Treatment High Rate Algal Ponds Research Articles

Size matters – Microalgae production and nutrient removal in wastewater treatment high rate algal ponds of three different sizes

High rate algal ponds for coupled wastewater treatment and resource recovery have been the focus of much international research over the last 15 years. Microalgal biomass productivity reported in full-scale studies (1-ha or greater) have often been substantially lower than that reported from smaller scale ponds in similar climates, regardless of the season or the dominant microalgal species used. The disconnect between smaller-scale and full-scale productivity is unclear and uncertainty remains regarding the applicability of smaller scale studies to full-scale systems. In order to better understand the differences in reported productivity, the performance of three different size wastewater treatment high rate algal ponds (5 m2, 330 m2 and 1-ha) were assessed with respect to nutrient removal and microalgal productivity over three seasons. Both daily areal nutrient removal and biomass production were affected by the size of the pond. NH4-N removal via nitrification/denitrification decreased with increasing pond size, with the highest removal rate in the 5 m2 pond and the lowest in the 1-ha. Microalgal areal productivity was maximal in the 330 m2 pond, suggesting that a combination of mixing frequency and higher photosynthetic potential under low light conditions were the main drivers of enhanced productivity in this pond compared to the 5 m2 (mesocosm) and 1-ha (full-scale) ponds. The lowest daily nutrient removal and biomass production occurred in the 1-ha (full-scale) pond. Our results suggest that, based on the current design and operation of high rate algal ponds, the optimum size for maximum productivity is considerably smaller than the current full-scale systems. This has implications for commercial scale systems, with respect to capital and operational costs.

Continuous low dosing of cationic polyacrylamide (PAM) to enhance algal harvest from a hectare-scale wastewater treatment high rate algal pond

ABSTRACTCationic polymers (e.g. polyacrylamide) having a high molecular weight require a much lower dose than inorganic metal salts (e.g. AlCl3, FeCl3, or Al2(SO4)3) to produce dense compact flocs with good settling characteristics. This study investigated the effect of a continuous low dosing of cationic polyacrylamide (PAM) to achieve efficient algal removal from a 1-ha High Rate Algal Pond (HRAP) treating domestic wastewater in New Zealand. Firstly, standard jar tests were conducted in laboratory conditions. A wide range of PAM dose rates (0.25–9 mg/L), mixing periods (60–250 s) and settling times (0–120 min) were tested on HRAP effluent samples to determine optimal operational parameters for the 1-ha HRAP in terms of TSS and turbidity removal. PAM addition effectively promoted flocculation of the wastewater algal biomass, removing 78% of TSS (from 135 to 30 mg/L) and 70% of turbidity (from 41 to 12 NTU) at a dose of only 4 mg/L. The jar tests determined that ∼120 s of vigorous mixing followed by 15-min of settling were sufficient to promote the formation of dense/settleable algal flocs. Continuous addition of PAM to the hectare-scale HRAP effluent at the 4 mg/L laboratory determined dose rate was found to improve TSS removal in the Algal Harvest Pond (AHP) by ∼55% (from 13% to 68%). The TSS concentration and turbidity of the AHP effluent were consistently maintained <50 mg/L and <18 NTU respectively. Furthermore, a ∼1-log E. coli removal (from 1 × 105 to 1 × 104 MPN/100 ml after settling) was also achieved in the AHP as a result of the algal floc formation and removal. Overall, this study demonstrated that addition of low levels of the chemical flocculant (e.g. cationic polyacrylamide) to the HRAP effluent can greatly improve algal solid removal (at least 50%) with a minimum chemical cost (∼$0.05 NZD/m3 of wastewater).

