Optimized aeration strategies for nitrogen and phosphorus removal with aerobic granular sludge

Biological wastewater treatment by aerobic granular sludge biofilms offers the possibility to combine carbon (COD), nitrogen (N) and phosphorus (P) removal in a single reactor. Since denitrification can be affected by suboptimal dissolved oxygen concentrations (DO) and limited availability of COD, different aeration strategies and COD loads were tested to improve N- and P-removal in granular sludge systems. Aeration strategies promoting alternating nitrification and denitrification (AND) were studied to improve reactor efficiencies in comparison with more classical simultaneous nitrification–denitrification (SND) strategies. With nutrient loading rates of 1.6 gCOD L⁻¹ d⁻¹, 0.2 gN L⁻¹ d⁻¹, and 0.08 gP L⁻¹ d⁻¹, and SND aeration strategies, N-removal was limited to 62.3 ± 3.4%. Higher COD loads markedly improved N-removal showing that denitrification was limited by COD. AND strategies were more efficient than SND strategies. Alternating high and low DO phases during the aeration phase increased N-removal to 71.2 ± 5.6% with a COD loading rate of 1.6 gCOD L⁻¹ d⁻¹. Periods of low DO were presumably favorable to denitrifying P-removal saving COD necessary for heterotrophic N-removal. Intermittent aeration with anoxic periods without mixing between the aeration pulses was even more favorable to N-removal, resulting in 78.3 ± 2.9% N-removal with the lowest COD loading rate tested. P-removal was under all tested conditions between 88 and 98%, and was negatively correlated with the concentration of nitrite and nitrate in the effluent (r = −0.74, p < 0.01). With low COD loading rates, important emissions of undesired N₂O gas were observed and a total of 7–9% of N left the reactor as N₂O. However, N₂O emissions significantly decreased with higher COD loads under AND conditions.     

N2O emission from a partial nitrification-anammox process and identification of a key biological process of N2O emission from anammox granules

Emission of nitrous oxide (N₂O) during biological wastewater treatment is of growing concern. The emission of N₂O from a lab-scale two-reactor partial nitrification (PN)-anammox reactor was therefore determined in this study. The average emission of N₂O from the PN and anammox process was 4.0 ± 1.5% (9.6 ± 3.2% of the removed nitrogen) and 0.1 ± 0.07% (0.14 ± 0.09% of the removed nitrogen) of the incoming nitrogen load, respectively. Thus, a larger part (97.5%) of N₂O was emitted from the PN reactor. The total amount of N₂O emission from the PN reactor was correlated to nitrite (NO₂⁻) concentration in the PN effluent rather than DO concentration. In addition, further studies were performed to indentify a key biological process that is responsible for N₂O emission from the anammox process (i.e., granules). In order to characterize N₂O emission from the anammox granules, the in situ N₂O production rate was determined by using microelectrodes for the first time, which was related to the spatial organization of microbial community of the granule as determined by fluorescence in situ hybridization (FISH). Microelectrode measurement revealed that the active N₂O production zone was located in the inner part of the anammox granule, whereas the active ammonium consumption zone was located above the N₂O production zone. Anammox bacteria were present throughout the granule, whereas ammonium-oxidizing bacteria (AOB) were restricted to only the granule surface. In addition, addition of penicillin G that inhibits most of the heterotrophic denitrifiers and AOB completely inhibited N₂O production in batch experiments. Based on these results obtained, denitrification by putative heterotrophic denitrifiers present in the inner part of the granule was considered the most probable cause of N₂O emission from the anammox reactor (i.e., granules).     

Source identification of nitrous oxide on autotrophic partial nitrification in a granular sludge reactor

