How to assess and improve water quality 

It is important to optimise the quality of water going into your system to increase the lifespan of the equipment and improve crop quality. 

If the quality of the water you use is poor, it can shorten the life of your irrigation and fertigation equipment, clog equipment, impact on crop growth, reduce crop quality, and increase waste.  

Many technologies are available to optimise the quality of water going into the fertigation system for irrigation/fertigation. Water from different water sources can have different treatment requirements but, as a first step, the most important thing you can do is to assess the quality of your water. 

Regular analysis of irrigation water, regardless of source, is important, and should be undertaken at least annually, if not every six months, particularly if introducing a new water source or if you are considering investment in new irrigation equipment. The results of analyses are used to make important decisions on how to change fertiliser programmes and what process to use. 

Three aspects of water quality should be considered: 

• Chemical composition 
• Suspended solids 
• Plant pathogen presence

Water samples should be at least 200 ml in volume, and should always be collected in a clean plastic bottle, securely sealed and labelled. If the sample cannot be sent off within 12 hours, it should be stored in a cool dark place to prevent algal growth. 

If acid is used to reduce the alkalinity of the irrigation water, it is important to submit a fresh sample of both the untreated and treated water for comparison, and ensure that the fresh water is from the same source as the acid-treated water. 

When sampling water containing soluble fertiliser, ensure the dilutor equipment is allowed to run for a short while, to make sure the liquid feed solution collected from the tap is a representative sample of that being applied to crops. 

As an approximate guide (based on a medium-sized, single site nursery in horticulture in 2016), irrigation water analysis (including analysis of water containing soluble fertiliser), allowing for four samples per year: £100–£150. This does not include testing for suspended solids or plant pathogens. 

For information on costs for substrate analysis or leaf tissue analysis, see factsheet Sampling methodologies and analysis interpretation. 

There are several things to consider when interpreting water analyses, including the crop, the water source, whether the crop is grown in soil or not, and whether the water is reused or not. For chemical analyses of water, information is provided on: 

• Alkalinity – the bicarbonate content of the water. A high bicarbonate content causes the pH to rise in substrates, and results in deposits on leaves and in irrigation equipment 
• Elements content – major and minor. The presence of high levels of iron or sulphates can lead to issues with water suitability 
• Conductivity – total salts content of the water. High conductivity levels can make water unsuitable for certain purposes 

This information comes from the factsheet: Sampling methodologies and analysis interpretation. It gives a summary of the highest, average and lowest criteria and element values from a number of actual irrigation water analyses. How to interpret alkalinity and potential methods of correction.

The water source you use has a big impact on the water quality – rainwater is considered the best source of water, whereas surface and ground water can contain minerals that should be taken into account when designing the fertiliser programme you use. The crop will have specific pH and nutrient requirements, and sometimes the pH or alkalinity of your water is not suitable for your crop. Below is a range of treatment options. For more information, please refer to The Fertigation Bible. 

Water can be categorised based on its alkalinity (bicarbonate content). The alkalinity of the water can cause changes in pH in substrates over time. Below is a table of water categories and suggested treatments: 

An important point to consider when treating irrigation water with acid, is its impact on other aspects of water quality. For example, treating water with nitric acid will add nitrogen to the water, making it in essence a dilute liquid feed, which needs to be taken into account when calculating how much soluble fertiliser to apply. 

High iron and manganese can block drippers. Aerating the solution oxidises them, causing them to precipitate out of solution. Both can then be removed by filtration (see ‘Removing particles’ below). 

• Electrophysical precipitation – solid-state electrodes disintegrate and combine with the particles in the water, which can then be filtered out 
• Capacitive Deionisation (CDI) – ions move out of the water and stick to electrodes when a potential difference is created. CDI typically recovers between 80% and 90% of water, and removes up to 99% of salts

Particles in water can clog irrigation equipment or introduce contaminants into crops, so filtration of the water may be required. 

When selecting a filtration method, consider how much water needs to be filtered to meet crop needs – some methods of filtration are slower than others. In some cases, a pre-filtering treatment may be needed, depending on the quality of the water. Also, bear in mind that some filtration methods produce backwash, which might contain nutrients and/or pesticide residues. It may not be possible to discard this water. Below are several methods of filtration. 

