Second Generation Atad A-Tad Better

“SECOND GENERATION” ATAD – A-TAD BETTER? – TWO CASE STUDIES OF CONVERSION FROM “FIRST GENERATION” TO “SECOND GENERATION” ATAD SYSTEMS

By David W. Oerke, P.E. (Principal Author) CH2M HILL, Denver, Colorado,

Chad Olsen, McMahon Associates; James Kirk, Grand Chute-Menasha West Sewerage Commission, Neenah, Wisconsin; and Chris Maines, Eagle River Water and Sanitation District, Vail, Colorado

ABSTRACT

Autothermal Thermophilic Aerobic Digestion (ATAD) is becoming more prevalent due to its advantages over conventional aerobic digestion, including the generation of a Class “A” product, a relatively small footprint, and reduced biosolids for end disposal.

Both the Grand Chute-Menasha West Wastewater Treatment Facility (WWTF) in Neenah, Wisconsin and the Edwards Wastewater Treatment Plant (WWTP) operated by the Eagle River Water and Sanitation District located in Edwards, Colorado have “First Generation” ATAD systems and are in the process of converting the systems to “Second Generation” ATAD systems.

The “Second Generation” ATAD systems include process modifications over the “First Generation” ATAD systems such as better mixing and process control that have resulted in higher volatile solids reduction, lower product volume, less odor generation and less sidestream impacts at several installations. This paper explores the advantages of the “Second Generation” ATAD systems over the original installations and the reasons why the two plants decided to convert their systems to the newer ATAD systems.

KEYWORDS

Autothermal Thermophilic Aerobic Digestion, storage nitrification/denitrification reactor, biofilter, odor control

INTRODUCTION

The handling, stabilization and reuse/disposal of biosolids are an increasingly costly portion of the wastewater treatment plant (WWTP) operation. Given the trend of decreasing availability and increasing cost of ultimate reuse/disposal options, WWTP residuals handling is becoming an even greater challenge. Autothermal thermophilic aerobic digestion (ATAD) is becoming more prevalent due to its advantages over conventional aerobic digestion, including the generation of a Class “A” product, a relatively small footprint, and reduced biosolids for end disposal. Both the Grand Chute-Menasha West Wastewater Treatment Facility (WWTF) in Neenah, Wisconsin and the Edwards WWTP operated by the Eagle River Water and Sanitation District located in Edwards, Colorado have “First Generation” ATAD systems and are in the process of converting the systems to “Second Generation” ATAD systems. This paper explores the advantages of the “Second Generation” ATAD systems over the original installations and the reasons why the two plants decided to convert their existing systems to the newer ATAD processing operations.

METHODOLOGY AND DISCUSSION

The ATAD process was first utilized in the United States in 1994 at the Grand Chute-Menasha West WWTF. There are approximately 50 “First Generation” ATAD installations in Europe and North America including designs based on equipment provided by Fuchs, Incorporated, equipment provided by Jet Tech, and installations that used a pumped-venturi mixing system designed by Dayton & Knight Consulting Engineers. Although some of the “First Generation” systems have had impressive volatile solids destruction rates, many have also had a history of significant odor problems, high ammonia concentration in sidestream recycle flows, foaming issues and relatively high dewatering costs. The problems associated with the “First Generation” ATAD systems have caused several facilities to discontinue operation. ATAD is commonly referred to as “liquid composting” because it operates at thermophilic temperatures comparable to conventional composting (50 to 70 degrees Centigrade [ºC] or 122 to 158 degrees [ºF]). The process is described as autothermal because, as it increases in temperature during start-up, and then it requires no other heat source (other than mixing energy) to maintain thermophilic temperatures. The heat released by organic decomposition during digestion typically sustains the thermophilic operating temperatures. Increased operating temperatures produce a more rapid digestion process when compared to conventional aerobic digestion. The increased digestion rate results in a decreased digester volume. Results from operating facilities show that the system, which is relatively simple, has low operation, maintenance, instrumentation, and energy requirements. A properly designed ATAD system has the following advantages over conventional biosolids digestion processes:  Production of a Class “A” biosolids product  Cost-effective compact facilities  Significant volume reduction  Better biosolids dewaterability The advantage of using a Class “A” stabilization process includes easier compliance, less monitoring and record keeping, less odor in the final biosolids product, and maximum flexibility for beneficial reuse and marketing options. A Class “A” biosolids material, similar to potting soil is produced from a well designed and operated ATAD facility (see photo of “Second Generation” ATAD Class “A” Product from Yorkville, IL in Figure 1). This material can be land applied or distributed and marketed without the restrictions associated with Class “B” biosolids. Figure 1 - Class “A” soil-like biosolids product from “Second Generation ATAD facility at Yorkville, Illinois

