Assuring water quality along multi-barrier treatment systems for agricultural water reuse

Based on three pilot- and demonstration-scale projects investigating agricultural irrigation practices with reclaimed water, risks associated with these water reuse practices are highlighted and processes and strategies to minimize associated microbial risks were evaluated. A number of treatment processes and combinations were tested regarding their ef ﬁ cacy for pathogen removal, representing the biggest threat to the quality of products from reuse irrigation practices. In addition, the importance of regrowth potential and different methods for monitoring risks associated with pathogens were discussed. One method for online monitoring is ﬂ ow cytometry. The results of an exemplary quantitative microbial risk assessment (QMRA) were discussed to determine the signi ﬁ cance of microbial risks. Multi-barrier approaches comprised of technical and administrative barriers can reduce the risks of water reuse signi ﬁ cantly. Quality management also needs to address all stakeholders involved in a reuse project, starting from source control in the sewershed to marketing of the ﬁ nal products. In addition, environmental risks of water reuse need to be addressed by quality management as well.


INTRODUCTION
Global challenges such as climate change, population growth, and an increasing shift toward urbanization are resulting in an increased demand of water resources for agricultural and industrial purposes. Thus, the gap between the growing need for water supply and the existing limited water resources is widening continuously. With this growing demand, fresh water supplies are becoming an increasingly scarce resource in many regions worldwide including areas with a high population density, e.g. mega cities. Due to perceptible climate change, the lack of available water resources will be of even greater importance worldwide in the near future, as the frequency of extended droughts and heavy rainfalls is becoming a more widespread phenomenon even in moderate climate zones. According to the FAO (), agricultural irrigation needs an average of 70% of fresh water supplies worldwide. Thus, water reuse focusing on reducing the amount of conventional fresh water supplies is increasingly considered to avoid conflicts due to local supply shortfalls while implementing a sustain-

MICROBIAL RISK ASSESSMENT AND MANAGEMENT
Water reuse for agricultural irrigation is associated with an elevated risk due to the presence of pathogens in municipal wastewater. Thus, it is paramount that the microbial risk in these applications is properly managed. Microbial risk assessment combines scientific knowledge about water- In order to properly manage the overall risk and to achieve the health-based target, a multiple barrier approach can be adopted which is based on a number of measures that together avoid or reduce the risk of contamination by the use of recycled water (WHO ). This is achieved by control measures at various locations along the entire production chain to establish effective barriers against the spread of pathogens (NHMRC ; Health Canada ).

