Drinking water must be free of pathogenic microorganisms. While the presence of indicator germs implies contamination, we now know that their absence is in no way proof of acceptable quality. This conundrum cannot be resolved without paying close attention to each stage of production. This is particularly important given the fact that some microorganisms responsible for water borne infections are highly resistant to disinfection treatment.
Over a century has passed since scientists discovered the link between cholera or typhoid epidemics and the contamination of water with fecal matter. Yet it took until the second half of the twentieth century to realise that pathogens present in feces were not the only cause of water contamination. Many microorganisms, and in particular viruses and parasites, were not taken into account.
Moreover, other interesting observations were made in the early 1950s. They showed, for example, that non-pathogenic microorganisms can indirectly promote infections. “We know that nitrates can be an indirect cause of methemoglobinemia, a disease
chiefly affecting babies who are breast-fed,” explains J. Vial, Former President of the water section of the Conseil supérieur d’hygiène publique de France during a session on the microbiology of water at the 1994 congress of the French association AGHTM (Association Générale des Hygiénistes et des Techniciens Municipaux). It is the reduction of nitrates to nitrites, which then react with haemoglobin, that produces this blood disorder. Numerous germs capable of reducing nitrates can be found in water and proliferate there. Almost all the cases observed result from the consumption of water of poor microbiological quality drawn from private wells. Today, affections of this kind have practically disappeared from western Europe.
At present, Legionella is a cause for concern. The bacteria colonise water distribution systems of buildings, swimming pools or balneotherapy centres. They can be inhaled in the form of infected aerosols, while taking a shower for example [A noter : d’après Ronald Droste, Theory and practice of water and wastewater treatment, New York, Wiley, 1997, les douches ne sont pas impliquées !] This is also the case with cyanobacteria (blue-green algae) which proliferate in drinking water reservoirs and can secrete toxins.
Most of the disorders caused by water of poor biological quality take the form of gastroenteritis, with diarrhoea, abdominal pain or vomiting. Alongside these short-lived, benign epidemics, other much more serious waterborne diseases occasionally occur (typhoid fever, bacillary dysentery, cholera, hepatitis A and E, poliomyelitis, etc.).
Since 1980, the rapid rise in the number of cases of gastroenteritis related to the consumption of drinking water has led to an increase in research into the presence of pathogens in water resources. Cryptosporidium and Giardia present a considerable public health risk. These protozoa have been implicated in numerous epidemics of waterborne origin. The most serious occurred in Milwaukee (U.S.A.) in 1993, causing over 400 000 cases of sickness and 112 deaths. Today, surveillance is carried out for bacteria such as E. coli, Campylobacter, Salmonella, and Shigella, infectious hepatitis viruses, Norwalk viruses, retroviruses and for the parasites Giardia and Cryptosporidium (and helminths in tropical countries). It is highly probable that the list of waterborne pathogens is far from definitive. There are still epidemics where the origin is unknown.
In order to eliminate these microorganisms, water treatment engineers use disinfectants. However, the response to these products differs from one organism to another. Generally speaking, viruses, parasites and some bacteria are more resistant than faecal microorganisms. This makes it extremely difficult or even impossible to apply a universal form of treatment. Its role as a barrier cannot be 100 % effective. The lack of certainty regarding the efficacy of treatment is an argument against using monobarrier procedures. Furthermore, it is impossible, even with highly sophisticated analyses, to prove that there are no pathogens in the entire water production process. There are therefore very clear arguments in favour of advising producers of drinking water to implement multibarrier treatment.
Multibarrier treatment
No single stage in the treatment process can single-handedly ensure the microbiological quality of the water supply. Today it is universally recognised that the production of good quality water requires the setting up of a series of physical and chemical barriers. This approach has, in fact, been recommended by the WHO since 1994. “By introducing an ozone disinfection reactor, the transport of ozonated gas towards the cell walls of microorganisms encounters several barriers that have to be crossed.
Using the multibarrier concept, the final disinfection is improved, with several microorganisms being removed at each point,” explains Antoine Montiel, Doctor of Science, in charge of the water quality scientific mission at Sagep (Société Anonyme des Eaux de Paris). The various treatments coming before the final disinfection with chlorine must be capable of producing very clear water of excellent microbiological quality. The final disinfection should merely be a last security step. Each stage in the treatment does not in itself constitute an absolute barrier, but leads to improvement stage by stage, until the water eventually meets the quality objectives.
