ADHAR - Association for development of human and community rebuilding

Technical know how:Horizontal Roughing filter/slow sand filter 

Horizontal Roughing Filter (HRF) is a pretreatment technology. HRF copies natural purification process. In HRF water runs in horizontal direction. This usually consists of differently sized relatively coarse filter material (ranging from 20 to 4mm) which successively decrease in size to treat highly turbid (300-400 NTU or even 1000 NTU for shorter duration) surface water over prolonged period. Sedimentation is the main solid separation process. It mainly reduces the fine solids efficiently and partly Suspended solid impurities from raw water. Bacteriological water quality improvement is also observed to a great extent. The resulting water quality from HRF found suitable for charging SSF. This also improves the performance of SSF which also helps in increasing the rate of filtration without affecting the water quality, ultimately resulting in economy. In HRF, the filtration rate ranges between 0.3 to 1.5 m/ hr. Length of filter is dependent on raw water turbidity. Filter cleaning is also carried out with hydraulic filter flush or in manual process which helps in periodical removal of accumulated solids from filter media. Cleaning of filter media

plays key role in long term and efficient roughing filter operation. Turbidity reduction can be achieved to the extent of 70-90 per cent and even in some cases up to 98 per cent, depending upon raw water characteristics. HRF is simple in construction, using locally available material and skills for operation. Neither mechanical parts nor chemicals are necessary for HRF, Where as SLOW SAND FILTERS (SSF) is find global application specifically in small community water supply schemes in developing countries, due to its added advantages of most qualitative, cost effective, simplest and reliable technology, which require limited professional skill and easy maintenance. SSF require few technical component and usually no chemicals. The performance of SSF is not controlled by mechanical system but by ecosystem of living organism.

Thus HRF technique is not only an appropriate and economical option for rejuvenation / augmentation of existing SSF where high turbidity is problem but is also most suitable for new systems in rural and semi urban areas with similar problems.

 HRF improved the water quality, meeting the requirement of SSF.

 SSF capacity augmented/enhanced by more than 50 per cent due to improved water quality.

 Cost effective in comparison to rapid gravity filters.

The introduction of horizontal roughing filter and slow sand filter (HRF/SSF) to treat surface water has proved to be a most successful treatment system and made safe drinking water available in many places. The major components of this filter are described below.

Filter Channel:- With an area of one square meter in cross section and eight meter length, to be laid across the tank embankment. The filter channel consisted of ten uniform compartments, in the first and second being packed with broken brick bats (different in size), the third and forth compartments being packed with sized gravel and granules, fifth compartment being packed with charcoal and on return way from sixth to ninth compartment being packed with coarse sand and flowed by fine sand. Last compartment (Tenth) will be kept empty fitted with a discharge tap for collecting filtered water. The HRF usually consists of filter material like gravel and coarse sand that successfully decreases in size from 25 mm to 4 mm. The bulk of solids in the incoming water are separated by this coarse filter media or HRF and SSF. At every outlet and inlet point of the channel, fine graded mesh is implanted to prevent entry of finer materials into the sump. The length of a channel varies according to the nature of the site selected for the sump.

Sump:- A storage provision to collect filtered water from the tank through the filter channel for storage and collection.

While HRF acts as a physical filter and is applied to retain solid matters, SSF is primarily a biological filter,

used to kill microbes in the water. Both filter types are of an equal technical level and their operation is characterized by stability. These make full use of the natural purification process of harvested surface water and do not require any chemicals.

Considering the scarcity of safe drinking water in the area, Association for Development of Human and Community Rebuilding (ADHAR) has initiated to introduce community rain water harvesting structure with the help of Gram Panchayat and successfully identified a few community ponds where salinity in storage water is very less and local people generally use that water for drinking, cocking and other domestic purpose. Based upon the findings ADHAR introduced Horizontal Roughing Filter / Slow sand Filter (HRF/SSF) and involved Gram Panchayat for renovation of identified ponds where rain water can be stored. Accordingly Gram Panchayat under took the renovation work under MGNREGS programme and ensured that the ponds should not be contaminated by any manner.Association for Development of Human.