Tertiary nutrient removal from wastewater by immobilised microalgae: impact of wastewater nutrient characteristics and hydraulic retention time (HRT)

Immobilising microalgal cells has been proposed as a process solution to overcome the barriers associated with the implementation of microalgae for wastewater remediation. This work evaluated the performance and remediation mechanisms of immobilised microalgae for continuous wastewater treatment under varying hydraulic retention times (HRT). Three domestic secondary wastewaters with differing concentrations of orthophosphate (PO4-P), ammonium (NH4-N) and nitrate (NO3-N) were treated by Scenedesmus obliquus immobilised within 2% calcium alginate. Trials were run in continuous operation at HRTs of 3, 6, 12 and 20 h. Removal rates for PO4-P improved with increasing HRT, with minimum residual concentrations of 0.3–3.1 mg·L−1 observed at 3 h and 0.01–0.2 mg·L−1 at 20 h. Ammonium remediation was not linked to HRT or NH4+ concentration with minimum residual concentrations of <0.001 mg·L−1. Reduction in NO3-N improved with increasing HRT, with minimum residual concentrations of ≤19.3 at 3 h and ≤0.4 mg·L−1 at 20 h. Remediation was achieved through a combination of mechanisms including biological uptake and precipitation as a by-product of photosynthesis and nutrient metabolism. As such, immobilised microalgae have been proven to be an effective alternative solution for PO43− and NH4+ remediation of wastewater effluents at HRTs of 6–12 h

Processi biologici avanzati per il trattamento di reflui ad alto contenuto di nutrienti e valutazione delle emissioni gassose

La diffusione di impianti di digestione anaerobica in ambito agricolo ha determinato la necessità di individuare post-trattamenti dei surnatanti, per ridurre le pressioni ambientali legate allo spandimento dei digestati, in accordo ai limiti in azoto N imposti dalla Direttiva 91/676/CEE. Le prestazioni del processo automatizzato di nitritazione-denitritazione in continuo in reattore unico sono state valutate su un pilota da 3 m3, alimentato distintamente con surnatanti anaerobici di origine bovina e suina, entrambi puri ed addizionati con siero di latte. I risultati hanno evidenziato, per ogni matrice, rimozioni di N tra l’80 ed il 90%, mentre il monitoraggio delle costanti cinetiche Kn e Kd ha confermato che il processo ha operato stabilmente al 90% via nitrito. I valori numerici delle costanti, circa tre volte i valori ottenibili da processi biologici convenzionali, sono variati a seconda del refluo trattato, diminuendo al ridursi dell’azoto ammoniacale N-NH4 influente. L’analisi dell’effluente ha evidenziato inoltre rimozioni del fosforo P dell’ordine del 72%, e riduzioni di COD e TSS comprese tra il 60 ed il 70%. Il monitoraggio in continuo delle emissioni gassose, calcolate come % dell’N-NH4 influente, ha permesso di riscontrare valori pressoché nulli di biossido di azoto NO2, ed in media inferiori allo 0,011% e 0,009%, rispettivamente per protossido (N2O) e monossido (NO) di azoto. I risultati mostrano che le emissioni aumentano al crescere della concentrazione di nitriti in fase liquida, per carichi maggiori di N-NH4 in ingresso. Inoltre, la rimozione terziaria dei nutrienti da parte della microalga Scenedesmus Obliquus, immobilizzata in alginato di calcio, è stata valutata su fotobioreattori a letto fluido ed impaccato, alimentati in continuo in modalità upflow, senza immissione di aria né CO2. I risultati hanno mostrato rimozioni di N e P superiori al 95% e facilità nella separazione della biomassa algale dal flusso liquido, per un possibile riutilizzo.The widespread application of the anaerobic digestion process for the stabilisation of zootechnical waste determined the need to define possible treatments techniques of the liquid supernatant, to reduce the nitrogen N content and hence the pollution due to its spreading, according to the 91/676/ECC Directive. The removal performances of the continuous nitritation-denitritation automated process were evaluated in a single reactor of 3 m3, separately fed with anaerobic supernatants from cattle and swine wastewater, both pure and added with dairy waste. The results showed N removal between 80 and 90%, for each matrix, while the monitoring of the kinetic parameters Kn and Kd confirmed the nitrites pathway, with stable NO2-N contributions of 90%. The numerical values of Kn and Kd were about three times the typical values of conventional biological processes, but a variation with the influent wastewater change was observed: basically the parameters reduced at lower ammonia nitrogen NH4-N influent load. Also, the effluent characterisation determined total phosphorous P removal of 72%, with COD and TSS reductions between 60 and 70%. The continuous monitoring of the gaseous emissions, expressed as % of the influent NH4-N, identified the almost absence of nitrogen dioxide (NO2), and amounts of nitrous oxide (N2O) and nitric oxide (NO) lower than 0.011% and 0.009% respectively. Data highlighted that emissions increased with the higher NO2-N concentration in the liquid phase, linked to greater NH4-N influent loads. Moreover, the polishing performance of the microalgae Scenedesmus Obliquus, immobilized in calcium alginate spheres, was evaluated on fluidised and packed bed photobioreactors, continuously fed, without air or CO2 addition. The results highlighted N and P removals on the average equal to 95%. In addition, once the treatment maximum capacity of the beads was reached, the separation of the algal biomass from the liquid effluent, for its reuse, was easily achieved

Seawater physics and chemistry along the Med-SHIP transects in the Mediterranean Sea in 2016

The Mediterranean Sea has been sampled irregularly by research vessels in the past, mostly by national expeditions in regional waters. To monitor the hydrographic, biogeochemical and circulation changes in the Mediterranean Sea, a systematic repeat oceanographic survey programme called Med-SHIP was recommended by the Mediterranean Science Commission (CIESM) in 2011, as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Med-SHIP consists of zonal and meridional surveys with different frequencies, where comprehensive physical and biogeochemical properties are measured with the highest international standards. The first zonal survey was done in 2011 and repeated in 2018. In addition, a network of meridional (and other key) hydrographic sections were designed: the first cycle of these sections was completed in 2016, with three cruises funded by the EU project EUROFLEETS2. This paper presents the physical and chemical data of the meridional and key transects in the Western and Eastern Mediterranean Sea collected during those cruises