Effect of hydraulic retention time on the treatment of secondary effluent in a subsurface flow constructed wetland

This study evaluates the potential of subsurface flow (SSF) constructed wetlands (CWs) for tertiary treatment of wastewater at four shorter HRTs (1–4 days). The CWs were planted with Typha angustata, which was observed in our earlier study to be more efficient than Phragmites karka and Scirpus littoralis. The CWs comprised four rectangular treatment cells (2.14 m × 0.76 m × 0.61 m) filled with layers of gravel of two different sizes (approximately 2.5 cm and 1.5 cm diameter) to a depth of 0.61 m. The inflow rates of the secondary effluent in the four cells were accordingly fixed at 300 L d^?1, 150 L d^?1, 100 L d^?1 and 75 L d^?1, respectively, for 1, 2, 3 and 4 days HRT. The hydraulic loads ranged between 59.05 mm d^?1 and 236.22 mm d^?1.The wastewater inflow into the CW system as well as the treated effluent were analyzed, using standard methods, at regular intervals for various forms of nitrogen (NH₄-N, NO₃-N and TKN), orthophosphate-P and organic matter (BOD and COD) concentrations over a period of five weeks after the development of a dense stand.The higher HRT of 4 days not only helped maximum removal of all the pollutants but also maintained the stability of the treatment efficiency throughout the monitoring period. For the nutrients (NH₄-N, NO₃-N and TKN), HRT played a more significant role in their removal than in case of organic matter (BOD₃ and COD). More than 90% of NO₃-N and TKN and 100% of NH₄-N were removed from the wastewater at 4 days HRT.At lower HRTs, the mass loading rate was higher with greater fluctuation. However mass reduction efficiency of the T. angustata CW for all forms of nitrogen was >80% with the HRTs of 2, 3 and 4 days.     

Potentials of anaerobic treatment for catalytically oxidized olive mill wastewater (OMW)

The catalytically oxidized olive mill wastewater (OMW) was subjected to continuous anaerobic treatment using two treatment schemes. The 1st step in both schemes was an up-flow anaerobic sludge blanket (UASB) reactor (2 0 l). The 2nd step was either a hybrid UASB reactor or a classical one (1 0 l, each). The 1st stage was operated at constant hydraulic retention time (HRT) of 24 h. The organic loading rate (OLR) varied from 3.4 to 4.8 kgCOD/m³ d depending on the quality of the pretreated wastewater. The results obtained indicated that, the 1st step UASB reactor achieved a COD percentage removal value of 53.9%. Corresponding total BOD₅ and TSS removal were 51.5% and 68.3%, respectively.The results obtained indicated that the hybrid UASB reactor as a 2nd step produced better quality effluent as compared to the classical one. This could be attributed to the presence of the packing curtain sponge with active biomass in the sedimentation part of hybrid UASB reactor which minimizes suspended solids washout, consequently enhancement of the efficiency of the reactor.Available data showed that a two stage system consisting of a classical and a hybrid UASB reactor operated at a total HRT of 48 h and OLR of 2.0 kgCOD/m³ d provided promising results. Removal values of COD_total, BOD_5 total, TOC, VFA, oil and grease were 83%, 84%, 81%, 93% and 81%, respectively. Based on the available data, the use of a two stage anaerobic system consisting of a classical UASB reactor followed by a hybrid UASB as a post-treatment step for catalytically oxidized OMW is recommended.     

