The WEF Sustainable Utilities Task Force presents a resource for utility managers seeking examples of succesful sustainability practices

Algae Harvesting With a High Rate Water Processing Technology: Dissolved Air Flotation


The algae-to-biofuel industry requires a cost effective means for separating a relatively dilute algae suspension from a large flow of water. Ideally, algae recovery should be economical and result in a thick algae concentrate with minimal extraneous and inert materials that would detract from the value of the resource. To a large extent, the economy of microalgae production depends on the method employed for harvesting and concentrating the algae. This paper discusses some of the possible algae separation techniques with an overview of the advantages and disadvantages of each. Emphasis is placed on the dissolved air flotation process because it still remains the most effective means of separating a relatively low concentration of algae from a large body of water. Some of the important design features of dissolved air flotation that promote effective algae separation and harvesting, as well as thickening of the separated algae, are discussed. The efficiency of recovery is a significant issue because it relates to the economics of the separation technique. However, it is not necessarily of overriding importance since the water from the separation unit can be recycled back to the algae growth units. The efficiency of recovery and operation of the separation technique depends to a large extent on the means by which the algae is conditioned. The paper discusses various inorganic and organic chemicals for algae coagulation, to facilitate algae separation by dissolved air flotation, or by other means. Inorganic coagulants tend to be the most effective, although some natural and synthetic polymers are effective. Chitosan is a natural polymer that does not detract from the animal feed option for the co-product from algae biofuel processing, although it does introduce significant quantities of inert material. Other organic polymers that are GRAS certified should be used preferentially to those products that are not certified. Source: WEFTEC 2009 Proceedings


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Posted: August 27th, 2010 | Filed under: Waste Water Treatment | Tags: , , , , , , , | No Comments »

Nutrient Removal & Greenhouse Gas Abatement with CO2 Supplemented Algal High Rate Ponds


High rate algae ponds fed clarified domestic wastewater and CO2-rich flue gas are expected to remove nutrients to concentrations similar to those achieved in mechanical treatment technologies, such as activated sludge. However, the energy intensity of wastewater treatment with CO2-supplemented high rate ponds (HRPs) would be less than that of mechanical treatments. In conjunction with anaerobic digestion of algal biomass and co-substrates, the algae-based system would produce a substantial excess of electricity. Greenhouse gas abatement from such CO2-HRP/digestion systems would stem mainly from energy conservation and the offset of fossil fuel electricity with biogas-derived electricity. Laboratory experiments showed nutrient removals of >98% for ammonium and >96% for phosphorus with mixed culture microalgae grown on CO2-supplemented primary wastewater effluent. An engineering numerical model for CO2-HRP/digestion facilities (based in part on large-scale algae production under southern California conditions) indicates a potential energy surplus of 330 kWh/ML (1,200 kWh/MG) from biogas-derived electricity, compared to the net energy consumption of about 760 kWh/ML (2,900 kWh/MG) at typical activated sludge facilities with nitrification/denitrification. Considering the net electricity production and energy savings of the CO2-HRP/digestion systems, a greenhouse gas abatement potential of 660 kg CO2eq/ML (2,500 kg CO2eq/MG) treated is expected for a 100-ha facility treating 20 MGD. Source: WEFTEC 2009 Proceedings


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Posted: August 27th, 2010 | Filed under: Waste Water Treatment | Tags: , , , , , , | No Comments »