Fixing broken ecosystems
29 November 2013 by Jags Pandhal
The world's growing population is increasing demand for resources like food and clean water - but more people means more waste, and that's having a direct impact on those resources. But all is not lost: Jags Pandhal and colleagues describe a new technique for cleaning up ecosystems that has knock-on economic benefits too.
With Earth's population predicted to exceed nine billion by the middle of this century, providing enough food, water, medicines and renewable energy is becoming a major challenge. At the same time, global economic growth means more waste from industry and agriculture is making its way into the environment, and this pollution damages the ecosystems that we rely on for so many of our resources.
One very visible example is eutrophication. This happens when too many nutrients such as phosphorus and nitrogen reach our water systems, causing a bloom of overfed algae. Algal blooms make water undrinkable and starve it of oxygen. This damages wildlife and biodiversity and in turn affects fisheries and tourism, and the toxins released can be a severe health hazard.
2013 saw China's largest algal bloom on record and the Chinese government has pledged £400bn over the next decade to meet increasing drinking-water demand. But blooms are on the increase everywhere; more than three-quarters of England's freshwater systems are classified as eutrophic, with estimated clean-up costs of up to £114m a year.
Climate change could add to the problem, as warmer temperatures and changing rainfall patterns can encourage algal growth.
No silver bullets
There are several ways to prevent or treat eutrophication, though each has limitations. Legislation to control the nutrients coming from industry, sewage or agriculture has made the future look brighter, but because stored phosphorus continues to be released it can take decades before we see any improvement.
There's ecological engineering, for example chemically fixing nutrients to the sediment to stop them causing damage, or protecting our natural wetlands so the plants can soak up excess nutrients before they can cause a bloom. Or there's bio-manipulation, for example encouraging the growth of algae's natural predators. Alternative methods include barley straw and ultrasound (see Planet Earth Summer 2013, pp30-1).
An attractive option is to remove the algae, but doing this efficiently is a challenge. The good news is there are economic as well as conservation reasons to get better at it; but despite the algal biofuel industry's efforts over the last decade, harvesting still accounts for as much as 30 per cent of industry costs. One of the most popular harvesting methods, dissolved air flotation (DAF), widely used by the water treatment industry, is considered too expensive to use on a large scale. But all that could be about to change.
Eco-technology: Microflotation with bubbles
DAF clarifies wastewater by removing solids suspended in it. It works by saturating water with air under pressure, then releasing it through a diffuser which causes bubbles to form. A chemical flocculent is added to make suspended particles in the water clump together, and the bubbles float the solids to the surface where they can be more easily removed. It works, but it uses a lot of energy.
Now our team at Sheffield, led by Professor Will Zimmerman, has developed a cheaper alternative. The breakthrough has been in how the bubbles are made. Conventional DAF mechanisms depend on their diffuser to generate microbubbles, which break off when they are big and buoyant enough. Our system, microflotation (MF), has no moving parts but instead uses the dynamic properties of the water itself. As air enters the system the water flows from side to side due to the so-called Coanda effect, which dictates the direction of fluidic flow. This fluidic oscillation releases bubbles at the infant stage, much sooner than if they had to break free by themselves.
Microflotation can produce the same effect at just 5-10 per cent of the cost.
With just a small amount of flocculent, these tiny bubbles can float microscopic particles like algae cells to the surface of the water.
Traditional bubble-based separation systems are expensive because they have to pump both gas and liquid to make their bubbles, and pumping liquid takes a lot of energy. MF only pumps gas so it uses a fraction of the energy. It doesn't use large saturation tanks and needs none of the expensive safety certification required for operating in high-pressure conditions; in fact it has no moving parts. Essentially, MF can produce the same effect as a standard DAF system but at just 5-10 per cent of the capital and operating costs.
So far so good: MF works well in the lab but how do we scale up the process to make it commercially viable? MF bubbles last for a long time and can efficiently process around 200 litres of water. Where a large body of water needs treating one option would be to have MF operating in a lagoon from which treated water is released. We're also looking at the use of organic flocculants and how these can be chosen to complement water characteristics, for example pH, and the types of algae being removed. Once these questions are resolved, microflotation could be a crucial tool for both conservation and the biofuel industry.
Good for the environment...
Many environmental problems we face today are an indirect result of mankind's engineering progress, so it's fitting that our engineering proficiency should help solve them. But before we use microflotation in the environment, we need to use all our scientific expertise to assess the possible effects on biodiversity and ecosystems, and make sure the technology does good and not more harm. As an important first step, we are developing techniques using proteins to give us a snapshot of what is happening to the water biology in lab experiments. We're combining this data with more traditional ecology approaches that will help us predict and control the effects of microflotation in the environment. Laboratory experiments must be moved into the field as natural ecology is a much more complex system.
...Good for the economy
The secret to making biomass removal even more cost effective is to find other ways of making money out of the process - and there are many. Microalgae grow very fast and are very efficient at converting the sun's energy into forms we can readily use. They store energy as lipids or fats (some species are up to 76 per cent lipids), which can be converted to bio-diesel and jet fuel, or used in cosmetics and drugs. Algal protein can be turned into animal feed or food supplements, and the carbohydrates can be converted to ethanol and methanol. The algae themselves can be directly fed into anaerobic digesters to produce methane, or used as a fertiliser which helps restore the structure of eroded soils.
In an age when we have to balance protecting the environment with the challenge of sustaining a growing human population, we need innovative technology more than ever. Algal blooms are becoming more prevalent around the world as a direct result of human waste and carelessness. Stopping them happening is paramount, but with smarter technology and the right financial drivers we can start to fix these broken ecosystems and benefit both the environment and the economy.
Jagroop Pandhal is the NERC/TSB Research Fellow for the Algal Bioenergy Special Interest Group (AB-SIG) and is based in the Department of Chemical & Biological Engineering at the University of Sheffield. This research was also supported by a Catalyst grant made under NERC's Resource Recovery From Waste programme.
Professor Will Zimmerman is an expert in microfluidics and inventor of the technology, in the same department.
James Hanotu is a Consulting Project Engineer for Perlemax Ltd, working on microflotation trials and the application of microbubbles in bioreactors.