Emission of nitrous oxide (N₂O) during biological wastewater treatment is of growing concern since N₂O is a major stratospheric ozone-depleting substance and an important greenhouse gas. The emission of N₂O from a lab-scale granular sequencing batch reactor (SBR) for partial nitrification (PN) treating synthetic wastewater without organic carbon was therefore determined in this study, because PN process is known to produce more N₂O than conventional nitrification processes. The average N₂O emission rate from the SBR was 0.32 ± 0.17 mg-N L⁻¹ h⁻¹, corresponding to the average emission of N₂O of 0.8 ± 0.4% of the incoming nitrogen load (1.5 ± 0.8% of the converted NH₄⁺). Analysis of dynamic concentration profiles during one cycle of the SBR operation demonstrated that N₂O concentration in off-gas was the highest just after starting aeration whereas N₂O concentration in effluent was gradually increased in the initial 40 min of the aeration period and was decreased thereafter. Isotopomer analysis was conducted to identify the main N₂O production pathway in the reactor during one cycle. The hydroxylamine (NH₂OH) oxidation pathway accounted for 65% of the total N₂O production in the initial phase during one cycle, whereas contribution of the NO₂⁻ reduction pathway to N₂O production was comparable with that of the NH₂OH oxidation pathway in the latter phase. In addition, spatial distributions of bacteria and their activities in single microbial granules taken from the reactor were determined with microsensors and by in situ hybridization. Partial nitrification occurred mainly in the oxic surface layer of the granules and ammonia-oxidizing bacteria were abundant in this layer. N₂O production was also found mainly in the oxic surface layer. Based on these results, although N₂O was produced mainly via NH₂OH oxidation pathway in the autotrophic partial nitrification reactor, N₂O production mechanisms were complex and could involve multiple N₂O production pathways.     

Microbial community distribution and activity dynamics of granular biomass in a CANON reactor

The application of microelectrodes to measure oxygen and nitrite concentrations inside granules operated at 20 °C in a CANON (Complete Autotrophic Nitrogen-removal Over Nitrite) reactor and the application of the FISH (Fluorescent In Situ Hybridization) technique to cryosectioned slices of these granules showed the presence of two differentiated zones inside of them: an external nitrification zone and an internal anammox zone. The FISH analysis of these layers allowed the identification of Nitrosomonas spp. and Candidatus Kuenenia Stutgartiensis as the main populations carrying out aerobic and anaerobic ammonia oxidation, respectively.Concentration microprofiles measured at different oxygen concentrations in the bulk liquid (from 1.5 to 35.2 mg O₂ L⁻¹) revealed that oxygen was consumed in a surface layer of 100-350 μm width. The obtained consumption rate of the most active layers was of 80 g O₂ (L_granule)⁻¹ d⁻¹. Anammox activity was registered between 400 and 1000 μm depth inside the granules. The nitrogen removal capacity of the studied sequencing batch reactor containing the granular biomass was of 0.5 g N L⁻¹ d⁻¹. This value is similar to the mean nitrogen removal rate obtained from calculations based on in- and outflow concentrations.Information obtained in the present work allowed the establishment of a simple control strategy based on the measurements of NH₄⁺ and NO₂⁻ in the bulk liquid and acting over the dissolved oxygen concentration in the bulk liquid and the hydraulic retention time of the reactor.     

Floatation and control of granular sludge in a high-rate anammox reactor

The granule floatation is a serious issue of the anammox (anaerobic ammonium oxidation) process when high loading rates are applied that results in instability or even system collapse. The present study reports the granule floatation in an anammox reactor when high loading rates were applied. The comparison of enlarged photos taken for the settling and floating granules showed that the two kinds of granules both contained macroscopic gas pockets accounting for 11 ± 14% of total volume. The settling granules had gas tunnels that could release the gas bubbles, while the floating granules did not. The presence of gas bubbles enclosed in the gas pockets led to the small density of 979.2 ± 15.8 mg L⁻¹ and flotation of anammox granules. Consequently, the flotation caused washout of anammox granules and the deterioration of anammox process (volumetric removal rate decreased from 4.00 to 2.46 kg N m⁻³ d⁻¹). The collection of floating granules, breaking them into small pieces and then returning to the anammox reactor proved an effective control strategy. The volumetric removal rate was finally up to 16.5 kg N m⁻³ d⁻¹ after the control strategy was put into use.     

Total and stable washout of nitrite oxidizing bacteria from a nitrifying continuous activated sludge system using automatic control based on Oxygen Uptake Rate measurements

