Pilot-scale High Rate Algal Ponds Research Articles

Optimization of some fermentation conditions for bioethanol production from microalgae using response surface method

BackgroundAlgal biomass fermentation is one of the promising alternatives for bioethanol production. The bioethanol yield relies on fermentation conditions as the algal biomass amount, the yeast volume (% v/v), and the fermentation time. In this work, algal biomass harvested from a pilot-scale high rate algal pond (HRAP) was fermented anaerobically using immobilized Saccharomyces cerevisiae (ATCC 4126). The HRAP was constructed at the Zenin wastewater treatment plant (WTP), Giza, Egypt. A separate hydrolysis fermentation process (SHF) was applied for algal biomass. The effect of the algal biomass amount, the yeast volume (% v/v), and the time of fermentation as three independent variables were studied simultaneously and analyzed statistically using Design-Expert software V6.0.8.ResultsThe harvested algal biomass from HRAP contains 45% carbohydrates and was dominated by Microcystis sp. The results revealed that optimum bioethanol yield 18.57 g/L is achieved by fermenting 98.7 g/L algae using 15.09% of the volume immobilized yeast for 43.6 h with a 95% confidence interval.ConclusionMicroalgae grown on wastewater are a promising source of bioethanol production. Maximizing the ethanol production is achieved by optimizing the fermentation parameters as algal biomass, fermentation time, and yeast volume percent. The simultaneous optimization of the parameters using a statistical program is an effective way to maximize the production and predict a model that describes the relationship between these parameters and their response. The prospective research is going to study the effect of these predicted parameters on continuous fermentation on the semi-pilot scale.

The effect of primary treatment of wastewater in high rate algal pond systems: Biomass and bioenergy recovery

The aim of this study was to assess the effect of primary treatment on the performance of two pilot-scale high rate algal ponds (HRAPs) treating urban wastewater, considering their treatment efficiency, biomass productivity, characteristics and biogas production potential. Results indicated that the primary treatment did not significantly affect the wastewater treatment efficiency (NH4+-N removal of 93 and 91% and COD removal of 62 and 65% in HRAP with and without primary treatment, respectively). The HRAP without primary treatment had higher biodiversity and productivity (20 vs. 15 g VSS/m2d). Biomass from both systems presented good settling capacity. Results of biochemical methane potential test showed that co-digesting microalgae and primary sludge led to higher methane yields (238–258 mL CH4/g VS) compared with microalgae mono-digestion (189–225 mL CH4/g VS). Overall, HRAPs with and without primary treatment seem to be appropriate alternatives for combining wastewater treatment and bioenergy recovery.

Impacts of operational conditions on oxygen transfer rate, mixing characteristics and residence time distribution in a pilot scale high rate algal pond.

Different combinations of operational parameters including water level, paddle rotational speed and influent flow rate were applied to investigate their impacts on mixing characteristics, residence time distribution and gas transfer rate in a pilot-scale high rate algal pond. In closed condition, the paddle rotational speed had a positive correlation with the Bodenstein number (Bo), water velocity and oxygen volumetric mass transfer coefficient (kLaO2) while increasing water level generated a negative impact on these parameters, although the impact of water level on water linear velocity was small. The amplification effect of water level and paddle rotational speed on the sensitivity of Bo and kLaO2 should be noticed. Moreover, paddle rotational speed had more impact on kLaO2 than on Bo. The study in open condition indicated that effective volume fraction had a positive correlation with inlet flow rate and negative correlation with paddle rotation, while the opposite was observed in the case of Peclet number. The impact of water level variation on these parameters was unclear. Both water level and paddle rotational speed had negative impacts on the short-circuiting index, while no correlation was observed when varying inlet flow rate. In this study, the optimal operational conditions included low water level (0.1 m) and medium paddle rotational speed (11.6 rpm).

Investigating why recycling gravity harvested algae increases harvestability and productivity in high rate algal ponds

It has previously been shown that recycling gravity harvested algae promotes Pediastrum boryanum dominance and improves harvestability and biomass production in pilot-scale High Rate Algal Ponds (HRAPs) treating domestic wastewater. In order to confirm the reproducibility of these findings and investigate the mechanisms responsible, this study utilized twelve 20 L outdoor HRAP mesocosms operated with and without algal recycling. It then compared the recycling of separated solid and liquid components of the harvested biomass against un-separated biomass. The work confirmed that algal recycling promoted P. boryanum dominance, improved 1 h-settleability by >20% and increased biomass productivity by >25% compared with controls that had no recycling. With regard to the improved harvestability, of particular interest was that recycling the liquid fraction alone caused a similar improvement in settleability as recycling the solid fraction. This may be due to the presence of extracellular polymeric substances in the liquid fraction. While there are many possible mechanisms that could account for the increased productivity with algal recycling, all but two were systematically eliminated: (i) the mean cell residence time was extended thereby increasing the algal concentration and more fully utilizing the incident sunlight and, (ii) the relative proportions of algal growth stages (which have different specific growth rates) was changed, resulting in a net increase in the overall growth rate of the culture.

