Gonzalez, Dana

Dana Gonzalez is a Principal Technologist at Carollo Engineers. Prior to her recent transition to Carollo, she was a Treatment Process Engineer at HRSD in southeast Virginia. Dana has led projects focused on nutrient removal, resource recovery, water reuse, solids handling, PFAS fate and transport, system and equipment optimization, design and tuning of process controls, and process modeling. Dana has a bachelor's degree from UCLA, a masters from Virginia Tech, and a PhD from the University of Virginia.

Titles from this speaker

Description: A Beautiful Biosolids Product in the Making: Dewatering Optimization and Aerobic...

A Beautiful Biosolids Product in the Making: Dewatering Optimization and Aerobic Curing of Thermally Hydrolyzed Solids at HRSD's Atlantic Treatment Plant

Abstract

Background In mid-2020 Hampton Roads Sanitation District (HRSD) transitioned to Class A biosolids production via thermal hydrolysis process (THP) using a Cambi Mark II B6-4 unit at the Atlantic Treatment Plant (ATP). THP increased solids capacity by allowing for higher digester loading, increased volatile solids reduction (VSR), and significantly improved final dewatering performance. This additional capacity will allow ATP to receive solids from other HRSD facilities, which will provide the opportunity to shut down multiple hearth incinerators at other plants. In addition, THP is near energy neutral and can be energy positive, especially with the addition of fats, oil, and grease (FOG) receiving at the ATP installation. The B6-4 system relies on steam injection within batch reactors to thermally hydrolyze solids prior to anaerobic digestion. Not only does this process kill pathogens within the solids, but it also reduces the viscosity of feed solids and allows for higher digester loading. Initial startup and seeding of the digesters with DC Water digestate occurred in May 2020; it took approximately six months to startup three anaerobic digesters after disinfection and seeding. Using the same dewatering centrifuges, final dewatering cake solids content increased from the baseline prior to THP (14-16% total solids (TS)) to a range of 20-25% TS with minimal dewatering optimization. Additional optimization at final dewatering resulted in reliable 30% TS final cake solids production without any change in polymer selection. Additional aerobic windrow turning of solids was shown to produce a low-odor product that was 55-60% TS. Purpose This presentation will focus on final dewatering centrifuge optimization that was conducted to reliably produce a 30% cake product. In addition, a series of curing pilots were conducted using the 30% cake and a windrow turner to determine the most efficient method for producing an aerobically cured product with low odor. Data from final dewatering optimization and the curing pilots were used to evaluate cost savings associated with each improvement. Final Dewatering Optimization Initial dewatering optimization focused on evaluating solids loading rates after transitioning to a thicker feed coming out of the THP digesters. After achieving optimal loading rates, staff focused efforts on polymer dosing locations and sludge-polymer mixing upstream of the final dewatering centrifuges. Adding 100% of the polymer upstream of the centrifuge at the sludge-polymer inline mechanical mixers allowed plant staff to minimize the mass of polymer required to get optimal capture. In addition, after testing various sludge-polymer mixing intensities, it was found that the optimal mixer speed was 900 rpm. This mixing intensity achieved the same or better recoveries with an even lower polymer usage rate (Figure 1). After capture optimization was completed, staff focused on cake dryness. To do this, the torque setpoint on the centrifuge was increased progressively and additional polymer was added to maintain equivalent capture. These data were used to compare the cost benefit of polymer usage vs. reduced hauling rates with increased cake dryness (Figure 2). Additional data will be presented to show the dewatering benefits of ferric addition upstream of the primary clarifiers. While plant staff initially added ferric to help minimize reduced sulfur compound odors in the final cake product, it was found that half of the ferric costs could be recouped with polymer savings in final dewatering. Curing Pilots Various iterations of curing pilots were executed to answer two main questions: (1) What are the characteristics of the final cured end-product? and (2) What is the most efficient way to achieve that end-product? Additionally, data collected during pilot studies were used to evaluate the business case for curing THP solids. End-Product Characteristics Figure 3 shows %TS and percent volatile solids (%VS) from several curing pilot experiments in which solids were aerated with a windrow turner at various frequencies, with and without the addition of cured cake to piles to 'seed' the process. When evaluated holistically, it appears that the final cured product tended to be 55-60% TS and 55-60% VS. Once the piles achieved those endpoints, plant staff found that the product was too dusty to continue turning. Seasonal VSR was evaluated using the Van Kleek method (Figure 4). During the winter, addtional VSR due to curing hovered around 10-15%, whereas it was seen as high as 40% in the spring and 45% in the summer. Production Efficiency Two main variables, seed addition and frequency of turning, were evaluated to determine if they impacted the speed at which cured cake could be produced. Seed addition consisted of adding variable amounts of already cured product to fresh cake by volume. Although this study did not determine the mechanism by which back mixing cured product with fresh product, two main explanations are likely: (1) Seed addition creates additional pore space for oxygen penetration into piles, similar to wood chip addition in the composting process, and (2) Seed addition kick starts the curing process by adding a large quantity of microorganisms adapted to aerobic curing. A variety of turning scenarios were employed within the five-day work week (10 times, 5 times, 3 times, and 2 times per week). Observations and data collected during these phases indicated that turning 5 times a week (once a day Monday to Friday) was sufficient to achieve the end-product. Seeding appeared to accelerate the curing process (Figure 5). While it took fresh cake-only piles approximately four weeks to reach the end point, the addition of 25% cured cake by volume reduced the curing time by at least one week. It should be acknowledged that using 25% of pad capacity for a 25% reduction in curing time is a breakeven point, however, qualitative observations during the process indicated that odors emanating from the curing piles were less offensive when 45% TS was achieved in the piles. Seed addition may therefore offer an additional benefit if that threshold is achieved faster in the process. Conclusions The combination of final dewatering centrifuge optimization and aerobic curing have been utilized at HRSD's Atlantic Treatment Plant to produce a low odor, high value soil amendment. Ultimately, dryer cake with lower odors will result in lower hauling costs and less truck traffic through neighborhoods surrounding the plant site.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerGonzalez, Dana

