A pretreatment process using phagotrophic algae shows potential to help wastewater treatment plants boost efficiency in aerobic and anaerobic digestion.
Clean-water plants increasingly look beyond the treatment of wastewater and toward resource recovery. A variety of technologies coming to market in recent years are designed to help further that objective.
Such processes are not limited to the liquid side of treatment. Solids treatment is a costly process in its own right, and plant operators and researchers are looking for ways to make it more efficient and increase its yield of usable products.
Now Ovivo has developed a solids pretreatment process that uses a special form of algae to help make sludges more digestible. The BioAlgaNyx process can shorten the time required to produce biogas and biosolids, bringing a range of potential benefits. Hiren Trivedi, director of strategic development for Ovivo’s municipal division in North America, talked about the process in an interview with Treatment Plant Operator.
TPO: What are the drivers behind bringing this technology to commercialization and into the wastewater treatment market?
Trivedi: Looking at the wastewater industry, we envision that in the next five to 10 years we will see more changes than in the previous 100 years. The primary reason is that we now realize that wastewater is not a nuisance — it contains valuable resources, and technologies are being put forward to recover them. The BioAlgaNyx technology is a step in that direction.
TPO: In the most basic sense, how does this technology work?
Trivedi: We use a unique form of algae called phagotrophic algae. Until now, most of the research done in the wastewater industry has been with photosynthetic algae, with the main objective of recovering nitrogen and phosphorus. Our main objective is not nutrient recovery but organic carbon recovery.
TPO: What exactly do you mean by phagotrophic algae?
Trivedi: Phagotrophic algae is a type that can consume not only dissolved organics in water but also particulate matter. That particulate matter can include organic carbon in the form of oil droplets, as well as organic carbon within bacteria cells and viruses. The algae cells have the capability to engulf bacteria, viruses and other small organic particles.
TPO: In the context of the wastewater treatment market, how do these algae function?
Trivedi: If you expose high-strength wastewater or municipal sludge to these algae, the algae cells will feed off the organic particles, bacteria and viruses and convert the carbon into fat, also known as lipids. Typically, the lipid content of the algae cells will be five to 15 times higher than that of a bacterial cell.
TPO: What practical applications does this have?
Trivedi: One is to capture organic carbon from high-strength industrial wastewaters along with the algae and make a saleable product. On this we are doing fundamental research at the university level, and the results have been fairly promising. So a juice manufacturer, for example, or a refinery, would be able to harvest the algae, and the fat content could be extracted to manufacture bioplastics or biofuels, or it could be used as animal feed additive.
The other involves biological sludge generated by municipal wastewater treatment plants. As a rule of thumb, a 1 mgd plant can generate a ton of solids every day.
Stabilization of that sludge is time-consuming and expensive. We can expose the sludge to the algae before either aerobic or anaerobic digestion. We convert some of the organic content, bacteria and viruses into algae biomass and send the mixture on to the digester. Because these algae cells have a high fat content, it makes the whole mass more biodegradable.
TPO: What is the benefit to the digestion process?
Trivedi: The digestion is very fast. We have done both aerobic and anaerobic digestion based on algae-converted biomass and have compared that to digestion of normal biological sludge. We find that we can speed up the digestion process by more than 50 percent.
TPO: How does that translate to an advantage for the wastewater treatment plant?
Trivedi: It’s twofold. If it’s a new plant, then there is a straight capital savings because they can start with smaller digesters. If it’s a retrofit, then within the same infrastructure they can get an expansion of capacity, or a longer retention time, which means higher volatile solids reduction and less biosolids to haul out.
TPO: How is the algae introduced to the process?
Trivedi: It’s a pretreatment process that involves a few steps. There could be a pH adjustment before we expose the biomass to the algae. Also, the biological sludge is flocculated, and we need to disturb that floc to get the particles and bacteria cells into the water where the algae can engulf them. Then there is the algae conversion step, which is an aerobic process. And then we thicken the sludge before sending it to the digester.
TPO: Are there any particular-size plants for which this is the most advantageous?
Trivedi: Our initial commercial applications will start with smaller plants, but actually the larger plants would have more benefit because they typically have anaerobic digestion with the objective of capturing organic carbon. Algae pretreatment should speed up that process and provide better organic carbon recovery. In addition, if they are doing co-digestion, bringing in food-grade waste, that becomes another source of organic carbon recovery.
TPO: In anaerobic digestion, how does the algae pretreatment affect biogas production?
Trivedi: It would speed it up, but it would not change the amount of biogas generated.
TPO: What has been done to test and prove out this approach?
Trivedi: We started more than two years ago working with the University of Akron where the concept was initiated. We have been funding research at the university level. We have done a nine-month pilot at the Canton Wastewater Treatment Plant in Ohio, and now we are running a pilot at the Akron Wastewater Treatment Plant. We are also doing laboratory-scale research on the use of algae with high-strength wastewater in different industries. Commercially, we are ready to move to market in applications involving enhancement of aerobic digestion.