Browsing by Author "Ho, Long"

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A closer look on spatiotemporal variations of dissolved oxygen in waste stabilization ponds using mixed models
(2018) Alvarado Martínez, Andrés Omar; Ho, Long
Dissolved oxygen is an essential controlling factor in the performance of facultative and maturation ponds since both take many advantages of algal photosynthetic oxygenation. The rate of this photosynthesis strongly depends on the time during the day and the location in a pond system, whose roles have been overlooked in previous guidelines of pond operation and maintenance (O&M). To elucidate these influences, a linear mixed effect model (LMM) was built on the data collected from three intensive sampling campaigns in a waste stabilization pond in Cuenca, Ecuador. Within two parallel lines of facultative and maturation ponds, nine locations were sampled at two depths in each pond. In general, the output of the mixed model indicated high spatial autocorrelations of data and wide spatiotemporal variations of the oxygen level among and within the ponds. Particularly …
An integrated mechanistic modeling of a facultative pond: parameter estimation and uncertainty analysis
(2019) Ho, Long; Alvarado Martínez, Andrés Omar; Larriva Vásquez, Josué Bernardo; Cássia Rocha, Pompeu; Goethals, Peter
Abstract Imitating natural lakes, pond treatment systems inherit a high complexity with interconnected web of biochemical reactions and complex hydraulic processes. As such, its simulation requires a large and integrated model, which has been a challenge for pond engineers. In this study, we develop an all-encompassing model to gain a quantitative and comprehensive understanding of the hydraulic, physicochemical and microbiological conversion processes in the most common pond, a facultative pond. Moreover, to deal with an evitable issue of large mechanistic models as overparameterization leading to poor identifiability, a systematic parameter estimation was implemented. The application of sensitivity analysis reveals the most influential parameters on pond performance. Particularly, physical parameters, such as vertical eddy diffusivity, water temperature, and maximum growth rate of heterotrophs induce the most changes of organic matters while microbial assimilation and ammonia volatilization appear to be main processes for nutrient removal. In contrast, the efficiency of phosphate precipitation and nutrient biological removal via polyphosphate accumulating organisms and denitrifying bacteria is limited. Identifiability problems are addressed mainly by the characterization of light dependence of algal growth, interaction between water temperature and its coefficient, and the growth of autotrophic bacteria while based on the determinant measures, the most important parameter subsets affecting model outputs are related to physical processes and algal activity. After the establishment of the influential and identifiable parameter subset, an automatic calibration with the data collected from Ucubamba pond system (Ecuador) demonstrates the effect of high-altitude climatic conditions on pond behaviors. An aerobic prevailing condition is observed as a result of high light intensity causing accelerated algal activities, hence, leading to the limitation of hydrolysis, anaerobic processes, and the growth of anoxic heterotrophs for denitrification. Furthermore, the output of uncertainty analysis indicates that a large avoidable uncertainty as a result of vast complexity of the applied model can be reduced greatly via a systematic approach for parameter estimation.
Exploring the influence of meteorological conditions on the performance of a waste stabilization pond at high altitude with structural equation modeling
(2018) Ho, Long; Van Echelpoel, Wout; Alvarado Martínez, Andrés Omar
Algal photosynthesis plays a key role in the removal mechanisms of waste stabilization ponds (WSPs), which is indicated in the variations of three parameters, dissolved oxygen, pH, and chlorophyll a. These variations can be considerably affected by extreme climatic conditions at high altitude. To investigate these effects, three sampling campaigns were conducted in a high-altitude WSP in Cuenca (Ecuador). From the collected data, the first application of structure equation modeling (SEM) on a pond system was fitted to analyze the influence of high-altitude characteristics on pond performance, especially on the three indicators. Noticeably, air temperature appeared as the highest influencing factors as low temperature at high altitude can greatly decrease the growth rate of microorganisms. Strong wind and large diurnal variations of temperature, 7–20 C, enhanced flow efficiency by improving mixing inside the ponds. Intense solar radiation brought both advantages and disadvantages as it boosted oxygen level during the day but promoted algal overgrowth causing oxygen depletion during the night. From these findings, the authors proposed insightful recommendations for future design, monitoring, and operation of high-altitude WSPs. Moreover, we also recommended SEM to pond engineers as an effective tool for better simulation of such complex system like WSPs.
