BIO 2008: Sensing the sewage assassin

Cleaning slime out of sewage pipes is a dirty job but, as the saying goes, somebody's got to do it. That somebody is Australian environmental scientist Ross Chandler, but thankfully the biochemical solution he has developed to reduce the level of harmful bacteria in Australia's sewage treatment systems means nobody has to get their hands dirty.

Chandler is the creator of Biosol, a liquid product made up of naturally occurring chemicals that is injected into sewage systems to control the behaviour of corrosion-inducing bacteria. The product was featured in an episode of The New Inventors on the ABC in February of this year, where it was received enthusiastically and awarded best invention by the show's panel of expert judges.

Wastewater is rife with bacteria, which gather at bends in the thousands of kilometres of pipes leading to water treatment plants. These collections of bacteria eventually form slimes, known as biofilms, which generate poisonous gas and corrosive acid that cause millions of dollars worth of damage to sewerage infrastructure every year.

It's no surprise to anyone that sewers smell bad. But the source of that odour, hydrogen sulphide gas (H2S), is also responsible for the corrosion of sewage pipes - and worse. Hydrogen sulphide gas is also one of the chief threats facing sewer maintenance workers, many of whom are injured or even killed by the gas every year. Chandler says developing nations with less-advanced sewage treatment systems lose three to five people a year "just from opening manhole covers".

Hydrogen sulphide gas arises from the breakdown of organic and inorganic sulphates made possible by the anaerobic conditions that exist in sewage pipes. This hydrogen sulphide gas is later oxidised by other bacteria to form sulphuric acid, the main culprit behind pipe corrosion.

Recent research in the US places the cost of sulphuric acid-induced corrosion in the area of $US14 billion a year, and Chandler estimates the damage to Australian wastewater infrastructure alone is in the order of $1.1 billion a year.

---PB--- Quorum sensing

Biosol has its origins in the discovery in the early 1990s that bacteria can communicate with each other, a process known as quorum sensing. Quorum sensing is the method by which bacterial cells "sense" that their numbers have reached a certain level and then co-ordinate their behaviour accordingly.

The process is triggered in response to autoinducers that accumulate in bacterial cells once they begin reproducing. As soon as the cells sense that a quorum has been reached they go on the attack, flipping their internal switch for manufacturing virulence factors, such as biofilms. These polysaccharide shells provide the bacteria with tough, self-protective mechanisms that are almost impossible to fight with antibiotics.

Most of the research into quorum sensing to date has been medical in nature, designed to restore the potency of antibiotic treatments and limit the development of antibiotic resistance in bacteria. Restoring the effectiveness of antibiotics is one of the hottest areas of biomedical research at the moment, attracting a great deal of investment dollars for pharmaceutical companies both great and small.

But Chandler, whose background is in agricultural science, opted to pursue a different path and instead saw an opportunity to apply the principles of quorum sensing to the burgeoning field of environmental science.

Chandler first became aware of quorum sensing in the early 1990s, when he headed an Australian government-sponsored delegation to deal with vegetation problems in the Middle East. In the UAE, Saudi Arabia and Kuwait, most vegetation systems are irrigated with treated sewage effluent, which invariably contains high levels of phosphorus detergents.

When this sewage effluent is applied to vegetation systems, it causes a disassociation of the normal symbiotic feeding relationship between soil mychorrizae and plant roots. That limits the uptake of specific plant nutrients and causes chlorosis, or yellowing of the plant leaves. The plant cannot feed properly, stresses and often dies.

Chandler's investigation into the symbiotic relationship between soil mychorrizae and plant roots led him to the pioneering research into quorum sensing conducted by institutions like the University of Nottingham in the UK and the University of Montana in the US.

"I started thinking that there were major odour problems associated with a lot of irrigation in the Middle East, so maybe if we slowed down microbial activity we might be able to stop the odours," Chandler says.

Chandler then took his idea one step further, hypothesising that microbes might be controlled in wastewater systems to prevent odour generation and improve sewage processing - doing some good for the planet while also tapping into a much larger target market.

"As far as sewage catchments go, the idea was to send bacteria from a multi-cellular biofilm form back to a single-cell, or 'planktonic', form," Chandler says. Chandler uses the analogy of sending bacteria into hibernation. "By sending the bacteria into hibernation you reduce both their metabolic rate and their reproduction rate," he says.

"This also stops the formation of the polysaccharide glues, which stick the bacteria together, and they slowly begin to disintegrate, releasing the sediments which are caught up in the biofilm sediment complexes that collect on pipe inverts."

---PB--- Up and down regulation

Biosol controls bacteria in the sewage treatment process in two ways: up-regulating and down-regulating. "Up-regulating" bacteria occurs mainly at sewage treatment plants and leads to quicker and faster processing of the sewage.

In its current incarnation, however, Biosol primarily employs down-regulators - signal blockers similar to the furanones produced by the marine macroalga Delisea pulchra - to dissolve the biofilm sediment complexes that clog sewage pipes and catchments.

Controlling the growth of biofilms not only reduces corrosion from sulphuric acid, it also reduces greenhouse gas emissions. Hydrogen sulphide gas impedes sewage processing, but the sulphides that arrive at sewage treatment plants also must be converted back to sulphates in the treatment process.

That requires twice as much oxygen per molecule; for every H2S molecule four atoms of oxygen are needed to form H2SO4. Where that oxygen is supplied by aeration - common practice in most wastewater treatment plants - large amounts of energy are expended.

As a result, decreasing the amount of sulphides that arrive at a treatment plant also reduces the plant's energy requirements and cuts down greenhouse gas emissions.

"If you remove biofilms, you remove the source of odour, you remove the source of corrosion and you reduce the energy needs of the treatment plant, thereby reducing greenhouse gas emissions," Chandler says.

However, Chandler is quick to point out that there are many other potential applications for microbial control which can benefit the environment. "It is a breakthrough to able to manufacture a product that solves odour control problems in low-flow, long-detention treatment systems, because they're the ones that have given us the greatest grief, but there is still an enormous array of problems out there relating to biofilms and microbial colonization," he says.

Methane gas, for instance, is one area where Chandler and his company are focusing their attention, and the company conducted trials to reduce methane emissions with Brisbane Water last year.

Then there is the problem of corrosion in pipes that carry plain water, which is even more costly than damage to sewage pipes. "You're looking at $19 billion in the US alone in terms of damage caused by infrastructure corrosion, and once again it's microbial activity that's creating it," he says.

"It's a bit like computers were back in the late 1970s or early 80s. We're only at the beginning of a technology revolution with this stuff."