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Table 3

SWOT analysis of water reuse in hydroponic systems for German conditions: Weaknesses

Weaknesses
MaterialImmaterial
Water treatment
  • Proximity of plant production to the area of wastewater treatment/irrigation water preparation can cause an additional airborne loading of plants with pathogens via aerosols.

  • Salinity of treated wastewater should not exceed the plant requirements. Removal of excess salinity, for example by reverse osmosis, could be advisable. In such cases, the management of residues might also be necessary.

  • The additional treatment of wastewater also cannot guarantee that there are no pharmaceutical residues in the products because there are too many of them and their range is continually changing, making holistic analyses impossible.

  • Technical challenges presented by a wide range of influent water situations (dry weather and heavy rainfall).

Plant production
  • Adequate plant nutrition on a minimum feeding level needs close attention and control systems.

  • Certain nutrients need to be added artificially to secure adequate plant nutrition.

Environment
  • If a conversion of agricultural cropland to a greenhouse area takes place in an uncontrolled way, there will be negative ecological impacts such as the reduction of biodiversity and groundwater recharge, if no countermeasures are undertaken. Moreover, the landscape quality might decrease (Bürgow et al. 2019).

 
Operation and management:
  • The system is highly complex as two goals have to be achieved: wastewater treatment and plant production. This requires a high level of competence and monitoring.

  • Due to this complexity, quality assurance plays a major role and requires competent and dedicated staff in a field where untrained staff are often employed.

  • So far, there is no comprehensive reference and therefore no experience of a combined system like this.

  • The conversion of traditional farming to greenhouse production requires new knowledge and experience, and also needs the establishment of new professional networks, including sales structures.

  • There is a dependency between wastewater treatment/irrigation water production and plant production. At the moment, the two systems are combined; individual decisions in one of the two subsystems always have an influence on the other and need to be coordinated.

Economy
  • Synthetic fertilisers are cheap and, in areas with sufficient freshwater resources, there is usually little economic incentive for water reuse.

  • Barriers to entry in this new business field are relatively high initial investment costs for the greenhouses and the dependency on partnerships with WWTPs. So far, there is no experience of such partnerships in Germany.

Actors and institutions (law)Values and standards (sustainability)
  • The operator of a WWTP has to change perspective from disposal to provision and deliver a product that meets qualitative standards and quantitative requirements.

 
Weaknesses
MaterialImmaterial
Water treatment
  • Proximity of plant production to the area of wastewater treatment/irrigation water preparation can cause an additional airborne loading of plants with pathogens via aerosols.

  • Salinity of treated wastewater should not exceed the plant requirements. Removal of excess salinity, for example by reverse osmosis, could be advisable. In such cases, the management of residues might also be necessary.

  • The additional treatment of wastewater also cannot guarantee that there are no pharmaceutical residues in the products because there are too many of them and their range is continually changing, making holistic analyses impossible.

  • Technical challenges presented by a wide range of influent water situations (dry weather and heavy rainfall).

Plant production
  • Adequate plant nutrition on a minimum feeding level needs close attention and control systems.

  • Certain nutrients need to be added artificially to secure adequate plant nutrition.

Environment
  • If a conversion of agricultural cropland to a greenhouse area takes place in an uncontrolled way, there will be negative ecological impacts such as the reduction of biodiversity and groundwater recharge, if no countermeasures are undertaken. Moreover, the landscape quality might decrease (Bürgow et al. 2019).

 
Operation and management:
  • The system is highly complex as two goals have to be achieved: wastewater treatment and plant production. This requires a high level of competence and monitoring.

  • Due to this complexity, quality assurance plays a major role and requires competent and dedicated staff in a field where untrained staff are often employed.

  • So far, there is no comprehensive reference and therefore no experience of a combined system like this.

  • The conversion of traditional farming to greenhouse production requires new knowledge and experience, and also needs the establishment of new professional networks, including sales structures.

  • There is a dependency between wastewater treatment/irrigation water production and plant production. At the moment, the two systems are combined; individual decisions in one of the two subsystems always have an influence on the other and need to be coordinated.

Economy
  • Synthetic fertilisers are cheap and, in areas with sufficient freshwater resources, there is usually little economic incentive for water reuse.

  • Barriers to entry in this new business field are relatively high initial investment costs for the greenhouses and the dependency on partnerships with WWTPs. So far, there is no experience of such partnerships in Germany.

Actors and institutions (law)Values and standards (sustainability)
  • The operator of a WWTP has to change perspective from disposal to provision and deliver a product that meets qualitative standards and quantitative requirements.

 
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