• Sieve bend screen filtration (removes particles 150μm – 5mm in size). This is often the first filtering step. Water is pumped into a sieve. Water escapes through the sieve while any solids are captured 
• Hydrocyclone filtration (>50μm). A pump creates a water vortex, causing the heavier particles to be pushed downwards and outwards, while the now clean water rises to the top and to an outlet. There are no moving parts, only a pump 

• Rapid sand filtration (10μm). A filter filled with coarse sand and other granular media. Incoming water flows through it under gravity or pressure 
• Cloth filtration (5–10μm). Hollow discs are arranged vertically in a tank and covered with cloth. As the tank fills, the pressure forces the water through the cloth into a central shaft, leaving particles behind on the cloth 
• Drum filtration (5–10μm). Water flows into a rotating drum with a fine mesh. As it rotates, water escapes through the mesh leaving particles behind. Some water is used to wash the particles off the mesh and is removed as a sludge. The particle sludge or cake can also be scraped off 
• Microfiltration (0.1–10μm). Uses a microporous membrane to remove particles and contaminants. It doesn’t use pressure to move the water through the membrane 
• Ultrafiltration (0.01μm) operates in a similar way to microfiltration, but uses pressurised flow. This method requires a pre-filtration treatment 
• Disc filtration (55–400μm). Water is forced through the grooves of discs, trapping debris and releasing clean water. These units are relatively small and have high throughput of water 

Algal blooms tend to appear in stagnant water, and will be worse at higher temperatures or at higher concentrations of nitrates and phosphates. Although some blue-green algae can contain toxins harmful to humans, it is highly unlikely that the toxins will come into contact with crops produced in fertigated systems. The main problem is clogging of irrigation equipment. Solutions can be divided into three categories:  

• Physical 
• Biological  
• Chemical 

It is worth considering physical control options first. Below are some of the options available. For more detailed information, explore The Fertigation Bible.

• Covering the storage structure (water silo, pond or reservoir) is an effective method of control, as algae needs light to grow. For large storage facilities, floating rather than fixed covers can be used
• Ultrasound creates a sound barrier that prevents algae from rising to the water surface and absorbing light for photosynthesis, preventing growth
• Water movement and aeration moves algae to darker areas preventing photosynthesis, and increases dissolved oxygen, encouraging beneficial bacteria, which then compete with the algae for any nutrients in the water

• Daphnia, or water fleas, feed on small algae. The temperature has to be high enough (16°C) for them to stay active and reproduce
• Grass or silver carp fish clear the water of algae
• Bacteria and enzymes dissolve the organic molecules from algae. The effect is only visible after 2–3 weeks
• Straw bales can be put in the water to rot down. The rotting process exudes toxins that kill the algae. The straw bales are good shelter for water fleas. The effect is visible after 6–8 weeks
• Some submerged aquatic plants produce substances that inhibit the growth of algae, and act as shelter for water fleas

• Blue dye filters out part of the light spectrum, preventing algae from photosynthesising
• Hydrated lime is mixed into the water and causes any phosphates to precipitate out
• Hydrogen peroxide and chlorine are strong oxidants effective against algae

To avoid introducing disease pathogens into your crop, it may be necessary to disinfect the water you use, particularly in closed, recirculating systems such as those found in some tomato or protected ornamental systems, depending upon the source of the water.

It is possible to use physical or biological methods to disinfect water, and these are worth considering before chemical oxidation methods, as they avoid the use of harsh chemicals, and require lower energy input. Explore The Fertigation Bible for details on operational conditions, advantages and disadvantages.

• Ultraviolet disinfection – UV-C light damages the DNA of microorganisms, either killing them or making them harmless. This works for fungi, bacteria, nematodes and viruses
• Thermal disinfection – the water is heated to 95–97°C for 30 seconds to inactivate pathogens. Heat treatment is expensive if you are using large volumes of water

• Slow sand filtration – water is filtered by several layers of granules ranging from sand to gravel. Pathogens such as Phytophthora and Pythium can be almost completely removed by this method. It is a low-cost solution
• Biofiltration – in a tank, water flows over a volcanic material, which has a biofilm (a thin layer of microorganisms) growing on it. The biofilm cleans organic matter and pathogenic fungi and bacteria. It can be made more effective by circulating air through the tank

• Chlorination – chlorine is added to water as sodium hypochlorite, calcium hypochlorite or chlorine gas
• Ozonisation involves the controlled addition and generation of oxidants (in this case, ozone) to waste water. Ozone is a highly toxic, unstable gas so must be produced on site and mixed immediately into the water being treated
• Peroxide is also unstable, and has oxidative activity. Hydrogen peroxide is most commonly used, and it oxidises proteins, membrane lipids and DNA, destroying microorganisms