High Free Ammonia Concentrations Research Articles

Nitrogen removal from urea effluent in an anammox sequencing batch biofilm reactor with valorised dishwashing scrubber as biocarrier

BackgroundThis study aimed for efficient nitrogen removal from urea wastewater in an anammox SBBR with valorised dishwashing scrubber as biocarrier. MethodsThe study (225 d) comprised three discrete phases: Phase I [1 – 26 d; Inf. NH4+-N (Simulated ww)], Phase II [27 – 131 d; Inf. NH4+-N (Real ww) + Inf. NH4+-N (Simulated ww)] and Phase III [132 – 225 d; Inf. NH4+-N (Real ww)] in which the reactor was supplemented with varied ratios (0 – 7) of influent NH4+-N reflecting varying combinations of real and simulated wastewater. Significant findingsThe reactor exhibited average total nitrogen removal efficiency of approximately 83 %, 81 %, and 80 % in the subsequent phases I, II, and III, respectively at varied nitrogen loading rates (100 – 232 g N m−3 d−1). The 16S rRNA gene sequencing revealed significant enrichment of Candidatus Kuenenia (from 41.69 % to 48.11 %) on the 225th day which was primarily attributed to significant decline in Nitrospira from 23.9 % to 1.24 % (due to nitrite oxidizing bacteria inhibition on account of high free ammonia concentrations in the reactor) and Nitrosomonas from 4.32 % to 0.01 %. Additionally, genera that were not initially present (on the 1st day) including Stenotrophobacter (17.34 %), Sphingobium (8.41 %), Rhodococcus (5.88%) and Ohtaekwangia (5.84 %) were observed to emerge by the end of the experiment (225 d).

Biofouling control of reverse osmosis membrane using free ammonia as a cleaning agent

Reverse osmosis (RO) is an important and widely-used membrane separation process for water recycling. However, biofouling is extensively considered a major problem for RO membranes due to the biofilm formation on the membrane surfaces. This study proposed and demonstrated a novel and sustainable chemical cleaning approach using a free ammonia (FA) solution for the removal of biofouling on RO membranes. The feasibility of FA solution for biofouling removal was investigated through a series of lab-scale soak cleaning tests and cross-flow cleaning tests on four fouled RO membranes (M1-M4) collected from municipal wastewater recycling plants. In soak cleaning tests on M1, FA concentrations of 65–560 mg NH3–N/L (pH = 8.9) can remove adenosine triphosphate (ATP) by 32–75 %, remove proteins by 18–47 % and remove polysaccharides by 31–74 %, which was up to 3.4 times of the removals by using NaOH solution under the same pH. FA solution of 310–560 mg NH3–N/L (pH = 8.9) even showed higher removals than NaOH solution with a higher pH of 11. In the cross-flow cleaning tests on M2-M4, FA solution of 310–560 mg NH3–N/L (pH = 8.9) removed the ATP by 82–100 %, removed proteins by 58–87 % and removed polysaccharides by 68–100 % in the fouling layers and increased the permeability by 8–16 %. Such cleaning effects in cross-flow tests were also positively correlated (R > 0.9, p < 0.05). Compared to the conventional anti-biofouling agent of NaOH solution (pH = 11), FA solution (310–560 mg NH3–N/L) showed significantly better cleaning performance. A high FA concentration of 560 NH3–N/L (pH = 8.9) could achieve comparable cleaning effect to 1 % EDTA solution (pH = 10). Additionally, increasing the frequency of cleaning or using a higher concentration of FA solution for biofouling removal will be more advantageous to prolong the membranes’ lifespan. The findings provide a promising alternative to using FA as a cost-effective and environmentally friendly solution for cleaning biofouling on RO membranes.

