Gaston Picard

Harvesting marine microalgae species by chitosan flocculation

Abstract Marine microalgae are still an important larval feeding source. One of the most promising harvesting techniques of the algae produced appears to be chemical flocculation. We report results obtained with chitosan flocculation of five marine species of microalgae of importance to mariculture (Skeletonema costatum, Dunaliella tertiolecta, Thalassiosira nordenskoldii, Chlorella sp. and Thalassionema sp.). The algae were grown in the laboratory in 20-liter batch cultures under normal conditions in artificial seawater. Without pH control, a 100% flocculation efficiency was reached at fairly high chitosan concentrations (above 40 mg liter−1). When the final pH was adjusted to around 7·8–8·0, a 100% flocculation efficiency was obtained with chitosan concentrations of 40 mg liter−1 or more. However, when pH was adjusted to around 7 or less, prior to chitosan addition for S. costatum and Chlorella sp., the concentration of chitosan required to obtain a 95–100% flocculation efficiency was reduced to 20 mg liter−1 for Chlorella and 2 mg liter−1 for S. costatum. The results are discussed in the light of the currently accepted theories on flocculation.

A comparative study of four systems for tertiary wastewater treatment by Scenedesmus bicellularis: New technology for immobilization

Immobilization appears to be one of the best techniques to separate physically micro-algal cells from their culture medium for the purpose of algal tertiary wastewater treatment. High operation costs and other drawbacks of large-scale physico-chemical methods of harvest led to a comparative study of biotreatment systems. Before treatment began, Scenedesmus bicellularis cells were conditioned (starved) under four different sets of conditions: 1) non-immobilized cells with air bubbling (NCA); 2) cells immobilized in alginate beads (CBW) and 3) cells immobilized on alginate screens (CSW), all conditioned in synthetic culture medium depleted in N and P; 4) cells immobilized on alginate screens but conditioned in air at 100% relative humidity (CSA). Starvation was started under a light:dark photoperiod of 16:8 h. Starved cells were then used to treat wastewater for a 2-h period. The performance of each system was evaluated by determination of residual NH4-N and phosphate ions and by growth (dry weight, total chlorophyll, cell count, protein content). We then tested the capacity of microalgae immobilized on screens to eliminate N and P from a secondary municipal wastewater effluent and examined the influence of temperature and starvation. The quality of treated effluents was improved considerably with the system using CSA or CSW model. For CSA model, the protein content was 22.4 pg cell-1 compared to 12.9, 9.5, 9.1 pg cell-1 for NCA, CBW and CSW models, respectively. The CBW and CSW models were efficient for chlorophyll synthesis. The residual ammonium content in natural wastewater after 2 h of treatment with CSA model was 39% at 6±2 °C and reached 100% removal at 18±2 °C. With the first 2 h, the removal of orthophosphate was inferior (53%) at 6±2 °C, but 88 to 100% at 18±2 °C depending on starvation times. Long starvation times (72 or 96 h) caused damage to cells and uptake of nutrients was lower than with 54 h starvation. This work demonstrates that by using immobilization on screens, removal of nutrients from wastewater was higher than with conventional biological tertiary wastewater treatments (free cells or bead-shaped alginate particles).

Intensive cultivation of freshwater microalgae on aerated pig manure

Abstract The aim of this work is to find a satisfactory method for biologically recycling pig manure with aerobic fermentation followed by freshwater microalgal ( Scenedesmus sp.) culture. The biomass produced could be incorporated into animal feeding. The results obtained with batch algal culture on manure aerated for 2 days are good for nutrient removal as well as for biomass production and can be improved by a semi-continuous mode of culture. Using a manure that has been aerobically fermented for 8 days as a substrate for very dense algal suspensions is, however, the best solution; dilution has to be reasonably small (1/4), initial NH 4 + concentration can be high (15 m m ) without interfering with the algal culture, the biomass of which is about 5·0 g dry weight litre −1 after 8 dayss. Nutrient removal is up to 3·3 mmol litre −1 day −1 (59 mg litre −1 day −1 ) for N - NH 4 + . It appears that the use of hyperconcentrated algal cell suspensions grown on slightly diluted aerated manure is a promising way of simultaneously treating pig waste and producing good quality biomass.

