Vertical Subsurface Flow Research Articles

Enhancement of total nitrogen removal through effluent recirculation and fate of PPCPs in a hybrid constructed wetland system treating urban wastewater.

The effect of effluent recirculation on the removal of total nitrogen (TN) and eight pharmaceuticals and personal care products (PPCPs) was evaluated during 9months in an experimental hybrid constructed wetland (CW) system applied in the treatment of urban wastewater. An Imhoff tank was followed by three stages of CWs (two 1.5-m2 vertical subsurface flow (VF) beds alternating feed-rest cycles, a 2-m2 horizontal (HF) and a 2-m2 free water surface (FWS) wetland in series). A fraction of the final effluent was recycled back to the Imhoff tank with a recirculation rate of 50% (hydraulic loading rate=0.37md-1). The system's performance varied throughout the study. In Period I (summer) consistently high load removal efficiencies of TN (89±5%) and a removal rate of 6.6±1.4gTNm-2d-1 were exhibited. In Period II (fall), the poor performance of the FWS during the senescence of macrophytes caused a large increase in organic matter, solids and nutrient concentrations, drastically deteriorating water quality. The determination of PPCPs was conducted during this period. Recalcitrant compounds, namely sulfamethoxazole, carbamazapine, TCEP and sucralose were negligibly removed in all CWs. However, noteworthy was the ≈30% removal of sucralose in the VF wetland. Caffeine (80%) and fluoxetine (27%) showed similar elimination rates in both VF and HF units, whereas trimethoprim and DEET were significantly better removed in the VF than in the HF. The concentration of the four latter compounds showed a severe increase in the FWS, indicating possible desorption from the sediment/biomass during adverse conditions. Harvesting of the aboveground biomass in this unit returned the system's performance back to normality (Period III), achieving 77±7% TN removal despite the winter season, proving effluent recirculation as an effective strategy for TN removal in hybrid CW systems when stringent restrictions are in place.

Effectiveness of pollutants removal in hybrid constructed wetlands – different configurations case study

In recent years, an increase in interest in hybrid constructed wetland systems (HCWs) has been observed. The aim of the paper is to compare different HCW configurations in terms of mass removal rates and efficiency of pollutants removal. Analysed data have been collected at multistage constructed wetlands in Poland, which are composed by at least two beds: horizontal subsurface flow (SSHF) and vertical subsurface flow (SSVF). The evaluation was focused on hybrid constructed wetlands performance with HF+VF vs. VF+HF configuration, where influent wastewater of the same composition was treated. In analysed HCWs, the effective removal of organic matter from 75.2 to 91.6% COD was confirmed. Efficiency of total nitrogen removal varied from 47.3 to 91.7%. The most effective removal of TN (8.3 g m −2 d −1 ) occurred in the system with VF+VF+HF configuration.

Effects of plant species on CH4 emission from integrated vertical subsurface flow constructed wetlands

Effects of plant species on CH4 emission from integrated vertical subsurface flow constructed wetlands

Application of subsurface vertical flow constructed wetlands to reject water treatment in dairy wastewater treatment plant

ABSTRACTThe paper presents the effects of applying subsurface vertical flow constructed wetlands (SS VF) for the treatment of reject water generated in the process of aerobic sewage sludge stabilization in the biggest dairy wastewater treatment plant (WWTP) in Poland. Two SS VF beds were built: bed (A) with 0.65 m depth and bed (B) with 1.0 m depth, planted with reeds. Beds were fed with reject water with hydraulic load of 0.1 m d−1 in order to establish the differences in treatment efficiency. During an eight-months research period, a high removal efficiency of predominant pollutants was shown: BOD5 88.1% (A) and 90.5% (B); COD 84.5% (A) and 87.5% (B); TSS 87.6% (A) and 91.9% (B); TKN 82.4% (A) and 76.5% (B); N-NH4+ 89.2% (A) and 85.7% (B); TP 30.2% (A) and 40.6% (B). There were not statistically significant differences in the removal efficiencies between bed (B) with 1.0 m depth and bed (A) with 0.65 m depth. The research indicated that SS VF beds could be successfully applied to reject water treatment in dairy WWTPs. The study proved that the use of SS VF beds in full scale in dairy WWTPs would result in a significant decrease in pollutants’ load in reject water. In the analyzed case, decreasing the load of ammonia nitrogen was of greatest importance, as it constituted 58% of the total load treated in dairy WWTP and posed a hazard to the stability of the treatment process.

