How Good is Wastewater For Irrigation?

Scott Trimble

October 21, 2020 at 9:42 pm | Updated March 16, 2022 at 1:28 pm | 7 min read

Water use is increasing at twice the rate of the population, and many places on earth are reaching the limit of their capacity to supply water. Agriculture that uses 70% of the water withdrawn can be a part of the solution to this problem. Using reclaimed water for agriculture is one of the possibilities. More research on wastewater’s suitability for different crops is urgently needed, given the issues of biological and chemical contamination. Precise field data collection supported by portable analytic tools is providing a much-needed impetus to the process.

Dealing with Water Scarcity

Part of the water scarcity is due to less rain, complicated by increasing temperatures that result in drought. There are also new demands for water that didn’t exist before. Though not all regions in the world suffer from drought, half the world population suffers from water scarcity at least for a month each year.

One of the current strategies receiving attention to alleviate water scarcity is the use of wastewater for irrigation, which frees up potable water for other purposes. This has other advantages, like reducing water pollution to local water bodies and the seas.

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Reclaimed Wastewater for Agriculture

The use of wastewater for agriculture is not new. However, at present, there is an emphasis on reclaiming or treating the water before using it for irrigation where possible. The wastewater is treated to kill microbes, especially if the source is municipal wastewater or sewage.

However, reclaimed water can still have higher than usual levels of salts, nutrients like nitrogen and phosphorus, microbes, and heavy metals.

Hence, reclaimed water has been deemed to be safe only for irrigation in case of landscapes—such as golf courses and lawns—and ornamental plants, and there is ongoing research to verify use for food crops.

Long-term use of reclaimed water for twenty years has shown to positively increase organic matter content. However, on the flip side, the concentrations of pathogens and heavy metals can be significantly higher. Wastewater is not homogeneous, and its composition will differ depending on its source, just as its utility will depend on the crop for which it will be used. Therefore, research into this interesting concept continues.

A range of new precise plant instrumentation is making the work of scientists easier by combining data collection with analysis. These new devices are small, portable, give rapid results, and replace laborious and expensive laboratory techniques. They can be used in farms, forests, greenhouses, and laboratories.

The role played by some of the new techniques in advancing research in wastewater irrigation is discussed below.

Tertiary Treated Wastewater Tested by Leaf Area

During the primary treatment, large solids and contaminants are removed from the municipal wastewater. In secondary treatment, natural biological processes that are aerobic or anaerobic, are used to breakdown the remaining solid wastes and nutrients. In the final and tertiary phase, the quality of water is improved. Here, microbes are disinfected and levels of inorganic nutrients, like nitrogen and phosphorus, are reduced.

In central Italy, the Pistoia’s Wastewater Treatment Plant (WWTP) is a conventional activated sludge treatment system. A new method of tertiary treatment, using filtration and peracetic acid + UV disinfection, was tested for its efficiency in improving water quality. Also, the scientists wanted to check the effect of the water to irrigate ornamental plants in a nursery.

Six species in containers were irrigated. The plants were the common evergreen Viburnum Tinus, Fraxinus excelsior, and Pittosporum tobira nana; conifers such as Cupressus sempervirens and Juniperus horizontalis, and the deciduous Weigelia florida. The control pots were irrigated with fertigated water, freshwater enriched with fertilizers. The effect of the two types of waters on the growth and physiology of the plants was observed.

One of the growth parameters studied was the leaf area. The scientists used the CI-203 Handheld Laser Leaf Area Meter, produced by CID Bio-Science Inc.

The CI-203 can take non-destructive measurements of leaves on plants, so it is ideal for growth experiments to trace differences over time. The device is a portable, handheld device that smooths the leaves and a laser beam scans it to record length, width, area, and perimeter. Using this data in pre-programmed formulae, the shape factor, aspect ratio, and void count are computed on the spot. The data can be stored and transferred later to a computer through a USB. The instrument can measure leaves of a wide range of thickness and width within seconds.

