Diederik P. L. Rousseau

Contaminant Removal Processes in Subsurface-Flow Constructed Wetlands: A Review

The main contaminant removal processes occurring in subsurface-flow constructed wetlands treating wastewater are reviewed. Redox conditions prevailing in the wetlands are analyzed and linked to contaminant removal mechanisms. The removal of organic matter and its accumulation in the granular medium of the wetlands are evaluated with regard to particulate and dissolved components and clogging processes. The main biological processes linked to organic matter transformation—aerobic respiration, denitrification, acid fermentation, sulfate reduction, and methanogenesis—are reviewed separately. The processes of removal of surfactants, pesticides and herbicides, emergent contaminants, nutrients, heavy metals and faecal organisms are analyzed. Advances in wetland modeling are presented as a powerful tool for understanding multiple interactions occurring in subsurface-flow constructed wetlands during the removal of contaminants.

CWM1: a general model to describe biokinetic processes in subsurface flow constructed wetlands

This paper presents the Constructed Wetland Model No1 (CWM1), a general model to describe biochemical transformation and degradation processes for organic matter, nitrogen and sulphur in subsurface flow constructed wetlands. The main objective of CWM1 is to predict effluent concentrations from constructed wetlands without predicting gaseous emissions. CWM1 describes aerobic, anoxic and anaerobic processes and is therefore applicable to both horizontal and vertical flow systems. 17 processes and 16 components (8 soluble and 8 particulate) are considered. CWM1 is based on the mathematical formulation as introduced by the IWA Activated Sludge Models (ASMs). It is important to note that besides the biokinetic model a number of other processes including porous media hydrodynamics, the influence of plants, the transport of particles/suspended matter to describe clogging processes, adsorption and desorption processes and physical re-aeration must be considered for the formulation of a full model for constructed wetlands.

Removal of estrone, 17α-ethinylestradiol, and 17ß-estradiol in algae and duckweed-based wastewater treatment systems

Background, aim, and scopeMany pollutants have received significant attention due to their potential estrogenic effect and are classified as endocrine disrupting compounds (EDCs). Because of possible ecological effects and increased attention for water reuse schemes, it is important to increase our understanding of the EDC removal capacities of various wastewater treatment systems. However, there has so far been little research on the fate and behavior of EDCs in stabilization pond systems for wastewater treatment, which represent an important class of wastewater treatment systems in developing countries because of their cost-effectiveness. The aim of this work is to study the fate and behavior of EDCs in algae and duckweed ponds. Because the synthetic hormone 17α-ethinylestradiol (EE2) and the natural hormones estrone (E1), as well as 17β-estradiol (E2), have been detected in effluents of sewage treatment plants and been suggested as the major compounds responsible for endocrine disruption in domestic sewage; E1, E2, and EE2 were therefore chosen as target chemicals in this current work.Materials and methodsBoth batch tests and continuous-flow tests were carried out to investigate the sorption and biodegradation of estrogens in algae and duckweed pond systems. The applied duckweed was a Lemna species. The applied algae was a mixture of pure cultures of six different algae genera, i.e., Anabaena cylindrica, Chlorococcus, Spirulina platensis, Chlorella, Scenedesmus quadricauda, and Anaebena var. Synthetic wastewater were used in all tests. The concentrations of estrogens were measured with three different enzyme-linked immunosorbent assay kits specific for E1, E2, or EE2. When the concentrations of estrogens in water samples were below the lowest quantitative analysis range (0.05xa0µg/l), preconcentration of the water samples were performed by means of solid phase extraction (SPE) with C18 cartridges.ResultsThe 6-day batch tests show that the presence of algae or duckweed accelerated the removal of the three estrogens from the synthetic wastewater. More estrogens were removed in the tests with duckweed than in tests with algae or with wastewater. In the sorption tests, a swift sorption of the three estrogens was observed when the estrogens were contacted with duckweed or algae, while the estrogen concentrations in tap water kept unchanged during the 3-h sorption tests. The mass balances indicated that only about 5% of the estrogens were bound to the algae sediment or duckweed at the end of the 6-day tests. Results of the continuous-flow tests revealed that the algae and duckweed ponds effectively removed E1, E2, and EE2 even at nanograms per liter level. Interconversion of E1 and E2 occurred both in batch and continuous-flow tests. E2 could be readily transformed to E1, especially in the tests with algae.DiscussionDifferent processes like sorption, biodegradation and photolytic degradation might play an important role in the removal of estrogens from the aquatic phase. The 3-h sorption tests support the importance of sorption for estrogen removal, in which a rapid initial sorption was observed over the first 2xa0min for E1/E2/EE2 to both duckweed and algae. In the 6-day batch tests, estrogens were sorbed by algae or duckweed during the early stage when algae and duckweed were contacted with the synthetic wastewater and the sorbed estrogens were further biodegraded by the microorganisms developed in the wastewater. The persistent estrogen concentrations in tap water, however, implied that no sorption, biodegradation, or photolytic degradation occurred in tap water under the specific experimental conditions. Under aerobic or anoxic conditions, E2 could be first oxidized to E1, which is further oxidized to unknown metabolites and finally to CO2 and water. Under anaerobic conditions, E1 can also be reduced to E2. However, the interconversion might be much more complex especially in the tests with algae because both aerobic and anaerobic conditions occurred in these tests due to the variation of the dissolved oxygen concentration induced by the light regime.ConclusionsThis study shows that estrogens, E1, E2, and EE2, can be effectively removed from the continuous-flow algae and duckweed ponds even when their concentrations are at nanograms per liter level. The presence of algae and duckweed accelerate the removal of estrogens from the synthetic wastewater because estrogens can be quickly sorbed on duckweed or algae. The sorbed estrogens are subsequently degraded by microorganisms, algae, or duckweed in the wastewater treatment system. E1 and E2 are interconvertible in both duckweed and algae pond systems. E2 can be readily transformed to E1, especially in the tests with algae.Recommendation and perspectivesBased on the tests performed so far, one can conclude that both sorption and biodegradation are important to the estrogens removal from stabilization pond systems for wastewater treatment. Further research using, e.g., radioimmunoassay is needed to investigate the biodegradation pathway of estrogens in algae and duckweed ponds.

