Environmental Science 4.12

 Mid-1800s, 25,000 people living along the River Thames in London died of cholera  River was declared “dead” by 1950  Walk along the Thames today no longer requires a “pocket full of posies” to mask the stench

 The Charles River in Boston was so polluted by 1900 that shad had disappeared  By 1940, the Delaware River was black w/ untreated sewage and chemical wastes  Chemical fumes overcame dockworkers  By 1950, level of DO was zero

 In 1927, workmen refused to work on construction along the Willamette River in Oregon  Untreated sewage directly to river  By 1967, most cities had primary and secondary treatment  Riverbed was still lined w/ decaying matter  90% of oxygen-demanding wastes were coming from the pulp and paper industries  2006 – Willamette was considered one of America’s most endangered rivers  Toxic chemicals is major concern Mercury, PCBs, sewage, agricultural sediment

 1972 CWA required cities to treat their sewage before dumping; industries were also required to use the “best practicable” technology to stop pollution  Significant progress, but 2000 National Water Quality Inventory reports 40% of streams, 45% of lakes, and 50% estuaries were not clean enough to support fishing and swimming

 Primary treatment mostly involves physical separation of liquids and solids  Screens remove large debris  Suspended solids settle out in primary settling tanks  Sludge – solids that sink to bottom  Floating grease and oils are skimmed off surface

 Secondary treatment is a biological process that increases oxygen in wastewater and allows time for organisms to remove organic matter  Activated sludge process Wastewater is piped to large tanks where air is pumped Air supplies bacteria w/ oxygen they need to eat organic matter Pumped to secondary settling tanks where the remaining solids settle out Activated sludge is returned to aeration tanks where the bacteria help eat more organic matter

 Trickling filter Tank filled w/ rock-like or grid system that is coated w/ slime, which contains algae, bacteria, and other pollution-tolerant organisms Organisms remove most of the organic matter and nutrients  After going through the activated sludge process or trickling filter process, the sludge is piped to digesters, which are large tanks that are heated Anaerobic bacteria in the waste can digest more of the organic material Natural gas (methane) is a waste product

 Biosolids – solids that are removed from sewage (dewatered)  Dried w/ heat or mechanically dried  Resulting biosolids are used as fertilizers or soil enhancers  Farmers can reduce fertilizer costs by applying biosolids  Must know nutrient level to calculate the amount that can be safely applied  Compost

 Purpose of primary and secondary sewage treatment is to reduce organic matter and lower the BOD of the effluent  Toxic wastes in the sewer may kill the bacteria and stop the treatment process  Industries are required to pretreat wastewater so that toxic wastes don’t interfere w/ the treatment process

 After secondary treatment, the wastewater is disinfected before discharged through an outfall pipe  Chlorination is the least expensive and most common method of disinfection  Disadvantages:  Chlorine reacts w/ organic compounds in the water to form trihalomethanes, which are carcinogenic  Chlorine is toxic to aquatic organisms

 Combined sewers carry both sewage and storm water  When they were built, no one anticipated problems at the sewage treatment plant  Sewage treatment plants are not designed to process such massive volumes of water  New sewer systems must have separate storm and sanitary sewers

 Secondary treatment at some sewage treatment plants does not produce effluent that meets EPA standards for nitrates and phosphates, so the effluent must be further treated

 Septic tank-soil absorption systems are the most common type of wastewater treatment used in rural areas  Septic tank – large tank buried in the ground to treat sewage from an individual home or business  Solids settle to the bottom, bacteria break down organic matter, and the effluent flows through pipes to the soil

 Bacteria normally digest about 50% of solids; remainder accumulates in the bottom  Sludge will build up and clog the outlet piping unless it is regularly removed from the tank  Septage – mixture of fluids and solids pumped from the tank