Gaseous fluxes in the nitrogen and carbon budgets of subsurface flow constructed wetlands.

In 2001 and 2002, fluxes of N(2)O, CH(4), CO(2) and N(2) were measured in two constructed wetlands (CW) for domestic wastewater treatment in Estonia. The difference between the median values of N(2)O, CH(4), and N(2) fluxes in the horizontal subsurface flow (HSSF) CWs was non-significant, being 1.3-1.4 and 1.4-4.1 mg m(-2) d(-1) for N(2)O-N and CH(4)-C, and 0.16-0.17 g N m(-2) d(-1) for N(2)-N respectively. The CO(2)-C flux was significantly lower (0.6 g C m(-2) d(-1)) in one of the HSSF filters of a hybrid CW, whereas the single HSSF and VSSF filters emitted 1.7 and 2.0 g C m(-2) d(-1). The median value of CH(4)-C emission in CWs varied from 1.4 to 42.6 g C m(-2) d(-1), being significantly higher in the VSSF filter beds. We also estimated C and N budgets in one of the HSSF CWs (312.5 m(2)) for 2001 and 2002. The total C input into this system was similar in 2001 and 2002, 772 and 719 kg C year(-1), but was differently distributed between constituent fluxes. In 2001, the main input flux was soil and microbial accumulation (663 kg C year(-1) or 85.8% of total C input), followed by plant net primary production (NPP) (10.2%) and wastewater inflow (3.9%). In 2002, 55.7% of annual C input was bound in plant NPP, whereas the increase in soil C formed 28.5% and wastewater inflow 15.7%. The main C output flux was soil respiration, including microbial respiration from soil and litter, and the respiration of roots and rhizomes. It formed 120 (97.5%) and 230 kg C year(-1) (98.2%) in 2001 and 2002 respectively. The measured CH(4)-C flux remained below 0.1% of total C output. The HSSF CW was generally found to be a strong C sink, and its annual C sequestration was 649 and 484 kg C year(-1) per wetland in 2001 and 2002 respectively. However, negative soil and microbial accumulation values in recent years indicate decreasing C sequestration. The average annual N removal from the system was 38-59 kg N year(-1) (46-48% of the initial total N loading). The most important flux of the N budget was N(2)-N emission (22.7 kg in 2001 and 15.2 kg in 2002), followed by plant belowground assimilation (2.3 and 11.9 kg N year(-1) in 2001 and 2002), and above-ground assimilation (1.9 and 9.2 kg N year(-1), respectively). N(2)O emission was low: 0.37-0.60 kg N year(-1)(.).

Dynamics of phosphorus, nitrogen and carbon removal in a horizontal subsurface flow constructed wetland.

The dynamics of nitrogen (N), phosphorus (P) and carbon (C) accumulation in the filter material of a horizontal subsurface constructed wetland (HSSF CW; established in 1997) and in a specially designed oil-shale ash filter (2002) for P retention have been studied. Concentrations of N, P and C in filter media (coarse sand) in the HSSF beds show an increasing trend. Both the annual accumulation of P and increasing outflow concentrations of P in the HSSF CW reflect the possible saturation of filter media with P after 8 years working. Tested ash material derived from oil-shale combustion demonstrated very high P removal efficiency in laboratory batch experiments. However, during the first 4 months of the in situ ash filter experiment, the efficiency of P removal was about 71% (an average outflow concentration of 1.9 mg L(-1) was achieved). Subsequently, the efficiency decreased to 10-20%, which might be a sign of saturation or clogging due to quick biofilm development on the ash particles. The increasing of hydraulic retention time and the improvement of design for maximal contact between material and wastewater are considered to be key factors that can provide optimal pH for the removal processes.

Bioaugmentation in a newly established LECA-based horizontal flow soil filter reduces the adaptation period and enhances denitrification.

