Wastewater use in algae production for generation of renewable resources: a review and preliminary results
© Dalrymple et al.; licensee BioMed Central Ltd. 2013
Received: 30 December 2011
Accepted: 20 December 2012
Published: 5 January 2013
Microalgae feedstock production can be integrated with wastewater and industrial sources of carbon dioxide. This study reviews the literature on algae grown on wastewater and includes a preliminary analysis of algal production based on anaerobic digestion sludge centrate from the Howard F. Curren Advanced Wastewater Treatment Plant (HFC AWTP) in Tampa, Florida and secondary effluent from the City of Lakeland wastewater treatment facilities in Lakeland, Florida. It was demonstrated that a mixed culture of wild algae species could successfully be grown on wastewater nutrients and potentially scaled to commercial production. Algae have demonstrated the ability to naturally colonize low-nutrient effluent water in a wetland treatment system utilized by the City of Lakeland. The results from these experiments show that the algae grown in high strength wastewater from the HFC AWTP are light-limited when cultivated indoor since more than 50% of the outdoor illumination is attenuated in the greenhouse.
An analysis was performed to determine the mass of algae that can be supported by the wastewater nutrients (mainly nitrogen and phosphorous) available from the two Florida cities. The study was guided by the growth and productivity data obtained for algal growth in the photobioreactors in operation at the University of South Florida. In the analysis, nutrients and light are assumed to be limited, while CO2 is abundantly available. There is some limitation on land, especially since the HFC AWTP is located at the Port of Tampa. The temperature range in Tampa is assumed to be suitable for algal growth year round. Assuming that the numerous technical challenges to achieving commercial-scale algal production can be met, the results presented suggest that an excess of 71 metric tons per hectare per year of algal biomass can be produced. Two energy production options were considered; liquid biofuels from feedstock with high lipid content, and biogas generation from anaerobic digestion of algae biomass. The total potential oil volume was determined to be approximately 337,500 gallons per year, which may result in the annual production of 270,000 gallons of biodiesel when 80% conversion efficiency is assumed. This production level would be able to sustain approximately 450 cars per year on average. Potential biogas production was estimated to be above 415,000 kg/yr, the equivalent of powering close to 500 homes for a year.
The United States (US) imports about 57% of the petroleum it consumes. Among all sectors, transportation accounts for 72% of all petroleum consumption . As energy consumption increases, the US dependence on foreign oil will also increase and compete heavily with the energy demands of rapidly growing economies such as China, India and Brazil. This will place tremendous pressure on global oil production and may decrease energy security. In addition, the wide and sustained use of petroleum-based fuels has been implicated as a major cause of increased atmospheric greenhouse gases, which may contribute to global climate change . These challenges have sparked the quest for alternative energy sources to serve as viable replacements to reduce dependence on fossil fuels and improve environmental sustainability. Among the many options, microalgae are receiving enormous attention as a source for the production of biofuels. Model estimates from Pacific Northwest National Laboratory have suggested that algal biofuels (particularly biodiesel) have the potential to meet as much as 17% of the transportation fuel demand . Microalgae oil production per unit area of land far exceeds other oil crops such as corn, soybean, coconut, and oil palm by as much as 2–3 orders of magnitude . Furthermore, they do not compete for arable land and can be produced year-round in suitable climates. They also grow much faster than traditional crops (doubling time can be as fast as 24 hours) and are likely to recover more quickly from adverse effects [5, 6].
Large-scale commercial production of algae, however, is potentially more costly than traditional crop production. Algae cultivation requires significant quantities of energy and water and the use of off-site generated carbon dioxide. One energy intensive process, for example, is the harvesting of the algal biomass, which can account for as much as 30% of the total cost of production [7–11]. In addition, water and nutrients are among the most critical variables in algal production [10, 12]. Fortunately, algae can be grown in both fresh water and seawater depending on species, but nutrient costs can be substantial. The main nutritional requirements for algal growth are nitrogen, phosphorous, and a number of micronutrients including potassium . Algae take up these nutrients along with CO2 and produce biomass via photosynthesis. Various combinations of fertilizers maybe used, including common field crop N-P-K fertilizer, but the associated costs can sometimes exceed the value of the final algae products .
