**Calculation of quantity of bleaching powder required per day**

To calculate the quantity of bleaching powder required per day for disinfecting 4 million liters/day, we need to consider the required dose of chlorine and the available chlorine content in the bleaching powder.

**Step 1: Determine the desired dose of chlorine**
The desired dose of chlorine for disinfection is given as 0.5 ppm (parts per million). This means that for every million parts of water, we need 0.5 parts of chlorine.

**Step 2: Convert the desired dose of chlorine to grams**
To convert the desired dose of chlorine from ppm to grams, we need to consider the molecular weight of chlorine. The molecular weight of chlorine is 35.45 g/mol.

0.5 ppm can be written as 0.5 g/10^6 g (since 1 ppm = 1 mg/L = 1 g/10^6 g).

Therefore, the desired dose of chlorine in grams can be calculated as follows:

Desired dose of chlorine (g) = 0.5 g/10^6 g * 4 million liters/day

**Step 3: Calculate the quantity of bleaching powder required**
The bleaching powder contains 30% available chlorine. This means that for every 100 grams of bleaching powder, we have 30 grams of available chlorine.

Let "x" be the quantity of bleaching powder required in grams.

Using the available chlorine content, we can set up the following equation:

30 grams of available chlorine/100 grams of bleaching powder = desired dose of chlorine (grams)/x grams of bleaching powder

Simplifying the equation:

30/100 = desired dose of chlorine (grams)/x

x = (desired dose of chlorine (grams) * 100)/30

x = (0.5 g/10^6 g * 4 million liters/day * 100)/30

**Step 4: Calculate the quantity of bleaching powder required in kilograms**
To convert the quantity of bleaching powder from grams to kilograms, we divide the value by 1000.

Quantity of bleaching powder required (kg) = x grams/1000

Therefore, the final quantity of bleaching powder required per day for disinfecting 4 million liters/day can be calculated using the above steps and equations.