In the natural environment, the majority of microorganisms are found in the form of aggregates or adsorbed onto particles in suspension or onto interfaces, such as biofilms. Preliminary treatment by filtration–sedimentation allows a number of them to be eliminated. For the remaining pathogens, additional oxidising treatments are needed. These may be complemented by one or more filtration steps.
Oxidants, such as ozone, chlorine and chlorine dioxide, act on microorganisms by means of an oxidation–reduction chemical reaction. The site of action depends on the nature of the microorganisms, which explains their varying degrees of resistance to the treatment. With bacteria, for example, inactivation occurs through a degradation of the cytoplasmic membrane. With viruses the main site of action is at the nucleic acid level. This leads to a reduction in the infectivity of the cells.
As they explain at Trailigaz, “most microorganisms have several different life-forms, some of which are highly resistant to chemical attack”. Bacteria that have survived under extreme conditions (lack of nutrients, repeated oxidation, raised temperatures) are more resistant to the action of oxidising agents than the same strains under optimal conditions for growth. Sporulation mechanisms enable germs to protect themselves under these extreme conditions by adopting slower life-forms.
Generally speaking, all microorganisms (bacteria, viruses, protozoa, fungi, yeasts) are sensitive to the oxidizing capacity of ozone. However, their resistance to ozone depends on their physicochemical structure. This treatment can be complemented by the use of chlorine. It serves as an auxiliary treatment right at the start of the process, but is also used at the end of the process, when the water has already undergone full-scale refining. This final treatment limits chlorine by-products, since the water contains practically no more components capable of consuming the oxidising agent. But a word of caution is needed: chlorine is no remedy for poorly conducted clarification. That is why it is so important for each stage to be correctly carried out, in order to achieve as high a level of elimination as possible at each point.
All this explains the new water treatment processes. They either use slow filtration—a technique which up until a few years ago was wrongly considered to be obsolete—the combination of ozone and activated carbon, or more recently membrane treatment. Particular care is needed to ensure that each treatment is specifically adapted to the water being treated.
Adapting the treatment
Treatment processes must be selected according to the physicochemical quality of the resource and the type of pollution involved. The technical choice of treatment cycle must therefore take into account the quality of the raw water, consumer requirements, changes in water quality in the distribution network, etc.
A case in point is the French water treatment plant at Ivry-sur-Seine, operated by Sagep. It is fed with surface water. The plant produces 300,000 m³ of drinking water daily using water from the river Seine and supplies a part of the drinking water for Paris. The production cycle is based on four successive filtration steps. The first is screening, which may be followed by preozonation and if necessary the addition of emergency reagents such as powdered activated carbon or ferrous iron. Next comes contact coagulation on biolite followed by coagulation on sand filters before the passage through slow biological filters. The coagulation-flocculation step is the most important and influences the remainder of the treatment. The choice of coagulants is crucial. These filtration phases capture numerous microorganisms. “There is an indirect relationship between the filtering treatment and the presence of Cryptosporidium”, explains Antoine Montiel.
This treatment is completed by ozonation, and then fresh filtration on granular activated carbon. Final chlorination ensures the
(1) Non-significant effectiveness
Cumulated elimination of pollutants during the treatment cycle (expressed as %)
| Particles / Dissolved matter | Sedimentation | Filtration through granular activated carbon | Ozone | Membrane procedure plus powdered activated carbon |
|---|---|---|---|---|
| Water particles | 85 | 95 | – | 100 |
| Bacteria | 90 | 99 | 99.9 | 100 |
| Viruses | 90 | – | 99.9 | 100 |
| Parasitic organisms (protozoa) | 90 | – | 99.9 | 100 |
| Algae, plankton | 99 | – | 99.9 | 100 |
| Mineral salts (sodium, calcium, potassium, etc.) | – | – | – | 90 |
| Dissolved organic matter | 50 | 60 | – | 90 |
| Organic micropollutants (pesticides, etc.) | – | 10 | 40 | 100 |
| Algae metabolites (responsible for tastes and odours) | – | 10 | 60 | 100 |
Safety of water during its transport through the network. This succession of barriers allows a level of turbidity to be reached at works outflow of between 0.06 and 0.1 NTU, which is far better than the French standard of 0.5 NTU.
A further example involves Switzerland, where the waters of Lake Zurich are invaded by algae every spring. The species involved, Stephanodiscus, accounts for 60 to 90 % of the biomass. Due to their shape and size these algae are very difficult to filter out. Only flocculation filtration, particularly if coupled with preozonation, allows an elimination level of over 80 % to be reached.
