Impacts of irrigation with industrial treated wastewater on soil properties

Wastewater reuse in agriculture is a widespread practice in developing countries, especially in urban areas where water shortage and poverty encourage people to use that marginal resource. Raw or treated wastewaters are used by farmers, but even treated wastewaters frequently do not meet WHO and FAO standards for irrigation water. Such practices may lead to health hazards, relatively well documented in the literature and to environmental damages. Adverse environmental impacts such as soil degradation and groundwater contamination are frequently associated with the use of wastewater from industrial sources. Previous studies have demonstrated that wastewater irrigation may decrease soil hydraulic conductivity and infiltration rate. Nevertheless, the effects on soil structural and chemical behaviors have been little studied so far and need further investigations. The impacts of irrigation with alkaline and sodic industrial wastewater previously treated in microphyte ponds on soil physical and chemical properties were studied downstream the sewage treatment plant of Kossodo in Ouagadougou, on plots cropped with eggplants. Plots irrigated with fresh water and non cropped, non irrigated plots were used as controls. Different soil properties were characterized: pore volumes determined by using shrinkage analysis, pH and electrical conductivity of water extracts, and major soluble and exchangeable cations (Ca, Mg, K, and Na). Organic matter characterization was performed by means of three dimensional fluorescence spectra analysis to determine its origin and evolution on irrigated soils. Plots irrigated with wastewater showed important structural damages, especially in the subsurface horizon where the soil pore network collapsed dramatically, resulting in a compact impermeable layer. Fluorescence spectra revealed that the organic matter contained in the wastewater was largely dissolved due to a sharp soil pH increase, resulting in black alkali formation at the surface: the soil became sodic, with an exchange complex dominated by sodium, whereas plots irrigated with fresh water kept properties comparable to that of non irrigated plots. Such a rapid soil sodication was seldom reported so far. The study emphasizes the need to carefully examine irrigation water quality and particularly calcite residual alkalinity and suggests that shrinkage analysis could be used to monitor the physical changes of soil properties upon sodication. Inadequate wastewater quality is likely to cause deep and irreversible damages to irrigated soils. (C) 2013 Elsevier B.V. All rights reserved.

