Over the last few years there have been many successful projects involving the use of algae to treat water that is highly polluted with agricultural waste. Not only is this an environmentally friendly technology, but the harvested algae biomass could be used to produce biodiesel.
by Dr L. Wang
The huge amount of agricultural waste that is generated from confined animal feeding operations provides opportunities for both potential economic gain and also for benefits from an environmental perspective, if the nutrients in the wastes can be recycled appropriately, such as through the use of growing lipid-rich algae.
When the price of petroleum hit a record high of $145/barrel in 2008, more attention started to be given to the field of renewable energy. The use of biofuels, focussing on resources which can be grown or are a biproduct of agricultural production, has the potential to be sustainable in the long term. The first generation of biofuels, mainly biodiesel from soybean and ethanol from corn, are already on the market. Although these technologies utilise resources that could be used to grow food, and have thus been deemed non-environmentally friendly (the energy input surpasses the output, and some life cycle analyses show that the technology can be carbon positive), they at least use renewable resources.
Second generation technologies to produce biofuels, e.g. biofuel from algae, lignocellulosic ethanol and microbial fuel cells (MFCs), are emerging rapidly. Growing microalgae for biofuel is a promising approach, especially when this is combined with wastewater treatment.
Advantages of growing algae in wastewater to produce biofuel
Firstly, the method is much less energy intensive for treatingwastewater compared to the conventional activated sludge (AS) process, which requires a lot of energy input for aeration and later sludge disposal. Secondly, as a biofuel feedstock, microalgae contain a satisfactory lipid content and have a much faster growth rate than soybean and corn. The algal residue after oil extraction also has a high protein content, which is potentially a good feed supplement. Thirdly, algae assimilate CO2 autotrophically or mixotrophically, removing at least some of the greenhouse gas that contributes to global warming.
Current research projects
In recent years there have been many research projects involving the growth of algae to treat highly polluting agricultural waste. Many of these projects, from various geographical regions, cultured green algae on manure. Such projects are no longer small scale lab investigations but have been scaled up to several hundred to several thousand litres in open raceway ponds (where algae, water & nutrients circulate around a 'racetrack'). de Godos et al in Spain used two 464-L high-rate algal ponds (HRAPs) to treat 20- and 10-fold diluted swine manure for a hydraulic residence time of ten days [1]. They were able to obtain biomass productivities ranging from 21 to 28 g/m2 •d with average chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal efficiencies of 76% ± 11% and 88% ± 6%, respectively, which is very encouraging if one considers the simple structure and low energy input involved compared with other activated sludge processes. Mulbry et al at the Dairy Research Unit in Maryland, USA used an outdoor algal turf scrubber (ATS) raceway system with a 3500 L complementary underground concrete tank to treat raw and digested dairy manure [2]. Mean algal productivity values increased from approximately 2.5 g/m2 •d at the lowest loading rate (0.3 g total nitrogen (TN) /m2 •d) to 25 g /m2 •d at the highest loading rate (2.5 g TN /m2 •d). From this project Mulbry concluded that projected annual operation costs were well below those cited for upgrading existing water treatment plants in sensitive watersheds, indicating that algal technology for dairy manure treatment was indeed very encouraging from the environmental standpoint.
Factors affecting efficiency
It can be observed from these studies that there are several factors that affect the efficiency of applying open pond algae systems for the treatment of manure for nutrient recovery and algal biomass production. Firstly, the seasonal variations of temperature and solar irradiation have a significant impact on productivity because they affect the rate of algal photosynthesis. Normally, a higher rate of biomass production is observed during the summer season. The second important factor is the loading rate of major nutrients (C, N, P), especially nitrogen. A higher loading rate corresponds to a lower retention time; if the loading rate is controlled within a certain range, the gain in productivity is directly proportional to the increase in loading rate. However if the loading rate goes beyond a certain level, which is the upper limit of a system, nutrients build up, which might be lethal to the algae and so result in the system collapse. There can also be a need to supplement CO2. When the organic carbon which can be assimilated by algae in the manure is not sufficient i.e. digested dairy manure, extra CO2 can help boost algal growth. 1-5% CO2-enriched air can be added, which also has a positive effect on stabilisation of the culture's pH.
The harvested algae biomass, which can represent a biofuel feedstock, could be processed to produce biodiesel or green diesel. This may not be economically viable compared with the price of petroleum at current levels. However, the additional advantages of waste remediation and carbon sequestration will surely make this an attractive technology in the future.
I often think that if, in the early 20th century, scientists hadn't developed the bubbling needed for wastewater treatment, followed by the activated sludge process, but instead had mimicked the water purifying process that occurs in natural waters (algae bloom), algae would have been applied to wastewater treatment a long time ago. This would have been far more environmental friendly than the currently prevailing use of the activated sludge process.
References
1. de Godos I, Blanco S, García-Encina P A, Becares E, Muñoz R. Long-term operation of high rate algal ponds for the bioremediation of piggery wastewaters at high loading rates. Bioresour Technol 2009; 100 (19): 4332-4339.
2. Mulbry W, Kondrad S, Pizarro C, Kebede-Westhead E. Treatment of dairy manure effluent using freshwater algae: Algal productivity and recovery of manure nutrients using pilot-scale algal turf scrubbers. Bioresource Technology 2008; 99 (17): 8137-8142.
The author
Liang Wang, Ph. D.
Postdoc Associate
Dept. of Bioproducts and Biosystems
Engineering, University of Minnesota
1390 Eckles Avenue,
St. Paul, MN 55108, USA
wangx739@umn.edu