To be sure of being able to resolve the problem, Ozonia recently equipped the Lake Zurich-Lengg waterworks with an ozone oxidation unit. After flocculation with 2 mg of aluminium sulphate per litre, followed by double filtration, 67 to 93 % of the biomass is removed. Treatment by ozonation at 1.8 mg/l, followed by filtration through granular activated carbon, guarantees removal of up to 99 % of the biomass.
In fact, all these treatments constitute a succession of physical and chemical barriers put in place to remove microorganisms and nutrients.
A succession of chemical and physical barriers
The Vigneux waterworks in France, which was inaugurated in 1997, produces 55 000 m³ to supply the Paris region. Fed with water from the river Seine, it is situated upstream from the conurbation, thus reducing the risk of pollution. As they explain at Lyonnaise des Eaux, “the river nevertheless remains a surface water of varying quality: algae in spring and a high level of turbidity in autumn and winter”.
The water intake, situated below the surface of the river, eliminates floating matter. Screening removes any substances of 5 cm and above. A system of screens then removes particles in suspension of over one millimetre. At this point, emergency chlorination may be implemented, if need be, to limit the quantity of oxidisable matter and the development of algae. This is done by injecting chlorine gas.
The raw water is then taken into the suction tank of the pumping station, before being delivered to the treatment works. At this point, the particles in suspension are so fine that they have difficulty in settling. A coagulant is therefore added to neutralise the negative charge. A second reagent facilitates flocculation by linking particles together. The resulting floc is eliminated in the settling tanks. The water thus obtained has been clarified. Filtration on an 80-cm-thick bed of granular activated carbon adsorbs the dissolved organic matter. As it leaves the carbon filter bed, the water is treated by ozonation to destroy any remaining bacteria or viruses, as well as organic material responsible for tastes and odours. At the polishing stage, the water finally passes through a Cristal process, combining a membrane-
Based ultrafiltration procedure and adsorption onto activated carbon. This final step eliminates all impurities including viruses, while maintaining a good balance of dissolved mineral salts. At this point the water is drinkable. A small dose of chlorine is added to preserve the quality of the water until it reaches the tap.
To ensure that it is fully effective, this procedure needs to be closely monitored.
A procedure that is closely monitored
To monitor the smooth running of the procedure, the treatment cycles have to be permanently controlled. The standard methods are unsuitable at this stage. Faster indicators are required, which can be continuously measured. For this reason, parameters such as pH, free chlorine or bound chlorine and turbidity are monitored since they provide an instant response. The effectiveness of disinfectants depends on the quality of the water. “Turbidity is an important parameter and must be carefully controlled,” points out Antoine Montiel, “its level must be continuously monitored.” Sagep has accumulated eleven years’ experience in monitoring this measurement, which many countries consider to be of major importance.
This is the case with the United States of America. American law is very strict: a value of 1 NTU is no guarantee. For turbidity, the concentration must be reduced by log 4. In the U.S.A. the maximum permitted level is 0.1 NTU, compared to 0.3 NTU in Germany and 0.5 NTU in France. However, the producer may adopt more stringent standards. At Sagep, for example, production is halted whenever the level exceeds 0.3 NTU. As a general rule, the plant operates at 0.15 NTU.
Various measurements are used to guarantee the quality of disinfection barriers. For example, for ozone, 90 % of the water must have been in contact with ozone at a concentration of 0.4 ppm for 10 to 12 minutes. For chlorine, on the other hand, contact with 0.5 ppm must have been effective for 30 minutes to guarantee disinfection. Furthermore, when the mixture contains chlorine in its free form in a ratio of free chlorine to total chlorine of less than 0.5, the pH must be higher than 8.
At Sagep, they point out that “by measuring turbidity, ozone, chlorine and the pH we can guarantee disinfection.” Seventy percent of installed instruments measure these parameters. Data obtained in the field are recorded. Any drop in performance sets off an alarm, allowing rapid intervention.
It is not, however, sufficient simply to monitor the process. The producer is responsible for the quality of the water that he produces, including its microbiological quality. He therefore needs to have direct or indirect means of knowing whether, at any given moment, the water leaving the works is fit to drink.
Production quality control
Under a law that came into force on 3 January 1989, water distributors are requi
Firstly, internal surveillance is defined under Article 13, which states that “the producer is required to continuously monitor the quality of water intended for human consumption”. This means that the producer must set in place a system of quality assurance with indicators that will enable him to guarantee the quality 24 hours a day.
He must also provide proof that the system has been implemented, in compliance with Article 14. “The producer shall place at the disposal of the said authority the results of the verification operations that he has conducted in respect of the continuous monitoring provided for under Article 13, and all other information relating to the quality of the water distributed.”
