used to characterize
wastewater
Respirometric Characterization of Aggregate Organic Constituents.
Determination of the BOD value and the corresponding rate constant k1 can be accom- plished more effectively using a respirometer as compared to using the bottle technique as described above (Young and Baumann, 1976a, 1976b; Young, et al., 2003). Respirometers are devices that are used to measure the rate of respiration of living microorganisms in aerobic, anoxic, and anaerobic environments.
Description. Modern headspace-gas respirometers work by maintaining a constant oxygen pressure over a sample containing microorganisms that are in the process of metabolizing an organic substrate by replacing the oxygen as it is consumed by the microorganisms. Oxygen replacement is accomplished by means of an electrolysis cell, a bubble-type flow cell, or by transducer-controlled pneumatic injection. An example of a EXAMPLE 2–11 Determination of BOD/COD, BOD/TOC, and TOC/COD ratios Determine
the theoretical BOD/COD, BOD/TOC, and TOC/BOD ratios for the compound C5H7NO2. Assume the value of the BOD first-order reaction rate constant is 0.23/d (base e) (0.10/d base 10).
Solution
1. Determine the COD of the compound using Eq. (2–57).
C5H7NO215O2S5CO21NH312H2O mw C5H7NO25 113, mw 5O25 160 COD 5 160/113 5 1.42 mg O2/mg C5H7NO2
2. Determine the BOD of the compound.
BOD
UBOD5(12e2k1t)5(12e20.2335)5120.3250.68
BOD 5 0.68 3 1.42 mg O2/mg C5H7NO25 0.97 mg BOD/mg C5H7NO2
3. Determine the TOC of the compound.
TOC 5 (5 3 12)/113 5 0.53 mg TOC/mg C5H7NO2
4. Determine BOD/COD, BOD/TOC, and TOC/BOD ratios.
BOD
COD5 0.6831.42 1.42 50.68 BOD
TOC 5 0.6831.42 0.53 51.82 TOC
COD5 0.53 1.4250.37
that they will change significantly with the degree of treatment the waste has undergone, as reported in Table 2–15. The theoretical basis for these ratios is explored in Example 2–11.
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typical commercially available respirometer is shown on Fig. 2–27(a). The principal advan- tages of modern headspace-gas respirometers over manometric respirometers, such as Gilson or Warburg respirometers (Tchobanoglous and Burton, 1991), are (1) the use of a large (1-L) sample that minimizes the errors of grab sampling and pipetting in dilutions and (2) oxygen consumption is measured continuously, thereby providing much more detail about the progress of the biological reaction.
Respirometric Applications. Used initially for the determination of the BOD and rate constants, respirometry is now used in a number of different applications in the field of wastewater treatment, including (1) monitoring oxygen uptake rates in activated sludge mixed liquors, (2) assessing biodegradability and treatability of industrial wastewaters, (3) assessing toxicity of industrial chemicals to wastewater treatment processes, and (4) assessing nutrient deficiencies (Young and Cowan, 2004). Biodegradation characteristics can vary among chemical types and wastewater sources, as illustrated on Fig. 2–27(b).
The curve labeled “control” represents oxygen uptake of readily biodegradable substances.
The curve labeled “inhibition” is characteristic of the oxygen uptake for chemicals that may be toxic or have low rates of biodegradation. When acclimation is required, a delay in oxygen uptake will occur, but the initial rates of oxygen uptake will be similar to that of the seed culture. Other patterns can occur depending on the type of stresses imposed on the seed culture.
Oil and Grease
The term oil and grease, as commonly used, includes the fats, oils, waxes, and other related constituents found in wastewater. The term fats, oil, and grease (FOG) used previ- ously in the literature has been replaced by the term oil and grease. The oil and grease content of a wastewater can be determined by several methods based on liquid-liquid extraction and solid phase adsorption followed by liquid extraction (Standard Methods, 2012). Following the extraction step, the solvent used in the extraction is evaporated and Figure 2–27
View of respirometer and response curves: (a) commercial headspace-gas respirometer
(courtesy of Respirometer Systems and Applications, LLC) and (b) typical oxygen uptake curves for wastewater samples having different biodegradation characteristics (Young and Cowan, 2004).
