Water treatment

Sterilising water treatments

Effective treatments which eliminate, or reduce to an acceptable level water-borne microorganisms prior to irrigating crops can be thought of as critical control points. Within reason, and from a purely microbiological viewpoint (i.e. making no considerations for chemical contaminants), an effective treatment can negate the impact of risky storage and application practices and help justify the use of poorer quality surface water sources. However, growers should be aware that not all of the decontamination systems routinely used for irrigation water clean-up can completely eliminate all of the microorganisms that are potentially present in water.  Consequently, practical steps should be taken to reduce or eliminate as many of the identified contamination risks/sources as possible prior to the treatment of the water, to ensure water applied to the crop is as clean as it can be.  The information provided below describes commonly-encountered water sanitation systems and their relative advantages and disadvantages. It is important to stress that for all of the systems listed below, a periodic check is required to confirm operational efficiency of the decontamination treatment.

Summary of main water treatment options (this is not an exhaustive list)

Chemical injection

  • Liquid hypochlorite 
  • Peroxyacetic acid 
  • Chlorine dioxide 
  • Hydrogen peroxide 
  • Liquid calcium hypochlorite Tablet Systems 
  • Calcium hypochlorite 
  • Ozonation

Physical treatment

  • Ultraviolet light 
  • Reverse osmosis (RO)
  • Physical filtration (membrane or other media) 

Chlorine

Chlorine (and related molecules such as chloramines, chlorine dioxide, sodium hypochlorite and hydrochloric acid) is an effective general sanitiser with a wide range of activities; even when used at low concentrations of 1 part per million. In water that is relatively clear of suspended organic material, chlorine will efficiently destroy most non spore-forming pathogens (e. g. E. coli O157, Salmonella and Listeria) and all routinely-encountered indicator species. When using chlorine, consideration needs to be made of the final concentration of chlorates in produce. Organic materials dissolved or suspended in water can reduce the effectiveness of chlorine. Also, if the water being treated has a high organic content, chlorine can react with these impurities to form carcinogenic (cancer-causing) side products such as trihalomethanes. Chlorine is most notably ineffective at killing protozoa such as Cryptosporidium parvum which is well established in the UK animal population and routinely isolated from UK watercourses. Some spore forming bacteria (e. g. Bacillus cereus) are resistant to the antimicrobial action of chlorine. A further disadvantage of chlorine is that if it is mixed with water at excessively high concentrations (upwards of 30 ppm), it will begin to corrode stainless steel and other materials used for valve constructions, joining hoses and sprinkler heads. Thus, in order to protect their infrastructures, growers need to tightly regulate their chlorine concentrations. In 2020, there were new laws enacted that require consideration of chlorine use in agriculture and the consequences for a substance called chlorate, which can build up in food generally, including fresh produce. Growers may want to consider a summary of these new laws which are available here.

Ultraviolet sterilisation

Ultraviolet radiation (UV) of between 200 and 300nm is strongly biocidal (destructive to life) because it changes the chemical structure of DNA, interfering with cellular metabolism and causing the death of single cell organisms such as bacteria. UV has a number of advantages over chemical treatments, most notably that it effectively kills protozoa such as Cryptosporidium parvum without leaving any chemical residues. However, spore forming bacteria can resist the sterilising effect of UV. Like chlorine, UV effectively lowers numbers of indicators in water and also zoonotic pathogens such as toxin-producing E. coli O157. The main advantages of UV are that it can be fairly cheap to set up for low volumes (units that can purify ~50 litres per minute start at around £600), although units that can sanitise larger throughputs are significantly more expensive. UV has low maintenance costs and requires no hazardous chemical storage. However, an electrical supply is required, which can be provided by a portable generator. In order to operate effectively, the water must have sufficient clarity for good UV light penetration.  Some water sources might require filtration before they can be sanitised by UV; a filtration unit might cost as much again as the UV sterilising unit.  In addition, UV lamps can take almost a minute before they generate biocidal levels of radiation and so there is period at start up when water treatment is ineffective in some systems. Growers should be aware that the effectiveness of UV lamps diminishes over time and that the lamps need periodic changing. In addition, build-up of dirt or limescale or large numbers of small scratches on the surface of the quartz lining of the sterilisation chamber can all reduce the effectiveness of UV sterilisation units. In recent years, commercial units have appeared that use UV in combination with titanium dioxide (TiO2) to achieve an enhanced kill. When irradiated with UV, titanium dioxide is activated to catalyse the formation of hydroxyl radicals which are highly reactive in a similar manner described for ozone below. TiO2 in combination with UV is a highly effective water treatment as long as the water has sufficient clarity to allow the UV to reach and activate the TiO2.

Ozone

Ozone is a trimer of oxygen (O3). The molecule is unstable, and quickly breaks down to atmospheric oxygen (O2) and an oxygen radical. The radical is highly destructive and will react with a wide range of materials including the molecules that form microorganisms. Ozone is a powerful disinfectant which can destroy indicators, pathogens and protozoa. It can also damage bacterial spores. Ozone is less likely to form carcinogenic compounds than chlorine-based sanitisers, but it has been shown to form bromate; which can cause cancers in laboratory animals. As for chlorine, water with low organic load reduces the likelihood of problematic side products. Ozone will oxidise (corrode) most metals and plastics. It is also hazardous to human lungs if inhaled. The primary advantage of ozone is that it is fast-acting, short-lived and leaves very little behind in the way of residues or harmful effects; most of the molecule breaks down to oxygen and is released into the air. Water purified by ozone has none of the taint or toxicity that is characteristic of water treated with chlorine.

