Fix salinity, save water: recycle!

A problem with hydroponics is dealing with nutrient build-ups and water salinity. Instead of using more water to flush away excess salts, recycling and filtering deals with the problem while using less water – a real win-win solution. Glenneis Erasmus reports.
Issue date: 2 March 2007

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Hydrogen peroxide in the blue tank is injected into the recycling system.

A problem with hydroponics is dealing with nutrient build-ups and water salinity. Instead of using more water to flush away excess salts, recycling and filtering deals with the problem while using less water – a real win-win solution. Glenneis Erasmus reports.

Hydroponics systems might result in more effective fertilisation than traditional systems, but a large amount of water and nutrients are also wasted in these systems. Over-fertigation can result in nutrient build-up in the soil, causing salinity and rendering growth mediums unsuitable for future production.

Most South African greenhouse producers try to wash away excessive salts through a 10% to 30% over-irrigation of growth mediums. However, water recycling allows producers to address the potential salinity problem while using less water, according to Boersma, agricultural specialist from the Western Cape Department of Agriculture’s Institute of Agricultural Training at Elsenburg.

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Nevill has been studying water recycling with hydrogen peroxide on tomato plants for the past eight months with highly successful results.
The experiment was carried out over only eight rows of plants, but the recycling system still managed to save around 8 300 litres of water for the project per month.

Nevill estimates that the system can reduce water use by at least 30% in commercial greenhouses. Recycling has also resulted in fertiliser cost savings, as nutrients are saved during the recycling process. What makes the system even more extraordinary is that dam water was used as raw water.

Trial conditions were similar to those experienced on farms. recycling studies make use of cleaner raw water sources. he main requirement for the system to work is that the electroconductivity (EC) of the original raw water should be lower than that of the recycled water and should preferably not be more than 0,5. EC requirements, however, vary from crop to crop. For example, tomatoes will be able to tolerate an of 0,7, while an above 0,3 would be too high for cucumbers.

A simple recycling system
Unabsorbed nutrients are drained from the plant growth medium into plastic gutters and through a pipe into a 1m3 water trap. Small papers, leaves, insects, sand and other small impurities are removed from the water in this trap.
Sand usually filters to the bottom of this tank, because it is heavier than water, while other light impurities filter to the top of the water. he water is then filtered through a finer homemade filter to take out impurities that has not been removed in the first filtering process. built this filter from a 12cm diameter PVC pipe.
Holes is drilled around the part of the pipe that is placed in the water trap. The end of the pipe is blocked off with a stopper and a mesh bag is wrapped around the pipe over the holes to act as sift and filter.
Water then moves through the sift and holes in the pipe and then to the dosage tank. he drained water is dosed with 100ppm of hydrogen peroxide in the dosage tank, which has a 1 000-litre holding capacity. The hydrogen peroxide helps to destroy nematodes, bacteria and fungal diseases, such as pythium and phytophora blight. solution should be left for at least eight hours to allow for the hydrogen peroxide oxidation process. “Hydrogen won’t be able to destroy pathogenic organisms if the chemical reaction is interrupted and might lead to the burning of the plant roots,” Nevill explains.

He prefers using hydrogen peroxide as it is relatively cheap and easily attainable. Chlorine is also cheap and easy attainable, but it is dangerous to use, can be toxic to plants and affects the pH of the growth medium.
 Ultraviolet filters are used to destroy pathogens in Europe, but the use of these filters is not economically feasible in South Africa. Nevill explains the practice requires the filtration of each water molecule with light. Hence the practice does not work well in murky water as is usual in South African conditions.
Water should only contain around and no less than 5ppm hydrogen peroxide after the chemical breakdown and when it is released for recycling. Nevill tests the hydrogen peroxide content with oxystix (chlorine dioxide test strips).

The purified drain water is then pumped into a 10 000-litre storage tank where it is mixed with nutrients before it is pumped back into the greenhouse for re-use. The addition of nutrients to the purified drained recycle water can result in a build-up of nutrients in the water.

The system at Elsenburg uses EC measurements to adjust the amount of nutrients added to the water after purification. Nevill points out that EC is only an indication of the salinity of water and it doesn’t indicate which nutrients are in oversupply. Adding nutrients can result in an excess of certain elements in the fertigation water – these elements can cause damage to plants.

Therefore, Nevill advises farmers who use this system to test the water at least three times a season to identify whether there is an excess of a specific element.

Unwanted chemical reactions may occur between elements in the recycled water and the hydrogen peroxide. EDTA – ethelynediaminetetraacetic acid – should therefore be added to the recycled water to stabilise the elements in the solution and to prevent these reactions. Around a teaspoon of EDTA can be used for every 10 000 litres of water.

Nevill estimates that setting up a commercial system would cost a farmer between R10 000 and R15 000. Maintenance and running costs are, however, very low after the initial capital investment. “Savings from lower water and fertiliser use more than justify the investment,” he says. For more information contact Nevill Boersma on 084 548 7912. |fw