Demonstration of a Separate Centrate Deammonification (SCAD) Process at the 26th Ward WWTP
- Wendell Khunjar PhD - Hazen and Sawyer
Sidestream deammonification has been identified as a cost effective strategy for performing nutrient removal with minimal supplemental chemical addition and reduced aeration requirements, as compared to conventional nitrification and denitrification. However, existing infrastructure and the cost of designing and building a sidestream deammonification facility is a challenge that must be navigated.
In this project, a separate centrate deammonification (SCAD) process was piloted at the 26th Ward WWTP in New York City. This SCAD process is different from commercial deammonification processes since the SCAD process does not require use of complex controls to achieve nitrite oxidizing bacteria (NOB) repression. Instead, the process is designed to exploit the natural predisposition of the mixed culture community to produce a mixture of nitrite and nitrate when oxygen is provided, and denitrify to nitrite in the presence of glycerol, in order to facilitate anaerobic ammonia oxidation.
Two main phases of testing were conducted.
Bench-scale – Experiments were performed at Manhattan College to demonstrate proof of principle. These experiments were conducted at bench-scale using sequential batch reactors and centrate from a local treatment plant in NYC. These experiments were focused on demonstrating that the SCAD process can stably remove nitrogen and to inform the pilot operation.
Pilot scale – An existing flocculation tank and plate settler were re-purposed as the pilot reactor (Vol = 1,700 gal) and solids separation device respectively. The system was configured to be operated as a sequential batch reactor with alternating aerobic and anoxic cycles. Total hydraulic retention/cycle time was 48 hours with the aerobic/anoxic phases lasting up to 24 hours each. These retention/cycle times were selected to mimic hydraulic retention times that would be experienced in the full-scale SCAD system. Aeration was provided via a single stage reciprocating air compressor (3 HP, Ingersoll Rand, Edison NJ) and delivered via a membrane disc diffuser (Silver series II, Sanitaire; Harrison, NY) installed at the base of the reactor. Airflow to the system was monitored using a rotameter (Dwyer RMC-103, Michigan City, IN). Total suspended solids (TSS) (Insite M15; Wayne, NJ) and dissolved oxygen (DO) (Insite M10; Wayne, NJ) were monitored using an Insite Model 2000 Process Analyzer (Insite; Wayne, NJ). pH (pHD sc, Hach; Loveland, CO) was also monitored using a Hach sc100 meter (Hach; Loveland, CO).
Ammonia, nitrate, nitrite, orthophosphate, chemical oxygen demand, suspended solids (total and volatile) were analyzed using standard methods (Eaton, Clesceri et al. 1995). Anammox activity tests were performed using protocols outlined elsewhere (Wett 2007; Khunjar 2015). Biomass samples for molecular analyses were centrifuged (8,000 x g for 20 min), supernatant was decanted and the biomass pellets were frozen at -200°C and shipped to Columbia University where quantitative PCR was used to assess the relative abundance of total bacteria, AerAOB, NOB, and AnAOB (Ma, Sundar et al. 2015; Park, Sundar et al. 2015). Gaseous nitrogen oxide emissions were measured in collaboration with Columbia University using online gas analyzers (nitric oxide (NO) – chemiluminescence, CLD-64 Eco Physics, Ann Arbor, MI; nitrous oxide (N2O) – gas-filter correlation, Teledyne API 320E, San Diego, CA;) (Jiang, Khunjar et al. 2015).
Results and Discussion
Proof of Principle Results: The results from the proof-of-principle experiments demonstrate that the SCAD process exploiting the glycerol nitrite lock can achieve consistent TN removals of up to 80% with proper carbon dosing and effective pH and dissolved oxygen control. The process required approximately 50% of the air and 25% of the supplemental carbon required for traditional BNR processes. As the process acclimated to higher nitrite levels due to partial nitritation and denitratation, the amount of glycerol needed was further reduced as a larger portion of the TIN was removed via shortcut denitrification..
Pilot Results: The SCAD pilot was operated from December 2014 to November 2015 with a target volumetric nitrogen loading rate of 0.2 kg N/m3-day (representative of conditions that would be observed in a full-scale SCAD application at the 26th Ward WWTP). Demonstration of the pilot process yielded the following insights into applying advanced nutrient removal technologies at the 26th Ward WWTP:
- The SCAD process was able to achieve 64 ± 13 % removal of total inorganic nitrogen (TIN) at a total nitrogen loading rate of 0.20 kg N/m3-day. This level of nitrogen removal is equivalent to the performance of the conventional SCT system.
- The SCAD process was able to recover rapidly from multiple periods of unstable operation resulting from centrate supply and quality issues. The resiliency of the SCAD process indicates that it is robust enough for application in real world settings where highly variable flow and centrate quality can be expected.
- The SCAD process allowed for significant reduction in aeration (57%), glycerol (90%) and alkalinity (50%) requirements versus conventional SCT operation.
- Nitric Oxide (NO) emissions accounted for 0.034 ± 0.034 % of the NH3-N fed to the SCAD system (corresponding to 0.066 ± 0.065% of the NH3-removed in the SCAD process). Nitrous Oxide (N2O) emissions accounted for 1.6 ± 2.1 % of the NH3-N fed to the system (corresponding to 3.1 ± 4.0 % of the NH3-N removed in the SCAD process).
- Molecular analyses indicated that the anaerobic ammonia oxidizer (AnAOB) population increased from < 0.1% in Nov. 2014 up to 1% in Sept. 2015 indicating a gradual enrichment of AnAOB over time. During this period, the aerobic ammonia oxidizer community comprised between 4 and 8 % of the total bacteria community.
Implementation of a deammonification based technology for sidestream nitrogen treatment at the 26th Ward WWTP would result in significant operational and energy savings as compared to the conventional SCT operation. Even though full-scale implementation of SCAD at the 26th Ward WWTP would require capital investments for infrastructure and operational modifications, savings associated with the reduction in energy and chemical addition would allow for a simple payback of capital investment within four to six years.
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