Irrigation: Nano-Bubbles Offer Big Benefits

Assoc. Prof. Surya Bhattarai from CQUniversity, Qld (right) and fellow researcher Dr Jay Dhungel from the University of South Australia.

Australian researchers are breathing new life into surface and subsurface drip irrigation practices, revealing how enhanced water aeration systems (‘oxygation’) can improve both crop yields and soil quality. And, as John Power reports, nano-bubbles are an important feature of the process.

As users of outdoor power equipment know, the presence of adequate, high-quality water is the starting point for practically all plant-based activity, from animal and crop farming to viticulture, golf and turf sports, as well as landscaping and gardens.

Inadequate water supplies, ineffective or inefficient water distribution methods, along with salinity, are the enemies of plant-based enterprises, so it should come as no surprise that a great deal of work is being devoted to improved irrigation methodologies.

One of the most interesting fields of research relates to drip irrigation (DI) and subsurface drip irrigation (SDI), which are far more efficient and controllable than furrow or overhead sprinkler watering systems. 

Whereas furrow and sprinkler watering systems are prone to water wastage from evaporation, wind blow and run-off, DI and SDI systems are protected from such hazards, providing superior water management through the optimal positioning (depth and distancing) of irrigation lines, or via the incorporation of chemical or gaseous additives to the lines to increase the overall effectiveness of water around root zones. Most SDI lines, for instance, are laid to a depth of 5cm–50cm depending on the soil types and root zone characteristics of prevailing crops. 

Regardless of the type of irrigation used, it is now clear that appropriate oxygen levels around root zones are just as important as moisture levels for maintaining robust and healthy plants – from turf species to cotton, chickpeas, apricots, and innumerable other commercially managed crops. Low oxygen levels, or ‘hypoxia’, in heavy, clayey soils (vertisols), according to Associate Professor Surya Bhattarai from Central Queensland University, result in ‘irrigation paradox’: water in, oxygen out. In vertisols, in particular, this occurrence often manifests itself as waterlogged, stagnant ground, which can stifle organic activity and lead to sick or dead plants.

Effective irrigation, therefore, has two separate elements: (1) appropriate and non-wasteful water penetration to the root zones of plants, and (2) sufficient subsurface oxygen around root zones to facilitate long-term healthy plant growth and good yields.

VENTURI AERATION

The challenge of maintaining strong water and oxygen levels at plant root zones is not new, Assoc. Prof. Bhattarai says. “People have been using different aeration techniques for more than 100 years,” he explains. “In its traditional form it’s all about tilling the soil, making it loose, so air can move around the root zone – the idea here is to mix in the air.”

Other techniques, he adds, involve tandem subsurface lines, with one line for water delivery and another containing pressurised air; however, uniform and consistent air transference using such methods has been of limited value.

By contrast, Assoc. Prof. Bhattarai’s approach is far more tantalising: “Our ‘oxygation’ solution has been to incorporate oxygen into pressurised DI and SDI irrigation lines using a low-cost venturi system.”

How do venturi systems work? The simplest venturi devices used in the irrigation industry involve a small air pipe fitted into a pressurised water line, with the air pipe inlet exposed to the open air. The water line sucks in air through the smaller pipe thanks to differential pressure (the greater the pressure differential across the venturi, the greater the air intake), and the result downstream is a heavily aerated water flow with a fizzy, milky or bubbly appearance. Aerated water then exits the irrigation line at regular intervals through ‘emitters’, i.e. small evacuation points through which the water percolates into the surrounding soil. If the air intake pipe is blocked (with a finger, for instance), the downstream water aeration ceases instantly and the water regains its clarity.

Assoc. Prof. Bhattarai has spent a considerable part of his career examining the many variables affecting venturi-based irrigation, including fieldwork to overcome the challenges of delivering uniformly aerated water throughout an entire irrigation system. One problem, for instance, is that when water is aerated with bubbles of different sizes, the larger bubbles tend to disperse early in the distribution line, leaving only smaller bubbles to reach the farthest extremities. “If you have bubbles of different sizes, most of the bigger bubbles come out in the first few metres, and then only the small bubbles move out along the line,” he says. This kind of variation can lead to non-uniform oxygen delivery to different areas of a site, and the problem may be aggravated if emitter junctions have penetrated too far into the water pipes, i.e. below the ascendant bubble line.

