Biological processes in groundwater deferrization technologies

The overwhelming majority of settlements in Belarus are supplied with water from groundwater sources and only three cities partially use surface water for centralized water supply (Minsk, Gomel and Grodno). In most cases, groundwater, being of high quality in sanitary and bacteriological terms, contains elevated concentrations of dissolved iron, which may be in combination with other substances whose concentrations exceed the established norms (manganese, ammonium nitrogen, carbon dioxide, hydrogen sulfide, methane, etc.). However, more than 50% of centralized water supply systems in cities and most rural settlements do not have deferrization stations and their water does not meet sanitary standards. Therefore, the main problem of providing the population of Belarus with water of the required quality is the problem of water treatment. Similar problems are relevant for most regions of Russia and other CIS countries.

In the practice of groundwater deferrization the most widespread are aeration methods. In Belarus and a number of other countries since the middle of 60th years of the last century the method of simplified aeration with the subsequent filtration in pressure or non-pressure versions is practically universally applied. It is the most studied and recommended for application at iron content in initial water up to 10 mg/l (including oxide not less than 70 %), hydrogen sulfide up to 0,5 mg/l, pH not less than 6,8 and alkalinity more than (1+Fe2+/28) [1].

In spite of numerous studies of this method, it draws attention to a significant scatter of limitations of its applicability for the given and a number of other indicators (ammonium nitrogen, oxidizability, carbon dioxide, manganese, etc.). This indicates the complexity of the processes taking place and the mutual influence of the composition of treated water on them [2-4].

Many scientists already in the first studies of aeration methods of groundwater deferrization noted a significant influence of spontaneously developing biological processes on their efficiency [4,5]. On the nozzles of contact cooling towers, in the thickness or on the surface of filter media, accumulations of iron bacteria were formed, which, as a rule, showed a catalytic effect and contributed to the efficiency of the deferrization process. Degrémont specialists note that "...the boundary between purely chemical and biological oxidation cannot be clearly defined and many facilities of the first type work effectively due to the growth of microorganisms" [6]. [6]. According to the results of examination of many deferrization stations, operating by the method of simplified aeration with filters washed with non-chlorinated water, iron bacteria are found in most of them. This is evidenced by their large number in the wash water and in the formed deposits on the surface and in the thickness of the filter media. The reason for the development of iron bacteria is their presence in practically all investigated water intake wells, the possibility of entering through the aeration system, from water conduits and the favorable conditions for biological processes [7-10].

Despite these facts, the participation of microorganisms in deferrization processes was underestimated for a long time, and their influence on technological and design parameters of deferrization stations in most cases is not taken into account until now. All kinetic constants and parameters of reagentless (aeration) methods are determined on the basis of reactions and processes of purely chemical oxidation of iron [2-5]. The reason for this is that as a result of both chemical and biological oxidation of iron, its hydrates of very complex structure are formed, which are very difficult to distinguish without special microbiological studies. But it is obvious that the rate constants of chemical and biological reactions are different.

M.N. Mencha conducted the following experiment. A layer of disinfected and thoroughly washed crushed stone of 5-10 mm coarseness was loaded into three model filters. The first filter was left with sterile loading, the second one was filled with chlorine water treated sediments from operating filters of deferrization stations, the third one was filled with the same fresh untreated sediments. Preliminary microbiological analysis of sediments showed that there was a mass development of iron bacteria, mainly of the genus Leptothrix. The initial water with iron concentration of 1.37 mg/l was continuously fed to three parallel filters with the same speed for two days. The dynamics of iron concentration change in the filtrate of model filters is shown in the figure. The analysis of the obtained results confirms that the rate of biological oxidation significantly exceeds the rate of chemical oxidation of iron, and washing of filters with chlorinated water acts oppressively on the ongoing processes.

This site uses various cookies. We use cookies to personalize content, provide social media features and analyze traffic to our site. Some cookies are placed by third parties that appear on our pages. For more information and choices, please see our Privacy Statement and Cookie Settings.

Accept Change