Greenhouse horticulture water management and it's challenges
In the past, irrigation water was not a regular discussion topic and was mainly regarded as a carrier for nutrients. Now, however, the subject of water, including irrigation water, is on the agenda of many greenhouse horticulture companies. A large number of factors have an influence on water, making it not only a practical focus but also a strategic issue. Water quality, availability and flooding are discussed in numerous forums. Moreover, water is a basic necessity of life for both flora and fauna.
A greenhouse horticulture company never works with just one kind of water: it comes in several different forms, all of which are related to irrigation water. This is also why such a multitude of factors play a part in a company’s water management. Because of all these various water streams, integrated water management is an absolute necessity today. Water management involves the entire chain, from the intake water through to treatment of discharge water. To ensure that water management is performed optimally, Van der Ende Group has developed the horticultural water chain model, consisting of an availability strategy, a recirculation strategy and a discharge strategy.
Strategies
Availability strategy
The easiest and cheapest form of intake water for use as irrigation water is rain water. As a result, rain water is the primary source of water for many horticulture companies. The availability of rain water depends greatly on various factors and is not constant throughout the year. One of these factors is climate change: the fluctuations in the climate mean that large amounts of rain are now falling in shorter periods, while no rain water whatsoever is available for longer periods due to protracted drought.
Most horticulture companies do not have an infinite capacity for rain water storage to cover periods of drought. A secondary water source is therefore needed to ensure that sufficient irrigation water is always available. There are several different kinds of secondary water sources, such as city water, groundwater, surface water and water that comes from other production processes. All of these sources require treatment to a greater or lesser extent before they can be used as irrigation water. We refer to this step in the model as “Availability”.
Recirculation strategy
Irrigation water is lost within a greenhouse horticulture company partly through plant uptake and partly through evaporation. The actual amount of uptake and evaporation of irrigation water depends on the growing conditions and strategy of the company, but the average is 70%. The remaining 30% of the irrigation water is returned in the form of drain water. What companies need is to maximise
recirculation of this drain water; it not only saves water but also the nutrients that are still present in the drain water. An important consideration when recirculating drain water is the hygiene aspect, which means that undissolved particles, viruses, bacteria and fungi must be removed. We refer to this step in the model as “Sustainability”.
Discharge strategy
The final step in the model is called “Legislation” and relates to the discharge of water. Continuously recirculating drain water results in accumulation of various elements, including sodium. When the drain water quality becomes too low for recirculation, it is often discharged. However, in many cases there are local laws that restrict the discharge of water. For example, some countries have completely prohibited the discharge of crop protection products. There may also be restrictions on the amount of nutrients or the quantity of water that can be discharged.
The restrictions that apply will depend on the country or region where the company is located. The general trend is that laws on what companies can discharge are becoming stricter worldwide, to combat the causes and consequences of climate change and water shortages. There are various ways to treat discharge water that make it possible to both comply with the local laws and meet the needs of a greenhouse horticulture company: for example, by re-using crop protection products or nutrients that may not be discharged.
Water streams
and their challenges
The water used in a horticulture company consists of several different water streams, each of which brings its own specific challenges.
Rain water
Generally speaking, rain water is an extremely clean water source and contains very few elements that cause problems for the irrigation water quality. However, it can contain a small quantity of certain elements, such as iron. This is because these elements are released by the drain pipes used to capture rain water that falls on the greenhouse roof. There is also a risk that algae, viruses, bacteria and fungi could grow in rain water reservoirs, or living organisms could find their way into them.
It is therefore essential to treat rain water by means of coarse filtration and disinfection before it can be used as high-quality irrigation water.
City water
City water contains various elements that may inhibit plant growth and/or combine with other elements at a certain pH value to form a chemical compound. Examples of elements that can form a chemical compound are calcium and bicarbonate. City water also contains sodium and chloride, which can inhibit plant growth. These elements must therefore be removed by desalination before city water is used as irrigation water.
This water can be treated using membrane technology, such as reverse osmosis.
Well water
The main challenge in the case of well water is the quantity
of salts, measured as conductivity or dissolved substances. Depending on the location and other external factors, the EC can be determined to some extent by sodium. However, well water also contains other elements, such as chloride, bicarbonate and calcium.
There can also be a high concentration of trace elements, such as iron, manganese, zinc and copper. These elements are especially challenging when oxygen is present in well water. This is known as an aerobic source, and it causes oxidation of heavy metals, such as iron.
This results in formation of suspended solids, which can lead to pipes becoming blocked. Other solids, such as sand or clay particles, can also be present in well water. All of these elements and solids need to be removed before the well water can be used as irrigation water.
Sand filters or multimedia filters are a good way to remove solids or oxidised trace elements. Dissolved major elements can be removed safely and effectively using membrane technology, such as reverse osmosis.
Surface water
Surface water is strongly influenced by many different factors. Its quality can be variable, depending on the seasons or contamination due to e.g. industrial activities, coarse or suspended particles, algae and/or mussels or other organisms. Moreover, surface water contains differing quantities of major and trace elements, viruses, bacteria and fungi. Numerous challenges are thus encountered when surface water is used as irrigation water. To effectively treat surface water to produce high-quality irrigation water, several filtration steps are therefore essential.
