New Methods for Better Wastewater Purification
Before wastewater reaches recipient waters, nutrients must be removed in order to avoid eutrophication and large algal blooms, which may result in serious damage to animal and plant life. Robert Almstrand at the Depertment of Chemistry and Molecular Biology at the University of Gothenburg, Sweden, has shown in his thesis that better removal of nitrogen from wastewater can be achieved by providing the bacteria that purify the water with alternating high and low levels of nutrients.
The emission of wastewater to oceans, lakes and rivers contributes nutrients in the form of nitrogen and phosphorus to these recipients. This causes large algal blooms, and to prevent this the wastewater must be purified in wastewater treatment plants before being released.
The removal of nitrogen is a biological process in which different groups of bacteria are used to convert the nitrogen compounds to nitrogen gas in a number of steps. The nitrogen gas is then emitted to the atmosphere. The first, and rate-limiting step is nitrification. Nitrification is carried out by bacteria that oxidize ammonia via nitrite to nitrate ions.
Almstrand shows in his thesis that the ability of the bacteria to carry out nitrification is improved if the bacteria receive alternating high and low levels of substrate (in the form of dissolved ammonium), rather than a constant low level. This will enable the wastewater treatment plant to cope better with variations in the concentration of nitrogen in the wastewater.
"Since the composition of the wastewater varies continuously, it was important to carry out the studies in a set-up that was as similar as possible to real wastewater treatment plants. For this reason we constructed a pilot plant at Ryaverket in Gothenburg", Almstrand says.
The bacteria grew in what are known as "biofilms", which are dense collections of microorganisms on surfaces. Almstrand used microscopy and digital image analysis to study how the bacterial colonies in the biofilms changed when exposed to different levels of nutrients.
"Different species of bacteria are promoted or inhibited to different extents by changes in their habitat", Almstrand says. "These properties are reflected in their positions in the biofilm, and so we developed new methods to analyse these in detail. The new methods are very flexible and can be used to analyse pretty much any type of biofilms."
Furthermore, Almstrand discovered that groups of bacteria that are normally considered to be the same species (Nitrosomonas oligotropha) were affected differently by the changes. It is, thus, important to understand the diversity within species of nitrifying bacteria, in order to improve the removal of nitrogen.