Info

2,800

Note: Concentrations and loading rates are on dry-weight basis. A February 25, 1984, Federal Register notice deleted chromium, deleted the molybdenum values for all but the ceiling concentration, and increased the selenium limit for monthly average concentration from 36 to 100.

Note: Concentrations and loading rates are on dry-weight basis. A February 25, 1984, Federal Register notice deleted chromium, deleted the molybdenum values for all but the ceiling concentration, and increased the selenium limit for monthly average concentration from 36 to 100.

distances greater than 150 meters. The limits for chromium and nickel also vary in a similar fashion (i.e., 200 to 600mg/kg for chromium and 210 to 420mg/kg for nickel). However, for sites with relatively impermeable liners (i.e., hydraulic conductivity values of 10-7 cm/sec or less) and leachate collection systems, the above limits do not apply.

Biosolid disposal sites must also comply with certain siting criteria and management practices. For example, biosolid landfills cannot impede the flow of a 100-year flood, be located in geologically unstable areas, or in a wetland unless a special permit is obtained. The landfills used for biosolid disposal must also be able to divert runoff from a 25-year, 24-hour storm event.

U.S. EPA requirements for biosolids that are incinerated include limits on metal concentrations and total hydrocarbons. Levels of beryllium and mercury emitted from biosolid incinerators must meet the National Emission Standards for Hazardous Air Pollutants. Arsenic, cadmium, chromium, and nickel must meet risk-specific concentrations, which range from 0.023 to 2.0 ug/m3 and are based on a combination of biosolid feed rates, dispersion factors, and incinerator control efficiencies.

Biosolid treatment and disposal can be time-consuming and costly. Biosolid handling prior to final disposal may involve collection, thickening, stabilization, conditioning, dewatering, heat drying, air-drying, lagooning, composting, and final disposal of the sludge.66 Figure 3.13 shows some sludge treatment processes and Table 3.20 gives some treatment design parameters. Sludges, including septic tank sludge, can be expected to contain numerous organic and inorganic chemicals and pathogens that can pose a hazard to agricultural produce, grazing animals, surface water, groundwater, and human health if not properly handled. Anaerobically digested sludge has been found to contain ascaris, toxocara, and trichuris ova, which remained viable in storage lagoons for up to 5 years.67

Biosolid thickening processes include gravity settling, flotation and centrifuga-tion. Biosolid stabilization is usually achieved by aerobic or anaerobic digestion, lime treatment, or composting. Digestion reduces the organic solids and pathogens in sludge. Anaerobic two-phase (first digester acid, second digester methane producer) digestion of municipal sludge at 127.4°F (53°C) for 10 days "reduces to essentially undetectable levels indicator bacteria (fecal coliforms, Escherichia coli, fecal streptococci), enterovisuses, and viable Ascaris eggs."68 Lime treatment and composting can also reduce pathogen levels. Also, sludge can be heated and mixed to accelerate the rate of digestion with sludge usually added at a rate of about 200 lb volatile solids per 1,000 ft3 per day.

Biosolids can also be conditioned, prior to thickening or dewatering, by the addition of chemicals. Heat treatment by means of a furnace or dryer reduces sludge moisture content. Dewatering is accomplished by means of drying beds, centrifuges, vacuum filters, continuous belt presses, plate and frame presses, or evaporation lagoons.

Final disposal of biosolids can be by composting, incineration in multiple-hearth or fluidized bed (Figures 3.16 and 3.17), pyrolysis, sanitary

Cooling air discharge Floating damper

Cooling air discharge Floating damper

Sludge feed

Auxiliary fuel

Rabble arm at each hearth Solids flow

Cooling air return

Rabble arm drive

Combustion air

Ash discharge

Cooling ' air fan

FIGURE 3.16 Cross-section of a multiple-hearth sludge incineration furnace. Temperature 1,400 to 1,500°F (769° to 816°C) in middle hearths. (Source: Environmental Regulations and Technology, Use and Disposal of Municipal Wastewater Sludge, U.S. EPA, Washington, DC, September 1984, p. 49.)

Combustion air

Sludge feed

Auxiliary fuel

Rabble arm at each hearth Solids flow

Cooling air return

Rabble arm drive

Ash discharge

Cooling ' air fan

FIGURE 3.16 Cross-section of a multiple-hearth sludge incineration furnace. Temperature 1,400 to 1,500°F (769° to 816°C) in middle hearths. (Source: Environmental Regulations and Technology, Use and Disposal of Municipal Wastewater Sludge, U.S. EPA, Washington, DC, September 1984, p. 49.)

landfill,* land application or reclamation, or sod growing. Composting may be by the (1) window method including 5 turnings over 15 days and mixture temperatures of not less than 131°F (55°C) 6 to 8 inches below the surface, (2) static pile method in which the pile is kept at a temperature of not less than 130°F (55°C) for at least three consecutive days, or 3) enclosed vessel method in which the mixture is maintained at a temperature not less than 130°F (55°C) for at least three consecutive days.69 Sawdust is often mixed with the finished compost.

