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2.1.17 Solids

Solids that find their way into wastewaters include the solids on the kitchen table: corn, vegetables, crab, rice, bread, chicken, fish, egg, and so on. In short, these are the solids flushed down the toilet. In addition, there are also solids coming from the bathroom such as toilet paper and human wastes. In the old combined sewer systems, solids may include the soils from ground eroded by runoff. Figure 2.3 shows a pictorial representation of the various components of total solids.

The total solids content of a wastewater are the materials left after water has been evaporated from the sample. The evaporation is normally done at 103-105°C. Total solids may be classified as filtrable and nonfiltrable. The filtrable fraction contains the colloidal particles and the dissolved solids that pass through the filter in a prescribed laboratory procedure. The nonfiltrable fraction contain the settleable and the nonsettleable fractions that did not pass through the filter.

The nonsettleable fraction of the nonfiltrable fraction is in true suspension; it is composed of suspended solids. On the other hand, the settleable fraction does not suspend in the liquid and, thus, these component solids are not suspended solids; they are settleable fractions because they settle. Solids retained on the filter (nonfiltrable solids) are, however, collectively (and erroneously) called suspended solids while those that pass the filter are collectively (and, also, erroneously) called dissolved solids.

The solids that pass through the filter are not all dissolved, because they also contain colloidal particles. Also, the solids retained on the filter are not all suspended,

FIGURE 2.4 Imhoff cones.

because they also contain settleable solids; however, the use of these terms have persisted. More accurately, the nonfiltrable-nonsettleable fraction should be the one called suspended solids. Since the nonfiltrable solids are composed of the true suspended solids and the "nonsuspended" suspended solids, nonfiltrable solids are also called total suspended solids.

The settleable fraction is the volume of the solids after settling for 30 minutes in a cone-shaped vessel called an Imhoff cone. The volume of solids that settled, in milliliters, divided by the corresponding grams of solids mass is called the sludge volume index, SVI. Settleable solids are an approximate measure of the volume of sludge that will settle by sedimentation. Figure 2.4 shows a photograph of Imhoff cones.

All the types of solids described previously can have fixed and volatile portions. The fixed portions of the solids are those that remain as a residue when the sample is decomposed at 600°C. Those that disappear are called volatile solids. Volatile solids and fixed solids are normally used as measures of the amount of organic matter and inorganic matter in a sample, respectively. Magnesium carbonate, however, decomposes to magnesium oxide and carbon dioxide at 350°C. Thus, the amount of organic matter may be overpredicted and the amount of inorganic may be under-predicted if the carbonate is present in an appreciable amount.

Example 2.3 A suspended solids analysis is run on a sample. The tared mass of the crucible and filter is 55.3520 g. A sample of 260 mL is then filtered and the residue dried to constant mass at 103°C. If the constant mass of the crucible, filter, and the residue is 55.3890 g, what is the suspended solids (SS) content of the sample?

Solution:

„„ _ 55.3890 - 55.3520 g _ 55.3890 - 55.3520nmmnmm _ 260 mL _ 260 (1000)(1000j

Even the purest water exhibits ionization. Kohlrausch, a German physical chemist, demonstrated this property by measuring the electrical conductivity of water using a very sensitive instrument. The existence of the electrical conductivity is a result of the chemical reaction between two water molecules as shown below:

The first term on the right-hand side of Equation (2.18) is called the hydronium ion; the second is called the hydroxide ion. These ions are responsible for the electrical conductivity of water. The concentrations of these ions are very small. At 25°C, for pure water, there is a concentration of 1 x 10 mole per liter of the hydronium ion and of the hydroxide ion, respectively. When the water is not pure, these concentrations would be different. In a large number of environmental engineering textbooks, the hydronium is usually written as H+. Also the hydronium ion is usually referred to as the hydrogen ion. In essence, the hydronium ion can be looked at as a hydrated hydrogen ion.

