New high-rate plug flow anaerobic digester technology for small communities

Transcription

1 New high-rate plug flow anaerobic digester technology for small communities C. H. Burnett* and A. P. Togna** **Total Solids Solutions, 53 Talais Drive, Little Rock, AR, USA ( **Shaw Environmental & Infrastructure, Inc., 17 Princess Road, Lawrenceville, NJ, USA ( ABSTRACT: A new and radically different anaerobic digester called the BioTerminator has been developed using plug flow hydraulics that achieves remarkable and unprecedented solids destruction efficiencies. It is designed to digest biological solids from wastewater treatment plants and treat high-strength organic wastewaters from industrial operations. The recently introduced commercial version of this digester is available in a capacity of 38 m 3 /day (10,000 gallons/day) solids treatment capacity, which will handle solids produced from a 7,600 m 3 /day (2 million gallons per day) wastewater treatment plant. The new BioTerminator digester now provides an anaerobic digestion option for these communities. The BioTerminator digester has the potential to provide low cost and sustainable advanced wastewater treatment for developing countries. Coupled with a simple earthen-basin sequencing batch reactor (SBR) wastewater treatment plant, the BioTerminator digester can produce methane gas from the waste sludge produced in the SBR. The digester gas can be combusted in a gas-fired internal combustion engine that is direct coupled to a low cost positive displacement blower to provide most of the process aeration needs. The paper describes the new digester technology in greater detail, and presents solids destruction results achieved at several locations. A further description of the application of the technology to a sustainable advanced treatment system using the SBR process is also described. KEYWORDS: Anaerobic digestion, high rate plug flow, sequencing batch reactor INTRODUCTION Anaerobic baffled reactors, which would include the BioTerminator digester, have been applied to industrial wastes in limited applications over the past 20 years. (Barber and Stuckey, 1999) The BioTerminator highrate plug flow anaerobic digester is designed for municipal as well as industrial applications. It consists of five separate chambers, each of which plays a specialized role in the progression of anaerobic digestion of feed solids from initial hydrolysis to, ultimately, generation of methane. The genera of microorganisms differ in each chamber according to their sequential function in the digestion process. The digester operates at mesophilic temperatures (i.e., approximately 35ºC). A simplified schematic showing the flow pattern through the Bioterminator is shown in Figure 1 and a photograph of a 1,000-gallon pilot unit is shown in Figure

2 BIOTERMINATOR DESCRIPTION Figure 1 Simplified Schematic Representing Flow Through BioTerminator Plug Flow Anaerobic Digester Figure 2 1,000-Gallon Pilot BioTerminator Unit The BioTerminator is a force-fed continuous flow reactor which involves the use of baffles to create multiple treatment zones or chambers in a single vessel. No mixing occurs within the reactor other than through gas generation. Proper selection of the dimensions characterizing each treatment chamber allows the build-up of a blanket of solids. The feed solids, together with new biomass solids formed during the process, remain in the reactor for a period longer than the hydraulic retention time (HRT). That is the solids retention time (SRT) is greater than the HRT. While flow is continuous, the upflow or downflow velocity in each chamber of the reactor is quite small, e.g., 0.06 cm/sec (0.002 ft/sec). At this velocity, solids will settle in the chamber while the liquids are slowly transported downstream. ANAEROBIC DIGESTION IN THE BIOTERMINATOR DIGESTER Anaerobic digestion in the Bioterminator occurs in separate chambers of the baffled reactor. The first chamber is largely populated with hydrolyzing and acid forming microorganisms that solubilize the feed organic solids leading to the formation of volatile fatty acids (VFAs) and other intermediate products. The influent organic solids will settle in the first chamber where hydrolysis occurs. Anaerobic digestion does not require the maintenance of the organic solids in suspension but does require mesophilic temperature conditions to maximize the various biological reactions. In the first chamber, the waste products generated by the hydrolyzing and acidifying bacteria are VFAs, a liquid, carbon dioxide, and hydrogen. As a liquid, the VFAs migrate downstream to the subsequent chambers where they become the food source for organisms responsible for fermentation and other reactions ultimately resulting in the formation of methane. As is the case with any anaerobic digester, most of the feed solids entering the BioTerminator disappear in this first intense digestion 384