(a) (b)
0 100 200 300 400 500
0 20 40 60 80 100 120 140
Oxygen uptake, mg/L
Time, h
ThOD or COD Control
Inhibition
Seed culture
Acclimation required
the residual oil and grease content is determined gravimetrically. Other extractable sub- stances include mineral oils, such as kerosene and lubricating and road oils. Oil and grease are quite similar chemically; they are compounds (esters) of alcohol or glycerol (glycerin) with fatty acids. The glycerides of fatty acids that are liquid at ordinary temperatures are called oils, and those that are solids are called grease (or fats).
If grease is not removed before discharge of treated wastewater, it can interfere with the biological life in the surface waters and create unsightly films. The thickness of oil required to form a translucent film on the surface of a water body is about 0.0003048 mm (0.0000120 in.), as given in the following table.
Film thickness Quantity spread
Appearance in. mm gal/mi2 L/ha
Barely visible 0.0000015 0.0000381 25 0.365
Silvery sheen 0.0000030 0.0000762 50 0.731
First trace of color 0.0000060 0.0001524 100 1.461
Bright bands of color 0.0000120 0.0003048 200 2.922
Colors begin to dull 0.0000400 0.0010160 666 9.731
Colors are much darker 0.0000800 0.0020320 1332 19.463
Source: Eldridge (1942).
Fats and oils are contributed to domestic wastewater in butter, lard, margarine, and vege- table fats and oils. Fats are also commonly found in meats, in the germinal area of cereals, in seeds, in nuts, and in certain fruits. The low solubility of fats and oils reduces their rate of microbial degradation. Mineral acids attack them, however, resulting in the formation of glycerin and fatty acid. In the presence of alkalies, such as sodium hydroxide, glycerin is liberated, and alkali salts of the fatty acids are formed. These alkali salts are known as soaps. Common soaps are made by saponification of fats with sodium hydroxide. They are soluble in water, but in the presence of hardness constituents, the sodium salts are changed to calcium and magnesium salts of the fatty acids, or so-called mineral soaps. These are insoluble and are precipitated.
Kerosene, lubricating, and road oils are derived from petroleum and coal tar and con- tain essentially carbon and hydrogen. These oils sometimes reach the collection system in considerable volume from shops, garages, and streets. For the most part, they float on the wastewater, although a portion is carried into the sludge on settling solids. To an even greater extent than fats, oils, and soaps, the mineral oils tend to coat surfaces. The particles interfere with biological action and cause maintenance problems.
Surfactants
Surfactants, or surface-active agents, are large organic molecules that are slightly soluble in water and cause foaming in wastewater treatment plants and in the surface waters into which the waste effluent is discharged. Surfactants are composed most commonly of a strongly hydrophobic group combined with a strongly hydrophilic group. Typically, the hydrophobic group is a hydrocarbon radical (R) made up of 10 to 20 carbon atoms. Two types of hydrophobic groups are used: those that will and those that will not ionize in water. Anionic surfactants are negatively charged [e.g., (RSO3N)2Na1], whereas cationic surfactants are positively charged [e.g., (RMe3N)1Cl2]. Nonionizing (nonionic) surfactants
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commonly contain a polyoxyethylene hydrophilic group (ROCH2CH2OCH2CH2 . . . OCH2CH2OH, often abbreviated REn, where n is the average number of -OCH2CH2- units in the hydrophilic group). Hybrids of these types also exist. In the United States, ionic surfactants amount to about two-thirds of the total surfactants used and nonionics to about one-third (Standard Methods, 2012).
Surfactants tend to collect at the air-water interface with the hydrophilic group in the water and the hydrophobic group in the air. During aeration of wastewater, these compounds collect on the surface of the air bubbles and thus create a very stable foam.
Before 1965, the type of surfactant present in synthetic detergents, called alkyl-benzene- sulfonate (ABS), was especially troublesome because it resisted breakdown by biologi- cal means. As a result of legislation in 1965, ABS has been replaced in detergents by linear-alkyl- sulfonate (LAS), which is biodegradable. Because surfactants come primar- ily from synthetic detergents, the foaming problem has been greatly reduced. It should be noted that so called “hard” synthetic detergents are still used extensively in many foreign countries.
Two tests are now used to determine the presence of surfactants in water and waste- water. The MBAS (methylene blue active substances) test is used for anionic surfactants.
The determination of surfactants is accomplished by measuring the color change in a standard solution of methylene blue dye. Nonionic surfactants are measured using the CTAS (cobalt thiocyanate active substances) test. Nonionic surfactants will react with the CTAS to produce a cobalt containing product which can be extracted into an organic liquid and then measured. It should be noted that the CTAS method requires sublimation to remove nonsurfactants and ion exchange to remove the cationic and anionic surfactants (Standard Methods, 2012).
