Alit Wiel-Shafran

Dr. Gabriel Rubanenko Prize for Excellence in Water Research, 2005.

Ever since childhood I have been captivated with the beauty and complexity of nature and deeply involved with the protection and conservation of the environment. As my father was among the founders of the first field center of the Society for Protection of Nature in Israel (SPNI) (at Ein-Gedi) I grow up between desert trails and the Kibbutz fields. As a youth I joined the youth-group in Eilat filed center, a group of youngsters that devoted its weekends and holidays to explore the landscapes of Israel. Indeed hiking and nature conservation remained my major areas of interest. Later, I served my army duty as a field guide in the Sede-Boqer field center were I devoted most of my time for educational activity with Israeli youths, introducing them to the natural landscape of the country and its unique natural history. When my duty service was over I stayed in the Sede-Boqer filed center and managed a restoration project of the Negev's ancient water holes. During my years in the Sede-Boqer field center and my work on the restoration project in particular, I acquired an intimate acquaintance with the water system in the desert as well as a deep comprehension of water-economy problems. My service at the SPNI also exposed me to the problems facing decision makers and especially the severe lack of knowledge they confront trying to depict conservation strategies. For me, specialization in water recycling systems is the natural succession of my involvement with the efforts to better understand and preserve Israel's unique natural resources.

Description of research:

Environmental effects of surfactants originating from reuse of graywater for irrigation
Supervisors: Dr. Amit. Gross, Prof. Eilon Adar, Dr. Zeev Ronen

My research goal is to study the environmental impact of irrigation with reused graywater (Domestic wastewater excluding the toilets) on the soil, and specifically the fate and effects of surfactants, which are abundant in graywater. The reuses of graywater for irrigation is becoming more and more common in arid and semi-arid areas throughout the world and especially in western countries where it is considered an "environmental friendly" practice. Unfortunately, crucial knowledge regarding the long-term effect of graywater usage is yet lacking and thus, there is an urgent need to assess the environmental impact, in order to set guidelines for safe and sustainable use.
Graywater is all the non-toilet wastewater produced in the average household including the water from bathtubs, showers, sinks, washing machines, and dishwashers. Detergents are the main source of surfactants in domestic wastewater, which have been recognized to be the most abundant organic chemical in municipal wastewater. Surfactants are organic molecules consisting of hydrophilic and hydrophobic groups. The hydrophobic group contains alkyl chain lengths of C10-C20. The hydrophilic group has an electrical charge, or is polarized, and can form hydrogen bonds. Surfactants in aqueous solutions tend to accumulate in the inter-phase of air/solution or solid/liquid, causing to reduction of water surface tension. Consequently capillary force is decrees and, the mobility of soil solution is enhanced as a result of a change of capillary pressure, causing changes in the distribution of water and air between and within soil pores of different sizes.
Capillary rise of water in soils is a phenomenon that has both beneficial and detrimental effects for agricultural soils. It is an important mechanism by which plants can draw water from below the root zone, but it is also a primary mechanism, which can contribute to the accumulation of salts in the soil.

The aims of the study were to monitor the accumulation of surfactants in soils irrigated with graywater and to determine its effect on capillary rise in loess and sand soils, commonly used for gardening.
Surfactant content in loess and sand-loess mixture from 3 gardens plots irrigated with greywater was determined, and compared with nearby plots that were not irrigated or irrigated with freshwater. Next, we mixed sieved oven dry, loess and sand soils that were never irrigated, with water containing known amount of laundry detergent solution (approximately 10% surfactant) to give 10% soil moisture content (w/w). The soil was placed into 25 cm column (2.5 cm diameter), and was covered with a fine net on one end. The column was attached to a balance and its bottom was located on the water surface of open reservoir containing freshwater. The weight change due to the capillary rise in the column was recorded with a data logger every 1 second.

Significant accumulation of surfactants in the graywater-irrigated plots was demonstrated (Fig. 1).

Fig. 1. Concentration of anionic surfactants found in loess soils, where "Graywater" indicates the average concentration found in soils irrigated with graywater. "No irrigation" and "freshwater" indicates the average concentration of native soils that were not irrigated or irrigated with freshwater respectively (n=5). Soils were irrigated for over 3 years. The different letters a, b indicate statistical significance (p<0.05).

The introduction of laundry solution to the soils caused a significant decrease in the capillary rise over a range of concentrations that is found in graywater irrigated soils. The effect was more noticeable in the sand over a narrow range of concentrations as compared to the loess soil (Fig. 2). This can be explained by the larger pore size and the better homogeneity of the sand matrix.

Fig. 2. The effect of laundry detergent solution mixed in sand and loess soils (final water content 10% w/w) on the capillary rise.