Presentation time

09:00:00

09:30:00

Session time

08:30:00

11:15:00

SessionTroubleshooting & Optimization

Session number26

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicClass A, Dewatering Optimization, Thermal Hydrolysis

Author(s)

Gonzalez, Dana

Author(s)D. Gonzalez1, J. Nicholson2, C. Wilson2, H. Matel2, C. Bott2

Author affiliation(s)Carollo Engineers 1; Hampton Roads Sanitation District 2; HRSD 2; Hampton Roads Sanitation District (HRSD) 2; HRSD 2;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159435

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count22

Description: Phosphorus Sequestration in Biosolids, Nuisance Struvite Control via PAD and...

Phosphorus Sequestration in Biosolids, Nuisance Struvite Control via PAD and Chemical Addition to TH-AD Digestate, and Downstream Effects

Abstract

Purpose: Struvite and other common precipitates have long posed persistent maintenance and operational challenges at wastewater treatment plants. These precipitates can be enhanced at plants that utilize biological phosphorus removal and can precipitate within anaerobic digesters and on final dewatering equipment which causes damage and decreases treatment efficiency. Struvite can be sequestered as nutrient rich biosolids if properly controlled to remain in the cake. In a digested solids storage tank (DSST) that is in between anaerobic digestion and final dewatering, precipitation can be controlled by manipulating variables such as mixing, aeration of digestate to achieve a shorter solids retention time (SRT) version of post aerobic digestion (PAD) and by chemical addition. Optimizing this via batch and pilot testing can control nuisance struvite and other crystal formation, enhance downstream effects such as increasing phosphorus in the cake, improve biosolids odor profile and cake dryness, reducing polymer demand, and achieve additional volatile solids reduction (VSR).
This research will review findings from batch and pilot scale tests and discuss the nature of these precipitates. Objectives include determining the mechanism that provides optimal phosphorus removal, odor control, cake characteristics, and analyzing the precipitated solids of interest. The presentation will discuss results across batch, pilot, and full-scales and examine the lessons learned of each.

Motivation and Background: The Atlantic Treatment Plant (ATP) in Virginia Beach, VA, operated by the Hampton Roads Sanitation District's (HRSD) is a high-rate facility with an A/O process configuration. In solids handling, a Cambi Thermal Hydrolysis Process (THP) is utilized prior to anaerobic digestion, leading to the formation of scaling precipitates and nuisance struvite. ATP is actively engaged in efforts to control these issues for optimal operational efficiency.
Precipitate formation is determined predominantly by the solids solubility product (Ksp) and the saturation in solution and can be controlled by pH [1]. Manipulation of pH via CO2 stripping from agitation and aeration has successfully been shown to control solubility reactions [2, 3]. Increasing the saturation index via chemical addition can also control precipitation reactions, decrease polymer demand, and improve cake dryness with an SRT of a few hours [4]. Likewise, PAD has been shown to achieve ammonia removal, additional VSR, and break down odor compounds formed in anaerobic digestion at a 5-6 day SRT [5]. Based on these findings, a pilot scale set up (Figure 1) will run at an SRT of 3 days to provide benefits from PAD, but with a longer reaction time that may be more beneficial for phosphorus sequestration. Various mixing, aeration rates, and chemical addition will be manipulated to determine the optimal operation conditions on phosphorus sequestration, cake characteristics, VSR and odors.