Long-term performance of an integrated constructed wetland for advanced treatment of mixed wastewater

The aim of this study was to investigate long-term performance of a full-scale integrated constructed wetland (CW) system for advanced treatment of sanitary and industrial mixed wastewater. The system consisted of a vertical subsurface flow (VSF) CW, a free water surface (FWS) CW, and a horizontal subsurface flow (HSF) CW in series. Results from six-year operation showed that the system resulted in average removal efficiencies of 70%, 70%, 34%, 52%, 45%, 74%, 21%, 43%, and 98% for COD, NH4-N, NO3-N, TN, TP, TSS, F−, Ni, and E. coli, respectively. Especially, NH4-N was effectively removed by the VSF CW. With relatively high hydraulic loading rates (18.0–43.6cmd−1) and short hydraulic retention times (1.04–2.51 d), the removal efficiencies of the system were quite stable for most of the water quality parameters, while the removal efficiency of TN increased from 49% to 60% during the six years. The removal efficiencies of the system were generally stable with the change of seasons. This study demonstrates that the VSF+FWS+HSF CW system can be a valuable approach for advanced wastewater treatment to meet the more stringent discharge requirement.

Different leachate phytotreatment systems using sunflowers

The use of energy crops in the treatment of wastewaters is of increasing interest, particularly in view of the widespread scarcity of water in many countries and the possibility of obtaining renewable fuels of vegetable origin. The aim of this study was to evaluate the feasibility of landfill leachate phytotreatment using sunflowers, particularly as seeds from this crop are suitable for use in biodiesel production. Two different irrigation systems were tested: vertical flow and horizontal subsurface flow, with or without effluent recirculation. Plants were grown in 130L rectangular tanks placed in a special climatic chamber. Leachate irrigated units were submitted to increasing nitrogen concentrations up to 372mgN/L. Leachate was successfully tested as an alternative fertilizer for plants and was not found to inhibit biomass development. The experiment revealed good removal efficiencies for COD (η>50%) up until flowering, while phosphorous removal invariably exceeded 60%. Nitrogen removal rates decreased over time in all experimental units, particularly in vertical flow tanks. In general, horizontal flow units showed the best performances in terms of contaminant removal capacity; the effluent recirculation procedure did not improve performance. Significant evapo-transpiration was observed, particularly in vertical flow units, promoting removal of up to 80% of the inlet irrigation volume.

Nitrogen transforming bacteria within a full-scale partially saturated vertical subsurface flow constructed wetland treating urban wastewater

The aim of this study was to characterize the nitrogen transforming bacterial communities within a partially saturated vertical subsurface flow constructed wetland (VF) treating urban wastewater in southern Brazil. The VF had a surface area of 3144m2, and was divided into four wetland cells, out of which two were operated while the other two rested, alternating cycles of 30days. The nitrifying and denitrifying bacterial communities were characterized in wetland cell 3 (764m2 surface area) over a period of 12months by using the FISH technique. Samples were collected monthly (from Feb 2014 to Feb 2015) from different layers within the vertical profile, during operation and rest periods, comprising a total of 6 sampling campaigns while the cell was in operation and another 6 when the cell was at rest. This wetland cell operated with an average organic loading rate (OLR) of 4gCODm−2d−1 and a hydraulic loading rate of 24.5mmd−1. The rest periods of the wetland cell presented influences on the abundance of ammonia-oxidizing bacteria (AOB) (8% and 3% for feed and rest periods, respectively), and nitrite-oxidizing bacteria (NOB) (5% and 2% for feed and rest periods, respectively). However, there was no influence of the rest periods on the denitrifying bacteria. AOB were only identified in the top layer (AOB β-proteobacteria) in both operational and rest periods. On the other hand, the NOB (Nistrospirae and Nitrospina gracilis) were identified in feed periods just in the top layer and during rest periods just in the intermediate layer. The denitrifying bacteria (Pseudomonas spp. and Thiobacillus denitrificans) were identified from the intermediate layer downwards, and remained stable in both periods. Based on the identified bacterial dynamics, the partially saturated VF wetland operated under low OLR enabled favorable conditions for simultaneous nitrification and denitrification.

