Total organic carbon | EPFL Graph Search

Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of water quality, or TOC of source rock may be used as one factor in evaluating a petroleum play. For marine surface sediments average TOC content is 0.5% in the deep ocean, and 2% along the eastern margins. A typical analysis for total carbon (TC) measures both the total organic carbon (TOC) present and the complementing total inorganic carbon (TIC), the latter representing the amount of non-organic carbon, like carbon in carbonate minerals. Subtracting the inorganic carbon from the total carbon yields TOC. Another common variant of TOC analysis involves removing the TIC portion first and then measuring the leftover carbon. This method involves purging an acidified sample with carbon-free air or nitrogen prior to measurement, and so is more accurat

Conception of a biological activated carbon filter for micropollutants removal in wastewater

Solène Gomez

The presence of micro pollutants in water is an important emerging problem because of their negative impacts on health and environment. The Swiss Federal Office of the Environment is planning to impose new regulations to reduce the load of micro pollutants in waste water. This paper assesses the use of biological activated carbon (BAC) to treat micro pollutants from small waste water treatment plant (WWTP). In a first part, articles were consulted to evaluate the state of the art of BAC filters: their properties, efficiencies, costs as well as the impact of parameters such as the empty bed contact time (EBCT), aeration and backwashing. In a second part, two BAC filters were designed for implementation in WWTP (Vidy, Lausanne, Switzerland) and laboratory. The dimensioned BAC filter for the WWTP has an EBCT of 90 min and treats 1.7 L day-1. The proposed carbon is the ‘DCL-G’ carbon type. Feed water is pumped from the aeration tank of the Vidy WWTP and rejected before the existing granular activated carbon filter. Backwashing consists in stirring the top layer of carbon bed weekly and no aeration is needed for the proposed implementation. Dissolved organic carbon and micro pollutants monitoring is planned. The BAC filters are expected to remove 90% of micro pollutants treated below a concentration of 10 ng L-1.

2013

Co-culture of microalgae and bacteria for wastewater treatment

Thierry Mounir

To address the problem of insucient wastewater treatment and eutrophication of lakes and rivers, the symbiotic association of microalgae and bacteria has shown great potential and numerous advantages. First, co-cultures can exhibit ecient treatment of domestic wastewater, with the combination of high nutrients (nitrogen and phosphorus) removal abilities of the microalgae and ecient degradation of organic matter by bacteria. Second, the symbiosis between microalgae and bacteria exchanging CO2 and O2 makes aeration not necessary, thus signicantly reducing the operating costs of the wastewater treatment plant. Third, recovered biomass of algae could be used as highpro t products like biofuels and food source. Fourth, the xation of the atmospheric CO2 by the microalgae through photosynthesis would contribute to greenhouse gas reduction in the atmosphere. Finally, the decayed microalgal biomass could serve as organic carbon source for denitrication by bacteria. However, this technology is not well-known yet, and further research on the in uence of the operating parameters have to be conducted. Furthermore, its main drawback is the harvesting of the microalgae. Due to their small size, microalgae demonstrate a low settling rate of 3.6 x 10􀀀3 m/h. A solution is to enhance aggregation of the biomass through occulation, followed by granulation to reach fully-developed microalgal-bacterial consortia. These granules exhibit high treatment performances and high settleability. The rst objective of this project was to identify the optimal microalgae:bacteria ratio for COD and nutrients removal. The second objective was to form fully-developed granules, starting from a co-culture, and assess the in uence of dierent agitation speeds on the granulation. The photobioreactor with a microalgae:bacteria ratio of 75:25 expressed the most promising results in COD removal eciency (95% after 8 days), nitrogen removal eciency (94% after 9 days), phosphorus removal eciency (93% after 7 days), and biomass productivity (144 mg/L/d). In this photobioreactor, nitrogen was removed by a combination of assimilation into microalgal biomass and nitrication/denitrication by the bacteria. The COD was removed by degradation by bacteria and mixotrophic microalgae that used the organic matter as carbon source. Finally, phosphorus was removed by assimilation into microalgal biomass. Although obvious aggregation between microalgae and bacteria could be observed, no fully-developed granules were formed after 89 batches of 24 hours. Because of low COD and nitrogen removals, no starvation period was experienced by the co-cultures. Therefore, extracellular polymeric substances were not released in sucient amounts to enhance the granulation. Furthermore, excessive stirring (200 rpm) may have lead to microalgal cells damage, thus promoting bacterial growth over microalgal growth. On the other hand, slower agitation speeds (80 and 120 rpm) seemed to enhance microalgal growth. Overall, the agitation speed appeared to have an indirect impact on the treatment performances and the settleability of the co-cultures, by signicantly in uencing the microalgal and bacterial growths.

2020

Soil-vegetation patterns in secondary slash and burn successions in Central Menabe, Madagascar

Alexandre Buttler, Jean-Michel Gobat

Slash and burn agriculture is a traditional and predominant land use practice in Madagascar and its relevance in the context of forest preservation is significant. At the end of a cycle of culture, the fields become mostly weed covered and the soil fertility starts to drop. As a consequence, these fields are abandoned (they are called “monka”) and the farmers, in the best case, re-use old surfaces where the vegetation has recovered to some extent. Nevertheless, some of the farmers continue to extend part of their cultures into the natural forest. In order to decrease deforestation, the paper focuses on the potential for agricultural re-use of monkas. To do so, we present the soil–vegetation pattern along a slash and burn successional gradient from newly cultivated surfaces to surfaces abandoned for 40 years. Vegetation relevés were carried out on 61 plots sampled on yellow and red soils, and soil variables such as loss of ignition, pH, total carbon content and total nitrogen content were measured. Results show that: (1) by the 10th year of abandonment woody species are increasing, and after 21–30 years herbaceous plants become less dominant, (2) the species richness increases with age of abandonment, but flattens out by 40 years, (3) by 20 years of fallow, the loss of ignition, total carbon and total nitrogen show similar values or even higher values than in cultivated surfaces, (4) the yellow soils are related to higher pH more than the red soils and are preferred for cultivation, but the higher pH of yellow soils is not associated with higher species richness. Given these results, we conclude that fields older than 20 years have recovered sufficient fertility to be re-used as agricultural land. This re-use would decrease impacts on natural forests. But beyond the nutrient perspective, critical problems remain, including the growing demand for arable land and the need for cultivation to control invasive weeds.

2010