ATAD is a very efficient stabilization process because its facilities are more compact (when compared to conventional aerobic digestion). This compactness:

 Reduces capital and some annual operation and maintenance costs. However, energy costs can be higher for ATAD facilities since the high ATAD temperatures typically provide poor oxygen transfer efficiency and the significantly higher degree of treatment typically requires more energy)

 Enables capture of off-gas for better odor control  Can provide those owners with existing available tanks significant tank capital cost savings The ATAD system has been successfully installed in existing retrofitted concrete and coated- steel aerobic digester tanks. Compared to conventional aerobic digestion, ATAD is a more effective digestion and pathogen and vector attraction reduction process. The ATAD process typically destroys 50 to 70 percent of the volatile solids in the biosolids. This reduces “downstream” solids support capacity requirements, such as dewatering facilities. Volume reduction saves hauling costs associated with land application and distribution and marketing of the final product. To effectively operate the ATAD process, the feed material must be thickened to a minimum 40,000 mg/L (4 percent solids) or greater chemical oxygen demand (COD). Some ATAD facilities with lower energy waste activated sludge (WAS) feed require a minimum of 5 to 6 percent solids concentration. Prior to ATAD, gravity belt thickeners, rotary drum thickeners, and centrifuges have been successfully used for thickening solids. Feed material with a lower solids concentration has been successfully treated by the ATAD process, but requires larger reactor tanks and typically more process control to consistently meet Class “A” stabilization criteria. “First Generation” ATAD Process Issues The total ATAD process retention time recommended in the “First Generation” ATAD process was only 5 to 8 days. This compares to a 20 to 60-day process retention time for aerobic digestion. However, the design criteria for the “First Generation” ATAD systems with only 5 to 8 days hydraulic retention time (HRT) were originally used for only primary solids digestion. Primary solids are much easier to “break down” compared to secondary solids. The shorter HRT has been inadvertently applied to ATAD systems with combined primary and secondary feed solids. Note that solids retention time (SRT) and HRT are the same since the ATAD process tanks are designed to be completely mixed. The “First Generation” ATAD design criteria generally required at least two to three reactors to optimize process performance, stability, and flexibility. The temperature is typically maintained at approximately 45ºC (113ºF) in the first “preheat” reactor and approximately 60ºC (140ºF) in the second and third “process” reactors. A fourth reactor is typically required for cooling and storage to enhance thickening and dewatering performance and supernatant/filtrate quality. Heat is typically extracted from the ATAD biosolids through heat exchangers to heat the incoming “cold” feed solids at the ATAD facilities with pumped-venturi mixing designed by Dayton & Knight Consulting Engineers. The “First Generation” ATAD processes are typically operated in batch, semi-continuous, or continuous mode. The “First Generation” ATAD systems typically provide only one level of oxygen supply during the entire process, regardless of the level of activity in the reactor. Foam can serve as important insulation at the liquid-surface layer where the highest amount of heat loss caused by evaporation can occur. Many times, ineffective process control in the “First Generation” ATAD systems has led to excessive foam generation and numerous housekeeping problems. Mechanical foam cutters were often used to control the foam generation. Ammonia concentrations in the recycled centrate/filtrate from “First Generation” ATAD facilities have also been significant (typical ammonia concentrations of 800 to 1,500 milligrams per liter [mg/L]) depending on the degree of volatile solids reduction and the temperature. In addition, impacts on the secondary treatment process had to be considered, especially at nutrient removal WWTPs. Odor generation was also a significant concern with “First