Production of algal biomass on electrochemically recovered nutrients from anaerobic digestion centrate

Microalgae cultivation on the nutrients in anaerobic digestion (AD) centrate represents a promising strategy for circular economy by converting wastes to valuable products. However, direct cultivation of microalgae in AD centrate would introduce some toxic compounds, bacteria, high turbidity, and coloration that could hamper biomass production. Herein, nutrients were extracted from AD centrate by an electrochemical nutrient recovery cell (ENRC) and then used to support microalgal growth. At an applied current density of 4 A m−2, the ENRC recovered 67.5 ± 1.2 % of NH4+-N and 93.1 ± 2.1 % of PO4−3-P from AD centrate at energy consumption of 16.49 ± 1.97 kWh kgN−1 or 177.49 ± 13.11 kWh kgP−1. The high free ammonia concentration of 214.46 mg L−1 in the nutrient recovery solution inhibited microalgal growth, and this effect could be reduced by dilution, which led to a maximum biomass concentration of 0.68 ± 0.04 g L−1with chlorophyll a concentration of 10.10 ± 0.20 mg L−1 at 35 times of dilution. The second mitigation strategy was to control the pH of nutrient recovery solution below the pK value of NH4+/NH3. At pH 8, the initial NH3-N concentration in the 5× solution was 12.14 mg L−1, resulting in an enhanced growth of Chlorella vulgaris with a biomass concentration of 1.02 ± 0.02 g L−1 and chlorophyll a concentration of 13.30 ± 0.34 mg L−1. To improve microalgal growth in the ENRC-based nutrient recovery, one should also consider the tradeoff between inorganic carbon supply and free ammonia formation under the influence of pH.

Dynamic calibration of process-wide partial-nitritation modeling with airlift granular for nitrogen removal in a full-scale wastewater treatment plant

A main challenge in rapid nitrogen removal from rejected water in wastewater treatment plants (WWTPs) is growth of biomass by nitrite-oxidizing bacteria (NOB) and ammonia-oxidizing bacteria (AOB). In this study, partial nitritation (PN) coupled with air-lift granular unit (AGU) technology was applied to enhance nitrogen-removal efficiency in WWTPs. For successful PN process at high-nitrogen-influent conditions, a pH of 7.5–8 for high free-ammonia concentrations and AOB for growth of total bacterial populations are required. The PN process in a sequential batch reactor (SBR) with AGU was modeled as an activated sludge model (ASM), and dynamic calibration using full-scale plant data was performed to enhance aeration in the reactor and improve the nitrite-to-ammonia ratio in the PN effluent. In steady-state and dynamic calibrations, the measured and modeled values of the output were in close agreement. Sensitivity analysis revealed that the kinetic and stoichiometric parameters are associated with growth and decay of heterotrophs, AOB, and NOB microorganisms. Overall, 80% of the calibrated data fit the measured data. Stage 1 of the dynamic calibration showed NO2 and NO3 values close to 240 mg/L and 100 mg/L, respectively. Stage 2 showed NH4 values of 200 mg/L at day 30 with the calibrated effluent NO2 and NO3 value of 250 mg/L. In stage 3, effluent NH4 concentration was 200 mg/L at day 60.

Inhibition Effects of Free Ammonia (FA) on the Rates of Growth, Photosynthesis and Respiration of Chlorella vulgaris

The potential use of microalgae in advanced biological wastewater treatment processes has been actively studied due to their excellent nitrogen and phosphorus removal capabilities. This study investigated the inhibitory effects of free ammonia (FA) on microalgae under autotrophic, heterotrophic, and mixotrophic growth conditions, since the inhibitory effects in heterotrophic and mixotrophic modes has not been reported yet. Chlorella vulgaris were exposed to 3–20 mg-N L−1 of FA in the autotrophic, heterotrophic, and mixotrophic modes and the effects on growth, photosynthesis, and respiration were studied. The microalgal growth rate was 2–3 times greater in the mixotrophic mode than the other modes. Microalgal growth in the autotrophic mode was most vulnerable to FA and the growth inhibition was observed at FA concentration of ≥ 9 mg-N L−1. Similarly, the photosynthesis and respiration of C. vulgaris were inhibited at FA concentrations higher than 12 mg-N LT−1. The removal of carbon and ammonia was greater in the mixotrophic mode than the other modes. The ammonia removal by microalgae increased in proportion to the FA concentration unlike the carbon removal. This was attributed to the ammonium accumulation in microalgae due to the proton imbalance and differences in intracellular and external pH conditions, which were caused by the presence of excess FA in water. Overall, the results show that the mixotrophic microalgae have greater tolerance to FA, and hence, greater potential for use in treatment of wastewaters having high FA concentrations.

