Water and Sewage Treatment

I am going to describe a safe and aseptic method using a limited amount of equipment whereby dirty, non-pure water is treated to produce water that is acceptable for drinking or washing. The final product will be checked on agar plates. Agar plates are used to identify the presence of bacteria.

Method

• Ensure that all the containers that will hold the water in this experiment are thoroughly sterilized.

• Collect the dirty water in a large measuring column.

The following three processes are all forms of Filtering.

• Sedimentation will be the first form of treatment that will be done to the dirty water. Sedimentation separates any microorganisms and any organic material. The water should be left for at least a day to allow the large and small particles to separate from each other. The large clay particles, stones, dirt etc will sink to the bottom under the effect of gravity forming sludge. The clean water is left on top and can be collected using a pipette or by gently pouring it into a beaker. Under ideal conditions a sedimentation tank is used.

In the water industry the wastewater passes into a sedimentation tank, in which organic materials settle out and are drawn off for disposal. The process of sedimentation can remove about 20 to 40 per cent of the BOD5 and 40 to 60 per cent of the suspended solids. In water industries, sludge from the primary settling tanks and aerobic processing is treated for two to three weeks in tanks in the absence of oxygen at a temperature between 30 and 40 °C. Anaerobic ‘digestion’ of the sludge by bacteria such as Mathanobacterium converts the organic material into methane, carbon dioxide and hydrogen gases along with water and minerals.

• In our experiment, after sedimentation, the effluent is passed through a column of different sized stones and sand. The largest stones should be at the bottom and the smallest at the top. Sand can also be used at the top of the column. Some important points to follow when making a rock column are to make sure the stones are clean and there are no gaps. This will remove any large insoluble particles that weren’t removed during sedimentation.

• The effluent that is collected after the rock column will be filtered at least twice through a filter funnel and filter paper.

 

In industrial plants a process called Trickling Filter is used. In this process, a waste stream is distributed intermittently over a bed or column of some type of porous medium. A gelatinous film of microorganisms coats the medium and functions as the removal agent. The organic matter in the waste stream is absorbed by the microbial film and converted to carbon dioxide and water. The trickling-filter process, when preceded by sedimentation, can remove about 85 per cent of the BOD5 entering the plant.

The following processes in our experiment are all forms of treatment following the filtering processes:

• You may treat the filtered water with liquid chlorine or laundry bleach.

NOTE: do not use scented laundry bleach, powdered bleach, or swimming pool chlorine. These contain additional chemicals that are poisonous.

 The chart below shows how much liquid bleach should be added to the water.

• Distillation will remove any bacteria in the water that hasn’t already been killed by the bleach.  It will also separate the pure water from salts and other chemicals left over by the bleach. Under ideal conditions the water can be boiled, passed through a condenser jacket and then collected in a beaker. But due to the limited amount of equipment we have been given, the water will have to be boiled using a Bunsen burner, and then collected by holding a cold beaker above the beaker of boiling water to collect the steam. The steam will then condense on the glass walls of the cool beaker. The beaker should be held at an angle so that the condensed water can drip into another beaker.

The beaker being used to collect the steam should be held with a clamp because it will get very hot.

• Once the sterilized effluent has been condensed and collected a few drops can be poured into an agar plate to test its purity. The agar plate must not stay open for very long otherwise bacteria from the atmosphere will enter. The few drops of effluent in the agar plate should be spread evenly using a glass rod that has been thoroughly sterilized by holding it under a blue Bunsen burner’s flame. The lid of the agar plate should quickly be put back and sealed with a few strips of tape. The tape shouldn’t be completely cover the lid of the agar plate otherwise oxygen will not be able to get inside, preventing any bacteria from the water multiplying. If there are any bacteria present and it doesn’t multiply we will not be able to assess the purity of the water.

• After at least two days the agar plate should be checked. If the agar plate is clear and there are no signs of bacteria present, then the water should be safe to drink. But if there are signs of colonies of bacteria forming then the water will be unsafe to drink.

Variables

• Instead of the sedimentation process the dirty water in our experiment can be passed through a series of sieves. The first filter would be the one with the largest pores and last one would be the sieve with the smallest pores. The sieves would mainly remove large pieces of material such as stones, sand dirt etc.
• An alternative treatment replacing the use of bleach is to use Iodine tablets.
• The purity of the water can be tested using agar plates after each of the individual processes in the experiment in order to see which one was the most effective.

Prediction

I think that the most effective form of treatment will be the distillation process and after a few days the agar plate will show no signs of bacteria present. If only the distillation process was carried out I would still predict that the distilled water would be pure. Distillation will separate the pure water from insoluble particles and anything else that was dissolved in the water. Any bacteria present will be eliminated due to the high temperature (over 100°C) reached.