Reuse in practice: a review of selected French case studies

Reuse of treated wastewater is currently practiced in approximately 40 sites across France and amounts to a daily volume of around 19,000 m³, which is quite low in comparison to Mediterranean countries (adapted from Jimenez and Takashi, 2008). Treated wastewater reuse is increasingly recognised as a sustainable water supply alternative to ease water stress in the context of global changes. In its recently published Blueprint the European Union (European Commission 2012), for instance, is proposing to identify suitable instruments to encourage reuse. Since Member States like France have already developed their own regulatory framework and implemented numerous reuse projects, it is considered of potential interest to examine these existing practices in order to identify whether any valuable lessons could be learned from this experience. This paper discusses selected examples of reuse projects in France focussing on issues such as authorisation processes, reuse drivers and risk management.

Until recently, reuse projects in France were authorised on a case by case basis by local health authorities on the basis of general recommendations made by a national health advisory body, the Conseil Supérieur d'Hygiène Publique De France (CSHPF, 1991). Pornic and Sainte-Maxime, where reuse has been implemented to irrigate golf courses since 1994 and 2005, respectively, provide interesting examples of reuse projects authorised under this regulatory framework.

Pornic is located in South-eastern Brittany near the Loire Estuary in a water scarce and environmentally sensitive area. The wastewater treatment plant (WWTP) is a 50,000 population equivalent (pop.eq.) facility that initially comprised an activated sludge step followed by disinfection (in summer only) and a 7,000 m³ lagoon. When the golf course was created in 1992, the lack of local water availability to fulfil the need for irrigation water drove the golf course owners towards implementing reuse. To protect both health and environment, the local health authorities who dealt with the project prescribed a set of physico-chemical and microbial standards for the reclaimed water that were largely based on CSHPF's recommendations. These are shown in Table 1.

Interestingly, the authorisation process in Pornic also involved the monitoring of airborne pathogens. Micro-organisms were counted on nutritive agar situated 1.50 m above the ground at nine control points. The results obtained under different humidity and wind speed conditions led to technical specifications being prescribed by local health authorities to minimise the exposure of nearby inhabitants by direct airborne pathogens inhalation and the ingestion of raw edible crops from the neighbouring gardens. These included the installation of plant screens, prohibiting irrigation during windy weather conditions (i.e. >wind force 4), and the use of low pressure aero-sprinklers near inhabited areas. On average, approximately 70,000 m³ of treated wastewater (12% of the volume produced) is now transported each year by a 5 km long pipe to the golf course and irrigates 20 hectares (48% of the golf course surface).

Sainte-Maxime is located along the Mediterranean coast on the French Riviera in a water scarce area, especially during summer periods when tourism boosts potable water demand. The WWTP is a 60,000 pop.eq. facility that comprises an aerobic biological treatment of oil and grease Biolix™, 2 lines of a high flow rate clarifier in which water is flocculated with microsand Actiflo™ followed by 6 aerated biofilters Biostyr™. To cope with increasingly frequent potable water shortages, the Sainte-Maxime golfcourse managers decided to establish a permanent source of irrigation water through reuse. The scheme was first authorised in 2005 and prescription by the local health authority included, as shown in Table 1, limit concentrations on faecal indicators and pathogens (salmonella, legionella pneumophila, nematode eggs) and on physico-chemical parameters. To conform to these requirements, a tertiary treatment step comprising 4 sand filters, UV disinfection and chlorination was installed. Sainte-Maxime's WWTP now supplies 2,000 m³ per day (i.e. 7% of its total capacity) of treated effluent to the golf course located 1.7 km away.

As illustrated in Table 1, comparison between Pornic and Sainte-Maxime's quality standards shows noticeable differences both in terms of regulated parameters and water quality thresholds. The risk assessment process (i.e. monitoring of airborne in Pornic) and the technical specifications required by the local health authorities (i.e. screening, weather conditions constrains and sprinkler specifications) also differ significantly between the two case studies described above. This illustrates how risk assessment carried out under a flexible legal framework may in practice lead to different water quality standards being required for similar projects and uses. These dissimilarities may simply reflect differences in terms of actual risk levels as evaluated during project risk assessment, but may also reflect differences in terms of individual health authorities’ attitude to risk management (e.g. acceptable risk level, application of precautionary principle, etc.).