Sulfidation forwarding high-strength Anammox process using nitrate as electron acceptor via thiosulfate-driven nitrate denitratation

A single up-flow thiosulfate-driven nitrate denitratation coupled with the sulfurized Anammox (TDSA) with the core–shell structure (S0@ Anammox granules) provided a chemical/energy-saving way for the removal of high-content ammonium with nitrate as electron acceptor. Approximately 83.66% total nitrogen removal efficiency (TNRE) could be achieved by the sulfurized Anammox encrusted by S0/Sn2− at a high loading rate (2.6 kg-N/(m3·d)) via resisting high concentration of free ammonia (FA) (22.35 mg/L), mainly through S2O32−, S0/Sn2− -driven partial denitrification-Anammox (PDN-Anammox) process. Moreover, S0/Sn2−-PDN-Anammox was largely restricted when intermittently aerated, but still resulting in 74.47% TNRE due to the partial nitrification-Anammox (PN-Anammox). The sequencing analysis revealed that Anammox bacterium (Candidatus_Kuenenia) and sulfur-oxidizing bacterium (Thiobacillus) coexisted, in which Anammox process occurred mainly via NO instead of NH2OH. This study provided a new perspective for high concentration nitrogen wastewater removal in engineering applications.

Bioaugmentation with syntrophic volatile fatty acids-oxidizing consortia to alleviate the ammonia inhibition in continuously anaerobic digestion of municipal sludge

Ammonia inhibition easily affects the performance of anaerobic digestion (AD) for municipal sludge and the oxidization of volatile fatty acids (VFAs) is the rate-limiting step of this process. Bioaugmentation is considered to be an effective method to alleviate ammonia inhibition of AD, but most study used the hydrogenotrophic methanogens as the bioaugmentation culture. In this study, bioaugmentation of mesophilic AD (MAD) and thermophilic AD (TAD) under ammonia inhibition with syntrophic acetate and propionate oxidizing consortia was investigated. The results showed that the bioaugmented reactors recovered earlier than control reactors with 20 (MAD) and 8 (TAD) days, respectively. The high-throughput 16S rRNA gene sequencing indicated that the relative abundance of carbohydrates fermenter (Lentimicrobium), syntrophic VFAs-oxidizing bacteria (Rikenellaceae_DMER64, Smithella and Syntrophobacter) and acetoclastic and hydrogenotrophic methanogens (Methanosaeta, Methanolinea and Methanospirillum) increased in MAD after bioaugmentation. However, part of the bioaugmentation culture could not adapt to the high free ammonia (FAN) concentration in MAD and the effect was weakened. In TAD, proteolytic bacteria (Keratinibaculum and Tepidimicrobium), syntrophic VFAs-oxidizing bacteria (Syntrophomonas) and hydrogenotrophic methanogen (Methanosarcina) were strengthened. The effect of bioaugmentation in TAD was durable even at higher organic loading rate (OLR), due to its positive influence on microbial community. These results suggested that the different bioaugmentation mechanism occurred in MAD and TAD, which are derived from the synergetic effects of ammonia tolerance and microbial interactions. Our study revealed the VFAs-oxidizing consortia as bioaugmented culture could be the potential strategy to alleviate the ammonia stress of AD.

