General Characteristics:
The iron bacteria are an unusual group of microorganisms found in industrial waters, streams, lakes, wells, and in potable water supplies. These organisms may appear alone or in different combinations with other species of bacteria, fungi, and algae.
“Iron bacteria” are considered to be capable of withdrawing iron present in their aquatic habitat and of depositing it in the form of hydrated ferric hydroxide on or in their mucilaginous secretions. Similar mechanisms are employed by bacteria utilizing manganese. The large amount of brown slime so produced will impart a reddish tinge and an unpleasant odor to drinking water which may render the supply unsuitable for domestic or industrial purposes. Such bacteria may also be the causative agents in the initiation of pitting and tuberculation of pipes.
Although the iron bacteria have not been studied as completely as some of the other organisms, this lack of study does not indicate that they have not caused serious difficulty in industrial and fresh water systems. There are many cases histories regarding the effect of these iron bacteria in city and industrial water systems. The published reports on the effect of iron bacteria in city water systems are many and are justifiably referred to as “water calamities.” The universal distribution of the iron bacteria is born out of the development of these water calamities in Europe and the United States. Recently, these microorganisms have gained recognition in industry by their effect upon the process equipment and on the finished product. Usually, the affected will become slightly turbid or acquire a light reddish tint and/or objectionable odor. As these microorganisms increase in number, the water may become more turbid, and the color of the water will become brick-red. Hence, the common reference to “red water.”
In addition to discoloring the water, this group of microorganisms produce undesirable accumulations in pipes, nozzles, ponds, etc. These deposits, in time, will slough and plug lines, foul pumps, valves and/or affect the quality of the finished product. Bacteria of this type, oxidize ferrous to ferric ion, which is precipitated as ferric hydrate. Iron may be obtained from the pipe itself or from the water being carried. The amount of ferric hydrate deposited is usually very large in comparison with the enclosed cells. Some bacteria belonging to this group which do not oxidize ferrous iron may, nevertheless, indirectly cause it to be dissolved or deposited. In their growth, they either liberate iron by utilizing organic radicals to which the iron is attached or they alter environmental conditions to permit the solution or deposition of iron. In consequence, less ferric hydrate may be produced but taste, odor, and fouling may be encouraged.
The growth and development of iron bacteria may manifest itself in several different ways. For example, it has been reported that these microorganisms have reduced the effect area of a 6” pipe to that of a 2” pipe in a matter of weeks. The accumulation in pipes, pumps, nozzles, etc., may be hard and crusty or it may be relatively light and spongy in appearance. Often, other biological slimes may be associated with iron bacteria and thus further reduce or restrict the flow of water through pipes or entrap other debris. The association of other species of microorganisms with the iron bacteria will increase turbidity, increase color, give rise to objectionable odors and taste in domestic water supplies and also cause sloughing of deposits in industrial systems which may affect the process or the product being produced.
The iron bacteria are usually considered as typical fresh water organisms. Recently, however, some of the iron bacteria has been isolated from high brines, which indicate the versatility of this class of organisms. These microorganisms are considered as aerobic, but they have also been found to grow in waters with very low oxygen content.
The principle distinguishing characteristics between the iron bacteria and other types of microorganisms is that they have the capacity to absorb and to accumulate iron and/or manganese when grown in environments which contain these elements. These organisms deposit iron and manganese salts around their cells which result in the characteristic reddish brown-black color.
The most common iron bacteria fall in the following genera: Sphaerotilus, Lepththrix, Toxothrix, Crenothrix, Clonothrix, Gallionella, Siderocapsa, Siderosphaera, Sideronema, Ferribacterium, Sideromonas, Naumanniella, Ochrobium, Siderococus, Siderobacter, and Ferrobacillus.
The iron bacteria are extremely difficult to grow in the laboratory using the usual cultural media. Therefore, it is not possible to evaluate the number of these organisms in different water supplies with the usual bacteriological procedures. The satisfactory procedure for examining the population index of these organisms in water supplies is a direct microscopic analysis of the water. In general, the iron bacteria prefer the lower temperatures but these organisms have been observed to grow at temperatures which range from 0°C. to 40°C. Their optimum temperature, however, will range from 6° to 25°C. The growth of iron bacteria is very much influenced by the hydrogen ion concentration. Although published reports indicate the different species have different requirements, the pH range for growth will vary from 5.5 to 8.2 with the optimum pH requirement being around 6.5. These organisms are not affected by light and have been found to grow in exposed areas, in shade and in complete darkness, such as in pipes, wells, etc. The growth of these organisms is very much influenced by the amount of dissolved iron in their immediate environment. These bacteria have been found in waters with as much as 30 parts per million of iron and they have also been found in waters which contain as low as .1 part per million of iron.
The Galionellas and Other Unicellular Forms
Like the coral animals, the Galionellas are more notable for their excreta than for themselves. They are small, curved rods widely distributed in steams and springs where iron is in solution. From the concave sides of the cell issues a flat twisted ribbon of iron oxide (Fe[OH]3) held together by some cement. The physiology of the Galionellas is unclear, but in the absence of CO2 or air, no growth will occur. These organisms have been classified as autotrophic reducing CO2 by the following reaction: 4FeCO
4FeCO3 + O2 + 6H2O – 4Fe(OH)3 + 4CO2(-F=+40,000Cal°)
A different unicellular iron bacterium, of simple rod shaped is Ferrobac. Ferro-oxidans which oxidizes Fe++ to Fe++ vigorously at pH 3 to 3.6 at 37°C.
Several other unicellular iron bacteria have been described, particularly Siderocapsa, a coccoid form embedded in a gelatinous capsule which becomes heavily encrusted with iron oxide which could provide anaerobic conditions at the bottom of the growth of sulfate-reducing bacteria.
The Chlamydobacteria
The second kind of iron bacteria comprises sheath filaments of a variety of types. The sheath becomes impregnated to varying degrees with Fe(OH)3, and sometimes with Mn(OH)3. Most of these seem to be forms of Sphaerotilus natans. This organism is widely encountered as a pest in sewage disposal plants, where its filamentous nature prevents the activated sludge from settling. A typical iron bacterium belonging to this group is Crenothrix which begins life a single, nearly spherical, non-motile cell which divides by fission and forms a chain, with the cell elongated in the process. These bacterial cells exude a “mucilaginous” tubular sheath which hardens and becomes impregnated with ferric hydroxide. Under good nutritional conditions and an adequate oxygen supply, this organism may multiply to such an extent as to completely retard the water flow through pipelines, or, in the case of water flood operations, may completely block formation.
Chlorination has been used for control of iron bacteria for many years; however, there are drawbacks in the use of this material. High chlorine demand due to iron and organic matter in the water and the possibility of damage to cooling tower lumber has shifted the emphasis to the use of non-oxidizing biocides such as the quaternary ammonium halides, the organo-sulfur compounds, the polychlorinated phenolics and the phenolic-amine compounds.
Treatment
In recirculating cooling water systems, iron bacteria can be controlled with the use of proprietary microbicide formulations; or a combination of chlorine and another microbicide. In any case, two materials should be used alternately to minimize the possibility of developing resistant microorganisms. The control of iron bacteria thus becomes a part of the total microbicide program for the system. Best results are obtained if the microbicide is fed intermittently rather than continuously. If possible, the bleed-off valve should be closed for several hours following biocide addition, for longer retention of the biocide.
For once-through systems, the microbicide should be fed for at least one hour per 8 hour shift. Treatment for potable water systems must be approved by the Public Health Service, thus iron bacteria control is limited to the use of chlorine only, in these systems.