Continuous removal of Cr(VI) from wastewater by phytoextraction using Tradescantia pallida plant based vertical subsurface flow constructed wetland system

This study investigated the performance of Tradescantia pallida in continuous removal of Cr(VI) from contaminated water using a laboratory scale vertical subsurface flow (VSSF) constructed wetland system. The effect of different hydraulic residence time (HRT), i.e. 1, 2 and 3 day HRT, was first studied at an influent Cr(VI) concentration of 20 mg/L. Later experiments were carried out at different influent pH (7 and 4) and at different influent Cr(VI) concentration (20 and 30 mg/L), but with the same HRT of two days. Best results were achieved for 2 d HRT and for an influent pH of 7 with a maximum Cr(VI) removal efficiency of 97.2–98.3% and a maximum total Cr removal efficiency of 86–88.2%. Cr(VI) was found to be uniformly distributed in different segments of the wetland units. Compared with the control unit in which no plants were grown, the units with the plant biomass showed substantially high amount of Cr(VI) in its soil (63–68%). Bioconcentration factor and translocation factor values ranged from 16.45 to 23.21 and 0.37 to 0.90 respectively, which revealed the excellent Cr uptake potential of T. pallida. The highest accumulated Cr concentration (464.33 mg/kg) was found in plants grown in the system at pH 7.0, whereas the minimum accumulated Cr concentration (249.21 mg Cr/kg) was in plants grown in the system at pH 4.0. Furthermore, 3–4% of the total Cr input into the system was accumulated in the plants at the end of the operation. All these results revealed an excellent performance of the T. pallida plant based constructed wetland system for continuous removal of Cr(VI) from contaminated water.

Performance of an Ultraviolet Mutagenetic Polyphosphate-Accumulating Bacterium PZ2 and Its Application for Wastewater Treatment in a Newly Designed Constructed Wetland.

Total phosphorus (TP) removal performance and application for wastewater treatment of polyphosphate-accumulating bacteria (PAB) in constructed wetlands (CWs) were investigated. In this study, a novel isolated ultraviolet (UV) mutant PZ2 with phosphate-accumulating ability was screened from domestic wastewater and identified as Pseudomonas putida by 16S ribosomal DNA (rDNA) sequencing analysis. The TP removal performance of PZ2 in the synthetic wastewater reached the highest of 93.95% within 45h. Two vertical subsurface flow CWs planted with two aquatic macrophytes Canna indica and Acorus calamus were newly designed. After inoculating PZ2 into two CWs within 45h, the average chemical oxygen demand (COD), TP, and ammonia-nitrogen (NH3-N) removal efficiencies reached 68.50, 60.22, and 66.81%, respectively. Vegetation type and filter size significantly influenced the TP removal capability of PZ2 in CWs. Meanwhile, considerable qualitative differences were found in the pollutant removal efficiencies of PZ2 with and without CWs in synthetic wastewater. These results could also indicate potential applications of the UV mutagenesis in PAB isolation and the newly designed CWs in wastewater treatments.

Removal of antibiotics and antibiotic resistance genes from domestic sewage by constructed wetlands: Effect of flow configuration and plant species

This study aims to investigate the removal of antibiotics and antibiotic resistance genes (ARGs) in raw domestic wastewater by various mesocosm-scale constructed wetlands (CWs) with different flow configurations or plant species including the constructed wetland with or without plant. Six mesocosm-scale CWs with three flow types (surface flow, horizontal subsurface flow and vertical subsurface flow) and two plant species (Thaliadealbata Fraser and Iris tectorum Maxim) were set up in the outdoor. 8 antibiotics including erythromycin-H2O (ETM-H2O), monensin (MON), clarithromycin (CTM), leucomycin (LCM), sulfamethoxazole (SMX), trimethoprim (TMP), sulfamethazine (SMZ) and sulfapyridine (SPD) and 12 genes including three sulfonamide resistance genes (sul1, sul2 and sul3), four tetracycline resistance genes (tetG, tetM, tetO and tetX), two macrolide resistance genes (ermB and ermC), two chloramphenicol resistance genes (cmlA and floR) and 16S rRNA (bacteria) were determined in different matrices (water, particle, substrate and plant phases) from the mesocosm-scale systems. The aqueous removal efficiencies of total antibiotics ranged from 75.8 to 98.6%, while those of total ARGs varied between 63.9 and 84.0% by the mesocosm-scale CWs. The presence of plants was beneficial to the removal of pollutants, and the subsurface flow CWs had higher pollutant removal than the surface flow CWs, especially for antibiotics. According to the mass balance analysis, the masses of all detected antibiotics during the operation period were 247,000, 4920–10,600, 0.05–0.41 and 3500–60,000μg in influent, substrate, plant and effluent of the mesocosm-scale CWs. In the CWs, biodegradation, substrate adsorption and plant uptake all played certain roles in reducing the loadings of nutrients, antibiotics and ARGs, but biodegradation was the most important process in the removal of these pollutants.

