Toxicity

Municipal wastewater treatment plants often treat a combination of industrial, commercial, and domestic wastewaters. Some municipal wastewater treatment plants also treat septage. These wastewaters and septage contain several significant components that are of concern to operators of municipal wastewater treatment plants (Table 19.1). Several of these components represent a risk of toxicity to aerobic, biological treatment units such as the activated sludge process and anaerobic, biological treatment units such as the anaerobic digester (Table 19.2).Although the discharge of toxic wastes in toxic amounts to municipal wastewater treatment systems is prohibited by Section 101(a) of the federal Clean Water Act, toxicity too often occurs in biological treatment units.

As National Pollution Discharge Elimination System (NPDES) permits become more restrictive for industrial wastewater discharges to municipal wastewater treatment plants, proposed chemical additions to the industrial wastewater may need to be monitored early to determine adverse impacts upon the biological processes of municipal wastewater treatment plants. Monitoring may need to include the toxic effects of an industrial wastewater.

Toxicity is the occurrence of an adverse impact upon the biomass in biological treatment units. By combining commercial, domestic, and industrial wastewaters in biological treatment units, the possibility occurs for the introduction of toxic wastes. Frequently, the presence of toxic wastes is sporadic, but the biomass of the treatment system can be seriously damage.

A toxic waste or toxicant is a compound or ion in the wastewater or sludge that has a deleterious effect on living organisms. Toxic wastes are known to cause toxicity or inhibition to cBOD removal and nBOD removal (nitrification) in the activated sludge process and cBOD removal and methane production in the

TABLE 19.1 Significant Components of Municipal Wastewater

Impact upon Biological

Component

Primary Source

Treatment Systems

Chelating agents

Industrial

Pass-through of heavy metals

Fats, oils, and grease

Commercial, domestic,

Foam production;

and industrial

Undesired growth of filaments; Toxicity in anaerobic systems

Heavy metals

Industrial

Toxicity

Oxygen demand,

cBOD

Commercial, domestic, and industrial

Dissolved oxygen consumption; Sludge production

Oxygen demand,

nBOD

Domestic, industrial

Dissolved oxygen consumption; Nitrification;

Denitrification (clumping)

Pathogens

(slaughterhouses); I/I (cat, dog, and rodent wastes)

Disease transmission

Salts

Domestic (water softeners); Industrial

Increased salinity

Solvents

Industrial

Toxicity

Surfactants

Domestic, commercial, industrial

Foam production; Dispersion of biomass; Toxicity

anaerobic digester. There are two groups of toxic wastes: inorganic and organic (Table 19.3).

Inorganic compounds and ions do not contain carbon (C) and hydrogen (H) and may be placed into two broad categories. The first category contains the highly toxic compounds and ions such as arsenic (As), chromates, chromium (Cr), cyanide, and zinc (Zn). Many of these compounds and ions (arsenic, cadmium, chromium, copper, cyanide, mercury, and nickel) are classified as priority pollutants. Fluoride (F), another toxic anion, is commonly found in wastewater from electronics manufacturing facilities. The second category contains compounds and ions that are necessary for cellular growth but can be toxic in high concentrations. These compounds and ions, usually nontoxic, may induce changes in the metabolic processes that alter cellular growth patterns at high concentrations.

Organic compounds do contain and hydrogen. Among the toxic organic compounds are some that also are sources or carbon and energy (substrates) for some bacteria. Phenol and some phenolic compounds are examples of toxic organic compounds that also serve as substrates for some bacteria. Although phenol and phenolic compounds are toxic to many genera of bacteria, many species in the genus Pseudomonas can degrade these compounds and often proliferate in biological treatment units that receive phenol or phenolic compounds.

Significant toxic organic compounds include aromatic compounds, halogentated compounds, oils, lipophilic solvents, and anionic surfactants. Aromatic compounds of concern include benzene, toluene, and xylenes. Many of these compounds are highly toxic because they are non-ionic in charge or structure and easily dissolve in the cell wall of many bacteria.

Examples of toxic, halogenated aliphatic compounds include chloroform (CH3Cl3) or trichloromethane, carbon tetrachloride (CCl4) or tetrachloromethane, 1,1,1-trichloroethane (CH3CCl3), and methylene chlorine (CH2Cl2) or dich-loromethane. Chloroform is used as a solvent, particularly in lacquers, and in

TABLE 19.2 Threshold Concentrations (mg/liter) of Commonly Occurring Pollutants that Are Toxic to Biological Treatment Processes

Biological Treatment Process Activated Sludge Activated Sludge

TABLE 19.2 Threshold Concentrations (mg/liter) of Commonly Occurring Pollutants that Are Toxic to Biological Treatment Processes

Biological Treatment Process Activated Sludge Activated Sludge

Pollutant

cBOD Removal

nBOD Removal

Anaerobic Digester

Inorganic

Ammonia

480

1500-3000

Arsenic

0.1

1.6

Cadmium

10-100

0.02

Chromium (Cr6+)