Nitrate removal rates in woodchip media of varying age

A variety of low-cost carbonaceous solids have been successfully tested in bioreactors designed for nitrate treatment. In many agricultural and wastewater settings, however, such reactors may be practical only if they are maintenance free for a number of years after installation. Although field installations have demonstrated consistent treatment over multi-year timeframes, the ability to accurately quantify slowly declining reaction rates in field settings is problematic because of variations in reactor flow rates, ambient temperatures and influent chemistry. In this study, laboratory column tests were undertaken on four samples of coarse wood particle media (woodchips), two that were fresh and two that had been in continuous operation in subsurface denitrifying bioreactors for periods of 2 and 7 years respectively. Four experimental runs were undertaken at increasing influent NO₃-N concentrations of from 3.1 to 48.8 mg N L^?1. Nitrate mass removal rates remained relatively constant and did not systematically increase in successive runs at higher NO₃ concentrations indicating that NO₃ was not the rate-limiting substrate at these concentrations. Thus, zero-order reaction kinetics were used to model the attenuation reaction (presumably denitrification). The 7-year-old media had a mean NO₃-N removal rate of 9.1 mg N L^?1 d^?1 (6.4 g N m^?3 media d^?1), which remained within 75% of the rate for the 2-year-old media (12.1 mg N L^?1 d^?1 or 8.5 g N m^?3 media d^?11) and within 40–59% of the rate for the fresh chips (15.4–23.0 mg N L^?1 d^?1 or 10.8–16.1 g N m^?3 media d^?1). Results support field experience indicating that woodchips loose about 50% of their reactivity during their first year of operation as soluble organic compounds are leached out, but then relatively stable rates persist for a considerable number of years thereafter.     

Nitrate removal and greenhouse gas production in a stream-bed denitrifying bioreactor

Denitrifying bioreactors are currently being tested as an option for treating nitrate (NO₃^?) contamination in groundwater and surface waters. However, a possible side effect of this technology is the production of greenhouse gases (GHG) including nitrous oxide (N₂O) and methane (CH₄). This study examines NO₃^? removal and GHG production in a stream-bed denitrifying bioreactor currently operating in Southern Ontario, Canada. The reactor contains organic carbon material (pine woodchips) intended to promote denitrification. Over a 1 year period, monthly averaged removal of influent (stream water) NO₃^? ranged from 18 to 100% (0.3–2.5 mg N L^?1). Concomitantly, reactor dissolved N₂O and CH₄ production, averaged 6.4 μg N L^?1 (2.4 mg N m^?2 d^?1), and 974 μg C L^?1 (297 mg C m^?2 d^?1) respectively, where production is calculated as the difference between inflow and effluent concentrations. Gas bubbles entrapped in sediments overlying the reactor had a composition ranging from 19 to 64% CH₄, 1 to 6% CO₂, and 0.5 to 2 ppmv N₂O; however, gas bubble emission rates were not quantified in this study. Dissolved N₂O production rates from the bioreactor were similar to emission rates reported for some agricultural croplands (e.g. 0.1–15 mg N m^?2 d^?1) and remained less than the highest rates observed in some N-polluted streams and rivers (e.g. 110 mg N m^?2 d^?1, Grand R., ON). Dissolved N₂O production represented only a small fraction (0.6%) of the observed NO₃^? removal over the monitoring period. Dissolved CH₄ production during summer months (up to 1236 mg C m^?2 d^?1), was higher than reported for some rivers and reservoirs (e.g. 6–66 mg C m^?2 d^?1) but remained lower than rates reported for some wastewater treatment facilities (e.g. sewage treatment plants and constructed wetlands, 19,500–38,000 mg C m^?2 d^?1).     

Nitrate removal and hydraulic performance of organic carbon for use in denitrification beds

Denitrification beds are a cost-effective technology for removing nitrate from point source discharge. To date, field trials and operational beds have primarily used wood media as the carbon source; however, the use of alternative more labile carbon media could provide for increased removal rate, lower installation costs and reduced bed size. While previous laboratory experiments have investigated the potential of alternative carbon sources, these studies were typically of short duration and small scale and did not necessarily provide reliable information for denitrification bed design purposes. To address this issue, we compared nitrate removal, hydraulic and nutrient leaching characteristics of nine different carbon substrates in 0.2 m³ barrels, at 14 and 23.5 °C over a 23-month period. Mean nitrate removal rates for the period 10–23 months were 19.8 and 15 g N m^?3 d^?1 (maize cobs), 7.8 and 10.5 g N m^?3 d^?1 (green waste), 5.8 and 7.8 g N m^?3 d^?1 (wheat straw), 3.0 and 4.9 g N m^?3 d^?1 (softwood), and 3.3 and 4.4 g N m^?3 d^?1 (hardwood) for the 14 and 23.5 °C treatments, respectively. Maize cobs provided a 3–6.5-fold increase in nitrate removal over wood media, without prohibitive decrease in hydraulic conductivity, but had higher rates of nutrient leaching at start-up. Significant difference in removal rate occurred between the 14 and 23.5 °C treatments, with the mean Q₁₀ temperature coefficient = 1.6 for all media types in the period 10–23 months.     