3 step. Subsequent steps of the digestion process produce new solids (biomass) that feed on the previously generated dissolved organics and other reaction products (e.g., hydrogen). The predominant microorganisms selected in the first chamber are hydrolytic and fermentative bacteria. In the subsequent reaction chambers, increasing percentages of acetogenic and methanogenic bacteria are selected. Methane gas rises to the top of the digester and can be collected as produced. By using naturally-occurring microorganisms and by compartmentalizing the selection of organisms that most effectively thrive on the material found in that particular compartment, the digester efficiently and rapidly digests the waste. It is well known that the acid phase of anaerobic digestion is a much more rapid process than methanogenisis. In conventional phased anaerobic digestion, the acid phase HRT may be as short as one day. Since conventional digesters are designed as completely mixed vessels, some acidifiers and raw organics are transferred out of the acid phase digester to the second phase methanogenic digester. In the BioTerminator, the acidifiers remain behind in the first chamber and form a sludge blanket, which may reach solids densities exceeding that of the influent organic solids. This biomass blanket grows proportionally to the mass of feed solids. In subsequent chambers of the BioTerminator, a biomass sludge blanket forms to produce specialized series of microbial colonies, each optimally growing and thriving on the waste products of the upstream population. The biomass remains in each chamber while the liquids slowly migrate through the reactor. The final result is a digester effluent almost totally devoid of solids and near total destruction of volatile matter. Fixed solids consisting of sodium, potassium, chloride, sulfate, silica, calcium, and magnesium will accumulate in the chambers and/or exit the reactor with the effluent in a dissolved state. Periodically, a small portion of the solids blanket in each chamber is wasted from the bottom of the reactor to a container filter. This removes settled grit and enough accumulated inert material to keep the process in balance. In the container filter, the wasted solids are allowed to drain by gravity and the contents are disposed in a landfill. Influent phosphorus contained in the feed solids will be solublized and exit the reactor with the effluent, as will any influent ammonia. Additional ammonia will be contributed to the effluent through conversion of organic nitrogen under anaerobic conditions. Subsequent treatment of the digester effluent will be required if phosphorus and ammonia must be removed from the process. To date, no testing has been performed to determine the liberation of phosphorus, if any, from solids treated with coagulants. DEVELOPMENT OF THE BIOTERMINATOR TECHNOLOGY The BioTerminator high-rate plug flow reactor was developed in the 1990s by Dr. Chino Srinivsasan a professor of microbiology at Louisiana State University (LSU), now deceased (Sheng et al., 2003). The first 1,000-gallon (3,785-liter) capacity prototype reactor was installed at the Central WWTP in Baton Rouge, LA in October 2000 and operated for five months digesting raw primary solids with an average feed concentration of 4%. This digester was unheated, yet still achieved very high solids reduction efficiencies, averaging 93% volatile solids (VS) removal at a two-day HRT. Several conclusions were drawn from these experiments. The digester was shown to function effectively on raw primary sludge. The digester discharged effluent containing primarily soluble COD, which can be readily converted by aerobic treatment processes. Thus the digester effluent was shown to be suitable for total and continuous recycle back to the head of the treatment works. Subsequently, the digester technology was licensed by LSU to Total Solids Solutions, L.L.C. (TSS) Monroe, LA. TSS developed a second 1,000-gal capacity reactor with assistance from the LSU researchers. This digester was heated, and was installed at a WWTP in Daphne, AL from February-April The sludge feed was 100% waste activated sludge from an aerated waste sludge holding tank with an average concentration of 0.75%. Good results were achieved, with total solids (TS) reduction averaging 86% (volatile solids were not measured) at a 1 day HRT (Burnett, 2005). Subsequent tests were performed in June 2004 at a poultry processing plant in Fort Smith, AR. The digester was installed as a pretreatment device on raw wastewater from the poultry plant. The digester proved to be a very powerful anaerobic reactor and achieved an average 86% reduction of the approximately 50,000 mg/l 385

