Sustainable Supplemental Carbon Sources

Transcription

1 Hunter Long*, Katya Bilyk, Wendell Khunjar Hazen and Sawyer Charles Bott, Bill Balzer, Jeff Nicholson HRSD Steven Chiesa Santa Clara University James Grandstaff Henrico County Water Reclamation Facility Jared Alder - OpenCel *hlong@hazenandsawyer.com Abstract In order to meet strict nutrient limits, wastewater treatment facilities with low influent COD:N may require supplemental carbon for denitrification, resulting in chemical costs of approximately $0.60-$1.00 per pound nitrate removed for a methanol supplemental carbon product. In efforts to reduce chemical costs, two wastewater treatment facilities in Virginia are investigating alternate methods for on-site production of supplemental carbon. The Nansemond Treatment Plant (NTP) is evaluating co-fermentation of primary sludge (PS) with truck hauled grease trap waste (GTW) to produce VFAs and soluble COD as a supplemental carbon source. The Henrico County Water Reclamation Facility (HCWRF) is evaluating the use of focused pulse treated (OpenCel TM ) thickened waste activated sludge (TWAS) as a carbon source for denitrification. OpenCel TM treated TWAS purportedly achieves soluble COD production comparable to fermentation but without the associated odor potential and operational challenges of running a biological process. Nansemond Treatment Plant Co-Fermentation Pilot The NTP located in Suffolk, VA and operated by the Hampton Roads Sanitation District (HRSD), discharges treated effluent into a tributary of the Chesapeake Bay. This 30 mgd facility currently has average annual total phosphorus and total nitrogen goals of 1.0 mg/l and 5.0 mg/l, respectively. More stringent effluent nutrient concentration limits are anticipated within the next five years. The NTP s primary effluent contains insufficient levels of readily biodegradable organic carbon to meet its biological nutrient removal targets without supplemental carbon addition. The plant currently purchases concentrated waste methanol to assist with denitrification in its modified five-stage BNR process system at approximately $500K per year in chemical cost. To potentially provide additional nutrient removal capacity and reduce the net cost of supplemental organic carbon addition, a pilot-scale investigation was conducted to examine the ability of blended settled primary solids and locally available grease trap waste to be co-fermented to produce a volatile fatty acid (VFA) -enriched product stream to enhance phosphorus and nitrogen removal. The plant currently receives, on average, 5,200 gpd of grease trap waste from local haulers and can receive additional grease trap waste by diverting such flows from other HRSD treatment facilities. This material has the high C:N and C:P ratios desired in a supplemental carbon source and a significant daily organic (COD) loading that makes it an attractive option for replacing a fraction of the purchased chemical currently used to augment nutrient removal. Methodology The NTP co-fermentation pilot design consists of two fermenter reactor trains; Train 1 ferments PS only and Train 2 ferments a combination of GTW and PS (Error! Reference source not found.). The fermenters are 300 gallon, completely impeller mixed rectangular intermediate bulk containers. Each fermenter overflows by gravity into a separate 30-inch diameter, 90-gallon conical induction tank acting as a gravity thickener and solids-liquid phase separator. Each thickener has a recycle pump that returns solids to the fermenter at gpd (1-2 x influent flow). The gravity thickeners overflow into mixed sampling buckets from which a composite sample is automatically collected. Sludge is wasted directly from the fermenter to control SRT and the solids in the thickeners are not included in the SRT calculation. Additionally the co-fermentation pilot system contains a short detention time blending/mixing tank which

