Biosolids: A New Road to Successful Recycling

• By Ann N. Clarke
• Dec 01, 2001

Florida's concentrated coastal populations and nutrient-sensitive surface waters caused the state to impose strict environmental regulations. Although these regulations, particularly on wastewater treatment facilities, will ensure the environmental well-being of the region for many years to come, they have created a new set of challenges for local municipalities.

For years, the South Plant wastewater treatment facility, located in Titusville, Fla., processed 540 tons of Class B biosolids, commonly known as sludge that has been treated. The city utilized a treatment process that required the solids extracted from the wastewater treatment process to be stored for up to 48 days in holding tanks to ensure the material was free of enough bacteria to meet Class B standards. Once the material was treated, the biosolids were hauled to neighboring farms for use as fertilizer. Excess material was dewatered and deposited in neighboring landfills.

The production of Class B biosolids created limited benefits for the city. Aside from having a limited amount of land to spread the material on, due to the region's geography, many of the local farmers were unwilling to participate in the biosolids program. This is due to additional monitoring, reporting and record keeping required when using Class B biosolids, which contain some degree of pathogens.

As the city's population increased and the production of wastewater solids intensified, the City of Titusville's biosolids management program became jeopardized.

In 1996, the city took the first step in developing a new biosolids management program by building the Blue Heron Wastewater Treatment Facility. The 4.0 million gallons per day (mgd) facility replaced the South Plant and was designed to operate with a strong focus on preserving the region's natural coastal and marine environment. The clean water extracted from the wastewater treatment process flows into 300 acres of wetlands that have become a habitat for a variety for native flora and fauna.

The city partnered with USFilter to install its autothermal thermophilic aerobic digestion (ATAD) process to treat the facility's wastewater solids. The ATAD process was chosen because of the technology's ability to produce a higher quality biosolids product that meets Florida's Class AA standards and the U.S. Environmental Protection Agency's (EPA) requirements for Class A material. In addition, the biosolids also meet Florida's required limits for various metals including cadmium, copper, lead, nickel and zinc.

Aside from removing pathogens by exposing liquid wastewater residue to extremely high temperatures and air, the two-tank ATAD system has the ability to reduce biosolids volume. At the South Plant, biosolids volume had been an issue due to limited storage space and the 48-day retention time required to eliminate pathogens. The ATAD unit not only reduced volume, but also lowered the biosolids retention time to an average of 7.5 days at the Blue Heron facility.

By producing Class A material, which is pathogen-free and has fewer land application restrictions, the city has also seen an increase in the number of parties interested in participating in its biosolids management program. Currently, the liquid biosolids are distributed to local farmers for use on pastureland, sodfarms and orange groves.

The Blue Heron facility was the first wastewater treatment plant to enter into an agreement with the Florida Department of Transportation to allow land application of treated biosolids along interstate highways and other roadsides. In Florida, land is especially limited, so the practice of applying liquid biosolids along highways helps plants, grass, wildflowers and trees grow in otherwise unsuitable roadside soils. Also, it serves as another beneficial use for the organic material. Cities and the state are saving money for other projects instead of spending it on fertilizer for roadside beautification efforts.

Overall, the city's biosolids management program has become more efficient and cost effective since installing the ATAD system. The ATAD system has brought the city annual cost savings estimated at $60,000 per year. This savings includes $1,900 in energy costs and $2,350 per month in sludge hauling (since the current volume of biosolids generated only requires one-half the hauling effort as before), and $800 in administrative costs, which were cut due to lowered monitoring and reporting requirements for Class A material.

ATAD has been studied since the 1960s and was significantly developed by the Fuchs company in the mid-1970s. The Fuchs ATAD design, provided by USFilter, is a proven system with nearly 20 years of operating experience handling primary, waste activated and trickling filter biosolids.

ATAD is an aerobic digestion process that operates within the thermophilic temperature range (55 to 65 degrees Celsius in the final reactor) without the introduction of supplemental heat. The typical ATAD system consists of covered insulated reactors containing aeration and mixing equipment.

Sludge, which has been thickened to a minimum of three percent solids, is fed to the reactors on a batch basis once every day. Batch feeding in one hour per day periods is preferred to eliminate the short-circuiting potential and to ensure that waste sludge is exposed to thermophilic temperatures for a sufficient period of time. Reactor temperatures are monitored with two temperature probes mounted on opposite sides of the reactor at different elevations.

Aspirating aerators mounted tangentially through the reactor wall provide aeration sludge mixing. A circulation aeration device mounted in the center of the reactor can be used in conjunction with the spiral aerators. The waste sludge aeration leads to the generation of surface foam that is controlled with foam cutters supported from the reactor roof. The foam cutters do not eliminate the foam, but instead produce a manageable foam layer that is beneficial as an insulator and in oxygen transfer.