Kinetic study of real landfill leachate treated by non-thermal plasma (NTP) and granular sequential batch reactors (GSBR)

The biological treatment of landfill leachate is a challenging process due to the high concentration of chemical oxygen demand (COD), nitrogen compounds, and other toxic compounds. Furthermore, nitrification inhibition may occur in landfill leachate due to the high concentration of free ammonia and inhibitors, such as poisonous compounds. Accordingly, this study aimed to use non-thermal plasma and aerobic granules to reduce organic loading rate and heavy metals of landfill leachate. We used a plasma reactor with a working volume of 0.5 l, quartz material for the pretreatment process. According to the results, the plasma reactor was most efficient in removing metals as well as d quasi-metals (e.g., arsenic, antimony, selenium, cobalt, iron, strontium, and silver) and least efficient in the removal of nickel, zinc, aluminum, and manganese. Our findings indicated that the COD removal efficiency decreased from 100% to 98%, from 93.5% to 89.18%, and from 92% to 88%, for 20%, 50%, plus 100% leachate, and three hydraulic retention times (6, 4.5, and 2.1 h), respectively. The amount of organic matter removal was a function of concentration and followed the second-order kinetic (R2 > 0.9) and Stover-Kincannon models (R2 > 0.97). Thus, it could be concluded that aerobic granules could effectively reduce e the organic load, COD, and heavy metals entering a GSBR reactor. Due to the good efficiency of the plasma reactor and GSBR, so we proposed full- scale studies at the landfill site for heavy metal and organic loading reduction. It also presents a new subject of research and concludes with an outlook on future investigation topics, such as different contaminants in leachate of landfills from different areas.

Insight into the mechanism of nitritation establishment through external fermented sludge addition

In this study, a novel strategy for nitritation establishment through external fermented sludge addition is proposed. The nitrifying activities under various fermentation times were assessed; batch tests results revealed that the decay rate of nitrite oxidizing bacteria (0.53 ± 0.08 d−1) was much higher than that of ammonia oxidizing bacteria (0.15 ± 0.04 d−1), during alkaline fermentation with high endogenous free ammonia concentrations (0.57–138.79 mg·L−1). The long-term effects of fermented sludge addition were constantly monitored in an anaerobic/oxic/anoxic sequencing batch reactor. During 60 days of operation, the nitrite accumulation rate reached above 90% and could be maintained stably. Total inorganic nitrogen removal achieved 94.7%, with an average external sludge reduction rate of 61.3%, after incorporation of the sludge addition. Illumina MiSeq sequencing demonstrated that nitrite oxidizing bacteria were successfully eliminated and the relative abundance of ammonia oxidizing bacteria remained at 0.39%, resulting in nitrite accumulation.

Role of syntrophic acetate oxidation and hydrogenotrophic methanogenesis in co-digestion of blackwater with food waste

Blackwater collected from toilets represents a type of sustainable bioenergy resource in the modern sanitation system, while its biomethane recovery efficiencies through anaerobic digestion were limited by slow hydrolysis and inhibited methanogenesis due to a large fraction of solid organics and high free ammonia concentrations. In the current study, food waste and blackwater co-digestion was performed in an up-flow anaerobic sludge blanket (UASB) reactor (35 °C). Co-substrates with increasing food waste proportions were stepwise applied to demonstrate the threshold organic loading rate (OLR). Co-digestion effectively enhanced substrate hydrolysis efficiency by up to 86.1% and methanisation rate by up to 39.7% compared to blackwater mono-digestion. Hydrogenotrophic methanogens showed predominance in both feeding conditions. The dominant bacterial groups shifted from genus Bacteroides to T78, and methanogenic groups shifted from genus Methanogenium to Methanoculleus and Methanospirillum when the operation system shifted from blackwater mono-digestion to food waste co-digestion. The microbial community structures and the isotopic carbon analysis for CH4 and CO2 in the produced biogas indicated that a combined syntrophic acetate oxidation (SAO) and hydrogenotrophic methanogenesis (HM) pathway was established throughout the operation. The enhanced substrates’ properties including higher carbon/nitrogen (C/N) ratios and more readily biodegradable organics in food waste and blackwater co-digestion system contributed to the enhancements in biomethane recovery and microbial development compared to blackwater mono-digestion. The OLR stress under the overloaded condition negatively affected the microbial community structure and resulted in process deterioration.