Removal Behavior of Ibuprofen and Diclofenac in Different Constructed Wetlands

Ibuprofen and diclofenac, two commonly used pharmaceuticals, were studied to evaluate the removal behavior of acid pharmaceuticals in constructed wetlands (CWs) with different flow types, vegetation and seasons. It was shown that flow types influenced the results significantly. The average removal efficiency of ibuprofen in horizontal subsurface flow (HSSF) and vertical subsurface flow (VSSF) CWs (69% and 60%, respectively) was significantly higher than that in surface flow (SF) CWs (26%). For diclofenac, SF CWs (58%) was significantly higher than HSSF and VSSF CWs (49% and 43%, respectively). In addition, the presence of plants improved the efficiency of ibuprofen, while it had no significant influence on the removal of diclofenac. Paired t-test found out that summer and autumn season variety had little impact on the removal. Furthermore, the removal of diclofenac and DO, as well as COD, showed very significant negative correlations. Opposite to diclofenac, the removal of ibuprofen had good positive correlations with dehydrogenase activities. The research indicated that aerobic biodegradation behavior mainly took place in ibuprofen, yet anaerobic biodegradation and photolysis behavior were expected for diclofenac.

Organic load and hydraulic regime influence over the bacterial community responsible for the nitrogen cycling in bed media of vertical subsurface flow constructed wetland

The aim of this study was to evaluate the influence of the organic loading rate (OLR) and the hydraulic regime on the nitrifying and denitrifying bacterial community present in the bed media of vertical flow constructed wetland (VFCW) employed as urban wastewater treatment solution. Two microcosms filled with sand (d10=0.3mm and uniformity coefficient=6.2) simulated the vertical profile of VFCW and were monitored for 360days. Microcosm 1 was operated with a constant OLR of 41g COD m−2d−1 and with an average hydraulic loading rate (HLR) of 72mmd−1. Microcosm 2 was operated with a constant HLR of 170.5mmd−1 and with an average OLR of 104g COD m−2d−1. After 180days feeding weekly, an intermittent regime with 30days of feeding followed by 30days of rest period was adopted. In total, 7 periods were monitored (4 periods of feeding and 3 periods of rest). At the end of each period, the total, nitrifying and denitrifying bacterial communities were characterized using FISH technique. At the end of the experiment, genome sequencing of the bacterial community was performed at the surface and bottom of the two microcosms. The OLR and the HLR significantly affected microcosms’ performance and bacterial community structure. Rest periods did not affect Bacteria domain abundance. However, they presented an influence on nitrifying bacteria abundance. Ammonia oxidizing bacteria abundance (Nitrososmas sp and AOB β-proteobacteria) was higher in microcosm 1 in feeding (15%) and rest (3%), periods when compared to microcosm 2 (9% and 1% for feeding and rest periods). The same dynamics were observed for nitrite oxidizing bacteria (Nitrobacter sp, Nitrospirae and Nistrospina gracilis), with relative abundance of 18% and 10% for microcosm 1, and 8% and 4% for microcosm 2, in feeding and rest periods, respectively. Differently from nitrifying bacteria dynamics, a higher abundance of denitrifying bacteria (Pseudomonas spp.) was observed in rest periods (12% for microscom 1 and 18% for microcosm 2) than in feeding periods (2% for microcosm 1 and 10% for microcosm 2). The identification of nitrifying and denitrifying bacteria in both microcosms can indicate the process of simultaneous nitrification and denitrification.

