A Complete Guide: Design, Working & Benefits of Slow Sand Filter

Slow Sand Filter (Gravity type) are most effective (98-99% bacteria removal) and widely used for purification of water in small villages & communities where water demand is comparatively low. Slow Sand Filter comes under the heading of Gravity Filtration system which are designed for  maximum daily demand of that area.

Gravity Filtration System 

Gravity system is one in which water flows under gravity or water head is required for water flow. Gravity System is generally classified into two system:

• Slow Sand Filtration 
• Rapid Sand Filtration

Slow Sand Filters

Slow Sand Filter Diagram

Slow Sand Filter utilize the effluent either from plain sedimentation or directly from sources. Sand Filters medium comprises of Sand, geo textile, gravels. A gravel layer is provided only to provide support to fine sand layer.

Various design specification of Slow Sand filter are as follows:

• Generally rectangular in shape.
• Depth of tank is in range of 2.5 to 3.5 m.
• Effective size (diameter) of Sand particle are of 0.2 to 0.3 mm.
• Coefficient of uniformity varies in range of 3 to 5.
• Top layer of sand (15 to 30 cm) out of total depth is of fine variety and rest is of uniform size.
• Depth of water equals to Depth of sand filter itself.
• Rate of filtration is in range of 2400- 4800 L/sq.m/day or 100-200 L/sq.m/hour. 

Working of Slow Sand Filter

The basic force on which entire filter works is Biological action. When the water is passed through the filter. After some time a gelatinous layer known as Schmutzdecke, is formed over the sand particle. It consist of a variety of bacteria, fungi and other tiny micro organism.It is the layer which provide effective cleaning of water. The Schmutzdecke layer capture the foreign matter which are present in water and then the micro organism decompose it.

Slow sand filter slowly lose their performance as the biofilms thickens which further lead to loss of water head. To increase the performance of slow sand filter again, Cleaning of sand filter become necessary. 

Cleaning of Slow Sand Filter

• Cleaning of Sand filter should be done when head loss become 0.7 to 0.8 times of depth of filter medium.
• Freshly cleaned filter had head loss of 10-15 cm that goes on increasing with time 
• For cleaning sand filter, top 15 to 30 cm of sand layer is removed with help of rack and washed with water.
• Once the cleaning is completed, Slow Sand Filter is again loaded with water for filtration but filtered water is not used for next 24 to 36 hours.
• Frequency of cleaning is 1-3 months.

Total number of Slow sand filters required for filtration process according to requirement of air are given below:

Requirement of Slow Sand Filter

Advantage 

The bacteriological efficiency is in range of 98-99 %. Effluents of filter has comparatively less nutrient. Hence if send in distribution system, The chances of growth of micro organism and algae is very less. 

Sand filter work on very less mechanical power and chemicals. Hence the minimal staff is required with basic training only.

Disadvantage

 Slow Sand Filters only work if turbidity of incoming water is less than 50 NTU because the filter get choke very frequently on working of water having turbidity > 50 NTU, require more cleaning. Hence increase operational cost of filter.