FAQ about Bioreactor
For the woodchip source – how many options have been trialed and are there any differences in the source or type of woodchip used? Trees are plentiful in the Nordic countries but have other species been considered for other countries? For countries where wood is not as available, could alternatives be used?
In the bioreactor studies conducted within the NITREM project and its predecessors, only pine woodchips have been tested. Other studies in the USA, Canada and New Zealand have tested hardwoods and also other materials (e.g. compost, rice husks, corn cobs, wheat straw); these studies indicated that alternative materials are at least as effective as pine woodchips in providing an energy and carbon source for the nitrogen-removing microbial community. However, these studies have also shown that many of these materials also promote the production of undesirable byproducts, such as nitrite and ammonium. If alternative carbon sources are selected for a bioreactor, laboratory experiments should be used to trial the new materials, in order to evaluate the potential production of denitrification byproducts.
Once the woodchips have been used up as an energy source, how could this material be disposed of? Would it be classed as hazardous waste?
In the bioreactor, the woodchips will eventually degrade to the point where they no longer provide an efficient source of energy and carbon for the microbial community. In addition, the hydraulic properties will also eventually change so that proper treatment is impaired. At this point, the expended woodchips will need to replaced.
The bioreactor is designed for the removal of nitrate through denitrification. This process in itself does not leave any residual compounds in the woodchips, and degraded woodchips can be disposed of as ordinary organic waste material (non-hazardous waste). However, if the mine drainage that has been treated with the bioreactor contains elevated concentrations of compounds such as heavy metals, these may accumulate in the organic substrate (either as adsorbed metals or precipitated solid phases). Therefore, it would be necessary to chemically analyze the composition of the woodchips in order to determine their waste classification, prior to disposal. If high concentrations of heavy metals have accumulated in the woodchips, it is possible that they would be classified as hazardous waste. It is also possible, however, that the metals would be of economic value after extraction from the woodchips.
Extremes of temperature – has there been any research on this? Sweden may well get colder temperatures than elsewhere – is there any understanding of how seasonal temperatures change the efficiency of the treatment? Would changes in temperature potentially require a longer/shorter residence time? Also – are there any upper or lower temperature bands that severely reduce efficiency?
Many studies have focused on precisely this point – temperature. It is well-established that the process of denitrification is temperature-dependent: the higher the temperature, the faster the reaction rate. If the water to be treated exhibits large variations in seasonal temperature, then you should expect that treatment efficiency will be affected by this. For colder temperatures, a longer residence time will be needed for the same level of treatment at higher temperatures.
Our bioreactor system in northern Sweden has a nearly constant year-round temperature of 3oC. Of course, a year-round temperature of 10oC would have been better for nitrate removal, but these are the conditions we are working with. We have considered the possibility of introducing heat into the system, but there are many obstacles for the implementation of such measures.
The system removes nitrate, but what about ammonium and nitrite? What are the actual reactions here?
The bioreactors are designed to remove nitrate through the process of denitrification. This is a microbially-catalyzed reaction that actually occurs in a number of steps:
The final product is thus nitrogen gas, which is harmless and dissipates into the atmosphere. As can be seen in the above sequence, nitrite can also be removed through denitrification. Ammonium is unaffected by the process of denitrification. If there are elevated concentrations of ammonium in the water, it may be necessary to first aerate the water so that ammonium first oxidizes to nitrate.
What are the concentrations that the system works at? If the N concentration is too high, or too low (if a water quality standard was very low), are there any problems with efficiency?
We have worked with nitrate-nitrogen concentrations as high as 100 mg/L N; high concentrations do not appear to be a concern with bioreactor treatment. Indeed, the reaction rate generally increases with increases in nitrate concentrations. At the other extreme, the reaction rate will be slower with low nitrate concentrations (< 2 mg/L N), so this must be considered in terms of water flow through the bioreactor.
Is there any information on iron from the recent trials? It would be interesting to know more about how this technology and any metals in the water.
Iron is not affected by the process of denitrification. Iron will simply pass through the bioreactor in the dissolved form. However, under certain conditions (e.g. short hydraulic residence times, complete removal of nitrate), sulfate-reducing conditions may be generated in the bioreactor, producing hydrogen sulfide. The reaction of dissolved iron with hydrogen sulfide will result in the precipitation of a solid-phase iron sulfide.