Islam, Shafkat

Shafkat Islam is an environmental engineer and researcher with experience in full-scale wastewater treatment, sludge fermentation, and nutrient recovery. He has worked with the RD team at DC Water, supporting process optimization at the Blue Plains Advanced Wastewater Treatment Plant. He completed his Masters in Civil and Environmental Engineering at George Washington University.

Titles from this speaker

Investigating Fermentation of Primary Sludge in Full-Scale Gravity Thickeners

Abstract

Introduction
Wastewater treatment plants face stricter nitrogen (TN) discharge limits, increasing reliance on external carbon (C) sources for nitrogen (N) removal, which drives up costs and external dependencies. Advances in short-cut N removal processes, like PdNA, reduce carbon use but still depend on significant external C dosing. Influent wastewater carbon is less effective for N removal due to aerobic consumption, whereas external carbon dosing avoids oxygen-related losses (Ladipo-Obasa et al., 2022). Primary sludge fermentate is a low-cost carbon source that could reduce reliance on external carbon, such as methanol (MeOH) (Ali et al., 2021). However, variable organic yields and fermentate quality present challenges (Sánchez Rubal et al., 2012). Factors like wastewater characteristics, seasonal conditions, and sewer fermentation activity affect consistency.

Preliminary bench-scale tests at Blue Plains showed potential yields of 0.2 g sCOD/g VSS but also increased phosphorus release and odor. This study expands fermentation tests in full-scale gravity thickeners to assess fermentate quality, nutrient release, and odor formation under realistic conditions, optimizing N and P removal processes.

Materials and Methodology
Fermentation tests were conducted in the gravity thickener (GT) at Blue Plains AWWTP over seven months with controlled sludge blanket height covering a range of SRTs and varying operational and seasonal conditions (Fig. 1). Grab samples were collected from the primary sludge influent, GT overflow, and three depths of sludge blanket. A complete elutriation assumption was applied to estimate weighted organic yields (sCOD), nutrient release (orthophosphate, ammonia, iron). Additionally, odor samples were tested for odor detection thresholds (ASTM E679 and EN13725) and total reduced sulfur and compositions (ASTM D5504-20), while a gas analyzer (Odalog) provided real-time H2S monitoring onsite.

Result and Discussion
SRT control in gravity thickeners
SRT in gravity thickeners was controlled by maintaining sludge blanket targets (Fig. 1A). Flow to the gravity thickener was stabilized to enhance consistency (Fig. 1B). Improved system stability typically led to higher sCOD yields, while instability, often during morning samples, resulted in lower yields (Fig. 1C, Fig. 1D).

Stability, assessed via carbon (C) and phosphorus (P) mass balance errors, was higher when errors stayed below 20%. Yields varied within the 1—1.5-day SRT range, with better stability crucial for optimizing fermentate quality in full-scale operations.

COD and nutrient yields (N, P, Fe) under full elutriation assumption
The COD and nutrient yields were based on full gravity thickener profiles, reflecting 100% elutriation since no elutriation occurred under current conditions. Typically, 20% of soluble compounds from fermentation are lost in the underflow (WRF Project 4975, 2023). sCOD yield increased with SRT (Fig. 2A), though variability beyond 1 day suggests other influencing factors. Among 11 data points (SRT 1—1.5 days), five showed high-quality fermentate (>0.2 g sCOD/g VSS), while six showed lower quality (<0.2 g sCOD/g VSS). High-quality fermentate correlated with mass balance errors <30%, while poor quality correlated with >30% errors, indicating instability in GT operations. Factors such as primary clarifier operation and GT underflow influenced stability, with lack of control strategies being a bottleneck. NH4-N yield varied without clear SRT trends (Fig. 2B), averaging 20 ± 7.5 mg N/L at SRT >1 day. P release increased with SRT (Fig. 2C), with 1.64 g Fe released per gram of OP-P, enabling 61% P removal using current Fe/P ratios.

GTO and future fermentate composition
Previous studies showed that increased SRT improved compression (SVI30 from 48 to 36 mL/g) but created smaller, harder-to-flocculate particles (TOF > 700 mg TSS/L) (Islam et al., 2024). Higher sludge blankets at SRT > 1.25 days raised GTO TSS (Fig. 2E). To balance sCOD and nutrient yields with settling, too long SRTs should be avoided.

