Filtration
This domain covers the design and operation of granular media filters, their performance monitoring, and backwash procedures. The exam tests your ability to interpret filter data, recognize breakthrough and upset conditions, and apply regulatory turbidity limits.
Filter Types and Media
- Rapid sand filters (single or dual media) use media such as sand, anthracite, and garnet. Media effective size (ES) and uniformity coefficient (UC) determine filtration performance. Typical sand ES is 0.45–0.55 mm with UC ≤1.5.
- Slow sand filters rely on a biological schmutzdecke layer; they are not backwashed but scraped. Loading rates are low (0.05–0.15 gpm/ft²).
- Membrane filters (microfiltration, ultrafiltration) provide absolute barriers; integrity testing is critical.
Filtration Mechanisms
- Straining, sedimentation, interception, diffusion, and electrostatic attraction remove particles. The dominant mechanism depends on particle size and filter velocity.
- Filter run ends when effluent turbidity exceeds limits, headloss reaches maximum, or a preset run time is reached.
Backwashing
- Backwash expands the media bed by 20–50% to remove captured particles. Backwash rates for sand are typically 15–20 gpm/ft²; anthracite requires lower rates (8–12 gpm/ft²).
- Air scour can improve cleaning. Filter-to-waste diverts initial filtrate after backwash until turbidity returns to acceptable levels.
- Backwash water must be managed; recycling can concentrate contaminants and may require treatment.
Regulatory Requirements
- Under the Surface Water Treatment Rules, combined filter effluent turbidity must be ≤0.3 NTU in 95% of monthly samples and never >1 NTU for conventional filtration. Individual filter effluent monitoring is required for systems with ≥2 filters; if turbidity exceeds 1 NTU for two consecutive 15-minute samples, it triggers a filter profile and potential follow-up.
Worked Example
A filter has a media area of 150 ft² and a backwash flow of 2,400 gpm. What is the backwash rate in gpm/ft²? Is it within the typical 15–20 gpm/ft² for sand?
- Rate = 2,400 gpm / 150 ft² = 16 gpm/ft².
- This is within the typical range for sand, so adequate bed expansion should occur.
- If the rate were 10 gpm/ft², expansion might be insufficient; if 25 gpm/ft², media loss could occur.
Common Traps
- Confusing effective size (ES) with uniformity coefficient (UC); ES is the sieve size through which 10% of media passes.
- Forgetting that filter-to-waste is needed after backwash to prevent turbidity spikes.
- Assuming all filters require the same backwash rate; media type and temperature affect expansion.
- Overlooking individual filter effluent monitoring triggers for systems with multiple filters.
- Neglecting that recycling backwash water can increase DBP precursors and turbidity.