Water treatment

Chlorine and related compounds (chloramines, chlorine dioxide, sodium hypochlorite, hydrochloric acid) are effective, broad-spectrum sanitisers, even at low concentrations (e.g. 1 mg/L). In water with minimal organic material, chlorine is highly effective against non-spore-forming pathogens such as Escherichia coli O157, Salmonella, and Listeria, as well as common indicator organisms.

However, organic matter in the water can reduce chlorine’s effectiveness and may lead to the formation of harmful by-products like trihalomethanes, which are carcinogenic. When using chlorine growers must consider final chlorate concentrations in produce. Chlorine is notably ineffective against protozoa such as Cryptosporidium parvum, a pathogen frequently found in UK watercourses, and some spore-forming bacteria like Bacillus cereus.

Chlorine at high concentrations (above 30 mg/L) can corrode stainless steel and other materials used in irrigation equipment, so accurate concentration control is essential to protect infrastructure. In 2020, new legislation was introduced requiring growers to assess the use of chlorine and its link to chlorate residues in food, including fresh produce. See a summary of these regulations.

Ozone is a triatomic form of oxygen (O₃). It is an unstable molecule that rapidly breaks down into atmospheric oxygen (O₂) and a highly reactive radical. This radical is extremely destructive and readily reacts with a broad range of substances, including the molecular structures of microorganisms. As such, ozone is a powerful disinfectant capable of destroying indicator organisms, pathogens and protozoa. It can also damage bacterial spores.

Compared to chlorine-based sanitisers, ozone is less likely to form carcinogenic by-products. However, it has been shown to produce bromate, a compound that has been associated with cancer in laboratory animals. Like chlorine, the likelihood of problematic by-product formation is reduced when the water being treated has a low organic load.

Despite its effectiveness, ozone is highly oxidative and will corrode most metals and plastics. Additionally, it poses a significant inhalation hazard, as it is harmful to human lungs when present in the air.

The principal advantage of ozone lies in its speed of action and the fact that it decomposes quickly, leaving behind minimal residues. Most of the ozone molecule reverts to harmless atmospheric oxygen and is released into the air. Water treated with ozone does not have the off-taste or toxicity sometimes associated with chlorine-treated water.

Hydrogen peroxide (H₂O₂) is increasingly being used in closed-loop systems such as vertical farming, where the same water is continuously recirculated. Its mechanism of action involves the production of free radicals as it decomposes into water and oxygen. These free radicals have strong oxidising properties, which enable them to deactivate or destroy pathogens.

However, the antimicrobial action of hydrogen peroxide in irrigation water is short-lived, as the active ingredients react readily with metals, organic matter and other substances present in the system. As a result, filtration of the water prior to treatment is essential. Pre-filtration not only improves the efficacy of the treatment but also reduces the risk of generating carcinogenic by-products as the free radicals decay.

Pure hydrogen peroxide is unstable and must be transported and stored in a stabilised form, most commonly using silver as a stabilising agent. The presence of such stabilisers can pose an additional risk of phytotoxicity, potentially causing damage to crops. A more effective and stable formulation can be achieved by combining hydrogen peroxide with glacial acetic acid to create peracetic acid. However, peracetic acid is extremely corrosive and is known to cause damage to stainless steel after only a few exposures.

One of the primary advantages of hydrogen peroxide, compared to other chemical disinfectants such as chlorine, is that it breaks down rapidly, leaving no sterilant residues in the water. This means it poses fewer environmental concerns. Although some hazardous side products may still form depending on the contaminants present in the water, these are generally minimal. Another advantage is that the effectiveness of hydrogen peroxide is relatively unaffected by changes in pH, which can be a significant limitation for other oxidising agents. Additionally, the decomposition of hydrogen peroxide produces oxygen, which can be beneficial to plants when delivered to the root zone.

Nonetheless, several disadvantages should be considered. Data on phytotoxicity, particularly for peracetic acid, are limited, creating uncertainty around safe use rates. Like other oxidising disinfectants, hydrogen peroxide-based treatments are less effective when the water contains high levels of organic material, which should be filtered out beforehand. Peracetic acid, while effective, is corrosive and may damage plumbing fixtures and metal infrastructure. Furthermore, safe storage and handling of these substances can be challenging, particularly at the farm level.

