http://fac.ksu.edu.sa/sites/default/files/Abdel-Raouf%20et%20al%2C2012.pdf

Microalgae and wastewater treatment.

Wastewater treatment high rate algal ponds for biofuel production.

Microalgae dewatering is a major obstruction to industrial-scale processing of microalgae for biofuel production. The dilute nature of harvested microalgal cultures creates a huge operational cost during dewatering, thereby, rendering algae-based fuels less economically attractive. Currently there is no superior method of dewatering microalgae. A technique that may result in a greater algal biomass may have drawbacks such as a high capital cost or high energy consumption. The choice of which harvesting technique to apply will depend on the species of microalgae and the final product desired. Algal properties such as a large cell size and the capability of the microalgae to autoflocculate can simplify the dewatering process. This article reviews and addresses the various technologies currently used for dewatering microalgal cultures along with a comparative study of the performances of the different technologies.

Dewatering of microalgal cultures: A major bottleneck to algae-based fuels

Microalgae dewatering is a major obstruction to industrial-scale processing of microalgae for biofuel prodn. The dil. nature of harvested microalgal cultures creates a huge operational cost during dewatering, thereby, rendering algae-based fuels less economically attractive. Currently there is no superior method of dewatering microalgae. A technique that may result in a greater algal biomass may have drawbacks such as a high capital cost or high energy consumption. The choice of which harvesting technique to apply will depend on the species of microalgae and the final product desired. Algal properties such as a large cell size and the capability of the microalgae to autoflocculate can simplify the dewatering process. This article reviews and addresses the various technologies currently used for dewatering microalgal cultures along with a comparative study of the performances of the different technologies.

Dewatering of microalgal cultures : a major bottleneck to algae-based fuels

Many researchers consider efficient harvesting is the major challenge of commercialising micro-algal biofuel. Although micro-algal biomass can be ‘energy rich’, the growth of algae in dilute suspension at around 0.02–0.05 % dry solids poses considerable challenges in achieving a viable energy balance in micro-algal biofuel process operations. Additional challenges of micro-algae harvesting come from the small size of micro-algal cells, the similarity of density of the algal cells to the growth medium, the negative surface charge on the algae and the algal growth rates which require frequent harvesting compared to terrestrial plants. Algae can be harvested by a number of methods; sedimentation, flocculation, flotation, centrifugation and filtration or a combination of any of these. This paper reviews the various methods of harvesting and dewatering micro-algae for the production of biofuel. There appears to be no one method or combination of harvesting methods suited to all micro-algae and harvesting method will have a considerable influence on the design and operation of both upstream and downstream processes in an overall micro-algal biofuel production process.

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A review of the harvesting of micro-algae for biofuel production

A Source of high-quality protein for animal feed, based upon algae recovered in the process of upgrading waste oxidation pond effluents and promising to be particularly economical, is being developed at the Technion. Unlike other types of single cell protein(SCP), the algal protein does not have to return the full production cost but only that of concentration and final processing. The balance is shared by the value of waste disposal and the reclaimed water. Whereas such systems as activated sludge require considerable mechanical energy to supply the oxygen needed for aerobically degrading organics in wastewater, oxidation ponds utilize solar energy for that purpose. The sludge obtained when their effluents are clarified consists largely of algae, bacteria, fungi, and zooplankton in relative proportions varying with operating conditions, and contains 40–60%(dry basis) high-quality protein. The high rate oxidation pond (a particularly intensive type of pond) produces on the average 34 g/m27sol;day solids, or over 100 tons/ha (hectare) annually. Two clarification routes have been found promising: centrifugation and alum flocculation followed by frothflotation. The latter route is less expensive in terms of both fixed and operating cost, and gives clarified effluent of higher quality, which can be seasonally stored with minimal eutrophication because the aluminum removes most of the phosphate from the effluent. A good product has been obtained by drum-drying the concentrate, and preliminary feeding tests have indicated that it can replace at least 1/4 of the soymeal in broiler rations and 2/3 of the fishmeal in carp feed. No ill effect of the aluminum in the product recovered by alum flocculation has been found so far a process for removing and recycling the aluminum has been developed nonetheless, in case ill effects do show up in further tests. The combined value of the benefits derived from a system centered around the high-rate oxidation pond with clarification by flocculation–flotation, in terms of waste treatment by alternative means, potable water saved, and soymeal replaced, significantly exceeds estimated cost.

Algal single cell protein from wastewater treatment and renovation process

Theoretical considerations confirmed by outdoor experiments indicated carbon limitation of biomass production in high-rate oxidation ponds at certain seasonal and operational conditions. Apparently, free carbon dioxide concentration in the pond is the major determinant of carbonlimiting algal photosynthesis. High concentrations of free CO2 are provided through bacterial respiration which is the main contributor to algal photosynthesis. At high photosynthetic activities and low organic loadings, free CO2 concentrations are low; its flux into algal cells determines photosynthesis and biomass production rate in the pond.

Carbon limitation of biomass production in high-rate oxidation ponds.

Treatment of municipal wastewater in High Rate ponds has been extensively studied in a multitude of outdoor pond units. The ponds are 35 to 50 cm deep with folding channels and are mixed and aerated by a rotor cage aerator. The suspended biomass is separated by flocculation and flotation yielding high quality effluent and algal “froth” containing between 5 to 10 percent solids to be further dewatered and drum dried. Average production of photoautotrophic algae reached 22gr m−2day−1 which constitute about 5.8 percent conversion efficiency of the photosynthetically available light. The photosynthetic oxygen supply reached over 35 gr m−2day−1 which constitute over two thirds of the oxygen demand for the aerobic degradation of the waste matter. Predominant algal species such as Scenedesmus dimorphus , Micractinium quadrisetum , Phytoconis sp . and Oocystis solitaria varied seasonally in the pond without any significant change in growth and productivity. Dried algae containing up to 4 percent aluminium (flocculant residue) were fed to carps ( Cyprinus carpio ) and to St. Peter's fish ( Tilapia galilea ) where 85 percent of the fish-meal portion of commercial diet (usually 15 percent of diet) was replaced by 30 percent algae in the feed (equal protein level). Fish weight gain and health were equal if not improved by this algae containing diet. The algae also served for feeding of young chicks (1–8 weeks) with no mal-effects where algae (containing aluminium) replaced 25 percent of the proteins in the diet. The economy of the overall system is based on the value of wastewater treatment, the added value of the final effluent high quality rendering its reuse for agricultural irrigation and the value of the harvested algae as a proteinaceous feed. Once the need exists for all three values, the system is highly competitive vs. any other wastewater treatment process where the proper climatic conditions and land availability exist.

R. Moraine

Papers

Algal single cell protein from wastewater treatment and renovation process

Carbon limitation of biomass production in high-rate oxidation ponds.

Combined algae production - wastewater treatment and reclamation systems

Nutrients Removal and Recovery in a Two-Stage High-Rate Algal Wastewater Treatment System