Generation” ATAD facilities. The “First Generation” ATAD facilities that do not have proper mixing and process control typically produce an odorous off-gas that includes high concentrations of ammonia, amines, and reduced sulfur compounds. Successful odor treatment systems for “First Generation” ATAD systems include multiple odor treatment technologies in series, including wet scrubbers, ozone, three- stage scrubbers and biofilters. Grand Chute WWTF ATAD Facility The first North American ATAD facility was installed at the Grand Chute-Menasha West (GCMW) WWTF, in Neenah, Wisconsin in 1994. Co-settled and thickened primary sludge and waste activated sludge (WAS) is pumped to the mixed sludge holding tank. Sludge is transferred from the mixed sludge holding tank once/day, 7 days per week to the three ATAD reactors operated in series. The “First Generation” ATAD reactors consists of 3, 10.7-m (35-foot) diameter, 3 m (10 foot) liquid depth insulated process tanks with a total of approximately 757 cu m (200,000 gallon) capacity. Each process tank included: 1) four 9 kW (12 HP) Fuchs air-aspirating aerators, and 2) eight 1 kW (1.5 HP) foam cutters suspended from the tank roof. A gravity thickener is used to co-settle the primary and WAS to provide a minimum feed solids concentration of 3.5 percent solids. A gravity belt thickener was provided but is rarely used. An approximately 1,703 cu m (450,000 gallon) Post-ATAD Storage Tank was also provided to decrease the process temperature prior to dewatering. This facility has consistently achieved greater than 50 to 65 percent volatile solids destruction [measured from the initial tank through the belt filter press (BFP)], and has never failed to meet Class “A” pathogen and vector attraction reduction requirements since start-up in 1994. Fecal coliform levels in the BFP cake have typically averaged 10-30 MPN/g TS (see table 1). Significantly levels of additional volatile solids destruction have been achieved through the Post-ATAD storage tank. ATAD off-gases are treated in a three-stage chemical scrubber followed by a biofilter with lava rock media. The odor control system was design and installed after the initial ATAD facility start-up due to significant odor generation. The ATAD reactors were designed to handle approximately 5,715 kg/d (12,600 lbs/d) total solids loading with a hydraulic retention time of 6.5 days at 115 cu m/d (30,250 gpd) at 3.5 percent solids concentration. The ATAD reactors have been loaded at approximately 70 percent of the design solids loading and 106 percent of the volumetric loading. Post-ATAD biosolids are discharged into the circular sludge thickener tank. BFP feed pumps draw suction from this tank to feed the BFPs. The dewatered solids cake concentrations of 23 to 29 percent have been achieved with a BFP. Previous cake solids concentrations for aerobically digested biosolids were 14 to 17 percent using the same BFPs. However, a higher polymer dose was required to dewater the ATAD biosolids. It should be stated that since the ATAD process has a higher volatile solids reduction than aerobic digestion, comparably less material must be dewatered. Therefore, the overall chemical dewatering cost is often times approximately the same. For example, if only the ATAD process produces one-half the biosolids quantity but twice the chemical dewatering dose is used, then the overall chemical use is approximately equal. Table 1- “First Generation” ATAD Plant Performance – Grand Chute-Menasha West and Edwards WWTPs Plant Year VSR VSR VSR ATAD Fecal Fecal Fecal (%) – (%) – (%) – #3 Coliform Coliform Coliform Across Through Through Reactor (MPN/g (MPN/g (MPN/g ATAD Post- BFP Temp. TS, GM) TS, GM) TS, GM) Tanks ATAD Cake (degrees – Across – Post- – BFP Storage C) ATAD ATAD Cake Tanks Storage Grand Chute- Menasha West WWTP, Neenah,WI 1995 35.4 50.2 57.0 * 74 139 69 1996 31.6 53.6 60.6 * 65 185 22 1997 30.7 50.1 60.7 * 61 121 11 1998 34.1 52.5 62.5 * 74 188 29 1999 34.4 49.7 58.2 * 87 158 18 2000 30.5 47.6 54.7 * 82 173 19 2001 29.0 48.3 51.5 58 64 125 15 2002 33.2 52.4 57.1 60 64 119 16 2003 34.6 54.8 58.4 58 71 122 14 2004 34.9 58.7 62.3 60 61 95 12 2005 38.1 61.9 65.7 63 65 99 21 2006 38.8 60.2 63.3 64 66 102 12 2007 33.7 56.7 58.4 58 84 131 15 2008 36.6 58.5 60.0 60 67 95 28