Swine wastewater treatment using vertical subsurface flow constructed wetland planted with Napier grass

Abstract This research aims to investigate the pollutant removal efficiencies in swine wastewater using a vertical subsurface flow constructed wetland (VSF CW) planted with two species of Napier grass. The grass productivities were also cultivated and compared in order to provide information for species selection. Twelve treatment units were set up with the VSF CWs planted with Giant Napier grass ( Pennisetum purpureum cv. King grass) and Dwarf Napier grass ( Pennisetum purpureum cv. Mott.) with 2 and 5 cm d −1 of hydraulic loading rates (HLR). Comparisons of removal efficiency and grass productivity were analyzed using Duncan's Multiple Range Test and t-test at the significant level 0.05. Both species of Napier grass performed more than 70% of removal efficiency of BOD and TKN. The VSF CW planted with Giant Napier grass at 5 cm d −1 HLR performed the highest BOD removal efficiency of 94 ± 1%, while the 2 cm d −1 HLR removed COD with efficiency of 64 ± 6%. The results also showed the effluent from all treatment units contained averages of BOD, COD, TSS, TKN and pH that followed Thailand's swine wastewater quality standard. Average fresh yields and dry yields were between 4.6 ± 0.4 to 15.2 ± 1.2 and 0.5 ± 0.1 to 2.2 ± 0.1 kg m −2 , respectively. The dry yields obtained from four cutting cycles in five months of CW system operation were higher than the ones planted with a traditional method, but declined continuously after each cutting cycle. Both species of Napier grass indicated their suitability to be used in the VSF CW for swine wastewater treatment.

Performance of Tropical Vertical Subsurface Flow Constructed Wetlands at Different Hydraulic Loading Rates

The hydraulic loading rate (HLR) plays a vital role in pollutant removal in constructed wetlands. This study evaluated the effects of HLR variation on pollutant removal in continuously fed vertical subsurface flow (VSSF) constructed wetlands in tropical conditions. Three VSSF wetland beds (Length: 1.4 m, Width: 0.5 m and Depth: 0.6 m), filled with 10–20 mm gravel media, were set up in a tropical region. Two beds were planted with a locally available emergent macrophyte narrow‐leaf cattail (Typha angustifolia), and the remaining bed was used as a control without plants. The performance of these wetland mesocosms was tested at different HLRs of 2.5, 5, 7.5, 10, 12.5, 15, 20, 25, and 30 cm day−1 changing at 2 week intervals in two phases over a period of 6 months. The results revealed that both VSSF wetland systems are capable of substantial reduction of pollutants with a good buffering capacity up to 25 cm day−1 HLR.

Carbamazepine removal in three pilot-scale hybrid wetlands planted with ornamental species

The aims of this one-year study, were to evaluate the removal of carbamazepine (CBZ) in three pilot-scale two-stage hybrid constructed wetlands as well as to evaluate the performance of three emergent species (Thypha latifolia, Iris sibirica and Zantedeschia aethiopica) planted as a polyculture. The systems included horizontal subsurface flow wetlands (HSSF-CW), vertical subsurface flow wetlands (VSSF-CW) and stabilization ponds (SP). The three different configurations were: HSSF-CWs followed by SPs, HSSF-CWs followed by VSSF-CWs and VSSF-CWs followed by HSSF-CWs, which were identified as system I (SI), system II (SII) and system III (SIII) respectively. In addition, measurements of DO, Eh and pH, were taken in situ in order to know the internal system conditions. Out of the three hybrid systems, two were equally effective (SI and SII) and superior to SIII (p>0.05). In these two systems (HSSF-CWs- SPs and HSSF-CWs-VSSF-CWs) the average mass removals were 62.5±4.5% and 59.0±4.5%, respectively. CBZ removal was more effective under reductive environment (∼−60 to +50mV) and near-anoxic conditions (<1.5mg/L of DO). In addition, the two ornamental plants exhibited a better capacity to tolerate and take up CBZ in comparison to T. latifolia. These results demonstrate that it is possible to obtain higher CBZ removal efficiencies in CWs than those reported so far, under subtropical climate by using hybrid systems planted with a polyculture that include ornamental species and using a ground local filter material.

Application research of constructed wetland treatment with nano materials for a rural middle school in Qingdao

ABSTRACTA 24 m3/d waste water treatment station including structures, buildings, and non-standard equipment was designed. The design considered the current situation that the wastewater discharged without any disposal of the rural middle school. It also combined with the provisions of the relevant national environmental protection, and was strictly in accordance with the relevant effluent discharge standards and design requirements. The process of vertical subsurface flow constructed wetland system with nano materials was adopted after comprehensive consideration of various factors. The operating cost of the wastewater treatment system is mainly a small amount of electricity bills, which is no more than 0.1 RMB per ton wastewater. The advantage of the project is based on biological treatment as the main technical route and low operating cost. Project commissioning condition and the treatment effects are good.

