In this study, concurrent operation of anammox and partial denitrification within a nonacclimated mixed culture system was proposed. The impact of carbon sources (acetate, glycerol, methanol, and ...ethanol) and COD/NO3−‐N ratio on partial denitrification selection under both short‐ and long‐term operations was investigated. Results from short‐term testing showed that all carbon sources supported partial denitrification. However, acetate and glycerol were preferred due to their display of efficient partial denitrification selection, which may be related to their different electron transport pathways in comparison with methanol. Long‐term operation confirmed results of batch tests by showing the contribution of partial denitrification to nitrate removal above 90% after acclimation in both acetate and glycerol reactors. In contrast, methanol showed challenges of maintaining efficient partial denitrification. COD/NO3−‐N ratio mainly controlled the rate of nitrate reduction and not directly partial denitrification selection; thus, it should be used to balance between denitrification rate and anammox rate.
Practitioner points
The authors aimed to investigate the impact of carbon sources and COD/NO3‐N ratio on partial denitrification selection.
All the carbon sources supported partial denitrification as long as the nitrite sink was available.
90% partial denitrification could be achieved with both acetate and glycerol in long‐term operations.
COD/NO3‐N ratio did not directly control partial denitrification but can be used to balance between denitrification rate and anammox rate.
Despite the increased research efforts, full‐scale implementation of shortcut nitrogen removal strategies has been challenged by the lack of consistent nitrite‐oxidizing bacteria out‐selection. This ...paper proposes an alternative path using partial denitrification (PdN) selection coupled with anaerobic ammonium‐oxidizing bacteria (AnAOB). A nitrate residual concentration (>2 mg N/L) was identified as the crucial factor for metabolic PdN selection using acetate as a carbon source, unlike the COD/N ratio which was often suggested. Therefore, a novel and simple acetate dosing control strategy based on maintaining a nitrate concentration was tested in the absence and presence of AnAOB, achieving PdN efficiencies above 80%. The metabolic‐based PdN selection allowed for flexibility to move between PdN and full denitrification when required to meet effluent nitrate levels. Due to the independence of this strategy on species selection and management of nitrite competition, this novel approach will guarantee nitrite availability for AnAOB under mainstream conditions unlike shortcut nitrogen removal approaches based on NOB out‐selection. Overall, a COD addition of only 2.2 g COD/g TIN removed was needed for the PdN‐AnAOB concept showing its potential for significant savings in external carbon source needs to meet low TIN effluent concentrations making this concept a competitive alternative.
Practitioner points
Nitrate residual is the key control parameter for partial denitrification selection.
Metabolic selection allowed for flexibility of moving from partial to full denitrification.
2.2 g COD/g TIN removed was needed for partial denitrification‐anammox process.
PdN‐AnAOB route for mainstream shortcut N removal without NOB out‐selection.
Primary sludge fermentate, a concentrated hydrolyzed wastewater carbon, was evaluated for use as an alternative carbon source for mainstream partial denitrification–anammox (PdNA) in a suspended ...growth activated sludge process in terms of partial denitrification (PdN) efficiency, PdNA nitrogen removal contributions, and final effluent quality. Fermenter operation at a 2‐day sludge retention time (SRT) resulted in the maximum achievable yield of 0.14 ± 0.05 g sCOD/g VSS without release of excessive ammonia and phosphorus to the system. Based on the results of batch experiments, fermentate addition led to PdN efficiency of 93 ± 14%, which was similar to acetate at a nitrate residual of 2–3 mg N/L. In the pilot‐scale mainstream deammonification reactor, PdN efficiency using fermentate was 49 ± 24%, which was lower than acetate (66 ± 24% during acetate period I and 70 ± 21% during acetate period II), most probably due to lower nitrate and ammonium kinetics in the PdN zone. Methanol cost‐saving potential for the application of PdNA as the main short‐cut nitrogen pathway was estimated to be 30% to 55% depending on the PdN efficiency achieved.
Practitioner points
Primary sludge fermentate was evaluated as an alternative carbon source for mainstream partial denitrification–anammox (PdNA).
Fermenter operated at a 1 to 2 day SRT resulted in the maximum achievable yield without the release of excessive ammonia and phosphorus to the system.
Although 93% partial denitrification efficiency was achieved with fermentate in batch experiments, around 49% PdN efficiency was achieved in pilot studies.
Application of PdNA with fermentate can result in significant methanol cost savings.
This paper showed the use of primary sludge fermentate as carbon source for partial denitrification anammox (PdNA). Pilot results showed the potential for 30‐40% methanol savings as a result of PdNA application. Based on kinetic testing under ideal conditions, increased methanol savings were estimated, thus indicating further room for optimization.
