Lizette,
You need to show us that you too have been doing some research, have found things out, and are working on a plan. You have posted in several places, but you never reply directly to the additional questions people have asked you in order to understand your project better and give you better answers.
In any case, I think you are mixing up two things that don't go together. One is cleansing liquid hospital waste of hazardous microorganisms, and the other is cleansing it of harmful chemicals. To cleanse it of harmful organisms, it might be enough to heat-sterilize it or do some kind of pasteurization (
http://en.wikipedia.org/wiki/Pasteurization) or flash pasteurization (
http://en.wikipedia.org/wiki/Flash_pasteurization), or some variant. I don't think this lends itself to a science fair project unless you have a lot of time and resources. Still, you may be able to do a project using UV germicidal irradiation (
http://en.wikipedia.org/wiki/Ultraviole ... rradiation) if you have access to a strong UV light. CAUTION: UV LIGHT CAN CAUSE PERMANENT DAMAGE TO EYES AND EXPOSED AREAS OF YOUR BODY.
Removing hazardous compounds from wastewater using plants to my mind sounds more interesting and lends itself to a variety of science fair projects. If you remove the word "hospital" from your project name and think of ways to use plants to remove chemicals from any arbitrary kind of wastewater, you can design a project using a
model system. This means you choose chemicals that are practical for you to test. The chemical(s) must be cheap, easily obtainable, not hazardous to you under the conditions of the experiment, and can be metabolized by a plant that you can easily obtain and grow within the time and resource limitations of your science fair project.
Perhaps even the idea of waste water can be changed into using plants to treat farmland that has too much salt and had to be abandoned. If you could research plants that could partly or completely remove salts from the soil, at least enough for useful crops to be harvested, then you would also have an interesting and socially useful project. Try this search:
http://www.google.com/search?hl=en&lr=& ... tnG=Search
If you are already committed to hospital wastes, you need to tell us, in this thread and without starting any additional threads, what specific things you want to remove from hospital wastewater. Also, you need to tell us where this wastewater is coming from. Is it being emitted by the hospital into the sewer system? Is it in metal drums? Is it in the plastic and paper items used to treat patients, such as in catheters, diapers, syringes, etc.? Is it the microorganims or the chemicals you want to eliminate?