Aerobic granulation in sequencing batch reactors with different reactor height/diameter ratios

Effects of reactor height/diameter ratios ranged from 24 to 4 corresponding to reactor settling velocities from 12 to 2 m h^?1 on aerobic granulation were investigated. It was found that granules appeared after 1-week operation and granule volume percentages exceeded 50% after 2–3 weeks in four reactors. In addition, similar granule fraction of 94–96% was found at steady state in all four reactors. Sludge volume index (SVI), average sludge size, biomass density and granule settling velocity at steady state were around 50 ml g^?1, 1800 μm, 53 g l^?1 and 40 m h^?1, respectively, in four reactors. Extracellular polymeric substances (EPS) and specific oxygen uptake rate (SOUR) were around 38 mg g^?1 VSS and 40 mg O₂ g^?1 VSS h^?1, respectively. Denaturing gradient gel electrophoresis (DGGE) fingerprint of sludge in four reactors showed the same microbial population shift during the start-up period and same microbial community structure during steady-state period. These results recommended strongly that reactor height/diameter ratio or reactor setting velocity in the used range in this study did not affect granule formation, physical characteristics, microbial community structure of granules and stable operation of granular sludge reactor. Reactor height/diameter ratio thus can be very flexible in the practice, which is important for the application of aerobic granule technology.     

Dairy wastewater polluting load and treatment performances of an industrial three-cascade-reactor plant

An industrial three-cascade-reactor plant treating 45 m³ d^?1 of dairy wastewater (DW) was monitored for approx. one year to investigate the effect of variable daily influent loads. It removed more than 85% COD, NH₄-N and non-ionic and anionic surfactants from DW within the loads 7–24, 0.4–2.3, 0.4–0.7 and 0.1–0.5 kg d^?1, respectively; NH₄-N removal, in particular, was almost quantitative. Although the degradation of the above parameters below the lower load thresholds declined to 78.7, 87.5, 50.2 and 64.7%, respectively, their residual concentrations met effluent discharge standards. The biomass settling properties, assessed as sludge volume index (SVI), were satisfactory (generally lower than 150 ml g^?1) regardless of the organic load of the influent. The depletion of the pollutant load took mainly place in the first reactor albeit a significant contribution to the removal of the slowly degradable organic matter fraction was given by the two subsequent reactors.     

Sewage treatment in a combined up-flow anaerobic sludge blanket (UASB)–down-flow hanging sponge (DHS) system

The performance of up-flow anaerobic sludge blanket (UASB) in combination with down-flow hanging sponge (DHS) system for sewage treatment at an average wastewater temperature of 15 °C has been investigated for 6 months. The results showed that a combined system operated at a total HRT of 10.7 h and total SRT of 88 days represents a cost effective sewage treatment process. The average COD_total and BOD_5 total concentrations measured in the final effluent of the total system (UASB + DHS) amounted to 43 and 3.0 mg/l, respectively, corresponding to the overall removal efficiency of 90% for COD_total and 98% for BOD_5 total. The total process provided a final effluent containing a low concentration of 12 mg/l for TSS.Eighty-six percent of ammonia was eliminated at space loading rate of 1.6 kg COD/m³ d and HRT of 2.7 h. The calculated nitrification rate of the DHS system according to the nitrate and nitrite production amounted to 0.18 kg/m³ d. The removal of F. coliform in the UASB reactor only amounted to 0.86 log₁₀. On the other hand, the F. coliform concentration dropped substantially, i.e. by 2.6 log₁₀ in the DHS system resulting only 2.7 × 10³/100 ml in the final effluent.The calculated average sludge production for UASB operated at an HRT of 8.0 h amounted to 30 g TSS/d, corresponding to sludge yield coefficient of 0.2 g TSS/g total COD removed, while it was indeed very low only 6.0 g TSS/d corresponding to sludge yield coefficient of 0.09 g TSS/g total COD removed, for DHS system.The DHS profile results revealed that in the first and second segment of DHS system, the COD_total, BOD_5 total and TSS was eliminated, followed by the oxidation of ammonia in the next segments.     

