The bed of a slow sand filter is 80-100 centimeters of clean fine sand with an effective size of 0.15-0.35 millimeters, resting on three grades of gravel and a perforated drain, and the sand does the treating while the gravel only carries the water away without losing sand. Clean sand costs only about 4.1-8.3 centimeters of head at 0.1-0.2 meters per hour for a typical 0.25 millimeter grain, so most of the 50-100 centimeters of water above the bed is spare head available to push water through as the surface clogs.
Part of Water Treatment at Scale
The filter bed is the stack of sand and gravel layers inside a slow sand filter, each layer with a specific grain size and thickness that determines how well water is treated and how fast the bed clogs.
Each Layer Has One Job, And The Sand Grain Size Sets The Trade-Off
The carrier gives the layers from bottom to top: an underdrain of 20-40 mm gravel 15 cm deep with a perforated pipe, support gravel of 5-10 mm and 5 cm, fine gravel of 2-5 mm and 5 cm, and filter sand of 0.15-0.35 mm effective size, 80-100 cm deep, with 50-100 cm of raw water above. The sand-filters page adds a uniformity coefficient under 3 (Sand Filters).
| Layer | Job | Fails by |
|---|---|---|
| Raw water above | Provides the head that drives flow; protects the sand surface from drying | Falling below the sand |
| Sand, 80-100 cm | Holds the living layer at the top and strains particles | Too fine: clogs fast; too coarse: poor removal; dusty: clouds the water |
| Fine gravel, 2-5 mm | Stops sand from falling into the coarser gravel | Sand sifts through |
| Support gravel, 5-10 mm | Carries the load and bridges between grades | Mixing with the layers above |
| Underdrain, 20-40 mm, with perforated pipe | Collects water evenly over the whole floor | Blocked or uneven, so flow concentrates |
“Effective size” is the grain diameter such that 10% of the sand by weight is finer. A sand of small effective size removes more but clogs sooner. The uniformity coefficient is D60 ÷ D10: a value near 1 means grains of nearly one size, which keeps the pore spaces open.
What You Need
- Sieves or screens of known aperture, to check the sand grade
- A sample of the sand weighed before and after sieving
- Washed gravel in the three grades
- A post or mark inside the box to record the starting sand level
- A measuring tape and a clear tube or gauge on the water level
Method: Check The Sand, Then Budget The Head
Step 1: Grade the sand. Weigh a sample, pass it through screens, and find the size that 10% passes. Reject sand that is much finer than 0.15 mm or much coarser than 0.35 mm.
Step 2: Check uniformity. The grains should be mostly one size, with D60 ÷ D10 under 3.
Step 3: Place the layers as in Filter Construction and mark the finished sand level.
Step 4: Budget the head. Available head for clogging = depth of water above the sand - head lost in the clean bed.
Step 5: Watch the sand level. A falling sand surface with no scraping means sand is migrating into the gravel.
Worked Example: The Head Budget
A clean bed resists flow in proportion to the viscosity of water, the bed depth and the flow, and in inverse proportion to the square of the grain size. The Carman-Kozeny relation estimates the clean-bed loss: h = 180 × ν × L × v × (1 - ε)² ÷ (g × ε³ × d²). The constant 180 and the porosity ε = 0.4 are textbook values I am recalling, not read from a source here, so use the result as an order of magnitude.
Inputs: viscosity ν = 1.0 × 10⁻⁶ m²/s (20 °C); bed depth L = 0.9 m; grain d = 0.25 mm = 0.00025 m; velocity v = 0.1 m/h = 2.78 × 10⁻⁵ m/s; g = 9.81 m/s².
- Numerator: 180 × 1.0 × 10⁻⁶ × 0.9 × 2.78 × 10⁻⁵ × 0.36 = 1.62 × 10⁻⁹.
- Denominator: 9.81 × 0.064 × (0.00025)² = 3.93 × 10⁻⁸.
- h = 1.62 × 10⁻⁹ ÷ 3.93 × 10⁻⁸ = 0.041 m = 4.1 cm.
At 0.2 m/h the loss doubles to 8.3 cm. For a finer grain of 0.15 mm the loss is (0.25 ÷ 0.15)² = 2.78 times higher: 11.5 cm at 0.1 m/h and 23 cm at 0.2 m/h. In cold water at 5 °C the viscosity is about 1.52 × 10⁻⁶ m²/s and the losses are about 1.5 times higher. Spare head = 50-100 cm of water - these figures: for 0.25 mm sand at 0.1 m/h that is 46-96 cm. As the schmutzdecke and the top sand clog, the loss at the surface grows until the spare head is used up; then the flow falls and the filter needs scraping.
Sand inventory. Between scrapings each removes 1-2 cm. From 90 cm down to the 60 cm minimum the carrier names is 30 cm, which is 30 ÷ 2 = 15 to 30 ÷ 1 = 30 scrapings before the stored sand must be put back.
Failure Modes
- Flow collapses within days: sand too fine or full of silt, or untreated muddy water.
- Cloudy effluent after cleaning: washed sand put back dirty, or the bed disturbed.
- Sand in the effluent or a sinking surface: sand migrating through the fine gravel; the jump from 0.15-0.35 mm sand to 2-5 mm gravel is large.
- Uneven flow and dry patches: an unlevel bed or a blocked underdrain.
- A “ripe” filter that is not: the living layer dried out (Biofilm Maturation).
Common Mistakes
- Choosing sand by the finest you can find. It removes more but clogs sooner.
- Mixing gravel grades. Each layer must be uniform.
- Forgetting to mark the sand level. You cannot tell loss from migration.
- Cleaning with a rake that digs deep. It destroys the living layer.
Safety
Wash the media and the box, and keep chlorine out of the bed
Do not use sand from a source that may be contaminated with sewage or chemicals; wash it well. Do not clean the bed with chlorinated or soapy water, because residues can harm the living layer and then pass into the drinking water. A deep box is a confined space: no entry without ventilation, a harness on a line held by a person outside, and a rule that the person outside never enters after a collapsed worker. Do not test the air with a flame.
What’s Next
- Learn next: Inlet and Outlet — keeping the bed wet and the flow steady
- Combine with: Biofilm Maturation — the living layer on top of the sand