Optimization of a novel single air-lift sequencing bioreactor for raw piggery wastewater treatment: Nutrients removal and microbial community structure analysis

In this study, a sequencing batch and intermittent air-lift bioreactor (SBIAB) was evaluated under the three independent variables to treat raw piggery wastewater. The effects of hydraulic retention time (HRT), air flow rate and sludge retention time (SRT) on the nutrients removal of SBIAB were researched. The optimum values of HRT, air flow rate and SRT were 8 d, 2 l/min and 20 d, respectively. Meanwhile, the removal rates of chemical oxygen demand (COD), NH4+-N, total nitrogen (TN) and total phosphorus (TP) were up to 89.5%, 93.5%, 61.1% and 57.3%, respectively. Generally, the nutrients removal performance could be enhanced with increasing HRT from 6 to 10 d, while it was inhibited at air flow rate of 3 l/min. Higher air flow rate caused the alkaline pH and high free ammonia concentration, which imposed restrictions on the process of wastewater treatment. In the SBIAB, a coupling of aerobic/anoxic/anaerobic zone was formed according to the changes of oxidation-reduction potential (ORP) values at the optimum condition. Microbial community structure analysis indicated that the functional microbes including Brachymonas, Prokaryote, Giesbergeria, Comamonadaceae bacterium, Clostridiales bacterium, Comamonas, Tissierella, Aequorivita were enriched in the SBIAB, which played a significant role in the removal of complex organics and nitrogen.

Urine nitrification robustness for application in space: Effect of high salinity and the response to extreme free ammonia concentrations

Urine nitrification is one of the possibilities for the future recovery of water and elements for soilless crop production in space systems. The start-up of artificial urine nitrification was conducted for over 85 days in a sequencing batch reactor (SBR). Two free ammonia (FA) incidents occurred, which gave the opportunity to demonstrate the impressive ability of nitrifiers to resist temporary inhibition by FA without long lasting adverse effects. The failures led to very high FA concentrations of 280 and 84 gN-NH3/m3, respectively. Sludge was exposed to FA for 19 and 27 h. It was possible to restore nitrification with simple remedy actions (dilution and pH restoration). No inhibitory effects on the nitrification rate were seen after implementation of the remedy actions and the nitrification rate increased considerably (up to 300% of the pre-failure value) due to decrease in salinity. After a few days of normal operation and salt concentration, the nitrification rate returned to the pre-failure values in both cases.

Suppression of nitrite-oxidizing bacteria under the combined conditions of high free ammonia and low dissolved oxygen concentrations for mainstream partial nitritation

Abstract The possibility of nitrite-oxidizing bacteria (NOB) suppression in mainstream wastewater (COD/N of 6.0, ammonia of approximately 50 mg N L−1) was investigated by controlling the ratio of dissolved oxygen to total ammoniacal nitrogen (DO/TAN) in a continuous suspended sludge reactor without prior COD removal. It was found that a combination of high free ammonia (FA) of approximately 20 mg N L−1 and low dissolved oxygen (DO) of 0.3 ± 0.1 mg L−1 (DO/TAN ratio of 0.003) established in the start-up period was an efficient way to inhibit the growth of NOB. The activity of NOB was more inhibited as FA concentration increased, particularly at the initial ammonia concentration of 175 mg N L−1 (FA of about 20 mg N L−1) Then, the influent ammonia was lower to 50 ± 2 mg N L − 1 (FA of 4.9 ± 0.2 mg N L−1), a typical nitrogen concentration of mainstream wastewater. After four months of operation, mainstream partial nitritation (PN) became stable with nitrite accumulation ratio over 87% and COD removal of above 94% (effluent COD of 18.4 mg L−1). Besides, the effluent NO 2 − -N/NH3-N ratio was 1.22 ± 0.12, which was suitable for the subsequent anaerobic ammonium oxidation (anammox) reaction. The relative abundance of ammonia-oxidizing bacteria (AOB) (e.g., Nitrosomonas) was 15 times higher than NOB. The abundance (0.3%) of Nitrospira (k-strategist NOB) was insignificant, while Nitrobacter (r-strategist NOB) and Nitrotoga (NOB) were not detected during the steady state of PN under low nitrogen load because NOB was suppressed and washed out of the reactor by the condition of extremely low DO/TAN ratio in the start-up period. Thus, it was noted that NOB suppression for stable PN could be accomplished by providing high FA and low DO concentrations, especially in the early stage.