2 overflows into the fermenter. This configuration was an attempt to address the many problems historically plaguing attempts to acidogenically ferment lipid-rich materials by allowing for separate control of hydraulic retention time and solids retention time, the use of a small input stream of anaerobically digested sludge from the NTP digesters for lipid pre-adsorption and pre-hydrolysis, and the ability to provide a well-mixed, continuously fed acid phase bioreactor with headspace gas evacuation for better hydrogen gas partial pressure control. To minimize methane production and potential full-scale system chemical costs, fermenter ph and temperature levels were left uncontrolled. The resulting acidic, ambient temperature growth environment was chosen in an attempt to selectively promote the growth of syntrophic, hydrogenotrophic bacteria for maximum VFA production. Elutriation water was added to the thickener to increase the VFA capture efficiency and moderate the system oxidation reduction potential (ORP) as needed. The two pilot systems were instrumented for flow rate, ph, temperature, and ORP at key locations and operated continuously to provide steady-state operation at each set of pre-established operating conditions. Hampton Roads Sanitation District Nansemond Plant Trap Grease Study Phase II: Process Flow Diagram Tanks Primary Sludge Treatment Train ID Name Operating Vol., gal TK-102 Grease Trap Waste Feed TK-103 Ana. Dig. Sludge Feed 5-50 TK-104 Blend Tank 2.5 TK-105 PS Fermenter 300 TK-106 GTW Fermenter 300 TK-107 PS Thickener 90 TK-108 GTW Thickener 90 TK-109 PS Fermentate Sample 3.5 TK-110 GTW Fermentate Sample 3.5 Primary Sludge Transfer Line Potable Water FI FI FI FI TK 105 Fermenter P-07 Timered TK 107 Gravity Thickener Composite Sampler Water Fermentate GTW Solids Multi Basket Strainer P-01 Waste P-05 Solids TK 109 Fermentate Sample Grease Trap Waste Treatment Train Ball Valve Strainer Ball Valve Peristaltic Pump Air Blower FI Rotameter Electric Motor Heat Exchanger TK 102 GTW Feed P-02 Timered TK 103 P-04 AD SludgeTimered Tk-104 TK 106 Fermenter P-08 Timered TK 108 Gravity Thickener Composite Sampler TK 110 Fermentate Sample P-03 Timered Waste P-06 Solids Figure 1 Both pilot systems were seeded with acid phase digester sludge from a nearby two-phase anaerobic digestion system. The initial phase of the pilot involved running both reactors without grease trap waste addition and confirmed the performance equivalency of the two trains. Operation of the co-fermentation unit operation was then adjusted to include grease trap waste. The particulate COD (pcod) conversion efficiency to scod was calculated using a waste streams total COD (tcod) and scod using Equation 1:

3 Equation 1 - % pcod Solubilization % #$ #$%&#&'()&"* =! #! #! #! (! #!! #! ) =! #! #! #! (! #! ) The denominator in this equation represents the mass feed rate of pcod into the system. calculation represents the difference between measured tcod and scod for a given sample. This To estimate the effective degree of pcod solubilization to scod products for the grease trap waste, an incremental yield analysis was employed using the control PS system as a baseline for comparison. The incremental yield represented the amount of additional scod produced per unit amount of additional pcod added with the GTW system. Inflows of individual COD fractions were quantified using contributions from the primary sludge feed, and any grease trap waste and anaerobic digester sludge added to the particular system. scod outflows were determined using contributions from both the elutriated system effluent stream and the waste sludge stream. After a 25 day digestion period, the minor contributions of pcod from the anaerobic digester sludge was considered biologically unavailable for solubilization and were excluded from calculations. The incremental scod yield for the GTW addition was calculated by Equation 2. Equation 2 - % #$%&%"'() "#$%#" # #$% =! #! #! #! #! #! #! #!!!! #! #! #! The two fermenter reactors and blend tank were operated as shown in Table 1 and Table 2 with one of the reactors being fed only PS and the other reactor being fed a mixture of PS, GTW and a small amount of ADS. Table 1 - Fermenter Operating Parameters Phase SRT Duration Temperature Train 1 (PS Only) Train 2 (PS + GTW) PS OLR GTW OLR PS OLR GTW OLR (days) (days) ( C) (kg (kg (kg (kg COD/m 3 /d) COD/m 3 /d) COD/m 3 /d) COD/m 3 /d) Phase Average HRT (days) Table 2 - Blend Tank Operating Parameters Temperature ( C) PS (gpd) GTW (gpd) ADS (gpd) GTW:ADS : :10