In Pistoia, the scientists found that the reclaimed sewage water had no adverse effect on the plants due to heavy metal contamination. Its effect was comparable to the fertigated water for most species tested. Hence, the scientists could approve the use of the effluents from the WWTP for the irrigation of ornamental plants. The bacterial count was well within limits for total coliform count set by the Italian Law, so people involved in watering plants would face no occupational hazard.

Leaf Area Increase Reflects Enhanced Nutrition from Reclaimed Wastewater

Another study also used the CI-203 Leaf Area Meter to test reclaimed water for landscape plants in Florence, Italy. The reclaimed water was, once again, from municipal sources. The focus of this experiment was to check the nutritional benefits of using recycled wastewater from municipal sewages for the plants.

Three species were used in this experiment: Abutilon, Viburnum tinus, and Weigelia florida.

The experiment compared the effects of irrigation with controls that were irrigated with well water. Another aspect of the investigation was to examine the impact of fertilizers given at the seedling stage on the plants.

Plant growth, leaf area, chlorophyll content, and ion uptake were measured for all plants. The CI-203 Handheld Laser Leaf Area Meter was used to quantify leaf morphological features.

The scientists found that the effluent water and its higher load of salts had no adverse effects on the growth of the plants. In fact, the nutrients and organic matter that is present in the effluents helped plants to grow better than plants watered with well water. The plants watered by effluents had a higher dry matter than well water plants. Interestingly, additional fertilizers added to effluent irrigated plants produced no difference in plant performance compared to plants that got no fertilizers. So, effluent water can be a source of nutrients that could replace fertilizer applications.

The response of all three species was not the same. Weigelia plants benefitted the most from wastewater irrigation, showing better growth, leaf size, and chlorophyll content than the Abutilon plants, which showed the least benefits from the treatment.

Olive Oil Mill Wastewater Improves Photosynthesis

The wastewater from olive oil mills contains water, oil, and crushed olives. The effluent is rich in biophenolics, many of which are not biodegradable. Hence, the wastewater can be phytotoxic and have a high chemical oxygen demand. While treatment to reclaim the wastewater is aimed at reducing these two effects, some experiments are studying the impact of direct application of the effluents without treatment to olive farms.

An experiment over three years supplied olive mill wastewater (OMW) as surface irrigation each month in four rates: one dose of 5 L m−2, 10 L m−2, and 20 L m−2, and four doses of 20 L m−2; control plots received no OMW. The effect on soil properties and plant performance were observed. Photosynthesis, fruit set, yield, oil content, and oil quality were tested in the plants.

Photosynthesis was measured by the CI-340 Handheld Photosynthesis System, made by CID Bio Science Inc. This is a portable device that analyzes gas exchange to measure not only photosynthesis but also transpiration, respiration, stomatal conductance, and internal CO2. It has ten leaf chambers of different volumes to accommodate leaves of a wide range of sizes and can be connected directly to the instrument to give quick measurements. Optional modules can control and regulate H2O, CO2, temperature, and light intensity. Photosynthesis is measured as the difference in the CO2 concentrations in the air before and after it enters the leaf chamber.

The scientist found that the levels of organic matter, phenolic compounds, potash (K), and microbial counts were higher in the irrigated plots than the control after three years. However, there were no adverse effects. OMW given at 10 L m−2 and 20 L m−2 actually improved the performance of plants by increasing photosynthesis, fruit set, and yield. There was no effect on oil quality. OMW at 10 L m−2 was recommended by the study to enhance crop performance and soil fertility.

Modern Tools

Modern analytic tools, rigorous enough to be used on the field but that also give results that rival sophisticated techniques in accuracy, are changing the way research is conducted in agriculture. By reducing time and money investment in chemical and physiological analyses, portable tools are increasing the reach of technology. This frees up time for the scientists to concentrate on other tasks or cover larger sample sizes, ultimately benefitting science.

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Tools:

See More:

CI-202 Portable Laser Leaf Area Meter

CI-203 Handheld Laser Leaf Area Meter

CI-340 Handheld Photosynthesis System