At what interval should rapid sand filters be cleaned by backwashing?

The primary purpose of aerating water is to remove which of the following?

A water sample has a pH of 9.25. The concentration of hydroxyl ions in the water sample is:

What are the factors affecting per capita water demand?

The disinfection efficiency of chlorine in water treatment depends on the pH value.

If the BOD (Biochemical Oxygen Demand) of a wastewater sample is 75 mg/L and the reaction rate constant (base e) is 0.345 per day, the amount of BOD remaining in the given sample after 10 days is

Hardness in water is caused by the presence of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Which are the least soluble forms of calcium and magnesium at normal water temperatures?

An industrial wastewater enters a stream having a BOD concentration of 10 mg/l and flow of 20 m³/s. If the flow of wastewater is 1.5 m³/s and its BOD concentration is 250 mg/l, what will be the BOD concentration in the stream at a point downstream of the point of confluence of the wastewater with the stream?

A student began an experiment of 5-day 20°C Biochemical Oxygen Demand (BOD) on Monday. Since the 5th day fell on Saturday, the final DO reading was taken on Monday. On calculation, the 7-day 20°C BOD was found to be 150 mg/L. What would be the 5-day, 20°C BOD (in mg/L)? Assume the value of the BOD rate constant (K) at 20°C as 0.23/day (base e).

Consider the following statements:

1. Grease data is a measure of effectiveness of skimming tanks.

2. Volatile acid data provide information concerning anaerobic degradation of organic matter.

3. Total and volatile solids data are indispensable in the design and operation of sludge digestion, vacuum filter and incineration units.

4. Ammonia and organic nitrogen data are helpful in controlling aerobic treatment costs to a minimum.

Which of these statements are correct?