Antifouling

Discussion at the Status Conference 2015 “German-Israeli Co-operation in Water Technology Research” held in Berlin on 23 March 2015.

Clean water is becoming an increasingly valuable commodity. Sophisticated technologies are required to obtain, purify, recycle, store and distribute it. This was clearly demonstrated at the Status Conference 2015 “German-Israeli Cooperation in Water Technology Research”, which took place in Berlin on 23 March 2015.

By Uta Deffke

German and Israeli scientists used this opportunity to exchange the latest findings of their projects, which the Federal Ministry of Education and Research (BMBF) and the Israeli Ministry of Science, Technology and Space (MOST) jointly support.

Harald Horn, professor of hydrochemistry and water technology at the Karlsruhe Institute of Technology (KIT) presented a black plastic object smaller than a finger nail. This “dripper” is the key component of the agricultural irrigation system, which, drop by drop, supplies each plant with water. A complex geometry, comprising minute labyrinthine pipes with a diameter of only a fraction of a millimetre each, allows each dripper to deliver the same amount of water, irrespective of its position in the pipe, which could be longer than 100 meters in places. This is an excellent example of the sophistication of the irrigation technology developed in Israel, which is now being used world-wide. Horn pointed out that Netafim, the company responsible for this technology, has been "one of the hidden champions" and is now the world market leader.

Such a precise dosage of irrigation water is essential as water resources are very scarce and valuable in Israel. "There is almost no other country in the world that plans its water supply so rigorously and with such farsightedness", says Horn. Israel's only freshwater source of note, the Sea of Galilee, fills up slower than in the past, and water conservation is therefore essential. Neither groundwater nor potable water is used in agricultural irrigation ― only biologically purified wastewater. Although efficient, it does lead to problems in these sensitive, fine and complex dripper irrigation systems. The purified wastewater contains a considerable proportion of carbon, which assists the growth of micro-organisms. This results in the growth of a biofilm on the dripper’s internal surface, which, within a few hundred hours, progressively leads to blockages. These blockages cause intermittent or obstructed water supply. Regular chlorination and rinsing with hydrogen peroxide (H2O2) are effective countermeasures, "although chlorination leads to undesirable by-products like unhealthy chloroform", explains Horn. This is the reason for the German-Israeli research team to examine how the usage of chlorine can be minimised by monitoring the biofilm growth in the "BioScIrr" project.

BioScIrr is one of more than 130 projects in the water technology field on which German and Israeli research groups have collaborated since 1974. In July 2013, the BMBF announced its support for this project for a period of three years. The project partners, i.e. Professor Yona Chen, head of the Faculty of Agriculture of the Hebrew University of Jerusalem, and Professor Harald Horn of the Department of Hydrochemistry and Water Technology at the Karlsruhe KIT, have known each other for a long time. Horn said that "there had already been an intensive and beneficial co-operation with my predecessor". In addition to the regular exchange of scientists, an Israeli doctoral candidate had also spent a rewarding research visit at KIT the previous year. The current project has two industry partners on board - Netafim from Israel, and Lagotec from Magdeburg, which developed the biofilm sensor Deposens a few years ago. This sensor determines the thickness of a biofilm via its heat isolating properties by measuring the heat development of a heat impulse over a small pipe section.

The scientists’ idea is to monitor the thickness of the biofilm with the Deposens sensor, activating a cleaning flush only when a critical threshold is exceeded. The sensor is attached to the pipe, and not to the sensitive and complex dripper. By using additional measurements of the flow rate and the water quality in the pipe, the researchers can deduce the state of the dripper. In order to research this, Harald Horn and his colleague, Michael Wagner, built a 14-meter long laboratory scale irrigation system at the KIT, through which they run artificially mixed waste water. They measure the drip rate, determine the effect of disinfection rinses and, with optical tomography, examine the internal biofilm expansion by means of a specially built model of the dripper. During the summer season, they will repeat their experiments at a real waste water treatment plant.

Simultaneously, Yona Chen's colleagues in Israel have conducted their first field experiments in a test facility at the agricultural station in Lachish, approximately 50 kilometres south of Jerusalem. With increased flow rates, and strongly fluctuating temperatures (between 10 and 50 degrees Celsius), the conditions for the sensors are more severe than in a laboratory.

Horn summarises as follows: “Our first results show that Deposens’ measurements correlate with the biofilm growth in the dripper; they therefore seem a reliable trigger for the cleaning flush, which can reduce the environmental impact and costs". Chlorine proved to be a more effective cleaning agent than hydrogen peroxide. Currently, the scientists are preparing further field experiments, and are equipping the sensor for usage in even more severe field conditions.

The growth of biofilm is also a problem where biologically treated waste water is further purified by means of filtering. The membranes used for filtering may also become clogged. A solution for this would be to coat the membrane with a thin polymer hydrogel film, which makes it more difficult for organic material to attach to the membrane. The scientists Viatcheslav Freger of the Israel Institute of Technology and Moshe Herzberg of the Ben-Gurion University of the Negev, together with Mathias Ulbricht of the University of Duisburg-Essen, are researching strategies to solve this problem. The industry partners BASF/Inge supply the membranes.

The concept: A two-step procedure with the simplest molecules possible. A thin adhesive coating, consisting of a single layer of molecules, is applied. These molecules are selected such that they not only connect well with the membrane surface area, but are also able to bond the polymers. The hydrogel film is then applied in the form of a watery solution, which is left to set for two hours. Mathias Ulbricht explains: "The advantage is that we do not need an external energy supply. This allows us to also coat the inside of capillary membranes".

The scientists could demonstrate that a stable and homogenous hydrogel layer develops that significantly reduces the susceptibility to biofilm growth on the membranes, which, in turn, simplifies purification procedures. Ulbricht summarises: "This will reduce the operating cost, increase the lifetime of the membranes and reduce energy input". Over and above the antifouling effect, the scientists also emphasise that the coating does not impair the membrane’s flow rate and filter qualities. Furthermore, the procedure is so simple that it can also be implemented in industrial membrane production.

While Mathias Ulbricht and Moshe Herzberg are researching porous ultrafiltration membranes, Viatcheslav Freger focuses on non-porous nanofiltration membranes. He stated that the hydrogel layer on the membrane is not always stable and uniformly fixed. The scientists suspect that the varying material characteristics of the membrane may possibly cause this. They are thus currently working towards an optimisation of the coating process.