Chemical Energy in Wastewater and Biosolids
The chemical energy content of the organic constituents in untreated wastewater, primary sludge, and biosolids can be determined by (1) using a full-scale boiler as a calorimeter, (2) using a laboratory bomb calorimeter, and (3) by calculation, if the elemental composi- tion is known. Because of the difficulty in instrumenting a full-scale boiler, most of the experimental data on the energy content of the organic constituents of wastewater, sludge, and biosolids are based on the results of bomb calorimeter tests (Shizas and Bagley, 2004;
Zanoni and Mueller, 1982).
The energy content of wastewater can be estimated from an elemental analysis of the constituents in organic compounds using the following expression, which is a modified form of the DuLong formula developed by Channiwala (1992), also Channiwala and Parikh (2002).
HHV (MJ/kg) 5 34.91 C 1 117.83 H 2 10.34 O 2 1.51 N 1 10.05 S 2 2.11A (2–66) Where HHV is the high heating value and C is the weight fraction of carbon; H of hydro- gen; O of oxygen; N of nitrogen; S of sulfur; and A of ash as derived from an ultimate analysis or from the chemical formula, if known. When the HHV is used, it is assumed that the water component is in liquid state at the end of combustion. Another estimate of the heating value of a combustible material is the lower LHV (lower heating value), in which it is assumed that latent heat of vaporization is not recovered. In general, the LHV is about 6 to 8 percent lower than the corresponding HHV. For stationary combustion units with exhaust heat recovered, use of the HHV is the most appropriate. Where exhaust heat is not recovered, use of the LHV is most appropriate. Also, in most of the European literature, LHVs are reported, whereas HHVs are reported in the American literature. The application of Eq. 2–66 is illustrated in Example 2–12.
EXAMPLE 2–12 Estimate the Chemical Energy Content of Untreated Wastewater and Biosolids Estimate the chemical energy content, on a COD basis, of (1) the organic fraction of untreated wastewater, composed of 50, 40, and 10 percent proteins, carbohy- drates, and fat, respectively and (2) biosolids comprised of bacterial cell biomass.
Assume the chemical composition of untreated wastewater is C7.9H13 O3.7NS0.04 with an ash content of 3 percent. The composition of the cell biomass is C5H7O2N (Hoover and Porges, 1952) with an ash content of 3 percent. Express results on the basis of MJ/kg organic fraction or biosolids COD.
Solution-Part 1 Untreated Wastewater
1. Determine the energy content of the wastewater using Eq. 2–66.
a. Determine the weight fractions of the elements and ash comprising the wastewater.
Component Coefficient
Molecular
weight Molecular mass Weight fraction
Carbon 7.9 12 94.8 0.50a
Hydrogen 13 1 13 0.07
Oxygen 3.7 16 59.2 0.31
Nitrogen 1 14 14 0.08
Sulfur 0.04 32 1.28 0.01
Ash 0 0.03
182.28 1.00
a (94.8/182.28) 3 0.97 5 0.50.
b. The energy content of the organic fraction using Eq. 2–66 is:
HHV (MJ/kg organic fraction) 5 34.91 (0.50) 1 117.83 (0.07) 2 10.34 (0.31) 21.51 (0.08) 1 10.05(0.01) 2 2.11 (0.03) HHV (MJ/kg organic fraction) 5 17.45 1 8.25 – 3.21– 0.12 1 0.10 2 0.06 5 22.41 2. Determine the COD of the organic fraction.
a. Write a balanced reaction for the chemical oxidation of the biomass neglecting sulfur.
C7.9H13NO3.71 8.55O2 S 7.9CO21 NH31 5H2O
182.28 8.55(32)
b. The COD of the organic fraction is
COD 5 8.55(32 g O2/mole)/(182.28 g organic fraction/mole) 5 1.50 g O2/g organic fraction
3. Determine the energy content of the biomass in terms of MJ/kg biosolids COD HHV (MJ/kg organic fraction COD) 5 (22.77 MJ/kg of organic fraction)
(1.50 kg O2/kg of organic fraction) 515.1 MJ/kg of organic fraction COD 1. Determine the energy content of the biosolids using Eq. 2–66.
a. Determine the weight fractions of the elements and ash comprising the biosolids.
Solution-Part 2 Biosolids