Question

Evaluate the physical and chemical methods of waste treatment? How efficient are they in and what is their effect on the environment (positive and/or negative)?

Answer 1

Firstly coming to the physical methods of waste treatment.They are as follows:

sedimentation , screening, aeration, filtration,flotation.skimming,degasification,equalization and ultra-violet treatment

secondly, the chemical methods include:

chlorination,ozonation,neutralization,coagulation,adsorption and ion exchange method

In real life water treatment the method used depends on the quality of the water required ,the cost of the treatment, time available in doing so and many other factors. Many times both physical and chemical methods are carried out hand in hand for efficient water treatment.

Physical methods include processes where no gross chemical or biological changes are carried out and strictly physical phenomena are used to improve or treat the wastewater. Examples would be coarse screening to remove larger entrained objects and sedimentation . In the process of sedimentation, physical phenomena relating to the settling of solids by gravity are allowed to operate. Usually, this consists of simply holding a wastewater for a short period of time in a tank under quiescent conditions, allowing the heavier solids to settle, and removing the clarified effluent. Sedimentation for solids separation is a very common process operation and is routinely employed at the beginning and end of wastewater treatment operations. While sedimentation is one of the most common physical treatment processes that is used to achieve treatment, another physical treatment process consists of aeration -- that is, physically adding air, usually to provide oxygen to the wastewater. Still, other physical phenomena used in treatment consists of filtration. Here wastewater is passed through a filter medium to separate solids. An example would be the use of sand filters to further remove entrained solids from a treated wastewater. Certain phenomena will occur during the sedimentation process and can be advantageously used to further improve water quality. Permitting greases or oils, for example, to float to the surface and skimming or physically removing them from the wastewaters is often carried out as part of the overall treatment process.

Chemical treatment consists of using some chemical reaction or reactions to improve the water quality. Probably the most commonly used chemical process is chlorination. Chlorine, a strong oxidizing chemical, is used to kill bacteria and to slow down the rate of decomposition of the wastewater. Bacterial kill is achieved when vital biological processes are affected by the chlorine. Another strong oxidizing agent that has also been used as an oxidizing disinfectant is ozone.  

A chemical process commonly used in many industrial wastewater treatment operations is neutralization. Neutralization consists of the addition of acid or base to adjust pH levels back to neutrality. Since lime is a base it is sometimes used in the neutralization of acid wastes.  

Coagulation consists of the addition of a chemical that, through a chemical reaction, forms an insoluble end product that serves to remove substances from the wastewater. Polyvalent metals are commonly used as coagulating chemicals in wastewater treatment and typical coagulants would include lime (that can also be used in neutralization), certain iron-containing compounds and alum.  

Certain processes may actually be physical and chemical in nature. The use of activated carbon to adsorb or remove organics, for example, involves both chemical and physical processes. Processes such as ion exchange, which involves exchanging certain ions for others, are not used to any great extent in wastewater treatment.

Now coming to the efficiency both the methods are equally efficient and both the methods remove the contaminants to a good measure as required. But all the methods are not equally efficient and the methods are used accordingly such as for coarse water treatment and fine water treatment.

Chemical methods sometimes have a negative impact on the environment, whereas physical methods have very less or zero negative impact on the environment. In chemical methods, if the amount of chemicals added is more, the quality of the water is spoiled and it also effects the health of the people drinking that water.