Partial nitrification (ammonium oxidation to nitrite) has gained a lot of interest among researchers in the last years because of its advantages with respect to complete nitrification (ammonium oxidation to nitrate): decrease of oxygen requirements for nitrification, reduction of COD demand and CO₂ emissions during denitrification and higher denitrification rate and lower biomass production during anoxic growth.In this study, an extremely high-strength ammonium wastewater (3000-4000 mg N L⁻¹) was treated in a continuous pilot plant with a configuration of three reactors in series plus a settler. The system was operated under the maximum possible volumetric nitrogen loading rate, at mild temperature (around 25 °C), with high sludge retention time (around 30 d) and significant nitrifying biomass concentration (average of 1800 ± 600 mg VSS L⁻¹). The implemented control loops transformed the system, which was operating with complete nitrification, into a continuous partial nitrification system. Nitrite oxidizing bacteria (NOB) washout was accomplished with local control loops for pH and dissolved oxygen (DO) with proper setpoints for NOB inhibition (pH = 8.3 and DO = 1.2-1.9 mg O₂ L⁻¹) and with an inflow control loop based on Oxygen Uptake Rate (OUR) measurements, which allowed working at the maximum ammonium oxidation capacity of the pilot plant in each moment. This operational strategy maximized the difference between ammonia oxidizing bacteria (AOB) and NOB growth rates, which is the key point to achieve a fast and stable NOB washout. The results showed a stable operation of the partial nitrification system during more than 100 days and NOB washout was corroborated with fluorescence in-situ hybridization (FISH) analysis.     

Nitrate removal, communities of denitrifiers and adverse effects in different carbon substrates for use in denitrification beds

Denitrification beds are containers filled with wood by-products that serve as a carbon and energy source to denitrifiers, which reduce nitrate (NO₃⁻) from point source discharges into non-reactive dinitrogen (N₂) gas. This study investigates a range of alternative carbon sources and determines rates, mechanisms and factors controlling NO₃⁻ removal, denitrifying bacterial community, and the adverse effects of these substrates. Experimental barrels (0.2 m³) filled with either maize cobs, wheat straw, green waste, sawdust, pine woodchips or eucalyptus woodchips were incubated at 16.8 °C or 27.1 °C (outlet temperature), and received NO₃⁻ enriched water (14.38 mg N L⁻¹ and 17.15 mg N L⁻¹). After 2.5 years of incubation measurements were made of NO₃⁻-N removal rates, in vitro denitrification rates (DR), factors limiting denitrification (carbon and nitrate availability, dissolved oxygen, temperature, pH, and concentrations of NO₃⁻, nitrite and ammonia), copy number of nitrite reductase (nirS and nirK) and nitrous oxide reductase (nosZ) genes, and greenhouse gas production (dissolved nitrous oxide (N₂O) and methane), and carbon (TOC) loss. Microbial denitrification was the main mechanism for NO₃⁻-N removal. Nitrate-N removal rates ranged from 1.3 (pine woodchips) to 6.2 g N m⁻³ d⁻¹ (maize cobs), and were predominantly limited by C availability and temperature (Q₁₀ = 1.2) when NO₃⁻-N outlet concentrations remained above 1 mg L⁻¹. The NO₃⁻-N removal rate did not depend directly on substrate type, but on the quantity of microbially available carbon, which differed between carbon sources. The abundance of denitrifying genes (nirS, nirK and nosZ) was similar in replicate barrels under cold incubation, but varied substantially under warm incubation, and between substrates. Warm incubation enhanced growth of nirS containing bacteria and bacteria that lacked the nosZ gene, potentially explaining the greater N₂O emission in warmer environments. Maize cob substrate had the highest NO₃⁻-N removal rate, but adverse effects include TOC release, dissolved N₂O release and substantial carbon consumption by non-denitrifiers. Woodchips removed less than half of NO₃⁻ removed by maize cobs, but provided ideal conditions for denitrifying bacteria, and adverse effects were not observed. Therefore we recommend the combination of maize cobs and woodchips to enhance NO₃⁻ removal while minimizing adverse effects in denitrification beds.     

Influence of manganese and ammonium oxidation on the removal of 17 alpha-ethinylestradiol (EE2)

Flow-through reactors with manganese oxides were examined for their capacity to remove 17α-ethinylestradiol (EE2) at μg L⁻¹ and ng L⁻¹ range from synthetic wastewater treatment plant (WWTP) effluent. The mineral MnO₂ reactors removed 93% at a volumetric loading rate (B_V) of 5 μg EE2 L⁻¹ d⁻¹ and from a B_V of 40 μg EE2 L⁻¹ d⁻¹ on, these reactors showed 75% EE2 removal. With the biologically produced manganese oxides, only 57% EE2 was removed at 40 μg EE2 L⁻¹ d⁻¹. EE2 removal in the ng L⁻¹ range was 84%. The ammonium present in the influent (10 mg N L⁻¹) was nitrified and ammonia-oxidizing bacteria (AOB) were found to be of prime importance for the degradation of EE2. Remarkably, EE2 removal by AOB continued for a period of 4 months after depleting NH₄⁺ in the influent. EE2 removal by manganese-oxidizing bacteria was inhibited by NH₄⁺. These results indicate that the metabolic properties of nitrifiers can be employed to polish water containing EE2 based estrogenic activity.     