First, a coarse filter is needed to remove coarse and suspended particles and algae. The next step is to remove viruses, bacteria and fungi using disinfection techniques, such as ultrafiltration, a membrane technology. Finally, major and trace elements can also be removed with membrane technology, such as reverse osmosis.
Water from other production processes
A less common secondary source is water that comes from other production processes within the greenhouse horticulture company or other industries in the vicinity. Examples include wastewater or cooling water from power stations, water from wastewater treatment plants or condensed water from a CHP (combined heat and power) system.
The challenges involved in treating these water streams depend very much on the specific source, but examples that are often encountered with this kind of water are a very high pH value and varying levels of major and trace elements. Another challenge can be the organic composition of these streams. Although these challenges are reasonably simple to resolve, they may be associated with high operating costs.
Drain water
Drain water has essential value for a present-day greenhouse horticulture company, but it entails a number of challenges. An important aspect is the removal of viruses, bacteria and fungi, to enable safe and responsible recirculation of drain water. Suitable treatment techniques are ozone dosing, UV disinfection or disinfection by heating. However, as a result of the high costs of energy and gas, the current trend for treatment techniques is changing; companies are increasingly choosing disinfection techniques with very low energy consumption, such as ultrafiltration, a membrane technology, which has many other advantages in addition to those in the area of energy.
Another challenge with recirculating drain water is contamination by solids and suspended particles, such as substrate material that has entered the water. These solids and suspended particles contaminate the watering system and can cause blockages, although they can easily be removed with coarse filters. Organic contamination can also occur when drain water is recirculated, due to organic matter. This organic contamination acts as a breeding well for pathogens; to combat this, the watering system is often cleaned with oxidising chemicals, such as hydrogen peroxide. Several techniques are available to remove this contamination, thus ensuring that the watering system is kept clean. This has the advantage of eliminating the breeding well for pathogens, and the risk of crop damage is considerably reduced.
Recirculation of drain water also results in accumulation of elements. The accumulation of sodium is a particularly prevalent challenge for greenhouse horticulture companies. If the sodium level of irrigation water is too high, the growth of plants is inhibited because sodium impedes the uptake of other nutrients, including potassium. It is therefore very important to maintain a low sodium level. This can be done by simply discharging drain water with a high concentration of sodium. However, this is not a sustainable solution, as a large quantity of water and important nutrients are also lost. A more sustainable method is to selectively remove sodium from the discharge water stream, which results in saving water and retaining valuable nutrients. Sodium can be effectively and selectively removed by means of the correct membrane technology.
An increasingly common phenomenon is root exudates, which reduce the production and yield of crops in greenhouse horticulture companies. Root exudates are substances secreted by the roots of plants, intended to inhibit the growth of root fungi and/or competing plants. Although they are toxic, they can only cause damage in high concentrations. Recirculating drain water causes the concentrations of root exudates in the drain water to rise, thus increasing the risk of damage to the plant. Root exudates can be removed using adsorption techniques, such as an activated carbon treatment.
Discharge water
The challenges involved with discharge water depend very much on the total water composition and the applicable laws. An example of one of the challenges in the Netherlands is crop protection products, the discharge of which has been prohibited since 2018. Crop protection products can be effectively removed using membrane technology or a combination of chemical dosing and a UV system.
Another challenge is removal of major and trace elements that would damage environmental biodiversity if they were discharged, such as nitrate and phosphate. These elements can be selectively removed from discharge water using ion exchange, an adsorption technique. A further advantage of selectively removing nitrate and phosphate is that these elements can then be re-used as fertiliser.
Summary
Irrigation water has become an important practical focus within greenhouse horticulture, and good water management is strategically important at the present time. A greenhouse horticulture company never works with just one kind of water, but rather with a range of different water streams. These water streams are influenced by many factors and they all have their own specific challenges. To ensure that these water streams are managed correctly, it is essential to have an availability strategy, a recirculation strategy and a discharge strategy. All of these strategies, water streams and challenges are incorporated in Van der Ende Group’s water management model.
The availability strategy relates to ensuring that sufficient irrigation water is available from the primary and/or secondary sources that are present in a greenhouse horticulture company. These sources can be, for example, rain water, city water, well water or surface water. Each source requires a different approach for its treatment.
The recirculation strategy relates to the recirculation of drain water. Several aspects are important in order to enable recirculation of drain water, such as the hygiene aspect, accumulation of sodium, and contamination by solids or organic matter. Treatment of drain water is therefore essential for responsible and sustainable use of water.
When the drain water quality has become so low that recirculation is no longer advisable, the water must be discharged. This is when the discharge strategy comes into play. The strategy required for treating discharge water depends on the local laws that are applicable. Laws that make it compulsory to treat discharge water for specific chemicals or elements, such as crop protection products or nitrate, are increasingly being imposed worldwide.
In this article, Van der Ende Group has aimed to present a complete and accurate picture of all the aspects involved in irrigation water management at greenhouse horticulture companies.
For more information on this topic or products in this area, please contact our water treatment specialists.