(E6) Water Treatment Sarah Black

Background: About ¾ of the Earth’s surface is covered by water. Water is a polar substance capable of hydrogen bonding, which allows it to dissolve many chemicals. Thus, some toxic substances, bacteria, and viruses can be carried by water. The purpose of sewage treatment is to remove these hazardous materials, reduce the BOD (Biological Oxygen Demand) of the sewage and kill microorganisms prior to discharge. Depending on the availability of resources, three separate levels of sewage treatment may occur. These all reduce the level of pollutants and the BOD, but the tertiary treatment is the most effective (also the most expensive to build and operate.)

Some Common Pollutants: Heavy Metals Cadmium  Rechargeable batteries, metal plating, pigments Copper  Household plumbing, copper mining, smelting Mercury  Batteries, mercury salts as fungicides, mercury cells in chlor-alkali industry, discharge from pulp and paper mills Pesticides (include DDT, fungicides, and herbicides) Agricultural practics Dioxins From by-products of industrial processes such as waste incineration, forest fires and burning fuels, manufacture of chlorinated pesticides; used in Agent Orange as a defoliant in Vietnam war

Some Common Pollutants (cont.): Polychlorobiphenyls (PCBs) Very stable compounds that were used widely as coolants and lubricants in capacitors, transformers, and other electrical equipment Organic Matter Waster treatment plants, decomposition of plants and animals, oil spills, industrial waste Nitrates Agriculture: chemical nitrate fertilizers for farming; from acid rain Phosphates Detergents and chemical phosphate fertilizers

Primary Sewage Treatment (Removes 30-40% of the BOD waste) 1st  passed through screens and traps which filter out large objects such as trash and debris and, from the surface, remove floating objects including grease. 2nd  passed through settling tanks where smaller heavier objects settle and can be transferred to land fills. 3rd  passed through holding or sedimentation tanks where it is allowed to settle and sludge is removed from the bottom. The addition of certain chemicals can speed this process up (called flocculation).

Secondary Sewage Treatment (Removes about 90% of BOD waste) 1st  Air enriched with oxygen is bubbled , using large blowers, through sewage mixed with bacteria-laden sludge (called an activated sludge process). This allows aerobic bacteria to thoroughly mix with the sewage in order to oxidize and break down most of the organic matter. 2nd  passed through a sedimentation tank where large quantities of biologically active sludge collect.

Tertiary Sewage Treatment These processes involve specialized chemical, biological, and/or physical treatment to further purify the water. It can remove remaining organic materials, nutrients, and substances not already taken out. Examples of tertiary treatment include: carbon bed, chemical precipitation, and biological processes.

Carbon Bed Method: Uses activated carbon black. Which consists of tiny carbon granules with large surface areas which have been treated and activated by high temperatures. Has the ability to readily adsorb (the attraction of a substance to the surface of a solid substance) organic chemicals. Effective against many toxic organic materials and charcoal filters are often used to further purify tap water for drinking purposes.

Chemical Precipitation Basically, the precipitation of toxic heavy metal ions as their sulfide salts (which have low solubility in water). Carefully controlled amounts of hydrogen sulfide gas are bubbled through a solution containing heavy metal ions and the corresponding sulfides can then be filtered out.

Anaerobic Biological Process This is denitrification (achieved by denitrifying bacteria), which turns the nitrogen in nitrates back to atmospheric nitrogen. This is a reduction process in which the oxidation number of nitrogen is reduced from +5 in the nitrate ion, NO3-, to 0 in N2.

Distillation Further methods of desalination (removal of salts) to produce fresh water are multi-stage distillation and reverse osmosis. Distillation allows the seperation of a volatile liquid from non-volatile materials. The water vapor can then be collected and separated as fresh water. Osmosis is the natural tendency of a solvent such as water to move from a region of high solvent concentration to one of lower solvent concentration through a semi-permeable membrane.