The possibility of enhancing the denitrification of a newly established LECA-based horizontal subsurface flow (HSSF) soil filter receiving pretreated wastewater from a vertical flow filter was studied. The pilot-scale experiment offers evidence regarding the survival and reproduction of introduced microbes taken from an LECA-based HSSF constructed wetland (CW) that has similar internal conditions, after bioaugmentation into newly established LECA-based HSSF CW mesocosms. Bioaugmentation resulted in a trend towards higher and more stable denitrification in the supplemented mesocosms during the nearly half-year study period.

Variation of microbiological parameters within planted soil filter for domestic wastewater treatment.

Microbial community structure was assessed in a horizontal subsurface flow planted sand filter treating domestic wastewater with molecular and culture-based methods. The diversity and spatial distribution of the microbial community was investigated using a PCR-DGGE (eubacterial and archaeal primers, ammonia-oxidizing bacteria, and ammonium monooxygenase specific primers), and spread plate and MPN counts. Significant differences were found in the spatial distribution of the microbial community structure. Data analysis revealed that different components of the microbial community possessed different spatial distribution patterns within the filter bed and depending on community type, relationships with soil chemical, and microbiological parameters varied. The most important spatial pattern in microbial community structure within the constructed wetland was related to the depth gradient, followed by differences between inflow and outflow. A comparison of a number of heterotrophic bacteria between inlet and outlet pipes as well as between two sampling depths showed no significant differences. In addition, the variation of the abundance of ammonia-oxidizing bacteria demonstrated no clear spatial pattern.

Microbial characteristics and nitrogen transformation in planted soil filter for domestic wastewater treatment.

We studied an experimental horizontal subsurface-flow planted sand filter in Kodijärve, Estonia. We measured the microbial biomass, nitrogen immobilization, potential nitrification, soil respiration, multiple carbon source utilization patterns of the microbial consortia of the soil samples, the carbon, nitrogen, and phosphorus content of the soil samples, the water quality and physicochemical indicators in water sampling wells as well as emissions of CO2, N2, NO2, and CH4 from the two beds (the dry bed and the wet bed) in the wetland. The potential nitrification of the upper layer of the dry bed could not be attributed primarily to autotrophic nitrification, or the nitrifying bacteria in this layer could be facultative heterotrophs, whereas autotrophic nitrification is predominant in the upper layer of the wet bed. It also was found that changing aeration conditions in the lower layer of the dry bed have resulted in a lower diversity of the microbial community and led to a relative depletion of easily degradable soil carbon resources.

Gaseous fluxes from subsurface flow constructed wetlands for wastewater treatment.

We measured nitrous oxide (N2O), dinitrogen (N2), and methane (CH4) fluxes in two constructed wetlands (CW) in Estonia using the closed chamber method and the He-O method in the period from October 2000 to March 2003. Emission rates of N2O-N, N2-N and CH4-C from both CWs varied significantly on a both spatial and temporal scale, ranging from 1 to 2,600, 170 to 130,000, and -1.7 to 87,200 microg m(-2) h(-1) respectively. The average flux of N2O from the microsites in the Kodijärve horizontal subsurface flow (HSSF) CW and Kõo hybrid CW ranged from 27 to 370 and from 72 to 500 microg N2O-N m(-2) h(-1), respectively, whereas the average dinitrogen flux from the microsites in the HSSF CW in Kodijärve was 2-3 magnitudes higher than the N2O flux, ranging from 19,500 to 33,300 microg N2-N m(-2) h(-1). The average methane emissions from the microsites in the Kodijärve HSSF CW and the Kõo hybrid CW ranged from 31 to 12,100 and from 950 to 5,750 microg CH4-C m(-2) h(-1), respectively. The highest emission values for all three gases were observed in the warm period. There was a significant relationship between emission rates and water table depth: CH4 and N2 emission increased and N2O emission decreased when the water table did rise. Although the emission of N2O and CH4 from CWs was found to be relatively high, their global warming potential (GWP) in the time horizon of 100 years is not significant, ranging from 4.5 to 16.3 tonnes of CO2 equivalents per ha per year in Kodijärve and from 12.1 to 17.3 t CO2 equivalents ha(-1) yr(-1) in Kõo.