Design Considerations for a Multiple-Reactor DEMON Process Treating Sludge Liquors from a Thermal Hydrolysis Anaerobic Digester

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1 Design Considerations for a Multiple-Reactor DEMON Process Treating Sludge Liquors from a Thermal Hydrolysis Anaerobic Digester Andrew Shaw, Black & Veatch Peter Thomson, Black & Veatch Beverley Stinson, AECOM Salil Kharkar, DC Water Sudhir Murthy, DC Water Chris debarbadillo, DC Water Nick Passarelli, DC Water Bernhard Wett, ARA Consult Geert Nyhuis, Cyklar-Stulz Blair Wisdom, Black & Veatch Black & Veatch 8400 Ward Parkway Kansas City, MO (913) ABSTRACT The DEMON treatment process is rapidly becoming an established technology for sidestream treatment due to its low energy use and no requirement for carbon addition to remove nitrogen completely through deammonification. A new filtrate treatment facility (FTF) based on the DEMON process and currently under design for the Blue Plains Advanced Wastewater Treatment Plant (AWTP) has several unique design aspects and treatment challenges due to its large scale and the fact that it will treat liquors from a digestion system that includes the CAMBI thermal hydrolysis process. This paper outlines the design, highlighting the unique features and the approach taken to overcome design constraints to meet the desired treatment performance, including consideration of: (1) space constraints, (2) multiple-reactor operation, (3) potential toxicity, (4) high temperatures, and (5) the aeration/mixing system. Design of the Blue Plains FTF is due to be completed in KEYWORDS Sidestream treatment, deammonification, DEMON, sequencing batch reactor, SBR, thermal hydrolysis INTRODUCTION The DEMON treatment process is an emerging technology that is gaining rapid acceptance as an energy-efficient, nitrogen removal option for high-strength nitrogen wastes such as sludge liquors from dewatering following anaerobic digestion. Since its initial full-scale development at the Strass WWTP in 2004 there are already more than 12 DEMON plants in operation (Table 1) and several more under construction around the world.

2 Table 1: DEMON Plants in Operation Location Loading kg N / d Strass (AT) Glarnerland (CH) Plettenberg (DE) Thun (CH) Gengenbach (DE) Heidelberg (DE) Etappi Oy (FI) 1, Balingen (DE) Apeldoorn (NL) 1, Limmattal (CH) Zalaegerszeg (HU) Alltech (Serbia) 2, Commissioned Year The DEMON process makes use of ammonia oxidizing bacteria (AOB) and ANaerobic AMMonia OXidizing bacteria (Anammox) that, when compared to conventional nitrification/denitrification processes, require less than half of the oxygen, no carbon addition and, typically, no supplemental alkalinity. Neethling (2012) describes the considerable benefits of using processes utilizing anammox bacteria and compares the different process configurations based on anammox including ANAMMOX, DEMON and AnitaMox. Table 2 lists typical performance ranges specifically for the DEMON treatment process for high strength ammonia wastes such as digested sludge liquors. Table 2: Typical Performance for DEMON Performance Parameter Units Typical Values Mass Loading Rates kg N/m 3 /day Nitrogen Removal % ~90% NH 3 -N Energy Use kw hrs/ kg NH 3 -N removed ~85% TN BLUE PLAINS DEMON DESIGN A new filtrate treatment facility (FTF) based on the DEMON process and currently under design for the Blue Plains AWTP has several unique design aspects and treatment challenges due to its large scale and the fact that it will treat liquors from a digestion system that includes the CAMBI

thermal hydrolysis process. This paper outlines the design, highlighting the unique features and the approach taken to overcome design constraints to meet the desired treatment performance.

3 thermal hydrolysis process. This paper outlines the design, highlighting the unique features and the approach taken to overcome design constraints to meet the desired treatment performance. Compact Multiple-Reactor Design Figure 1 is an aerial view of the Blue Plans AWTP showing the proposed location for the new DEMPON FTF in orange and the new anaerobic digesters, CAMBI and combined heat and power (CHP) system indicated in green. Figure 1: Aerial Photograph of Blue Plains AWTP Showing Proposed Location of the New DEMON FTF (orange) and New Digesters, CAMBI and CHP (green). Almost all DEMON facilities are single sequencing batch reactors (SBR) or dual reactors which operate autonomously (i.e. they have their own individual blowers and feed pumps). However, the Blue Plains facility will have an estimated filtrate flow of 4,200 m3/d (1.1 mgd) and liquor ammonia-nitrogen concentrations of up to 3000 mg/l to give a loading up to 12,600 kgn/d (27,000 ppd). A plant of this size requires multiple reactors and therefore 6 reactors were selected for Blue Plains. Figure 2 shows the proposed layout of 6 deep filtrate reactors (labeled FR1-6) using rectangular common-wall construction located in the tight space between existing treatment stages shown on Figure 1. A maximum water depth of 7.9 meters (26ft) was selected to make the best use of the available footprint and yet enable conventional blower systems to be used for aeration. Figure 2 also shows the location of potential future filtrate reactors, feed tanks, sedimentation tanks for pre-treatment of liquors to remove solids and phosphorus feed tanks for nutrient addition.

