Heavy Metals

Russian studies on the influence of biosolids on harvest, crop quality, and migration of heavy metals from biosolids into soil and from soil into plants (Goldfarb et al., 1983; Turovskiy, 1988) established that absorption of heavy metals by plants depends on the following:

• Mobility of heavy metals in sludge. Nickel, cadmium, and zinc are the most movable metals, but different methods of treatment may change the mobility of heavy metals in sludge. When treated by lime, for instance, the major part of the heavy metals does not migrate into plants.

• Types of soils. In acidic soils, the mobility of heavy metals is significantly higher than in alkaline soils. Organic and exchangeable cations in soils facilitate withholding of heavy metals. Until nitrogen is acting as a fertilizer, the influence of heavy metals on the growth of pants is insignificant.

• Types of plants. Crops have different absorptions of metals. For instance, during the study of the mineral composition of potatoes, carrots, and garden radishes grown on lands where sludge from Kiev (Ukraine) was applied, researchers found that manganese was absorbed by all experimental crops; zinc and copper by only potatoes and carrots; iron by only carrots and garden radishes; and lead by only garden radishes. Iron and molybdenum decreased in potatoes, and nickel decreased in carrots. Other elements did not change significantly.

In the grain and straw of millet grown on black soils where turf-sludge fertilizers were applied, the content of copper and nickel increased. The content of chromium, lead, titanium and molybdenum in the ash of all plants were on the level of control.

Distribution of heavy metals in plants is uneven—the concentration is greater in plant's organs (stems, leaves) and smaller is the grain. The age of plants also affects the accumulation of heavy metals; there are more heavy metals in older tissues than in younger ones.

It was found that the conditions under which the sludge was applied affected the accumulation of heavy metals in plants. Applying sludge to soils in the spring tended to cause an increase in iron, barium, molybdenum, nickel, cobalt, and chromium in potato tubers.

Experiments to determine the toxicity and migration of heavy metals in plants were conducted because of the high content of heavy metals in the thermally dried sludge at the wastewater treatment at the city of Orechovo-Zuevo. The content of heavy metals (in mg/kg of dry substance) was as follows: lead, 200; chromium, 700; cadmium, 100; manganese, 500; copper, 200; zinc, 2500; and nickel, 100. The results of the experiments are shown in Table 10.5.

Because of the possibility of accumulating toxins in plants, it was decided to restrict the dosage of biosolids in the soil. Criteria for such restrictions were standard for limited concentration values in soil (PDS) for several heavy metals and toxins that were developed in Russia and approved by Russia's Ministry of Health. PDS values in dry soil were developed for the following metals (in mg/kg): lead, 20; cadmium, 9; arsenic, 20; nickel, 50; chromium, 100; mercury, 2.1; manganese, 1500; vanadium, 150; manganese + vanadium, 1000 + 100; and superphosphate (P2O5), 200.

The maximum rate of adding biosolids to soil is usually determined by a calculation that takes into account the possibility of embedding harmful impurities in the soil. The calculation is based on an assertion that after adding sludge to the soil, the total content of heavy metals in the soil (considering the dispersion in the plowed layer) should not exceed the permissible dose of heavy metals in soil (PDS) in mg/kg, defined by the equation

where F is the prime content of heavy metal in soil (mg/kg) and D is the additional supply of the same metal into the plowed layer of soil with fertilizer (mg/kg)

TABLE 10.5 Content of Heavy Metals in Plants Grown with the Use of Thermally Dried Sludge from Orechovo-Zuevo, Russia

Content"

Ash Content

(% of

mass of dry substance)

Sample Analyzed

Cu

Zn

Mn

(%)

Barley

Controlb

0.008

0.002

0.004

2.7

50 metric tons/ha TDS

0.001

0.003

0.0026

2.2

50 metric tons/ha TDS + N180

0.002

0.004

0.005

5.4

50 metric tons/ha TDS + P120

0.002

0.003

0.002

5.7

50 metric tons/ha TDS + K180

0.002

0.0017

0.0036

2.2

50 metric tons/ha

0.007

0.004

0.003

2.5

TDS + N180 + K180 + P120

Oat

Controlb

0.0016

0.006

0.014

4.0

10 metric tons/ha TDS

0.0015

0.008

0.012

3.4

20 metric tons/ha TDS

0.0005

0.006

0.007

2.5

Corn

Controlb

0.01

0.016

0.005

10.5

10 metric tons/ha TDS

0.01

0.008

0.003

10.7

20 metric tons/ha TDS

0.007

0.007

0.005

10.2

Perennial herbs

Controlb

0.005

0.004

0.003

4.1

10 metric tons/ha TDS

0.004

0.003

0.003

5.0

" In addition to the elements listed, traces of Cr and Cd were found. b Control = plant grown on soils without biosolids.

