TX249_Frame_C02.fm Page 125 Friday, June 14, 2002 1:51 PM Constituents of Water and Wastewater Given a wastewater, what process should be applied to treat it: biological, chemical, or physical? Should it be treated with a combination of processes? These questions cannot be answered unless the constituents of the wastewater are known Thus, before any wastewater is to be treated, it is important that its constituents are determined On the other hand, what are the constituents of a given raw water that make it unfit to drink? Are these constituents simply in the form of turbidity making it unpleasant to the eye, in the form of excessive hardness making it unfit to drink, or in the form bacterial contamination making it dangerous to drink? Water and wastewater may be characterized according to their physical, chemical, and microbiological characteristics These topics are discussed in this chapter 2.1 PHYSICAL AND CHEMICAL CHARACTERISTICS The constituent physical and chemical characterizations to be discussed include the following: turbidity (physical), color (physical), taste (physical) temperature (physical), chlorides (chemical), fluorides (chemical), iron and manganese (chemical), lead and copper (chemical), nitrate (chemical), sodium (chemical), sulfate (chemical), zinc (chemical), biochemical oxygen demand (chemical), solids (physical), pH (chemical), chemical oxygen demand (chemical), total organic carbon (chemical), nitrogen (chemical), phosphorus (chemical), acidity and alkalinity (chemical), fats and oils and grease (chemical), and odor (physical) The characterization will also include surfactants (physical), priority pollutants (chemical), volatile organic compounds (chemical), and toxic metal and nonmetal ions (chemical) These constituents are discussed in turn in the paragraphs that follow 2.1.1 TURBIDITY Done photometrically, turbidity is a measure of the extent to which suspended matter in water either absorbs or scatters radiant light energy impinging upon the suspension The original measuring apparatus that measures turbidity, called the Jackson turbidimeter, was based on the absorption principle A standardized candle was placed under a graduated glass tube housed in a black metal box so that the light from the candle can only be seen from above the tube The water sample was then poured slowly into the tube until the candle flame was no longer visible The turbidity was then read on the graduation etched on the tube At present, turbidity measurements are done conveniently through the use of photometers A beam of light from a source produced by a standardized electric bulb is passed through a sample vial © 2003 by A P Sincero and G A Sincero TX249_Frame_C02.fm Page 126 Friday, June 14, 2002 1:51 PM 126 Physical–Chemical Treatment of Water and Wastewater The light that emerges from the sample is then directed to a photometer that measures the light absorbed The readout is calibrated in terms of turbidity The unit of turbidity is the turbidity unit (TU) which is equivalent to the turbidity produced by one mg/L of silica (SiO2) SiO2 was used as the reference standard Turbidities in excess of TU are easily detected in a glass of water and are objectionable not necessarily for health but for aesthetic reasons A chemical, formazin, that provides a more reproducible result has now replaced silica as the standard Accordingly, the unit of turbidity is now also expressed as formazin turbidity units (FTU) The other method of measurement is by light scattering This method is used when the turbidity is very small The sample “scatters” the light that impinges upon it The scattered light is then measured by putting the photometer at right angle from the original direction of the light generated by the light source This measurement of light scattered at a 90-degree angle is called nephelometry The unit of turbidity in nephelometry is the nephelometric turbidity unit (NTU) 2.1.2 COLOR Color is the perception registered as radiation of various wavelengths strikes the retina of the eye Materials decayed from vegetation and inorganic matter create this perception and impart color to water This color may be objectionable not for health reasons but for aesthetics Natural colors give a yellow-brownish appearance to water, hence, the natural tendency to associate this color with urine The unit of measurement of color is the platinum in potassium chloroplatinate (K2PtCl6) One milligram per liter of Pt in K2PtCl6 is one unit of color A major provision of the Safe Drinking Water Act (SDWA) is the promulgation of regulations This promulgation requires the establishment of primary regulations which address the protection of public health and the establishment of secondary regulations which address aesthetic consideration such as taste, appearance, and color To fulfill these requirements, the U.S Environmental Protection Agency (USEPA) establishes maximum contaminant levels (MCL) The secondary MCL for color is 15 color units 2.1.3 TASTE Taste is the perception registered by the taste buds There should be no noticeable taste at the point of use of any drinking water The numerical value of taste (or odor to be discussed below) is quantitatively determined by measuring a volume of the sample A (in mL) and diluting it with a volume B (in mL) of distilled water so that the taste (or odor) of the resulting mixture is just barely detectable at a total mixture volume of 200 mL The unit of taste (or odor) is then expressed in terms of a threshold number as follows: A+B TON or TTN = A © 2003 by A P Sincero and G A Sincero (2.1) TX249_Frame_C02.fm Page 127 Friday, June 14, 2002 1:51 PM Constituents of Water and Wastewater 127 where TON = threshold odor number TTN = threshold taste number 2.1.4 ODOR Odor is the perception registered by the olfactory nerves As in the case of taste, there should be no noticeable odor at the point of use of any drinking water The secondary standard for odor is Fresh wastewater odor is less disagreeable than stale wastewater odor but, nonetheless, they all have very objectionable odors Odors are often the cause of serious complaints from neighborhoods around treatment plants, and it is often difficult for inspectors investigating these complaints to smell any odors in the vicinity of the neighborhood The reason is that as soon as he or she is exposed to the odor, the olfactory nerves become accustomed to it and the person can no longer sense any odor If you visit a wastewater treatment plant and ask the people working there if any odor exists, their responses would likely be that there is none Of course, you, having just arrived from outside the plant, know all the time that, in the vicinity of these workers, plenty of odors exist The effect of odors on humans produces mainly psychological stress instead of any specific harm to the body Table 2.1 lists the various odorous compounds that are associated with untreated wastewater The determination of odors in water was addressed previously under the discussion on taste Odors in air are determined differently They are quantitatively measured by convening a panel of human evaluators These evaluators are exposed to odors that have been diluted with odor-free air The number of dilutions required to bring the odorous air to the minimum level of detectable concentration by the panel is the measure of odor Thus, if three volumes of odor-free air is required, the odor of the air is three dilutions It is obvious that if these evaluators are subjected to the odor several times, the results would be suspicious For accurate results, the evaluators TABLE 2.1 Malodorous Compounds Associated with Untreated Wastewater Compound Ammonia Butyl mercaptan Crotyl mercaptan Diamines Ethyl mercaptan Hydrogen sulfide Indole Methyl amine Methyl mercaptan Methyl sulfide Phenyl sulfide Skatole Formula Threshold (ppm) Odor Quality NH3 (CH3)3CSH CH3(CH2)3SH NH2(CH2)4NH2, NH2(CH2)5NH2 CH3CH2SH H 2S C H 7N CH3NH2 CH3SH (CH3)2S (C6H5)2S C H 9N 18 — — — 0.0003