Needless to say, there are a large number of physical practicalities to consider when installing venturi-based aeration systems, including the design and size of the venturi devices, water pressure, as well as line placement.

To date Assoc. Prof. Bhattarai has refined his methodologies to a high level, suggesting that aerated irrigation lines with a maximum length of 240m–250m are better suited to uniform oxygen delivery than longer lines of, say, 500m. Drip operating pressures of 10–15psi are a prerequisite for reliable aeration over such distances.

He has also invested time and energy examining bubble behavior with a view to creating improved forms of aeration that will conserve bubble uniformity and retention over a given irrigation block.

“We have started looking at nano-bubbles; if you have a bubble size that is extremely small, say less than 100 microns, they tend to be less buoyant – so the bubbles stay in the water longer, they don’t move up and down too far. We have been working on nano-bubbles for some time now, and we have a set of equipment at Narrabri, where our CQU team is working with the CSIRO to look at the effects on cotton.”

IMPROVED CROP YIELDS

‘Oxygation’ involves the aeration of drip irrigation water to increase the amount of oxygen reaching the root zones of plants.

Recently, Assoc. Prof. Bhattarai and fellow researchers from the Queensland Department of Agriculture announced promising results relating to the SDI oxygation of chickpeas. Analysis of historical data gathered during a 2006–07 crop trial in Emerald revealed that those areas irrigated with SDI oxygation equipment had yields between 10% and 27% higher than conventionally irrigated plots. 

These findings mean that advanced aeration practices like SDI oxygation, once considered too expensive for broadacre cropping, might now become commercially viable in light of contemporary water shortages and enhanced demand for specific crops; Assoc. Prof. Bhattarai says global demand for pulse crops like chickpeas is likely to rise by 30% (or 289m tonnes) before 2050.

The oxygated trial plots yielded between 2.05 and 3.24 tonnes per hectare compared to the long-term national average of just 1.13t/ha.

“These results were consistent with those for cotton on the same site, providing further justification for the capital investment required for oxygated subsurface drip irrigation systems,” Assoc. Prof. Bhattarai and colleagues reported.

FUTURE RESEARCH

According to Assoc. Prof. Bhattarai, work continues apace to refine the mechanical systems underpinning DI and SDI oxygation processes.

At present, there are two main research fronts involving (1) the production of nano-bubbles to improve the uniform delivery of oxygen to root zones, and (2) methods to increase the oxygen levels introduced into irrigation systems.

As discussed, nano-bubbles offer improved aeration performance by dispersing oxygen more evenly through water profiles.

The next step, logically, is to somehow enhance the level of oxygen that is introduced into the water. 

Standard air has an oxygen content of 21%, which normally defines the threshold for oxygen concentrations introduced into an irrigation system using an open-air venturi device.

However, Assoc. Prof. Bhattarai says he and his colleagues are experimenting with methods to raise oxygen levels to 90%. This is achieved with a device – an oxygen concentrator – which removes nitrogen from incoming air, leaving an oxygen concentrate to feed into the irrigation line. The effects on plants of enhanced dissolved oxygen levels via nano-bubble delivery represent an exciting new field of endeavor, particularly for high-value horticulture.

Other side effects of this research, so far unexplored, are worthy of speculation. For example, the effective irrigation and aeration of soils through SDI could have huge ramifications in turf applications such as sports ovals, golf tees and greens, bowls greens, as well as civic reserves, potentially leading to: less need for verticutting and coring, an end to damaged sprinkler heads caused by mowing equipment, reduced labour costs associated with handling above-ground hoses and sprinklers, and reduced downtime (at golf courses, for instance) waiting for above-ground irrigation systems to complete their cycles. 

Commercial crops like cotton and chickpeas have shown increased yields following oxygation.

Such potential side effects are incidental, of course, to the main goals of saving water and improving plant health, but they offer a glimpse into the far-reaching consequences of DI- and SDI-related ‘underground research’!