Design of waste stabilization pond systems: A review

A better design instruction for waste stabilization ponds is needed due to their growing application for wastewater purification, increasingly strict environmental regulations, and the fact that most of previous design manuals are outdated. To critically review model-based designs of typical pond treatment systems, this paper analyzed more than 150 articles, books, and reports from 1956 to 2016. The models developed in these publications ranged from simple rules and equations to more complex first-order and mechanistic models. From a case study on all four approaches, it appeared that rules of thumb is no longer a proper tool for pond designs due to its low design specification and very high output variability and uncertainty. On the other hand, at the beginning phase of design process or in case of low pressure over land and moderate water quality required, regression equations can be useful to form an idea for pond dimensions. More importantly, mechanistic models proved their capacity of generating more precise and comprehensive designs but still need to overcome their lack of calibration and validation, and overparameterization. In another case study, an essential but often overlooked role of uncertainty analysis in pond designs was investigated via a comparison between deterministic and uncertainty-based approaches. Unlike applying a safety factor representing all uncertainty sources, probabilistic designs quantify the uncertainty of model outputs by including prior uncertainty of inputs and parameters, which generates more scientifically reliable outcomes for decision makers. Based on these findings, we advise engineers and designers to shift from the conventional approaches to more innovative and economic tools which are suitable for dealing with large variations of natural biological systems.

Functional Response (FR) and Relative Growth Rate (RGR) Do Not Show the Known Invasiveness of Lemna minuta (Kunth)

Growing travel and trade threatens biodiversity as it increases the rate of biological invasions globally, either by accidental or intentional introduction. Therefore, avoiding these impacts by forecasting invasions and impeding further spread is of utmost importance. In this study, three forecasting approaches were tested and combined to predict the invasive behaviour of the alien macrophyte Lemna minuta in comparison with the native Lemna minor: the functional response (FR) and relative growth rate (RGR), supplemented with a combined biomass-based nutrient removal (BBNR). Based on the idea that widespread invasive species are more successful competitors than local, native species, a higher FR and RGR were expected for the invasive compared to the native species. Five different nutrient concentrations were tested, ranging from low (4 mgN.L-1 and 1 mgP.L-1) to high (70 mgN.L-1 and 21 mgP.L-1). After four days, a significant amount of nutrients was removed by both Lemna spp., though significant differences among L. minor and L. minuta were only observed at lower nutrient concentrations (lower than 17 mgN.L-1 and 6 mgP.L-1) with higher nutrient removal exerted by L. minor. The derived FR did not show a clear dominance of the invasive L. minuta, contradicting field observations. Similarly, the RGR ranged from 0.4 to 0.6 d-1, but did not show a biomass-based dominance of L. minuta (0.5 ± 0.1 d-1 versus 0.63 ± 0.09 d-1 for L. minor). BBNR showed similar results as the FR. Contrary to our expectations, all three approaches resulted in higher values for L. minor. Consequently, based on our results FR is sensitive to differences, though contradicted the expectations, while RGR and BBNR do not provide sufficient power to differentiate between a native and an invasive alien macrophyte and should be supplemented with additional ecosystem-based experiments to determine the invasion impact.

River Water Pollution Status and Water Policy Scenario in Ethiopia: Raising Awareness for Better Implementation in Developing Countries

Despite the increasing levels of pollution in many tropical African countries, not much is known about the strength and weaknesses of policy and institutional frameworks to tackle pollution and ecological status of rivers and their impacts on the biota. We investigated the ecological status of four large river basins using physicochemical water quality parameters and bioindicators by collecting samples from forest, agriculture, and urban landscapes of the Nile, Omo-Gibe, Tekeze, and Awash River basins in Ethiopia. We also assessed the water policy scenario to evaluate its appropriateness to prevent and control pollution. To investigate the level of understanding and implementation of regulatory frameworks and policies related to water resources, we reviewed the policy documents and conducted in-depth interviews of the stakeholders. Physicochemical and biological data revealed that there is significant water quality deterioration at the impacted sites (agriculture, coffee processing, and urban landscapes) compared to reference sites (forested landscapes) in all four basins. The analysis of legal, policy, and institutional framework showed a lack of cooperation between stakeholders, lack of knowledge of the policy documents, absence of enforcement strategies, unavailability of appropriate working guidelines, and disconnected institutional setup at the grass root level to implement the set strategies as the major problems. In conclusion, river water pollution is a growing challenge and needs urgent action to implement intersectoral collaboration for water resource management that will eventually lead toward integrated watershed management. Revision of policy and increasing the awareness and participation of implementers are vital to improve ecological quality of rivers.

