United States Environmental Protection Agency Municipal Environmental Research Laboratory Cincinnati OH 45268 Research and Development EPA-600/S2-84-142 Sept. 1984 4>EPA Project Summary Demonstration of Thermophilic Aerobic-Anaerobic Digestion at Hagerstown, Maryland Oscar W. Haas A thermophilic aerobic-anaerobic digestion system with a nominal secondary sludge capacity of 16,400 gallons per day was designed and constructed at the Hagerstown, Maryland, Wastewater Treatment Plant. This project establishes the process performance of the dual digestion system in a full-scale design. The system included a short (approximately 1 day) retention time aerobic digester followed by a high-rate anaerobic digester. Thickened, air-activated sludge was autothermally heated by the aerobic oxidation of organic substrates in the first step and then fed to the anaerobic second step, where the sta- bilization process was completed with the formation, of methane gas. Data were collected to evaluate the system's performance regarding volatile solids destruction, oxygen consumption, power draw, heat production, and process stability. Analysis of pathogens and indicator organisms were also made to determine the effectiveness of the aerobic step to inactivate pathogenic bacteria, viruses, and para- sites. Thermophilic temperatures (greater than 45°C) were rapidly achieved upon start-up of the dual digestion system and were maintained in the aerobic reactor at a hydraulic retention time of approximately 1 day. The high shear aeration device demonstrated suffic- ient oxygen transfer capacity to achieve and maintain these high temperatures at reasonable power densities in the aerobic reactor. The system responded well to variations in feed flow and solids concentration as well as to operational upsets. Analyses were performed that illustrate the ability of the dual digestion system to achieve significant reductions in the level of pathogenic organisms in sewage sludge. Finally, over the course of some 20 weeks of operation, the dual digestion system proved itself to be an effective sludge stabilization process, achieving an overall volatile solids reduction of 41.6 percent, with weekly averages in the 24 to 58 percent range. This Project Summary was developed by EPA's Municipal Environmental Research Laboratory. Cincinnati, OH, to announce key findings of the research project that is fully document- ed in a separate report of the same title (see Project Report ordering informa- tion at back). Introduction The proper treatment and disposal of wastewater sludges has become a problem of increasing concern (and expense) throughout the world. At present, the two most widely practiced biological processes for sludge stabiliza- tion are anaerobic and aerobic digestion. Anaerobic digestion, long a mainstay of wastewater treatment plant design, is a low-rate process typically operating with hydraulic retention times in the 15- to 30- day range. An advantage of the anaerobic process is that it requires little mechani- cal energy input to maintain operation, and in fact it produces a combustible methane gas. On the other hand, aerobic digestion, a relative newcomer, is a faster and more stable process but it is very energy intensive. The dual digestion ------- system, by combining a short-retention- time, autothermal, aerobic first step with a high-rate anaerobic second step, provides a novel approach to the biological stabilization of sludge that incorporates the advantages of each traditional method and minimizes their drawbacks. Process Description The dual digestion system designed for the Hagerstown, Maryland, Wastewater Treatment Plant consists of a short- retention-time, autothermal, aerobic digester followed in series by an existing anaerobic digestion step. The primary purpose of this process is to stabilize and pasteurize the thickened waste second- ary sludge produced by the air-activated sludge plant through the reduction of volatile matter in the sludge and the production of methane in the second step. The pasteurized residue can then be trucked for safe disposal on land. Figure 1 presents a process flow schematic for the dual digestion system. The first step of the dual digestion system consists of a high-rate autothermal aerobic digester contained in an insulated, covered, concrete tank of cylindrical geometry (Figure2). The tank is 11.5 feet in inside diameter with a maximum sludge sidewater depth of 24 feet and a minimum freeboard height of 2 feet. An adjustable external telescoping valve and overflow box maintains the liquid height in the aerobic digester at a depth of