Existing wastewater treatment and carbon capture technologies often entail costly transportation and storage procedures and are energy-intensive. This is bad news because improper or lack of wastewater treatment results in the production of methane (CH4) and carbon dioxide (CO2) without energy recovery and carbon sequestration, with harmful effects for global warming. If wastewater treatment is not extended to all urban areas worldwide, methane and carbon dioxide emissions associated with wastewater discharges will reach alarming levels in the near future. The benefits of carbon sequestration in wastewater treatment have huge potential, which adds an energy conservation incentive to upgrading existing facilities to complete wastewater treatment. Biogas conversion energy recovery and biomass sequestration are the two main strategies for emission offset and carbon sequestration, respectively.
For environmental sustainability, proper wastewater management should be a major goal in the design of any treatment facility. Unfortunately, the current technology has only managed to reduce emissions but has not managed to use wastewater for active carbon dioxide capture and utilization. However, it has been found that algae can be used effectively in wastewater treatment without the costs or risks typically associated with disposal. Furthermore, wastewater can be used to cultivate algae on non-agricultural land. Algae culture is of increasing value given that algae can provide a high yield on a per unit of light irradiated area, and algae contain starch and oil making it possible to be used for the production of high quality biodiesel. Additionally, the growth of algae requires nutrients and carbon dioxide that can be obtained from fossil fuel combustion and from wastewater. Thus, algae can contribute to wastewater treatment
, CO2 fixation, and can be a source of bioenergy.
Algae concentrate bicarbonate in the cell and have active bicarbonate pumps. The bicarbonate is spontaneously dehydrated – or can be dehydrated by carbonic anhydrase – and through the Calvin-cycle activity the resulting carbon dioxide is captured in the form of algal biomass. Every gram of algal biomass captures between 1.6 and 2 grams of carbon dioxide produced. To have the effect on the accumulation of greenhouse gas, the captured carbon dioxide must be sequestered over very long time intervals. A lower-risk strategy for sequestering carbon dioxide is to convert it chemically into stable solid or liquid salts. The algal carbon sequestration systems provide two options; either to bury processed or extracted carbon-rich salts/fractions from algal biomass, or to bury permanently total algal biomass in deep geologic formations.
The scale of micro-algal production facilities necessary to capture carbon-dioxide emissions from stationary point sources such as cement kilns and power stations has been found to be manageable. And algae’s ability to transport bicarbonate into cells has also been found to make them well suited to capture carbon. Bicarbonate or Carbon dioxide-capturing efficiencies as high as 90 percent have been reported in open ponds; thus, there’s no question as to whether algae can potentially be exploited for carbon dioxide capture and sequestration. The commercial use of algal cultures dates back to around 1940’s with application in treatment of wastewater and in mass production of different strains such as Dunaliella and Chlorella. Today, biologists’ understanding of the ecology and biology of large-scale algal cultures has led to the engineering of efficient algal harvesting methods and large-scale culture systems, all of which are aimed at the operation of high rate algal cultures to produce high-value products, such as genetically engineered and Pharmaceuticals products.
The use of algae in wastewater treatment is particularly attractive because of their photosynthetic capabilities. Algae are able to convert solar energy into useful biomasses and incorporating nutrients such as phosphorus and nitrogen causing eutrophication. This fascinating idea launched some 55 years ago in the United States has since been intensively tested in many countries with positive results.
The efficiency of carbon dioxide capture by algae varies according to the pond chemistry, the state of the algal physiology, and temperature. Under optimal conditions and with gas residence times as short as two seconds, carbon dioxide capture efficiencies as high as 99 percent are achievable. Since the micro-algal wastewater treatment systems require large tract of land, efforts are being made to develop wastewater treatment systems that use hyperconcentrated algal cultures. This has been shown to be highly efficient means of removing phosphorus and nitrogen within very short periods of times. The algal systems can treat livestock wastes, human sewage, agro-industrial wastes, and industrial wastes. Also, micro-algal systems for the treatment of other wastes such as piggery effluent, the effluent from food processing factories and other agricultural wastes have been studied. Also under consideration are algae-based system for the removal of toxic minerals such as cadmium, lead, scandium, mercury, bromine, tin, and arsenic.