Edwards WWTP, Edwards, CO 2007 * 52 * 44 * * * 2008 * 39 * 44 * * * VSR = Volatile Solids Reduction; GM = Geometric Mean; * Unknown Despite excellent performance, there are issues with the current ATAD design that are being addressed with the “Second Generation” ATAD facility modifications that are under construction: 1. Incomplete digestion which results in high reduced sulfide levels result from the short retention times (only 5 to 6.5 days) and high temperatures. “Second Generation” ATAD systems are typically designed for 12 to 14-day ATAD reactor retention time. 2. Better volatile solids destruction and a more stable operation in the ATAD reactors is anticipated for the “Second Generation” ATAD facility. 3. The Fuchs design does not account for the significant oxygen demand immediately after feeding the reactors, resulting in generation of reduced sulfide odor compounds. “Second Generation” ATAD system design criteria matches oxygen demand with oxygen supply, using ORP control to automate the system. This results in more complete digestion and virtually eliminates odors from sulfur compounds. 4. The BFP filtrate adds approximately 295 kg/d (650 lbs/d) of ammonia to influent wastewater. The operating staff at the WWTF is currently having difficulties meeting their effluent ammonia limits, especially on the days that the BFP is in operation. “Second Generation” ATAD systems provide a Post-ATAD/Storage Nitrification/Denitrification Reactor (SNDR) reactors should reduce the ammonia concentration in the digested biosolids to below 500 mg/L and reduce the impact of the ammonia in the dewatering filtrate on the secondary treatment process. The installation of the Post ATAD/SNDR reactors has allowed the GCMW Sewerage Commission to install a smaller secondary treatment system. 5. With a SNDR tank - cooling, nitrification, and denitrification are provided. The combination of cooling and post aeration to rebalance the cation to anion ratio in the biosolids results in a drier dewatered product with less polymer addition. The nitrification and denitrification process also reduces the nitrogen load returned in the filtrate.

6. Use of “Second Generation” ATAD odor control systems will include a water scrubber followed by a biofilter that will allow for the elimination of the triplex chemical scrubber odor control system, thus eliminating sulfuric acid, hypochlorite and caustic addition. 7. The addition of the new “Second Generation” ATAD system and related new solids processing facilities allows the GCMW Sewerage Commission to handle more solids production corresponding to an increased influent wastewater flow from growth within the service area. The new facilities are designed to process an average annual solids production of 7,606 kg/d (16,734 lbs/d).

Edwards WWTP ATAD Facility The original design of the “First Generation” ATAD system for the Edwards Wastewater Treatment Plant (EWWTP) was based on an average solids production of 2,517 kg/d (5,550 lbs/d); a maximum month solids production of 2,812 kg/d (6,200 lbs/d); and a peak week solids loading of 3,402 kg/d (7,500 lbs/d). The peak week solids production of (3,402 kg/d (7,500 lbs/d) was used to size the tanks assuming a total HRT of 4.5 days. The recommended HRT design value for other ATAD installations with lower odor generation compared to the EWWTP is 12 days. The actual dimensions of the four ATAD tanks are as follows: total height – 5.86 m (19.25 feet) to top of hatch; overflow height – 5.2 m (17 feet) for a capacity of 156 cu m (41,208 gallons); actual working height – 4.3 m (14 feet) to account for foam for a capacity of 129 cu m (34,000 gallons).

The current pumped-venturi mixing system is inadequate. The staff typically can maintain an ORP value between a low of –350 millivolts (mV) to a high of –200 mV that results in anaerobic conditions that creates significant odor. The ORP values should be between –50 mV and +50 mV to keep the process aerobic and minimize odor generation.

The coarse bubble aeration system that includes a 186 kW (250 HP) positive-displacement blower added by the District staff approximately 1 year ago is not effective in mixing the ATAD solids above 3.5 to 4 percent solids concentration; however, it could be effective if the solids concentration was decreased to approximately 1.5 to 2.5 percent solids.

The benefits for converting from the “First Generation” to the “Second Generation” ATAD facility at the Edwards WWTP that is currently under construction:

1. Compliance with Colorado Department of Public Health and the Environment (CDPHE) regulations that state that the ATAD tanks must have a minimum 10 day detention time to meet Class “A” standards. Stricter ATAD design criteria have been adopted by CDPHE after the design of the initial “First Generation” ATAD facility at Edwards WWTP.

