Roger Samsó

Bacteria distribution and dynamics in constructed wetlands based on modelling results.

Bacteria communities growing in constructed wetlands play a major role on the removal of pollutants from wastewater and the presence of a stable community is a critical factor affecting their performance. With this work we aimed at finding how long it takes for bacterial communities to stabilise in constructed wetlands and at answering specific questions regarding their abundance, spatial distribution and their relative importance on the treatment processes. To this end the numerical model BIO_PORE was used to simulate the dynamics of 6 functional bacteria groups (heterotrophic, autotrophic nitrifying, fermenting, acetotrophic methanogenic, acetotrophic sulphate reducing and sulphide oxidising bacteria) within a wetland for a period of 3 years. Three indicators of bacterial stabilisation were used: 1) total biomass; b) effluent pollutant concentrations and c) Shannons diversity index. Results indicate that aerobic bacteria dominated the wetland until the 80th day of operation. Anaerobic bacteria dominated the wetland from that moment and until the end of the studied period. Bacteria stability was reached between 400 and 700 days after starting operation. Once the wetland reached stability, sulphate reducing bacteria accounted for the highest biomass of all bacterial groups (46%). The distribution of bacterial communities obtained after bacterial stability is consistent with available experimental results, and was clearly controlled by dissolved oxygen (SO) concentrations and H2S toxicity. After stability, the progressive accumulation of inert solids pushed the location of the active bacteria zone towards the outlet section.

Modelling bioclogging in variably saturated porous media and the interactions between surface/subsurface flows: Application to Constructed Wetlands.

Horizontal subsurface Flow Constructed Wetlands (HF CWs) are biofilters planted with aquatic macrophytes within which wastewater is treated mostly through contact with bacterial biofilms. The high concentrations of organic carbon and nutrients being transported leads to high bacterial biomass production, which decreases the flow capacity of the porous material (bioclogging). In severe bioclogging scenarios, overland flow may take place, reducing overall treatment performance. In this work we developed a mathematical model using COMSOL Multiphysics™ and MATLAB(®) to simulate bioclogging effects in HF CWs. Variably saturated subsurface flow and overland flow were described using the Richards equation. To simplify the inherent complexity of the processes involved in bioclogging development, only one bacterial group was considered, and its growth was described using a Monod equation. Bioclogging effects on the hydrodynamics were taken into account by using a conceptual model that affects the value of Mualems unsaturated relative permeability. Simulation results with and without bioclogging were compared to showcase the impact of this process on the overall functioning of CWs. The two scenarios rendered visually different bacteria distributions, flow and transport patterns, showing the necessity of including bioclogging effects on CWs models. This work represents one of the few studies available on bioclogging in variably saturated conditions, and the presented model allows simulating the interaction between overland and subsurface flow occurring in most HF CWs. Hence, this work gets us a step closer to being able to describe CWs functioning in an integrated way using mathematical models.

Subsurface Flow Constructed Wetland Models: Review and Prospects

Numerical models are recognized nowadays as a powerful tool to increase the understanding of the internals of constructed wetlands and to help improve their design. Over the last decade many models have been developed, and many simulation studies have been published. Despite diversity is generally a positive thing, having so many different models can be confusing for potential users and may also hinder further development of the existing ones. The aim of this paper is to summarize the state of the art of this discipline, focussing the attention on the most feature-rich process-based models for constructed wetlands for urban wastewater treatment. Their description is combined with a feature comparison in a tabular format to facilitate the selection of one or another based on the specific needs of the potential user. Moreover, a discussion is made regarding the advantages of each reviewed model regarding features, licencing and expected evolution of each of them. Later in the document, we describe the essential phenomena, parameters and processes that we believe that future generation of constructed wetlands models should incorporate, to guide further research on this discipline. Although this paper is focused on models used in academic circles, a model developed to optimize the design of combined sewer overflow wetlands is presented as an example of the potential of design-focused wetlands. At the end of the paper we provide an overview of the past, present and future of constructed wetlands models and analyse were we stand and which is the way to go and the main goals in the near future.

Effect of key design parameters on bacteria community and effluent pollutant concentrations in constructed wetlands using mathematical models

Constructed wetlands are currently recognized as an effective environmental biotechnology for wastewater treatment, but the influence of their design parameters on internal functioning and contaminant removal efficiency is still under discussion. In this work, the effect of aspect ratio and water depth on bacteria communities as well as treatment efficiency of horizontal subsurface flow constructed wetlands (HSSF) under the Mediterranean climate was evaluated, using a mathematical model. For this purpose, experimental results from four pilot-scale wetlands of equal surface area but different aspect ratios and water depth were used. The HSSF system was fed with municipal wastewater. The experimental data were simulated using the BIO_PORE model, developed in the COMSOL Multiphysics™ platform. Simulations with the BIO_PORE model fitted well to the experimental results, showing a higher removal efficiency for the shallower HSSF for COD (93.7% removal efficiency) and ammonia nitrogen (73.8%). The aspect ratio had a weak relationship with the bacteria distribution and the removal efficiency. In contrast, the water depth was a factor. The results of the present study confirm a previous hypothesis in which depth has an important impact on the biochemical reactions causing contaminants transformation and degradation.

Changes in bacteria composition and efficiency of constructed wetlands under sustained overloads: A modeling experiment

The average organic and hydraulic loads that Constructed Wetlands (CWs) receive are key parameters for their adequate long-term functioning. However, over their lifespan they will inevitably be subject to either episodic or sustained overloadings. Despite that the consequences of sustained overloading are well known (e.g., clogging), the threshold of overloads that these systems can tolerate is difficult to determine. Moreover, the mechanisms that might sustain the buffering capacity (i.e., the reduction of peaks in nutrient load) during overloads are not well understood. The aim of this work is to evaluate the effect of sudden but sustained organic and hydraulic overloads on the general functioning of CWs. To that end, the mathematical model BIO_PORE was used to simulate five different scenarios, based on the features and operation conditions of a pilot CW system: a control simulation representing the average loads; 2 simulations representing +10% and +30% sustained organic overloads; one simulation representing a sustained +30% hydraulic overload; and one simulation with sustained organic and hydraulic overloads of +15% each. Different model outputs (e.g., total bacterial biomass and its spatial distribution, effluent concentrations) were compared among different simulations to evaluate the effects of such operation changes. Results reveal that overloads determine a temporary decrease in removal efficiency before microbial biomass adapts to the new conditions and COD removal efficiency is recovered. Increasing organic overloads cause stronger temporary decreases in COD removal efficiency compared to increasing hydraulic loads. The pace at which clogging develops increases by 10% for each 10% increase on the organic load.