Production of polyhydroxyalkanoates in open, mixed cultures from a waste sludge stream containing high levels of soluble organics, nitrogen and phosphorus

In this study, the production of polyhydroxyalkanoates (PHAs) from waste activated sludge (WAS) was evaluated. PHAs were produced from fermented WAS pretreated via high-pressure thermal hydrolysis, a stream characterised by high levels of nutrients (approximately 3.5 g N L⁻¹ and 0.5 g P L⁻¹) and soluble organics. PHA-storing organisms were successfully enriched at high organic loading rates (6 g COD_sol L⁻¹ d⁻¹) under aerobic dynamic feeding in sequencing batch reactors at a sludge retention time of 6 d with a short feast length less than 20% of the cycle, and a maximum substrate concentration during feast of 1 g COD_VFA L⁻¹. The biomass enrichment, characterised by a decrease in species evenness based on Lorenz curves, provided a biomass that accumulated 25% PHA on a dry-biomass basis with yields on VFA of 0.4 Cmol Cmol⁻¹ in batch tests. The PHA consisted of ∼70 mol% 3-hydroxybutyrate and ∼30 mol% 3-hydroxyvalerate, and presented high thermal stability (T_d = 283-287 °C) and a molecular mass ranging from 0.7 to 1.0 × 10⁶ g mol⁻¹. Overall PHA storage was comparable to that achieved with other complex substrates; however, lower PHA storage rates (0.04-0.05 Cmol PHA⁻¹ Cmol X⁻¹ h⁻¹) and productivities (3-4 Cmol PHA L⁻¹ h⁻¹) were probably associated with a biomass-growth and high-respiration response induced by high levels of non-VFA organics (40-50% of COD_sol in feed) and nutrients. PHA production is feasible from pretreated WAS, but the enrichment and accumulation process require further optimisation. A milder WAS pretreatment yielding lower levels of non-VFA organics and readily available nutrients may be more amenable for improved performance.     

Nitrous oxide emissions for early warning of biological nitrification failure in activated sludge

Experiments were carried out to establish whether nitrous oxide (N₂O) could be used as a non-invasive early warning indicator for nitrification failure. Eight experiments were undertaken; duplicate shocks DO depletion, influent ammonia increases, allylthiourea (ATU) shocks and sodium azide (NaN₃) shocks were conducted on a pilot-scale activated sludge plant which consisted of a 315 L completely mixed aeration tank and 100 L clarifier. The process performed well during pre-shock stable operation; ammonia removals were up to 97.8% and N₂O emissions were of low variability (<0.5 ppm). However, toxic shock loads produced an N₂O response of a rise in off-gas concentrations ranging from 16.5 to 186.3 ppm, followed by a lag-time ranging from 3 to 5 h ((0.43-0.71) × HRT) of increased NH₃-N and/or NO₂⁻ in the effluent ranging from 3.4 to 41.2 mg L⁻¹. It is this lag-time that provides the early warning for process failure, thus mitigating action can be taken to avoid nitrogen contamination of receiving waters.     

Effect of pest controlling neem and mata-raton leaf extracts on greenhouse gas emissions from urea-amended soil cultivated with beans: A greenhouse experiment