Stability of chitosan gel as entrapment matrix of viable Scenedesmus bicellularis cells immobilized on screens for tertiary treatment of wastewater

An experiment with immobilized microalgal cells was carried out using high and low viscosity chitosan and konjac flour to enhance the stability of hardened gels during tertiary treatment of wastewaters containing a high concentration of phosphate salts (1 m Na3PO4). Mechanical properties as well as compression testing, moisture retention in gels and the uptake of nutrients, such as nitrogen and phosphorus, from artificial wastewater were studied in the immobilized gel state. The disruption due to chemical interactions of pure high and low viscosity chitosan or mixed gels with konjac flour was examined daily by microscopic counting of the total cells released in the medium during seven weeks. The removal of NH+4-N and PO3−4-P was monitored during four nutrient starvation-removal cycles. A complete cycle consisted of starving immobilized cells for 48 h in air saturated at 100% relative humidity, followed by removal of nutrients from wastewater. The immobilization obtained from 2% (m/v) high viscosity chitosan gels showed a more significant chemical stability (P ≤ 0.05) in medium with 1 m Na3PO4 that contained viable cells than low viscosity chitosan or mixed gels. Sodium pyrophosphate was the best chelating agent used for cross-linking and did not affect the diffusion of inorganic nutrients through the hardened gels or the microalgal growth inside the network of chitosan immobilized cells. After the second uptake, the elimination of NH+4-N and PO3−4-P was 100% within 3 h, whereas after the fourth uptake, the nitrogen disappearance was 99% within 2 h and 100% for phosphorus with the same incubation time. After more than one month incubation of chitosan gel beads containing Scenedesmus bicellularis cells that had been soaked daily in a fresh 1 m Na3PO4 solution, the stability of hardened gels was not affected and no significant release (P ≥ 0.05) of entrapped cells was observed in the medium.

Effect of intermittent CO2 enrichment during nutrient starvation on tertiary treatment of wastewater by alginate-immobilized Scenedesmus bicellularis

A method of accelerating the removal of ammonium and phosphate by the unicellular microalga Scenedesmus bicellularis is presented for municipal tertiary wastewater treatment using immobilized cells to obtain a high quality of effluents. Microalgal cells grown in defined medium were harvested by centrifugation and stored at 4°C in the dark for 8 months before immobilization. The concentrated cell suspension was then immobilized in alginate films supported on polypropylene screens. Immobilized cells were incubated in a water-saturated air stream enriched with CO2 at 750, 1,000, or 1,500 ppm for 3 h periods followed by 2 h periods without enrichment. The quantitative effects of these three CO2 enrichments on nutrient uptake from secondary municipal wastewater effluent were compared to a control laboratory air at 320 ppm under the same conditions of illumination, photoperiod, and humidity. The exposure cycle of 48-h nutrient deprivation in air with CO2 enrichment followed by 2 h of nutrient uptake from wastewater was repeated three times with a residual NH4N content dropping to 0% after 105 min for the 1,500 ppm CO2 treatment and to 34% of the initial level after 120 min for the control treatment. Complete PO4P removal required more than 2 h. The chlorophyll a contents obtained with 1,000 and 1,500 ppm CO2 enrichments were comparable. This study establishes that intermittent CO2 enrichment during nutrient deprivation of immobilized microalgal cells in a water-saturated air stream may accelerate tertiary wastewater treatment.

Influence of agitation and aeration modes on biomass production by Oocystis sp. grown on wastewaters

Abstract Production of biomass with simultaneous wastewater treatment is feasible through recycling using algal culture. The productivity of such a system depends upon many parameters; two of them, turbulence and CO 2 regeneration, are studied here at the laboratory scale. Mechanical agitation with an air atmosphere over the culture leads to a low biomass production whereas bubbling with air produces a steady biomass increase, the rate of bubbling being less important than is suspected. Enrichment with 5% CO 2 in the bubbling air is an efficient way of obtaining a good biomass, but mechanical agitation of the algal culture with a 5% CO 2 : air mixture in a closed atmosphere over the culture is as good. Results are also presented on the relative effects of intermittent or continuous air bubbling.