Heavy Metals

Metals that have a significant detrimental impact upon biological treatment units are referred to as "heavy" metals. These metals cause the following undesired consequences when present in excessive quantities:

• Toxicity to organotrophic bacteria in aerobic treatment reactors

• Toxicity to organotrophic bacteria in anaerobic treatment reactors

• Toxicity to nitrifying bacteria in aerobic treatment reactors

• Interruption of floc formation

• Permit violations

• Accumulation of metals in sludges

• Increased operational costs

Metals such as cadmium (Cd), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni), silver (Ag), and zinc (Zn) are well-known, toxic heavy metals. These metals can occur in a variety of wastes and cause acute or chronic toxicity. In order for heavy metals to exert their toxic impact, the metals must be soluble and must be present as a free ion (e.g., Cu2+) or a metal oxide (e.g., CuO). As free ions or metal oxides the metals can be absorbed by bacteria and then exert their toxic effects (Figure 19.11).

The adsorption or uptake of heavy metals is very rapid. The rapid uptake is considered to occur over 30 minutes in two phases (Figure 19.12). During the first phase of metal uptake (approximately 10 minutes), most of the metals in a biological process are removed from the bulk solution through adsorption to bacterial fibrils. During the second phase of metal uptake (approximately 20 minutes), most of the

Soluble heavy metal

Soluble heavy metal

FIGURE 19.11 Heavy metal uptake by a bacterial cell. Heavy metals are transported into a bacterial cell through their adsorption to bacterial fibrils and then absorption into the cytoplasm.

Time (minutes)

FIGURE 19.12 Two-phase adsorption of heavy metals by bacteria in the activated sludge process. Most heavy metals are removed efficiently in the activated sludge process over a two-phase adsorption period. During the first phase of 10 minutes, over 70% of the influent metals to the activated sludge process are adsorbed to bacterial fibrils or inert solids. During the second phase of 20, many of the remaining heavy metal are adsorbed to bacterial fibrils or inert solids.

Time (minutes)

FIGURE 19.12 Two-phase adsorption of heavy metals by bacteria in the activated sludge process. Most heavy metals are removed efficiently in the activated sludge process over a two-phase adsorption period. During the first phase of 10 minutes, over 70% of the influent metals to the activated sludge process are adsorbed to bacterial fibrils or inert solids. During the second phase of 20, many of the remaining heavy metal are adsorbed to bacterial fibrils or inert solids.

remaining metals in the bulk solution are adsorbed to bacterial fibrils. The uptake of heavy metals in biological processes is very efficient. In the activated sludge process, uptake or removal efficiency for many metals often approaches 99%. The rapid and efficient removal of heavy metals is indicative of the susceptibility of biological processes to heavy metal toxicity.

There are approximately 55 heavy metals. Several of these metals are probably the most studied toxic wastes with respect to their impact upon biological wastewater treatment processes. There are two definitions that are used to describe heavy metals. First, the metals can be precipitated by hydrogen sulfide in an acidic solution. Second, the metals are members of the metals and metalloids in the groups IB, IIB, IIIB, IVB, VB, VIB, and VIII of the Periodic Table of Chemical Elements.

Although some metals such as nickel are oxidized by bacterial cells and are incorporated into bacterial enzyme systems to enhance enzymatic activity, excess soluble metals may cause toxicity. When metals are incorporated into enzyme systems, the metals are referred to as additives or activators.

The major source of toxic metals is industrial wastewater. For example, metal plating processes can discharge aluminum (Al), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), nickel (Ni), silver (Ag), and zinc (Zn) in addition to cyanide. Steel mill wastewater can contains aluminum, iron, and magnesium (Mg).

Metals in the influent of a wastewater treatment plant are removed or partitioned in different treatment tanks.Approximately 20% of the influent metals are removed in sedimentation tanks or primary clarifiers (Figure 19.13). Metal removal here is physical. Settling of metals occurs in primary clarifiers, if the metals are in the insoluble or precipitated form or are adsorbed to solids that settle.

Metals pass through the primary clarifiers and enter the activated sludge process because the metals are in solution as free ionic state species (e.g., Cu2+) or metal oxides, or they are bonded to chelating agents or ligands (Table 19.24). Ligands are soluble and may be simple or complex in structure and may be inorganic or organic in composition. Regardless of the structure or chemical composition of ligands, ligands hold metals in solution. If the metals stay in solution, the metals cannot cause toxicity but do pass through the wastewater treatment plant.

Primary Aeration Secondary clarifier tank clarifier

Primary Aeration Secondary clarifier tank clarifier

Waste activated sludge (WAS) FIGURE 19.13 Partitioning of heavy metals.

TABLE 19.24 Ligands Commonly Found in Wastewater Treatment Plants

Ligand Strength

Examples

Weak binding ligands

Ionized ammonia (NH4+) Carbonate (CO32-) Chloride (Cl-) Hydroxide (OH-) Phosphate (PO43-) Sulfate (SO42-) Fulvic acid (soil component) Humic acid (soil component)

Strong binding ligands

Metals may remain bonded to ligands or may be removed from the ligands by solids in the biological process, if solids have a stronger affinity for the metals than the ligands have. If the metals remain bonded to the ligands, toxicity is prevented, but the metals pass through the biological process to the receiving body of water. If metals from the ligands, free ionic state species of metals, or metal oxides are adsorbed by solids in the biological process, toxicity may occur.

For metal toxicity to occur in a biological treatment unit, metals must be adsorbed by the bacteria. The adsorbed metals then must be absorbed and must attack enzymes within the bacterial cells. Insoluble metals, metals bonded to ligands, and soluble metals that are adsorbed by nonbacterial or inert solids such as particulate materials do not cause toxicity.

Metals removed in the activated sludge process through their adsorption to bacteria and inert solids are concentrated in the settled solids or sludge in the secondary sedimentation tanks or clarifiers. Biological processes, especially the activated sludge process, are capable of removing approximately 80% of the influent metals. Metals in secondary sludge and primary sludge are transferred to an aerobic digester or anaerobic digester where they may cause operational problems, if they become soluble in the sludge.

Was this article helpful?

0 0
Losing Weight Without Starving

Losing Weight Without Starving

Tired of Trying To Loose Weight And It Never Works or You Have To Starve Yourself Well Here's A Weight Loss Plan That takes Care of Your Weight Problem And You Can Still Eat. In This Book, You’ll Learn How To Lose Weight And Not Feel Hungry! In An Easy Step-By-Step Process That Enables You To Feel Good About Loosing Weight As Well As Feeling Good Because Your Stomach Is Still Full.

Get My Free Ebook


Post a comment