Extent estimates and land cover relationships for functional indicators in non-wadeable rivers

Functional indicators are being increasingly used to assess waterway health but their responses to pressure in non-wadeable rivers have not been widely documented or applied in modern survey designs that provide unbiased estimates of extent. This study tests the response of river metabolism and loss in cotton strip tensile strength across a land use pressure gradient in non-wadeable rivers of northern New Zealand, and reports extent estimates for river metabolism and decomposition rates. Following adjustment for probability of selection, ecosystem respiration (ER) and gross primary production (GPP) for the target population of order 5–7 non-wadeable rivers averaged −7.3 and 4.8 g O₂ m⁻² d⁻¹, respectively, with average P/R < 1 indicating dominance by heterotrophic processes. Ecosystem respiration was <−3.3 g O₂ m⁻² d⁻¹ for 75% of non-wadeable river length with around 20% of length between −10 and −20 g O₂ m⁻² d⁻¹. Cumulative distribution functions of cotton strength loss estimates indicated a more-or-less linear relationship with river km reflecting an even spread of decay rates (range in k 0.0007–0.2875 d⁻¹) across non-wadeable rivers regionally. A non-linear relationship with land cover was detected for GPP which was typically <5 g O₂ m⁻² d⁻¹ where natural vegetation cover was below 20% and greater than 80% of upstream catchment area. For cotton strength loss, the relationship with land cover was wedge-shaped such that sites with >60% natural cover had low decay rates (<0.02 d⁻¹) with variability below this increasing as natural cover declined. Using published criteria for assessing waterway health based on ER and GPP, 232–298 km (20–29%) of non-wadeable river length was considered to have severely impaired ecosystem functioning, and 436–530 km (42–50%) had no evidence of impact on river metabolism.     

Schoolhouse wastewater purification in a LWA-filled hybrid constructed wetland in Estonia

This paper analyses the purification efficiency and mass removal of organic material, suspended solids, nitrogen and phosphorus in a hybrid constructed wetland (CW) system treating wastewater from a basic school in Paistu, Estonia. The CW consists of two subsurface flow filter beds using lightweight aggregates (LWA): a two-chamber vertical subsurface flow (VSSF) filter bed followed by a horizontal subsurface flow (HSSF) filter bed, with a total area of 432 m². This CW was constructed in summer 2002 by the Centre for Ecological Engineering in Tartu (CEET). Eighteen series of water samples (from 30.10.2003 to 15.10.2005) were undertaken. The analyses show the outstanding purification effect of the system: for BOD₇ the average purification efficiency is 91%; for total suspended solids (TSS)—78%, for total P—89%, for total N—63%, and for NH₄N—77%. The average outlet values for the above-listed parameters were 5.5, 7.0, 0.4, 19.2 and 9.1 mg L⁻¹, respectively. According to our results, the purification parameters meet the standards set by the Water Act of Estonia for wastewater treatment plants of 2000–9999 PE: 15, 25, and 1.5 mg L⁻¹ for BOD₇, TSS and total P, respectively. The results show that hybrid CW systems consisting of subsurface flow filter beds can work efficiently in conditions of changing hydraulic loading and relatively cold climate. We did not find significant differences between the removal efficiency, mass removal, and values of the first-order rate-constant k for most water quality indicators during the warm (May–October) and cold (November–April) periods. Locally produced LWA as a filter material in CWs has shown good hydraulic conductivity and phosphorus sorption capacity (k = 17.1 ± 12.4 m yr⁻¹). The Paistu CW, with its proper design and outstanding purification results, can be considered one of the best systems in Estonia.     