2. Additional solids processing capacity to handle future maximum month 6,033 kg/d (13,300 lbs/d) solids production and eliminate the need to haul liquid raw sludge to the Avon WWTP or to the Biosolids Containment Facility (BCF) operated by the Eagle River Water and Sanitation District (ERWSD) in Wolcott, Colorado.

3. To increase the capacity of the ATAD system, it was recommended that new larger ATAD tanks with better mixing facilities be installed. The existing pumped-venturi ATAD tank mixing system is inadequate and underpowered to completely mix the thickened solids with a solids concentration in the 6 to 8 percent range. The staff typically can maintain an ORP value between a low of –350 millivolts (mV) to a high of –200 mV that results in anaerobic conditions (especially in the corners of the square tanks) that creates significant hydrogen sulfide, mercaptans, and other odor causing compounds. The ORP values should be between –50 mV and +50 mV to keep the process aerobic and minimize odor generation.

4. Over 25 operating “Second Generation” ATAD facilities provided by TPS have shown that superior mixing, longer detention time, better process control to maintain aerobic conditions result in significantly less odor generation, and better sidestream treatment, including one at the Yorkville, Illinois WWTP. Several ERWSD staff members have had the opportunity to tour this facility and have expressed positive feedback on the facility performance.

5. With a SNDR tank - cooling, nitrification, and denitrification are provided. The combination of cooling and post aeration to rebalance the cation to anion ratio in the biosolids results in a drier dewatered product with less polymer addition. The nitrification and denitrification process also reduces the nitrogen load returned in the centrate.

6. Potential for different and more efficient odor control technologies (mist scrubber and two-stage biofilter) to replace existing biotrickling filters, ozone oxidation and three- stage chemical scrubber resulting in less odor control chemical consumption (now at $30,000 per year), lower operating costs and less non-potable water recycle flow (currently 1,136 cu m/d (300,000 gallon/d) to the Edwards WWTP that takes valuable plant capacity.

7. Reduce ammonia loading in the recycle to the aeration basins during peak diurnal loading.

8. The equalization of centrate allows for the centrate flow to be recycled over a 24-hour basis instead of the full load being applied during the operation of the centrifuge.

9. Recycle of nitrate to the aeration basins will decrease the overall oxygen demand for the aeration basins resulting in significant electrical savings and will increase alkalinity resulting in less soda ash addition.

10. Additional WAS storage is provided to improve WAS equalization and to allow for thickening and dewatering equipment operational flexibility and significant reduced labor and polymer costs.

Overall Advantages of the “Second Generation” ATAD Process The “Second Generation” ATAD ThermAer™ process differs substantially from the “First Generation” systems mentioned previously. Thermal Process Systems (TPS), Crown Point, Indiana, has patented the “Second Generation” system and introduced it in 1997. TPS has 25 successfully operating “Second Generation” facilities and has converted seven “First Generation” facilities to “Second Generation” ATAD facilities. The ATAD process design has evolved during this time, especially in regard to thermophilic/mesophilic staged operation, detention time, reactor covers and foam control. Previous studies (Scisson, 2009) have shown good to exceptional solids reduction and dewaterability from 8 operating installations. The municipal plants range in size from 0.1 cu m/s (2.5 MGD) to approximately 0.53 cu m/s (12 MGD). Information and process performance collected by Scisson from several “Second Generation” ATAD facilities is shown in Table 2. Table 2 - “Second Generation” ATAD Facility Characteristics