In Table 1, GTO and thus fermentate composition at a SRT of 1-1.25 days are calculated assuming 80% elutriation. This can lead to current GTO flows and denitrification stoichiometry and yield coefficient of 5.4 sCOD/g NO3-N (Lee et al., 2024) to a potential MeOH saving of 22±8%. When using fermentate for PdNA, this MeOH saving can be increased to 28 to 62% at PdNA efficiencies of 30-60%.

Odor emissions
Odor is a key side effect of fermentation, with H2S measured continuously and odor units/organic sulfur assessed at intervals. H2S levels reached 8537 ppbv, and odor detection peaked at 360,000 OU/m3 for SRT <1.25 days. Odor emissions increased in level and variability at SRT >1.5 days (Fig. 3), correlating with higher SRTs. Maintaining an optimal SRT (1—1.25 days) minimizes excessive microbial degradation and odor emissions while preserving fermentate quality. Further investigation is needed to understand variability and improve odor treatment design without overestimating for worst-case scenarios.

Conclusion
This study quantified fermentation yields, nutrient release, and odor production during full-scale gravity thickening under varying conditions. It provides guidelines to optimize fermentate quality with high sCOD yield, advancing fermentation to reduce MeOH use and leveraging GT as a fermenter.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

11:00:00

11:15:00

Session time

10:30:00

12:00:00

SessionFull-Scale Primary Sludge Fermentation

Session locationMcCormick Place, Chicago, Illinois, USA

TopicLiquid Stream Treatment - Nutrient Removal and Recovery

Author(s)

Islam, Shafkat, Ngo, Khoa Nam, Zaveri, Jaydev, Fitenko, Alexander, Mensah, Joshuan, Riffat, Rumana, Gu, April, Massoudieh, Arash, De Clippeleir, Haydee

Author(s)S. Islam1, K. Ngo2, J. Zaveri3, A. Fitenko3, J. Mensah4, R. Riffat1, A. Gu3, A. Massoudieh4, H. De Clippeleir2, , , ,

Author affiliation(s)George Washington University1, DC Water and Sewer Authority2, Cornell University3, The Catholic University of America4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2025

DOI10.2175/193864718825160025

Volume / Issue

Content sourceWEFTEC

Word count10

Optimizing Side Impacts of Using Primary Sludge Fermentate for Shortcut N removal in Chemical P Removal Facilities

