Autothermal Thermophilic Aerobic Digestion

Autothermal thermophilic aerobic digestion (ATAD) is a sludge digestion process that is capable of achieving a high degree of stabilization and pathogen reduction. The process is characterized by high reaction rates achieved at a thermophilic temperature of 40 to 70°C. The temperatures are attained by using the heat released by the exothermic microbial oxidation process. Approximately 15,000 kJ of heat is generated per kilogram of volatile solids destroyed. In a completely mixed and aerated environment, the thermophilic temperatures attained are sustained without the addition of supplemental heat (other than the heat introduced by aeration and mixing) by conserving the heat released during biological oxidation. Figure 4.7 illustrates the heat balance in an ATAD reactor. If sufficient insulation, HRT, and adequate solids concentrations and mixing are provided, the process can be controlled at the thermophilic temperatures to achieve greater than 38% volatile solids destruction and sufficient pathogen reduction to meet the U.S. EPA regulations for PFRPs or the 40 CFR Part 503 class A designation.

ATAD is a technology widely applied in Europe since the 1970s and more recently in North America. The major advantages of ATAD over conventional aerobic digestion processes are:

• Significantly reduced SRT (5 to 8 days) to achieve a volatile solids reduction of 40 to 50%

• Possibility of reducing pathogenic viruses, bacteria, viable helminth ova, and other parasites to below detectable levels, thus meeting the pathogen reduction requirements of class A biosolids

Heat Loss to Surroundings

Feed Sludge

Mixing Heat Input

Heat Loss to Surroundings

Feed Sludge

Mixing Heat Input

Heat Loss to Effluent Sludge

Influent Gas (Air)

Figure 4.7 Heat balance in an Autothermal thermophilic aerobic digester. (From

Heat Loss to Effluent Sludge

Influent Gas (Air)

Figure 4.7 Heat balance in an Autothermal thermophilic aerobic digester. (From

• Destruction of all weed seeds, making the biosolids highly suitable as a soil amendment or fertilizer for lawns

• Approximately 25% lower oxygen requirement because few, if any, nitrifying bacteria exist in temperatures above 40°C

• Heat possibly recoverable for heating buildings

The disadvantages of ATAD are:

• High capital and operating costs

• Need for feed sludge to be thickened to a minimum solids concentration of 4% (preferably 5 to 6%)

• Requirement for extremely efficient aeration for systems using air instead of high-purity oxygen

• Objectionable odors

• Foam breakers required, due to the high degree of foaming in the reactors

• Poor dewatering characteristics of digested biosolids

Process Theory The aerobic destruction of volatile solids occurs in thermo-philic aerobic digesters as illustrated by equation (4.2). Because of the destruction of nitrifying bacteria from the high temperatures in ATAD, the subsequent reactions outlined in equations (4.3) through (4.7) do not materialize. Equation (4.2) with the energy component added is as follows:

C5H7O2N + 5O2 ^ 4CO2 + H2O + NH4HCO3 + energy (4.11)

This equation indicates that approximately 1.5 kg of oxygen is required per kilogram of volatile solids destroyed. The system needs a hydraulic detention time of only about 5 to 8 days. The ammonia produced in the reaction reacts with water and carbon dioxide to form ammonium bicarbonate, resulting in increased alkalinity. Because nitrification does not occur, the pH will be in the range 8 to 9, higher than in conventional aerobic digesters. The energy produced is approximately 15,000 kJ/kg of volatile solids destroyed. As long as the system is well mixed and sufficient oxygen is provided, the temperature in the reactor will rise until a balance occurs (the heat lost equals the heat input from exothermic reaction and mechanical energy input). The temperature will continue to rise until the process becomes oxygen mass-transfer limited. Some of the ammonia nitrogen produced will be in the off-gas and in solution with concentrations of several hundred mg/L in each. Most of this ammonia will be returned to the treatment plant in the sidestreams from the off-gas odor control system in the biosolids dewatering facilities.

Process Design A typical ATAD system is shown in Figure 4.8. Key elements of this process are feed sludge characteristics, reactors, detention time, feed cycle, aeration and mixing, temperature and pH, foam control, digested biosolids storage and thickening, and odor control. Typical design criteria for the ATAD are given in Table 4.3.