Greenhouse gas dynamics in an urbanized river system: influence of water quality and land use
(2022) Ho, Long; Jerves Cobo, Ruben Fernando; Barthel, Matti; Seis, Johan; Bodé, Samuel; Boeckx, Pascal; Goethals, Pedro
Rivers act as a natural source of greenhouse gases (GHGs). However, anthropogenic activities can largely alter the chemical composition and microbial communities of rivers, consequently affecting their GHG production. To investigate these impacts, we assessed the accumulation of CO2, CH4, and N2O in an urban river system (Cuenca, Ecuador). High variation of dissolved GHG concentrations was found among river tributaries that mainly depended on water quality and land use. By using Prati and Oregon water quality indices, we observed a clear pattern between water quality and the dissolved GHG concentration: the more polluted the sites were, the higher were their dissolved GHG concentrations. When river water quality deteriorated from acceptable to very heavily polluted, the mean value of pCO2 and dissolved CH4 increased by up to ten times while N2O concentrations boosted by 15 times. Furthermore, surrounding land-use types, i.e., urban, roads, and agriculture, could considerably affect the GHG production in the rivers. Particularly, the average pCO2 and dissolved N2O of the sites close to urban areas were almost four times higher than those of the natural sites while this ratio was 25 times in case of CH4, reflecting the finding that urban areas had the worst water quality with almost 70% of their sites being polluted while this proportion of nature areas was only 12.5%. Lastly, we identified dissolved oxygen, ammonium, and flow characteristics as the main important factors to the GHG production by applying statistical analysis and random forests. These results highlighted the impacts of land-use types on the production of GHGs in rivers contaminated by sewage discharges and surface runoff.
Improving water management education across the Latin America and caribbean region
(2019) Domínguez Granda, Luis Elvin; Hampel, Henrietta; Lam, Norris; Nolivos Alvarez, Indira Yadira; Reinstorf, Frido; Lyon, Steve W.; Ho, Long; Rodríguez Tejeda, Raymundo C.; Vázquez Zambrano, Raúl Fernando; Goethals, Peter; Schneider, Petra
Education can help secure inclusive and resilient development around water resources. However, it is difficult to provide the latest science to those managing water resources (both now and in the future). Collectively, we hypothesize that dissemination and promotion of scientific knowledge using students as central agents to transfer theoretical knowledge into practice is an efficient way to address this difficulty. In this study, we test this hypothesis in the Latin America and Caribbean (LAC) region as a representative case study region. First, we use a literature review to map a potential gap in research on education around water resources across the LAC region. We then review potential best practices to address this gap and to better translate water resources education techniques into the LAC region. Integral to these efforts is adopting students as agents for information transfer to help bridge the gap between the global state-of-the science and local water resources management. Our results highlight the need to establish a new standard of higher educational promoting exchange between countries as local populations are vulnerable to future shifts in climate at global scales and changes in land usage at regional scales. The new standard should include peer-to-peer mentoring achieved by jointly exchanging and training students and practitioners in water management techniques, increasing access to water data and pedagogic information across the region, and lowering administration roadblocks that prevent student exchange
Integrated mechanistic and data-driven modeling for risk assessment of greenhouse gas production in an urbanized river system
(2021) Ho, Long; Jerves Cobo, Ruben Fernando; Forio, Marie Anne Eurie; Mouton, Ans; Nopens, Ingmar; Goethals, Peter
Surrounded by intense anthropogenic activities, urban polluted rivers have increasingly been reported as a significant source of greenhouse gases (GHGs). However, unlike pollution and climate change, no integrated urban water models have investigated the GHG production in urban rivers due to system complexity. In this study, we proposed a novel integrated framework of mechanistic and data-driven models to qualitatively assess the risks of GHG accumulation in an urban river system in different water management interventions. Particularly, the mechanistic model delivered elaborated insights into river states in four intervention scenarios in which the installation of a new wastewater treatment plant using two different technologies, together with new sewage systems and additional retention tanks, were assessed during dry and rainy seasons. From the insights, we applied fuzzy rule-based models as a decision support tool to predict the GHG accumulation risks and identify their driving factors in the scenarios. The obtained results indicated the important role of new discharge connection and additional storage capacity in decreasing pollutant concentrations, consequently, reducing the risks. Moreover, among the major variables explaining the GHG accumulation in the rivers, DO level was considerably affected by the reaeration capacity of the rivers that was strongly dependent on river slope and flow. Furthermore, river water quality emerged as the most critical variable explaining the pCO2 and N2O accumulation that implied that the more polluted and anaerobic the sites were, the higher were their GHG accumulation. Given its simplicity and transparency, the proposed modeling framework can be applied to other river basins as a decision support tool in setting up integrated urban water management plans.
