A single uniform seedling was transplanted into each pot. So that salt would not accumulate in the root zone, a 15% leaching fraction was allowed whenever irrigation was conducted; irrigation water was therefore applied at more than the amount of field capacity . The water that drained from each pot was collected in an empty container located beneath the pot. That water was weighed and considered to represent the volume of penetration. Salinity treatments and media were imposed on 17 June 2012 and 9 July 2013. Complete Grow-More fertilizer  was added at 1.5 g/L after appearance of the fourth leaf. An automatic weather station was installed in the central part of greenhouse to measure net radiation, air temperature, and relative humidity. The maximum and minimum temperatures of in the greenhouse were determined by a maximum and minimum thermometer and recorded once in each 24 h. The relative humidity was measured by a hydrograph and recorded every 2 h. In addition, a thermometer measured temperature continuously and was read when necessary. A pyrgeometer CM7B and an albedometer CG1/2 were used to measure and record long and short wave lengths and net sun radiation once each 24 h during the growing period. These devices were connected to four integrators  to record the data. The Penman–Mantith-FAO method was used to estimate the reference plant evapotranspiration, after Harmanto, Salokhe, and Tantauc , who used this method for measuring evapotranspiration in greenhouses.

The equation used Savvas and Lenz  and Unlukara, Kurunc, and Yurtseven  found similar results, showing that the salinity threshold tolerable for eggplant in hydroponic cultivation is 1.5 dS m1,whereas Moazed et al. suggested 2.5 dS m
1 ; our results suggest the same value 2.5 dS m1 . The relationship between yield and evapotranspiration under saline condition showed that the eggplant salinity stress coefficients in coco-peat and the coco-peat–perlite mixture are higher  than those in perlite . Stewart and Hagan  have proposed a model to predict crop yield by using evapotranspiration rate during the plant growing season. According to this model, flood and drain table the relation between relative evapotranspiration and relative yield is that the yield may decrease due to water stress. According to the repots  the yield response factor  has been used to evaluate plant tolerance to water stress . As reported , when Kyr1, it indicates that the plant is tolerant to water stress and if KyZ1, it indicates that the plant is sensitive to water stress. Other scientists  have used same analytical method for salinity related studies.  found that eggplant is moderately sensitive to salinity. coco-peat’s extended and fine porosity, although it leads to greater accumulation salt content and higher salinity saturated extract than shown by perlite, also permits greater water-holding capacity under the same salinity conditions. Of the three growth media we used, coco-peat provided the best growth conditions because of its physical and chemical properties, including water- and air-holding capacity and low volumetric weight. coco-peat has 50–100 times the cation exchange capacity of perlite. It provides more stability of pH, which affects absorption of nutrients . The highest yield was obtained in coco-peat and the lowest in perlite, perhaps because of coco-peat’s higher field capacity as measured by gravimetric methods. Increasing electrical conductivity of irrigation water leads to decreased plant evapotranspiration and water use efficiency.

Allen et al.  reported that presence of salt in soil water solution decreased evapotranspiration, causing the plant to use more energy to obtain water from soil. The presence of salt decreases the potential energy of soil water solution. The interaction effects between salinity and growth medium showed that coco-peat and the coco-peat–perlite mixture did not differ significantly in production of yield, whereas perlite produced less yield.The climate of the Nordic countries presents several challenges to agriculture practices. In Finland, the winter season can last from 4 to 6 months and is often characterized by freezing temperatures, a lack of natural sunlight and heavy snowfall. To cope with this harsh environment, a significant part of horticulture production occurs in greenhouses equipped with artificial light and regulated temperature systems, guaranteeing year-round food availability. Hydroponic farming is often preferred to improve water management compared to traditional practices, and to reduce the need for mineral fertilizers. Nonetheless, hydroponic greenhouse effluents are still overloaded with nutrients, containing up to 1000 mg L− 1 of nitrate and over 200 mg L− 1 of phosphate, with much room for improvement in dosage procedures. Recently, novel strategies have been proposed to mitigate pollution caused by agriculture waste waters. These involve using photosynthetic micro-algae to simultaneously: recover nutrients, purify water streams, and generate biomass for industrial application. Photosynthetic micro-algae and cyanobacteria are a diverse group of microorganisms able to convert sunlight into chemical energy. Through this process, the cells assimilate inorganic carbon and nutrients while generating biomass rich in organic molecules such as proteins, lipids, carbohydrates and pigments. Despite the industrial potential of these bio-active molecules, the large-scale production of micro-algae is still limited by high operation costs. This currently limits the potential use of micro-algal biomass, which has been proposed as alternative sources of food or nutrition, animal feed, commodity chemicals or bio-fuels.

Several life cycle assessment  studies have identified the costs associated with the supply of nutrients, fresh water and energy as some of the most important constraints to fiscal viability. These parameters, together with the scale of production and market applications, can drastically increase the cost of production to values ranging from 69€ to over 400€ kg− 1 DW. In order to reduce production costs, liquid waste streams containing a suitable nutrient composition  could be used as substitute for mineral fertilizers and fresh water. When combined with optimized cultivation and a suitable production scale, this alternative approach can decrease the cost of production to just 1.4€ kg− 1 DW. Biologically, the demonstrated resilience of photoautotrophs to different wastewater streams indicates their aptitude to nutrient reclamation and environmental remediation processes. Therefore, the use of micro-algae and cyanobacteria to recirculate agricultural effluents presents an opportunity to mitigate environmental pollution, while simultaneously decreasing the costs of biomass production. Under harsh Nordic conditions, the existing greenhouse infrastructure, already targeted to optimal food production, likely provides advantageous conditions for the concomitant farming of micro-algae and cyanobacteria. Use of artificial light for the production of micro-algae enables optimized productivity and can limit the biochemical variability of the biomass product which might otherwise result from natural climatic fluctuations. Thus, the use of greenhouse infrastructure to improve the yield of a cultivation system presents an opportunity to decrease production costs and ensure the reproducibility and reliability of biomass supply. Indeed, overall, it appears that the sustainability of horticulture cultivation can be greatly improved by integrating micro-algae cultivation into greenhouses for bio-remediation of hydroponic effluents. In this study, the feasibility of using Nordic cyanobacteria and micro-algae for bio-remediation of hydroponic effluents from a commercial cucumber greenhouse was evaluated. A laboratory scale rapid screening of the Nordic culture collection was performed to prove the general suitability of a variety of Nordic strains to growth in hydroponic effluent. Pre-treatments were then investigated to identify suitable options to deal with microbial communities that are commonly found in waste streams.