Free ammonia is the primary stress factor rather than total ammonium to Chlorella sorokiniana in simulated sludge fermentation liquor

Microalgal growth was inhibited in high-strength ammonium wastewater, but the actual stress factor and their physiological responses remain to be elucidated. In this study, compared to either the total ammonium or pH alone, free ammonia (FA) concentration clearly exhibited negative correlation with the growth of Chlorella sorokiniana cultivated in the simulated sludge fermentation liquor (SFL), which could be well described by an exponential decay model, indicating of the primary stress factor. Subsequently, six concentrations of FA were selected for further investigation and samples were monitored for: 1) growth and ammonium removal; 2) changes in photosynthetic system; 3) changes in reactive oxygen species (ROS) and soluble extracellular polymeric substances (S-EPS); as well as 4) biomass biochemical composition. Both the growth performance and photosynthetic parameters Fv/Fm and YII negatively correlated with FA concentrations. The highest biomass production was observed at the lowest initial FA concentration (2.06 mg/L) and almost no growth was observed in the highest FA concentration (110.97 mg/L).Ammonium nitrogen was removed by algae uptake and evaporation. Cell stress was indicated by significantly increased production of ROS (2.53–12.92 FI/1010cells) and secretion of S-EPS (144.13–160.86 mg/g DW). High FA concentrations induced the accumulation of cellular carbohydrates and palmitic acid. The stress responses to FA observed provide a guide in strategies for microalgal adaptation and growth in high-strength ammonium wastewater.

Partial nitrification-reactor configurations, and operational conditions: Performance analysis

• Essential parameters influencing partial nitrification were evaluated. • Bioreactors configurations and control strategies for nitritation were summarized. • Up-to date full-scale application of partial nitrification was demonstrated. • Research gaps for partial nitrification was presented and discussed. This comprehensive literature analysis evaluated essential partial nitrification (PN) parameters including various reactor configurations, control strategies on dissolved oxygen (DO) and aeration, pH, inhibition via free ammonia (FA) and free-nitrous acid (FNA), and oxidation reduction potential (ORP) in both pilot and full-scale systems. The median growth rates for ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) at 20 °C are 0.74 and 0.65 d −1 , respectively. The median first-order decay coefficient for both AOB and NOB was 0.14 d −1 while the average biomass yields were 0.17 and 0.08 mg chemical oxygen demand (COD) mg N −1 . The COD/N ratio is one of the critical factors for partial nitrification, particularly for attached growth systems. The optimum COD/N ratio for partial nitrification appears to be system-specific, as it is impacted both by system design parameters and operational conditions. The most widely used PN technology is the sequencing batch reactor (SBR) despite significantly lower volumetric nitrogen loading rates due to the ease of NOB suppression by the relatively high FA and FNA concentrations during the early stage of aeration. This review identified several knowledge gaps including better understanding of PN mechanisms in attached growth systems, combination of PN and enhanced biological phosphorous removal (EBPR), and development of process control strategies for technologies other than SBRs. Mainstream low-temperature application and combination with EBPR, particularly in light of the inhibitory impact of nitrites on phosphorus accumulating bacteria, are two major challenges hindering the widespread application of partial nitrification.

