By Paul Williams, ONDEO Degrémont
Groundwater is an important source of municipal drinking water for many small and medium-sized communities in Canada. Many favour groundwater over surface water because of its excellent and consistent quality, and because, generally, it requires little or no treatment before consumption. Unfortunately, many groundwater supplies are contaminated by varying levels of iron and manganese in concentrations that exceed the Canadian Drinking Water Guidelines. Although they have no negative effects on human health and the established limits for these elements are only aesthetic objectives (AO), the presence of iron and manganese in municipal potable water sources can be linked to the following problems:
Even though small amounts of iron and manganese are necessary for human health, the aesthetic and economic consequences have motivated many municipalities to invest in treatment systems that remove these metals from their well sources.
Iron and manganese when present in a ground source are generally found in their chemically reduced or soluble form. In this state, the metals do not present any problems. However, when the metals come in contact with oxygen in the air or other oxidants, they pass to their oxidized or precipitated form. This oxidized form is problematic because of its ability to foul, stain, and discolour.
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| Figure 1. Waterloo, QC, Ferazur, Mangazur® Filtration System. |
Conventional iron removal systems
Various chemical oxidants can be used to oxidize and precipitate iron and manganese from water, thus allowing them to be filtered. However, the rate of reaction is highly influenced by several factors including the concentration of the metal, the pH, the temperature of the water, the presence of other ions that are easily oxidized, and the presence of inhibiting elements and/or complex substances like humic acids, polyphosphates, and silica.
Any combination of these factors can significantly slow the oxidation of iron and/or manganese by mineral oxidants or by specific catalyzing media like green sand and manganese sand. For the removal of iron, often an intense aeration with a brief contact time will oxidize the iron and render it filterable by conventional single or dual media filters. The oxidation of manganese by oxygen alone is very slow, but the reaction is catalyzed by the presence of previously oxidized manganese (manganese dioxide). The use of strong oxidants such as ozone and potassium permanganate allow the reaction to take place in a reasonable length of time.
Conventional iron and manganese removal systems have been used in Canada for years with varying degrees of success but some have the following inconveniences:
With the advent of biological filtration for iron and manganese removal in Canada, municipalities are able to overcome most of the problems associated with conventional iron and manganese removal systems.
Naturally occurring bacteria, when placed in the right environment and given the appropriate oxidation-reduction potential are able to catalyze the oxidation of iron and manganese. These bacteria, which are already found in most iron and manganese containing wells, allow the simultaneous oxidation and filtration of these metals at rates two to four times higher than conventional systems. These 'biological filters' must be backwashed periodically to remove the accumulated metal oxides, but otherwise can be operated indefinitely. The systems also allow for the elimination of all strong oxidants, and they have much lower backwash requirements, resulting in reduced backwash water quantities.
Case study 1: Woodstock, New Brunswick
In the mid 1990s, an intensifying manganese problem in its well water forced the town of Woodstock, New Brunswick to begin looking at manganese removal systems. To compare the performance and operating costs of the different units, the town decided to pilot test several technologies including green sand, ozone, and a biological system.
Although all the systems were able to remove the manganese from the water adequately, the biological system was able to operate at higher filtration rates, had a lower capital cost, and operated without any chemical addition. It was the ability to operate without chemicals that was particularly important for sentimental reasons, because for over 100 years the town's water had been ''chemical free.''
After the pilot study, the new Mangazur® filtration system supplied by ONDEO Degrémont Ltd. (of Dorval, QC) was installed in the newly expanded well house. The new system consisted of two, ten foot diameter pressure filters filled with the specially designed filter media "Biolite." It was equipped with a raw water backwash system and control panel (the biological process allows the backwashing of the filters with raw water in certain cases). The system was designed with automatic valves and sensors to limit operator intervention during operation and backwashing.
After a short five-week seeding period, manganese removal efficiency had reached 90%, and the concentration of manganese in the filtered water was, and continues to be, 0.02 - 0.03 mg/l which is lower than the Canadian Drinking Water Guidelines Aesthetic Objective of 0.05 mg/l.
Since the installation of the Woodstock system, two other municipalities in New Brunswick have installed biological manganese removal filters for the treatment of their potable well water.
Case study 2: Waterloo, Quebec
The town of Waterloo, Quebec, had struggled for years with iron and manganese problems in the two wells that feed the town's distribution network. The municipality decided to study four different treatment methods including a UV/ozone system, conventional green sand filters, a system using potassium permanganate with an oxidation tower, and also biological filtration.
A pilot study was carried out using the biological system, and, after a subsequent economic evaluation, this system proved to be lower in capital and operating costs than the other systems. The elimination of chemical oxidants and the high filtration rates used in the biological system strongly influenced the outcome of the economic evaluation.
The presence of both iron and manganese in the water required the use of two biological filtration steps: one for the removal of iron and another for the removal of manganese. The system included an intermediate aeration column to adjust the pH and provide oxygen to the process. Although the seeding process for the two filters was slightly longer than anticipated, the plant reached full efficiency 15 weeks after start-up.
The concentrations of iron and manganese at the outlet of the full-scale plant are both significantly lower than the Canadian Drinking Water Quality Guidelines of 0.3 mg/l for iron and 0.05 mg/l of manganese.
Even though the biological system was not designed for turbidity removal, during periods of high surface influence in one of the wells where the raw water turbidity was as high as 12.8 NTU, the turbidity reduction was excellent and the filtered water turbidity was below 2 NTU (85% reduction). This was achieved without the use of any coagulant which was felt to be an impressive achievement for a single media pressure filter. During normal operation the turbidity is below 0.5 NTU at the filter outlet.
The biological system has great potential for small- and medium-sized communities because of the low capital and operating costs, simple operation without chemical addition, and the stable, reliable quality of treated water.
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