Plant Capacity Date in Process New or Liquid or Type of Total Volatile Dry Service replaced Retrofit cake solids Solids Solids Mg/D storage/ processed Reduction Reduction (TPD) reuse (percent) (percent) Three 3.6 (4) 2002 Anaerobic Retrofit Cake Primary + 45 55 Rivers, Digestion TWAS + MI septage Yorkville, 1.8 (2.2) 2004 Aerobic New Cake TWAS 50 Unknown IL Digestion Bowling 7.3 (8) 2005 Aerobic Retrofit Liquid Co-settled 61 72 Green, Digestion plus storage Primary + OH New plus WAS + Tanks dewatering septage Morehead, 2.7 (3.3) 2005 Anaerobic New Cake Co-settled unknown Unknown KY Digestion Primary + WAS Delphos, 4.1 (4.4) 2006 Aerobic New Cake TWAS 55 65 OH Digestion from MBR Heart of 10 2007 Anaerobic Retrofit Liquid Co-settled 56 63 the (13.5) Digestion storage Primary + Valley, WAS WI Marshall, 5.4 (6) 2006 Anaerobic Retrofit Liquid Co-settled 60 65 MN Digestion storage and Thickened Primary + WAS Updated After Scission, 2009 . Excellent TS and VS reduction values for all sites is shown in Table 2 with exceptional TS and VS values from the Bowling Green, Marshall and Heart of the Valley WWTPs. These three plants all process primary and secondary biosolids. Primary solids are more degradable than secondary solids and are assumed to contribute to the superior performance. The good performance at all these plants indicates that the “Second Generation” ATAD system performance should perform well at other treatment plants. The lower TS and VS reduction data from the Three Rivers WWTP is probably due to the very thin feed solids concentration that results in a shorter than desired HRT. Much of the thin solids concentration is due to the relatively high percentage of septage received at the Three Rivers WWTP (approximately 56.8 to 94.6 cu m/d (15,000 – 25,000 gallons/d).

The Marshall ATAD facility operates only two to three days per week with co-settled and thickened primary and WAS material from a gravity thickener. When the ATAD operating temperatures drop to below 58 degrees C (135 degrees F), then the staff co-thickens the primary and WAS in a gravity belt thickener to 5 to 6 percent solids concentration. The Morehead ATAD facility also experienced ATAD operating temperatures drop to below 51 degrees C (120 degrees F) when co-settled primary and WAS material to only 3 percent solids concentration. The Morehead WWTP staff has decided to install a gravity belt thickener to increase the feed solids concentration and address the operating issue.

The “Second Generation” ATAD systems have the following overall advantages over the original “First Generation” ATAD processes:

 A longer solids retention time (12 to 14 days) compared to the “First Generation” ATAD process of 5 to 8 days. This allows for better stability and volatile solids reduction, especially for the more difficult to digest secondary waste activated sludge (WAS) feed material.  Jet aeration that thoroughly mixes the ATAD process tanks “from the bottom up” and keeps aerobic conditions rather than aspirating or pumped-venturi systems that tend to create anaerobic and high odor conditions at times. The mixing system also uses conventional out-of-basin pumps and compressors with variable-speed drives so that the mixing energy can be varied to match the actual mixing energy required (See “Second Generation” Delphos, OH equipment gallery photo in Figure 2). Figure 2 - Delphos, OH “Second Generation” ATAD Facility Equipment Gallery Photo Showing Mixing Pumps with Variable Speed Drives

 Use of a single reactor with oxidation-reduction potential (ORP) control that matches oxygen supply to variable process demands and provides a more stable and complete digestion process with minimal odor generation.

 A unique and patented foam Splashcone™ system that controls foam with hydraulic energy and has no internal moving parts to maintain (see figure 3). The Splashcone is a cone that is suspended from the ATAD roof. Biosolids are recirculated from the ATAD reactor and piped to the Splashcones™. The cone disperses the biosolids around it, and the biosolids “beat down” the foam layer, controlling foam depth.