Let the symbol {H+} be read as "the effective concentration or activity of H+" and the symbol [H+] be read as "the concentration of H+." The effective concentration {H+} refers to the ions of H+ that actually participate in a reaction. This is different from the concentration [H+], which refers to the actual concentration of H+ , but not all the actual concentration of this H+ participate in the chemical reaction. Effective concentration is also called activity. The effective concentration or activity of a solute is obtained from its actual concentration by multiplying the actual concentration by an activity coefficient, f (i.e., {H+} = f [H+]).

When the concentration of a solute such as H+ is dilute, the solute particles are relatively far apart behaving independently of each other. Because the concentration is dilute (particles far apart), the particles participating in a reaction are essentially the concentration of the solute. Therefore, for dilute solutions, {H+} is equal to [H+].

The existence of the hydronium ion is the basis for the definition of pH as originated by Sorensen. pH is defined as the negative logarithm to the base 10 of the hydrogen ion activity expressed in gmols per liter as shown below.

The product of the activities of H+ and OH- at any given temperature is constant. This is called the ion-product of water, Kw, which is equal to 1 x 10_14 at 25°C.

Sorensen also defined a term pOH as the negative of the logarithm to the base 10 of the hydroxide ion activity expressed in gmols per liter.

In Equation (2.19), when [H+] is equal to one mole per liter, the pH is equal to zero. When the concentration is 1 x 10_14 mole per liter, the pH is 14. Although the pH could go below 0 and be greater than 14, in practice, the practical range is considered to be from 0 to 14. Low pH solutions are acidic while high pH solutions are basic. A pH equal to 7 corresponds to a complete neutrality. The range of pH from 0 to 14 corresponds to a range of pOH from 14 to 0.

The ion product of water, Kw, is

Taking the logarithm of both sides to the base 10, pH + pOH = pKw pH = pKw -pOH

pH is an important parameter both in natural water systems and in water and wastewater engineering. The tolerable concentration range for biological life in water habitats is quite narrow. This is also the case in wastewater treatment. For example, nitrification plants are found to function at only a narrow pH range of 7.2 to 9.0. In water distribution systems, the pH must be maintained at above neutrality of close to 8 to prevent corrosion. Above pH 8, the water could also cause scaling, which is equally detrimental when compared with corrosion.

Example 2.4 10 2 mole of HCl is added to one liter of distilled at 25°C. After completion of the reaction, the pH was found to be equal to 2. (a) What is the solution reaction? (b) What are the concentrations of the hydrogen and hydroxide ions?

Solution:

(a) The solution reaction is acidic.

2.1.19 Chemical Oxygen Demand

The chemical oxygen demand (COD) test has been used to measure the oxygen-equivalent content of a given waste by using a chemical to oxidize the organic content of the waste. The higher the equivalent oxygen content of a given waste, the higher is its COD and the higher is its polluting potential. Potassium dichromate has been found to be an excellent oxidant in an acidic medium. The test must be conducted at elevated temperatures. For certain types of waste, a catalyst (silver sulfate) may be used to aid in the oxidation.

The COD test normally yields higher oxygen equivalent values than those derived using the standard BOD5 test, because more oxygen equivalents can always be oxidized by the chemical than can be oxidized by the microorganisms. In some types of wastes, a high degree of correlation may be established between COD and BOD5. If such is the case, a correlation curve may be prepared such that instead of analyzing for BOD5, COD may be analyzed, instead. This is practically advantageous, since it takes five days to complete the BOD test but only three hours for the COD test. The correlation may then be used for water plant control and operation.

The chemical reaction involved in the COD test for the oxidation of organic matter is as follows:

Organic matter + Cr2O7- + H+ -Cr3+ + CO2 + H2O (2.24)

heat

From this reaction, chromium as reduced from an oxidation state of +6 to an oxidation state of +3. The oxidation products are carbon dioxide and water. The oxidation state is a measure of the degree of affinity of the atom to the electrons it shares with other atoms. A negative oxidation state of an atom indicates that the electrons spend more time with the atom, while a positive oxidation state indicates that the electrons spend more time with the other atom.

