By S. Muralidhararao and T.N.V.V. Rao

Drinking water is tapped from two different sources in general: surface water (rivers, lakes and ponds) and groundwater (bore and open wells). Depending on the nature of the soil, the season and other factors, particulate matter (especially sand, silt, clay, etc.) will be present in the water. Due to the presence of these particulates, water tends to be hazy (turbid) which impairs the quality of water and reduces its acceptability to the consumer.

To improve the quality of water, we use sediment filtration. People used cloth to remove sediments in olden days. Over time, mankind used different media for filtration purposes; notably sand filtration, which has been in use for treating water on a large scale. This method is very inconvenient and ineffective for domestic filtration; however, it is a common practice at the residential level in India to use ceramic filter candles, which are popularly known as stainless steel filters. These are economical, yet they are cumbersome to use and maintain due to very low filtration output and the need for frequent cleaning followed by boiling. Very fragile in nature, they are prone to breakage while handling.

There are different types of advanced sediment filtration methods that have emerged over the years. This article outlines the various sediment filters for domestic residential purposes, which have evolved over a period of time through Eureka Forbes, which manufactures water purifiers in India.

In general, domestic water purifiers that are used on-line (with running water) need to be fast flowing, easy to clean and have a long life. Depending on the water quality, various types of sediment filters have been developed and used for on-line purposes.

Polypropylene molded candle
In the early 1980s, Aquaguard was introduced into the Indian market. Now Aquaguard has become the generic name for a water purifier in this country. In those days, Aquaguard consisted of a polypropylene molded filter candle as the first stage of purification to tackle particulate matter in the water. This filter was generally used to remove sediments consisting of macro particles. It was a nominal filter of five-micron pore size, very easy to clean by backflushing (reversing the flow of the water to unload the accumulated sediments). The consumer himself could perform backflushing.

This filter was effective in regions with low turbidity. As the water quality in India varies from place to place, it was essential to have a filter that performed more effectively and also one that tackled the turbidity of the water across the country. Based on this need, a depth filtration candle was developed (polypropylene yarn-wound).

Polypropylene yarn-wound candle
This is a depth filter, which is made of polypropylene yarn and wound over a perforated core. The pore size of the filter is five micron (nominal). However, the construction of the filter’s crisscross matrix makes it capable of trapping particles less than five microns in size as well. This is one of the most widely used filters nowadays. In-house manufacturing facilities were developed including quality control testing. The polypropylene yarn-wound filter can he backwashed by reversing the water flow. In this type of filter, eight-inch and 10-inch sizes are available today.

These filters include, a silver (bacteriostatic) impregnated activated carbon layer, incorporated to prevent bacterial proliferation. This helps in the eradication of biological fouling that accumulates on the filters.

Nationally, these filters are widely accepted thanks to consistent performance. Effective life of these filters is proven to be anywhere from 10,000 to 11,000 liters of water.

There are certain specific regions in the country where water has colloidal particulates of about one micron in size. In these locations, the filtration efficacy of the polypropylene yarn filters is inadequate. This fact necessitated the development of a much finer filter.

Cellulose filters
Cellulosic filters are much finer depth filters with a tortuous matrix for filtration. These filters are the most effective solution for colloidal particulates and provide clear water. In this category, the filters available can filter particles of sizes 0.2, one, two and five microns. The surface of these filters can be cleaned by gentle scrubbing. On average, they will last four to six months depending on the input water quality (turbidity).

About the authors
Dr. S. Muralidhararao is the Senior Manager, Water Analysis and Depollution and Dr. T.N.V.V. Rao is the Senior Manager, Chemicals, with Eureka Forbes, Prakash Narayan Kotnis Marg, Mumbai, India 400 016. For additional information, contact the company at +91 22 2444 3436 or visit

This article has been adapted with the permission of Everything About Water Magazine® May/June 2005. Any reuse or republication, in part or whole, must be with the written consent of the Publisher.



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