4 Figure 2: Blue Plains FTF Layout Showing 6 SBRs (Also Showing Potential Future SBR positions) Designing for Flexible Multiple-Tank Operation The feed system for a single DEMON reactor is straightforward and usually consists of a feed tank and a single variable speed feed pump. However, a more complex system is required for multiple reactors. For the Blue Plains FTF a feed system was developed which is based on a single pumped flow loop with valves to each reactor, enabling the reactors to fill whenever they call for flow. The flow loop is maintained at a constant pressure using a manometric leg with the elevated top level at atmospheric pressure. This approach replaced an earlier concept which used a pressure-sustaining valve which a HAZOP identified as being a critical single-point of failure for the system. The new system has a better intrinsic reliability. Sizing of the feed system and evaluating the sequencing of the filling and emptying of the reactors was carried out using the spreadsheet shown in Figure 3. The example shown is for SBRs operated as pairs, however other operating configurations were checked to determine the maximum flows possible for the feed system. Unlike typical multi-reactor SBR systems that share common blowers and fill in sequence, the feed design for the Blue Plains FTF SBRs allows each reactor to fill "on demand" and individual reactors have their own aeration system which allows them to operate semi-autonomously. In order to provide this flexibility in feed flow ranges, a detailed system pump curves were developed and is shown on Figure 4.

Sequence Timings Entered in Top-Left Table. Colored Chart at Top Right Shows 5-minute Intervals for SBR Sequencing.

5 The one restriction on the sequencing that was deemed necessary was that no more than two SBRs can decant simultaneously so that the discharge piping would not have to be massively oversized. Figure 3: Spreadsheet Used to Investigate SBR Sequencing. Sequence Timings Entered in Top-Left Table. Colored Chart at Top Right Shows 5-minute Intervals for SBR Sequencing. Graph Shows Feed Flow (blue) and Decant Flow (red). Example Set Up Shows Impact of Operating SBRs in Pairs. Figure 4: Pump Curves Developed to Maintain an Even Flow in the Reactor Feed Line

6 Designing for Potential Toxicity Pilot testing was carried out to test removal rates for sludge liquors from a CAMBI system in order to determine if the DEMON process would be inhibited by them (Figdore, 2011). It was found that significant inhibition did occur, most notably to the AOBs, but that diluting the CAMBI liquors 1:1 with plant water reduced the inhibition to an acceptable level that enabled the DEMON process to remove ammonia at a volumetric loading rate of 0.6 kgn/m3/d. A dilution system using plant water (plant effluent) was included in the FTF design to facilitate this. Designing for High Temperature The temperature of the sludge liquor is expected to be in the range of 30-35ºC, which is an acceptable range for the DEMON process; however the slightly exothermic reactions coupled with high ammonia concentrations may push the reactor temperature above 35ºC. Heat calculations that included ambient conditions, dilution water temperature, blower air impacts and other thermal considerations were carried out to determine the potential maximum temperature for the reactors was 38ºC. This high temperature requires special consideration for the aeration system design and was one of the factors in the decision to use the Invent aeration system rather than membrane diffusers. Table 3 shows an example heat balance output for the reactors with dilution water added in the summer to ensure the reactor temperature does not exceed 38ºC. The most significant energy input is due to biological activity (327,000 MJ/d) and the major contributors to reducing the heat energy are atmospheric cooling, addition of filtrate that is marginally cooler than the reactor and addition of dilution water which is plant water assumed to be at a summer high temperature of 27 ºC. The magnitude of the heat fluxes is such that cooling using other means than dilution (e.g. chillers) was impractical. Table 3: Example Heat Balance Inputs (top) and Calculated Heat Transfer Components (bottom) for Summer Conditions. Note, negative heat added = heat removed from the system. Heat Balance Parameter Value Filtrate Flow 4200 m 3 /d Ambient Air Temperature 27⁰C Filtrate Temperature 35⁰C Dilution Water Temperature 27⁰C Reactor Temperature 38⁰C Required Dilution Water Flow 3400 m 3 /d Heat Transfer Components Total Biological Heat Added to Reactor MJ/d Atmospheric Heat Added to Reactor MJ/d Mechanical Mixing Heat Added to Reactor MJ/d Filtrate Heat Added to Reactor MJ/d Process Air Heat Added to Reactor MJ/d Dilution Water Heat Added to Reactor MJ/d Net Heat Transfer 0 MJ/d

control of the intermittently aerated SBRs; robust performance at high temperatures; lower maintenance costs; ease of installation.

