Steele, Paul, Steele

Steele, Paul

Paul Steele serves as Jacobs' Americas East Regional Market Solutions Leader for Wastewater. He is based out of the Nashville, TN and has roughly 20 years experience planning, modeling and designing water resource recovery facilities. When he's not working on a WRRF, you might find him planning or competing in variuos types of off-road endurance races.

Titles from this speaker

Green Energy, Less Power, Fewer Chemicals, Cleaner Water, Class A Biosolids: A Transformational Program at Chattanooga's MBEC With a Financial Payback

Abstract

Introduction The Moccasin Bend Environmental Campus (MBEC) in Chattanooga treats wastewater from a combined sewer system and currently has capacity of up to 140 million gallons per day (MGD). The liquids train treatment processes include primary clarification, high purity oxygen activated sludge (HPOAS) basins, secondary clarification, and chlorine disinfection. The solids treatment train includes gravity thickening, anaerobic digestion of a small fraction (~10 percent) of the total solids generated through temperature-phased digesters, centrifuge dewatering and post-lime stabilization of all biosolids to produce Class B biosolids. MBEC generates 55 dry tons of biosolids per day that is hauled offsite and land applied. Energy Audit MBEC recently completed a comprehensive Energy Audit to determine ways that both energy consumption and chemical use at the facility could be reduced. The audit evaluated energy and chemical savings from three potential major improvements in addition to several minor ones. The major improvements are based on:

*Installing thermal hydrolysis (THP) to enhance the digestion process and produce more biogas;

*Achieving complete nitrification to avoid very large hypochlorite expenditures due to nitrite lock; and,

*Converting the plant to diffused aeration with blowers and fine bubble diffusers. First, THP was evaluated as an anaerobic digestion enhancement alternative due to limited digester capacity and tight footprint at MBEC. Adding THP to the existing digestion system allows all sludge to be processed through the anaerobic digestion system, cuts biosolids generated in half (~$1.5M annual savings in hauling costs), eliminates lime use (~$570K annual savings in lime costs), produces Class A biosolids and boosts biogas production. The analysis recommended renewable natural gas (RNG) as the most favorable use of the biogas, given the potential for revenue generation. The net biogas available for RNG production is approximately 570 scfm and could account for $4M in annual revenue for the City at current RIN rates. Operational cost savings in the solids train are shown in Figure 1. Nitrification is difficult to achieve under the current MBEC configuration in dry weather conditions due to the short retention time and low pH conditions that are endemic to HPOAS systems. Meeting the 15 mg/l monthly average permit limit is difficult. In addition, the low pH conditions of HPOAS-based treatment can inhibit the growth of nitrite oxidizing bacteria, leading to high concentrations of nitrite in the secondary effluent. Nitrite has a high chlorine demand is responsible for over 50% of the chlorine consumed at MBEC. The alternatives analysis found that the most effective means of meeting MBEC's nitrification permit is by using membrane aerated bioreactor (MABR) technology. Utilizing MABRs avoided nitrite production, as the nitrifiers were fixed on submerged membrane cassettes inside the anoxic zone of the existing system, which has a higher pH, and results in ammonia removal without nitrite formation. Projected effluent quality from the plant SUMO model is shown in Figure 2. In addition, the analysis found that the lowest lifecycle cost option for replacing the oxygen plant at MBEC was to install blowers and fine-bubble diffusers as part of a conversion to aeration. The change to aeration supports the nitrification MABR recommendation, as the MABR reduces the soluble COD entering the aeration zones (reducing demand) and the switch to aeration removes the low pH conditions, reducing the residual nitrite formation. The annual average energy consumption from secondary treatment after the changes will be ~7 million kWh annually, a roughly 65% drop from the nearly 20 million kWh used at present. Conclusion The improvements listed here for MBEC result in very large operational cost savings throughout the treatment plant. The estimated capital cost for the program are $136M. See Figures 3-5 for capital cost estimates for the solids train, liquid train, and the combined program, respectively. The major operational cost savings are estimated as:

