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Environmental biotechnologies can make water pollutants part of the path to mitigating climate change [1]
['Bruce E. Rittmann', 'Biodesign Swette Center For Environmental Biotechnology', 'Arizona State University', 'Tempe', 'Az', 'United States Of America']
Date: 2023-03
To slow and ultimately reverse global climate change, society needs to replace fossil sources of energy and chemicals with renewable forms. Environmental biotechnologies, which utilize microbial communities that can provide human society with sustainability services, can play key roles towards this goal in two ways that are the focus of this perspective. First, technologies that employ anaerobic microbial communities can produce renewable, carbon-neutral energy by transforming the energy contained in the organic matter in wastewaters to methane gas, hydrogen gas, or organic chemicals used in the chemical industry. High-strength organic wastewaters are common from many facets of our systems of food supply: e.g., animal farms, food processing, uneaten food, and biosolids from sewage treatment. While anaerobic digestion of sewage biosolids is a long-standing method for making renewable methane, new, more-advanced environmental biotechnologies are making energy-generating anaerobic treatment more reliable and cost-effective for treating the wide range of organics-bearing wastewaters and for producing output with greater economic benefit than methane. Second, photovoltaic, wind, battery, and catalytic technologies require large inputs of critical ninerals and materials: e.g., Rare Earth Elements, Platinum Groups Metals, gold, silver, lithium, copper, and nickel. Environmental biotechnologies can create new, renewable sources of the critical materials by recovering them from wastewaters from mining, ore-processing, refining, and recycling operations. When provided with hydrogen gas as an electron donor, anaerobic bacteria in biofilms carry out reduction reactions that lead to the formation of nanoparticles that are retained in the biofilm and can then be harvested to serve as feedstock for the photovoltaic, wind, battery, and catalytic technologies. This perspective describes both ways in which environmental biotechnologies will help society achieves it sustainability goals.
Copyright: © 2023 Bruce E. Rittmann. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
An important new source of renewable fuels, chemicals, and CMM can be wastewaters that contain pollutants that can become tomorrow’s renewable resources. Here, I propose that environmental biotechnology presents an opportunity for recovering renewable fuels, chemicals, and CMM from wastewaters. I define environmental biotechnology as forming partnerships with microbial communities so that they provide human society with sustainability services [ 3 ]. Environmental biotechnology is a “win-win” strategy: Human society becomes more sustainable, while the microbial communities gain a “great microorganism life.”
Replacing fossil sources will demand large investments in photovoltaic, wind, battery, and catalytic technologies that are carbon-neutral or -negative. Those technologies will require large inputs of Critical Minerals and Materials (CMM) that already are in short supply [ 2 ]: Rare Earth Elements (REE), Platinum Groups Metals (PGM), gold, silver, lithium, copper, and nickel. Thus, the corollary challenge is to develop new and renewable sources for CMM.
While humans in the 21 st Century face many pressing challenges, the one that is the most pervasive and, perhaps, most difficult to overcome is slowing and eventually reversing the buildup of greenhouse gases in the Earth’s atmosphere [ 1 ]. Not meeting the challenge will mean that humans will endure the increasingly catastrophic effects of global climate change. The most important step will be replacing fossil sources of fuel and organic chemicals in ways that ensure energy and economic security.
Converting organic pollutants to renewable methane gas
Organic pollutants embed renewable energy in their carbon. The measure of the energy potential is the Chemical Oxygen Demand (COD), which represents electron equivalents that can be transferred from the original organic materials into a chemical form readily used in human society. Two prime examples of readily usable products are methane gas (CH 4 ) and hydrogen gas (H 2 ), and this section addresses CH 4 .
Waste streams with significant concentrations of organics are plentiful. Most of these waste streams stem from our agriculture and food system: e.g., animal manures, food- and beverage-processing wastewaters, wasted food, human sewage, and the biosolids generated from treating these wastewaters. Today, only a tiny fraction of the energy potential in the wastewaters is being realized, because the anaerobic processes that convert COD to CH 4 or H 2 are perceived as too expensive, large, and unreliable. However, recent science and technology advances are turning that perception upside-down by making modern anaerobic treatment less expensive, smaller, and more reliable than conventional approaches.
The conversion of organic matter to CH 4 gas, i.e., methanogenesis, has been used for many decades to treat sludges at wastewater treatment facilities [4,5]. The conventional approach treats a slurry of organic solids in a large, well-mixed anaerobic digester [3,6,7]. Anaerobic microbial communities hydrolyze the organic solids to soluble organics that are then fermented in multiple steps to form CH 4 , which bubbles out of the liquid and can be used to replace fossil natural gas [3,7,8]. Due to the needs to hydrolyze the input solids and to retain slow-growing methanogens, conventional anaerobic digesters have large volumes: e.g., hydraulic retention times of at least 15 days and often much longer [3,7]. Although methanogenesis is well-established, its characteristics lead to the well-known drawbacks: large and expensive reactors that are subject to upsets that compromise performance [3,7,9,10].
Fortunately, several advanced processes for producing CH 4 are overcoming the cost, size, and performance limitations. The advanced processes employ one or more of three innovations: pre-treatment to make the organic solid more rapidly hydrolyzed, filtration membranes to remove all solids from the effluent, and biofilm carriers to accumulate more biomass per reactor volume.
