Types of Solids in Wastewater
This demonstration will show the types of solids found in wastewater; Grit, dissolved, colloidal, and settable, as will as grease and oil. It will be explained how their physical properties are used to remove them.
- Vegetable oil
- Milk (1% or dry)
- Five -500 ml beakers or any clear container such as pickle jars.
- Spoon to stir with.
- Label the beakers; Sand, oil, oatmeal, milk, sugar
- Add 500mls water to each beaker
- For the dry materials (sand, oatmeal, sugar, and dry milk) place 1 to 2 teaspoons full of each material in their respective beaker. For liquid materials (milk and oil) place approximately 75 mls in their beakers.
- Mix the milk and sugar. The oatmeal will wet on its own and slowly settle to the bottom. Note the speed the oatmeal settles compared to the sand.
- Observe each beaker, note clarity and what happened to the material--settled, floated, or stayed suspended.
- Provide discussion on each beaker.
Sand represents the material known as grit in wastewater (e.g., eggshells, sand, coffee grounds). This material is heavy and is removed by slowing the water to allow settling of heavy inorganic matter. If grit is not removed from wastewater it will settle in process tanks taking up space and its abrasive characteristics will wear pumps and equipment. Oil and oatmeal representing primary treatment After preliminary treatment, primary treatment removes the settleable and floatable matter. This is done by slowing the water to a still motionless state. In this undisturbed condition, floatable and settleable matter separates where it can be collected and removed from the top and bottom of a clarifer.
After preliminary treatment, primary treatment removes the settable and floatable matter. This is done by slowing the water to a still motionless state. In this undisturbed condition, floatable and settable matter separates where it can be collected and removed from the top and bottom of a clarifer.
Some solids are so small they stay suspend in water. Milk proteins are such solids. (You can coagulate the milk solids with 15 mls vinegar.) Other solids dissolve in water; Sugar, salt, and alum are some examples. Colloidal and dissolved solids are difficult to remove from water. Some can be removed by chemical addition, but it is preferred to use microorganisms, which use the solids as food. During secondary treatment, microorganisms are grown under optimum conditions. They eat the dissolved and colloidal solids and in turn multiply producing more microorganism mass. These microorganisms can be settled from the wastewater and removed. Thus, colloidal and dissolved solids are removed by converting them into settable microorganisms that can be removed from the water.
Solids are removed during wastewater treatment by taking advantage of their physical properties (floating and settling). When the solid's physical properties make them difficult to remove, they are converted to a form that can be precipitated in biological secondary treatment.
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