1-10

0.25

5-50

Chromium (Cr3+)

50

50-500

Copper

0.1-1

0.005-0.5

1-10

Cyanide

0.1-5

0.3

4

Iron

1000

5

Magnesium

1000

Lead

0.1

0.5

Mercury

0.1-5

1300

Nickel

1-2.5

0.25

Potassium

3500

Sodium

3500

Sulfate

500

Sulfide

0.3

0.01

150

Zinc

0.3-1

0.01-0.5

5-20

Organic

Acetone

840

Benzene

100

Carbon tetrachloride

10-20

Ceepryn (surfactant)

100

Chloroform

10

10-16

Hydrazine

58

Nacconol (surfactant)

200

Phenol

50

4-10

Skatole

16

Toluene

200

TABLE 19.3 Significant Toxic Inorganic Compounds and Ions and Toxic Organic Compounds

Group

Compound/Ion

Inorganic

Organic

Ammonia Chlorine Cyanide Heavy metals Sulfide

Halogenated compounds Oils

Phenol and phenolic compounds

Solvents

Surfactants the manufacturing of plastics. Carbon tetrachloride and 1,1,1-trichloroethane are used as degreasing agents. Methylene chlorine is used as a solvent for fats, oils, grease, and waxes.

Because neutrally charged (non-ionic) molecules move more easily and more quickly across cell membranes than charged (anionic and cationic) molecules,

TOXICITY

Free-living nematodes

i

i

Rot

fers

TOXICITY

FIGURE 19.1 Toxicity in the activated sludge process. With few exceptions, toxicity in biological treatment units such as the activated sludge process "attacks" all organisms.

TOXICITY

FIGURE 19.1 Toxicity in the activated sludge process. With few exceptions, toxicity in biological treatment units such as the activated sludge process "attacks" all organisms.

organic molecules produce a more rapid toxic impact in biological processes than inorganic molecules and ions. Because the cell wall of many bacteria (especially methane-forming bacteria) contains lipids, lipid-soluble or fat-soluble organic compounds easily dissolve in the cell wall and exert toxicity.

The undesired impacts of toxic wastes often are complex. The impacts include loss of treatment efficiency, permit violations, and increased operational costs. Usually, wastewater treatment plants are not designed for the undesired impacts of toxic wastes. Therefore, there is a need for operators to be (1) aware of potential toxic wastes, (2) able to monitor and detect the occurrence of toxicity and (3) able to make appropriate operational changes to prevent or reduce the impact of toxicity.

Often, the presence of toxic wastes in biological treatment units is sporadic. The discharge of toxic wastes in undesired quantities may be difficult to avoid, and the problem is complicated by the design of municipal plants to treat only nontoxic wastes. Although biological treatment units are susceptible to toxic wastes, the systems are resilient and have a large diversity of bacteria that may enable the system to tolerate or even degrade toxic wastes.

There are two terms that are used to express toxicity in biological treatment processes. These terms are "acute" toxicity and "chronic" toxicity. Acute toxicity is toxicity that is severe enough to damage a biomass within a relatively short period of time—that is, <48 hours. Chronic toxicity is toxicity that damages a biomass for a relatively long period of time.

Generally, toxic wastes are not specific; that is, the toxic wastes do not "attack" just one group of organisms. With exceptions, toxic wastes attack all organisms (Figures 19.1 to 19.3). However, some groups of organisms may be more susceptible to toxic wastes that other groups of organisms.

Toxic wastes attack bacteria, protozoa, and metazoa in activated sludge processes and bacteria in anaerobic digesters. The toxic attack upon bacterial cells results in

FIGURE 19.2 Susceptibility of aerobic and anaerobic processes to the toxic effects of heavy metals. Because anaerobic bacteria in the digester obtain very little energy from the degradation of cBOD as compared to the aerobic and facultative anaerobic bacteria in the activated sludge process, anaerobic bacteria have very little energy available to repair damage caused by toxicity. Therefore, anaerobic digesters are much more "sensitive" to a toxic upset than the activated sludge process.

Increasing soluble metal concentration

FIGURE 19.2 Susceptibility of aerobic and anaerobic processes to the toxic effects of heavy metals. Because anaerobic bacteria in the digester obtain very little energy from the degradation of cBOD as compared to the aerobic and facultative anaerobic bacteria in the activated sludge process, anaerobic bacteria have very little energy available to repair damage caused by toxicity. Therefore, anaerobic digesters are much more "sensitive" to a toxic upset than the activated sludge process.

FIGURE 19.3 Anaerobic digester toxicity and methane-forming bacteria. Methane-forming bacteria obtain very little energy from the production of methane. Compared to other bacteria in the anaerobic digester, methane-forming bacteria have the least amount of energy available to repair damage caused by toxicity and experience different forms of toxicity that other bacteria do not.