*$4M in annual RIN revenue from RNG

*$2.2M annually in reduced sodium hypochlorite use

*$1.5M annually in reduced cake hauling costs

*$900k annually in secondary treatment power costs Note that a $570k reduction in lime addition costs are roughly balanced out by increased polymer use, increased maintenance and a small increase in power in the solids train. These savings total $8.6M annually. The end result of the savings is a rare outcome in the industry — a major capital improvement program with a financial payback. A simple payback analysis has the entire improvement program paying for itself within 15 years. Further, the payback for the improvements is not solely dependent on generating revenue from RIN sales. The estimated simple payback without RIN revenue is 30 years. When the true costs of a 'do nothing' option are also included ($25M to replace the end-of-life oxygen plant, and an alternate solution to meet increasing difficult ammonia limits and sludge disposal challenges) the true payback becomes even shorter. However, this is not a project pursued solely due to a favorable payback. The City will also be more reliably able to meet their NPDES permit, will replace aging assets, and will be generating both cleaner water and Class A biosolids

This paper was presented at the WEFTEC 2024 conference in New Orleans, LA October 5-9.

SpeakerSteele, Paul

Presentation time

08:30:00

08:50:00

Session time

08:30:00

10:00:00

SessionEnergy Conservation: From a Want to a Must

Session number305

Session locationRoom 346

TopicEnergy Production, Conservation, and Management, Facility Operations and Maintenance, Intermediate Level, Resilience, Safety, and Disaster Planning

Author(s)

Steele, Paul, Ohemeng-Ntiamoah, Juliet, Shen, Emma, Constantine, Tim, Johnson, Thomas

Author(s)P.M. Steele1, J. Ohemeng-Ntiamoah1, E. Shen2, T. Constantine3, T. Johnson4

Author affiliation(s)1Jacobs, TN, 2Jacobs, ON, 3Jacobs, MT, 4Jacobs, NC

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159720

Volume / Issue

Content sourceWEFTEC

Word count22

Using Gas-Phase Modeling To Track The Fate Of Pure Oxygen At A Large HPO Facility

