Metals

Metals at trace-level concentrations are found in all wastewaters and sludges. Industrial and commercial activities are the major sources, but wastewater from private residences can also have significant metal concentrations. The metals of greatest concern are copper, nickel, lead, zinc, and cadmium, and the reason for the concern is the risk of their entry into the food chain or water supply. A large percentage of the metals present in wastewater will accumulate in the sludges produced during the wastewater treatment process. As a result, metals are often the controlling design parameter for land application of sludge, as described in detail in Chapter 9. Metals are not usually the critical design parameter for wastewater treatment or reuse, with the possible exception of certain industries. Table 3.13 compares the metal concentrations in untreated municipal wastewaters and the requirements for irrigation and drinking-water supplies.

TABLE 3.13

Metal Concentrations in Wastewater and Requirements for Irrigation and Drinking Water Supplies

TABLE 3.13

Metal Concentrations in Wastewater and Requirements for Irrigation and Drinking Water Supplies

Untreated

Drinking

Wastewatera

Water

Irrigation (mg/L)

Metal

(mg/L)

(mg/L)

Continuous1,

Short-Term

Cadmium

<0.005

0.01

0.01

0.05

Lead

0.008

0.05

5.0

10.0

Zinc

0.04

0.05

2.0

10.0

Copper

0.18

1.0

0.2

5.0

Nickel

0.04

_

0.2

2.0

c a Median values for typical municipal wastewater. b For waters used for an infinite time period on any kind of soil. c For waters used for up to 20 years on fine-textured soils when sensitive crops are to be grown.

a Median values for typical municipal wastewater. b For waters used for an infinite time period on any kind of soil. c For waters used for up to 20 years on fine-textured soils when sensitive crops are to be grown.

Source: USEPA, Process Design Manual Land Treatment of Municipal Wastewater, EPA 625/1-81-013, U.S. Environmental Protection Agency, Center for Environmental Research Information, Cincinnati, OH, 1981.

3.5.1 Aquatic Systems

Trace metals are not usually a concern for the design or performance of pond systems that treat typical municipal wastewaters. The major pathways for removal are adsorption on organic matter and precipitation. Because the opportunity for both is somewhat limited, the removal of metals in most pond systems will be less effective than with activated sludge — for example, where more than 50% of the metals present in the untreated wastewater can be transferred to the sludge in a relatively short time period. Sludges from pond systems can, however, contain relatively high concentrations of metals due to the long retention times and infrequent sludge removal. The metal concentrations found in lagoon sludges at several locations are summarized in Table 3.14. The concentrations shown in Table 3.14 are within the range normally found in unstabilized primary sludges and therefore would not inhibit further digestion or land application as described in Chapter 9. Table 9.4 and Table 9.5 in Chapter 9 list other characteristics of pond sludges. The data in Table 3.14 are from lagoons in cold climates. It is likely that sludge metal concentrations may be higher than these values in lagoons in warm climates that receive a significant industrial wastewater input. In these cases, the benthic sludge will undergo further digestion, which reduces the organic content and sludge mass but not the metals content so their concentrations should increase with time.

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