FIGURE 19.3 Anaerobic digester toxicity and methane-forming bacteria. Methane-forming bacteria obtain very little energy from the production of methane. Compared to other bacteria in the anaerobic digester, methane-forming bacteria have the least amount of energy available to repair damage caused by toxicity and experience different forms of toxicity that other bacteria do not.

TABLE 19.4 Damage to Bacterial Structures and Activities

Damage

Description

Structure

Damage to enzymatic structure Damage to genetic material

Damage to structural components of the cell membrane

Damage to structural components of the cell wall

Damage to structure of cellular fibrils

Inhibition of enzymatic activity

Interference in carbohydrate synthesis

Interference in lipid synthesis

Interference in protein synthesis

Activity

TABLE 19.5 Undesired Operational Conditions Produced by Toxicity

Decreased cBOD removal or treatment efficiency Foam production and accumulation Increased operational costs Inhibition of bacterial flocculation

Inhibition of nitrification (nBOD removal or treatment efficiency) Loss of fine solids in the effluent of the biological treatment system Pass-through of toxic wastes to receiving waters Permit violations

Production and accumulation of nitrite (NO2-)

damage to bacterial structures or essential bacterial activities (Table 19.4). The resulting damage produces undesired operational conditions (Table 19.5).

The earliest undesired impacts of toxicity upon biological treatment systems following exposure to toxicity occurs at the cellular level. Here, toxic wastes damage structural components within cell walls, cell membranes, genetic material, and macromolecules (carbohydrates, lipids, and proteins). For example, carbon tetrachloride (CCl4) oxidizes the lipids in cellular membranes. This oxidation hinders the ability of the cellular membrane to regulate the flow of materials in and out of the cell.

Additionally, toxic wastes inhibit enzymatic activity and cellular metabolism. For example, mercury (Hg) alters the shape of enzymes by binding with thiol groups (—SH). Once the shape of an enzyme has been altered, the enzyme can no longer "wrap" around substrate (cBOD or nBOD) and degrade the substrate.

Toxic wastes attack organisms in activated sludge processes and anaerobic digesters. Principal organisms of concern in activated sludge processes are aerobic and facultative anaerobic bacteria. Principal organisms of concern in anaerobic digesters are facultative anaerobic and anaerobic bacteria.

The most important organisms in any biological treatment process are the bacteria. The bacteria are present in millions per milliliter of wastewater and billions per gram of bacterial solids. The bacteria are primarily responsible for the degradation of cBOD, degradation of nBOD, and removal of most fine solids—colloids, dispersed growth, and particulate material. The bacteria also are responsible for degrading or removing toxic wastes.

TABLE 19.6 Generation Times, Sludge Yields, and Energy Rank of Significant Bacterial Groups

Generation

Sludge Yield

Energy Available

Time

(#VSS/#BOD

for Cellular

Group

(Approximate)

Degraded)

Repair (Rank)

Organotrophs, activated sludge

30 minutes

0.6

1

Chemolithoautotrophs, activated sludge

2-3 days

0.06

2

Methane-forming bacteria

3-30 days

0.02

3

There are two significant groups of bacteria with respect to the degradation of cBOD and nBOD. These groups are the organotrophs and the chemolithoau-totrophs. Organotrophs degrade organic compounds or cBOD in activated sludge processes and anaerobic digesters. Bacteria that degrade the cBOD in activated sludge processes include aerobic and facultative anaerobic bacteria. Bacteria that degrade the cBOD in anaerobic digesters include facultative anaerobic and anaerobic bacteria. A major group of anaerobic bacteria found in anaerobic digesters is the methane-forming bacteria. Chemolithoautotrophs or nitrifying bacteria degrade the ionized ammonia (NH4+) and nitrite (NO2-) in activated sludge processes.

Organotrophs and chemolithoautotrophs degrade cBOD and nBOD, respectively, to obtain energy for cellular activity including growth and cellular repair. However, there is a significant difference in the quantity of energy obtained by these bacteria from the degradation of their respective BOD. Organotrophs obtain far more energy than chemolithoautotrophs when they degrade equivalent quantities of substrate.

Organotrophs and chemolithoautotrophs obtain more energy from the degradation of cBOD and nBOD, respectively, in the activated sludge process than methane-producing bacteria obtain from the degradation of cBOD in the anaerobic digester when these bacteria degrade equivalent quantities of substrate. With more energy available for growth and cellular repair, the activated sludge bacteria reproduce more frequently and in larger numbers than methane-forming bacteria (Table 19.6).

Because bacteria in the activated sludge process obtain more energy than methane-forming bacteria from the degradation of their respective substrates, the activated sludge bacteria can repair cellular damage caused by toxic wastes more often and more quickly. Therefore, activated sludge processes are more tolerant (less susceptible) to toxic wastes (e.g., heavy metals) than anaerobic digesters.

In the activated sludge process, organotrophs are more tolerant of toxic wastes than chemolithoautotrophs (nitrifying bacteria). For example, the minimum concentration of some heavy metals tolerated by organotrophs often are 10-50 times greater than those tolerated by nitrifying bacteria (Table 19.7).

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