Sewage Treatment Design Lab Essay Sample

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At the tertiary treatment of sewage, nitrogen and phosphorus compounds are removed but the water is also disinfected to kill all micro-organisms to make sure no pathogens are released in nature. In this experiment, a harmless strain of E. coli (HB 101) will simulate the presence of bacteria in water. Different methods of disinfection such as boiling, UV light exposure and chlorination will be used. The physical methods which are primary disinfectants only, will be compared to each other. Likewise the chemical methods which also have a residual effect will be compared to each other. After treatment, a Colilert testing kit will be used which would detect the presence of coliforms in water by a colour change to yellow. By the use of a spectrophotometer, the percentage transmittance relative to a positive test from a control will give quantitative data to determine which method was most effective in killing the E. coli.


* Independent variables:

1. For the physical methods of disinfection (boiling and UV light) the contact time to act will be changed. Time periods: 30 seconds, 2 minute and 8 minutes.

2. For the chemical methods of disinfection (sodium hypochlorite and calcium hypochlorite) the dosage, that is, the concentration will be changed. Concentrations: 5%, 10% and 20% (m/v)

* Dependent variable:

The percentage transmittance after the Colilert test. The spectrophotometer will be set at the wavelength of the colour of a positive test result using a control. The lower the transmittance, the more effective the method of disinfection was.

* Controlled variables:

1. The temperature of all trials will be constant. During the experiment, all steps will be carried at room temperature and the incubator will also have a constant temperature.

2. The approximate number of E. coli bacteria will be somewhat controlled. 2 mL of the E. coli culture will be placed in 10 mL of water.

3. The volume of solutions added for the chemical methods will be constant. 2 mL of the solutions will be added to the appropriate trials.

4. The time the chemicals are allowed to act unaffected will be 5 minutes for each trial. After that time, the Colilert reagent will be added.

5. The testing for bacteria will be the same for all trials. 10 mL of the Colilert reagent will be added to each trial.

6. The time allowed for Colilert test to act in incubation. 24 hours, same for all trials.


* Beakers

* Graduated cylinder

* Pipets

* Retort stand

* Ring clamp

* Thermometer

* Stopwatch

* Heating plate

* Waterproof marker

* Spectrophotometer

* Distilled water

* E. coli culture

* Incubator

* Colilert Test kit

* Sodium hypochlorite

* Calcium hypochlorite

* UV lamp


1. For the purpose of a control, place 10 mL of water in a beaker and add 2 mL of the E. coli culture to it, mix gently. Then add 10 mL of the Colilert reagent and place it within the incubator. Label it as the control.

2. Prepare solutions of sodium hypochlorite and calcium hypochlorite by mixing correct amounts of each salt to produce solutions of 5%, 10% and 20% m/v. 5 mL of each is sufficient.

3. Place 10 mL of water in a beaker, add 2 mL of the E. coli culture and mix gently.

4. Add 2 mL of 5% m/v sodium hypochlorite solution to it, mix and wait for 5 minutes using the stopwatch before adding 10 mL of the Colilert reagent. Label it accordingly and place it in the incubator.

5. Repeat step 3 and 4 but add 10% and 20% m/v solution to the water instead.

6. Repeat step 3 to 5 but use the calcium hypochlorite solutions.

7. Set up the heating plate, with a larger beaker acting as a water bath. Heat the water until it boils, check temperature with thermometer. Set retort stand and ring clamp to hold the smaller beaker.

8. Repeat step 3 and place the beaker in the clamp

9. Lower the beaker with the clamp into the bath and time 30 seconds with the stopwatch before removing it and adding 10 mL of Colilert reagent. Label the beaker accordingly and place it in the incubator.

10. Repeat steps 8 and 9 but time 2 minute and 8 minutes for each trial.

11. Repeat step 3 and place it on the table.

12. Shine the UV lamp upon it for 15 seconds. Then add 10 mL of Colilert reagent before labelling it and placing it in the incubator.

13. Repeat step 11 and 12 but time 2 minute and 8 minutes for each trial.

14. Come back in 24 hours. Remove the control and place a sample through the spectrophotometer, setting the wavelength as the baseline value.

15. Take a sample of each trial and place it within the spectrophotometer, recording the readings carefully.

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