" In addition to the elements listed, traces of Cr and Cd were found. b Control = plant grown on soils without biosolids.

The value of permissible addition to soil of one or another toxin Dtot may be determined by the equation

where 3000 is the mass of the plowed layer of soil in metric tons/ha recalculated for dry substance.

Depending on the type of soil, the magnitude of permissible supply of harmful impurities Dtot is usually decreased by the reducing factor K, which can be determined by the formula

where

K1 = factor based on the content of humus (H) in the soil (when H = 0.5 to 1%, K1 = 0.6; when H = 1 to 2%, K1 = 0.8; when H = 2 to 3%, K1 = 0.9; when H > 3%, K1 = 1)

K2 = factor based on the mechanical composition of soil (for sandy and sandy-loam soils, K2 = 0.7; for loamy soils, K2 = 0.9; for all other soils, K2 = 1)

K3 = factor based on the concentration of hydrogen ion in the soil (for soil with pH < 5 and biosolids with pH < 6, K3 = 0.4; for soil with pH = 6.5 to 7.0 and biosolids with pH < 6, K3 = 0.5; for soil with pH = 7.0 to 7.5 and biosolids with pH > 6, K = 0.8)

The average annual rate of adding biosolids to soil, Dav (in metric tons/ha), can be calculated using the equation

av TCN

where T is the maximum time for adding sludge to the same site (years) and CN is the concentration of the element in biosolids (g/ton of dry solids). The maximum rate of adding biosolids to soil (Dmax) at a frequency of once in five years will be 5Dav ton/ha • yr dry solids. Dmax is also restricted by the level of nitrogen in the soil, which should not exceed NP = 300 kg/ha • yr.

Current specifications for thermally dried biosolids or compost used as a fertilizer limit the quality of the biosolids as follows:

content as percentage of the mass of dry substance : organic : >40 nitrogen (total N): >1.6 phosphorus (as P2O5): >0.6 potassium (as K2O): >0.2 quantity of dry fractions D < 250 microns; % for sludge does not contain fraction more than 30 mm: <10 average density (kg/m3): 500 to 700

Several articles have described how to meet 40 CFR Part 503 regulations (Foess and Siger, 1993; Siger, 1993; McDonald, 1995; U.S. EPA, 1995). The requirements for heavy metals content in biosolids become less restrictive when they are used for shrubs, flowers, fast-growing trees, development of low-productive soils, soil stabilization of ravines and hillsides, and planting trees and shrubs on former industrial waste sites.

Experiments performed in Russia, Belarus, and Ukraine showed that bio-solids as fertilizers applied to the soil once continue to work for several years, although the effect decreased in subsequent years. Table 10.6 shows the data from those experiments for growing wheat and barley.

Compared to mineral fertilizers applied every year, biosolids applied once were effective for several years, as evidenced by the average harvest over

TABLE 10.6 Effect of Biosolids on Future Harvests

Condition

First-Year Harvest (kg/ha)

Second-Year Harvest (kg/ha)

Third-Year Harvest (kg/ha)

Average 3-Year Harvest (kg/ha)

Comments

Wheat

Without fertilizers

Mineral fertilizers,

N90P90K60

Compost,

40 tons/ha Thermally dried, 30 tons/ha Barley Without fertilizers

Mineral fertilizers,

N90P90K60 Compost,

40 tons/ha Thermally dried, 30 tons/ha

3100-4800

4200-5400 5510-5900 4980-5190

1850-2970

2440-3070 5100-4190 3100-4010

3000-3250

3780-4970 3980-5000 3860-4370

1150-2450

2250-3150 1890-3970 1890-3970

2310-2700

3450-4300 3500-3710 3120-3270

1200-2300

2100-2920 1620-2980 1890-2900

3200

4360 4600 4130

1990

2660 3290 2960

Average of experiments with different soils Used every year

Applied only in the first year Applied only in the first year

Average of experiments with different soils Used every year

Applied only in the first year Applied only in the first year several years. Experiments also showed that compost and thermally dried biosolids were more effective with crop rotation every other year, such as potatoes in the first year and wheat in the second year, or barley in the first year and corn in the second year. Thermally dried biosolids are very effective when they are used to grow silage corn. For example, when sludge from the city of Orechovo-Zuevo, Russia (lime and ferric chloride conditioned, dewa-tered by vacuum filters, and thermally dried) was used as fertilizer to grow silage corn, the harvest increased almost fourfold. It was also found that bio-solids as fertilizers are most effective when applied in autumn. For growing barley and corn, the best results are obtained when biosolids are used in combination with mineral fertilizers.

As a rule, application of biosolids to a field is recommended only once every five years, in a dosage from 5 to 20 dry tons/ha. This method provided higher average yields than did those using commercial fertilizers.

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