In this article, I will discuss the Differences between Rapid Gravity Filters and Slow Sand Filters.
The difference between the two lies not only in the filtration rate but also in the concept of the washing process. Slow sand filtration is a biological process in nature and rapid sand filtration is a physical cleaning process.
Rapid sand gravity filters and slow sand filters are two types of filters commonly used in water treatment. Slow sand gravity filters remove a higher percentage of bacteria and impurities, but filtration is very slow and expensive. On the other hand, high-speed sand gravity filters are ubiquitous because they help disinfect and decolorize the water coming directly from the clarifier.
| Sl. No. | Rapid Gravity Filters | Slow Sand Filters |
|---|---|---|
| 1 | Water should coagulate, flocculate and settle. | Raw water can be supplied without treatment or after settling. |
| 2 | Sand is supported by a gravel foundation. The bottom also distributes water evenly over the sandy surface. The depth is 0.6 to 0.9 m, and the size is 3 to 40 mm. | Sand is supported by a gravel foundation. The depth ranges from 0.3 to 0.75 m, and the size is from 3 to 65 mm. |
| 3 | Effective sizes range from 0.35 mm to 0.55 mm, and the coefficient of uniformity is from 1.2 to 1.8. | Effective sizes range from 0.2 mm to 0.4 mm, and uniformity coefficients range from 1.8 to 3. |
| 4 | The footprint is small and ranges from 10 to 80 square meters (5 m x 8 m). | The footprint is huge and ranges from 100 to 2000 square meters (30 m x 60 m). |
| 5 | Difficult to construct. | Easy to construct. |
| 6 | Outstanding performance, ie. 3000 to 6000 l/h per square meter of filter area. | Low performance, i.e. 100 to 200 l/h per square meter of filter area. |
| 7 | The initial loss is 0.3 m, and if cleaning is required, it is about 2.5-3.5 m. | The initial loss is 0.1 m, and if cleaning is required, it is about 0.8 to 1.2 m. |
| 8 | Filters are very flexible as they can accommodate reasonable variations. | It is not flexible because it cannot accommodate changes in demand. |
| 9 | These filters are well known for treating turbid water. It removes only 80-90% of bacteria but effectively removes color and haze. | This filter is 98-99% effective at removing bacteria, but not very effective at removing color and fog. |
| 10 | A drainage system is laid to receive filtered water as well as pass water for backwashing. | A drainage system has been installed to receive purified water. |
| 11 | Disinfection is essential. In addition to this, various treatments can be performed as needed. | The resulting water is very clean but can be disinfected. No other treatment is required. |
| 12 | It is economical and inexpensive due to the low initial cost. Operation and maintenance (O&M) costs are high. | The initial cost of land and materials is high, but the cost of operation and maintenance is low. |
| 13 | The cost of depreciation is very high. | The cost of depreciation is very low. |
| 14 | Skilled supervision is needed. | No need for expert supervision. |
| 15 | Frequent cleaning should be done within 1-3 days. Purification requires 1-5% filtered water. Relatively easy to clean as the sand grains need to be shaken and backwashing is possible with or without compressed air. | Cleaning should be done within 1-3 months. Purification requires 0.2-0.6% filtered water. Cleaning is relatively labor-intensive as it requires scraping, removing, and rinsing with a hose that is 1.5 to 3 cm thick. |