Valerie McCarthy

Nitrogen removal in an integrated constructed wetland treating domestic wastewater

The nitrogen (N) removal performance of a 3.25 ha Integrated Constructed Wetland (ICW) treating domestic wastewater from Glaslough village in County Monaghan, Ireland, was evaluated in this study. The ICW consists of two sludge ponds and five shallow vegetated wetland cells. Influent and effluent concentrations of two N species, namely, ammonia-nitrogen (NH3-N) and nitrate-nitrogen (NO3-N), which were measured weekly over 2 years, together with hydrology of the ICW provided the basis for this evaluation. The influent wastewater typically contained 40 mg L−1 NH3-N and 5 mg L−1 NO3-N. Concentrations of N in the ICW effluent were typically less than 1.0 mg L−1 for both species. Overall, a total load of 2802 kg NH3-N and 441 kg NO3-N was received by the ICW and a removal rate of 98.0 % and 96.9 %, respectively, was recorded. Average areal N loading rate (245 mg m−2 d−1 NH3-N and 38 mg m−2 d−1 NO3-N) had a significant linear relationship with areal N removal rate (240 mg m−2 d−1 and 35 mg m−2 d−1, respectively) for both species. The areal first-order N removal rate constants in the ICW averaged 14 m yr−1 for NH3-N and 11 m yr−1 for NO3-N. Temperature coefficients (θ) for N reduction in the ICW were lower and less than unity for NO3-N, suggesting that the variability in N removal by the ICW was marginally influenced by temperature.

Geophysical and hydrogeological characterisation of the impacts of on-site wastewater treatment discharge to groundwater in a poorly productive bedrock aquifer

Contaminants discharging from on-site wastewater treatment systems (OSWTSs) can impact groundwater quality, threatening human health and surface water ecosystems. Risk of negative impacts becomes elevated in areas of extreme vulnerability with high water tables, where thin unsaturated intervals limit vadose zone attenuation. A combined geophysical/hydrogeological investigation into the effects of an OSWTS, located over a poorly productive aquifer (PPA) with thin subsoil cover, aimed to characterise effluent impacts on groundwater. Groundwater, sampled from piezometers down-gradient of the OSWTS percolation area displayed spatially erratic, yet temporally consistent, contaminant distributions. Electrical resistivity tomography identified an area of gross groundwater contamination close to the percolation area and, when combined with seismic refraction and water quality data, indicated that infiltrating effluent reaching the water table discharged to a deeper more permeable zone of weathered shale resting on more competent bedrock. Subsurface structure, defined by geophysics, indicated that elevated chemical and microbiological contaminant levels encountered in groundwater samples collected from piezometers, down-gradient of sampling points with lower contaminant levels, corresponded to those locations where piezometers were screened close to the weathered shale/competent rock interface; those immediately up-gradient were too shallow to intercept this interval, and thus the more impacted zone of the contaminant plume. Intermittent occurrence of faecal indicator bacteria more than 100m down gradient of the percolation area suggested relatively short travel times. Study findings highlight the utility of geophysics as part of multidisciplinary investigations for OSWTS contaminant plume characterisation, while also demonstrating the capacity of effluent discharging to PPAs to impact groundwater quality at distance. Comparable geophysical responses observed in similar settings across Ireland suggest the phenomena observed in this study are more widespread than previously suspected.

Nitrogen transformations and mass balance in an integrated constructed wetland treating domestic wastewater

Nitrogen (N) transformations and removal in integrated constructed wetlands (ICWs) are often high, but the contributions of various pathways, including nitrification/denitrification, assimilation by plants and sediment storage, remain unclear. This study quantified the contributions of different N removal pathways in a typical multi-celled ICW system treating domestic wastewater. Findings showed near complete average total N retention of circa 95% at 102.3 g m⁻² yr⁻¹ during the 4-year period of operation. Variations in total N and NH4-N removal rates were associated with effluent flow volume rates and seasons. According to the mass balance estimation, assimilation by plants and sediment/soil storage accounted for approximately 23% and 20%, respectively, of the total N load removal. These were the major N removal route besides microbial transformations. Thus, the combination of plants with high biomass production offer valuable opportunities for improving ICW performance. The retrieval and use of sequestered N in the ICW sediment/soils require coherent management and provide innovative and valuable opportunities.