Recyclage des eaux usées en irrigation

Yeli Mariam Sou

The present study takes place in Ouagadougou (Burkina Faso), a typical Sub-Saharan city where water shortage and food crisis necessitate wastewater reuse in urban agriculture. Different issues linked to this practice were studied in two components separated in two experimental sites. The first part treats about agronomic and sanitary aspects and take place in the 2iE (International Institute for Water and Environmental Engineering) experimental site of Ouagadougou. The second component was emphasised on industrial effluents impacts on irrigated soil quality and takes place in Kossodo, an industrial suburb of Ouagadougou. The 2iE site is supply with treated wastewater from domestic origin. The effluent was used to realise the following objectives: (1) managed wastewater fertilizers to fit the plant needs; (2) assess different crops sanitary quality and the effectiveness of methods used for this assessment. Three treatments were tested: (i) irrigation with the treated wastewater (ii) irrigation with the treated wastewater and fertilizers management, adjusted to plant needs and (iii) conventional treatment i.e. fresh water irrigation and total doses of commercial fertilizers. Each treatment was tested on three different crops: lettuce (Lactuca sativa), carrot (Daucus carota) and eggplant (Solanum melongena). The first objective was achieved by comparing crops yields (on basis of wet and dry matter) obtained with the different treatments. The three crops, irrigated with only wastewater, led to yield reduction between the first and the second year. At the contrary, the adjusted treatment showed higher yield for lettuce, with comparable values to conventional treatment yield. However, the effects on carrot and eggplant did not demonstrate better results; indicating that further investigations must be completed for crops with longer growing periods. Most likely, the fertilizers management should better correspond to the crop period demand. The sanitary assessment (second objective) was performed by measuring the heavy metals concentration (Cd, Cu, Pb, Ni, Cr) and the faecal levels (faecal coliform, Escherichia coli and four pathogens among which Salmonella typhimirium and Vibrio cholerae). Chromocult Coliform Agar medium was used for simultaneous detection of faecal coliform and Escherichia coli whereas pathogens were analysed by Polymerase Chain Reaction (PCR). From methodological point of view, faecal coliform were not sufficiently discriminatory in the case of this study since they were naturally present in the soil as well as in the fresh water. At the contrary, Escherichia coli were better linked to wastewater contamination. However, some "false positive" results were detected meaning that each presumptive colony of Escherichia coli must be confirmed. Nevertheless, microbial quality assessment indicated no Escherichia coli on edible parts of carrot or eggplant irrigated with wastewater. Carrots were expected to have the poorest microbiological quality. Therefore carrot root bacterial quality must be further examined. Results on eggplant are not surprising as these vegetables are somewhat far from the ground and they receive significant solar radiation which acts like a disinfectant. Lettuce from wastewater treatment plots, however, exhibited presence of Escherichia coli, meaning that lettuce is likely to be contaminated by pathogens. However, no pathogens were detected neither on crops irrigated with fresh water nor on those irrigated with wastewater. Heavy metals results demonstrated that the crop's consumption did not involve higher risk for consumers when the vegetables were irrigated with domestic wastewater, in comparison with fresh water irrigation. Kossodo treatment plant receives mainly industrial wastewater, which are characterized by high levels of sodium and bicarbonates. The treated wastewater supplies a market garden in which space has been defined for the study of this second component. The main objective was to assess the impact of treated wastewater on soil quality during two years of irrigation. Early in the second year, the presence of black spots, suspected to be black alkali (dissolved organic matter), appeared at the surface of plots irrigated with treated wastewater. These indications led to put forward the hypothesis of alkalinisation/sodisation on soil receiving wastewater. The methodological approach used was based on in situ measurement of infiltration at the end of the second year of irrigation, supplemented by shrinkage curve modelling and chemical analysis. The hypothesis of an alkalinization/sodisation and dissolved organic matter (black alkali) presence in plots irrigated with treated wastewater were validated by the chemical analysis. Shrinkage curve modelling showed a reduction of the structural porosity on wastewater irrigated plots. The reduction was more marked in the subsurface horizons than in surface, which is probably caused by dispersed clay leached in depth. These structural changes were confirmed in situ by an infiltration capacity greatly reduced on plots irrigated with treated wastewater, compared to control plots (irrigated with fresh water) where the infiltration capacity is higher and comparable to that of non-irrigated soil. The study confirmed the fertilizing value of domestic wastewater while emphasizing the need to concentrate future investigations on sustainable effluent/fertilizers management. The sanitary assessment with different crops regarding their respective distances to the soil was very useful since it showed the strong interaction of soil microbial community on the crop microbial quality. It also revealed the importance to determine a faecal indicator for the crop, specifically related to wastewater contamination. Soil quality assessment reiterated the necessity to verify wastewater quality before its reuse in agriculture. In the case of this study, the sodic and alkaline properties of the industrial effluents make them definitively unfit for irrigation.

EPFL2009

Fonctionnement hydrique de différents types de placages sableux dans le sahel burkinabè

