Microbes In Human Welfare

Lac is a natural, resinous substance secreted by a tiny scale insect, \textit{Kerria lacca} (or \textit{Laccifer lacca}). The insect produces this resin as a protective coating around itself on the twigs of specific host trees (like Ber, Palas, and Kusum). The twigs covered with this encrustation are known as sticklac, which is the raw material harvested. Economic Importance of Lac: Lac is a versatile product with numerous commercial applications, primarily after being processed into shellac.

Varnishes and Polishes: Shellac is used to make high-quality varnishes and polishes for furniture and musical instruments, providing a durable and glossy finish.
Food Industry: Food-grade shellac is used as a glazing agent on candies, pills, apples, and coffee beans to make them shiny and increase their shelf life.
Electrical Industry: Due to its excellent insulating properties, it is used as an insulator in electrical components and cables.
Adhesives and Cements: It is a key ingredient in manufacturing sealing wax and special cements used for fixing lenses and mirrors in optical instruments.
Cosmetics Industry: It is used in products like nail polishes, hair sprays, and mascara.
Printing and Inks: It is used in making printing inks, lithographic ink, and for waterproofing.
Lac Dye: Historically, a scarlet dye extracted during the washing of sticklac was a valuable product for dyeing textiles like silk and wool.

The preliminary (or primary) treatment of sewage is a physical process designed to remove large solid materials. It involves the following steps:

Screening: Raw sewage first passes through screens or bar racks. These are meshes of varying sizes that physically block and remove large floating objects like plastic bags, cans, sticks, and rags.
Grit Removal: After screening, the sewage enters a grit chamber. Here, the speed of the sewage flow is slowed down significantly. This allows heavy, inorganic particles like sand, grit, and small stones to settle at the bottom through sedimentation, while the lighter organic solids remain suspended.
Primary Sedimentation: The sewage then flows into a large tank called a primary settling tank or clarifier. In this tank, the flow is very slow, and it is held for a period of several hours. This allows most of the suspended organic solids to settle to the bottom by gravity, forming a mass called primary sludge. Lighter materials like grease and oil float to the top, forming a layer called scum, which is skimmed off. The remaining liquid, now called the primary effluent, is then sent for secondary treatment.