UV radiation (200–300 nm) is biocidal due to its capacity to damage DNA, disrupting cell metabolism and killing single-celled organisms. UV has notable advantages over chemical methods, including the ability to destroy protozoa such as Cryptosporidium parvum without leaving chemical residues. However, spore-forming bacteria may resist UV treatment.

UV is effective at reducing both indicator organisms and zoonotic pathogens like E. coli O157. For small-scale use (50 litres per minute), UV systems can be relatively affordable (£600), although higher capacity systems are significantly more expensive.

UV irradiation requires low maintenance and does not require hazardous chemical storage however, it requires a consistent electricity supply (portable generators are an option). The treatment is effective only if the water is clear enough to allow UV penetration, in some cases pre-filtration may be necessary. One big consideration is the fact that UC lamps require almost a full minute after switching on to achieve biocidal levels of UV irradiation and thus may be ineffectively treating the water for a short period of time. Another consideration is the fact that UV lamps degrade over time and need replacement. Additionally, dirt, limescale, or scratches on quartz tubes reduce efficacy. Some commercial systems combine UV with semi-conductor photocatalysts (e.g. titanium dioxide [TiO₂], carbon nitrite [C3N4], or zinc oxide [ZnO]), which catalyses the formation of highly reactive hydroxyl radicals, further enhancing disinfection. While promising, this technology is currently at proof-of-concept stage and still a number of years away from field deployment. 

Reverse osmosis (RO) systems function by forcing water through a semi-permeable membrane, which has a small pore size and a large surface area. For example, a spiral-wound filter tube measuring 50 cm in length and 10 cm in diameter may contain a membrane with a surface area comparable to that of a football pitch. As water is pushed through the membrane, impurities and microorganisms are retained on one side, while purified water collects on the other.

Reverse osmosis has the potential to produce extremely high-quality water, and certain units are capable of desalinating seawater. Theoretically, RO can remove all microorganisms from water. In practice, however, the membranes used in these systems are not perfect. Small imperfections can allow trace amounts of microorganisms and other contaminants to pass through to the purified side.

To function effectively, RO systems require that the input water is largely free of particulates. This necessitates a sequence of pre-filters and screens that must be regularly cleaned and replaced. Many commercial RO units also include a charcoal column, designed to remove chemical impurities that could otherwise damage the membrane. Like the pre-filters, this charcoal component requires periodic replacement.

A significant drawback of reverse osmosis is the volume of wastewater it generates. While clean water is produced on one side of the membrane, a concentrated solution of impurities collects on the other. This waste stream must be disposed of in a way that does not compromise the original water source. A well-functioning RO unit typically returns around 50% of the input water as waste.

Some commercial RO units incorporate UV sterilisation systems to further enhance microbial control and produce near-sterile water. However, RO units are generally more expensive than standalone UV systems, require a substantial and consistent power supply, and incur ongoing operational costs due to the need to replace filters and membranes. Despite these limitations, the quality of water produced by RO systems can be exceptionally high.

Although primarily designed for the removal of chemical contaminants such as nitrates, reed beds can also reduce microbial loads, although the benefits are time-dependent. The reduction in microbial numbers occurs due to the length of time water remains in the system, mirroring the natural decline of microbial populations over time. In this respect, reed beds function in a manner similar to water storage systems. Many of the same considerations that apply to stored water, including flow rate, retention time and protection from recontamination, apply equally to reed beds, and are discussed in further detail elsewhere.

Flocculation involves the use of either air or aggregating chemicals to cause suspended particles in water to clump together and settle out. While this process may result in some reduction in microbial load, it is not considered a reliable method for reducing microorganism levels to a safe threshold. Its primary utility lies in the removal of suspended solids from irrigation water prior to a more effective sterilising treatment.

Sedimentation is a passive process in which water is left to stand with minimal disturbance, allowing suspended materials to gradually settle to the bottom over time. Although this reduces the concentration of suspended nutrients that could support bacterial growth, it does not reliably remove pathogens. It is a useful pre-treatment step but should not be considered a form of disinfection on its own. Sedimentation can be enhanced when combined with flocculation.

Filtration is typically achieved by passing water through sand or another granular medium. The process physically traps suspended solids, thereby reducing the concentration of organic matter that could support bacterial growth. In addition, there is evidence that slow sand filtration can lower the number of pathogenic bacteria in the water (1).