The provisions of this subparagraph are close to the reference system required for a quality-assurance system of the type specified in ISO 9002.
Furthermore, external surveillance in the form of sanitary controls is carried out by State officials. Analyses are conducted in Ministry of Health approved laboratories. They provide a means of verifying the effectiveness of the quality system.
Biological quality standards for drinking water cover thermoduric coliforms, fecal streptococci, aerobic bacteria that are reactivated at 22 °C and 37 °C and sulphur-reducing bacteria spores. At the production site, the frequency of testing depends on variations in the parameters. In general, several tests a year are sufficient as long as no problems are detected.
Surveillance is adapted to the quality of the water being treated and the quality of the treatment. For raw water, for example, thermoduric coliforms and fecal streptococci are measured. The crucial point is to detect any fecal contamination before problems arise.
As for the surveillance of drinking water quality, the official method consists of culturing a sample drawn from the pipes, to detect any presence of coliform bacteria. This is an extremely lengthy process. It takes 48 to 96 hours to obtain the result, which is far too long for field analyses.
To improve surveillance, numerous players are endeavouring to find ways of reducing the analysis time. Trials have been conducted in France by Saur, l’Agence de l’eau Seine Normandie, Ysebaert and CRECEP (Centre de recherche et de contrôle des eaux de Paris) to develop an automated E. coli detector for use on site.
The chosen method is based on the capacity of E. coli to rapidly acidify a glucose-enriched culture medium. The lifespan of an E. coli bacterium in 100 ml of water is around ten hours in an inhibitory medium at 44 °C.
Cirsee-Citep, a research centre at Suez-Lyonnaise des Eaux, is currently working on a method to monitor the microbiological quality of water based on molecular biology. It involves identification of the organism by its DNA or RNA sequence, which is specific to any living being. The method is clearly of very great interest. It is fast and the organism can be identified in a matter of hours. Furthermore, it ensures specific quantification of any microorganism, whether or not it can be cultured.
As for Groupe Générale des Eaux, its partnership with the Institut Pasteur since 1996 has been marked by the creation of the Aquabiolab laboratory. Set up to develop molecular methods for the detection and identification of bacteria in water, it conducts research missions in accordance with precise specifications. According to Générale des Eaux, “the preliminary results are encouraging. After one year, we have succeeded in defining new marking techniques and identifying four species of bacteria indicative of fecal contamination.” The corresponding information can now be obtained in 4 hours instead of the previous average of 72 hours.
Moreover, another partnership was signed with the Natural History Museum in January 1998. It is aimed at developing surveillance and classification measures for blue-green algae (cyanobacteria) and the toxins released by these aquatic organisms.
On an international level, Générale des Eaux is a member of the American Water Works Association Research Foundation (AWWARF), a research foundation of the American water distributors’ association. Membership has led to its participation in a research programme on algal toxins, in partnership with American, Australian and French teams.
The concerns of water treatment specialists are not confined to the production site. The aim is to ensure the delivery of good quality water all the way to the tap.
Delivering good quality water all the way to the tap
Specialists in the water supply sector have long been aware of the importance of taking into account the deterioration of water that may occur during transport.
Studies on changes affecting water as it passes through the network have shown that chlorination as it leaves the waterworks is not enough to ensure the good quality of the product being distributed.
It is also essential for water to be of irreproachable quality in terms of organic matter and particles in suspension, since particles can transport germs and organic matter provides them with available nutrients.
To be able to manage water quality in distribution networks, it is important to understand and describe what takes place.
The increase in bacterial numbers that sometimes occurs along the network results from the activity of bacterial flora attached to the inner wall of the pipes. These bacteria multiply and become detached as a result of the hydraulic conditions, increasing the number of bacteria present in the water. Bacterial activity depends, among other factors, on temperature, the supply of nutrients and the presence of residual chlorine in the network.
The complexity of the phenomena involved means that mathematical modelling is necessary to define the impact of the different parameters. The work carried out at Cirsee has led to the development of a model, named Piccobio, to predict bacterial growth.
This computer modelling application is used in conjunction with the Piccolo hydraulic calculation software created by Safege. Taking into account the hydraulic conditions, the network geometry and descriptive parameters of water quality on entry into the network, such as bacterial count and nutrients (BDOC, temperature, pH, residual chlorine), it is possible to predict bacterial development for the entire network and determine which zones are at risk.
These are displayed on a plan, allowing any malfunction to be more easily diagnosed. This model has been validated in several French networks of varying size and complexity, such as Marseille and Cholet.