Enhancing biomass energy yield from pilot-scale high rate algal ponds with recycling

This paper investigates the effect of recycling on biomass energy yield in High Rate Algal Ponds (HRAPs). Two 8 m3 pilot-scale HRAPs treating primary settled sewage were operated in parallel and monitored over a 2-year period. Volatile suspended solids were measured from both HRAPs and their gravity settlers to determine biomass productivity and harvest efficiency. The energy content of the biomass was also measured. Multiplying biomass productivity and harvest efficiency gives the ‘harvestable biomass productivity’ and multiplying this by the energy content defines the actual ‘biomass energy yield’. In Year 1, algal recycling was implemented in one of the ponds (HRAPr) and improved harvestable biomass productivity by 58% compared with the control (HRAPc) without recycling (HRAPr: 9.2 g/m2/d; HRAPc: 5.8 g/m2/d). The energy content of the biomass grown in HRAPr, which was dominated by Pediastrun boryanum, was 25% higher than the control HRAPc which contained a mixed culture of 4–5 different algae (HRAPr: 21.5 kJ/g; HRAPc: 18.6 kJ/g). In Year 2, HRAPc was then seeded with the biomass harvested from the P. boryanum dominated HRAPr. This had the effect of shifting algal dominance from 89% Dictyosphaerium sp. (which is poorly-settleable) to over 90% P. boryanum in 5 months. Operation of this pond was then switched to recycling its own harvested biomass, which maintained P. boryanum dominance for the rest of Year 2. This result confirms, for the first time in the literature, that species control is possible for similarly sized co-occurring algal colonies in outdoor HRAP by algal recycling. With regard to the overall improvement in biomass energy yield, which is a critical parameter in the context of algal cultivation for biofuels, the combined improvements that recycling triggered in biomass productivity, harvest efficiency and energy content enhanced the harvested biomass energy yield by 66% (HRAPr: 195 kJ/m2/day; HRAPc: 118 kJ/m2/day).

Modification, calibration and verification of the IWA River Water Quality Model to simulate a pilot-scale high rate algal pond

We implemented the IWA River Water Quality Model No. 1 (Reichert et al., 2001. River Water Quality Model No. 1, IWA Scientific & Technical Report No. 12) to simulate water-quality characteristics in two pilot-scale High Rate Algal Ponds. Simulation results were compared with two years' of data from the ponds. The first year's data from one pond were used for model calibration; the remaining data were used for validation. As originally formulated and parameterized, the model consistently yielded summer-time algal biomass concentrations which were too low – with consequent failures in its reproduction of dissolved oxygen, pH and nutrient dynamics. We experimented with various structural/parametric changes to improve the model's performance. The most effective strategy was to greatly increase the respiratory losses suffered by the heterotrophic osmotrophs (thereby giving the algae access to a larger fraction of the incoming dissolved organic carbon and nitrogen). This suggests that CO2-bubbling alone cannot entirely preclude resource-limitation of algal production. We doubt that our parameterization of heterotrophic osmotrophs is correct and infer that the algae derive a large fraction of their nutrition by direct osmotrophic uptake of dissolved organic matter. This inference is supported by the literature concerning the physiology of the dominant algal species in our ponds.

Performance of a pilot-scale high rate algal pond system treating abattoir wastewater in rural South Australia: nitrification and denitrification

As part of a study examining the efficacy of high-rate algal pond treatment of high-strength abattoir wastewater, the impact of pond configuration and loading rate on nitrification was determined. The extent of nitrification in all ponds was consistent with mass balance estimates of oxygen demand and availability. Deeper ponds were more stable nitrifying systems, with shallow ponds displaying greater variation in response to changes in nitrogen loading. In a separate experiment the pond system was modified by covering a part of an in-series HRAP to exclude light, providing conditions suitable for denitrification. Specific denitrification rates were often within the range typical for endogenous carbon sources, with mass balance calculations indicating removals of up to 95%.

Production and separation of algae in a high-rate ponds system

Pilot scale high-rate algal ponds followed by settling tanks were developed to produce algal biomass from wastewater for greenery development and agricultural use in arid zones, like Kuwait. The high-rate ponds designed and operated by instructions resulting from the authors' research work can produce algal biomass of 300–400 kg/ha. d in summer and 200–300 kg/ha.d in winter seasons. The actual yield depends on the efficiency of the floc formation and settling. A minimum of a 2-h residence time in the settling tanks is needed to achieve adequate separation of the phases, assuming healthy algae culture that are susceptible to flock formation. The flock formation and settling properties are sensitive to the weather and operational parameters (e.g., sunshine, hydraulic conditions in settling tank, variations of the flow rate, and pond depth). If conditions are inadequate, 20–30% of the biomass can be lost. Satisfactory biomass production can be achieved only with careful plant operation and maintenance. The sensitivity of the algae separation by settling operation indicated the development of a modified sand filter system for algae trapping.