Effect of sulfide concentration on the location of the metal precipitates in inversed fluidized bed reactors

The effect of the sulfide concentration on the location of the metal precipitates within sulfate-reducing inversed fluidized bed (IFB) reactors was evaluated. Two mesophilic IFB reactors were operated for over 100 days at the same operational conditions, but with different chemical oxygen demand (COD) to SO(4)(2-) ratio (5 and 1, respectively). After a start up phase, 10mg/L of Cu, Pb, Cd and Zn each were added to the influent. The sulfide concentration in one IFB reactor reached 648 mg/L, while it reached only 59 mg/L in the other one. In the high sulfide IFB reactor, the precipitated metals were mainly located in the bulk liquid (as fines), whereas in the low sulfide IFB reactor the metal preciptiates were mainly present in the biofilm. The latter can be explained by local supersaturation due to sulfide production in the biofilm. This paper demonstrates that the sulfide concentration needs to be controlled in sulfate reducing IFB reactors to steer the location of the metal precipitates for recovery.

Reversing clogging in subsurface-flow constructed wetlands by hydrogen peroxide treatment: two case studies

One of the most frequently encountered operational problems in subsurface-flow constructed wetlands is clogging. Traditionally, the restoration procedure is to remove the clogged gravel or sand and replace it with clean material. This method, while effective, is costly and may require sections of the facility to be taken offline for extended periods of time. Another common remediation strategy is to have a resting period for each wetland cell, although this is not an option for very small systems which often consist of only one treatment cell. Recently, a more radical approach has been tested on a number of lab-scale and pilot-scale setups which consists of an aggressive oxidation of organic matter by means of hydrogen peroxide (H(2)O(2)). Results indicate that after treatment, clogging was substantially reduced and that H(2)O(2) did not appear to have a long-term negative effect on plants and biofilms. The outcomes of two full-scale tests are discussed in this paper.

Performance comparison and economics analysis of waste stabilization ponds and horizontal subsurface flow constructed wetlands treating domestic wastewater: a case study of the Juja sewage treatment works.

The performance, effluent quality, land area requirement, investment and operation costs of a full-scale waste stabilization pond (WSP) and a pilot scale horizontal subsurface flow constructed wetland (HSSF-CW) at Jomo Kenyatta University of Agriculture and Technology (JKUAT) were investigated between November 2010 to January 2011. Both systems gave comparable medium to high levels of organic matter and suspended solids removal. However, the WSP showed a better removal for Total Phosphorus (TP) and Ammonium (NH4(+)-N). Based on the population equivalent calculations, the land area requirement per person equivalent of the WSP system was 3 times the area that would be required for the HSSF-CW to treat the same amount of wastewater. The total annual cost estimates consisting of capital, operation and maintenance (O&M) costs were comparable for both systems. However, the evaluation of the capital cost of either system showed that it is largely influenced by the size of the population served, local cost of land and the construction materials involved. Hence, one can select either system in terms of treatment efficiency. When land is available other factor including the volume of wastewater or the investment, and O&M costs determine the technology selection.

Performance Evaluation of Horizontal Subsurface Flow-Constructed Wetlands for the Treatment of Domestic Wastewater in the Tropics

AbstractThe lack of information on constructed-wetland performance in the tropics is among the factors that have hindered the adoption of low-cost wastewater-treatment technologies as alternatives to conventional wastewater treatment. A pilot scale study was undertaken in Juja (Kenya) to assess the performance of horizontal subsurface flow–constructed wetlands (HSSF-CWs) under tropical conditions. Primary domestic wastewater effluent was continuously fed into three replicate wetland cells, each with an area of 22.5u2009u2009m2 (7.5×3u2009u2009m) and with gravel as substrate. The study revealed successful performance of the wetlands in terms of compliance with local discharge standards with respect to chemical-oxygen demand (COD), BOD5, total suspended solids (TSS), and SO42−-S at an average mass removal efficiency between 58.9 and 74.9%. Moderate removal of NH4+-N and total phosphorus (TP) was recorded. The estimated first-order aerial-rate constant and the BOD5 background concentration showed the HSSF-CW to be area-requ...