In 2010, the French regulatory framework for reuse in irrigation was clarified through the publication of a decree specific to reuse. This piece of legislation is inspired from the WHO Guidelines for safe use of wastewater, excreta and greywater (WHO 2006). It establishes water quality standards and technical prescriptions relative to the use of treated municipal wastewater for the irrigation of crop and open spaces that vary depending on crop type, irrigation systems and practices. The standards include concentrations limits for physico-chemical parameters and E. coli as well as treatment performance targets for microbial indicators. Prescriptions vary depending on crop type and practices (see Table 2). To ensure satisfactory environmental condition and health and safety, the following specific measures are also required:

• - Prohibition of some reuse practises (e.g. raw wastewater irrigation, no effluent with sludge quality non-compliance, irrigation in groundwater source protection zones …).

• - Respecting minimum distances to existing water activities (e.g. recreational water, fisheries, shellfish farms…).

• - Field constrains (e.g. only drip irrigation when slope is greater than 7%, no irrigation on waterlogges soils).

• - Monitoring programme of treated water quality and soil quality.

Interestingly and despite the fact that treated wastewater quality may in some cases be much better than surface water used for irrigation, the decree only authorised spray irrigation on experimental grounds pending more thorough risk assessment by the national health authorities. This meant that new project proponents had to undergo a more complicated authorisation process and to provide more comprehensive risk assessment studies, resulting in very few new projects having been proposed since the regulation came into force. Four years later and on the basis of the results of risks studies carried out on existing experimental sites, the national health authorities amended the 2010 decree by simplifying some of the requirements for spray irrigation and introducing new constraints on irrigation practices (e.g. minimum distances to housing and roads and maximum wind speed). It remains to be seen if these amendments will boost reuse projects across the country.

For existing projects, the 2010 decree also required that compliance with the new quality standards and associated technical specifications be assessed and, if necessary, corrective action be implemented within a year. In practice, many sites found it difficult to comply with some of the requirements especially on treatment plant performance, as these do not take into account the natural variation of the influent water contamination levels. In Sainte-Maxime, compliance with the new regulation has led to an upgrade of the disinfection step with the number and power of UV lamps being increased. In Pornic, compliance with the new regulation and the need to improve effluent discharge quality to protect existing recreational water activities led to an upgrade of the entire process. The WWTP now comprises screening, aeration ponds, a membrane bioreactor with immerged UF membranes and UV disinfection.

Beyond compliance with water quality targets, safe operation of reuse projects requires that health risks be assessed and dealt with both from the raw wastewater to the point of use and from project design to full-scale operation. This is fully recognised by WHO, who promotes the development and implementation of Sanitation Safety Plans in order to control these risks (WHO 2010). The recent upgrade of the WWTP in Pornic has provided an opportunity to test such an approach. Indeed, to ensure that hazards are dealt with from the raw wastewater to the point of use, an internal risk evaluation and management tool has been developed and applied to this site. This tool follows the methodology proposed by WHO and consists of

• - A detailed hazard check list, including 102 microbial and physico-chemical hazards and their possible origin. This list is then simplified by parameter groups to achieve a check list of 20 hazards.

• - A risk evaluation matrix based on a scoring system (occurrence probability times potential impact).

• - An evaluation sheet to validate effectiveness of control measures and surveillance means.

• - A risk re-evaluation sheet that proposes corrective actions to deal with residual risks.

A series of questions following the multi-barrier concept guides the user through the evaluation of control measures from raw water to endpoint use. Gaps in the treatment process, potential failures and good practices concerning health and environmental protection may thereby be highlighted. For instance, hazardous events linked to UV operation, such as lamp degradation or increase of effluent turbidity, are controlled through maintenance and operational actions and checked by online measurements (see Table 3). In Pornic, the management plan shows that adequate surveillance of critical points (e.g. operational monitoring of turbidity of treated wastewater, UV dose of tertiary effluent) strengthens the effectiveness of control measures thereby ensuring better safety, especially in respect to microbial contamination.

Experience in France also shows that practical implementation of reuse projects requires careful consideration of different types of risks in order to be successful and economically viable. Whereas health risks and to some extend environmental risks are generally adequately covered by reuse regulations, some business risks can easily be overlooked if not adequately assessed and mitigated throughout the reuse project. Sainte-Maxime, where reuse has replaced potable water in a water scarce area to irrigate a golf course since 2007, provides an interesting example of how this type of risk has been successfully dealt with through collaboration between the golf course management team and the WWTP operator.