Efficient poultry manure management: anaerobic digestion with short hydraulic retention time to achieve high methane production

The efficient treatment or appropriate final disposal of poultry manure (PM) to avoid serious environmental impacts is a great challenge. In this work, the optimization of a 2-stage anaerobic digestion system (ADS) for PM was studied with the aim of reaching a maximal methane yield with a short hydraulic retention time (HRT). Three activities were performed: The first activity, ADS 1, consisted of evaluating the effect of the substrate concentration and the HRT on the process, with a constant organic loading rate (OLR) of 3.66 ± 0.21 gVS L−1 d−1. The second activity, ADS 2, consisted of decreasing the HRT from 9.09 to 2.74 d with a constant substrate concentration. In the third activity, ADS 3, the substrate concentration was increased from 10.09 ± 1.41 to 35.25 ± 6.20 gVS L−1 with an average HRT of 4.66 ± 0.11 d.Maximal methane yields of 0.22, 0.21, and 0.22 LCH4 gVS−1 were reached for ADS 1, ADS 2, and ADS 3, respectively, at a low HRT (3.38 to 4.66 d) and high free ammonia concentration (between 323.05 ± 56.48 and 460.93 ± 135.40 mgN-NH3 L−1). These methane yields correspond to the production of 40.36 and 42.28 cubic meters of methane per ton of PM, respectively, and a laying hen produces between 47.45 and 54.75 kg of PM per year in Chile.Finally, this is the first study of the separate and combined effects of OLR, HRT and substrate concentration on the anaerobic digestion of PM. The results demonstrate the technical feasibility of the two-stage ADS treatment of PM with a short HRT; the system tolerates variations in the total ammonia nitrogen concentration of PM throughout the year and achieves a high methane yield when the correct operational conditions are selected.

Comparison Between Single and Two-Stage Anaerobic Digestion of Vegetable Waste: Kinetics of Methanogenesis and Carbon Flow

This study aims to compare the performance and kinetics between the single-stage anaerobic digestion (SAD) and the two-stage anaerobic digestion (TAD) of vegetable waste (VW). The SAD was performed using continuously stirred tank reactors. Meanwhile, the TAD experiment was set up using a combined system involving a continuously stirred tank for hydrolysis/acidogenesis and an upflow reactor for methanogenesis. The hydrolytic reactor operated as a batch process with a retention time (RT) of 9 days, while the methane reactor was a continuous process operation with RT of 20 days. Both TAD and SAD were controlled at a temperature of 36 °C. The SAD experiments lasted for 143 days, and were characterised by the kinetic rate constant k = 0.02 day−1 which was much lower than that for the TAD (k = 0.66 − 2.16 day−1). The SAD seemed to be inhibited by high concentration of free ammonia and low inoculum to substrate ratio; herein, only 17.8–22.3% of the initial carbon could be converted into biogas (equivalent to 91–110 Nml/g-VSadded) with low methane content (44.1–48.7%). Meanwhile, TAD converted 41.67% initial carbon to biogas (equivalent to 299.0–374.6 Nml/g-VSadded) with high methane content (71.68–81.0%). Moreover, methanogenesis in the TAD was highly stable which enabled the digestion process to return to normal state within a few days, even though the concentrations of the influent increased to double (6.5–24.5 g-COD/l). As per these results, the TAD was much more stable, faster, and stronger than the SAD.

