Watson, Ian

Ian Watson is the Technology Development Manager at USP Technologies with a chemical engineering background and 23 years in wastewater treatment. His...

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Description: Enhancing Sludge Dewaterability and Phosphate Removal Through a Novel Chemical...

Enhancing Sludge Dewaterability and Phosphate Removal Through a Novel Chemical Dosing Strategy Using Ferric Chloride and Hydrogen Peroxide

Abstract

Enhancing Sludge Dewaterability and Phosphate Removal Through a Novel Chemical Dosing Strategy using Ferric Chloride and Hydrogen Peroxide Ian Watson c, Vahid Ghodsi a,b, Siva R. Sarathy a,b, John R. Walton c, Elsayed Elbeshbishy d, Domenico Santoro b,c a Trojan Technologies, London, Ontario Canada b Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada c USP Technologies, Atlanta, Georgia United States d Department of Civil Engineering, Ryerson University, Toronto, Ontario Canada Introduction Sludge processing and disposal constitutes 50-60% of the total treatment cost in WRRFs, and dewatering operations represent a large fraction of those costs. Those dewatering costs may be direct (as in the trade-off between polymer dose and cake solids content, and how that impacts disposal costs) or indirect (as in the removal of sulfide and phosphorus from the return streams, and how that impacts aeration and solids processing costs). A particular inefficiency confronting many WWRFs is the recycling of phosphorus into the water line via the dewatering return stream, which can comprise 25-50% of the total P entering the aeration unit. In this study, we aimed to develop a sludge conditioning process for Anaerobically Digested Sludge (ADS) with the goal of breaking this phosphorus recycle loop, and doing so within the constraints of current operations (i.e., with minimal capital investment and quick implementation within existing budgets). The Higgins Modified Centrifugation Test (MCT) was used as it allows a solids cake to form atop a screen platform while capturing the centrate below. The initial work was to calibrate the MCT parameters (screen mesh size, centrifugation speed and time, and polymer dose) to plant operational performance. Then, different dosages of ferric chloride (FeCl3), hydrogen peroxide (H2O2) and/or cationic polymer were applied and performance measures were collected for cake solids content and centrate TSS, sulfide, and o-phosphate. Results Conditioning ADS with H2O2 only (without polymer added). At a 200 mg/L H2O2 dose (Figure 1a), centrate o-phosphate levels dropped by 30% after 10 minutes, and by 50% after 20 minutes (no significant phosphate reduction was observed at higher H2O2 doses). Such incomplete phosphate elimination indicates insufficient and/or unavailable endogenous iron, and suggests a need to supplement by externally dosing FeCl3. Conditioning ADS with FeCl3 only (without polymer added). FeCl3 was added to ADS and centrate o-phosphate levels were measured (Figure 2). Dosing 90 mg/L Fe resulted in approx. 40% decrease in o-phosphate levels, increasing to 80% removal when dosing 220 mg/L Fe. In the case of FeCl3 dosing, five minutes was enough for Fe to react with o-phosphate (Figure 2). Conditioning ADS with FeCl3 and H2O2 (without polymer added). Figure 3 shows a 40% drop in o-phosphate levels after dosing 90 mg/L FeCl3, which increased to nearly 60% after dosing 300 mg/L H2O2. Raising the FeCl3 dose to 155 mg/L and 220 mg/L, with the same 300 mg/L H2O2 dose, increased o-phosphate removals to about 80% and 85%, respectively. Similarly, a 90 mg/L Fe dose reduced o-phosphate levels by 40%, increasing to nearly 60% when combined with a 400 mg/L H2O2 dose. Raising the Fe dose to 155 mg/L and 220 mg/L (with the same concentration of H2O2) increased P-removal to about. 80% and 85%, respectively. Conditioning ADS with FeCl3 and H2O2 (with polymer added). As expected, higher concentrations of polymer yielded a drier cake and lower centrate TSS levels (Figure 4a). Centrate TSS levels dropped by nearly 70% when the polymer dosage increased from 0.8% to 1.6%. Also, the cake solids content of the ADS treated with the combination of FeCl3 and H2O2 was about two percentage points higher (compared to unconditioned ADS with no FeCl3 and no H2O2). This indicates potentially significant cost savings on polymer consumption and/or solids disposal. Compared to ADS that was not conditioned, the centrate from ADS conditioned with FeCl3 and H2O2 had far lower o-phosphate levels (Figure 4b). This confirms that Fe (III) is capable of playing several simultaneously roles in wastewater treatment, such as a phosphate precipitant and a TSS flocculant. Economic assessment The results in Figures 3 and 4 can be combined to estimate the additional costs of different H2O2 / FeCl3 combinations to reach increasing levels of treatment (P-removal) — see Table 1. The first conclusion for this particular study is that the benefit of adding FeCl3 exceeds that of adding H2O2. This conclusion could change, however, were: 1) the endogenous Fe content in the feed sludge to increase (due to e.g., upstream Fe addition in the collection system or WRRF); or 2) the sulfide levels in the feed solids significantly increase (consuming FeCl3) or if durational odor control (at the sludge disposal facility) was needed. The second finding is that the higher cake solids content achieved by H2O2 and FeCl3 dosing can offset the costs for these chemicals by 25-30% in this particular case. This amount will be more for situations where disposal costs are higher than $50 per WT or where the degree of dewaterability improvement (2% points higher cake solids) is greater. Conclusions The results of this work confirmed that iron is key to binding phosphorus into the dewatered cake, though it is often not present in anaerobic digested solids in the amounts or forms needed to bind both sulfide and phosphorus. In those cases, the endogenous Fe bound to sulfide can be regenerated by adding an oxidizing agent (H2O2) or, if the Fe content is still insufficient, the regenerated Fe can be supplemented with FeCl3. In this study, the latter was shown to be the case: combined H2O2 and FeCl3 dosing was shown to greatly reduce centrate sulfide and phosphorus levels while producing a dewatered cake with two percentage points higher solids content (at the same polymer dose).