4 The PS and GTW feed characteristics are summarized in Table 3. Table 3 - PS and GTW Feed Characteristics PS GTW Parameter Avg. Std. Dev. Avg. Std. Dev. ph TS, % VS, % tcod, mg/l 11, ,900 44,700 scod, mg/l ,500 2,280 Temperature, C 18 to 27 C 37 2 TKN, mg/l NH 3, mg/l TP, mg/l OPO 4, mg/l A regular sampling schedule for constituents including key COD concentrations, solids levels, soluble product concentrations, and inorganic nutrient concentrations was used to assess system performance and identify steady state conditions. The majority of the analytical tests; TS/VS, TSS/VSS, TKN, NH 3, TP, OPO 4, tcod, scod and VFA by distillation were performed according to the standard methods (APHA et al., 1989). VFAs were also measured by GC-FID and LCFA was measured using Fatty Acid Methyl Ester Analysis by Capillary Column GC/FID method. Results Figure 2 provides a summary of fermentation performance for fermenter Train 1 (T1) and Train 2 (T2). T1 was fed only PS while T2 was fed a combination of PS, GTW and ADS during phases 3 and 4. The total effluent scod, total effluent VFA and increase in scod and VFA across the system are higher in the GTW fed fermenter T1- Phase 1 T2- Phase 1 T1- Phase 2 T2- Phase 2 T1- Phase T2- Phase ph Effluent scod (lbs/day) scod (lbs/day) Effluent VFA (lbs/day) as COD VFA (lbs/day) as COD T1- Phase 4 T2- Phase 4 Figure 2 - Summary of Fermentation Performance

5 Figure 3 demonstrates that the pcod conversion (calculated from Equation 1) has averaged around 7-10% over the course of the pilot, with the highest conversion rates (>10%) coming during the longer SRT operation (Phase 1). 16% pcod Conversion (%) 11% 6% 1% 3/18/2013 4/8/2013 4/29/2013 5/20/2013 6/10/2013 7/1/2013 7/22/2013-4% Figure 3 - pcod Conversion to scod Figure 4 displays the incremental GTW pcod conversion to scod (calculated by Equation 2). The incremental GTW pcod conversion appears to have shown an increase during Phase 4 compared to Phase 3. During Phase 4 the blend tank (prior to the fermenter) was operated with a longer HRT, a higher ADS:GTW blend ratio, and without PS, resulting in a warmer, higher ph environment that may be more suitable for GTW pretreatment. A negative number indicates periods when the PS fermenter was producing more scod than the GTW fermenter. Incremental GTW pcod Conversion (%) 20% 15% 10% 5% - 10% - 15% Discussion T1 - pcod Conversion T2 - pcod Conversion Phase 1 Phase 2 Phase 3 Phase 4 0% 4/16/2013 5/6/2013 5/26/2013 6/15/2013 7/5/2013 7/25/2013 8/14/2013-5% Incremental GTW Conversion Phase 3 Phase 4 Figure 4 - Incremental GTW pcod Conversion to scod