QUALITY MANAGEMENT IN AGRICULTURE
In the technical debate, it is assumed that the 'fit for purpose' approach entails the production of reclaimed water to a quality that meets the needs of the intended end-use (ISO :). In traditional commercial water supply sectors, quality control typically involves monitoring of the quality of the water produced at one or more locations in the process. For this purpose, the supplier either sets quality standards itself or negotiates them with its customers.
Another possibility is that the standards are specified by technical norms or legal requirements. In the commercial agricultural business context, quality management is increasingly built up very systematically and comprehensively. However, extending these protocols to water reuse applied in the context of agricultural irrigation has not been developed systematically. A successful water reuse project not only depends on reliable treatment processes employed but also on proper organizational procedures and strict compliance with regulatory requirements while also meeting customer expectations continuously. These different aspects can be addressed under the framework of quality assurance. Quality assurance is comprised of an early identification of hazards or the avoidance of hazards.
If this is practised in an organized fashion, it is referred to as quality management. Quality management comprises the initiation and safeguarding of organizational measures that serve to ensure the provision of services and the underlying procedures and processesand thus also the quality of the final product. Guaranteeing desired quality requirements while also involving all stakeholders in quality management is, therefore, a central prerequisite for the consideration of water reuse in agriculture. The EU Commission has stressed this point as a major obstacle to further growing agricultural water reuse across Europe (Council of the EU ).
In this paper, we discuss how effectively water quality can be assured in a multi-barrier approach by analyzing the practices of different agricultural water reuse practices as part of three large-scale research projects ('HypoWave', 'EPoNa', and 'MULTI-ReUse') funded by the German Federal Ministry of Education and Research (BMBF). For these water reuse case studies, an overview of the technical and administrative barriers is given and their effectiveness is assessed. As far as process-related barriers are concerned, we investigated to what extent quality management can improve the process results. In addition, risk management is applied to assess the entire chain of wastewater collection, treatment, and conveyance up to the final application to agricultural products.
Dilution was done by deionized water (after 0.2 μm sterile filtration). Three replicates per sample and per bacteria group examined were generated, and corresponding blank samples were collected for each day of analysis. The analysis was based on eight sampling days. Taking the replicates into account, a total of 24 measuring results of each sampling point and each bacteria group was obtained.
How the HypoWave concept can be implemented has been examined in the context of a case study in Germany.
The wastewater of the village of Weißenberge (Gifhorn county, approximately 500 inhabitants, 26,400 m 3 /a) is currently being discharged into a system of treatment ponds ( Figure 2). In case of dry weather conditions, the residence time in the system is more than 90 days. The effluent of the plant still contains relatively high concentrations of nutrients such as nitrogen and phosphorus, but due to the long residence time only low levels of pathogenic microorganisms. In an activated carbon biofilter, the nitrogen, which is largely present as ammonium-nitrogen, will be oxidized to nitrate by aeration and thus be made more readily available to plants. In order to reduce the risk of pathogens, the UV-disinfection process will be used after the biofilter.
The treated water will then be used in hydroponic vegetable production, which also will utilize and remove the remaining nutrients. Eggplants, zucchini, tomatoes, peppers, and lettuce are potential crops for this application. For the case study of Weißenberge, a QMRA was carried out.
(B) In the EPoNa project, the town of Outapi in Northern Namibia has been selected. The town has flush toilets and a gravity sewer system in its formal districts. The household wastewater is discharged to waste stabilization ponds (WSP). Population growth and the overflowing of the ponds during the rainy season are challenges for the functionality of the system. Since the mere evaporation of the water is effectively wasting a valuable resource, water reuse for irrigation purposes is considered to be a resource-efficient option in the semi-arid region according to the Namibian Water Resources Management Act (Act No. 24 of 2004).
The WSP system comprises two parallel treatment trains For this reason, the OOWV started working on concepts for a more efficient water supply to deliver their customers with a water quality adapted to their needs.
Beside drinking water, this includes different advanced treated water types of a defined quality using tertiary effluent from the municipal WWTP Nordenham (36,000 population

RESULTS AND DISCUSSION
Multi-barrier systems Table 1 provides a summary of barriers employed for the projects EPoNa, HypoWave, and MULTI-ReUse. In addition to wastewater treatment, the barriers in the area of wastewater collection and agricultural application are also displayed. In addition to technical barriers, administrative barriers were also used in all projects; for example, in some applications, the risk of hazardous substances in the sewage is reduced by monitoring the connections to the sewer network, and in other applications, the crops that are irrigated are not for human use. In the following section, different technical barriers regarding pathogens are discussed, an exemplary QMRA is executed followed by a brief discussion of quality management and administrative barriers.