between 20 and 24 feet. During this demonstration program, the liquid level was maintained at 20 feet, creating a reactor volume of 16,400 gallons which includes the volume of the drainage well at the bottom of the tank. The aerobic reactor can be maintained at thermophilic temperatures (greater than 45°C) through the conservation of heat generated by biological oxidation of degradable organic matter in the waste sludge and does not require the use of an external heat source. High-purity oxygen is introduced into the aerobic reactor by means of static course bubble diffusers located at the bottom of the tank. Oxygen transfer from the gas to the liquid phase is enhanced by means of a rotating high- shear device that breaks up the oxygen gas bubbles as they rise through the sludge. The use of high-purity oxygen, in addition to enhancing the oxygen transfer rate, minimizes the volume of water- saturated gas (mainly carbon dioxide and oxygen) vented to the atmosphere. This low gas volume minimizes latent and sensible heat losses from the aerobic step and allows the desired operating temperature to be maintained more efficiently. The major mode of temperature control in the first step is the adjustment of the oxygen feed rate and thus the extent of heat-producing biological oxidation that occurs. Typically, only 5 to 15 percent of the volatile solids content of the sludge needs to be j| metabolized to produce the required quantity of heat, depending on the desired operating temperature, environmental heat losses, and influent sludge characteristics. The latter was a crucial factor during the Hagerstown program because of persistently low feed solids concentration (<4%TS). This factor necessitated the occasional use of sludge flow adjustment as another mode of temperature control when feed solids concentrations dropped below critical levels. By decreasing the sludge flow rate, the amount of heat lost with the aerobic sludge effluent was reduced. Despite the undesirability and inconveni- ence of low sludge concentrations, the ability to control the process was effectively demonstrated. The second step of the dual digestion system consists of a high-rate anaerobic digester that receives the heated and partially digested sludge from the autothermal aerobic pretreatment step. The anaerobic reactor completes the stabilization process by further breaking down volatile matter into carbon dioxide and methane gas. For purposes of this investigation, an existing fixed-cover anaerobic digester with a single 50-foot- diameter tank and a sidewater depth of 25 feet (nominal capacity of 396,550 gallons) was used. To keep anaerobic hydraulic retention times within a reasonable range and yet maintain a gas Anaerobic Vent Gas Influent Sludge Shear Blades Anaerobic Pump Effluent* Sludge Aerobic Step Anaerobic Step Figure 1. Dual digestion system process flow schematic. 2 ------- Figure 2. Liquid oxygen storage tank (left) and first step aerobic reactor. seal over the mid-depth influent sludge pipe, the reactor was operated at slightly more than half of its total volume. The reactor cover was sealed to prevent gas leakage at this reduced liquid level. A 7.5 horsepower pump was installed for sludge wasting and recirculation mixing. Process Observations The Hagerstown dual digestion demonstration plant was started on June 11, 1980, with the aerobic reactor being filled to the 20-foot level (approximately 16,400 gallons) with thickened secondary sludge. Beginning June 13, a secondary sludge feed was initiated at a rate sufficient to maintain a 1 -day aerobic retention time. As can be seen from Figure 3, by the end of the seventh day of operation, the aerobic digester tempera- ture had risen to the thermophilic region (greater than 45°C). By the ninth day the temperature had reached 55°C without the aid of external heating. At that point, the oxygen feed rate was lowered to Stabilize the sludge temperature and increase oxygen utilization. Thermophilic temperatures were maintained in the aerobic step despite a wide range of process and operating conditions. Table 1 summarizes phase average values of key parameters for the 20 weeks of continuous operation docu- mented in this report. In addition to the data listed, information was also obtained concerning heat and oxygen balances around the aerobic reactor, kinetic rate constants, high-shear power draw, pas- teurization performance (bacteria, viruses, and parasites), and process stability and flexibility. Conclusions The Hagerstown dual digestion system has successfully demonstrated that thermophilic temperatures may be autothermally