In a previous laboratory experiment, extracts of neem (Azadirachta indica A. Juss.) and Gliricidia sepium Jacquin, locally known as mata-raton, used to control pests on crops, inhibited emissions of CO₂ from a urea-amended soil, but not nitrification and N₂O emissions. We investigated if these extracts when applied to beans (Phaseolus vulgaris L.) affected their development, soil characteristics and emissions of carbon dioxide (CO₂) and nitrous oxide (N₂O) in a greenhouse environment. Untreated beans and beans planted with lambda-cyhalothrin, a commercial insecticide, served as controls. After 117 days, shoots of plants cultivated in soil amended with urea or treated with lambda-cyhalothrin, or extracts of neem or G. sepium were significantly higher than when cultivated in the unamended soil, while the roots were significantly longer when plants were amended with urea or treated with leaf extracts of neem or G. sepium than when treated with lambda-cyhalothrin. The number of pods, fresh and dry pod weight and seed yield was significantly higher when bean plants were treated with leaf extracts of neem or G. sepium treatments than when left untreated and unfertilized. The number of seeds was similar for the different treatments. The number of nodules was lower in plants fertilized with urea, treated with leaf extracts of neem or G. sepium, or with lambda-cyhalothrin compared to the unfertilized plants. The concentrations of NH₄⁺, NO₂⁻ and NO₃⁻ decreased significantly over time with the lowest concentrations generally found at harvest. Treatment had no significant effect on the concentrations of NH₄⁺ and NO₂⁻, but the concentration of NO₃⁻ was significantly lower in the unfertilized soil compared to the other treatments. It was found that applying extracts of neem or G. sepium leaves to beans favored their development when compared to untreated plants, but had no significant effect on nitrification in soil.     

Biological removal of 17α-ethinylestradiol by a nitrifier enrichment culture in a membrane bioreactor

Increasing concern about the fate of 17α-ethinylestradiol (EE2) in the environment stimulates the search for alternative methods for wastewater treatment plant (WWTP) effluent polishing. The aim of this study was to establish an innovative and effective biological removal technique for EE2 by means of a nitrifier enrichment culture (NEC) applied in a membrane bioreactor (MBR). In batch incubation tests, the microbial consortium was able to remove EE2 from both a synthetic minimal medium and WWTP effluent. A maximum EE2 removal rate of 9.0 μg EE2 g⁻¹ biomass-VSS h⁻¹ was achieved (>94% removal efficiency). Incubation of the heterotrophic bacteria isolated from the NEC did not result in a significant EE2 removal, indicating the importance of nitrification as driving force in the mechanism. Application of the NEC in a MBR to treat a synthetic influent with an EE2 concentration of 83 ng EE2 L⁻¹ resulted in a removal efficiency of 99% (loading rates up to 208 ng EE2 L⁻¹ d⁻¹; membrane flux rate: 6.9 L m⁻² h⁻¹). Simultaneously, complete nitrification was achieved at an optimal ammonium influent concentration of 1.0 mg NH₄⁺-N L⁻¹. This minimal NH₄⁺-N input is very advantageous for effluent polishing since the concomitant effluent nitrate concentrations will be low as well and it offers opportunities for the nitrifying MBR as a promising add-on technology for WWTP effluent polishing.     

Halophyte filter beds for treatment of saline wastewater from aquaculture

The expansion of aquaculture and the recent development of more intensive land-based marine farms require efficient and cost-effective systems for treatment of highly nutrient-rich saline wastewater. Constructed wetlands with halophytic plants offer the potential for waste-stream treatment combined with production of valuable secondary plant crops. Pilot wetland filter beds, constructed in triplicate and planted with the saltmarsh plant Salicornia europaea, were evaluated over 88 days under commercial operating conditions on a marine fish and shrimp farm. Nitrogen waste was primarily in the form of dissolved inorganic nitrogen (TDIN) and was removed by 98.2 ± 2.2% under ambient loadings of 109-383 μmol l⁻¹. There was a linear relationship between TDIN uptake and loading over the range of inputs tested. At peak loadings of up to 8185 ± 590 μmol l⁻¹ (equivalent to 600 mmol N m⁻² d⁻¹), the filter beds removed between 30 and 58% (250 mmol N m⁻² d⁻¹) of influent TDIN. Influent dissolved inorganic phosphorus levels ranged from 34 to 90 μmol l⁻¹, with 36-89% reduction under routine operations. Dissolved organic nitrogen (DON) loadings were lower (11-144 μmol l⁻¹), and between 23 and 69% of influent DON was removed during routine operation, with no significant removal of DON under high TDIN loading. Over the 88-day study, cumulative nitrogen removal was 1.28 mol m⁻², of which 1.09 mol m⁻² was retained in plant tissue, with plant uptake ranging from 2.4 to 27.0 mmol N g⁻¹ dry weight d⁻¹. The results demonstrate the effectiveness of N and P removal from wastewater from land-based intensive marine aquaculture farms by constructed wetlands planted with S. europaea.     