Nitrogen transformations in vertical flow systems with and without rice (Oryza sativa) studied with a high-resolution soil–water profiler

Nitrogen transformations were studied in flooded and non-flooded vertical flow columns with and without a rice plant. Influent (average concentration: NH₄⁺-N: 40 mg L^?1; NO₃^?-N: 0.15 mg L^?1; and NO₂^?-N: 4.0 mg L^?1) was supplied at 1.25 cm d^?1 during stage 1 (20 May–5 August) and at 2.50 cm d^?1 at stage 2 (6 August–26 October), which resulted in an average nitrogen loading of 156 g m^?2 during the entire experimental period. Total nitrogen (T-N) removal efficiencies exceeded 90% in vertical flow systems with rice plants. Nitrogen assimilated by the rice plants in the flooded column accounted for 60% of the total input nitrogen, while that in the non-flooded column accounted for 36% of the total input. The remaining nitrogen appeared to be removed through biogeochemical pathways. Although some nitrogen flowed out, most input nitrogen was also removed even in the flooded and non-flooded unplanted columns.A high-resolution vertical distribution investigation showed the changes of nitrogen forms in soil water. In the flooded condition, there were high ammonium and high nitrite concentrations in the upper layers. The concentrations of ammonium and nitrite simultaneously decreased with depth increasing, suggesting that anaerobic ammonia oxidation (anammox) may occur in these anaerobic conditions. In contrast, the distributions of nitrogen in the non-flooded columns with elevated water level suggested that nitrification–denitrification route was the major removal mechanism, whether or not rice plants were present.     

High-rate denitrifying sulfide removal process in expanded granular sludge bed reactor

An expanded granular sludge bed (EGSB) reactor was adopted to incubate bio-granules that could simultaneously convert 4.8 kg-S m^?3 d^?1 of sulfide in 97% efficiency; 2.6 kg-N m^?3 d^?1 of nitrate in 92% efficiency; and 2.7 kg-C m^?3 d^?1 acetate in 95% efficiency. Mass balance calculation of sulfur, nitrogen, and carbon over the EGSB reactor confirmed the performance results. This noted reactor performance is much higher than those reported in literature. Stoichiometric relation suggests that the nitrate was reduced to nitrite via autotrophic denitrification pathway, then the formed nitrite was converted via heterotrophic denitrification pathway to N₂.     

Comparison of the treatment performances of blast furnace slag-based and gravel-based vertical flow wetlands operated identically for domestic wastewater treatment in Turkey

In 2001, to foster the practical development of constructed wetlands (CWs) used for domestic wastewater treatment in Turkey, vertical subsurface flow constructed wetlands (30 m² of each) were implemented on the campus of the METU, Ankara, Turkey. The main objective of the research was to quantify the effect of different filter media on the treatment performance of vertical flow wetlands in the prevailing climate of Ankara. Thus, a gravel-filled wetland and a blast furnace granulated iron slag-filled wetland were operated identically with primarily treated domestic wastewater (3 m³ d⁻¹) at a hydraulic loading rate of 0.100 m d⁻¹, intermittently. Both of the wetland cells were planted with Phragmites australis. According to the first year results, average removal efficiencies for the slag and gravel wetland cells were as follows: total suspended solids (TSS) (63% and 59%), chemical oxygen demand (COD) (47% and 44%), NH₄⁺–N (88% and 53%), total nitrogen (TN) (44% and 39%), PO₄³⁻-P (44% and 1%) and total phosphorus (TP) (45% and 4%). The treatment performances of the slag-filled wetland were better than that of the gravel-filled wetland in terms of removal of phosphorus and production of nitrate. Since this study was a pioneer for implementation of subsurface constructed wetlands in Turkey using local sources, it has proved that this eco-technology could also be used effectively for water quality enhancement in Turkey.     

A key cultivation technology for denitrifying granular sludge

Well-formed denitrifying granular sludge with a biomass concentration of 24.8 gVSS L^?1 and a specific nitrate removal rate of 0.19 gNO₃-N gVSS^?1 d^?1 was obtained in an upflow sludge blanket (USB) reactor by cultivating seeded aerobic flocculent sludge for 6–8 weeks. Regularity phenomena exist in the granulation including flotation of flocculent sludge, formation of fine granules, occurrence of channelling, and formation of mature granular sludge. The granulation is similar to crystal growth, that the non-denitrifying bacteria evolve into the carriers (fine granules), on the surface of which denitrifying bacteria proliferate and develop into mature granular sludge.There are several key parameters that must be considered when developing a good denitrifying granular sludge. First, the proper seed sludge must be chosen (VSS/SS at 0.65–0.75, SRT over 25 days) to accelerate the granulation process. Secondly, any floating sludge should be stirred, and the sludge loading rate should be within the range of 0.05–0.15 gNO₃-N gVSS^?1 d^?1 until fine granules emerge. Additionally, spontaneous gas agitation or interval air-blowing should be used to effectively eliminate channelling; Finally, the sludge loading rate should be less than 0.25 gNO₃-N gVSS^?1 d^?1 until dense, mature granular sludge appears. This study could support and promote the full-scale application of denitrifying granular sludge.     

