Please consult following links;

cchealth.org/eh/small-water/.../storage_tank_disinfection_procedures.pdf‎

http://www.cdc.gov/safewater/chlorination-faq.html

WHO recommends 4 drops per liter of 5% bleach to clear or clarified water, Wait 30 minutes for water at ~ 25 C. See p110 in 4th edition of WHO Guidelines for Drinking Water Quality. Joe Cotruvo

This depends on the chlorine demand, which is that water can exist Iron (II), manganese, organic matter and ammonia, then their presence may consume chlorine and previously should evaluate how much chlorine is consumed and depending on bond this remnant is added regularly the minimum value recommended free chlorine is 0.3 mg / L, this after having reached the breaking point, which is where you will start to measure free chlorine. As comments Cotruvo consider necessary contact time and a minimum half hour is recommended, however this depends upon the desired microorganism inactivation, as in the case of Giardia is necessary to contact two hours.

Dear you can do different sets of experiments by using 2 ppm- 10 ppm in water. 

No need to do trial and error as suggested above. See Table 6.1 p 109  in the newly released WHO amended 4th edition of the Guidelines for Drinking Water Quality. Free download at        http://www.who.int/water_sanitation_health/publications/drinking-water-quality-guidelines-4-including-1st-addendum/en/

The USEPA CT value for Giardia 3 log reduction is 104 mg min/l  which means you would need 1 mg/l for  52 minutes for water at 10 C and pH 7. Two log reduction is about 35 minutes contact time. Warmer water would be less time.

correction. It is 104 minutes for Giardia (worst case except for crypto which chlorine does not touch) @ 1 mg/L. If no giardia or crypto atre present the 4 log CT for virus is 6 under the  same conditions. That means 1 mg/L for 6 minutes., 4 minutes for 3 logs.

what does it mean "sodium hypochlorite 1% W/V" ???? I want to use this solution as a disinfectant for cleaning desks and chairs where students using frequently? any suggestions for using it and or alternative chemical

Use 10 parts of sodium hypochlorite to disinfect 1L of water.

This is because 1L of water contains 1000g, then since the container of sodium hypochlorite shows 4% w/v it means there is 4g of sodium hypochlorite in every 100g of the solution (presumably water). Hence measure 10 parts of the sodium hypochlorite solution which will contain 40g of the active chemical to 1000g or 1L of water.

Thank you.

NO! NO!! 40 grams is absurd. WHO recommends adding 4 drops of 5% bleach per liter of clear water about 25 C and wait 30 minutes.

Muhammad Bilal Asif , in the CDC link that you have mentioned, they say 5-10 mL for 20 L of water. Any idea what the chlorine concentration is for the hypochlorite solution they are talking about?