Top Crop’s top water treatments

Top Crop Nursery has implemented two water treatments to let them use surface run-off water. Synthetic wetlands worked until they couldn’t keep up with the nursery’s requirements, then the nursery turned to chemical treatments. Robyn Joubert spoke to owner Mike Kruger.
Issue date: 29 May 2009.

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Top Crop Nursery has implemented two water treatments to let them use surface run-off water. Synthetic wetlands worked until they couldn’t keep up with the nursery’s requirements, then the nursery turned to chemical treatments. Robyn Joubert spoke to owner Mike Kruger.

It’s essential that all farmers get their houses in order when it comes to water, says Top Crop Nursery owner Mike Kruger. “South Africa is a dry country and water is going to get scarcer with the intensification of global warming,” he warned farmers at the farmers’ day of the Seedling Growers Association of SA at Cedara on 7 May.
“Groundwater can’t be replenished as fast as it’s used. At some point in the future, the use of borehole or groundwater will be more strictly controlled. As a result, the use of surface water is going to become more important and water recycling critical.”
Top Crop Nursery produces many millions of vegetable, forestry, ornamental and turf seedlings annually. An ample supply of good quality water is critical. To this end, they’ve tapped a source normally considered unsuitable for a nursery: surface water.
Situated 25km from Pietermaritzburg, Top Crop has a permit to draw surface water from the Umgeni River. Water quality varies tremendously. It’s low in salts, which is a big advantage. But it also has a highly unsafe E.coli count and contains phytophtora, pythium, rhizoctonia and many more “bugs” that are detrimental to humans as well as plants.
Most untreated surface water will contain pathogens so water preparation is key. Top Crop Nursery has implemented two water purification techniques to ensure a steady supply of water in the years to come. A synthetic wetland system cleans water through slow filtration. The second method cleans and sterilises it chemically.
Synthetic wetlands
Wetlands are nature’s way of purifying water after contamination. “Preserving wetlands is now a national concern,” Mike says. “But artificial wetlands can be constructed to mimic the natural process and filter water to a reasonably high quality” (see Figure 1: synthetic wetland).
In its wetland system, Top Crop excavated a reservoir 25m across and 3m deep. The reservoir is divided into three layers, each 1m deep. The top soil layer is composed of about 40% clay, the middle layer is composed of sand, and the bottom layer is composed of gravel or stone. Reeds and bulrushes are planted on the surface of the reservoir and dripper piping is installed.
Embedded at the very foundation of the stone layer are slotted pipes which radiate out of a block well in the centre. From the central well, a pump sucks the clean water up through a suction line into a reservoir.
“Water is fed into the wetland slowly via drippers, at a flow rate of 2 000â„“/hour. This gives time for the water to percolate through the soil,” says Mike.
The various layers have different functions. The plants suppress the pathogens and use some of the salts. The high-clay soil has a high cation action and the water’s slow movement through the soil allows a lot of the salts to be removed. This layer also collects organic matter. The sand layer acts as a filter. Lastly, the stone layer allows clean water to collect before it’s drawn through the central well.
Mike says the wetland system is very inexpensive to run, removes the organic component without the use of chemicals, and is environmentally friendly. It can also be used as a water storage system. “But though the wetland system worked very efficiently for 12 years, we discontinued it as the water yield was too slow to meet our needs,” he explains. “Building a larger reservoir would have been fairly expensive.”
The filtered water also contains some chemical residue, such as manganese. The wetland does need to be managed over time. “The clay soil accumulates organic matter and salts, and needs to be removed after a few years,” says Mike. “One could consider replacing the clay portion with pine bark or another material with a high cation exchange.”

Chemical treatment
Top Crop Nursery now relies on chemical treatment to remove the water’s organic component. This is fairly straightforward.
“We use aluminium sulphate (alum) to remove the organic matter,” explains Mike. “Adding alum to the water sets off a reaction known as flocculation – the fine organic compounds clump together into ‘flakes’, or floc, which then sink to the bottom.”
First the dirty water is pumped into a tank where the alum is added, then into a separator which slows its rate of flow (see Figure 2: a separator). This allows enough time for the alum to react with the water and for the organic matter to be dumped. Clean water is siphoned off from the top of the separator and pumped into a chlorination tank.
“To calculate how big the separator should be, work out the volume of water needed and divide by three,” advises Mike. “The amount of water required needs to be held in the separator for three hours for the flocculation process to be completed. So if the flow rate is 1 000â„“/hour the separator should hold a minimum of 3 000â„“ of water. If the separator volume is 10 000â„“, it can clean 10 000â„“ of water divided by three hours, or about 3 333â„“/hour.”
Chlorine is then added to destroy pathogens. This is in the form of Calcium hypochlorite (Ca-(CLO)2), as found in the pool cleaner HTH, not sodium hypochlorite (Na-(CLO)2), as found in household bleaches like Jik.

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Properly sanitised
“Cleaning water properly, either by wetland or chemical means, is crucial,” explains Mike. “Only once the organic component has been removed can chlorine be added to destroy pathogens. If the organic matter is removed efficiently, tests have proven over 90% of the pathogens will be removed with it.”
Before cleaning the water, a jar test must be performed to understand the water’s alum and calcium hypochlorite requirements. Mike recommends starting by adding alum at 50 parts per million (ppm) increasing to 100ppm.
“Let the water stand for a few hours to let the organic matter settle. Then siphon the clean water off the top and place it into a new jar. Perform a pH test on it to calculate the chlorine requirements. A lower pH means less chlorine is required,” he explains.
To evaluate the addition of calcium hypochlorite, add 1ppm of calcium hypochlorite, stir, and then test for free chlorine. “Keep adding 1ppm at a time, testing after each addition until free chlorine is shown,” says Mike. “At this stage there should be enough chlorine to reduce all waterborne plant pathogens.
“It’s essential to follow the correct instructions when reading the chlorine levels. Critically, only free chlorine is read, not total chlorine. This should be done within 30 seconds after adding the correct chemical, such as DPD powder, to the water.
The chlorinated water should be allowed to stand for about six hours before it’s used, to ensure no pathogens survive.
Mike says separators and tanks need ongoing maintenance. “The organic matter must be removed daily. The chlorine contact tanks need monthly cleaning to remove floc. But, at the end you’ll have clean water without pathogens and no phytotoxicity. That’s essential for crop farmers and nurseries.”
Mike says Top Crop have found a water solution that works for them. “Both the synthetic wetland and the chemical treatment systems have succeeded to varying degrees. Any farmer can adopt them with relative ease,” he concludes.
Contact Mike Kruger on 083 560 2639 or (033) 569 1333, or e-mail [email protected] or [email protected]     |fw