Finally, a pilot-scale cultivation in a real greenhouse environment was used as a proof of concept to demonstrate the feasibility of integrated micro-algal wastewater reclamation and greenhouse cultivation in a Nordic climate. A total of 13 strains of cyanobacteria and eukaryotic micro-algae from NordAqua Nordic Culture Collection database  were selected for a screening trial . These strains belong to the University of Helsinki Culture Collection  and the Norwegian Culture Collection of Algae.Strains were maintained in 100 mL Erlenmeyer flasks with 50 mL of media. Z8 medium was used for NORCCA strains, while HAMBI strains were kept in Z8X  or BG11 medium. The cultures were maintained and pre-cultivated in continuous low-intensity light  5–10 μmol m− 2 s− 1 at room temperature  before the experimental screening. The strains were pre-cultivated in the respective culture media to guarantee good proliferation rates. The medium Z8 has a N:P ratio of 24 while Z8x contains only residual amounts of N-NO3− from its trace metal stock solution. Additionally, the nitrogen composition of the hydroponic effluent was mainly dominated by the same N-source and the pH was similar to the culture media. For those reasons,rolling bench an adaption period before the experiment was not performed, as the purpose of the trial was to identify robust strains that could survive and proliferate in unadjusted hydroponic effluent. A tubular PBR was constructed in a research greenhouse belonging to the Natural Resources Institute of Finland in Kaarina, Finland. A detailed description of the PBR can be found elsewhere . Briefly, the tubular PBR made of PLEXIGLAS-XT acrylic holds a volume of 65 L and is equipped with electronic CO2 solenoid valve injection and inline sensors that allow an effective real-time monitoring of OD at 880 nm , OD at 680 nm , temperature, pH and flow. All sensors are controlled via NI LabView based software which records data points at 10 s intervals and displays a real-time graphic representation of each parameter. The footprint of the PBR is 2 m by 0.5 m. The LUKE glass greenhouse is equipped with artificial light, heating and cooling systems that are programmed to maintain constant abiotic conditions. The heating system circulates hot water through a pipeline spread across the facilities.

Cooling is achieved either by roof vents that allow air circulation, or fan cooling with ethylene glycol circulation. These systems maintained a constant temperature of 20–25 ◦C. The greenhouse compartment is also equipped with 15 high pressure sodium bulb lights  evenly distributed in three rows. The height of the light bulbs was adjusted to provide an average PPFD of 200 μmol m− 2 s− 1 at the top surface of the PBR. The PPFD was adjusted at night, using a PPFD meter  and a quantum sensor . The system was programmed to provide a 17 h photoperiod, following the conditions of cucumber cultivation. The greenhouse has a dedicated meteorological station which monitors and records the outside weather throughout the year. In order to prevent photo inhibition due to excessive light, a threshold of 500 W m− 2 of solar irradiance was established upon which the high-pressure sodium bulb lights would automatically shut down. Six cyanobacterial and seven micro-algal strains from HAMBI and NORCCA were screened to evaluate the growth performance in the hydroponic effluent. The effluent was freshly collected prior to each experiment, and therefore nutrient concentrations varied during the study. Table 1 shows the chemical composition of the greenhouse hydroponic effluent used in the screening trial. The N:P ratio of the effluent  was slightly above the general optimum Red field ratio for phytoplankton growth. The nitrogen composition of the effluent was mainly dominated by a single N-source, nitrate, whereas both N-NH4+ and N-NO2− concentrations were negligible. The effluent used in this study had a lower content of organic matter  than other reported sources of agricultural greenhouse effluents. The screening trial demonstrated that the hydroponic effluent from cucumber cultivation is suitable for the growth of eukaryotic micro-algae and cyanobacteria . The genus Tetradesmus demonstrated the best growth with OD750 = 0.77 on the fifth day followed by Selenastrum and Apatococcus genera which demonstrated steady growth but reached a slightly lower OD . The strains of Scenedesmus genus showed a similar performance achieving a maximum value of OD750 of 0.53 and 0.55 respectively by the end of the fifth day. The genera Monoraphidium and Chlorococcum displayed slow growth throughout the course of the experiment reaching a final OD750 of 0.45 and 0.42, respectively. Among the studied cyanobacterial strains, UHCC0492 showed the most rapid growth reaching an OD750 similar to the best eukaryotic strains by the fifth day . The Microcystis genus had a lag phase of two days followed by a short phase of linear growth, and reached a stationary phase before the end of the trial. The two filamentous, heterocystous N2-fixing Nostoc strains both displayed slow growth, although no lag phase was observed. This outcome will be further investigated as it would be expected that a sudden availability of macro-nutrients from the hydroponic effluent would improve the growth performance of these strains.