Pilot Design and Preliminary Operation: The pilot set up at ATP consists of four tanks. Each tank is 11 feet tall with a volume of about 63 gallons. Simulating the DSST, the tanks are operated as daily batch fed continuously stirred tank reactors (CSTRs) maintained by pump recirculation with a 3-day SRT. The tanks are aerated with fine bubble diffuser membranes which can be operated at constant or intermittent air flow rates, or via Dissolved Oxygen (DO) or pH set points. Preliminary operation has included mixing and aerating tank 1 at a constant airflow rate of 5 L/min, while recording online measurements of temperature, DO, and pH (Figure 2). At this operation, about 50% OP removal across the pilot has been measured with minimal NH3 removal due to lack of pH increase at this airflow rate. Future plans include tracking COD across the pilot, increasing aeration, and the addition of Ca(OH)2 or Mg(OH)2 at various dosages. Batch Testing Results: Prior to running pilot scale tests, some objectives can be mechanistically answered by preliminary batch testing. Chemical Addition: The following test was conducted on a Phipps and Bird PB-900 series Jar Tester at a continuous mixing speed of 300 rpm. Five jars of digestate consisting of a control with no chemical addition, two jars dosed with a carbide lime slurry at Ca2+ + Mg2+/P ratios of 0.5:1 and 1:1, and two jars dosed with thioguard, similarly, at Ca2+ + Mg2+/P ratios of 0.5:1 and 1:1. Chemical dose was injected as a spike at time zero of the test. Analytical measurement of OP-P and pH were determined daily via spectrophotometry (Phosphorus HACH TNTplus Vial Test UHR 2-20 mg-PO4-P/L) and pH bench meter, respectively) (Figure 3).
The pH of the digestate sample was about 7.4 and increased up to a maximum of 8.7 within 24 hours likely due to CO2 stripping from the mixing intensity. OP removal in the control condition with no chemical addition was measured as 68%. The addition of chemical helped to increase OP removal as high as 95%. The digestate pH increasing towards alkaline within the first test day showed the greatest impact on OP removal rate. The higher Ca2+ + Mg2+/P molar ratio tested of 1:1 for both the carbide lime slurry and thioguard had higher OP removal rates than those dosed at lower ratios of 0.5:1. Thioguard compared to the carbide lime slurry overall had a slightly more significant effect on OP removal. The pH decrease that is measured after 24 hours may be due to struvite's production of hydrogen ions in the formation reaction, however, future plans of repeat experiments as well as X-Ray Diffraction for precipitate analysis will further determine this.

Cake Characteristics: A separate test was completed to look at aeration effects on cake characteristics. A sample from the DSST was conditioned, dewatered, and analyzed to evaluate polymer demand and cake solids. Polymer was tested on two buckets of digestate - a gently mixed control bucket and a bucket aerated at about 5 L/min. After 72 hours, polymer was used to condition each digestate. After polymer was added to the solids, it was mixed, and a capillary suction time (CST) instrument was used to find the optimal dose of polymer. The solutions with the lowest CSTs were identified to be the optimal dose of polymer needed and were then dewatered by the centrifuge cup method and measured for cake solids and solids capture, per the Higgins method [6]. In Table 1, the Aerated bucket had a higher polymer optimal dose, TS simulated cake value, and TSS simulated filtrate value compared to the Control bucket while the Control bucket had a higher VS simulated cake value. From these results, the aerated digestate had poorer solids capture when dewatered with the polymer compared to the Control bucket. In addition to cake dryness, improving cake odors will be researched in the future by the characterization of odor profiles and gas sampling the reactor head space using Chromatography Mass Spectrometry solid phase microextraction and Gastec Tubes.

Conclusion: This paper will present the challenges that ATP faces as well as batch and pilot research conducted to combat these issues. Optimization learned on this scale will advance the discussion on similar issues shared by wastewater treatment plants across the country. Further advancements have the possibility to improve plant efficiency and save plant capital costs.

This paper was presented at the WEF Residuals and Biosolids Conference, June 18-21, 2024.

SpeakerHarris, Caitlyn

Presentation time

13:30:00

14:00:00

Session time

13:30:00

15:00:00

SessionNutrient Considerations and Digestion

Session number16

Session locationOklahoma City Convention Center, Oklahoma City, Oklahoma

TopicAnaerobic Digestion, Pilot Scale, Struvite

Author(s)

Harris, Caitlyn

Author(s)C. Harris1, M. Garcia1, D. Gonzalez2, A. Williamson1, B. Ward3, J. Nicholson1, H. Matel1, C. Bott1, C. Wilson1

Author affiliation(s)HRSD 1; HRSD 1; Carollo Engineers 2; HRSD 1; HDR 3; Hampton Roads Sanitation District 1; Hampton Roads Sanitation District (HRSD) 1; HRSD 1; HRSD 1;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jun 2024

DOI10.2175/193864718825159414

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count19

Base text size -

Gonzalez, Dana

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