Relationship between microbial activity and microbial community structure in six full-scale anaerobic digesters

High activity levels and balanced anaerobic microbial communities are necessary to attain proper anaerobic digestion performance. Therefore, this work was focused on the kinetic performance and the microbial community structure of six full-scale anaerobic digesters and one lab-scale co-digester. Hydrolytic (0.6–3.5 g COD g^?1 VSS d^?1) and methanogenic (0.01–0.84 g COD g^?1 VSS d^?1) activities depended on the type of biomass, whereas no significant differences were observed among the acidogenic activities (1.5–2.2 g COD g^?1 VSS d^?1). In most cases, the higher the hydrolytic and the methanogenic activity, the higher the Bacteroidetes and Archaea percentages, respectively, in the biomasses. Hydrogenotrophic methanogenic activity was always higher than acetoclastic methanogenic activity, and the highest values were achieved in those biomasses with lower percentages of Methanosaeta. In sum, the combination of molecular tools with activity tests seems to be essential for a better characterization of anaerobic biomasses.     

Nitrification rates of algal–bacterial biofilms in wastewater stabilization ponds under light and dark conditions

The objective of this study was to investigate nitrification rates in algal–bacterial biofilms of waste stabilization ponds (WSP) under different conditions of light, oxygen and pH. Biofilms were grown on wooden plates of 6.0 cm by 8.0 cm by 0.4 cm in a PVC tray continuously fed with synthetic wastewater with initial NH₄-N and Chemical Oxygen Demand (COD) concentrations of 40 mg l^?1 and 100 mg l^?1, respectively, under light intensity of 85–95 μE m^?2 s^?1. Batch activity tests were carried out by exposure of the plates to light conditions as above (to simulate day time), dim light of 1.8–2.2 μE m^?2 s^?1 (to simulate reduced light as in deeper locations in WSP) and dark conditions (to simulate night time). Dissolved oxygen (DO) concentration and pH were controlled. At some experiments, both parameters were kept constant, and at others they were left to vary as in WSP. Results show biofilm nitrification rates of 945–1817 mg-N m^?2 d^?1 and 1124–1615 mg-N m^?2 d^?1 for light and dark experiments. When the minimum DO was 4.1 mg l^?1, the biofilm nitrification rates under light and dark conditions did not differ significantly at 95% confidence. When the minimum DO in the dim light experiment was 3.2 mg l^?1, the nitrification rates under light and dim light conditions were 945 mg-N m^?2 d^?1 and 563 mg-N m^?2 d^?1 and these significantly differed. Further decrease of DO to 1.1 mg l^?1 under dark conditions resulted in more decrease of the nitrification rates to 156 mg-N m^?2 d^?1. It therefore seems that under these experimental conditions, biofilm nitrification rates are significantly reduced at a certain point when bulk water DO is between 3.2 mg l^?1 and 4.1 mg l^?1. As long as bulk water DO under dark is high, light is not important in influencing the process of nitrification.     

A comparative study of surface and subsurface flow constructed wetlands for treatment of combined sewer overflows: A greenhouse experiment

The use of surface flow (SFCWs) and subsurface flow constructed wetlands (SFCWs) for the treatment of combined sewer overflows was assessed at pilot scale. Synthetic wastewater was applied in three batches with decreasing concentrations to mimic concentration profiles that are obtained in the field during overflow events. Three simulated combined sewer overflows were applied on each wetland. Composite water samples (60 in total) were taken for a period of 8 days to study the removal of total nitrogen (Ntot), NH₄–N, NO₃–N, total COD (CODtot) and total phosphorus. Redox potential, which was monitored at various locations along the wetlands, was more negative in the SSFCWs. In general, removal occurred faster in the SSFCWs and the final concentrations were lower. The removal of Ntot was only 36.6 ± 3.3% in the SFCWs due to nitrification-limiting conditions. The conditions in the SSFCWs, in contrast, seemed to promote Ntot removal (removal efficiency 96.7 ± 1.9%). The removal of P was hampered in both wetland types by reducing conditions. P that was initially removed was released again from the substrates later on. First-order removal rate constants were derived for the removal of both CODtot (SSFCWs: 1.1 ± 0.3 m d^?1; SFCWs: 0.17 ± 0.06 m d^?1) and Ntot (SSFCWs: 0.4 ± 0.1 m d^?1; SFCWs: 1.7 ± 0.5 m d^?1).     

