Slow sand filtration — allowable raw-water turbidity: A slow sand filter is generally used when the maximum turbidity of the raw water is less than which value (use standard unit mg/L or NTU)?

Introduction / Context:
Slow sand filters (SSFs) rely on biological schmutzdecke formation and fine filtration through a deep sand bed. They require relatively low and stable influent turbidity to avoid rapid clogging and to sustain effective biological action without frequent scraping.

Given Data / Assumptions:

• Conventional SSF without upstream coagulation.
• Typical municipal raw-water quality variation.
• Standardized turbidity units: mg/L (≈ NTU for exam purposes).

Concept / Approach:
Textbook practice recommends slow sand filtration where raw-water turbidity is generally below about 20–30 NTU, with many authorities citing 30 NTU as an upper guideline. Higher turbidities lead to excessive headloss and short filter runs unless pre-sedimentation or pre-coagulation is provided. Therefore, 30 mg/L (≈ 30 NTU) is widely recognized as the practical upper limit for direct SSF application.

Step-by-Step Solution:

Verification / Alternative check:
Utilities often include pre-sedimentation lagoons or roughing filters where raw-water turbidity routinely exceeds 30 NTU.

Why Other Options Are Wrong:
10 or 20 mg/L are conservative but not a general “maximum”; 40–50 mg/L are too high for sustained SSF operation without pretreatment.

Common Pitfalls:
Confusing SSF with rapid gravity filters (RGF) that accommodate higher turbidity with coagulation; mixing g/L with mg/L—here mg/L (or NTU) is the correct magnitude.

Final Answer:
30 mg/L (≈ 30 NTU)