The first innovation is pre-treatment of the wastewater to make the organic solids more rapidly biodegradable. Thermal, mechanical, electro-mechanical, acid, and enzymatic methods disrupt the physical and chemical structures of the organic solids, which accelerates their hydrolysis and increases the fraction of the organic matter that is converted to CH 4 [11,12]. As illustrated in Fig 1, these solids-treatment methods mimic the mechanisms humans use to make their food more digestible: cutting and chewing, cooking, and enzymes and low pH in the stomach. A key concern for any pre-treatment technology is whether its energy and financial costs are exceeded by its benefits from more solid destruction and CH 4 production.
The most common pre-treatment today is a heat treatment, such as the Cambi process, which uses heat (150–165°C), steam pressurization in a closed reactor, and rapid decompression to disrupt the solids and accelerate hydrolysis [12]. Cambi clearly can increase the rates of solids hydrolysis and conversion to CH 4 , but the heating and pressurization incur energy costs, which may challenge the process’s overall sustainability. In addition, heat treatment can release toxins that inhibit methanogenesis and that may increase the level of hazardous organic molecules in the output biosolids [13,14].
Other pre-treatment methods have proven to offer substantial net benefits. For example, a full-scale trial of the Pulsed Electric Field (PEF) technology (an electro-mechanical approach) provided a 60% increase in CH 4 production from thickened primary plus activated sludge, which led to an increase in energy output that was 2.7- to 5.2-fold the energy input and a payback period of 3 years [11]. Laboratory testing also demonstrated an 80% increase in CH 4 generation from pig manure and a 100% increase with waste activated sludge [15].
The second innovation is employing filtration membranes to removal all solids from the effluent, creating an Anaerobic Membrane BioReactor (AnMBR) [16–18]. The AnMBR is derived from the aerobic MBR (AeMBR), which is now well-established for aerobic treatment of low-strength wastewaters, such as domestic sewage, particularly for small installations [19–21]. AeMBRs and AnMBRs share certain features. In particular, perfect solids removal by the membranes can make the solids retention time (SRT) much greater than the HRT, which enhances hydrolysis of organic solids, retains slow-growing microorganisms (e.g., methanogens in an AnMBR), and increases the concentration of the mixed-liquor suspended solids (MLSS), which makes the reactor smaller and less expensive. The major difference between the AnMBR and the AeMBR is supplying oxygen gas (O 2 ). The AeMBR must be vigorously aerated to provide O 2 for aerobic respiration and to minimize fouling of the membranes; vigorous aeration is energy consuming, which is a well-known drawback of AeMBRs. In contrast, the AnMBR excludes O 2 and produces CH 4 gas, making it a net energy producer.
While still a relatively new technology, the AnMBRs is in full-scale practice. By using anaerobic metabolism, the AnMBR accentuates the benefits of solids retention and a long SRT. On the one hand, anaerobic microorganisms have much lower biomass yields than do aerobic microorganisms [3], which means that very long SRTs can be obtained while still having a realistic MLSS concentration. On the other hand, realistic MLSS concentrations in AeMBRs are limited by O 2 delivery [3], which is not relevant for an AnMBR. Thus, AnMBRs can operate with large SRT/HRT ratios that lead to relatively short HRTs and small reactor volumes. Perfect solids removal and retention of slow-growing methanogens also lead to reliably good effluent quality and CH 4 generation in AnMBRs, compared to anaerobic reactors without membranes.
The main challenge of the AnMBR (like all MBRs) is fouling of the membranes by the accumulation of solids and biofilm on the membrane, which leads to excessive pressure drops across the membranes. Fouling by a cake layer may be accentuated in AnMBRs as the MLSS concentration is increased, which means that means to minimize or reverse cake formation are essential. The simplest means to mitigate cake fouling is to sparge biogas into the membrane modules to dislodge excess solids accumulation [16–18].
The third innovation is adding mobile biofilm carriers, which enhance the retention of slow-growing microorganisms; adding biofilm carriers creates an Anaerobic Moving Bed Biofilm Reactor (AnMBBR). Aerobic MBBRs (AeMBBRs) are well-established (e.g, [22,23]), and the AnMBBR has gained interest more recently (e.g., [24–28]). The carriers, which usually are made of light-weight plastic, are held in the reactor by screens; the right side of Fig 2 shows two examples of mobile biofilm carriers. The carriers retain most biomass, especially the slow-growing methanogens in an AnMBBR, which keeps the concentration of suspended biomass low. Thus, a main advantage of the AnMBBR is that biofilm accumulation on the carriers increases the allowable biomass density in the reactor, but without having a high MLSS concentration.
The ultimate strategy is to combine membranes and mobile carriers to form an Anaerobic Biofilm Membrane BioReactor (AnBfMBR); Fi 2 illustrates two types of AnBfMBfRs. A particularly good example of an AnBfMBR is the Staged Anaerobic Fluidized Membrane BioReactor (SAF-MBR), which was developed in South Korea by Professors Perry McCarty and Jaeho Bae [29–33]. As shown on the left side of Fig 2, the SAF MBR includes granular activated carbon (GAC, ~ 1.1-mm diameter) as a fluidized biofilm carrier in both stages, with membranes in the second stage. Large-scale testing with domestic wastewater documented that the SAF-MBR’s effluent could meet secondary-treatment standards and minimize energy inputs for treatment. A major advantage of the SAF-MBR is that the fluidized GAC particles naturally scour solids from the membrane surface, which helps maintain a low pressure drop for permeate flow. A drawback is that the GAC abrades the polymeric membranes, leading to the need to replace the membranes [34].
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https://journals.plos.org/water/article?id=10.1371/journal.pwat.0000105
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