Start-up of a Three-stage PN/A Granular Sludge Reactor for Treating Wastewater with High Concentrations of Ammonia

In order to apply partial nitritation-ANAMMOX (PN/A) technology to treat wastewater with high concentrations of ammonia, autotrophic nitrogen-removing granular sludge was crushed and inoculated into a three-stage continuous flow reactor. The nitrogen loading rate (NLR), dissolved oxygen (DO) concentration, and free ammonia (FA) levels in each compartment of the reactor were controlled over a 106-day period. Results showed that the nitritation process occurred with the inoculated granules during the initial phase. A limited aeration strategy was employed in the reactor at relatively high NLRs. Given the effective suppression of nitrite-oxidizing bacteria and the prevention of ANAMMOX bacteria from high DO conditions, the compact structure and nitrogen-removal activity of the granules could be improved. When the ammonia-nitrogen concentration was increased in the influent to 350 mg·L-1, the adverse impacts of high FA concentrations on the functional microbe activity in the first compartment should be eliminated. This occurs by reducing the influent pH and alkalinity dosage. This occurs by reducing the influent pH and degree of alkalinity. As a result, a total nitrogen removal rate of 7.2 kg·(m3·d)-1 was achieved in the reactor, which is 50 to 100 times higher than that of conventional activated sludge systems. The consistent improvement in the nitrogen-removal activity of the granules was demonstrated by batch testing at different aeration intensities. This showed that activity was greatest in the first compartment, which showed the highest granular maturity. In addition, a clear linear correlation (R2>0.97) was observed between the amount of extracellular polymeric substance and the specific nitrogen removal rate. This indicated that the dense granules played a positive role in enhancing the performance of the reactor.

Mechanistic understanding of the NOB suppression by free ammonia inhibition in continuous flow aerobic granulation bioreactors

A partial nitritation continuous flow reactor (CFR) was operated for eight months demonstrating that partial nitritation granular sludge can remain stable under continuous flow conditions. The ammonia oxidizing bacteria (AOB)-to-nitrite oxidizing bacteria (NOB) activity ratios were determined for a series of granule sizes to understand the impact of mass diffusion limitation on the free ammonia (FA) inhibition of NOB. When dissolved oxygen (DO) limitation is the only mechanism for NOB suppression, the AOB:NOB ratio was usually found to increase with the granule size. However, the trend is reversed when FA has an inhibitory effect on NOB, as was observed in this study. The decrease in AOB:NOB ratio indicates that smaller granules, e.g. diameter <150 μm, are preferred for nitrite accumulation when high FA concentration is present, as in the partial nitritation process. The trend was further verified by observing the increase in the apparent inhibition coefficient as granule size increased. Indeed, this study for the first time quantified the effect of diffusion limitation on the apparent inhibition coefficient of NOB in aerobic granules. A mathematical model was then utilized to interpret the observed suppression of NOB and predicted that NOB suppression was only complete at the granule surface. The NOB that did survive in larger granules was forced to dwell within the granule interior, where the AOB growth declines due to DO diffusion limitation. This means FA inhibition can be taken advantage of as an effective means for NOB suppression in small granules or thin biofilms. Further, both FA inhibition and DO limitation were found to be required for the suppression of NOB in mainstream aerobic granules.

Evaluation of predominant factor for shortcut biological nitrogen removal in sequencing batch reactor at ambient temperature.

The shortcut biological nitrogen removal (SBNR) process requires less aeration and external carbon due to the oxidization of ammonia into nitrite and its direct denitrification to nitrogen gas during the biological nitrogen removal process. However, this process produces a poor effluent containing NH4+, since the system has to maintain a high free ammonia (FA, NH3) concentration. To overcome this drawback, in this study, the solid retention time (SRT) and the dissolved oxygen (DO) concentration were controlled to achieve both a high ammonia removal rate and nitrite accumulation in the sequencing batch reactor (SBR) process, which can remove nitrogen from wastewater to the desired concentration and provide high free ammonia inhibition and continuous shock loading. When sufficient DO was supplied, nitrite did not accumulate with a 20-day SRT, but the wash-out of nitrite oxidizers in a shorter SRT resulted in a high nitrite accumulation. When DO acted as a limitation, nitrite accumulated at all SRTs. This indicates that nitrite accumulation is more highly influenced by SRT and DO concentration than by FA inhibition. Also, as nitrite accumulated over a 10-day SRT regardless of DO concentration, the accumulation was more highly influenced by SRT than by DO concentration.