Adaptive neuro-fuzzy inference system for real-time monitoring of integrated constructed wetlands

Monitoring large-scale treatment wetlands is costly and time-consuming, but required by regulators. Some analytical results are available only after 5 days or even longer. Thus, adaptive neuro-fuzzy inference system (ANFIS) models were developed to predict the effluent concentrations of 5-day biochemical oxygen demand (BOD5) and NH4-N from a full-scale integrated constructed wetland (ICW) treating domestic wastewater. The ANFIS models were developed and validated with a 4-year data set from the ICW system. Cost-effective, quicker and easier to measure variables were selected as the possible predictors based on their goodness of correlation with the outputs. A self-organizing neural network was applied to extract the most relevant input variables from all the possible input variables. Fuzzy subtractive clustering was used to identify the architecture of the ANFIS models and to optimize fuzzy rules, overall, improving the network performance. According to the findings, ANFIS could predict the effluent quality variation quite strongly. Effluent BOD5 and NH4-N concentrations were predicted relatively accurately by other effluent water quality parameters, which can be measured within a few hours. The simulated effluent BOD5 and NH4-N concentrations well fitted the measured concentrations, which was also supported by relatively low mean squared error. Thus, ANFIS can be useful for real-time monitoring and control of ICW systems.

Geophysical assessment of contamination from a wastewater treatment system in the Milltown lake catchment, Ireland.

Residential on-site wastewater treatment systems (OSWTS), where contaminated wastewater discharges to the subsurface, act as the dominant means of domestic wastewater disposal in rural Ireland; septic tanks constitute the most common technology employed. The objective of this study was to determine the effectiveness of a number of non-invasive geophysical techniques, employed in conjunction with hydrogeological data, for characterising the three dimensional extent of a contaminant plume generated by septic tank effluent discharging to glacial-till subsoils at a test site within the Milltown Lake Catchment, Co. Monaghan, Republic of Ireland. It was found that the integrated use of three geophysical techniques, electromagnetics, electrical resistivity tomography and seismic refraction with existing hydrogeological and water quality data, significantly improved our understanding of the contaminant plume and associated subsurface contaminant pathways. The geophysical results, when combined with hydrogeological data suggest that the majority of wastewater contamination originating from the septic tank is being transported through a fractured zone of bedrock, far quicker than originally expected.

Mechanisms influencing zooplankton community structure in a shallow marl lake

Zooplankton play a central role in the pelagic food web oflakes. Populations are characteristically regulated by a combination of predation (top-down) and resource limitation (bottom-up) (McQUEEN et al. 1986). Influence of predation or resource limitation varies both between and within lakes and is dependent on fish recruitrnent and nutrient supply. Size-selective predation by fish can determine zooplankton community structure (BROOKS & DonSON 1965). Heavy predation by visually-foraging planktivorous fish can result in reduced zooplankton biomass, reduced size of individual zooplankton species and a reduction in large taxa such as Daphnia. Zooplankton are influenced strongly by both the quantity and quality o f their phytoplankton food supply. Food availability is often an important factor influencing zooplankton population dynamics. Daphnia, for example, are capable of responding to dramatic fluctuations in their food supply with considerable accuracy (GEORGE & REYNOLDS 1997). Food quality is also important. The elemental ratios of algae can vary considerably and will be dependent on the availability of these nutrients in the environment (SOMMER 1989). Heterotrophs, in contrast, maintain a comparatively constant, species-specific, ratio of cellular C:N:P (STERNER & HESSEN 1994). This homeostatic stoichiometry could be a significant factor influencing zooplankton community dynamics (HESSEN 1992). Species with high P requirements such as Daphnia (ANDERSEN & HESSEN 1991), will experience reduced growth, fecundity and survivorship when fed P-limited food (STERNER et al. 1993). Direct nutrient limitation could, therefore, be an important, but little studied aspect of primary consumer structure and production in pelagic foodwebs. This study investigated the principal drivers of zooplankton community structure in a shallow marl lake in westem Ireland. Both top-down and bottomup controls were considered, and their relative importance on the zooplankton population dynamics ofthe lake assessed.