Dial Niang

The study was performed in the Sahelian part of Burkina Faso. The country is subject to difficult climatic conditions, a strong demographic growth and a continuous decrease in soil fertility. The resulting degradation affects the processes which govern ecological systems and leads, on the long run, to a modification of the ecosystems. This is particularly the case of the eolian formations, the only ecological units susceptible of being useful in the area, since they present good infiltration capacity and support the main part of the vegetation. Field measurements (rainfall, runoff, saturated hydraulic conductivity, bulk density, hydraulic conductivity function, soil water content and pressure head) have been done to asses the impacts of the environmental degradation on the hydrodynamic behavior of these formations. Within this context, an experimental device made of seven measurement sites has been installed in three contrasting zones. The results show significant differences in the hydrodynamic behavior of the soils, according to their surface properties. On one hand, the sites located on an erosion crust as well as on a drying crust in transition towards an erosion crust, are characterized by a low infiltration capacity which favors runoff. The quantity of infiltrated water is very low when compared to total precipitation. Water is stored near the soil surface (in the first 30 cm) favoring evaporation in the days following rainfall events. Thus, little water is available for plants. The resulting water stress causes a weak density, or even complete absence of plants which have difficulties to colonize such surfaces. Surface runoff is favored with runoff coefficients ranging from 50 to 80 %. On the other hand, the sites situated on drying crusts are characterized by good hydraulic conductivity and infiltration capacity. The storage of water in the root zone is more important. Water flows deeper into the soil and, occasionally, drains at the reference depth of 50 cm. The runoff coefficients were found to be lower than in the other sites (30 to 40 %). This differentiated behavior is one of the main factors affecting the formation of different landscapes in the area through the existence of a relationship between soil surface properties, the type of landscape and the ratio runoff/infiltration. The sites located on an erosion crust and those in transition towards an erosion crust behave like impluviums, while those situated on drying crusts are more favorable to infiltration. Observation of the evolution of the hydraulic properties of the soil surface shows a progressive alteration that frequently leads to strongly degraded mediums unfavorable to plants. Control and rehabilitation methods considered in the study, namely "restoration" and "mise en défens" have only partial and time-limited effects; they do not make it possible to stop durably degradation or to favor rehabilitation. Among the other measures assessed on the INERA plots, only the restitution to the soil of the beforehand mown plants causes an improvement of the soil surface properties; the restitution causes the formation of an important macroporal system favoring water infiltration and circulation in the soil. The effects of those different measures occur mainly in the first years after their introduction and concern especially vegetation regeneration, soil protection against degradation factors, improvement of hydrodynamic soil properties, reduction of runoff and erosion. They make it possible to slow down soil degradation, but not to stop it definitively. Within this context, proposals of further studies focussed on other measures to control and rehabilitate degraded areas have been suggested, taking into account the specificity of the sahelian zone, the endogenous knowledge of the agro-pastors, as well as socio-economic and ecological conditions.

EPFL2006

Design approach for the improvement of the economic and environmental performances of potable water supply

François Vince

Triggered by the Kyoto Protocol and by the aggravation of freshwater scarcity, environmental impacts should soon become key decision criteria for the planning of potable water supply projects, especially when advanced systems such as seawater desalination are at stake. In order to foster the transition of the water industry towards sustainable practices, the present work proposes an integrated design approach dedicated to potable water supply that targets both economic and environmental objectives. At first, the current industrial practices (Chapter I) and the technical characteristics of potable water supply systems (Chapter II) are analyzed via the modeling of the process units used for potable water supply: pumping systems, conventional water treatment processes (e.g. clarification, filtration, disinfection) and advanced water treatment processes (e.g. membrane processes, thermal processes). Using the ISO 14040 standardized Life Cycle Assessment (LCA) method, Chapter III presents the development of a performance indicator that assesses the environmental impacts generated through all stages of the life cycle of potable water supply (i.e. from "cradle to grave"): from the construction of the potable water treatment plant, to its operation and decommissioning. This LCA-based indicator provides a holistic overview of all potential environmental impacts (e.g. green house gases (GHG) emissions, impacts on ecosystems and on human health). It allows to stress out the impact sources and the penalizing steps within potable water supply (Chapter IV): The production of electricity and chemicals required by the potable water treatment plant is highlighted to generate respectively 75% and 15% of the total impacts generated during the life cycle of potable water supply. Different potable water supply scenarios (e.g. potable water supply from ground water, from surface water, seawater desalination, water import from distant water resources) are benchmarked, in order to identify the best solutions as a function of the local context (e.g. type of electricity supply, topographic conditions, feed water quality). Based on the results of the environmental assessment, Chapter V proposes measures for the improvement of the industrial practices of the water sector, targeting energy and chemicals management, electricity sourcing and effluent disposal. Within the same perspective, Chapter VI details an optimization method for the design of the reverse osmosis (RO) membrane process, i.e. the key treatment process for desalination and wastewater reclamation. This method systematically synthesizes RO process configurations and evaluates their performances on economical (total annualized costs), technical (energy requirement, water conversion rate) and environmental criteria (GHG emissions). Evolutionary algorithms are then used to optimize the design of these configurations (process layout and operating conditions) and to identify those featuring the best trade-offs between economical costs and environmental impacts. As case study, the optimization method is applied on a brackish water reverse osmosis (BWRO) desalination project. The characteristics of the optimal BWRO process configurations are calculated as a function of the project constraints (e.g. economical and technical settings, minimum potable water quality), in order to illustrate how this method may support process engineers for the design of desalination plants.

EPFL2008