Doing More With Less – Leveraging Existing Assets to Manage Capital and Operating Costs
Last Modified: Jun 06, 2018
- Phill Yi, Wendell Khunjar, Ron Taylor, Janice Carroll, Paul Pitt, Sarah Lothman, Mike Latham, Michael R. Rumke, Rick Zaepfel
The Broad Run Water Reclamation Facility (BRWRF), owned and operated by Loudoun Water, is an 11 mgd advanced treatment facility that employs a 5-Stage biological nutrient removal (BNR) process, followed by membrane filtration and granular activated carbon (GAC) to meet monthly effluent concentration limits. The BRWRF must meet stringent limits such as, a COD limit of 10 mg/L and TKN limit of 1 mg/L as it is close in proximity to a downstream raw water intake for a drinking water treatment plant. The BRWRF must also comply with annual nutrient waste load allocations of 134,005 lbs total nitrogen (TN)/year (4 mg TN/L at 11 mgd) and 3,350 lbs TP/year (0.1 mg TP/L at 11 mgd). The BRWRF also provides reclaimed water for cooling tower purposes.
By 2014, the BRWRF was operating at approximately at 50% and 75% of the of the installed liquid and solids design capacities, respectively. Due to rapid and continued growth in the service area, Loudoun Water was faced with liquids and solids capacity limitations and needed to consider expansion of the overall treatment facility. Loudoun Water commissioned a master plan to identify the best path forward for the BRWRF. A key aspect of the master plan involved identifying opportunities that would allow Loudoun Water to maximize the capacity and performance of the existing infrastructure to reduce capital and operating cost investments while still meeting stringent effluent limits. This presentation will document the manner in which Loudoun Water successfully utilized chemically enhanced primary treatment and optimization of methanol addition to offset the need to build an additional bioreactor basin (deferred capital cost of $17 million). Additionally, we will discuss how Loudoun Water utilize rapid small-scale column testing to re-rate existing granular activated carbon contactors and defer the need for constructing new columns (deferred capital cost of $7 million).
APPROACH FOR LEVERAGING EXISTING ASSETS
A combination of process modeling, computational fluid dynamics modeling, bench and full-scale testing was used to validate capacity and performance for the unit processes at the BRWRF. Financial modeling was also performed to better understand how changes to capital and operating cost expenditures would impact overall project costs and rate payer costs.
Maximization of Nitrogen Removal in BRBs
The first phase of this work focused on identifying the maximum nitrogen removal that could be achieved in BRBs. Results from Phase 1 (Figure 1) demonstrated that the existing BRB configuration can be operated to help the BRWRF achieve a final effluent of approximately 2.0 mg/L without the need for enhanced tertiary treatment processes, assuming complete ammonia conversion to oxidized nitrogen (nitrate and nitrite, NOx) before the post-anoxic zone and maintained or improved removal of dissolved organic nitrogen across GAC.
Phase 2 was performed to demonstrate optimized methanol dosing strategies for cost effective nitrogen removal. In this phase, full-scale demonstration of flow versus nitrate-paced methanol dosing to the post-anoxic zone for denitrification was performed and resulted in approximately 10% savings in total methanol costs at the BRWRF as presented in Figure 2. Additionally, operating in nitrate-paced mode was demonstrated to allow for tighter control of nitrate and total nitrogen concentrations versus flow placed mode. This minimized periods of over and underfeeding of carbon to the BRBs and improved the consistency and reliability of nitrogen removal in the BRBs which is critical for the BRWRF.
Optimized Chemically Enhanced Primary Treatment
The BRWRF currently practices chemically enhanced primary treatment (CEPT) with the addition of aluminum sulfate (alum). Optimized CEPT was identified as a strategy for unlocking BRB and aeration system capacity at the BRWRF in such a manner that Loudoun Water could potentially defer construction of new BRBs in the immediate expansion. Bench-scale CEPT jar testing was performed to evaluate CEPT performance of alum, polyaluminum chloride (PACl), in the presence and absence of a polymer aid. Bench scale testing indicated that CEPT with alum (10 mg Al/L) or PACl (15 mg/L) could achieve TSS removals greater than 75%. Polymer addition did not improve CEPT performance. Results from bench-scale testing combined with unique PACl pricing initially suggested that Loudoun Water would potentially be able to reduce operating costs by switching to PACl. However, full-scale testing with PACl demonstrated that alum addition was superior to PACl at the BRWRF with respect to TSS, organic carbon and phosphorus removal (Figure 3).
Rapid Small-Scale Column Tests for GAC
The BRWRF relies on GAC treatment to meet weekly average and monthly average effluent COD limits of 15 mg/L and 10 mg/L, respectively. GAC treatment also removes approximately 50% of the TKN concentration remaining in membrane permeate, thus helping to maintain an effluent TKN concentration below the 1.0 mg/L monthly average limit (1.5 mg/l weekly average limit). The BRWRF Master Plan identified that Loudoun Water could potentially avoid the construction of new GAC contactors if the minimum acceptable empty bed contact time (EBCT) were reduced from 20 minutes to 15 minutes. Rapid small-scale column tests (RSSCT) were conducted and confirmed that the existing GAC treatment process at the BRWRF can achieve the same performance at a minimum acceptable EBCT of 15 minutes in terms of contaminant removal via sorption as presented in Figure 4. This means that the no new contactors would need to be constructed if the GAC contactors are operated in sorption mode.
SIGNIFICANCE OF WORK
This work documents a proven approach for identifying and validating practical strategies to maximize capacity, defer capital costs, and reduce operating costs. Strategies reviewed in this work will allow utility managers to see how practical testing approaches can be used to more effectively and efficiently operate a facility as well as defer some capital expenditure for expansion. At the BRWRF, this approach resulted in the deferment of $24 million dollars associated with a capacity expansion from 11 to 16.5 mgd. A ten percent reduction in methanol operating costs was also demonstrated.
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