(BOD)remaining = (BOD)initial e-kt

• At any time, the BOD consumed or degraded is:

(BOD)consumed = (BOD)initial (1 - e-kt)

• In England, where the concept of BOD was developed in the early 1900's, rivers are relatively short. So consumption of the initial BOD was never reached, because it would require more time (and space).

• Thus, the 5-day BOD was taken as the practical measure of the target consummable BOD. This concept was adopted throughout the world.

(BOD)consumed in 5 days = (BOD)initial (1 - e-5k)

• For example, for initial BOD = 300 mg/L, and k= 0.2, the BOD₅ is:

(BOD)consumed in 5 days = 190 mg/L

• For example, for initial BOD = 300 mg/L, and k= 0.2, the BOD₁₀ (BOD consumed in 10 days) is:

(BOD)consumed in 10 days = 259 mg/L

• Domestic wastewater usually contains high concentrations of ammonium (15 mg/L), due to insufficient time to allow nitrification (conversion to nitrate).

• Oxidation of the ammonium represents a nitrogenous oxygen demand (NOD), which sets in after a while.

• Primary treatment normally removes 35% of BOD, 30-50% of TSS, and only 10-20% of N and P.

• Secondary treatment should remove 85% of TSS and BOD to concentrations of 30 mg/L for each.

• Tertiary treatment is required for the removal of substantial quantities of P.

• P can be removed by chemical precipitation, or lime.

• N can be removed through denitrification, or ion exchange.

• Coliform bacteria are usually removed by chlorination after primary or secondary treatment.

• TSS concentrations for stormwater and sewage are similar.

• Metals (Zn, Cu, and Pb) are higher in runoff than in sewage effluent.

• The metals in urban runoff originate in the corrosion of vehicles and piping systems, as well as from atmospheric emissions.

• Lead in runoff comes from gasoline, but has declined in the U.S. since the advent of lead-free gasoline in the 1970's.

• Sulphite paper plants produce very high BOD's; effluents can have BOD₅ of 30 000 mg/L.

• Phenols are an important constituent of oil refinery wastewater.

• Waste from food processing industries consist of TSS and BOD.

• Mining wastes include Cu, Zn, and Fe.

• Acid rain is caused by sulphur dioxide from fossil-fuel burning power plants and nitrogen oxides from auto emissions later oxidized in the atmosphere.

• SO₄^2- is more significant than NO₃^- in acidifying surface waters, because nitrate is a limiting nutrient in forested areas and is neutralized in biological uptake and growth.

• Toxic wastes can enter the environment via leachate from solid-waste landfills, atmospheric emissions over broad areas, pesticides, and accidental spills.

• Some common toxic wastes include the polychlorinated biphenyls (PCB), polybrominated biphenyls (PBB), chlorinated hydrocarbon insecticides, and dioxins (chlorinated organics).

• PBCs were used as an insulating material in transformers and capacitors; PBBs were used as a fire retardant; chlorinated hydrocarbons such as DDT and other insecticides were used extensively in the U.S. until the 1970's.

5.4  AGRICULTURAL WASTES

• Wastewater from agriculture is varied.

• Cultivation results in increased erosion and high TSS in streams.

• With TSS comes nutrients and pesticides.

• BOD/TSS wastes from livestock enclosures can be high concentrated.

• Attempts to control this type of pollution include minimizing erosion, minimizing fertilizer applications, and use of improved irrigation technologies.