The following conference paper was presented at Residuals and Biosolids 2021: A Virtual Event, May 11-13, 2021.

SpeakerWatson, Ian

Presentation time

15:15:00

15:30:00

Session time

15:00:00

16:15:00

SessionInnovative Biosolids Process Enhancements

Session number2

Session locationSimu-Live

TopicChemical Treatment, Dewaterability, Phosphorus Removal and Recovery

Author(s)

I. WatsonV. GhodsiS. SarathyJ. WaltonE. ElbeshbishyD. Santoro

Author(s)I. Watson1; V. Ghodsi2; S. Sarathy3; J. Walton4; E. Elbeshbishy5; D. Santoro6

Author affiliation(s)USP Technologies 1; Trojan Technologies 2; Trojan Technologies 3; USP Technologies 4; USP Technologies 6;

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2021

DOI10.2175/193864718825157960

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count19

No More Odors Here on the Streets of Bakersfield

Abstract

Faced with increasing costs for controlling hydrogen sulfide in its wastewater collection system, the City of Bakersfield, California commissioned an evaluation of its program in 2019. Among the consultant recommendations was to consider using a more efficient treatment (ferrous chloride) in place of certain of the existing Calcium Nitrate dosing stations where costs were constrained. Of priority were stations those along the higher flow interceptor segment where chemical demands and community sensitivities were highest. An expected benefit of iron dosing is its durational control capability, which should translate into fewer dosing stations. Consequently, field testing was suggested to assess the cost-benefit. As siting dosing stations for ferrous chloride solutions can be limited (due to the corrosivity of the product), a low-hazard form of ferrous chloride, SulFeLox®, was used. The study was conducted from April to June 2021 and consisted of 12 days monitoring of the baseline Calcium Nitrate scenario, followed by 60 days of SulFeLox dosing at different feed rates, and concluding with 14 days of no chemical dosing (i.e., the true baseline scenario). The interceptor segment selected for the test was a 10-mile long (5-6 hour retention) trunkline leading to the City's Plant No. 3 (Figure 1). Wastewater flows increase from approximately 0.3 mgd at the Romero PS input (S1) to 12 mgd at the McCutcheon PS (S3). In this study, the SulFeLox dosing station replaced two Calcium Nitrate stations: 1) at the Romero site (S1), as well at the Buena Vista site (S2). Comparing the Calcium Nitrate and SulFeLox results shows 66-83% lower vapor-H2S peaks at the Buena Vista (S2) and treatment plant (S4) sites, with less benefit at the McCutcheon site (S3) where vapors are impacted by H2S by other flows into the station. (Figure 2a,2b,2c). Site 2 (Buena Vista PS) is in a particularly sensitive area, and prior tests suggested upstream Calcium Nitrate feed rates would need to be 500 gpd (or more) to maintain vapor-H2S levels below 25 ppm. Figure 3 shows the vapor-H2S results in this test, where SulFeLox feed rates of 120-210 gpd achieved the compliance targets and provided H2S reductions of 84-95% (relative to 146 gpd Calcium Nitrate). On a performance basis, the field results confirmed the consultant recommendation that (for system-wide H2S control) iron dosing from one site could provide improved results relative to Calcium Nitrate dosing from two sites. SulFeLox feed rates were determined that provide interceptor-wide H2S control to 25 ppm and 10 ppm. However, a direct cost comparison between the two treatment chemistries is complicated given that system-wide H2S control could not be demonstrated with Calcium Nitrate (per the 500 gpd test in a prior study). This present study did show, however, that the cost to control to target H2S levels using SulFeLox was $600-$1000 per day, where the recurring costs for Calcium Nitrate was $850-950 per day (to control to 100-140 ppm H2S). Subsequent to this field test, the City continued to feed SulFeLox at S1 (Romero PS) and has since replaced seven other Calcium Nitrate sites with three SulFeLox sites, reducing the total number of chemical feed sites from nine to four. As a result of this work, the City is now spending approximately $663k/yr for SulFeLox to meet its system-wide performance targets, where it was previously spending $990k/yr (for Calcium Nitrate). Further, with now a year's experience with dosing iron into the collection system, the treatment plant has observed operational benefits including the elimination of the ferrous chloride feed to their digesters for sulfide control. Early in 2022, the City entered into a five-year supply agreement for providing SulFeLox iron product and related services and equipment. Future work is planned to evaluate iron regeneration ahead of the treatment plant (at McCutcheon PS, S3), recognizing that this trunkline comprises 70% of total plant flow but 100% of influent iron. Oxidizing the spent iron (as FeS) to hydrous ferric oxide (using hydrogen peroxide) is expected to provide additional benefit to treatment plant operations.

This paper was presented at the WEF/WEAT Collection Systems and Stormwater Conference, July 15-18, 2025.

Presentation time

10:45:00

11:15:00

Session time

10:45:00

11:45:00

SessionStrategies for Odor and Corrosion Control

Session number03

Session locationGeorge R. Brown Convention Center, Houston, Texas, USA

TopicChemical Treatment, Collection Systems, Odor and Corrosion Control

Author(s)

Watson, Ian

Author(s)I. Watson1

Author affiliation(s)USP Technologies, 1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Jul 2025

DOI10.2175/193864718825159891

Volume / Issue

Content sourceCollection Systems and Stormwater Conference

Word count10