Application of the gas tracer method for measuring oxygen transfer rates in subsurface flow constructed wetlands

The oxygen transfer rate (OTR) has a significant impact on the design, optimal operation and modelling of constructed wetlands treating wastewater. Oxygen consumption is very fast in wetlands and the OTR cannot be determined using an oxygen mass balance. This problem is circumvented in this study by applying the gas tracer method. Experiments were conducted in an unplanted gravel bed (dimensions L x W x d 125 x 50 x 35 cm filled with a 30-cm layer of 10-11-mm gravel) and a planted horizontal subsurface flow constructed wetland (HSSFCW) (L x W x d 110 x 70 x 38 cm filled with a 30-cm layer of 3.5-mm gravel with Phragmites australis). Tap water saturated with propane as gas tracer (pure or commercial cooking gas, depending on the test) was used. The mass transfer ratio between oxygen and commercial propane gas was quite constant and averaged R = 1.03, which is slightly lower than the value of R = 1.39 that is usually reported for pure propane. The OTR ranged from 0.31 to 5.04 g O(2) m(-2) d(-1) in the unplanted gravel bed and from 0.3 to 3.2 g O(2) m(-2) d(-1) in the HSSFCW, depending on the hydraulic retention time (HRT). The results of this study suggest that the OTR in HSSFCW is very low for the oxygen demand of standard wastewater and the OTR calculations based on mass balances and theoretical stoichiometric considerations overestimate OTR values by a factor that ranges from 10 to 100. The gas tracer method is a promising tool for determining OTR in constructed wetlands, with commercial gas proving to be a viable low-cost alternative for determining OTR.

Numerical Modelling of Waste Stabilization Ponds: Where Do We Stand?

Waste stabilization pond (WSP) technology has been an active area of research for the last three decades. In spite of its relative simplicity of design, operation and maintenance, the various processes taking place in WSP have not been entirely quantified. Lately, modelling has served as an important, low-cost tool for a better description and an improved understanding of the system. Although several papers on individual pond models have been published, there is no specific review on different models developed so far. This paper aims at filling this gap. Models are compared by focussing on their key features like the presence and comprehensiveness of a water quality sub-model in terms of aerobic/anoxic and anaerobic carbon removal and nutrient removal; the type of hydraulic sub-model used (0D, 1D, 2D or 3D); the software used for implementation and simulation; and whether or not sensitivity analysis, calibration and validation were done. This paper also recommends future directions of research in this area. In-depth study of the published models reveals a clear evolution over time in the concept of modelling, from just hydraulic empirical models to 3D ones and from simple first-order water quality models to complex ones which describe key biochemical processes as a set of mathematical equations. Due to the inherent complexity, models tend to focus only on specific aspects whilst ignoring or simplifying others. For instance, many models have been developed that either focus solely on hydrodynamics or solely on biochemical processes. Models which integrate both aspects in detail are still rare. Furthermore, it is evident from the review of the different models that calibration and validation with full-scale WSP data is also scarce. Hence, we believe that there is a need for the development of a comprehensive, calibrated model for waste stabilization ponds that can reliably serve as a support tool for the improvement and optimization of pond design and performance.

Use of Gisenyi Volcanic Rock for Adsorptive Removal of Cd(II), Cu(II), Pb(II), and Zn(II) from Wastewater

Volcanic rock is a potential adsorbent for metallic ions from wastewater. This study determined the capacity of Gisenyi volcanic rock found in Northern Rwanda to adsorb Cd, Cu, Pb and Zn using laboratory scale batch experiments under a variety of experimental conditions (initial metal concentration varied from 1 to 50xa0mg/L, adsorbent dosage 4xa0g/L, solid/liquid ratio of 1:250, contact time 120xa0h, particle size 250–900xa0μm). The adsorbent had a surface area of 3xa0m2/g. The adsorption process was optimal at near-neutral pHxa06. The maximal adsorption capacity was 6.23, 10.87, 9.52 and 4.46xa0mg/g for Cd, Cu, Pb and Zn, respectively. The adsorption process proceeded via a fast initial metal uptake during the first 6xa0h, followed by slow uptake and equilibrium after 24xa0h. Data fitted well the pseudo second-order kinetic model. Equilibrium experiments showed that the adsorbent has a high affinity for Cu and Pb followed by Cd and Zn. Furthermore, the rock is a stable sorbent that can be reused in multiple sorption–desorption–regeneration cycles. Therefore, the Gisenyi volcanic rock was found to be a promising adsorbent for heavy metal removal from industrial wastewater contaminated with heavy metals.