6 Primary sludge fermentation has been well documented in the literature and this pilot has been able to confirm the expected 8-10% conversion of pcod to scod. GTW fermentation however has not been well documented in the literature. GTW has been demonstrated to significantly boost digester gas production when co-digested with municipal sewage sludge (Long et al., 2011). However, previous studies have also indicated that at low SRT conditions (<10 days) β-oxidation of LCFA is the rate limiting step (Masse et al., 2002; Miron et al., 2000). It is expected that LCFA β-oxidation is the limiting process in the current GTW fermentation pilot, however a 10 day SRT may result in significant methane production, which is undesirable for the fermentation process. Other methods, such as increasing the ratio of ADS:GTW in the blend tank are currently being investigated to increase the rate of β-oxidation and thus increase the incremental GTW conversion rate. Conclusions Primary sludge fermentation is a proven process to produce VFAs and soluble COD for denitrification or biological phosphorus removal. The Nansemond Treatment Plant fermentation pilot has confirmed the ability to consistently achieve 7-10% scod yields in a primary sludge fermenter over a range of SRTs and temperatures. GTW has been demonstrated to have similar scod yields as PS. Furthermore a fermentation pretreatment in which the GTW is pre-fermented with a significant quantity of ADS appears to have improved the conversion of GTW pcod to scod and VFAs. Detailed LCFA analysis is currently being conducted to further evaluate the effect of pretreatment on the GTW fermentation process. Henrico County Water Reclamation Facility OpenCel Pilot The second pilot study is at the Henrico County (Virginia) Water Reclamation Facility (HCWRF). The HCWRF is a 75 mgd facility which has strict effluent discharge requirements related to the Chesapeake Bay restoration efforts which require denitrification to achieve the necessary total nitrogen limits. For the specific characteristics of the wastewater at this facility, this process requires a supplemental carbon source to drive the denitrification reaction. Current operations utilize a glycerin product for this application but there is a potential to use the OpenCel focused pulse (FP) pretreatment technology on thickened and un-digested WAS to release bioavailable carbon from the WAS. A portion of TWAS is pumped through the OpenCel process where the TWAS is subject to a focused electric pulse causing the release of bioavailable carbon from the activated sludge cells. This released carbon source would then be introduced to the pre-anoxic zones of the biological treatment process to replace the chemical (glycerin) addition and potentially result in a cost savings. Methodology The objectives of the HCWRF OpenCel pilot are as follows: 1. Determine chemical and physical characteristics of TWAS before and after FP treatment. 2. Characterize the short-term impact of FP treated TWAS on denitrification activity. 3. Characterize the long-term impact of OpenCel treated TWAS on denitrification activity. 4. Characterize the impact of FP treated TWAS addition on nutrient removal performance at HCWRF. Objectives 1 and 2 have been completed and are summarized below and objectives 3 and 4 are ongoing. The long-term impacts of FP treated TWAS on denitrification will be determined using sequencing batch reactors (SBRs) at the HRSD lab. The impact of FP treated TWAS addition on nutrient removal performance at HCWRF will be tested in the spring of 2014.

7 During the pilot, HCWRF will operate two parallel basins to allow for direct comparison of performance results. Figure 5 shows the differences between the testing basin and the control basin, of which the main difference is the addition of the FP treated TWAS into the pre-anoxic zone. The ability to add glycerin to the pre-anoxic zone will be maintained to ensure that the plant can meet its effluent TN goals while also verifying that the OpenCel effluent is being utilized for denitrification. There is also expected to be some acclimation of the biomass to the FP treated TWAS stream before it can be fully utilized. This acclimation period is planned to help prevent significant process upsets, and potential permit violations, during the early period of the pilot test operation. The intent of the testing, and any potential full-scale implementation, is not to completely replace an external supplemental carbon source for the facility as it is anticipated that glycerin would still be required in the post-anoxic zone. The location of the post-anoxic zone, downstream from any significant aeration capacity, does not lend itself to the use of the FP treated TWAS due to the potential for nitrogen and phosphorus to be released through the pretreatment process. Without sufficient aeration downstream of the addition point, these constituents have the potential to pass through the biological reactor untreated, thus impacting the total nitrogen and phosphorus content in the plant effluent. Test Basin PE Treated TWAS Indicates profile point Glycerin RAS TB1 TB2 TB3 TB4 TB5 TB6 TB7 Glycerin storage Glycerin storage Control Basin Glycerin CB1 CB2 CB3 CB4 CB5 CB6 CB7 Glycerin storage Figure 5 - HCWRF Pilot Test Configuration Results Two TWAS samples were collected from the HCWRF and sent to the lab for analysis before the OpenCel unit was located onsite. Table 4 provides a summary of the TWAS characteristics before and after OpenCel focused pulse treatment. The TWAS Semi Soluble COD (soluble and colloidal COD) showed an almost four-fold increase after treatment demonstrating a yield of nearly 0.09 mg sscod/mg TS. This yield is comparable to the NTP pilot fermenter yield, however the present configuration of the OpenCel