The importance of uncertainties in scenario analyses--A study on future ecosystem service delivery in Flanders

Land use is rapidly changing and is significantly affecting ecosystem service delivery all around the world. The socio-economic context and political choices largely determine land use change. This land use change, driven by socio-economic pressures, will impact diverse elements of the environment including, for example, air quality, soil properties, water infiltration and food and wood production, impacts that can be linked to the provisioning of ecosystem services. To gain more insight into the effects of alternative socio-economic developments on ecosystem service delivery, land use change models are being coupled to ecosystem service delivery models to perform scenario analyses. Although the uncertainty of the results of these kind of scenario analyses are generally far from negligible, studies rarely take them into account. In this study, a cellular automaton land use change model is coupled to Bayesian belief network ecosystem service delivery models to facilitate the study of error propagation in scenario analysis. The proposed approach is applied to model the impact of alternative socio-economic developments on ecosystem service delivery in Flanders, Belgium and to assess the impact of land use allocation uncertainty on the uncertainty associated to future ecosystem service delivery. Results suggest that taking into account uncertainties may have an effect on policy recommendations that come out of the scenario analysis. However, in this study, uncertainties in the applied ecosystem service models were dominant, reducing the importance of accounting for land use allocation uncertainty.

Realistic environmental mixtures of hydrophobic compounds do not alter growth of a marine diatom

In this paper we determine whether a realistic mixture of hydrophobic chemicals affects the growth dynamics of a marine diatom and how this effect compares to the effect of temperature, light regime and nutrient conditions. To do so, we examine the specific growth rate of Phaeodactylum tricornutum in a 72 h algal growth inhibition test using a full factorial design with three nutrient regimes, two test temperatures, three light intensities and three chemical exposures. Passive samplers were used to achieve exposure to realistic mixtures of organic chemicals close to ambient concentrations. Nutrient regime, temperature and time interval (24, 48 and 72 h) explained 85% of the observed variability in the experimental data. The variability explained by chemical exposure was about 1%. Overall, ambient concentrations of hydrophobic compounds present in Belgian coastal waters, and for which the passive samplers have affinity, are too low to affect the intrinsic growth rate of P. tricornutum.

Relative contribution of persistent organic pollutants to marine phytoplankton biomass dynamics in the North Sea and the Kattegat

In this paper, we use concentrations of persistent organic pollutants (POPs) and of chlorophyll a to infer POP-induced effects on marine primary production in the Kattegat and the North Sea between the 1990s and the 2000s. To do so, we modelled phytoplankton dynamics using four classical drivers (light and nutrient availability, temperature and zooplankton grazing) and tested whether extending this model with a POP-induced phytoplankton growth limitation term improved model fit to observed chlorophyll a concentrations. Including monitored concentrations of PCBs and pesticides did not lead to a better model fit, suggesting that POP-induced growth limitation of marine phytoplankton in the North Sea and the Kattegat is small compared to the limitations caused by the classical drivers. In an attempt to more fully represent the multitude of POPs in the marine environment, the monitored concentrations were multiplied with a factor 10 and 100. Under these two configurations, region-specific contributions of POPs in the phytoplankton growth limitation were found. The inferred contribution of POPs to phytoplankton growth limitation was ca. 1% in Belgian marine waters, but in the Kattegat POPs explained ca. 10% of the phytoplankton growth limitation. These results suggest that there are regional differences in the contribution of POPs to the phytoplankton growth limitation.

Multidecadal Field Data Support Intimate Links between Phytoplankton Dynamics and PCB Concentrations in Marine Sediments and Biota

We analyzed three decades of field observations in the North Sea with additive models to infer spatiotemporal trends of chlorophyll a concentration, sediment organic carbon content, and polychlorinated biphenyls (PCBs) concentrations in mussels and sediments. By doing so, we separated long-term changes in PCB concentrations from seasonal variability. Using the inferred seasonal variability, we demonstrated that phytoplankton blooms in spring and autumn correspond to the annual maxima of the organic carbon content (r = 0.56; p = 0.004) and the PCB concentrations in sediments (r = 0.57; p = 0.004). Furthermore, we found a negative correlation between the PCB concentrations in sediments and in blue mussels (Mytilus edulis; r = -0.33, p = 0.012), which is probably related to the cleansing of the dissolved PCB phase driven by sinking organic matter during phytoplankton blooms and the filter-feeding behavior of the blue mussel. The present research demonstrates the role of seasonal phytoplankton dynamics in the environmental fate of PCBs at large spatiotemporal scales.

