Wastewater could be defined as any water that is influenced by pathogenic organisms (they can cause cholera, polio, typhoid, hepatitis, etc.) which may include any wastes from human activities such as environmental, chemical, biological, etc. The wastewater can be disinfected with the help of various chemicals (relatively limited use), biodegradation and filtrations systems (the filtration of wastewater through the soil), etc. There are certain positive and negative outcomes of wastewater disinfection. The positive outcomes are of course reduced pathogens. Chlorine and bromine are used commonly to disinfect the wastewater. The negative outcome includes the fact that the use of bromine, for instance, may not be very environmentally friendly. The bromine may also disinfect the wastewater for a very short period of time because of which chlorine is preferred as a disinfection chemical. The emerging technologies to disinfect wastewater are ozonation and UV lights.
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The wastewater treatment consists of certain stages such as pre-treatment, primary, secondary, and tertiary. In the pre-treatment stage, the wastewater (from numerous places – buildings or industrial units) is transported with the help of pipes to a treatment plant. The harmful contaminants are removed informally to have them transported safely to the treatment plant. In the primary treatment, harmful objects, stones, sticks, and other material (that can block the tank) are removed. Other harmful contaminants such as oil, are brought to the surface and skimmed off. The secondary treatment then takes the wastewater and offers few methods to clean it further. The idea is to break the organic material present in the water and settle it out. The most common method used to do this task is activated sludge that uses micro-organisms to break down organic pollutants. The tertiary process is the last stage for cleaning the wastewater which requires the water to pass through the sand filters and the remaining particles are removed and the water is considered clean.
The activated sludge process contains solids in the system and it takes days to clear the solid material, this span of time is known as solids retention time (SRT) which is also commonly known as sludge age. θc is a mean cell residence time which is the average time for which biomass remains in the system. Operating the plant depends on how the organisms in the plant are operated. The variations in the SRT directly influence the performance including the sludge production and stability, the presence of oxygen, etc. Increasing the SRT will increase the requirements of the oxygen in the process to have stability and it could result in more denitrification.
The F/M ratio is the food to micro-organisms ratio in the activated sludge process. Many of the operators have not understood how critical the F/M ratio control is, it is partially because there are numerous methods to calculate it. The important methods are the F/M ratio, the sludge age, and mean cell residence time. All these methods calculate the same thing which is the ratio of the weight of food to the weight of micro-organisms in the tank. The F/M ratio is considered opposite to the sludge age method while the mean cell residence time is more complicated. There is a relationship between the F/M ratio and mean cell residence time but both these methods are used to measure the same value while one being more commonly used with less complexity and the other one being complicated.
Davis, M.L. & Cornwell, D.A. (2006). Introduction to the Environmental Engineering 4th Edition. McGraw Hill. (n.p.).