For algal biofuels to achieve their full potential, inputs to algal cultivation must be inexpensive allowing for the economical mass production of feedstock. A convenient and cheap source of nutrients is municipal, industrial and agricultural wastewaters. Nutrient removal is an important aspect of wastewater treatment because rich nutrient streams discharged into natural water bodies can result in eutrophication. Furthermore, centrate (a nutrient-rich effluent stream from the anaerobic digestion process) is generally recycled to the head of the wastewater treatment plant and can increase the cost and destabilize the overall treatment process due to phosphorus accumulation. Since algae are known to grow in wastewater, a possible synergistic solution is to co-locate and integrate algal production with treatment of nutrient-rich wastewater and utilization of CO2 from power plant flue gas. This approach essentially reduces the cost of algal production, while preventing eutrophication and mitigating CO2 emissions [13–16].
Florida, and particularly the Tampa Bay area, has been identified as an ideal location for the development of algal feedstock and biofuel production because it receives significant sunshine, and demonstrates a relatively uniform seasonal evaporation loss compared to many other areas of the country . The latter is particularly important for open pond cultivation systems that lose significant amounts of water via evaporation. In this study, wastewater use for algae production is reviewed, particularly for renewable energy generation. A preliminary assessment of the potential to produce algal feedstock from wastewater is presented for two Tampa Bay cities. These include the City of Tampa and the City of Lakeland. All the wastewater from the City of Tampa is treated at the Howard F. Curren Advanced Wastewater Treatment Plant (HFC AWTP). HFC AWTP has a designed average daily flow capacity of 96 million gallons per day (MGD) and employs high-purity oxygen aeration for biochemical oxygen demand (BOD) removal followed by nitrification and denitrification. Lakeland’s municipal wastewater is treated by two traditional wastewater treatment plants and the secondary effluent is released into a 1,400-acre wetland treatment system (WTS) to achieve permissible nutrient reduction levels. The average daily flow rate into the wetland is 5.2 MGD. The WTS consist of a series of wetland cells connected by engineered discharged structures. Effluent from the WTS is discharged to the Alafia River. A wide cross-section of freshwater algal species thrives in the WTS.
Most of the electricity supplied to the Bay Area comes from Tampa Electric Company (TEC), which has a power plant located about 15 miles south of the Lakeland WTS and another plant across from the HFC AWTP. Together, these two power plants emit approximately 5.5 million metric tons of CO2 annually. Further, to lessen the burden on scarce freshwater resources, TEC and the City of Lakeland entered into a reclaimed water agreement in 2009 that allows TEC to use reclaimed effluent from the WTS commencing at the end of 2012. TEC will install a water treatment system to ensure that the effluent meets its cooling water standards.
The location of these facilities presents a potentially viable opportunity to explore synergy for algal feedstock production using wastewater and industrial CO2. A preliminary assessment was made to determine the quantity of algal feedstock that can be generated. The analysis was guided by experimental work on the growth of algae in enclosed bench-scale photobioreactors. The aim was to assess algae growth rate, nutrient uptake and lipid production using anaerobic digestion centrate from HFC AWTP and the Lakeland WTS.
Inoculum collection and scale-up
Wild-type algae were harvested from a secondary clarifier at the HFC AWTP in Tampa, Florida. Samples were transferred to 1-L flasks and bubbled with 2% CO2 in air during an 18-hr light/dark cycle under artificial light conditions of 310 μmol m-2 sec-1. Anaerobic digestion sludge centrate from the same facility was used as the scale-up medium after removal of suspended matter with a filter cloth. There were no nutrient additions to the centrate. Inoculum was grown until the culture biomass was 2 g dry wt L-1 as determined by total suspended solid (TSS) analysis with 5 mL algae suspension according to the standard method . University of Florida Environmental Biotechnology Laboratory analyzed samples and determined that the main algal species were Chlorella sp. and Scenedesmus sp.