Effects of wetland construction on water quality in a semi-arid catchment degraded by intensive agricultural use

Many factors can influence the improvement of water quality in surface-flow constructed wetlands (SFW). To test if water quality was improved, especially in nutrient and salt content, after passage through SFW, 11 wetland plots of various sizes (50, 200, 800 and 5000 m²) were established within constructed wetlands on agricultural soils in the Ebro River basin (NE Spain) that had been affected by salinization. A set of 15 water quality parameters (e.g., nutrients, salts, sediments, and alkalinity) was obtained from samples collected at the inflow and outflow of the wetlands during the first 4 years after the wetlands were constructed. NO₃-N retention rates were as high as 99% in the largest (5000 m²) wetlands. After 4 years, total phosphorus was still being released from the wetlands but not salts. Over the same period, in small wetlands (50, 200, and 800 m²), retention rate relative to the input of NO₃-N increased from 40% to almost 60%. Retention of NO₃-N amounted to up to 500 g N m^?2 per year, for an average load concentration at inflow of ～20 mg l^?1. Release of Na⁺ declined from 16% to 0–2% by volume, for an average load concentration at inflow of ～70 mg l^?1. At the current retention rate of NO₃-N (76–227 g m^?2 per year), 1.5–4% of the catchment should be converted into wetlands to optimize the elimination of NO₃-N.     

Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands

In constructed wetlands, solids accumulation may have two consequences with opposing effects on treatment efficiency: it decreases the longevity by reducing void space and it enhances biological activity by favoring biofilm development. The goal of our study was to estimate the effect of plants (presence and species) and artificial aeration on solids accumulation (volatile and inorganic). The horizontal and vertical distribution of solids was sampled using solids traps in 12 constructed wetland mesocosms (5 years old). Microbial density and activity were estimated in the biological fraction of the sampled solids. The effect of plant presence reduced accumulated solids by 26% and sulphide content by 50% sulphide content. There was more solids accumulation in Typha angustifolia units than in Phragmites australis. Also, T. angustifolia generated more biological activities at the surface and close to the inlet while conditions were more homogeneous throughout P. australis units. Aeration (1) stimulated biofilm development at the inlet of planted beds, (2) seemed to reduce mineral matter accumulation and (3) generated the same pattern of activities in planted beds enabling to reach a total nitrogen removal rate of up to 0.65 g N m^?2 d^?1.     

Long-term phosphorus and nitrogen removal processes and preferential flow paths in Northern constructed peatlands

Four treatment peatlands were studied in Northern Finland in order to determine peat P, Al and Fe concentration distributions and to find removal parameters for nutrient modelling. The sites had been under loading for 10–16 years. About 20 peat samples for analyses of oxalate-extractable and total P, Al and Fe were collected from the depth 0–10 cm in each peatland. The peat P concentration ranged from 0.097 mg g^?1 to 14 mg g^?1 being 1.7 mg g^?1on average. P accumulated in preferential flow path areas. Although P concentrations were locally high, DSSP (the index of potential soil P release from peat to water) was very low in all studied peatlands, indicating that peat was not saturated by phosphorus. The results indicate that Al-based precipitation chemicals increase substantially P retention capacity of peatland and maintain a stabile P reduction in spite of varying P loads. The results also show long-term phosphorus accumulation in peatlands polishing municipal wastewater from activated sludge treatment. The regression analyses showed that k-value for N removal depends on N loading and hydraulic loading. The first-order area model together with regression analysis of the rate constant result in a good agreement between observed and calculated nitrogen concentration. The NH₄-N loading to the peatland should be below 0.10 mg m² d^?1 in order to achieve a high reduction of 70%.