Raw Water Filtration: Some Technical Tips For Major Users

Water is the prime resource which concerns every one. It is one natural resource which every one think to much but understand little. Its value is realized when it is not available as required or available in a manner not suitable for the intended use. Water becomes a concern only when it creates a problem.

I do not wish to write more about the politics and philosophy of water here. In this blog I wish to present some technical aspects of water filtration.

Water in its chemical form H2O is never available naturally in its purest form. As it is a universal solvent or cleaning agent water available in nature contains many other organic and inorganic material in varying concentrations. These impurities could be in the form of dissolved solids or gases, collodidal solids, liquids or gases or suspended solids, gases or liquids.

Impurities in water get into it from various sources when it gets in contact with earth or other contaminations either naturally or through the activities of people. The natural process of purification and distribution of water takes place on a perpetual manner in the form of natural evaporation and condensation. The rain distributed water from the land areas either percolates down to underground water bodies or surface water bodies which in turn forms the natural fresh raw water. Fresh water contains much lower levels of dissolved solids in the form of salts of sodium, potassium, calcium, magnesium, etc where as sea water contains a much higher level of these dissolved solids.

Fresh water from rivers and ponds on the other hand contains a higher level of suspended and colloidal solids which reduces its clarity.

The best and simple way to make water relatively free of suspended and colloidal solids and to make it suitable for most industrial and domestic consumption is to filter it appropriately.

Filtration of water is one of the main treatment processes in any industrial or municipal water treatment facility. The most common water filtration method is the sand bed filtration. However, there are various types of designs of  sand filtration systems and equipment. Rapid Sand Gravity filtration and Pressure Sand Filtration are two very common types of water filtration technology.

Unfortunately very few people concerned with the decisions regarding the selection and implementation of a water treatment facility understand the relative advantages and disadvantages of various water treatment design arrangements. This one factor often makes a water treatement plant facility to costs much more with regard to its initial investment costs and subsequent operation and maintenance costs.

Conventional gravity sand filtration and pressure sand filtration systems are batch operating process units. The filters after operating for a certain length of time in filtration mode need to be taken out of filtration service to clean the filter beds from the accumulated suspended impurities it filtered out from the water. The cleaning is often effected by expanding the sand bed using a heavy reverse water flow using some of the filtered water. In water treatment, this process is called the filter backwash mode.

These operations require a number of heavy sized valves to be operated either manually or electrically. The number of valves and their sequencing often become a complex operation prone to much supervision and frequent maintenance. Moreover, this enhances the cost of installation and upkeep of the filtration system. Manpower and electric power become essential requirements for a water filtration plant making water filtration a costly affair for the owner of the treatment facility.

It is here that the 'Automatic Valveless Gravity Sand Filters' AVGSF or AVGF or VAG filters play an important role. This wonderful filter design was patented some decades ago in the US by the pioneering water treatment company called 'Permutit'.

Considering the relative advantages of this design, this water filter should have become the standard for water filtration. However for various reasons it has not. Unfortunately very few people have used this technology advantageously. AVGF filters do not require any man power or electricity for its operation or back wash. It is completely and fully automatic without the use of valves or costly programmable logic controllers for its automation.

I had adopted this technology for large industrial and drinking water treatment plant purposes some decades ago as a plant designer. Those plants are smoothly running all these years without much problems.

The following gives a typical constructional feature of a VAG filter for water treatment:

It is a wonderful design concept. Though simple, it has certain very important technological design details which needs special attention.

A few companies are offering filters designed on this concept. They are:

1. M/s Siemens

2. M/s Otoklin

3. M/s Ion Exchange India

Water filtration using this technology would be of great advantage for small and medium scale drinking water treatment plants of municipalities as well as for industries that operate closed loop cooling water circulation systems. In the latter, addition of a filtration unit adopting the VAG technology to continuously filter suspended impurities from about 10% of the circulation water would greatly improve the performances of the heat transfer process equipment and enhance their life. 

Engineers and technologists who are connected to water treatment may make use of this technology so that the water treatment facilities they design would be cost effective and simple in operation and maintenance.