In water treatment using slow sand filters, what is the maximum raw-water turbidity (in mg/L) that can generally be handled effectively without prior coagulation or pre-treatment?

Introduction / Context:
Slow sand filtration is a biological-physical process used for treating relatively clear surface sources. It relies on a biologically active layer (schmutzdecke) and fine sand to remove suspended and colloidal matter along with pathogens. This question tests practical design knowledge about the turbidity range that slow sand filters can accept without pre-treatment.

Given Data / Assumptions:

• Standard slow sand filters with effective size sand and typical filtration rates of 0.1 to 0.3 m/h.
• Typical design guidance that slow sand filters treat low-turbidity water directly.
• No coagulation–flocculation or other pre-treatment assumed.

Concept / Approach:
Slow sand filters are best suited for low and fairly stable turbidity. When turbidity is too high, the schmutzdecke clogs rapidly, head loss rises quickly, and filter runs become uneconomical. Many practice-oriented texts indicate that raw-water turbidity should generally be below about 50 mg/L; a conservative upper bound commonly used in exam problems is about 60 mg/L without pre-treatment.

Step-by-Step Solution:

Verification / Alternative check:
Where turbidity is frequently higher, plants add pre-treatment (coagulation–flocculation–sedimentation) before slow sand filtration. This aligns with operational experience worldwide.

Why Other Options Are Wrong:

• 75 mg/L and 150 mg/L: Too high for direct slow sand filtration; would cause rapid clogging.
• 100 g/L: Unit error and unrealistically large turbidity.
• None of these: Incorrect because 60 mg/L is a standard practical upper bound used in design questions.

Common Pitfalls:

• Confusing slow sand (biologically driven) with rapid gravity filters that follow coagulation.
• Ignoring head-loss and short filter runs at higher turbidity.

Final Answer:
60 mg/L