4 COD influent at a 3 day HRT. A third test with the second digester was performed at the Fourche Creek WWTP in Little Rock, AR from July 26, 2004 to September 9, 2004, and showed promising results on a mixed primary/secondary solids feed that averaged 1.7% TS, with VS reduction through the digester averaging 90% at a 1-2 day HRT (Burnett, 2005). In 2005, the process was sublicensed by TSS to Shaw Environmental & Infrastructure, Inc., Lawrenceville, NJ (Shaw E&I), a subsidiary of The Shaw Group, a Fortune 500 company headquartered in Baton Rouge, LA. Shaw E&I subsequently developed a third prototype 1,000 gal digester that incorporated more advanced instrumentation and control features, a gas measuring system, and provided greater equipment reliability. This unit was installed at a WWTP in Mosinee, WI and operated from August to October Results of this test were mixed; possibly due to the viscous nature of the 2.5% mixed primary and secondary feed solids that contained a large component of rags and stringy material. This resulted in frequent interruptions of the sludge flow, and demonstrated the need to provide screened sludge to the BioTerminator. The BioTerminator is designed to operate at a steady-state feed and temperature, and these conditions were not achieved in the Mosinee test a majority of the time. A second 1,000 gal (3,785 L) digester system was constructed by Shaw E&I and has been operating at the Airport WWTP in Galveston, TX since mid-december 2006 on 100% secondary solids (see Figure 2). The operation has been intermittent due to handling problems with the feed sludge. Steady state flow was ultimately achieved in late February 2007 and has been averaging 81% total solids removal and 76% volatile solids removal through mid-march FULL SCALE SYSTEM IMPLEMENTATION The original technology licensee, Total Solids Solutions, has constructed a 10,000-gallon (38 m 3 ) capacity digester that would be suitable to process solids, after partial thickening, from a 2 MGD (7,600 m 3 /day) WWTP (see Figure 3). This unit will be installed by Shaw E&I at the Daphne, AL WWTP that conducted the original testing. Figure 3 10,000-Gallon (38 m 3 /day) BioTerminator Reactor Ultimately, three units will be required at Daphne to process solids for their 4 MGD (15,000 m 3 /day) plant capacity. Methane produced by the digester is used to preheat the sludge using a dual-fuel boiler, with natural gas or propane serving as a backup. A gas flare is also provided to dispose of excess gas, although it can also be beneficially used by the plant. Another 1,000 gal/day (3,785 L/day) system is currently in the testing phase on screened dairy manure containing 4% solids. If successful, larger capacity systems will be required for this application. Larger units constructed primarily from concrete and designed to process solids from 5 10 MGD (4,000-8,000 m 3 /day) WWTPs are in the initial design stages. Results of ongoing tests on the BioTerminator digestion system as well as the first full scale installation will be available in

5 ENERGY RECOVERY The biogas produced by the BioTerminator has a heating value in the range of 20,000-24,000 kilojoule/m 3 ( BTU/cf), depending on the specific characteristics of the feed solids. Typically, biogas has a heating value that is approximately 64% of the heating value of natural gas. Some of the biogas is used to preheat incoming sludge while the rest can be used to generate energy. Assuming the biogas is utilized in an internal combustion reciprocating engine with heat recovery from the engine exhaust, only about 10% of the biogas is needed to preheat incoming sludge. The remaining 90% can be used to operate other machinery or produce electricity. A calculation of the energy production capability of one 10,000 gpd BioTerminator reactor is shown on Table 1. Table 1 BIOTERMINATOR APPLICATION TO A SUSTAINABLE ADVANCED WASTEWATER TREATMENT SYSTEM The BioTerminator high-rate sludge digester has proven to be an efficient means of reducing wastewater solids. The processing time is only 24 hours, so the size of the tank is quite small compared to a conventional digester that is typically 20 times larger or more in size. Because the tank is small, it is relatively inexpensive to construct compared to the much larger conventional digester tank. Correspondingly, the BioTerminator is especially well suited for smaller communities where expensive conventional waste reduction technologies are not affordable. In developing countries, the BioTerminator digester may have potential application to the thousands of communities that are in need of a low technology, sustainable wastewater treatment system. Figure 4 1,000 gal/day (3.8 m 3 /day) Test Unit The BioTerminator digester has the potential to provide low cost and sustainable advanced wastewater treatment for developing countries. Coupled with a simple earthen-basin sequencing batch reactor (SBR) wastewater treatment plant, the BioTerminator digester can produce gas from the waste sludge produced in 387