Figure 3 - Splashcones™ For Foam Control on “Second Generation” ATAD Facility

 A patented post digestion nitrification/denitrification reactor (SNDR) that provides optimum temperature, pH, alkalinity, and aeration conditions for nitrification and denitrification of processed ATAD biosolids. The cooled biosolids are sprayed down into the tank headspace, losing heat and dissolving ammonia from the air in the headspace back to the tank liquid contents. The SNDR decreases ammonia and soluble COD while reducing the overall dewatering chemical and polymer consumption. The SNDR has been shown to reduce ammonia concentrations from 800 to 1,500 parts per million (ppm) to 200 to 300 ppm or less, which can minimize sidestream impact on the secondary treatment process. pH set points are used to track and control the nitrification and denitrification process by creating an aerobic or anoxic condition, as warranted. In addition, the SNDR provides additional detention time that improves solids reduction. The use of a SNDR has shown to improve biosolids dewatering in two ways. First, the ammonia reduction improves the effectiveness of polymer. Polymer dose is affected by the ratio of monovalent cations to divalent (or multivalent) cations, with divalent cations being more desirable. By reducing the concentration of monovalent cations by denitrification, the water chemistry for polymer effectiveness is improved. Secondly, the reduction of COD in the SNDR can potentially eliminate the need for the use of a metal salt coagulation in the dewatering step. It has demonstrated that thermophilic digested biosolids have high polymer demand for effective dewatering. These high polymer doses seem to correlate with the biopolymers lysed from the cells during thermophilic digestion. Biopolymer concentration in biosolids correlates well with the soluble COD concentration. To reduce polymer dose, the biopolymers have to be reduced or coagulated. Iron or aluminum salts often are used to coagulate the biopolymers to reduce polymer dose for dewatering. The SNDR reduces the soluble COD by 65 percent thereby reducing the biopolymers in the digested biosolids, and reducing the required polymer dose. The SNDR can also act as part of the odor control system if the off gas from the ATAD reactor is piped to the SNDR headspace. The off-gas is directed to the spray from the Splashcones™ that provide up to a 50 percent reduction in ammonia and humidify the off gas for further treatment in the biofilter.

 A combined water scrubber and two-stage inorganic/organic biofilter BiofiltAer™ odor control system (see Marshall, MN odor control system photo in Figure 4). High ammonia concentration, sometimes created during upset conditions if not significantly reduced in the SNDR, can be further reduced in the water scrubber. The remaining off-gas, typically no higher in concentration than 300 ppm ammonia is then sent to the biofilter. The biofilter airflow distribution, temperature, humidity and pH are controlled by controlling various set points such as influent velocity, ammonia scrubber saturation and air temperature, and the washing of the media periodically with plant effluent water. The two-stage biofilter with both inorganic and organic media removes primarily hydrogen sulfide and other reduced sulfur compounds.

Figure 4 – BiofiltAer™ Odor Control System Showing Water Scrubber and Two-Stage Biofilter at the “Second Generation” ATAD Facility at Marshall, MN CONCLUSIONS

The benefits of the ATAD process vary depending on the size, layout, and configuration of the WWTP. A site-specific WWTP evaluation is recommended that includes both biosolids stabilization and reuse/disposal issues and costs. Several of the projects that have been retrofitted with the “Second Generation” process have reported a 60 to 70 percent reduction in biosolids product hauled from the WWTP site due to both greater mass destruction and drier cake solids concentration (less water) as compared to conventional aerobic and anaerobic digestion processes. The “Second Generation” ATAD systems has proven to be an effective biosolids conditioning process, producing total solids reductions as high as 60 percent and reliably producing exceptional quality biosolids products.

The ATAD process has been historically most applicable to small- to medium-sized WWTPs. However, the “Second Generation” ATAD process has been installed in WWTPs up to 1.3 cu m/s (30 MGD) with combined primary and secondary feed solids and up to 4.4 cu m/s (50 MGD) with 100 percent secondary feed solids. ATAD makes sense for many communities that have little available onsite land. A Class “A” product is often necessary to increase access to end user markets. Some of the municipal clients that have installed either the ”First Generation” and “Second Generation” ATAD systems give away the biosolids product as topsoil and others sell the product to topsoil producers and farmers. In a time when stricter regulations, growth, and higher unit costs are making biosolids disposal/beneficial reuse more difficult, ATAD should be considered.

ACKNOWLEDGEMENTS

We would like to acknowledge James R. Kirk, Grand Chute-Menasha West Sewerage Commission, Neenah, Wisconsin and Parker Newbanks and Candance Burbridge, Eagle River Water and Sanitation District, Vail, Colorado, for collection of the “First Generation” ATAD performance data from their respective WWTPs. I would also like to acknowledge Jim Scission for the summary of data on “Second Generation” ATAD facilities that is included as Table 2.

REFERENCES

Scisson, J. (2009) As Good as the Hype: An Overview of the Second Generation ATAD Performance. Proc. WEF Residuals and Biosolids Specialty Conference 2009, Water Environment Federation, May 3-6, Portland, OR, USA.