Feed Sludge Characteristics The autothermal thermophilic digesters can effectively digest a mixture of primary and secondary sludge. Blending before feeding to an ATAD is optional. The solids concentration of the feed sludge should be at least 3%, the minimum necessary to attain and maintain ther-mophilic conditions; and a maximum of 6%, the upper limit for efficient aeration and mixing. The feed must contain a minimum volatile solids content of between 25 and 40 g/L COD. Fine screening or grinding and good grit removal are required to remove or grind the plastic and stringy materials and to minimize abrasion on aerators and mixers.

Reactors Typically, two rectors in series are installed. Concrete and steel have been used in the construction of the tanks. Steel tanks are less susceptible to heat stress and less costly to construct than concrete tanks. However, steel tanks require 140 mm (about 6 in.) of mineral insulation and are clad with ribbed aluminum sheeting on the sides and plane sheeting on the top to protect the insulation from the elements and for aesthetic purposes. Access hatches are provided on top of the tanks. The entire tank is constructed above grade on a concrete foundation. The height-to-depth ratios vary from 0.5 to 1.0. Actual ratios depend on the aerators used and their effectiveness for good mixing.

Heat exchange is not necessary for process requirements but has been incorporated into some facilities for energy recovery and for preheating the feed sludge before the first-stage reactor. Heat can be recovered during bio-solids cooling with heat exchangers or by a water-cooling loop installed within

Rotary Drum Thickener

Rotary Drum Thickener

Wastewater Sludge

Holding Tank Treated Sludge

Waste Sludge Holding Tank

(a) Schematic of a System

Waste Sludge Holding Tank

Holding Tank Treated Sludge

(a) Schematic of a System

Foam breaker

Insulated reactor

Thickened sludge

Biological heat production

Off-gas

Foam breaker

Insulated reactor

Thickened sludge

Off-gas

Digested biosolids (to second-stage ATAD reactor)

Biological heat production

Digested biosolids (to second-stage ATAD reactor)

Air or high-purity oxygen

(b) Schematic of a Reaction

Figure 4.8 Autothermal thermophilic aerobic digestion system. [Part (a) from Fuchs, Cary, NC; part (b) from Metcalf & Eddy, 2003.]

TABLE 4.3 Design Criteria for Autothermal Thermophilic Aerobic Digestion

Parameter

SI Units

U.S. Customary Units

Value

Units

Value

Units

Hydraulic detention time

5-8

d

5-8

d

Feed sludge solids concentration

3-6

%

3-6

%

TSS loading

5-8

kg/m3 • d

320-500

lb/103 ft3-d

VSS loading

3.2-4.2

kg/m3 • d

200-260

lb/103 ft3-d

Temperature

Stage 1

40-50

OC

104-122

OF

Stage 2

50-70

OC

122-158

OF

Aeration and mixing

(aspirating mixer type)

Air input

4

m3/h/m3

Oxygen transfer efficiency

2

kg O2/kWh

4.4

lb O2/kWh

Energy requirement

130-170

W/m3

5.0-6.4

hp/103 ft3

Source: Adapted in part from Metcalf & Eddy, 2003.

Source: Adapted in part from Metcalf & Eddy, 2003.

the reactor shell. Heat can also be recovered from the off-gas. At the treatment plant in Salmon Arm, British Columbia, heat recovery of 1.2 J/s is used to heat the thickener building (WEF, 1998).

Detention Time Reactor hydraulic detention time (HRT) is from 5 to 8 days, with 2.5 to 4 days per reactor. About 40 to 50% of the volatile solids reduction takes place in the first reactor. German design standards include 5 to 6 days of HRT to comply with the pathogen destruction requirements.

Feed Cycle The influent sludge can be introduced into the first reactor continuously, intermittently, or in batches, although batch mode provides assurance in meeting class A pathogen reduction requirements.

In the continuous and intermittent feed modes, as sludge is introduced into the first reactor, the contents of the first reactor overflow to the second reactor, and from the second reactor to the biosolids holding tank. If aspirating aerators are used in the reactors, it is important to have a constant liquid level to ensure uniform and consistent oxygen transfer.

For batch mode, the system is designed to feed a one-day volume of the sludge to the reactor in less than 1 hour to ensure that the feed solids are exposed to the reactor temperature continuously for a minimum of 23 hours. This enhances pathogen destruction. Digested biosolids are withdrawn prior to introducing sludge from the first reactor to ensure that the effluent biosolids have been maintained at thermophilic temperatures for at least 24 hours and to minimize the possibility of contaminating the treated biosolids with partially treated sludge. After the transfer of sludge from the first reactor to the second reactor is completed, raw sludge is introduced again to the first reactor.

