Solids are removed from water through the unit operation known as solids transfer. This process involves the separation of solid particles from a liquid stream, typically water, by physical means. Solids transfer is an important step in water treatment and various industrial processes to ensure the purity and quality of the water.

There are several methods and equipment used for solids transfer, depending on the specific application and the characteristics of the solid particles being removed. Some common techniques include sedimentation, filtration, and centrifugation.

- Sedimentation: This method utilizes gravity to separate solid particles from water. The water is allowed to stand in a tank or basin, and over time, the heavier solids settle to the bottom due to their higher density. The settled solids, also known as sludge, can then be removed using various mechanisms such as scrapers or pumps.

- Filtration: Filtration involves passing the water through a porous medium, such as sand, gravel, or a filter media, to trap and remove solid particles. The size of the filter media and the filtration rate can be adjusted to effectively remove different sizes of solids. Filtration can be done using various types of filters, including rapid sand filters, multimedia filters, and membrane filters.

- Centrifugation: Centrifugation is a technique that utilizes centrifugal force to separate solid particles from water. The water is spun rapidly in a centrifuge, causing the heavier solids to move towards the walls of the centrifuge while the water remains in the center. The separated solids can then be discharged from the centrifuge.

- Other methods: There are also other methods used for solids transfer, such as flotation, where air bubbles are introduced into the water to attach to and carry the solid particles to the surface for removal. Electrocoagulation is another technique where an electric current is applied to the water, causing the formation of coagulated solids that can be easily separated.

In summary, solids transfer is the unit operation used to remove solid particles from water. This process can be achieved through sedimentation, filtration, centrifugation, flotation, or electrocoagulation, depending on the specific requirements and characteristics of the solids being removed.