7 Aerator/Mixer Design A detailed assessment of different aeration system options was carried out. Ultimately it was determined that the Invent Mixer/Aerator provided several benefits over a conventional diffused aerator system, including: combined mixing/aeration functionality which gives good control of the intermittently aerated SBRs; robust performance at high temperatures; lower maintenance costs; ease of installation. Figure 5 is a schematic showing the main components of the invent Mixer/Aerator and Figure 6 is a photograph of an example installation. Figure 7 is the proposed layout for four 30kW (40hp) mixer/aerators with provision made for a possible fifth unit if required in the future. In the mixing mode, the units will run at a slow speed, drawing approximately 6.3kW (8.5 hp); in the aeration mode, air will be provided to the ring sparger beneath the mixer/aerator by high-speed gearless turbo blowers and the unit will be run at a higher speed. Figure 5: Invent Mixer/Aerator Schematic (courtesy Invent) Figure 6: Example Installation Photograph for an Invent Mixer/Aerator System (courtesy Invent) Figure 7: Proposed Layout of Invent Mixer/Aerators for the FTF

8 SUMMARY The DEMON process is gaining in popularity and has rapidly become an established process option for sidestream treatment. The DEMON plant proposed for the Blue Plains FTF was the largest of its kind and the first to treat liquors from a CAMBI thermal hydrolysis system when it was conceived. This has resulted in some unique design features including consideration of: 1. Space constraints 2. Multiple-reactor operation 3. Potential toxicity 4. High temperatures 5. Aeration/mixing system Table 3 summarizes the FTF design parameters, noting the constraints and comments described in more detail in the previous sections of this paper. The design is due to be completed in 2013 with construction and commissioning starting soon after. In parallel with the design, pilot testing is being conducted to investigate inhibition and temperature effects using liquors generated from Blue Plains sludge. REFERENCES Figdore, B., Wett, B., Hell, M. and Murthy, S. (2011) Deammonification of Dewatering Sidestream from Thermal Hydrolysis-Mesophilic Anaerobic Digestion Process Proceedings of WEFTEC 2011 Neethling, J.B. (2012) Deammonification Compendium Water Environment Research Foundation Report, December 2012

9 Table 3: Design Parameter Summary Values Constraints/Comments Flow 4200 m 3 /d (1.1 mgd) Design flow estimated from sludge production and limiting ammonia concentration in digesters to 3000 mg/l using dilution of Cambi treated sludge. Reactor Dimensions Number of reactors: 6 Length: m (80 ft) Width: m (60ft) Small-footprint available, therefore deep tanks used. Depth limited to enable normal blowers to be used. Maximum SWD: 7.93 m (26ft) Minimum SWD: 6.71 m (22ft) Loading 0.6 kgn/m 3.d ( lb/d/ft 3 ) Typical loading for DEMON is 1.0 kgn/m 3.d, however toxicity concerns require a reduced loading. Acclamation may enable higher loadings to be achieved than current design values. Further pilot testing to be conducted to check this. Temperature Maximum 38 C Dilution water is plant effluent with a temperature of 27 C in the summer. Cooling of the reactors to 38 C is feasible with a reasonable dilution of Aeration System Feed System Equalization/Dilution Invent Mixer/Aerator 4x30kW (40 hp) Units AOR = 404 kg/h (890 pph) during aeration cycle Max loop flow: 3600 m3/hr (16000 gpm) Minimum loop flow: 110 m3/hr (470 gpm) Dilution range for inhibition 1:1 Dilution range for cooling: 2.5:1 Feed tank volume: (0.58 MG) up to 2.5 times the influent flow. High temperatures and mixing requirements for granular sludge, amongst other factors made diffused aeration less favorable than the Invent system. Required AOR depends on aeration period time and overall SBR sequence timings. Longer cycles = lower AOR but limits the flow of filtrate that can be treated. The need to be able to feed up to 6 SBRs simultaneously over a wide range of dilutions creates a wide range of flows that have to be accommodated by the feed flow loop 2.5 times the influent flow maximum available for temperature control. Flows in excess of this would require multiple decanters in the SBRs to handle the extra flow.

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