Incineration can be combined with other industrial processes, such as cement manufacturing, to reduce the cost of disposal. This approach was used by the Los Angeles County Sanitation district to dispose of a portion of the 1,250 tons of biosolids they produce each day. Since 1996, the county has agreed to provide a local cement manufacturer with between 240 and 480 tons of biosolids per day. The biosolids are injected into the cement plant's hot exhaust gases where ammonia in the biosolids reduces plant nitrogen oxide emissions by up to 45 percent.

* Sludge dewatered to at least 20 percent solids.

FIGURE 3.17 Cross-section of a fluidized-bed sludge incineration furnace. Temperature of bed 1,400 to 1,500°F (769° to 816°C). (Source: Environmental Regulations and Technology, Use and Disposal of Municipal Wastewater Sludge, U.S. EPA, Washington, D. C., September 1984, p. 49.)

While land disposal of stabilized sludge can promote the growth of vegetation and control erosion, certain precautions must be taken to ensure that sludge contaminants do not endanger the public health. For example, cadmium and zinc are known to accumulate in food crops grown on sludge disposal sites. For that reason, U.S. EPA has set limits under the Part 503 Rule for both maximum and average monthly metal levels that are not to be exceeded (see Table 3.21) for biosolids that are applied to the land.

Authority for implementation of the Part 503 Rule biosolid disposal requirements has been delegated to the states. As a result, a number of states had imposed more restrictive limits for the specified pollutants (13 states) and required testing for additional pollutants (22 states).70 In some cases, communities (e.g., Kern County in California) have responded imposition of additional restrictions that have essentially prohibited the application of Class B biosolids to land. In response, many municipalities have converted to Class A treatment of biosolids. The treatment options usually selected for upgrading to Class A standards have been heat drying, composting, lime pasteurization, the N-Viro process (an alternative type of lime pasteurization), and thermophilic aerobic digestion.71

Cost of Sewage Treatment

The cost of sewage treatment systems can vary widely based on location, system size, and degree of treatment required. In general, costs can be divided into two categories: capital, and operation and maintenance. Cost estimates can be adjusted to present-day costs using the Engineering News-Record or other appropriate construction cost index (see Table 3.23).

The cost of individual septic tank systems will vary based on dwelling size, site conditions, and type of system. Typical costs (2006) for various on-site sewage treatment systems are given in Table 3.24.

For sewage treatment plants, typical costs can be estimated by adding 10 to 15 percent to the construction cost for contingencies. To these costs an additional 15 to 20 percent for engineering and 2 to 3 percent for legal/administrative costs needs to be added. The resulting total would be considered the total project construction cost. To this total cost an additional 3 to 6 percent needs to be added for financing costs. Taken together, these additions increase the total project cost by between 36 to 48 percent from the construction cost.

Cost comparisons should also consider the total annual costs—that is, the initial cost of construction and the annual cost of operation and maintenance (O&M). Typical average capital, O&M, and unit costs for selected sewage treatment processes are presented in Table 3.25.

Sometimes advanced wastewater treatment (Figure 3.13) is desired without fully realizing the large additional cost to obtain a small incremental increase in plant efficiency. Advanced wastewater treatment to remove an additional 3.8 to 10 percent BOD, 5.2 to 13 percent suspended solids, and approximately 61 to 68 percent phosphorus and ammonia-nitrogen has been found to increase capital costs by 42 to 99 percent and O&M costs by 37 to 55 percent.72 This finding suggests that the other more cost-effective alternatives should be explored before making a decision to add advanced wastewater treatment.

Because the cost of advanced treatment can be high, some operators have opted to use natural or constructed wetlands for effluent polishing. The cost of constructing and, in particular, operating such systems can be significantly lower than those for advanced treatment processes. Typical construction and O&M costs for the natural treatment systems are given in Table 3.26 (See also Tables 3.27 and 3.28).

Many treatment plant operators have installed computer-based monitoring and control systems in an effort to reduce their operational costs. Also, the Internet revolution has offered the means for not only accessing real-time operational data,

TABLE 3.25 Cost Indices (Average per Year)

Year Marshall & Engineering

Handy-

Engineering

Chemical

Organic Gardeners Composting

Organic Gardeners Composting

Have you always wanted to grow your own vegetables but didn't know what to do? Here are the best tips on how to become a true and envied organic gardner.

Get My Free Ebook


Post a comment