1. a. The greatest errors in laboratory tests are usually caused by improper or nonrepresentative sampling, or poor preservation of samples prior to analysis.
2. False. It is good technique to flush the waterline, but this should be done at a convenient flow for sampling (usually about one-half a gallon per minute) for two to five minutes. Violent flushing with the faucet wide open may disturb sediment and incrustations in the line which may take longer to flush out before the sample may be collected.
3. b. Pipe volume = 3.1416 x (0.75 inches ÷ 2 ÷ 12 inches per foot)2 x 30 feet x 7.48 gallons per cubic foot = 0.7 gallons.
Flushing time = 0.7 gallons x 2 ÷ 0.5 gallons per minute = 2.8 minutes or about 3 minutes.
4. False. Free residual chlorine refers to free elemental chlorine (Cl2), hypochlorous acid (HOCl) and the hypochlorite ion (OCl–). Combined residual chlorine generally refers to the chlorine-ammonia compounds of monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3). The chloramines have slower and longer-lasting disinfecting power, however, dichloramine and trichloramine, along with chlorophenol and other chlorinated organics, are also taste and odor producers.
5. True. Because of the time-consuming procedure, special techniques and equipment necessary to test for specific pathogens, water is generally analyzed for coliform bacteria. Fecal coliform are discharged in great numbers in human and animal wastes. Coliforms are typically hardier than true pathogenic bacteria, and their absence from water is thus a good indication that the water is bacteriologically safe for human consumption.
6. True. The coliform group bacteria is defined as all oxygen-using, gram-negative, nonspore-forming and rod-shaped bacteria that ferment lactose (a sugar) and produce gas from the lactose within 48 hours at 95° F. However, the fecal coliform can grow at a higher temperature (112° F) than the nonfecal coliform.
7. True. The Membrane Filter (MF) Method has been an approved test for coliform bacteria since 1960. The test uses a cellulose ester membrane filter, the 0.45 micron pore size of which can be manufactured to close tolerances.
8. False. Hardness is caused principally by calcium and magnesium ions because of their relative abundance in natural waters. However, hardness may also be caused by other divalent and trivalent metallic cations such as strontium, iron, manganese, aluminum and zinc. Because only calcium and magnesium are present in significant concentrations in terms of hardness, the total concentration of calcium and magnesium ions expressed as the calcium carbonate (CaCO3) equivalent is used to define the practical amount of water hardness.
9. False. The anion hardness ions are associated with determines whether the hardness is carbonate or noncarbonate hardness. The anions bicarbonate (HCO3–) and carbonate (CO32-) associated with hardness cations cause carbonate hardness — sometimes called temporary hardness because it can be caused to precipitate by prolonged heating or boiling. Sulfate (SO42-), chloride (Cl–) and nitrate (NO3–) anions associated with calcium or magnesium cause noncarbonate or permanent hardness that cannot be removed or precipitated by boiling or heating.
10. (a) Calculate the mean.
Mean, most probable number (or MPN)/100 ml = sum of all MPNs ÷ number of MPNs = (48 + 60 + 4 + 21 + 6 + 7 + 26) ÷ 7 = 172 ÷ 7 = 25 MPN/100 ml.
(b) To determine the median, rearrange the data in increasing order and select the middle value.
Order 1 2 3 4 5 6 7
MPN/100 ml 4 6 7 21 26 48 60
Median, MPN/100 ml = middle value of the group of data = 21 MPN/100 ml.
(c) Calculate the geometric mean for the given data.
Geometric mean, MPN/100 ml = (48 x 60 x 4 x 21 x 6 x 7 x 26)1/7 = (264,176,640)1/7 = 16 MPN/100 ml.