Photobioreactor setup and operation
Each day, 1 L of the reactor volume was replaced with centrate collected from the HFC AWTP. The nutrient content of the centrate was analyzed prior to feeding the reactors. A data-logger (Onset® HOBO U12) was used to record irradiance, ambient temperature, culture temperature and relative humidity every 15 minutes.
A 1-L batch reactor was also operated with wetland water from the City of Lakeland WTS. The WTS contained a native population of algae, whose diversity was previously analyzed by GreenWater CyanoLab (Palatka, FL) and shown to include Bacillariophyta, Chlorophyta and Cyanobacteria groups. Air with 2% CO2 was fed to the reactor in like manner as the plastic reactors. A low-nutrient media was maintained by semi-continuous addition of 50 mL of 22.5 mg L-1 K2HPO4 and 60.71 NaNO3 to the batch reactor. The batch was operated for 3 weeks. Similar nutrient analyses were performed as previously described. All nutrients used in the study were obtained from Sigma Aldrich (St. Louis, MO).
Pretreatment and wastewater characterization
Nutrient content of centrate used as growth media for mixed algae species
200 – 250 mg L-1
2 – 75 mg L-1
100 – 250 mg L-1
Biomass and nutrient monitoring
Measurements of TSS and pH were performed daily. Nutrient removal analyses were performed every week for TN, ammonia (NH3), nitrate (NO3−), TP and chemical oxygen demand (COD) according to Standard Methods . TSS was determined by filtering a 5-mL algae suspension followed by drying in an oven for 24 hours.
Lipid content analysis
Temperature and pH
Biomass development and production rates
Algae grown on the high strength centrate had very low lipid content (<10%) compared to the 65% lipid content of Lakeland WTS algae consortium.
Algae biomass production potential from wastewater resources
This study was conducted to assess the potential of cultivating algae using wastewater as a nutrient medium. The consortium of algal species, including Scenedesmus sp. and Chlorella sp. grew favorably on anaerobic sludge centrate from the HFC AWTP. There was relatively high nutrient uptake for phosphorous and ammonia. Total nitrogen uptake was much lower because organic nitrogen was most likely not assimilated by the culture. The mean productivity obtained for the entire cultivation period was 3.3 ±1.5 g dry wt m-2 d-1. These results are similar to Woertz et al.  who report an algae production rate of 3 g dry wt m-2 d-1 for Chlorella sp. grown on wastewater. Li et al.  report a biomass production rate of 13 g dry wt m-2 d-1 for algae grown on centrate. Their results showed that by the end of a 14-day batch culture 94% ammonia, 89% TN and 81% TP was removed. Their system was continuously operated at 50% daily harvesting rate, compared to 14% used in this study. Zhou et al.  also grew algae on full strength anaerobic sludge centrate and obtained a biomass production rate of 12.8 g dry wt m-2 d-1. The lipid content reported by Li et al.  was ca. 11%, similar to these results. This is a downside of growing algae, especially Chlorella sp., in high strength nitrogen media. The caloric content which is linked to lipid production is significantly reduced . In general, high lipid content is achieved when the organisms are “starved” of nitrogen [4, 22, 23].
Potential application to large-scale algal production
Photobioreactor optimization can potentially increase biomass production, as observed from improving only air bubbling in this study. Improved air delivery was achieved by changing from spherical to cylindrical ceramic diffusers, resulting in better mixing. Work by Richmond , Richmond and Zou  and Qiang and Richmond  indicates that highly productive and efficient enclosed algal systems can be obtained by optimizing cell density and mixing rate in relation to photon flux density, particularly when nutrients are not limited. In addition, better aeration promotes increased mass transfer allowing for the removal of oxygen, which can become inhibiting at high concentrations .