Retrofitting conventional denitrification filters into partial denitrification‐anammox (PdNA)‐ or anammox (AnAOB)‐based filters will reduce the needs for external carbon addition. The success of ...AnAOB‐based filters depends on anammox growth and retention within such filters. Studies have overlooked the importance of media selection and its impact on AnAOB capacity, head loss progression dynamics, and shear conditions applied onto the AnAOB biofilm. The objective of this study was to evaluate viable media types (10 types) that can enhance AnAOB rates for efficient nitrogen removal in filters. Given the higher backwash requirement and lower AnAOB capacity of the conventionally used sand, expanded clay (3–5 mm) was recommended for AnAOB‐based filters in this study. Owing to its surface characteristics, expanded clay had higher AnAOB activity (304‐ vs. 104‐g NH4+‐N/m2/day) and higher AnAOB retention (43% more) than sand. Increasing the iron content of expanded clay to 37% resulted in an increase in zeta potential, which led to 56% more anammox capacity compared to expanded clay with 7% iron content. This work provides insight into the importance of media types in the growth and retention of AnAOB in filters, and this knowledge could be used as basis in the development of PdNA filters.
Practitioner Points
Expanded clay showed the lowest head loss buildup and most likely will result in longer runtime for full‐scale PdNA applications
The highest AnAOB rates were achieved in expanded clay types and sand compared with smaller media typically used in biofiltration
Expanded clay resulted in better AnAOB retention under shear, whereas sand could not withstand shear and required more frequent backwashing
Expanded clay iron coating enhanced AnAOB enrichment and retention, most likely due to increased surface roughness and/or positive charge
This study provided insight into the importance of media types in the growth and retention of anammox in polishing filters. Expanded clay (3–5 mm) was recommended for anammox‐based or partial denitrification‐anammox (PdNA) polishing filters. Owing to its higher surface roughness and higher iron content, expanded clay had three times more anammox activity and 43% more anammox retention than the conventionally used sand.
This study quantifies volatile sulfur compound (VSC) emissions from primary settling tanks and investigates their mechanisms of generation. Hydrogen sulfide (H2S) and methyl mercaptan (MM) ...concentrations in the off‐gas were dominant among the VSCs analyzed, while dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) were under their odor threshold for most sampling dates. H2S emission in primary settling tanks was mainly the result of the stripping of dissolved sulfide (64%) generated in the sewers. Results indicate that MM emission was more dependent on the conditions in the primary clarifiers (only 16% stripping). Prevention of odor emission in primary settling tanks can be achieved by managing biofilms and microbial reactions in the sewer network. Controlling the biomass seeding and fermentation product availability in the primary settling tanks is essential to significantly minimize the kinetics of H2S and MM generation. Overall, the management of sludge blanket heights and thus avoiding time at low oxidation–reduction potential minimized odor emission independent of sewer conditions.
Practitioner points
H2S emission from primary clarifiers mainly originated from the stripping of the dissolved sulfide formed in the sewers.
MM emission contributed for 89% to overall odor emitted from primary clarifiers.
Seeding of active biomass from the sewer into the primary clarifiers was be the main driver for both MM and H2S formation.
Increased availability of fermentation products or fermenters increased MM production.
Interactions and impacts of MM and H2S formation in sewer network on primary clarification emissions
This study demonstrates the potential of an innovative anaerobic treatment technology for municipal biosolids (IntensiCarb), which relies on vacuum evaporation to decouple solids and hydraulic ...retention times (SRT and HRT). We present proof‐of‐concept experiments using primary sludge and thickened waste activated sludge (50–50 v/v mixture) as feed for fermentation and carbon upgrading with the IntensiCarb unit. IntensiCarb fully decoupled the HRT and SRT in continuously stirred anaerobic reactors (CSAR) to achieve two intensification factors, that is, 1.3 and 2, while keeping the SRT constant at 3 days (including in the control fermenter). The intensified CSARs were compared to a conventional control system to determine the yields of particulate hydrolysis, VFA production, and nitrogen partitioning between fermentate and condensate. The intensified CSAR operating at an intensification factor 2 achieved a 65% improvement in particulate solubilization. Almost 50% of total ammonia was extracted without pH adjustment, while carbon was retained in the fermentate. Based on these results, the IntensiCarb technology allows water resource recovery facilities to achieve a high degree of plant‐wide intensification while partitioning nutrients into different streams and thickening solids.
Practitioner Points
The IntensiCarb reactor can decouple hydraulic (HRT) and solids (SRT) retention times in anaerobic systems while also increasing particulate hydrolysis and overall plant capacity.
Using vacuum as driving force of the IntensiCarb technology, the system could achieve thickening, digestion, and partial dewatering in the same unit—thus eliminating the complexity of multi‐stage biosolids treatment lines.
The ability to partition nutrients between particulate, fermentate, and condensate assigns to the IntensiCarb unit a key role in recovery strategies for value‐added products such as nitrogen, phosphorus, and carbon, which can be recovered separately and independently.
Intermittent feed and vacuum extraction with the IntensiCarb reactor provide an avenue to recover water and volatile organics, while minimizing reactor volume.
