Question

Draw a schematic of a wastewater treatment plant, explain the primary and secondary treatment stages. Identify the major pollutants removed in each stage.

( Please no too long answers )

Answer 1

Schematic Diagram of Wastewater treatment plant

Primary Treatment Stage

The primary treatment includes processes for the removal of materials that will either float or readily settle out by gravity. The processes such as screening, comminution, grit removal, and sedimentation.

1. Screening: This process removes large floating objects such as rags and sticks that might block the pipe or damage the treatment equipment.

2. Comminution: It is an alternative to screening. In a comminutor, the suspended solids are ground in order to reduce the particle size.

3. Grit removal: After the completion of screening, the sewage is passed through a grit chamber where cinders, sand and small stones that may cause wear and tear on pumps and other plant equipment are settled down.

4. Sedimentation: Suspended solids that go through screens and grit chambers are expelled from the sewage in sedimentation tanks. These tanks are also called primary clarifiers.

Secondary Treatment Stage

This stage removes the soluble organic matter that cannot be treated in primary treatment. More suspended solids are also removed in this stage. This removal is done by certain biological processes that includes the methods such as trickling filter, activated sludge process, oxidation pond and rotating biological contacter.

1. Trickling filter: It is a tank filled with deep bed of stones. The settled sewage is allowed to flow on the top of the stones and is then trickles to the bottom where it is collected for further treatment. It will help in reducing the biochemical oxygen demand (BOD) of the sewage. Trickling filters are followed by settling tanks known as secondary clarifiers which removes the microbes that are washed off the rocks by the flow sewage.

2. Activated sludge: The activated sludge treatment framework comprises of an air circulation tank followed by a secondary clarifier. Settled sewage, blended in with crisp muck that is recycled from the secondary clarifier, is brought into the air circulation tank. Packed air is then infused into the blend through permeable diffusers situated at the base of the tank. As it air pockets to the surface, the diffused air gives oxygen and a quick blending activity. Air can likewise be included by the beating activity of mechanical propeller-like blenders situated at the tank surface. This will reduce the BOD of the sewage.

3. Oxidation Pond: These are also called lagoons or stabilization ponds. Oxidation ponds are large shallow ponds designed to treat wastewater through the interaction of algae, bacteria, and sunlight. Algae grow through the process of photosynthesis by consuming carbon dioxide and utilizing the energy from the sun and release oxygen needed by the aerobic bacteria.

4. Rotating Biological Contacter: In this treatment process, a progression of enormous plastic plates mounted on an even shaft are in part submerged in primary effluent. As the shaft rotates, the disks are exposed then again to air and wastewater, permitting a layer of bacteria to develop on the plates and to utilize the organics in the wastewater.

These are the primary and secondary treatment stages of wastewater treatment

Effluents removed in each stage:

The effluent removal in each process of the primary and secondary stages are listed above. In general,

1. Primary treatment stage: Removes settleable organic and inorganic solids.

2. Secondary treatment stage: Removes residual organics and suspended solids. It also includes the removal of biodegradable dissolved and colloidal organic matter.

3.Teritiary Treatment or advanced treatment: It is done based on the problem requirement and type of wastewater. It is employed when there are specific constituents in the wastewater that cannot be removed by secondary treatment. This includes nitrogen removal by selective ion exchange, phosphorus removal by chemical precipitation, suspended solids removal by chemical coagulation, organics and metal removal by carbon adsorption, dissolved solids removal by reverse osmosis or electrodialysis or distillation etc..