Addition of Fe2+ increase nitrate removal in vertical subsurface flow constructed wetlands

Fe2+ can assist in the process of denitrification as electron donor to enhance nitrogen removal in constructed wetlands (CWs). In this study, Fe2+ was added to the influent of CWs aiming to investigate the effects of external Fe2+ addition on denitrification efficiency at different carbon/nitrogen (C/N). Community-level physiological profile (CLPP) was assessed according to substrate utilization patterns with Biolog Eco Plates™. Results showed that external Fe2+ remarkably improved the capability of denitrification in constructed wetlands. Nitrate removal rate was largely increased after the addition of 30mg/L Fe2+ to the CWs reactor for a 2-day hydraulic retention time (HRT) at the presence of organic carbon source with low C/N ratio of 2. In reactor without adding organic carbon source, Fe(II)–Fe(III) cycle reaction could not be completed, thus forming hydroxide precipitation leading to low effluent Fe2+ and total iron. However, the enhancement of Fe2+ on denitrification was not obvious at high C/N of 6 and 4 in that organic carbon acted as dominant electron donor in participating denitrification processes. Substrate microbial community structure and diversity in CWs were influenced by organic carbon source and Fe2+. The Shannon, Pielou and Simpson indices in the CWs with external Fe2+ were higher than the control. Principal component analysis (PCA) showed that the CLPPs in constructed wetlands of different influents were significantly different. The constructed wetlands with added organic carbon sources and Fe2+ were grouped together, while the wetlands with the low C/N and the wetlands with high C/N were detached.

ROLE OF PURUN TIKUS IN VERTICAL SUBSURFACE FLOW CONSTRUCTED WETLAND IN TREATING MANGANESE (Mn) FROM COAL MINE DRAINAGE

Constructed wetland (CW) is one of the passive methods used to process coal mine drainage. In this study, we used vertical (VF) subsurface flow (SSF) constructed wetland (CW) with purun tikus (Eleocharis dulcis). The purpose of the study was to assess the role of purun tikus in increasing the efficiency of constructed wetland to remove Mn from Coal Mine Drainage. The constructed wetland operated under batch hydraulic loading regime. The results showed that the efficiency of the VF-SSFCW System without purun tikus was 15.53% and VF-SSFCW with purun tikus 78.94%.

Vertical and lateral subsurface preferential flow along an agricultural catena

Despite all efforts, agricultural contaminants remain at alarming concentrations in Quebec surface waters. Preferential flow (PF) of soluble contaminants has been suggested as a contributing factor but has not been specifically studied in the humid climate of eastern Canadian soils. Three tracers were surface applied on plots along a catena on a loamy sand under intensive agricultural production in Beauce, Quebec. Tracer distribution in soil profiles was monitored three times over a 12-month period. At the summit, finger flow rapidly transported tracers into the subsurface. The subsurface preferential lateral flow rapidly brought the tracers downslope. Narrow points of preferential seepage and discharge and underground lateral PF were observed at the footslope. The summit and the backslope of the catena showed strong vertical and lateral subsurface PF, which made their contribution to subsurface tracer movement toward surface water equal to or greater than that of the footslope, in part because of the hydrological connectivity between summit, backslope, and surface water. PF and matrix flow were both significant in all parts of the catena. Therefore, all parts of a catena, even those far from surface water, should be considered when evaluating potential belowground contaminant transport toward surface water.

Nitrification cessation and recovery in an aerated saturated vertical subsurface flow treatment wetland: Field studies and microscale biofilm modeling

In aerated treatment wetlands, oxygen availability is not a limiting factor in sustaining a high level of nitrification in wastewater treatment. In the case of an air blower failure, nitrification would cease, potentially causing adverse effects to the nitrifying bacteria. A field trial was completed investigating nitrification loss when aeration is switched off, and the system recovery rate after the aeration is switched back on. Loss of dissolved oxygen was observed to be more rapid than loss of nitrification. Nitrate was observed in the effluent long after the aeration was switched off (48h+). A complementary modeling study predicted nitrate diffusion out of biofilm over a 48h period. After two weeks of no aeration in the established system, nitrification recovered within two days, whereas nitrification establishment in a new system was previously observed to require 20–45days. These results suggest that once established resident nitrifying microbial communities are quite robust.