However, there are limits to the photosynthetic conversion of sunlight energy into algal biomass in large-scale outdoor cultures. Under light-limited growth, there is an upper limit for light conversion efficiency of a large-scale culture. In practice, this usually translates to a maximum potential yield of 30–40 g dry wt m-2 day-1 under ideal outdoor sunlight conditions for short periods and considerably less for longer durations. This indicates that the non-optimized operation in this preliminary assessment was able to achieve 10% of the maximum. However, the cultures were grown under conditions of reduced light. It is possible to cultivate algae outdoor and improve light utilization through vertical reactor orientation, while keeping peak temperature down due to mutual shading of reactors .
Production in high rate algal ponds (HRAP) is possible and has shown commercial production rates as high as 40 g dry wt m-2 d-1. Craggs et al.  provide a good summary of production in HRAP. There is a wide variability of production rates achieved based on wastewater source, type, location and culture conditions. Algae growth in HRAPs has also been shown to achieve greater than 75% nutrient removal . Production was shown to improve with CO2 addition from 10.6 to 15.2 g dry wt m-2 d-1. Li et al.  and Zhout et al.  scaled up their wastewater-grown algal with 25-L BIOCOIL reactors and obtained net biomass productivity of 13 and 12.8 g dry wt m-2 d-1 respectively.
In the above equations, the chemical formula C106H263O110N16 represents algal biomass . According to the stoichiometry, 1 g of ammonia-nitrogen (NH3-N) or nitrate-nitrogen (NO3− − N) produces about 15.8 g of biomass and consumes 18.1 and 24.34 g of CO2 in the process, respectively. In addition to nutrient availability, algal biomass production also depends on light energy (hv). In the absence of nutrient limitation, photosynthesis increases with increasing irradiance until the maximum algal growth rate is attained as described my Michaelis-Menten kinetics [24–26]. A condition known as photoinhibition can occur when the irradiance is increased beyond the saturation point resulting in damage to algal light receptors and a decrease in the photosynthetic rate and productivity [24, 25].
Potential biomass production estimates for algae grown on wastewater nutrients in the Tampa Bay area, FL
Flow rate (MGD)
Nitrogen (mg L-1)
Algae biomass (tons yr-1)
CO2 consumed (tons yr-1)
Indoor area (ha)
Outdoor area (ha)
Algal production is restricted by available land close to the HFC AWTP. Approximately 200 hectares of suitable land area is available onsite and is located within close vicinity to where centrate is generated. Therefore, the flow rate has been chosen to reflect the land restriction for the indoor production. It is assumed that algae grown on moderate and low strength nutrient are nutrient limited and hence, their productivities are not affected by increasing light beyond a certain value. However, for algae grown on high strength centrate, the outdoor production area can be reduced since the algae are not nutrient limited.
Energy production and revenue potential
Annual biofuel production estimates derived from algae growth in wastewater nutrients in the Tampa Bay area, FL
Algae biomass (tons yr-1)
Biofuel (gal yr-1)
Total revenue (US$ yr-1)1
Biogas production estimates for anaerobic digestion of algae biomass grown on wastewater nutrients
Algae biomass (tons yr-1)
Biogas production (kg yr-1)
Total energy (MJ yr-1)
The above calculations assumed that the total production of algae goes toward digestion. It is also possible to extract lipids and attempt to derive biogas from spent biomass. The combination of algae production on the wastewater nutrient sources shows the potential for energy generation that can power close to 500 homes.
This work shows that there are important benefits to be derived from integrating algal production systems with nutrient-rich waste streams. The feedstock potential of the HFC AWTP and the Lakeland WTS is estimated to be approximately 71 tons ha-1 yr-1 of algal biomass, 270,000 gal hr-1 of liquid biofuel, and 415,000 kg yr-1 of methane. Renewable energy derived from algae will play a significant role in providing energy security while important services such as water treatment can be synergistically achieved by these systems. Even though the analysis has been preliminary, it shows that there is good potential for algal feedstock production in the Tampa Bay area. However, there are many important factors to be considered to assess whether algal production systems would be competitive. These include analysis of energy and cost associated with harvesting and extraction for example. It is hoped that with further research many of these challenges can be overcome.
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