Effective treatments which eliminate 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 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 cleanup can completely eliminate all of the microorganisms that are potentially present in water. 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.
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 (1 part per million). Chlorine will efficiently kill most non spore-forming pathogens (e.g. E. coli O157, Salmonella and Listeria) and all routinely-encountered indicator species. 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 also resistant to the antibacterial 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 control their chlorine concentrations. 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. However chlorine remains the cheap and effective treatment of choice for water with low organic content or water which has been pre-filtered to reduce particulates (NB: filtration is commonly used by water companies to remove Cryptosporidium parvum prior to chlorine treatment).
Ultraviolet sterilisation Ultraviolet radiation (UV) of between 200 and 300nm is strongly biocidal 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. However, spore forming bacteria can resist the sterilising effect of UV. Like chlorine, UV effectively lowers numbers of indicators in water and also zoonotic agents such as E. coli O157. The main advantages of UV are that it is fairly cheap to setup (units that can purify 50 litres per minute cost around £500), has low maintenance costs and requires no hazardous chemical storage. However, an electrical supply is required which can be expensive to provide in some locations. In order to operate effectively, the water must have sufficient clarity for good UV light penetration. 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 scale 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; most of the molecule breaks down to oxygen and is released into the air. Water purified by ozone has none of the taint 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. 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 periodic cleaning or 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.
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 infectious agents declines in water over time, the process is analogous 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 colloidal solids from water. Although flocculation can remove some microbial load from water, it is not considered a terribly 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 above.