Filter Backwash Efficiency and Media Loss
A conventional filtration plant with six dual-media filters (anthracite over sand) is experiencing shortened filter runs and increased turbidity breakthrough. Each filter is 20 ft by 30 ft, with a design loading rate of 4 gpm/ft². The backwash sequence uses air scour at 3 scfm/ft² for 3 minutes, followed by water backwash at 15 gpm/ft² for 8 minutes, and a 2-minute rinse. Recently, operators have noticed anthracite media accumulating in the backwash troughs and a gradual loss of media depth. The filter effluent turbidity after backwash is 0.8 NTU, which is above the plant's internal goal of 0.3 NTU. The backwash water is supplied from a clearwell that also serves as chlorinated finished water storage; the backwash pump is rated at 3,000 gpm. The plant runs at 8 MGD, and the filters are backwashed every 24 hours. The operator must determine why media is being lost and why turbidity breakthrough is occurring, and what corrective actions to take.
1. What are the likely causes of anthracite media loss during backwash, and how can the operator confirm and correct the issue?
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Anthracite media loss during backwash is typically caused by excessive backwash flow rate, insufficient freeboard, or improper air scour operation. The backwash rate is 15 gpm/ft², which is high for anthracite (typical design is 10–15 gpm/ft², but anthracite has a lower specific gravity than sand and can be washed out if the rate is too high or if the water temperature is low (less dense water requires lower rate). Also, if the air scour is left on during water backwash, it can cause media to be carried over. The operator should: 1) Verify the actual backwash flow rate using a calibrated flow meter; the pump is rated at 3,000 gpm, which for a 20x30 ft filter (600 ft²) gives 5 gpm/ft²-that seems low, but the pump may serve multiple filters? Actually, the backwash pump is 3,000 gpm; if it backwashes one filter at a time, the rate is 3000/600 = 5 gpm/ft², which is below typical. But the problem states backwash at 15 gpm/ft², so there is a discrepancy. The operator should check the actual flow rate. If the rate is indeed 15 gpm/ft², that is at the high end; reducing to 12 gpm/ft² may help. 2) Check the backwash trough elevation and freeboard; media loss occurs if the filter is overfilled or if the troughs are too low. 3) Ensure air scour is turned off before water backwash begins (a common mistake is to leave air on). 4) Inspect the media; if anthracite depth is significantly reduced, media may need to be replaced. 5) Check for bed expansion; anthracite should expand 15–20% during backwash. If expansion is excessive, reduce flow. 6) Consider installing a media retention cap or adjusting the backwash sequence. To confirm, the operator can measure the media depth and compare to design, and observe the backwash visually to see if media is being carried over. Also, check the backwash water temperature; colder water is more viscous and requires higher velocity to achieve the same expansion, but also has lower density, so media may be more easily carried over if flow is too high.
2. Why is the filter effluent turbidity after backwash high, and what steps can be taken to reduce it?
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High filter effluent turbidity after backwash (0.8 NTU vs. goal 0.3 NTU) can be caused by several factors: 1) Incomplete removal of backwash solids, leading to a dirty filter bed that releases particles during the next run. This could be due to inadequate backwash (too low flow or too short duration) or poor media cleaning. 2) Media loss leading to thinner anthracite layer, reducing filtration efficiency and allowing particles to pass. 3) Filter ripening: after backwash, the filter may take time to re-establish the schmutzdecke and remove particles; the initial turbidity spike can be mitigated by filter-to-waste or slow start. 4) Backwash water quality: if the backwash water is chlorinated finished water, chlorine may react with media or organics, but more importantly, if the backwash water contains particles, it could contaminate the filter. 5) Air scour may be causing media disturbance and fines migration. 6) The backwash sequence may not include a proper rinse; the 2-minute rinse may be insufficient. Corrective actions: 1) Optimize backwash: ensure adequate bed expansion (15-20% for anthracite), verify flow rate and duration, and ensure air scour is followed by a water backwash with sufficient time to remove solids. 2) Implement a filter-to-waste or slow-start after backwash: run the filter at a reduced rate for a period or divert initial filtrate to waste until turbidity drops below 0.3 NTU. 3) Check media depth and replace lost anthracite to restore design depth. 4) Consider adding a coagulant aid or filter aid to improve particle capture. 5) Monitor filter effluent turbidity continuously and adjust backwash frequency; if filters are backwashed every 24 hours but runs are shortened, maybe they need more frequent backwash. 6) Inspect underdrain and support gravel for disturbances. 7) Ensure backwash water is not chlorinated if it is stored in a clearwell with chlorine; chlorine can damage anthracite over time, but more importantly, it may not be the cause of turbidity. The operator should also check the filter influent turbidity; if it is high, the filter may be overloaded. The goal is to achieve <0.3 NTU after backwash, so filter-to-waste is a common solution.