Which of the following need to be done before chemical treatment and biological treatment?
A. Screening only
B. Filtration only
C. Boiling of water
D. Both A and B

Hint: Chemical and biological treatment is usually performed to treat waste or sewage water. Wastewater is the water utilized. It contains substances including certain human waste, food debris, fuels, detergents, and chemicals. Wastewater treatment, also recognized as waste management, elimination of contaminants from sewage or wastewater before it hits aquifers or natural water bodies, like rivers, streams, lakes, and seas.

Complete answer: Wastewater screening is the very first unit operation to be used for wastewater treatment. Screening clears items like rags, paper, plastics, and metal to minimize damage and blocking up of downstream appliances, pipelines, and appurtenances. There seem to be different kinds of wastewater screens, namely coarse screens, fine screens, and grinders. Coarse screens isolate solid particles, rags, and litter from wastewater and often have pores of 6 mm over about. Fine screens are commonly used to remove contaminants that could create issues in installation and management in downstream processes, especially in networks that do not have primary treatment. Typical opening sizes for fine screens are between 1.5 and 6 mm. A filtration device is being used to regulate the wastewater secondarily. The type of filtration device employed relies on the service, however, the filter cartridge (also defined as activated sludge process) is more often used. Wastewater is injected into the system in this process. This will stimulate the growth of bacteria. The bacteria adhere to surfaces, but then the bacteria absorb and decompose the substance which travels through the area. After this, the water flows through the next round of treatment. After quitting the sediment transport tank at the secondary level of the treatment, the water will be sent to the tanks where it would be subjected to chlorine, which could eradicate up to 99 percent of the microbes.
So, the correct answer is (D).

Note: Now, we know that throughout the wastewater management process, screening is just an initial phase that eliminates 'larger' biological matter, and then the filtering step is one which prevents certain impurities further.