Reverse osmosis

Reverse osmosis-based units operate by forcing water through a semi-permeable membrane with a small pore size and a large surface area.  A spiral wound filter tube unit of 50cm length and 10 cm diameter can contain a membrane that has the same surface area as a football pitch.  Impurities in the water, including microorganisms, are left on one side of the membrane and pure water is accumulated on the other side. Reverse osmosis (RO) has the potential to generate very high-quality water, with some units being capable of turning seawater into fresh water. RO can theoretically remove all microorganisms from water; however the reality is that the membranes used are imperfect, and small imperfections in them allow some microorganisms and impurities to pass into the clean water side of the membrane. The other issues with RO are that the water being filtered has to be largely free of particulates and so a series of screens and pre-filters are required (which require maintenance of cleaning and periodic changing). In addition, most commercial units have a charcoal column designed to remove chemical impurities that could damage the filtration membrane; and this also requires periodic changing. Water is wasted by the RO process. RO generates fresh water on one side of the membrane and a concentrated solution of impurities on the other side. These dirty side wastes require disposal in a manner that doesn’t contaminate the water source being purified. An efficient RO unit will return 50% of the inputted water as waste. A number of commercially available RO units include a UV purification system and therefore produce high quality water that is near-sterile. Units themselves are more expensive than UV sterilisers, have a requirement for a substantial electrical supply and have operational costs in the form of replacement of filters and columns. However, the output water generated can be of very high quality.

Hydrogen peroxide and peracetic acid

Hydrogen peroxide (H2O2) is a chemical treatment that is being increasingly being used to treat water in vertical farming systems where the water contained within a closed loop system.

The basis for deactivation or destruction of pathogens with hydrogen peroxide-based products is the oxidizing effects of the free radicals generated as the compound breaks down to water and oxygen.  The destructive effects of hydrogen peroxide-based products are short lived-in irrigation water due to the active ingredients reactivity with metals, organic matter, and other substances in irrigation systems.  Pre-Filtration is an essential pre-requisite before peroxide treatment to help prevent the creation of carcinogens by the free radicals as they decay.  Pure H2O2 is unstable and is required to be transported in a stabilsed form, commonly using silver as a stabiliser.  There is an increased risk of phytotoxicity (plant damage) from the stabilising agents. Significantly increased efficacy and stability can be achieved when hydrogen peroxide is combined with glacial acetic acid to form peracetic acid.  Peracetic acid is highly corrosive and will begin to dissolve stainless steel after just a few exposures.

Advantages

  • Unlike some other chemical treatments such as chlorine, hydrogen peroxide degrades quickly, leaving no residues from the sterilising agent, and as such poses fewer environmental concerns.  Depending on the nature of the contaminants in the water small quantities of hazardous side products may be created.
  • The effectiveness of peroxide is relatively unaffected by pH changes, compared with some other oxidisers.
  • When hydrogen peroxide products degrade, oxygen, which can be beneficial to plants when located in the root zone, is produced.

Disadvantages

  • Lack of phytotoxicity data, even more so than for peracetic acid products.
  • Like other oxidisers, water should be filtered beforehand to remove organic material that would otherwise reduce treatment efficacy and increase the likelihood of toxic side products.
  • Peracetic acid is corrosive and so may damage certain plumbing fixtures and metal infrastructure.
  • Like other oxidising disinfectants, safe handling and storage may be difficult.

Non-sterilising water treatments

Reed beds

Although reed beds can be effective in removing chemical contamination such as nitrates from water, the microbiological benefits are derived from the amount of time that water takes to pass through the reed bed. Since the numbers of bacteria and other pathogens declines in water over time, the process is similar to storing water as a way of reducing the microbial load associated with a water supply. Important considerations relating to water storage, which apply equally to reed beds, are discussed here.

Flocculation 

Flocculation involves the use of air or aggregating chemicals to remove suspended solids from water. Although flocculation can remove some microbial load from water, it is not considered an effective method of reducing numbers of microorganisms. Flocculation is better suited to removing suspended solids from irrigation water prior to treatment by one of the sterilising methods described.

Sedimentation

Sedimentation is a passive process that involves letting water stand without significant agitation or flow. Any suspended materials in water stored in this manner will gradually settle out of solution over time, lowering the levels of potential bacterial nutrients contained within the water.

Filtration

Filtration is commonly undertaken by passing water through a quantity of sand. The sand filters out, and retains, solids suspended in the water thereby lowering the levels of potential bacterial nutrients in the water.  In addition, there is evidence that the numbers of contaminating bacteria in the water are lowered by slow sand filtration (Fujioka et al., 2019). There is general guidance from the WHO on approaches that can be used to filter water.

Reference

Fujioka, T., Ueyama, T., Mingliang, F. and Leddy, M. (2019) Online assessment of sand filter performance for bacterial removal in a full-scale drinking water treatment plant. Chemosphere 229, 509-514.