Simultaneous nitrification, denitrification and carbon removal in microbial fuel cells

Microbial fuel cells (MFCs) can use nitrate as a cathodic electron acceptor, allowing for simultaneous removal of carbon (at the anode) and nitrogen (at the cathode). In this study, we supplemented the cathodic process with in situ nitrification through specific aeration, and thus obtained simultaneous nitrification and denitrification (SND) in the one half-cell. Synthetic wastewater containing acetate and ammonium was supplied to the anode; the effluent was subsequently directed to the cathode. The influence of oxygen levels and carbon/nitrogen concentrations and ratios on the system performances was investigated. Denitrification occurred simultaneously with nitrification at the cathode, producing an effluent with levels of nitrate and ammonium as low as 1.0 ± 0.5 mg N L⁻¹ and 2.13 ± 0.05 mg N L⁻¹, respectively, resulting in a nitrogen removal efficiency of 94.1 ± 0.9%. The integration of the nitrification process into the cathode solves the drawback of ammonium losses due to diffusion between compartments in the MFC, as previously reported in a system operating with external nitrification stage. This work represents the first successful attempt to combine SND and organics oxidation while producing electricity in an MFC.     

Emission of CO2 and N2O from soil cultivated with common bean (Phaseolus vulgaris L.) fertilized with different N sources

Addition of different forms of nitrogen fertilizer to cultivated soil is known to affect carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions. In this study, the effect of urea, wastewater sludge and vermicompost on emissions of CO₂ and N₂O in soil cultivated with bean was investigated. Beans were cultivated in the greenhouse in three consecutive experiments, fertilized with or without wastewater sludge at two application rates (33 and 55 Mg fresh wastewater sludge ha^− 1, i.e. 48 and 80 kg N ha^− 1 considering a N mineralization rate of 40%), vermicompost derived from the wastewater sludge (212 Mg ha^− 1, i.e. 80 kg N ha^− 1) or urea (170 kg ha^− 1, i.e. 80 kg N ha^− 1), while pH, electrolytic conductivity (EC), inorganic nitrogen and CO₂ and N₂O emissions were monitored. Vermicompost added to soil increased EC at onset of the experiment, but thereafter values were similar to the other treatments. Most of the NO₃⁻ was taken up by the plants, although some was leached from the upper to the lower soil layer. CO₂ emission was 375 C kg ha^− 1 y^− 1 in the unamended soil, 340 kg C ha^− 1 y^− 1 in the urea-amended soil and 839 kg ha^− 1 y^− 1 in the vermicompost-amended soil. N₂O emission was 2.92 kg N ha^− 1 y^− 1 in soil amended with 55 Mg wastewater sludge ha^− 1, but only 0.03 kg N ha^− 1 y^− 1 in the unamended soil. The emission of CO₂ was affected by the phenological stage of the plant while organic fertilizer increased the CO₂ and N₂O emission, and the yield per plant. Environmental and economic implications must to be considered to decide how many, how often and what kind of organic fertilizer could be used to increase yields, while limiting soil deterioration and greenhouse gas emissions.     

Treatment of a sanitary landfill leachate using combined solar photo-Fenton and biological immobilized biomass reactor at a pilot scale

A solar photo-Fenton process combined with a biological nitrification and denitrification system is proposed for the decontamination of a landfill leachate in a pilot plant using photocatalytic (4.16 m² of Compound Parabolic Collectors - CPCs) and biological systems (immobilized biomass reactor). The optimum iron concentration for the photo-Fenton reaction of the leachate is 60 mg Fe²⁺ L⁻¹. The organic carbon degradation follows a first-order reaction kinetics (k = 0.020 L kJ_UV⁻¹, r₀ = 12.5 mg kJ_UV⁻¹) with a H₂O₂ consumption rate of 3.0 mmol H₂O₂ kJ_UV⁻¹. Complete removal of ammonium, nitrates and nitrites of the photo-pre-treated leachate was achieved by biological denitrification and nitrification, after previous neutralization/sedimentation of iron sludge (40 mL of iron sludge per liter of photo-treated leachate after 3 h of sedimentation). The optimum C/N ratio obtained for the denitrification reaction was 2.8 mg CH₃OH per mg N-NO₃⁻, consuming 7.9 g/8.2 mL of commercial methanol per liter of leachate. The maximum nitrification rate obtained was 68 mg N-NH₄⁺ per day, consuming 33 mmol (1.3 g) of NaOH per liter during nitrification and 27.5 mmol of H₂SO₄ per liter during denitrification. The optimal phototreatment energy estimated to reach a biodegradable effluent, considering Zahn-Wellens, respirometry and biological oxidation tests, at pilot plant scale, is 29.2 kJ_UV L⁻¹ (3.3 h of photo-Fenton at a constant solar UV power of 30 W m⁻²), consuming 90 mM of H₂O₂ when used in excess, which means almost 57% mineralization of the leachate, 57% reduction of polyphenols concentration and 86% reduction of aromatic content.     