6 the SBR. The digester gas can then be combusted in a simple gas-fired internal combustion engine that is direct coupled to a low cost positive displacement blower. A conceptual layout of this simple but advanced closed loop treatment system is shown on Figure 5. Figure 5 Closed Loop Advanced Wastewater Treatment Plant The positive displacement rotary blower is a good match for a typical low rpm digester gas engine. Enough digester gas will be produced to provide the aeration needs of the SBR most of the time. In this way, advanced treatment of municipal wastewaters will be possible in developing countries at an affordable cost. A fullyloaded 2 MGD (7,600 m 3 /day) SBR, for example, would require average and peak aeration power requirements of approximately 100 and 120 kw, which compares favorably to the energy value of the digester gas of approximately138 kw from the same plant. The modern SBR process consists of a single treatment tank partitioned into two or more cells. All wastewater treatment takes place in these cells through alternating cycles of fill-aerate-settle-decant. Because only a single multiple-cell tank is required, the SBR is cost effective to construct. Additionally, it is capable of advanced wastewater treatment to remove BOD, solids, nitrogen, and phosphorus to very low levels. As such it is highly protective of the receiving streams or rivers for the plant effluent. Waste sludge produced by the SBR would be further processed in the BioTerminator digester to eliminate the requirement for sludge disposal and to create energy to run the plant. Modern SBRs are completely computer controlled, so that little operator attention or intervention is required. This is accomplished with a programmable logic controller (PLC). As long as the electricity supply is not interrupted, the SBR process will provide reliable treatment. If the electricity supply is temporarily interrupted, the effluent quality will deteriorate, but once the power is restored the system will self-correct and eventually restore normal treatment levels. A conceptual layout of the sustainable advanced wastewater treatment plant, consisting of the earthen basin SBR coupled to a BioTerminator high-rate anaerobic digester and aeration power provided by digester gas is shown on Figure 6. In addition to the facilities shown, a gravity thickening cell is also required. 388

7 Figure 6.Closed Loop Advanced WWTP Conceptual Site Layout CONCLUSIONS The BioTerminator high-rate anaerobic digester provides significant benefits compared to conventional aerobic or anaerobic digesters. These are: Achieves 85% reduction of total solids. Digester effluent is continuously recycled to plant influent. Generates recoverable methane for beneficial use. Needs only a small tank which reduces cost. Process is simple to operate, requiring only a steady state preheated sludge flow. The digester is readily adaptable to incorporate into an integrated modern wastewater treatment facility, such as the Sequencing Batch Reactor process, to produce a sustainable advanced low cost treatment system. REFERENCES Barber, W.P. and Stuckey, D.C. (1999) The Use of the Anaerobic Baffled Reactor (ABR) for Wastewater Treatment: A Review, J. Water Research, Vol. 33, No. 7, pp Hawkes, F.R. et al. (1987) Chapter 12: Anaerobic Digestion, in Basic Biotechnology, J. Bu Lock and B. Kristiansen, eds., Academic Press, Orlando, Florida, pp Burnett, C. (2005) Pilot Test Results of the BioTerminator High-Rate Plug Flow Anaerobic Digester; Proceedings, Joint Residuals and Biosolids Management Conference, Water Environment Federation, Nashville, TN. Levenspiel, O. (1972) Chemical Reaction Engineering, Second Edition; John Wiley & Sons, Inc. New York. Sheng, Y; Sansalone, J.; Srinivsasan, V.; Taylor, H. (2003) Pilot-Scale Sludge Treatment by a Force-Fed Serpentine Plug-flow Reactor (SPFR); Proceedings, 76 th Annual Conference of the Water Environment Federation, San Diego, CA. 389