This study evaluated the application of recuperative thickening (RT) to enhance anaerobic digestion (AD) performance for AD systems with thermal hydrolysis pretreatment (THP). RT was applied for two ...different reasons: (a) for increasing the sludge retention time (SRT) to degrade slowly hydrolyzable materials more efficiently and (b) for maintaining SRT at decreased hydraulic retention time (HRT) thus showing potential for increased AD throughput rates. A SRT increase from 15 to 30 days by RT application did not improve AD performance or hydrolysis rates significantly as 15‐day SRT was already a factor 2 higher than the estimated washout SRT. When applying RT to increase throughput rates (HRT of 7 days) while maintaining 15‐day SRT, no negative impact on biogas production or hydrolysis kinetics was observed. It was estimated that RT application on THP digesters can increase digester throughput by 100% and thus show clear potential for further AD intensification.
Practitioner points
Increased SRT from 15 to 30 days through recuperative thickening application did not improve biogas production.
A lower required minimum SRT (6–7 days) was estimated in THP‐AD systems compared to conventional AD.
Operation at decreased HRT by RT application resulted in similar AD performance under constant organic loading rates.
A 100% increase in throughput rates can be applied using RT without decreasing AD performance.
Recuperative thickening applied on anaerobic digestion system fed with thermal hydrolysis pretreated sludge.
This study evaluated the role of bulk and substrate physics on hydrolysis rates and biogas yields in anaerobic digestion (AD) pretreated by thermal hydrolysis (THP). Although THP decreases sludge ...viscosity, no evidence was found that bulk viscosity impacted the biogas yield or hydrolysis kinetics. In addition, no significant difference between the biogas yields for different total solids concentrations nor floc sizes was detected. However, increased mixing speeds did increase biogas yields. As a result of thermal treatment, the model protein, bovine serum albumin, was harder to degrade in terms of both overall biodegradability and hydrolysis rates when their macrostructures were changed from liquid to gel and to solid structures; the opposite was true for the model polysaccharide, amylopectin. These results demonstrated that hydrolysis in THP‐AD systems was impacted mostly by the physical properties of the substrate (gelation) rather than the bulk physical properties within the digester.
Practitioner points
Bulk viscosity does not significantly impact hydrolysis efficiency (biogas yield). However, mixing speed impacts hydrolysis beyond biogas holdup effect.
Increasing the amount of substrate–microbe collisions through increasing biomass concentration does not have an impact on hydrolysis efficiency or biogas yield.
Proteins are harder to degrade when macrostructure changes from liquid to gel/solid as a result of heat treatment.
Polysaccharides are easier to degrade when macrostructure changes from liquid to gel/solid as a result of heat treatment.
The time required for digesters to reach peak biogas production rates increased with decreasing specific surface available on gel and solid structures.
The impact of the bulk physical properties of sludge within anaerobic digesters and the hydrolysis substrates' molecular structures were investigated in batch‐scale experiments. The physical properties studied, the floc size, number of substrate‐microbe collisions, and viscosity of the sludge, did not have a significant impact on hydrolysis in mesophilic anaerobic digestion. Varying hydrolysis substrates' molecular structures through subjecting them to increased temperatures resulted in significant changes in hydrolysis performance in batch‐scale anaerobic digestion.
This study evaluated the impact of ammonia on mesophilic anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) treating a mixture of primary sludge and waste activated sludge and ...operated under constant organic loading rate of 9 kg COD/m3/d. Free ammonia concentrations in the digesters were varied between 37 and 966 mg NH3‐N/L, while maintaining all other operational conditions constant. A decrease in volatile solids reduction from 54 ± 5% (at <554 mg NH3‐N/L) to 35 ± 6% at the maximum free ammonia concentration of 966 mg NH3‐N/L was observed at steady‐state conditions. No impact of free ammonia on final dewaterability was detected. Free ammonia thus mostly limited methanogenesis. A free ammonia Monod inhibition constant of 847 ± 222 mg NH3‐N/L for methanogens was estimated based on the digester steady‐state methane rates dynamics. This study showed that current THP AD digesters (typically 110–260 mg NH3‐N/L) operate under 12%–18% ammonia inhibition for methanogenesis. Operation under SRT of 15 days, about 2 times more than needed to retain methanogens, can compensate for lower methanogens rates and avoid performance impacts. The later shows a good potential to operate under higher free and total ammonia concentration without jeopardizing performance.
Practitioner points
Only from a free ammonia concentration above 554 mg NH3‐N/L, decreased volatile solids reduction and biogas yield were observed.
A volatile solids reduction of 35 ± 6% at maximum free ammonia concentration of 966 mg NH3‐N/L was still achieved.
A Monod inhibition constant for methanogens of 847 ± 222 mg NH3‐N/L was estimated.
It was estimated that current THP AD systems (110–260 mg NH3‐N/L) operate under 12%–18% NH3 inhibition for methanogenesis.
Current THP AD systems (110‐260 mg NH3‐N/L) operate under 12‐18% NH3 inhibition for methanogenesis and have great potential for increased AD capacity and co‐digestion.
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