2.1.20 Total Organic Carbon

The polluting potential and strength of a given waste may also be assessed by measuring its carbon content. Because carbon reacts with oxygen, the more carbon it contains, the more polluting and stronger it is. The carbon content is measured by converting the carbon to carbon dioxide. The test is performed by injecting a known quantity of sample into an oxidizing furnace. The amount of carbon dioxide formed from the reaction of C with O2 inside the furnace is quantitatively measured by an infrared analyzer. The concentration of the total organic carbon (TOC) is then calculated using the chemical ratio of C to CO2.

2.1.21 Nitrogen

Nitrogen is a major component of wastewater. People eat meat and meat contains protein that, in turn, contains nitrogen. Every bite of hamburger is a source of nitrogen and every fried chicken you buy is a source of nitrogen. Nitrogen in protein is needed by humans in order to survive which, in turn, produces wastewater that must be treated.

Protein contains about 16% nitrogen. The nitrogen in protein is an organic nitrogen. Organic nitrogen, therefore, is one measure of the protein content of an organic waste. When an organic matter is attacked by microorganisms, its protein hydrolyzes into a type of ammonia called free ammonia. Thus, free ammonia is the hydrolysis product of organic nitrogen. The nitrites and nitrates are the results of the oxidation of ammonia to nitrites by Nitrosomonas and the oxidation of nitrites to nitrates by Nitrobacter, respectively. The sum of the organic, free ammonia, nitrite, and nitrate nitrogens is called total nitrogen. The sum of ammonia and organic nitrogens is called

Kjeldahl nitrogen. Of all the species of nitrogen, ammonia, nitrite, and nitrate are used as nitrogen sources for synthesis. They are to be provided in the correct amount in wastewater treatment. They also cause eutrophication in receiving streams.

The free ammonia may hydrolyze producing the ammonium ion according to the following reaction:

At pH levels below 7, the above equilibrium is shifted to the right and the predominant nitrogen species is NH+, the ionized form. On the other hand, when the pH is above 7, the equilibrium is shifted to the left and the predominant nitrogen species is ammonia. The unionized form is most lethal to aquatic life. Ammonia is determined in the laboratory by boiling off with the steam after raising the pH. The steam is then condensed absorbing the ammonia liberated. The concentration is measured by colorimetric methods in the condensed steam.

The nitrite nitrogen is very unstable and is easily oxidized to the nitrate form. Because its presence is transitory, it can be used as an indicator of past pollution that is in the process of recovery. Its concentration seldom exceeds 1 mg/L in wastewater and 0.1 mg/L in receiving streams. Nitrites are determined by colori-metric methods.

The nitrate nitrogen is the most oxidized form of the nitrogen species. Since it can cause methemoglobinemia, (infant cyanosis or blue babies), it is a very important parameter in drinking water standards. The maximum contaminant level (MCL) for nitrates is 10 mg/L as N. Nitrates may vary in concentrations from 0 to 20 mg/L as N in wastewater effluents. A typical range is 15 to 20 mg/L as N. The nitrate concentration is usually determined by colorimetric methods.

2.1.22 Phosphorus

Phosphorus can be found in both plants and animals. Thus, bones, teeth, nerves, and muscle tissues contain phosphorus. The nucleic acids DNA and RNA contain phosphorus as well.

The metabolism of food used by the body requires compounds containing phosphorus. The human body gets this phosphorus through foods eaten. These include egg, beans, peas, and milk. Being used by humans, these foods, along with the phosphorus, therefore find their way into wastewaters. Another important source of phosphorus in wastewater is the phosphate used in the manufacture of detergents.

In general, phosphorus occurs in three phosphate forms: orthophosphate, condensed phosphates (or polyphosphates), and organic phosphates. Phosphoric acid, being triprotic, forms three series of salts: dihydrogen phosphates containing the _ 2_ H2PO4 ions, hydrogen phosphate containing the HPO4 ions, and the phosphates

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