Modified First Stage of French Vertical Flow Constructed Wetlands performance during extreme operation conditions
(INRAE, INSA, 2022) Arévalo Durazno, María Belén; García Zumalacarregui, Jorge Alejandro; Ho, Long; Narváez, Andrea; Alvarado, Andrés
This study applied two Modified First Stage of French Vertical Flow Constructed Wetlands pilots for the treatment of sewage coming from a combined sewer system under extreme hydraulic loads and medium temperatures at high altitude. Particularly, the pilot-scale experiment was conducted within nine months after their start-up period of four months. During this period, two different high hydraulic loading rates (0.94 m d-1 and 0.56 m d-1) representing 2.5 and 1.5 times the design values respectively, were applied to investigate the impact of HLRs on the performance of the treatment system. The observed performances for COD and TSS removals were satisfactory, with average values of 53% and 66% respectively. Moreover, reducing the HLR resulted in higher removal efficiency for COD (from 46% to 64%) but not for TSS (from 67% to 64%). From these findings, it is suggested that high HLRs can be applied for the First Stage of the French System without compromising the operation of the units and obtaining satisfactory results, especially in case of diluted wastewater and severe area restrictions.
Spatial and temporal variations of greenhouse gas emissions from a waste stabilization pond: effects of sludge distribution and accumulation
(2021) Ho, Long; Jerves Cobo, Ruben Fernando; Morales Matute, Oscar Patricio; Larriva Vasquez, Josue Bernardo; Arevalo Durazno, Maria Belen; Barthel, Matti; Six, Johan; Bode, Samuel; Boeckx, Pascal; Goethals, Peter
Due to regular influx of organic matter and nutrients, waste stabilization ponds (WSPs) can release considerable quantities of greenhouse gases (GHGs). To investigate the spatiotemporal variations of GHG emissions from WSPs with a focus on the effects of sludge accumulation and distribution, we conducted a bathymetry survey and two sampling campaigns in Ucubamba WSP (Cuenca, Ecuador). The results indicated that spatial variation of GHG emissions was strongly dependent on sludge distribution. Thick sludge layers in aerated ponds and facultative ponds caused substantial CO 2 and CH 4 emissions which accounted for 21.3% and 78.7% of the total emissions from the plant. Conversely, the prevalence of anoxic conditions stimulated the N 2 O consumption via complete denitrification leading to a net uptake from the atmosphere, i.e. up to 1.4 ±0.2 mg-N m −2 d −1 . Double emission rates of CO 2 were found in the facultative and maturation ponds during the day compared to night-time emissions, indicating the important role of algal respiration, while no diel variation of the CH 4 and N 2 O emissions was found. Despite the uptake of N 2 O, the total GHG emissions of the WSP was higher than constructed wetlands and conventional cen- tralized wastewater treatment facilities. Hence, it is recommended that sludge management with proper desludging regulation should be included as an important mitigation measure to reduce the carbon foot- print of pond treatment facilities.