Water Quality and Purification

Which of the following is the most commonly used coagulant in water treatment plants across India?

What is the maximum permissible limit (MPL) for total hardness in drinking water according to Indian Standards (IS 10500:2012)?

A 28-year-old woman from a rural village in Uttar Pradesh presents with a 3-week history of progressive neurological symptoms: tremor, ataxia, and personality changes. Her family reports that she has been drinking water from a hand pump in their village. On examination, she has coarse tremor, dysarthria, and emotional lability. Laboratory investigations reveal elevated urinary mercury levels (45 µg/L; normal <5 µg/L). The village water supply was recently found to be contaminated with mercury from an upstream industrial discharge. What is the most appropriate immediate intervention to prevent further cases in this community?

A 5-year-old boy from a slum settlement in Mumbai presents with recurrent episodes of acute watery diarrhea over the past 2 months. Stool culture isolates Vibrio cholerae. His mother reports that the family collects water from a community tap that is located 10 meters downslope from a sewage outlet. The water appears clear but has a faint odor. On testing, the water shows: pH 7.2, turbidity 2 NTU, residual chlorine 0.1 mg/L, total coliforms present in 40% of samples, E. coli absent. What is the most critical water quality parameter that has failed in this supply?

A 28-year-old man from a village in Rajasthan presents with a 2-month history of loose stools (4–5 times daily), abdominal cramping, and weight loss of 3 kg. He reports that the village hand pump water tastes slightly salty. Stool microscopy reveals cysts of *Entamoeba histolytica* and *Giardia lamblia*. The village water supply is tested: pH 8.2, turbidity 5 NTU, chlorine residual 0 mg/L, total coliform count 40 CFU/100 mL, and fecal coliform count 15 CFU/100 mL. What is the primary water quality defect responsible for the disease transmission in this village?