Match each of the terms with the best description. each response is used once. a settling tank b trickle filter c bod d activated sludge system e imhoff tank f floc 1. a particle of organic matter that sinks to the bottom of a tank. these particles particles consist of little pieces of organic matter covered with microorganisms. the microbial population of the particles is a mixed community of bacteria and fungi that degrade organic compounds and protozoa that eat the smaller microorganisms. sometimes alum is added to the sewage, this increase the formation of larger particles that settle out faster. 2. an abbreviation that stands for biochemical oxygen demand. this is a measure of the total amount of organic matter that is dissolved in a sewage sample. 3. a principle tool of primary sewage treatment. lightweight solids, grease and floating particles are skimmed off the top and heavier materials sink to the bottom as sludge. 4. a secondary sewage treatment system used in smaller communities. effluent from the primary treatment slowly flows through a shallow artificial stream. the water is oxygenated as it flows over the rocks. the rocks become covered with a zooglial mass of aerobic microorganisms that break down organic matter to co2. 5. a secondary sewage treatment system used in larger communities. effluent from the primary treatment is seeded with sludge that contains a high concentration of metabolizing bacteria and vigorously aerated. aerobic metabolism breaks down organic matter that is dissolved in the water to co2. growth of microorganisms produces more sludge, which is allowed to settle out. 6. a container for the anaerobic digestion of sludge. anaerobic bacteria ferment organic matter producing co2, h2 and small organic acids such as acetic acid and formic acid. then anaerobic archaea convert the small acids, co2 and h2 to methane.