In Sainte-Maxime, operational difficulties encountered when switching from potable water to reuse included pond eutrophication, soil salinisation (1,100 ppm of soluble salts in reclaimed water), grass rooting and nutrient amendment. These difficulties have required changes to be made in irrigation practices which provide useful examples for other projects. To address eutrophication, for instance, a surface aerator, a stirrer, a wave diffuser and plankton eating fish species were installed in the pond instead of using chemical algaecides. To address soil salinisation, over irrigation of the turf has to be carried out in order to allow salt leaching from the plant root zone. The choice of salt resistant turf grass species also allowed preventing yellowing of the grass (see Figure 1). To improve grass rooting, which had drastically reduced during the first year of irrigation with reclaimed water, the soil aeration frequency was increased and soil porosity improved through the addition of sand, thereby reducing the occurrence of puddles and allowing a better grass rooting. Finally, the golf keepers adapted soil amendments to fit to treated wastewater contributions. The nitrogen's supply of reclaimed water, for instance, partly fulfils grass needs (see Table 4) and the use of synthetic fertiliser is now only needed during rainy winters when rainwater leaches nutrients from the soil. However, reclaimed water does not bring sufficient phosphorous, which still has to be addeed and calcium supplement is also added during aeration to overcome sodisation.

Overall, implementation of the above-mentioned good practices has enabled grass quality and visual aspect to be maintained while decreasing fertiliser use. The major benefit of the Sainte-Maxime project is the preservation of source water during droughts. More than 280,000 m³ per year of drinking water is now replaced by reclaimed water. This creates an economic gain for the golf course, as the price of reclaimed water (∼0.3 €/m³) is significantly lower than potable water. The nutrient content of reclaimed water also enables a significant decrease in synthetic fertiliser use. As a result, the 300k€ investment made by the golf course has been amortized over a 5 year period.

Finally, because water stress is moderate and unevenly distributed, water scarcity is not the only driver of reuse projects in France. Other drivers may include one or a combination of the following: increasing water demand, environmental protection and enhancement, socio-economic factors, etc. Château Renault, where reuse has been implemented to spray irrigate agricultural crops since 2010, provides an interesting example of a project where environmental protection was the major driver. In Château Renault, the WWTP (10,000 pop.eq.) was optimised in 2009 and a 50,000 m³ lagoon was created both to prevent effluent discharges in the nearby river during low-flow conditions and provide storage capacity for crop irrigation. The process now includes a screening step, an activated sludge process, a clarifier and a phosphate removal step with FeCl₃. Treated water is discharged to the lagoon located 1.5 km from the WWTP from end of June until beginning of September. The lagoon acts as a natural retention pond where retention time exceeds 20 days. The annual volume of 150,000 m³ is then spray irrigated on agricultural fields covering an area of 170 hectares.

Overall, reuse practice in Chateau Renault has enabled both to limit treated effluent discharge in the nearby river during the low-flow season and to reduce groundwater withdrawals by providing local farmers with an alternative source of irrigation water. This not only represents environmental benefits for the area, but also economic benefits mainly through productivity gains, especially for the farmers who did not irrigate previously. Wheat yield increases from 2 to 2.5 tonnes per hectare have been observed during the first year and reclaimed water irrigation has also allowed crop diversification, through introduction of rapeseed and barley. In terms of plant amendment, no operational changes have been observed so far, as N and P concentrations in the reclaimed water are quite low.

The French experience with reuse described above suggests that, although regulation is necessary to ensure health and environmental risks associated with reuse are adequately dealt with on the project level, it should be designed in such a way as to encourage reuse and not discourage it. It is argued here that a set of minimum quality standards for irrigation water irrespective of its source (groundwater, surface water or treated wastewater) complemented by a risk-based approach at the project level would be preferable to ensure both consistency and flexibility in terms of how risks are managed. Practical implementation of reuse projects also requires careful consideration of health, environment and other business risks in order to be successful and economically viable. Having a multi-disciplinary team of experts in the field in connection with strong operational know-how are thus necessary to address such key issues as demonstrated in our three case studies.