Performance of treatment processes regarding pathogens
The main goal of these investigations was to highlight how water could be reused in agriculture while properly managing associated health risks. Thus, treatment processes as technical barriers have been tested with a focus on pathogen removal. The feed water used in the HypoWave project was characterized by the following bacteria concentrations: total aerobic count ranging from 10 6 to 10 7 CFU/ml, EB and total coliforms from 10 5 to 10 6 CFU/ml, and E. coli from 10 4 to 10 5 CFU/ml. These concentrations are representative of municipal raw wastewater exhibiting typical E. coli concentration of 10 3 to 10 6 CFU/ml and 10 5 to 10 7 CFU/ml for total coliforms, respectively (WHO ; Asano et al. for an agricultural product provided for raw consumption, the focus of the QMRA was on a high exposure to health risks for the consumer. If the residual risk of the Gifhorn concept is below 10 À6 DALYs, every other hydroponically grown product in the same system could be considered to be hygienically safe as well.
Under the assumption that 100 g of lettuce is consumed in a 2-day rhythm, the risks of infection with Cryptosporidium, Campylobacter, and Norovirus are shown in Figure 7. While the risk of infection with Campylobacter is lowest, the risk of an exposed person becoming infected by a virus seemed to be highest.
In order to verify whether these QMRA results concerning the consumption of hydroponically grown lettuce complied with the maximum additional health risk of 10 À6 DALY, the DALY value was converted into a maximum tol- Nevertheless, the QMRA was applied repeatedly and proved to be a reliable and multifunctional method of risk forecast and risk assessment. Taking into account that the quality of the irrigation water is a decisive factor for the hygienic quality of the agricultural product, it is recommended not to exceed a value of 10 À5 concerning the infection risk per person per year for all pathogen groups. Risks for the staff working in the greenhouses, e.g. for maintenance and harvesting, have to be taken into account as well.
These results suggest that conventional quality assurance must be supplemented by further measures to ensure that quality targets are reliably achieved in practice. In this context, flow cytometry is seen as a suitable technology for online quality and performance assurance due to its high data resolution and degree of automation. Although The guarantee of desired quality requirements vis-à-vis the cooperation partners is a central prerequisite for the consideration of water reuse in agriculture. In the agricultural business context, quality management is increasingly built up very systematically and at the same time in a far more comprehensive way than is necessary for quality assurance.
This approach has not yet been systematically developed for water reuse in agriculture (Schramm et al. ).
Risk assessments in accordance with QMRA can be incorporated into the quality assurance systems for food production introduced at the European level. This can be used to ensure the quality of the irrigation water. For the European Union, the regulation (in course of legislation) on water reuse will provide a uniform framework for the first time; disinfection barriers will now be mandatory for most applications (Council of EU ). The approach of risk-based and process-oriented management of EN ISO 9001 can be used as a basis.
With regard to the scope of quality management, it is necessary to make an explicit distinction between environmental prevention and health care. Environmental policy is primarily interested in environmental precautions. The minimization of environmental risks has so far been given only limited consideration in the existing instruments, as the EU regulation on water reuse makes clear.

CONCLUSIONS
The prime focus of quality management in agricultural water reuse is on the removal of pathogens. A number of technical processes for the elimination of pathogens from water exist and have been applied in the involved research projects. Membrane filtration, e.g. UF, has proved to be a reliable barrier against pathogens. Multi-barrier approaches combine the advantages of single processes and make the reuse concept more resilient against disturbances.
Especially aerobic treatment processes and stabilization ponds have a strong additional removal efficiency for pathogens, but also slow filtration processes, UV and ozone help reduce the concentration of pathogens in water. According to the individual reuse case, an appropriate combination of these processes should be chosen. In addition to technical barriers, administrative barriers are important to ensure efficient and safe water reuse. For administering and managing water quality, the question arises as to which institutional arrangements are needed in order to make quality management durable and with which processes it can be successfully coordinated. If municipal wastewater is reused in agriculture, different actors usually carry out quality assurance in parallel. In the long run, it will make sense to coordinate the quality assurance of the various actors along the emerging 'water Monitoring can be another barrier, too, especially if the results are available in time to react accordingly. The assessment of the microbiological water quality status directly after treatment should be supplemented by measurement of the regrowth potential. Regrowth potential is currently not assessed on a regular basis, but a standardized procedure for initial testing would prove highly beneficial.
Despite standardization, sufficient flexibility should be provided as the relevant conditions (e.g. temperature) of the particular water application have to be considered. Flow cytometry is a valuable diagnostic tool to assess whether bacteria originate from regrowth or from the wastewater.
Digital process control systems can contribute to quality management. QMRA is a valuable tool to identify potential threats for a given water reuse case. The barriers can be chosen based on the results of this analysis.