An ecosystem service approach to support integrated pond management: a case study using Bayesian belief networks--highlighting opportunities and risks

Freshwater ponds deliver a broad range of ecosystem services (ESS). Taking into account this broad range of services to attain cost-effective ESS delivery is an important challenge facing integrated pond management. To assess the strengths and weaknesses of an ESS approach to support decisions in integrated pond management, we applied it on a small case study in Flanders, Belgium. A Bayesian belief network model was developed to assess ESS delivery under three alternative pond management scenarios: intensive fish farming (IFF), extensive fish farming (EFF) and nature conservation management (NCM). A probabilistic cost-benefit analysis was performed that includes both costs associated with pond management practices and benefits associated with ESS delivery. Whether or not a particular ESS is included in the analysis affects the identification of the most preferable management scenario by the model. Assessing the delivery of a more complete set of ecosystem services tends to shift the results away from intensive management to more biodiversity-oriented management scenarios. The proposed methodology illustrates the potential of Bayesian belief networks. BBNs facilitate knowledge integration and their modular nature encourages future model expansion to more encompassing sets of services. Yet, we also illustrate the key weaknesses of such exercises, being that the choice whether or not to include a particular ecosystem service may determine the suggested optimal management practice.

Comparison of the abiotic preferences of macroinvertebrates in tropical river basins

We assessed and compared abiotic preferences of aquatic macroinvertebrates in three river basins located in Ecuador, Ethiopia and Vietnam. Upon using logistic regression models we analyzed the relationship between the probability of occurrence of five macroinvertebrate families, ranging from pollution tolerant to pollution sensitive, (Chironomidae, Baetidae, Hydroptilidae, Libellulidae and Leptophlebiidae) and physical-chemical water quality conditions. Within the investigated physical-chemical ranges, nine out of twenty-five interaction effects were significant. Our analyses suggested river basin dependent associations between the macroinvertebrate families and the corresponding physical-chemical conditions. It was found that pollution tolerant families showed no clear abiotic preference and occurred at most sampling locations, i.e. Chironomidae were present in 91%, 84% and 93% of the samples taken in Ecuador, Ethiopia and Vietnam. Pollution sensitive families were strongly associated with dissolved oxygen and stream velocity, e.g. Leptophlebiidae were only present in 48%, 2% and 18% of the samples in Ecuador, Ethiopia and Vietnam. Despite some limitations in the study design, we concluded that associations between macroinvertebrates and abiotic conditions can be river basin-specific and hence are not automatically transferable across river basins in the tropics.

Additive modelling reveals spatiotemporal PCBs trends in marine sediments

We developed generalised additive mixed models (GAMMs) to infer spatiotemporal trends of environmental PCB concentrations from an extensive dataset (n=1219) of PCB concentrations measured between 1991 and 2010 in sediments of the Belgian Coastal Zone (BCZ) and the Western Scheldt estuary. A GAMM with time, geographical zone, periodicity and the organic carbon - water partition coefficient as covariates explained 49% of the variability in the log transformed PCB sediment concentrations. The time trends unraveled two to threefold PCB concentration decreases in the BCZ during the last 20 years. However, in the Western Scheldt estuary, time trends were spatially heterogeneous and not significantly decreasing. These results demonstrate that international efforts to cut down emissions of PCBs have been effective to reduce concentrations in open water ecosystems like the BCZ but had little effect in the urbanised and industrialised area of the Scheldt estuary.

Development and assessment of an integrated ecological modelling framework to assess the effect of investments in wastewater treatment on water quality

Worldwide, large investments in wastewater treatment are made to improve water quality. However, the impacts of these investments on river water quality are often not quantified. To assess water quality, the European Water Framework Directive (WFD) requires an integrated approach. The aim of this study was to develop an integrated ecological modelling framework for the River Drava (Croatia) that includes physical-chemical and hydromorphological characteristics as well as the ecological river water quality status. The developed submodels and the integrated model showed accurate predictions when comparing the modelled results to the observations. Dissolved oxygen and nitrogen concentrations (ammonium and organic nitrogen) were the most important variables in determining the ecological water quality (EWQ). The result of three potential investment scenarios of the wastewater treatment infrastructure in the city of Varaždin on the EWQ of the River Drava was assessed. From this scenario-based analysis, it was concluded that upgrading the existing wastewater treatment plant with nitrogen and phosphorus removal will be insufficient to reach a good EWQ. Therefore, other point and diffuse pollution sources in the area should also be monitored and remediated to meet the European WFD standards.