Aeration and Mixing The key to effective ATAD performance is aeration and mixing. The aeration system must be designed to (1) transfer sufficient oxygen to meet the high demand of the digestion process, (2) supply the oxygen required while minimizing the latent heat loss in the exhaust air, and (3) provide adequate mixing of the sludge to ensure complete stabilization. Aeration and mixing systems include aspirating aerators, Venturi aeration equipment, jet aeration, and immersible mechanical aerators.

Nearly all ATAD systems utilize aspirating aerators to introduce air and provide the mixing. Typical installations have a minimum of two aerators mounted on the side of each reactor. Larger installations may require a third unit on top of the roof at the center or additional wall-mounted units. The advantage of this type of aerator is that the motors and bearings are located outside the reactor. The design criteria for aspirating aerators are given in Table 4.3.

A combination recirculation pump and Venturi arrangement with air supplied to the Venturi has been used successfully. The main advantage of this aerator is that both the pump and the Venturi are located outside the reactor.

However, solids handling, pumps with corrosion-resistant impellers, and pump volute linings are required.

Temperature and pH Process operating temperature range in the first reactor is 40 to 50°C. During feeding, a drop in temperature will occur in this reactor. With an aspirating aeration system, the typical temperature recovery rate is 1°C/h (1.8°F/hr). The temperature in the first reactor should not be allowed to drop below 25°C (60°F), to avoid biological adaptation problems. Process operating temperature range in the second reactor is 50 to 70°C (102 to 158°F).

Typically, pH need not be controlled in an ATAD system because alkalinity is increased by the biological oxidation of cell mass, and no nitrification occurs to lower the alkalinity. Normally, pH in the first reactor is above 7 and in the second reactor is above 8.

Foam Control Substantial amounts of foam are generated in ATAD reactors because of cellular proteins, lipids, and oil and grease materials breaking down and releasing into solution. Control of the foam layer is important; however, the exact role of the foam layer has not been completely explained. The foam seems to improve oxygen utilization, provides insulation, and enhances biological activity. However, excessive foam inhibits air from entering the digesting sludge mass.

A freeboard of 0.5 to 1.0 m (1.65 to 3.3 ft) should be provided in the reactors to use as volume for foam development and control. Mechanical foam cutters suspended in the reactors at fixed elevations are used most commonly for foam control. Other methods include vertical mixers and spray systems. The design and operation of foam cutters are empirical and must consider the surface area of the reactors, solids concentration of sludge in the reactors, and the type and intensity of aeration.

Digested Sludge Storage and Thickening Cooling of the digested biosolids is necessary to achieve solids consolidation. A minimum of 20 days of detention is required for cooling and thickening. Detention time can be reduced substantially if heat exchangers are used for cooling the biosolids discharged from the reactors. Detention tanks are typically open-top, unmixed, and fitted with decant capability. Odor control is normally not required. Biosolids thickening by gravity thickeners can produce solids concentrations of 6 to 10%.

Odor Control Because there is no nitrification and because of the high temperatures in the ATAD systems, relatively high concentrations of ammonia are released. Reduced sulfur compounds, such as hydrogen sulfide, carbonyl sulfide, methyl mercaptan, ethyl mercaptan, dimethyl sulfide, and dimethyl disulfide, also result from the ATAD process.

Odors can be controlled if proper operating temperatures are achieved and the reactors are adequately mixed and aerated. Odor control systems may include wet scrubbers, biofilters, compost/soil filters, and diversion of off-gas to the activated sludge process.

Performance and Operation To meet the class A biosolids requirements of the Part 503 regulations, the requirement that needs to be demonstrated is (1) fecal coliform densities are less than 1000 MPN/g of total solids on a dry weight basis, or (2) Salmonella sp. bacteria concentrations are below detention limits of 3 MPN/g of total solids on a dry weight basis. Batch mode operation is better suited for meeting these requirements. In the continuous feed mode, it is possible that some pathogens pass through the system. Two or more reactors are required to ensure that all solids in the reactors are subject to the time and temperature requirements.

Part 503 regulations' vector attraction reduction requirement is a minimum 38% volatile solids reduction, or a SOUR of less than 1.5 mg of oxygen per gram per hour on a dry weight basis at a temperature of 20°C. Limited data are available on the ability of the ATAD process to meet these requirements. Volatile solids reduction is influenced by the feed sludge characteristics, HRT, operating temperature, and reactor loading. Data show volatile solids reduction ranging from 30 to 60%. Therefore, the ability of the process to meet the minimum requirement of 38% volatile solids reduction depends on a properly designed and operated system.

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