achieved and maintained in a full-scale system at relatively short (0.95 to 2.25 days) aerobic retention times. Despite wide variations in sludge feed flow rates, solids concentrations, and degradability, control of aerobic ------- 80 70 60 50 I 40 30 20 10 Start Tank Fill, QQ~16 cfm. Purge On False LEL Alarm, Mixer+O2 Off 4 End Tank Fill. Mixer Started ~ 32 rpm. Purge Off Mixer On+Off for Strain Gaging. kW meter Begin Sludge Feed, R.T. ~ 1 Day QG ~ 22 cfm. Mixer Off for Short Periods Mixer+Oi Restarted QG Reduced to 17.5 dm Aerobic Reactor 11 12 13 14 15 16 17 June 1980 18 19 20 21 22 Figure 3. Process start-up. reactor temperature was maintained through simple adjustment of the oxygen feed flow rates and hydraulic retention times. Volatile solids removal rates averaged 27.2 percent across the aerobic step and 41.6 percent over the entire system. These values agree well with historical, mesophilic, anaerobic digester perform- ance at much longer (20 to 50 days) retention time. Oxygen consumption ratios in the range of 1.6 to 1.8 pounds oxygen per pound of volatile solids removed were found by mass balances around the aerobic step. The aerobic heat of reaction was found to be a function of the extent of volatile solids removal and varied in the 3000 to 9500 Btu per pound of volatile solids removed. These values are consistent with expected results for operation on waste secondary air-activa- ted sludge at relatively low influent solids concentrations (1.9 to 2.9 percent volatile solids). The results confirm design model predictions and allow extrapolations to higher solids concentrations with confi- dence. The high-shear aeration device successfully demonstrated sufficient oxygen transfer capacity to achieve and maintain thermophilic temperatures in the aerobic step at short hydraulic reten- tion times and reasonable power densities (0.5 to 1.1 shaft horsepower per 1000 gallons). The capability of the device for efficient oxygenation of thickened, waste-activated sludge was also proved. Oxygen use exceeded 65 percent of the feed gas flow when the shear device was operated at 32.2 rpm. Furthermore, the data suggested that the efficiency of the high-shear device would be enhanced at higher influent sludge concentrations. Solids and temperature profiles indicated that the contents of the aerobic reactor were reasonably well mixed under all conditions. Preliminary information obtained from an aerobic digestor temporarily used as the second process step indicates that good overall volatile solids destruction, process stability, and acceptable methane purities are possible with the dual digestion system. Total alkalinity and volatile acid concentrations for the anaerobic step averaged 3559 and 265 mg/L, respectively, during the course of the test program. Data collected on bacteria, virus, and parasite kills demonstrate the ability of the dual digestion system to reduce significantly (by several orders of magni- tude) the levels of human pathogens when operated at thermophilic tempera- tures. ------- Table 1. Test Program Process Conditions Parameter Phase Average Range Aerobic retention time, days Anaerobic retention time, days Temperature, °C Feed Aerobic Anaerobic Ambient Solids concentration, % TS VS Volatile solids removal, % Aerobic Overall Oxygen flow rate, CFM NTP Gas purities, % Aerobic vent O2 Anaerobic vent CHt CO, 02 1.38 19.9 23.8 50.5 41.7 21.5 3.02 2.41 27.2 41.6 19.2 68.9 49.1 33.1 1.6 0.6 - 8.8 9.6 -34.0 16.8 -28.3 42.3 - 60.0 37.5 -47.0 5.0 - 40.0 1.50- 4.11 1.23 - 3.33 15 24 -51 -57 10.3 -33.4 31.5 -91.5 35.5 - 62.5 24.2 -49.5 0 - 4.5 Feed Aerobic Anaerobic 6.00 7.09 7.30 5.5 - 6.9 6.6 - 7.7 6.7 - 7.65 Recommendations The performance of the dual digestion system under operating conditions closer to typical design values should be investigated further. In particular, the long-term operations at a 1 -day aerobic, 8-day anaerobic retention time with high influent solids concentrations (greater than 3 percent volatile solids) is necessary to demonstrate the system's advantages completely. The full report was submitted in fulfillment of Grant No. S805823-01 by Union Carbide Corporation under the sponsorship of the U.S. Environmental Protection Agency. Oscar Haas is with Union Carbide Corporation, Tonawanda, NY 14150. B. Vincent Salotto is the EPA Project Officer (see below). The complete report, entitled "Demonstration of Thermophilic A erobic-A naerobic Digestion at Hagerstown, Maryland. "(Order No. PB 84-238 252; Cost: $13.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield. 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