spending a lot of energy to neutralize it* isn't as efficient as it could be. Alternatives are anaerobic digestion and microbial electro-chemical cells—these approaches allow for much of the energy in the BOD to be converted to a form we can capture and use.
  • He talked about how water can have contaminants "on both sides of the coin": oxidized (can accept electrons) or reduced (can donate electrons) chemical states. Hydrogen (H2) is a great electron donor so he's been developing a technology to supply hydrogen gas to a biofilm growing on a membrane to treat oxidized contaminants (e.g. perchlorate was mentioned).
  • Toward the end of the interview he has some good thoughts on process modelling.

*conventional WWTPs use a lot of energy to run aerators to satisfy the oxygen demand in the wastewater

There are a couple of other episodes from the same podcast that I've listened to recently that are also worth checking out (approx. 20 min. each):

Finally, if microbiology interests you, here is a blog to take a look at. This post was an interesting read.

Disclaimer: This is just a reminder that the views expressed in this blog are mine alone; I write it in my own capacity and not as part of my job.