High-rate denitrification using polyethylene glycol gel carriers entrapping heterotrophic denitrifying bacteria

This study evaluated the nitrogen removal performance of polyethylene glycol (PEG) gel carriers containing entrapped heterotrophic denitrifying bacteria. A laboratory-scale denitrification reactor was operated for treatment of synthetic nitrate wastewater. The nitrogen removal activity gradually increased in continuous feed experiments, reaching 4.4 kg N m⁻³ d⁻¹ on day 16 (30 °C). A maximum nitrogen removal rate of 5.1 kg N m⁻³ d⁻¹ was observed. A high nitrogen removal efficiency of 92% on average was observed at a high loading rate. In batch experiments, the denitrifying gel carriers were characterized by temperature. Nitrate and total nitrogen removal activities both increased with increasing temperature, reaching a maximum at 37 and 43 °C, respectively. Apparent activation energies for nitrate and nitrite reduction were 52.1 and 71.9 kJ mol⁻¹, respectively. Clone library analysis performed on the basis of the 16S rRNA gene revealed that Hyphomicrobium was mainly involved in denitrification in the methanol-fed denitrification reactors.     

Phosphorus limitation in nitrifying groundwater filters

Phosphorus limitation has been demonstrated for heterotrophic growth in groundwater, in drinking water production and distribution systems, and for nitrification of surface water treatment at low temperatures. In this study, phosphorus limitation was tested, in the Netherlands, for nitrification of anaerobic groundwater rich in iron, ammonium and orthophosphate. The bioassay method developed by Lehtola et al. (1999) was adapted to determine the microbially available phosphorus (MAP) for nitrification. In standardized batch experiments with an enriched mixed culture inoculum, the formation of nitrite and nitrate and ATP and the growth of ammonia-oxidizing bacteria (AOB; as indicated by qPCR targeting the amoA-coding gene) were determined for MAP concentrations between 0 and 100 μg PO₄-P L⁻¹. The nitrification and microbial growth rates were limited at under 100 μg PO₄-P L⁻¹ and virtually stopped at under 10 μg PO₄-P L⁻¹. In the range between 10 and 50 μg PO₄-P L⁻¹, a linear relationship was found between MAP and the maximum nitrification rate. AOB cell growth and ATP formation were proportional to the total ammonia oxidized. Contrary to Lehtola et al. (1999), biological growth was very slow for MAP concentrations less than 25 μg PO₄-P L⁻¹. No full conversion nor maximum cell numbers were reached within 19 days. In full-scale groundwater filters, most of the orthophosphate was removed alongside with iron. The remaining orthophosphate appeared to have only limited availability for microbial growth and activity. In some groundwater filters, nitrification was almost totally prevented by limitation of MAP. In batch experiments with filtrate water from these filters, the nitrification process could be effectively stimulated by adding phosphoric acid.     

Long term partial nitritation of anaerobically treated black water and the emission of nitrous oxide

Black water (toilet water) contains half the load of organic material and the major fraction of the nutrients nitrogen and phosphorus in a household and is 25 times more concentrated, when collected with a vacuum toilet, than the total wastewater stream from a Dutch household. This research focuses on the partial nitritation of anaerobically treated black water to produce an effluent suitable to feed to the anammox process. Successful partial nitritation was achieved at 34 °C and 25 °C and for a long period (almost 400 days in the second period at 25 °C) without strict process control a stable effluent at a ratio of 1.3 NO₂-N/NH₄-N was produced which is suitable to feed to the anammox process. Nitrite oxidizers were successfully outcompeted due to inhibition by free ammonia and nitrous acid and due to fluctuating conditions in SRT (1.0-17 days) and pH (from 6.3 to 7.7) in the reactor. Microbial analysis of the sludge confirmed the presence of mainly ammonium oxidizers. The emission of nitrous oxide (N₂O) is of growing concern and it corresponded to 0.6-2.6% (average 1.9%) of the total nitrogen load.