Abstract

Introduction
Biological process models for water resource recovery facilities are now standard tools in the industry. Commercially available modeling software at present incorporates aeration calculations into the model to simulate oxygen transfer. However, these simulators offer limited support for simulating oxygen transfer for high-purity oxygen activated sludge (HPOAS) facilities. Often, a design engineer will have to change the composition of the supply gas manually, zone-by-zone, through the treatment basin to reflect the gradual depletion of oxygen in the supply along the length of the treatment basin. Some more recent simulators do allow oxygen gas to transfer zone by zone through the process while reflecting gas transfer via surface aerators, which are significant improvements. However, this model still assumes that 100% of the fed oxygen is either dissolved or vented at the downstream end of treatment basin, and this may not reflect full-scale systems. Most HPOAS plants in the US are housed in structures that are at least 30 years old and perfectly sealed enclosed gas headspaces for these facilities is not a reasonable assumption. The objectives of this paper are to demonstrate the importance of utilizing gas-phase modeling when simulating an HPOAS plant and to demonstrate the benefits of gas-phase modeling with a case-study of a large facility in Tennessee. <
b>Background
The Moccasin Bend Wastewater Treatment Plant (MBWWTP) is located in Chattanooga, TN. The plant treats influent from a combined sewer and is currently capable of treating up to 140 MGD through the primary and secondary treatment systems. Secondary treatment is accomplished through a four-train UNOX HPOAS activated sludge facility, each train with eight cells in series. A biological process model was built in SUMO based on the existing unit processes at the MBWWTP to simulate process performance. The model included the primary clarifiers, equalization basin, UNOX bioreactors, secondary clarifiers, and the solids train. Critically, the model utilized Jacobs' proprietary model units for gas-phase modeling through the UNOX basins. These units allowed the model to replicate the feed pure oxygen from the oxygen plant and the vented oxygen at the downstream end of the UNOX basins. A process flow diagram of the SUMO model is depicted in Figure A.
Methodology
To overcome these weaknesses, a new enclosed CSTR module was developed using the SUMO simulation platform, by Dynamita, that includes an enclosed headspace and separate inputs and outputs for gas and liquid. The enclosed CSTR uses SUMO's embedded aeration calculations and is compared to a typical HPOAS facility schematic in Figure B. These inputs and outputs allow for a design engineer to accurately model the fate of the oxygen gas in an operating HPOAS facility, accounting for real world conditions such as leaks, variation in surface aerators efficiency, and the realities of vent purity control. The MBWWTP process model was calibrated with historical diurnal plant data provided for the month of February 2021 and validated with data from November 2021, following the IWA Good Modeling Practice Guidelines.
Findings
Liquid Phase Model Calibration Model calibration was achieved in both steady-state and with dynamic daily data. Average influent conditions used in the model calibration are shown in Table A. The model calibration process went through several steps, the first of which was approximating the actual efficiency of the installed surface aerators to obtain a rough match between recorded and simulated DO concentrations. Kinetic adjustments were also made to account for nitrification inhibition. Calibration and validation results are shown in Tables B and C, respectively.
Gas Phase Model Calibration For the gas phase, a comparison of the simulated oxygen vent purity data from the downstream end of the UNOX basins with the measured vent purity showed substantial discrepancies. Initially, the SUMO model was set up so that all the oxygen fed into the system was either transferred or vented, which led to a predicted vent purity well over 80%. Vent purities for UNOX 1 and 2 trend around 70% while UNOX trains 3 and 4 were substantially lower. Raw data from the vent purity meters for February 2021 is provided in Figure C. UNOX 3 vent purity data read a constant 21% nearly every day, raising suspicions that the meter was reading atmospheric oxygen conditions. Subsequent investigation showed that the UNOX 3 vent pipe had been clogged for a significant time, implying that the vent gas was using another exit. Further investigation with a lower explosive limit (LEL) sensor identified roof leaks on the seams between UNOX reactor zones. The LEL produced an out-of-range reading when placed on these seams, implying that the open air in that location had an O2 content above 30% (Figure D).
The roof leaks were incorporated into the model to match the lower vent purity data readings. Simulating UNOX 3 required assuming 100% of the exhaust gas was leaked through roof seams and UNOX 4 would have required simulating leaks above 90% of non-transferred oxygen. These values are extreme, and as a result, the model was calibrated against the average vent purity from trains 1 and 2, as those trains appeared to have a significant mass of oxygen reaching the vent. The data for UNOX trains 1 and 2 were still substantially lower than a leak free model would predict so three different gas leaks were added to simulate the leaked gas from UNOX 1 and 2. The Sumo model with the leaks is depicted in Figure E and the mass of oxygen leaking through the roof in the simulation is shown in Table D. The model predicted that approximately 52% of the oxygen fed to the UNOX is lost to the system through roof leaks in the two best performing trains in the plant. The vent purity output from the model is compared to the averaged data from the two trains in Figure F.
Status
The modeling study has been completed with a short-term recommendation that the City seal the roof seams to mitigate the leaks.
Significance
- It is important to understand the mechanics of unit processes included in a process model. Understanding the limitations of existing HPOAS simulators lead to a more realistic unit process model for handling HPOAS facilities. - Models that do not simulate cell to cell gas transfer do not provide a realistic simulation of an HPOAS facility. Similarly, models that assume that all gases in an enclosed headspace are transferred to the subsequent cell are not realistic simulations of aged facilities. - Gas leaks at pure oxygen plants can be very large. The scale of the leaks at MBWWTP are now estimated to be an order of magnitude greater than previously assumed, representing a substantial energy loss.

The objectives of this paper are to demonstrate the importance of utilizing gas-phase modeling when simulating an HPOAS plant and to demonstrate the benefits of gas-phase modeling with a case-study of a large facility in Tennessee.

SpeakerSteele, Paul

Presentation time

15:30:00

15:55:00

Session time

15:30:00

17:00:00

TopicIntermediate Level, Energy Production, Conservation, and Management, Facility Operations and Maintenance, Municipal Wastewater Treatment Design

Author(s)

Steele, Paul

Author(s)Paul Steele1; Juliet Ohemeng-Ntiamoah1; Bruce Johnson2; Tom Johnson3

Author affiliation(s)Jacobs, Nashville, TN1; Jacobs, Denver, CO2; Jacobs, Charlotte, NC3