Technology improves water efficiency

With climate change increasing uncertainty about future water availability and the need to apply for a licence to abstract water for any type of irrigation, including drip irrigation, more and more growers are looking to maximise the efficient use of water and to capture or reuse as much water as possible, writes Richard Crowhurst.

“In terms of greenhouse water use, recycling and roof collection are becoming more common,” says Mark Burt of New Leaf Irrigation. “A lack of water can be a limiting factor to business growth, particularly with the competition for supplies with other industries and domestic demand, together with strict licensing structures for abstraction. Alternative water supplies, more accurate application, and the use of drip versus sprinkler irrigation can all help in combating these challenges.”

“The main sources of water used for professional greenhouse irrigation depends largely on the area,” points out Kathryn Smethurst of Priva. “Greenhouse irrigation is mostly from rainwater, but growers also use boreholes and surface water; with additional treatment to ensure these sources are suitable as irrigation water.” She adds that open reservoirs or storage basins are the most common form of water storage on nurseries and that, depending on the circumstances algae can grow in such conditions. “Outside exposure also leads to more organic pollution and the need for greater filtration of the water before use.”

Mains water, costing £1-2 per cu. m is also used by nurseries, while licenced abstraction from open water sources, such as rivers, is rare in the greenhouse sector. Mark Burt of New Leaf Irrigation warns that growers abstracting large quantities of groundwater (more than 20 cu. m a day) will also need an abstraction licence and that this accounts in part for the popularity of rainwater collection. “In simplistic generalised terms, all the time rainwater from roofs remains within pipework it belongs to the grower, but once it hits the soil it is in the environment.

“Using gravity rainwater can be diverted directly from gutters to closed tanks or open reservoirs. Where no space can be made available close to the roof capture area, rainwater can be transferred to underground sumps or smaller holding tanks, and from these it can be pumped to main storage tanks at a more convenient location, although such a system involves more energy use.”

Mark also points out that water captured from roofs often has coarse debris filtered out en route to storage, and that using covered tanks to prevent light ingress will prevent algal growth. “The decision on whether to use direct or indirect storage and tanks or reservoirs will be governed by space and how much water can be captured and saved,” he stressed.

Depending on the crop, water source and how the water is stored, treatment of the water may be required before it is applied. “Pre-treatment of water ensures the quality of the irrigation water, making sure it has the right pH or that it’s free of any organisms such as viruses and bacteria which can be harmful for the plant,” says Kathryn. “The exact demand for water treatment depends on a more in depth analysis of the water quality on site, but the main options for growers include UV treatment, reversed osmosis, several forms of filtration.

Roof water is, as a rule, cleaner with low silt load and lower risk of plant pathogens such as Phytophthoraspp. and Mark points out that sending water samples for laboratory testing is inexpensive. “Doing it at different times of the season will give a good picture of what issues are in the water and how to erase them,” he advises. “If UV filtration and filters, or chlorine injection, is necessary, then a feasibility budget should be prepared so that capital payback time for the system can be calculated.”

Priva supplies its own UV system which uses proven technology from drinking water systems, providing guaranteed capacity and levels of disinfection to log 3 (or 99.9 per cent), without the use of chemicals such as chlorine or hydrogen peroxide which can affect crop quality. “The lifecycle return on investment for re-cycling treated drain water through the Vialux M-line is excellent, exceeding the initial capital investment many times over for most growers,” adds Kathryn.

Another option is ultra-filtration, which was recently demonstrated at Amsterdam GreenTech by Mienis Water. “Ultra-filtration is a membrane technology,” explained technical director Jochum Genuït. “This membrane let salts pass but filters out all organics. The membrane can block viruses up to a log 5.34 and bacteria even to a log 9.15. Our filter is not sensitive to turbidity, we can handle up to 150 microns, and colour is no problem.”

One of the key uses for the technology is water recycling, and the system is so effective that Jochum says the first users, whose systems were installed two and a half years ago, have not discharged any water from their sites since. “[It is] the safest and possibly the best way to recycle drain water,” he continues. “Because of the low operational costs, our UF systems have the cheapest operational costs. Even better, it is also the safest technology. The return on investment is often around a year. Our systems are located at nurseries and propagation companies, and they experience now major improvements in crop results and considerable cost savings.”

Having cleaned the water from whichever source is being used, it is then necessary to schedule and apply it in an efficient manner. John Walker of ETS Controls points out that, “Evapotranspiration estimates of how much water a crop is using and so can be used to decide when we need to irrigate. This not only saves precious water, it can help avoid damaging crops by over-or under-watering. Investing in technology to match irrigation to crop demand not only helps reduce water bills and meet regulatory requirements for water use efficiency, it can also improve crop quality, something which is particularly true for ornamentals.”

In the past irrigation scheduling relied largely on estimating or calculating the ET for the crop based on environmental factors such as temperature and light levels, as well as the stage of crop development. However, new systems such as the EvapoSensor from ETS Technology allow much of this process to be automated, as well as improving the accuracy of management decisions.

“One of the best ways to save on water is to apply the right amount in an accurate and uniform manner,” agrees Mark Burt. “Many systems on nurseries apply water in a non-uniform manner and consequently the grower applies too much in an attempt to get sufficient water in the drier patches. Additionally, manual application by hoses can be a massive extra cost and unless undertaken by experienced and capable staff is generally very inefficient. The result is high water bills, high wage bills, dry and wet patchy crops and a recipe for disease growth in boggy greenhouses and tunnels.”

He says that the use of well designed micro-sprinklers, pressure compensated drippers, drip tapes on capillary matting and ebb flood table systems will all enable growers to apply both water and the fertilizer in the water accurately and cost effectively.

Automation specialist Rotomation recently designed a system for micro greens producer Gro-Well Salads. “The grower wanted to water into cells below the leafy crop without wetting the crop,” explained Rotomation’s Jamie Ashton. “We designed a system based on our boom irrigation technology and the customer has been very impressed with the system.” In fact the system has been so successful that it is preferred to using the ebb and flood benches used elsewhere on the nursery as the boom wastes very little water and can control the quantity that each area receives with a much higher degree of accuracy using the ‘zonal irrigation’ feature. “This has led to much more uniformity in the crop and has also led to significant gains in harvesting efficiency and reduction to crop damage due to the crop previously still being wet when harvesting,” explains Jamie. “The customer no longer has to leave the crop to dry before harvesting, so the time between the last watering cycle and the harvesting has been eradicated, allowing the customer to react to new orders much more quickly for a product that is so sensitive.”

At the start of this year Rotomation installed a boom system to irrigate five acres for Sandiacre Nurseries: “They use rainwater collection into tanks and two header pipes were used in order to allow the customer to feed with one header and water with the other and automatic solenoid valves allow remote switching between the two. The customer can walk the crop, and select which areas to water or feed on a remote control, thus significantly increasing accuracy whilst reducing water usage.”

A further system in use at ornamentals producer Ivan Ambrose & Co. Ltd uses a collection reservoir and large collection tanks in order to run 22 booms at the same time. “This allows the grower to walk through and water six acres of crop in less than one hour,” adds Jamie. “This gives each variety the correct amount of water, and allows areas without crop to be bypassed. A Dosatron is used to apply chemical and feed, further saving on labour.”