Temperatures in the reactors can reach up to 65 degrees Celsius, as the sludge aerobically digests and heat is released, which creates an autothermal environment. The temperature of the reactors inhibits nitrification and limits the oxygen demand. At this temperature, the decomposition and destruction of pathogens is rapid, creating a Class A product.

At the Blue Heron facility, the ATAD system is comprised of two connected tanks or reactors that are each equipped with an aspirating circular aerator at the center of each reactor. Four additional spiral aerators are mounted inside the reactor walls to aerate and mix the biosolids contained within the reactor. A roof-mounted foam controller maintains a constant foam layer that acts as an insulator.

The U.S. Environmental Protection Agency (EPA) has spent numerous years developing guidelines and regulations pertaining to the safe and beneficial use of biosolids. The term biosolids was coined in 1991 to replace the commonly used description, sludge, and was adopted by the EPA in 1996 to better reflect the positive effects of more than 20 years of effort to clean up the nation's waters through a massive wastewater treatment program. The result of EPA's efforts was the 40 Code of Federal Regulations (CFR) Part 503 regulations for the use or disposal of biosolids. Included in these regulations were clear definitions of Class A and Class B biosolids treatment methods and requirements for such treatment.

Over the past few years, the production of Class B biosolids has created limited benefits for municipalities around the country. Furthermore, an increasing number of municipalities have a limited amount of land to spread the material on due to a number of influences. Among these influences are geographic location and local farmers' unwillingness to participate in a biosolids program. It requires additional monitoring, reporting and record keeping when using Class B biosolids, which contain a higher degree of pathogens. Additionally, public concern for safety and associated liabilities has forced certain areas to abandon conventional biosolids treatment methods for producing Class B biosolids.

According to the EPA standards for use or disposal of sewage sludge, 40 CFR Part 503, the biosolids must meet certain requirements with regard to pathogen reduction in order to be classified as "Class A" biosolids. The pathogen reduction can be accomplished by addition of lime or by maintaining the sludge temperature at a specific value for a period of time.

For sludge with a solids content of less than seven percent, the temperature and time period shall be determined using the following equation:

H=50,070,000 X 24 divided by 10 to the 01400t

Where,

H = time in hours

t = temperature in degrees Celsius

In addition to meeting the time-temperature requirements, EPA also has certain indicator organisms for pathogen monitoring. The limit of these organisms is stated as:

< 1,000 Fecal Coliform MPN/g TS

or

< 3 Salmonella MPN/4 g TS

Additionally, according to the EPA guidance document "Control of Pathogens and Vector Attraction in Sewage Sludge," vector attraction reduction must occur. This volatile solids reduction (VSR) must occur either simultaneously or immediately following the pathogen-reduction step. In an ATAD process, pathogen reduction and VSR occur simultaneously.

Class A biosolids contain minuscule levels of pathogens. To achieve certification as Class A biosolids, some form of heating, composting, digestion or pH increase is included in the treatment process to kill pathogens found in the biosolids before the material leaves a facility. Processes for treating biosolids to Class A standards include:

• Thermophilic Aerobic Digestion (USFilter: ATAD);
• Thermal Treatment (USFilter: Dragon Dryer);
• Pasteurization (USFilter: BioPasteur Process);
• Heat Drying (USFilter: J-Vap Unit);
• Composting (USFilter); and
• Lime Stabilization Process.

Some treatment processes will change the composition of the biosolids to a pellet or granular substance, also referred to as dried cake, which can be used as a commercial fertilizer. Class A designation is required for biosolids that are sold or given away in a bag or container, or applied to a residential lawn or garden.

Class B biosolids have less stringent standards for treatment and contain small amounts of bacteria. Treated in a wastewater treatment facility, these solids go through heating or composting processes before they leave the plant, but at less stringent levels. As a result, they contain some bacteria. For Class B biosolids, treatment can continue when the semi-solid material is exposed to the natural environment when spread as a fertilizer, where heat, wind and soil microbes naturally kill bacteria. There are restrictions for crop harvesting, grazing animals and public contact for all forms of Class B biosolids. Processes for treating biosolids to Class B standards include:

• Composting;
• Air Drying;
• Aerobic Digestion;
• Anaerobic Digestion; and
• Lime Stabilization.

Washington State University -- www.gardening.wsu.edu/stewardship/biosolid/biosolid.html.

U.S. Environmental Protection Agency, Office of Water -- www.epa.gov/owm/genqa.html.

U.S. Environmental Protection Agency, Office of Waste -- www.epa.gov/osw.

Northwest Biosolids Management Association -- www.nwbiosolids.org.

Water Environment Federation -- www.wef.org/PublicInfo/FactSheets/biosolidsrecycling.

Seattle & King County Washington, Public Health -- www.metroke.gov/health/prevcont/biosolid.html.

This article originally appeared in the 12/01/2001 issue of Environmental Protection.