Treatment of raw table olive processing wastewater with mixed microalgae-bacteria cultures in open ponds

In this research study, the biological treatment of agro-industrial wastewater using photosynthetic microorganisms was investigated. The wastewater originated from a table olive processing company and consisted mainly of brine generated during the processing procedure, characterized by high salinity and high organic and phenolic loads. Given that, a marine microalgae culture, naturally derived from high salinity seawater environment, was employed. Two different wastewater streams were examined, originating from the “Kalamon” and “Halkidiki” olive varieties. Both wastewater streams were used without sterilization and were diluted with filtered natural seawater prior to treatment.

Experiments were conducted in a pilot-scale open pond reactor made of stainless steel, with a working volume of 40L. Mixing and aeration were provided by a rotor-driven paddle-wheel, while continuous illumination was supplied by LED lamps. Batch reactor operation was investigated under similar initial dissolved chemical oxygen demand (d-COD) concentrations of about 500 mg/L for both wastewater streams over a 12-day experimental period.

The removal of organic load and other pollutants was monitored over time, while biomass production and bioproduct accumulation were simultaneously evaluated. The d-COD removal efficiency was similar for both wastewater streams, reaching approximately 55%. Phenolic content was reduced by 55-60%, while N-NO₃^- removal reached up to 70%. Color removal was satisfactory but relatively limited, at approximately 30%. The final biomass concentration reached up to 255 mg/L, while chlorophyll-a content increased to around 1.5% of dry biomass. In addition, protein and lipid contents were relatively high, reaching up to 35% and 37% of dry biomass, respectively.

Only a limited number of studies have investigated the use of microalgae for the treatment of highly polluted olive-processing wastewaters of this type. Mixed cultures of microalgae and indigenous bacteria demonstrated good tolerance to these substrates, achieving significant pollutant removal while simultaneously accumulating high contents of high added-value bioproducts.