8 process does not include a solids separation step such as the gravity thickener in the fermentation process and the FP treated TWAS would return more solids to the BNR process than the co-fermentation process. However, initial full scale OpenCel testing at HCWRF resulted in a yield of 0.01 mg sscod/mgts. Table 1The system is currently being optimized to achieve a higher soluble COD yield. Table 4 - Focused Pulse Treated TWAS Batch denitrification tests were conducted to characterize the short-term impact of FP treated TWAS on denitrification activity. Figure 6 demonstrates that there is no significant inhibition to short-term denitrification rates. Additionally, this result demonstrates that the cells can immediately use the carbon generated onsite for denitrification, thereby reducing the need for a lengthy acclimation period. 1.6 Specific denitro rate (mg NO 3 -N/g VSS day) Ctrl TWAS FP TWAS Glycerol Ctrl centrate FP centrate Figure 6 - Specific Denitrification Rate Discussion

9 FP treated TWAS has demonstrated high yield (0.09 mg sscod/mgts) in the laboratory and has shown impressive denitrification rates in batch testing. However, the pilot testing has revealed a few important considerations for applying focused pulse treated TWAS to the first anoxic zone at the HCWRF. The FP-TWAS will contain a significant amount of TSS and will increase the MLSS in the BNR system if the WAS rate is constant. In order to maintain a consistent MLSS concentration the WAS rate must be increased which would result in an effective decrease in the system SRT. Wastewater process simulations using the BioWin process model have indicated that the increased WAS rate would not have a negative effect on nitrification or denitrification if OpenCel is not inactivating biomass and the recycled TWAS can act as a bioaugmentation. However, if OpenCel is completely inactivating the biomass then the lower SRTs associated with the higher WAS rate may affect the ability to nitrify, particularly during cold weather. Conclusions The potential benefit of FP treated TWAS will be weighed against the potential impacts to the treatment process and its ability to meet the strict total nitrogen limitations. This application has the potential for significant process impacts if not implemented correctly due to the return of additional solids to the biological treatment process and the possibility to increase the nitrogen and phosphorus loading to the treatment facility. The pilot testing monitoring plan includes an assessment of these specific considerations to ensure that there are no adverse impacts on the final effluent from the facility. The final evaluation will include an economic assessment of the potential benefits, mainly in the form of offset supplemental carbon costs, as compared to the capital and operational costs of the OpenCel process. Both PS and GTW cofermentation and FP treated TWAS have the potential to offset a portion of the respective facility s supplemental carbon demand, significantly reducing chemical costs. There are ongoing efforts to optimize the operation of both pilots; increasing the yield and reducing the negative side effects (solids return, NH3 and TP return). References APHA, AWWA, WPCF, Standard Methods for the Examination of Water & Wastewater, 17 th ed. American Public Health Association, Washington D.C. Long, J.H., Aziz, T.N., de los Reyes III, F.L., Ducoste, J.J. (2011). Anaerobic Co-digestion of Fat, Oil, and Grease (FOG): A Review of Gas Production and Process Limitations. Process Safety and Environmental Protection. 90 (3), Masse, L., Masse, D.I., Kennedy, K.J., Chou, S.P. (2002). Neutral Fat Hydrolysis and Long Chain Fatty Acid Oxidation During Anaerobic Digestion of Slaughterhouse Wastewater. Biotechnol. Bioeng. 79 (1): Miron, Y., Zeeman, G., Van Lier, J.B., Lettinga, G (2002). The Role of Sludge Retention Time in the Hydrolysis and Acidification of Lipids, Carbohydrates and Proteins During Digestion of Primary Sludge in CSTR Systems. Water Research. 34 (5):