Physico-chemical and biological characterization of anopheline mosquito larval habitats (Diptera: Culicidae): implications for malaria control

BACKGROUND

A fundamental understanding of the spatial distribution and ecology of mosquito larvae is essential for effective vector control intervention strategies. In this study, data-driven decision tree models, generalized linear models and ordination analysis were used to identify the most important biotic and abiotic factors that affect the occurrence and abundance of mosquito larvae in Southwest Ethiopia.

METHODS

In total, 220 samples were taken at 180 sampling locations during the years 2010 and 2012. Sampling sites were characterized based on physical, chemical and biological attributes. The predictive performance of decision tree models was evaluated based on correctly classified instances (CCI), Cohen's kappa statistic (κ) and the determination coefficient (R2). A conditional analysis was performed on the regression tree models to test the relation between key environmental and biological parameters and the abundance of mosquito larvae.

RESULTS

The decision tree model developed for anopheline larvae showed a good model performance (CCI = 84 ± 2%, and κ = 0.66 ± 0.04), indicating that the genus has clear habitat requirements. Anopheline mosquito larvae showed a widespread distribution and especially occurred in small human-made aquatic habitats. Water temperature, canopy cover, emergent vegetation cover, and presence of predators and competitors were found to be the main variables determining the abundance and distribution of anopheline larvae. In contrast, anopheline mosquito larvae were found to be less prominently present in permanent larval habitats. This could be attributed to the high abundance and diversity of natural predators and competitors suppressing the mosquito population densities.

CONCLUSIONS

The findings of this study suggest that targeting smaller human-made aquatic habitats could result in effective larval control of anopheline mosquitoes in the study area. Controlling the occurrence of mosquito larvae via drainage of permanent wetlands may not be a good management strategy as it negatively affects the occurrence and abundance of mosquito predators and competitors and promotes an increase in anopheline population densities.

Integrating data-driven ecological models in an expert-based decision support system for water management in the Du river basin (Vietnam)

In this study, classification trees were combined with the Water Framework Directive (WFD)-Explorer, a modular toolbox that supports integrated water management in a river basin to evaluate the impact of different restoration measures on river ecology. First, the WFD-Explorer toolbox analysed the effect of different restoration options on the abiotic river characteristics based on the water and substance balance embedded in the simulation environment. Based on these abiotic characteristics, the biological index Biological Monitoring Working Party for Vietnam was then predicted by classification trees that were trained on biological and abiotic data collected in the Du river basin in northern Vietnam. The ecological status of streams in the basin ranged from nearly pristine headwaters to severely impacted river stretches. Elimination of point sources from ore extraction and decentralised domestic wastewater treatment proved to be the most effective measures to improve the ecological condition of the Du river basin. The combination of the WFD-Explorer results with data-driven models enabled model application in a situation where expert knowledge was lacking. Consequently, this approach appeared promising for decision support in the context of river restoration and conservation management.

A comparison of the short-term toxicity of cadmium to indigenous and alien gammarid species

Amphipods play an important role in many aquatic ecosystems and are commonly used in ecotoxicology and ecosystem health assessment. Several alien gammarids have been introduced in many regions of the world during the last decades. In this study, we investigated if differences in cadmium sensitivity occurred between (1) different species belonging to the family Gammaridae and (2) different populations of the same species originating from a polluted or a non-polluted site. The acute cadmium toxicity to two indigenous (Gammarus pulex and Gammarus fossarum) and four alien (Dikerogammarus villosus, Echinogammarus berilloni, Gammarus roeseli and Gammarus tigrinus) gammarids occurring in Belgium was tested. Significant differences (P < 0.05) in median lethal concentrations (LC(50)) were found between the different species, with 72 h-LC50s ranging from 6.3 to 268 μg/l and 96 h-LC50s from 4.7 to 88.9 μg/l. No clear trend in Cd sensitivity was found when comparing indigenous and alien gammarids. D. villosus, an alien invasive species, was the most sensitive to Cd toxicity and E. berilloni, another alien species, the least sensitive. In addition, larger Gammarid species were more sensitive to Cd toxicity than smaller ones. No significant differences were found between populations of the same species originating from metal polluted sites or non-polluted sites. Overall, our results showed that considerable differences in Cd sensitivity exist between gammarid species, which should be taken into consideration in environmental risk assessment and water quality standard setting. Finally, our data suggest that alien gammarids would not have an advantage over indigenous gammarids in Cd contaminated environments.

