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Sludge treatment processes are often the most difficult and costly part of wastewater treatment. Untreated sludge is odorous and contains pathogens. Sludge stabilization processes reduce odors, pathogens, and biodegradable toxins, as well as bind heavy

metals to inert solids, such as lime, that will not leach into the groundwater. The re-sulting biosolids can be used or disposed of safely.

TYPES OF RESIDUALS. Wastewater residuals include primary, secondary, mixed, and chemical sludge, as well as screenings, grit, scum, and ash. Between 40 and 60% of influent TSS is primary sludge. It typically has a concentration of 2 to 6% solids when removed from the primary clarifiers. Secondary (biological) sludge is composed largely of microorganisms. Between 0.5 and 2.0% of TSS is biological sludge. Mixed residuals—

commingled primary and secondary sludge—typically makes up 1.0 to 3.5% of influ-ent TSS. The concinflu-entration and characteristics of chemical sludge depend on the treat-ment chemicals (alum, ferric salts, or lime) used. It typically is found at treattreat-ment plants that have tertiary treatment, such as phosphorus removal.

The use or disposal method for residuals depends on how much treatment they have received. Biosolids are residuals that have been stabilized so they can be benefi-cially used as a soil amendment. Combustible residuals, such as screenings, may be incinerated or landfilled. Noncombustible residuals, such as grit, may be landfilled.

TREATMENT PROCESSES. Typical solids treatment processes include thicken-ing, stabilization, digestion, chemical, compostthicken-ing, dewaterthicken-ing, incineration, and heat drying. These processes are briefly described below.

Thickening. Thickening processes remove water to reduce the volume of liquid sludge, but the material retains the characteristics of a liquid (e.g., it flows). A thickened sludge typically contains between 1.5 and 8% solids. Thickening is intended to reduce the vol-ume of sludge so sludge treatment, storage, and hauling processes, equipment, and costs can also be reduced.

There are three types of thickening:

• Pre-thickening (thickening before stabilization and dewatering),

• Post-thickening (thickening after stabilization but before beneficial use), and

• Recuperative thickening (thickening biosolids and returning them to the stabi-lization process).

Pre-thickening processes include gravity thickeners, dissolved air floatation (DAF), centrifuges, gravity belt thickeners, and rotary belt thickeners. Gravity thickeners work best on primary and chemical sludges; they do not work well with combined sludges.

DAF is typically used to thicken WAS. Mechanical thickeners, such as centrifuges,

gravity belt thickeners, and rotary belt thickeners, are used on all types of sludges.

They remove more water from sludges than a gravity thickener or DAF does.

Stabilization. Stabilization via digestion or chemical stabilization reduces the sludge’s pathogen content, making the material a biosolids suitable for beneficial use.

Digestion. Aerobic and anaerobic digestion reduce the volatile solids and pathogen con-tent of sludge, thereby reducing odors and producing an environmentally acceptable soil amendment. Aerobic digestion uses microbes in open or closed vessels or lagoons to oxidize the sludge’s organic matter into carbon dioxide, water, and ammonia. It is becoming popular to operate these digesters at temperatures higher than 55 °C (131 °F) because doing so reduces the pathogen level in biosolids.

Anaerobic digestion uses microbes in a closed tank containing little or no oxygen (Figure 2.1). The tank is typically operated at temperatures between 35 and 38 °C (95 and 100 °F), but may be operated at more than 55 °C (131 °F) to further reduce pathogens

FIGURE2.1 An example of anaerobic digestion in a closed tank.

and solids. Anaerobic digestion destroys more than 40% of volatile solids and produces an offgas containing about 65% methane, which may be collected and used as a fuel.

Both types of digestion can produce a Class A or Class B biosolids suitable for beneficial use, depending on time, temperature, and flow configurations involved (40 CFR 503).

Chemical Stabilization. Chemical stabilization typically involves using lime to raise the sludge’s pH to 12.0 for 2 hours. This reduces pathogens and odors. Rules regarding chemical stabilization can be found in 40 CFR Part 503 rules. This stabilization method can also produce a Class A or Class B biosolids suitable for beneficial use (40 CFR 503).

Composting. Composting—windrow, aerated windrow, static pile, aerated static pile, and in-vessel—involves using microorganisms, a bulking agent (such as wood chips, leaves, or sawdust), and a controlled environment [typically 55 to 60 °C (131 to 140 °F)]

to decompose organic matter in sludge, as well as reduce its volume and odors. The moisture and oxygen levels are also controlled to minimize odors during the process.

Biosolids are typically used in windrow systems, while sludge can be composted in aerated static piles or an in-vessel system. Composting is gaining popularity because the finished material is an excellent soil conditioner.

Dewatering. Dewatering further reduces the volume and weight of biosolids via air-drying (on sand); vacuum-assisted air-drying beds; or mechanical dewatering equipment, such as belt filter presses, centrifuges, plate-and-frame filter presses, and vacuum fil-ters. When used with polymers, belt filter presses and centrifuges can produce a bio-solids “cake” that contains 15 to 25% bio-solids. Sand drying beds typically produce a cake containing between 10 and 50% solids. Vacuum-assisted beds produce a cake contain-ing 10 to 15% solids (when polymers are used). Plate-and-frame presses can produce a cake containing 30 to 60% solids (when lime, ferric chloride, or fly ash is used). Vac-uum filters can produce a cake containing 12 to 30% solids (when the biosolids have first been conditioned with lime, ferric chloride, or polymers).

Heat Drying. Heat drying processes, such as low-temperature heat drying, flash dry-ing, rotary kiln drydry-ing, indirect drydry-ing, vertical indirect drydry-ing, direct–indirect drydry-ing, and infrared drying, reduce the volume of secondary sludge and destroy pathogens.

They typically produce a commercially marketable biosolids. Digestion is typically not a prerequisite.

Incineration. Incineration typically involves firing biosolids at high temperatures in a multiple-hearth or fluidized-bed combustor, turning them into ash, and destroying volatile solids and pathogens in the process. It degrades many organic chemicals but

can form others (e.g., dioxin), so products of incomplete combustion must be con-trolled. Air emissions also must be concon-trolled. Metals are not degraded; they concen-trate in the ash. Most incinerated municipal sludge will produce nonhazardous ash, which can be landfilled (40 CFR 258). It also can be used as an aggregate in concrete or a fluxing agent in ore processing. If inorganic or organic constituents exceed Appendix II to Part 258, disposal in a hazardous waste landfill will be required (http://www.epa .gov/tribalmsw/pdftxt/40cfr258.pdf).

Beneficial Use. Unstabilized sludge that is landfilled with other waste must comply with 40 CFR 258. Once stabilized into biosolids, however, 40 CFR 503 requires that the material be used or disposed of via one of the following environmentally acceptable al-ternatives: land application, surface disposal, or incineration. Treatment plant staff should assess local land availability, public acceptance, and transportation constraints before selecting a use or disposal alternative.

Land application involves spreading biosolids on the soil surface or injecting it into soil (Figure 2.2). The material, which adds organic matter and nutrients to soil, must

FIGURE2.2 An example of land-applying biosolids.

meet Class A or Class B standards to be land-applied (40 CFR 503). Class A biosolids require more stabilization to reduce pathogens below detectable limits, but can be com-mercially marketed or land-applied without any pathogen-related restrictions.