Abstract

Introduction Traditionally, primary sludge fermentate has been utilized in biological-P removal plants to stabilize and enhance P removal (WRF Project 4975 Report, 2023). However, there is an emerging interest in employing fermentate in chemically enhanced P removal plants to support nitrogen removal and decrease the dependency on external carbon. Especially with the development of novel nitrogen removal systems such as partial denitrification anammox (PdNA), the use of fermentate can result in significant carbon removal savings (Ali et al., 2021; Ladipo-Obasa et al., 2022). It was estimated that for a plant like Blue Plains that relies on methanol (MeOH) as an external carbon source for N removal, the use of primary sludge fermentate can replace 15% of MeOH when deployed for full denitrification or 30-40% of MeOH when deployed for PdNA. To maximize N removal with fermentate, the fermentation yield in terms of volatile fatty acids or overall soluble COD yields plays a big role. This yield is dependent on the operational conditions of the fermenter with solids retention time (SRT) being one of the most important factors. The relationship between SRT and sCOD or VFA yields is shown in Figure 1A & 1B. Typically, an SRT of 3 days is needed to maximize yields (Fig. 1A & 1B). However, it should be noted that nutrients (NH4, OP) are also released (Fig 1C & 1D). Typically, in a non-chemical P plant, a yield of around 0.0055 gNH4-N/gVSS and 0.000842 gPO4-P/gVSS is expected (Fig 1C & 1D). However, when relying on ferric dosing, it is expected that enhanced P release will be observed due to the iron reduction in the fermenter (Fig 1D). This could cause a challenge as now P first bound with Fe, needs to be recaptured to meet discharge limits. This paper provides an assessment of primary sludge fermentation side impacts that need to be accounted for fermentate use for N removal in chemical-P plants. This includes impact of P recycles loads, solids load due to settling changes and odor emissions. Material and methods: Primary sludge was collected from Blue Plains Advanced Wastewater Treatment Plant. Fermentation tests were conducted in batches during different seasons. The last two seasons (Spring and Summer) results will be included in the full paper. Fermentate underwent characterization in terms of chemical composition and was tested for settling parameters (Ngo et al., 2021). Additionally, a full-scale batch fermentation was conducted in the gravity thickener during winter conditions. Results and discussion: Side impact 1: P release yields in chemical P removal plants All tests of Blue Plains primary sludge yielded consistent results in terms of sCOD and NH4 yields. Overall, these yields were a little lower than reported literature (Fig. 1A & 1C), most likely due to the long sewer systems and significant fermentation and loss of soluble organics before the sewage reaches the plant. For P yields, a larger variability existed (Fig. 1D), which might be related to the fermentation conditions (pH variability) or Fe dosing rates in the plant. Overall, the P concentration expected in the fermentate is about 30.9+/-12.6 mg P/L or 0.74+/-0.3 mg Peq/L in the mainstream. Given our TP limit of 0.18 mg TP/L, this contribution is significant and needs to be addressed. Potential avenues to look into to resolve this are optimization of fermentation conditions, potentially operating under pH <5 (Ali et al., 2021). A second avenue is to re-oxidize fermentate from ferrous to ferric and bind this back to FePO4. Current measurements show the availability of 0.72 mol Fe/P, which would allow us to decrease P content to 0.21 mg Peq/L in mainstream/L theoretically or 0.38 mg Peq in mainstream/L at current plant stoichiometries. Side impact 2: Settling changes during fermentation Spring and summer tests show a deterioration in flocculation behavior (Fig. 2A). Even though threshold of flocculation value was different, the trends were consistent. The decreased collision efficiency resulted in increased turbidity and non-settling fraction in the fermentate. This was consistent with observations from literature (Bouzas et al., 2007). It should also be noted that TOF reached the higher levels at the highest sCOD yields and thus we need to anticipate an increase in particulates at increased yields. Typically, we are seeing solids levels of 325+/-230 mg TSS/L in fermentate. While flocculation deteriorated, hindered settling properties improved with the decrease trend of SVI30 (Fig. 2B). This illustrated the interpretation of floc morphology and behavior between flocculation and hindered settling properties (Fig. 3). Side impact 3: Odor formation During fermentation, odor is formed and its composition under current gravity thickener operation and fermentation is showed on table 1. Increased odor is a result of microbial reactions under deep anaerobic conditions. Hydrogen sulfide formation remained low due to the presence of ferrous. Understanding the odor compounds can support evaluation of odor treatment and mitigation strategies. Conclusions Overall, this study quantified the expected increase in P loading coming from fermentate, the expected impacts on settling and odor formation. These three factors must be taken into account when considering primary sludge fermentate for N removal.

This study investigates using primary sludge fermentate in CEPT plants to support N removal and reduce using external carbon. At Blue Plains AWWTP, fermentate can replace 15% of methanol for full denitrification or 30-40% for PdNA. SRT correlates with sCOD and VFA yields, peaking at 2-3 days. The research also explores side effects like P loading, altered settling behavior, and odor formation, emphasizing the need to optimize fermentation conditions and implement effective odor treatment.

SpeakerIslam, Shafkat

Presentation time

16:00:00

16:30:00

Session time

15:30:00

17:00:00

SessionEnhancing Nitrogen Removal: Insights Into Carbon Sources and Mechanisms

Session number420

Session locationRoom 340

TopicAdvanced Level, Facility Operations and Maintenance, Municipal Wastewater Treatment Design, Nutrients, Research and Innovation

Author(s)

Islam, Shafkat, Ngo, Nam, Lapidus, David, Mendoza, Sara, Pathak, Bipin, Kozeracki, Emilia, Riffat, Rumana, Azam, Hossain, Massoudieh, Arash, De Clippeleir, Haydee

Author(s)S. Islam1, N. Ngo2, D. Lapidus3, S. Mendoza4, B. Pathak5, E. Kozeracki6, R. Riffat7, H.M. Azam3, A.N. Massoudieh8, H. De Clippeleir9

Author affiliation(s)1The George Washington University, DC, 2DC Water, VA, 3University of the District of Columbia, DC, 4Carollo Engineers, VA, 5DC Water & Sewer Authority, DC, 6The Catholic University of America, CT, 7George Washington University, VA, 8Catholic University of America, DC, 9DC Water & Sewer Authority, VA

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2024

DOI10.2175/193864718825159525

Volume / Issue

Content sourceWEFTEC

Word count18

Base text size -

Islam, Shafkat

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