ARTICLE

Removal of iron and manganese using biological filtration in Argentina

By Virginia A. Pacini, Rosario, Argentina

Centro de Ingeniería Sanitaria
Universidad Nacional de Rosario
Riobamba 245 bis 2000 Rosario
tel/fax:+54-341-4808546

Virginia Alejandra Pacini is a PhD student of Engineering Science at Cordoba National University, Argentina. She graduated in Sanitary Engineering at Rosario National University, Argentina. At present, she is working on developing of appropriate technologies at the Sanitary Engineering Center and since 1999 she has been working on research projects for iron and manganese biological removal for drinking water.

Introduction: The situation in Argentina

Even when iron (Fe) and manganese (Mn) do not involve any direct risk for health, they cause a lot of aesthetic and organoleptic problems that affect the life standards of local inhabitants who use groundwater containing these metals. From a sanitary point of view these metals produce chloride consumption and promote biofouling and microbiological induced corrosion (MIC) in the distribution network.

In general, the bigger cities of Argentina are supplied by superficial source without any Fe and Mn problems. However, in several region of Argentina there are many towns and small villages that use groundwater containing Fe and Mn at higher levels than those permitted in the drinking water standard. In Santa Fe Province (Argentina) there are around 30 drinking water suppliers and villages that cannot meet the drinking water standards for Fe and Mn, having Fe concentrations between 0,5 and 3 mg/l and Mn between 0,3 and 1,5 mg/l.

The Alimentary Code of Argentina rules that the maximum value of Fe shall be 0.30mg/l and that of Mn 0.10mg/l, while the Santa Fe Province Drinking Water Standard set by Act 11.220 requires the values shown in Table 1 (Santa Fe is the province where the Center of Sanitary Engineering is located).

Table 1: Santa Fe Province, Argentina, Drinking Water Standard

Options for treatment

The processes available for Fe and Mn removal may be either be based on physico-chemical or biological processes. The main physico-chemical conventional treatment processes are: aeration followed by sand filtration with addition of chemicals; chemical oxidation (without pre-aeration) followed by filtration; conventional treatment combined with lime softening. All of them require the use from three to four chemical products.

The biological removal of Fe and Mn used in Europe, consists of two biological rapid filtration steps with or without aeration and with or without pH correction in case of Fe or Mn and Fe are simultaneously present. Bacteria which are present in raw water (e.g. Gallionella, Leptothrix, Siderocapsa, Sphaerotilus) can multiply on sand filters under appropriate conditions and are able to oxidize divalent ions (Fe²⁺ and Mn²⁺) to oxided forms (Fe ³⁺ and Mn⁴⁺) and then be settled (see Figure 1).

The advantages of biological treatment compared to the conventional physical-chemical treatment can be summarized as follows: no use of chemicals, higher filtration rates, possibility of using direct filtration and lower operation and maintenance costs.

Figure 1: Bacteria Gallionella from a wash water roughing filter, Avellaneda, Santa Fe, Argentina

Iron and manganese removal by biological processes has still not been used in Argentina. In Santa Fe province, there are only four (4) treatment plants in operation but all of them are based on physico-chemical processes. Out of these four plants, three are using chemical oxidation, pH adjustment and pre-aeration, followed by settlement and filtration. The fourth plant is based on conventional treatment combined with lime softening, and is using four chemical products.

All of these communities are placed far from the big cities, around 400 to 700 km, and for this reason transporting of chemical products is very expensive and difficult. After the economic crisis in Argentina in 2001, when the national currency plunged in value, most of this kind of facilities have economic losses due to operating costs (chemical, transport and energy costs) which increased around three times. If these existing plants could be converted into biological treatment plants, they could save 80% of their operating costs.

The main purpose of our study was to develop an appropriate treatment for Fe and Mn removal from groundwater by means of biological filtration processes, specially applicable to these small villages. The system studied comprises the following stages: aeration, up-flow roughing filtration and filtration (slow or rapid sand filtration). Pilot plant tests were carried out from April 2000 to December 2002 using two different natural waters. The first stage of trials (Avellaneda city, Santa Fe) was performed using natural groundwater with the following characteristics: total Fe 0,50 mg/l, total Mn 0,20 mg/l, pH 6,95 and oxide-reduction potential 320 mV.

The second stage of trials (Las Garzas town, Santa Fe) was carried out using another natural groundwater presenting the following features: total Fe 0,15 mg/l, total Mn 1,50 mg/l, pH 6.8 and oxide-reduction potential 420 mV. Before starting to operate the pilot plants, the presence of Fe and Mn bacteria (Gallionella and Leptothrix) was detected in both raw water. The startup period for Fe was a few days, where the removal efficiency reached 95% and kept constant along the trial. The startup period for Mn was eight weeks, where the Mn removal efficiency increased from 60% to 90%.

Figure 2: % Removal Efficiencies of total Fe and Mn during Start Up Period Avellaneda, Santa Fe, Argentina. A: first cleaning of roughing filter; B: Second cleaning of roughing filter

Results: The system works fine

The trials proved that up-flow roughing filtration had a significant role in the treatment line since a very high efficiency was achieved, and simultaneous removal of iron and manganese can be reached in one step. The method requires a very simple washing that consists in a botton drainage and, when the concentrations of Fe and Mn aren't very high, it can be used without the last step of filtration.

The results obtained proved the great advantages offered by the use of up-flow roughing filtration for Fe and Mn removal by biological processes. It is a very efficient process with low operation costs due to the following reasons:

• Very long filter run periods: forty five days for the roughing filter and ninety for the slow sand filter, or two weeks for the rapid filter.
• The cleaning operation of the roughing filter is very simple.
• It does not require skilled operators.
• No chemical product is used.
• Maintenance costs are very low since no mechanical equipment is required.
• The volume of water used for cleaning is not relevant in comparison with the quantity of water produced.
• Intermittent operation does not affect the removal process.

Conclusion: The biggest difficulty is to convince the people

The most difficult stage, to convince the people about the advantages of the process. Now, the research team is developing public awareness of their work, and is preparing several technical workshops and meetings to provide information for decision makers (water suppliers, water authorities, etc.). The team has found some constraints to the acceptance of this "new" appropriate technology because there aren't any biological plants working in Argentina yet. Another problem is that, on one hand, that there are several well known enterprises suppliers of physico-chemical treatment plants who are not willing to share the business and , on the other, the government is not coordinated enough to bring technical support or promote the nexus between the different University Technical Assistance Offices, like our Center.

The technology developed combines simple and low cost operation and maintenance with high removal efficiencies. As a result of the research, design parameters are available to be used for full scale plants design. This technology could be innovative in Latin America, as it has been proved reliable with low costs due to the use of no chemicals and minimal investment and operation costs.