Traits-based approaches in bioassessment and ecological risk assessment: strengths, weaknesses, opportunities and threats

We discuss the application of traits-based bioassessment approaches in retrospective bioassessment as well as in prospective ecological risk assessments in regulatory frameworks. Both approaches address the interaction between species and stressors and their consequences at different levels of biological organization, but the fact that a specific species may be less abundant in a potentially impacted site compared with a reference site is, regrettably, insufficient to provide diagnostic information. Species traits may, however, overcome the problems associated with taxonomy-based bioassessment. Trait-based approaches could provide signals regarding what environmental factors may be responsible for the impairment and, thereby, provide causal insight into the interaction between species and stressors. For development of traits-based (TBA), traits should correspond to specific types of stressors or suites of stressors. In this paper, a strengths, weaknesses, opportunities, and threats (SWOT) analysis of TBA in both applications was used to identify challenges and potentials. This paper is part of a series describing the output of the TERA (Traits-based ecological risk assessment: Realising the potential of ecoinformatics approaches in ecotoxicology) Workshop held between 7 and 11 September, 2009, in Burlington, Ontario, Canada. The recognized strengths were that traits are transferrable across geographies, add mechanistic and diagnostic knowledge, require no new sampling methodology, have an old tradition, and can supplement taxonomic analysis. Weaknesses include autocorrelation, redundancy, and inability to protect biodiversity directly. Automated image analysis, combined with genetic and biotechnology tools and improved data analysis to solve autocorrelation problems were identified as opportunities, whereas low availability of trait data, their transferability, their quantitative interpretation, the risk of developing nonrelevant traits, low quality of historic databases, and their standardization were listed as threats.

Application of artificial neural network models to analyse the relationships between Gammarus pulex L. (Crustacea, Amphipoda) and river characteristics

This study aimed at analysing the relationship between river characteristics and abundance of Gammarus pulex. To this end, four methods which can identify the relative contribution and/or the contribution profile of the input variables in neural networks describing the habitat preferences of this species were compared: (i) the "PaD" ("Partial Derivatives") method consists of a calculation of the partial derivatives of the output in relation to the input variables; (ii) the "Weights" method is a computation using the connection weights of the backpropagation Artificial Neural Networks; (iii) the "Perturb" method analyses the effect of a perturbation of the input variables on the output variable; (iv) the "Profile" method is a successive variation of one input variable while the others are kept constant at a fixed set of values. The dataset consisted of 179 samples, collected over a three-year period in the Zwalm river basin in Flanders, Belgium. Twenty-four environmental variables as well as the log-transformed abundance of Gammarus pulex were used in this study. The different contribution methods gave similar results concerning the order of importance of the input variables. Moreover, the stability of the methods was confirmed by gradually removing variables. Only in a limited number of cases a shift in the relative importance of the remaining input variables could be observed. Nevertheless, differences in sensitivity and stability of the methods were detected, probably as a result of the different calculation procedures. In this respect, the "PaD" method made a more severe discrimination between minor and major contributing environmental variables in comparison to the "Weights", "Profile" and "Perturb" methods. From an ecological point of view, the input variables "Ammonium" and to a smaller extent "COD", were selected by these methods as dominant river characteristics for the prediction of the abundance of Gammarus pulex in this study area.

Application of automated measurement stations for continuous water quality monitoring of the Dender river in Flanders, Belgium

During the summer of 1999, two automated water quality measurement stations were installed along the Dender river in Belgium. The variables dissolved oxygen, temperature, conductivity, pH, rain-intensity, flow and solar radiation were measured continuously. In this paper these on-line measurement series are presented and interpreted using also additional measurements and ecological expert-knowledge. The purpose was to demonstrate the variability in time and space of the aquatic processes and the consequences of conducting and interpreting discrete measurements for river quality assessment and management. The large fluctuations of the data illustrated the importance of continuous measurements for the complete description and modelling of the biological processes in the river.