Environmental drivers that influence microalgal species in fullscale wastewater treatment high rate algal ponds

In the last decade, studies have focused on identifying the most suitable microalgal species for coupled high rate algal pond (HRAP) wastewater treatment and resource recovery. However, one of the challenges facing outdoor HRAP systems is maintaining microalgal species dominance. By increasing our understanding of the environmental drivers of microalgal community composition within the HRAP environment, it may be possible to manipulate the system in such a way to favour the growth of desirable species. In this paper, we investigate the microalgal community composition in two full-scale HRAPs over a 23-month period. We compare wastewater treatment performance between dominant species and identify the environmental drivers that trigger change in community composition. A total of 33 microalgal species were identified over the 23-month period but species richness (the number of species present at any given time) was low and was not related to either productivity or nutrient removal efficiency. Species turnover of the dominant microalgae happened rapidly, typically <1 week. Changes in the influent NH4-N concentration and zooplankton grazer numbers were significantly associated with species turnover, accounting for 80% of the changes in dominant species throughout the 23-month study period. Both nutrient removal and biomass production did not differ between the two HRAPs when the dominant species was the same or differed in the two ponds. These results suggest that microalgal functional groups are more important than individual species for full-scale HRAP performance. This study has increased our understanding of some of the environmental drivers of the microalgae within the HRAP environment, which may assist with improving wastewater treatment and resource recovery.

Control of zooplankton populations in a wastewater treatment High Rate Algal Pond using overnight CO2 asphyxiation

High Rate Algal Ponds (HRAPs) with addition of CO2 are open pond wastewater treatment systems that recover nutrients as microalgal biomass. Such ponds are vulnerable to contamination by opportunistic zooplankton species able to survive the wastewater HRAP environment. The high food availability and a near neutral pH can promote the rapid development of high densities of zooplankton that can reduce treatment performance by consuming microalgae. Zooplankton control using night time CO2 asphyxiation was selected from promising zooplankton control methods previously screened at laboratory and mesocosm scales, and used to control zooplankton densities in an 8m3 HRAP over 14months. Increasingly higher flow rates (1 to 6L/min) of pure CO2 were tested by using 13 control treatment events. CO2 was injected during night time, and treatment events were repeated for a number of consecutive nights sufficient to control zooplankton density to ≤10% of that before treatment. Treatments with higher CO2 flow rates promoted more rapid reductions of zooplankton density (12 nights to 1), and were associated with higher maximum CO2 concentrations (100 to 420mg/L), and lower pH (~6 to ~5). Compared to the control HRAP, CO2 treatment decreased the average population densities of some zooplankton species over the experimental period: Moina tenuicornis (41.3%), Paracyclops fimbriatus (43.9%), Filinia longiseta (59.8%), but was associated with higher average population densities of others: Heterocypris incongruens (174.4%), Asplanchna sieboldi (177.8%), Cephalodella catellina (200.0%), and Brachionus calyciflorus (234.9%). However, the population densities of the rotifers B. calyciflorus and C. catellina were always reduced following CO2 treatments with flow rates ≥2L/min. The cladoceran Daphnia thomsoni and the rotifer Brachionus urceolaris established only in the control HRAP. Zooplankton control by CO2 asphyxiation improved the overall performance of the treated WW HRAP compared to the control in several ways, including increasing algal biomass (VSS) (150.8%), productivity (151.4%), chlorophyll-a concentration (161.8%), particle size (MCSA) (115.8%), and average settleability efficiency (189.2%). Overnight CO2 asphyxiation showed the potential to control zooplankton and to promote better WW HRAPs performance.

Assessment of potential zooplankton control treatments for wastewater treatment High Rate Algal Ponds

Cladocerans and rotifers rapidly consume beneficial microalgae and reduce the performance of High Rate Algal Ponds (HRAPs) for wastewater treatment and algal production. Potential zooplankton control treatments for HRAPs have been proposed and tested at a laboratory scale including CO2 asphyxiation, biological control using competitor species, filtration, and mechanical disruption using hydrodynamic shear stress. This paper aims to validate these treatments using outdoor mesocosms with physicochemical and operational conditions similar to those of full scale HRAPs. A continuous CO2 concentration of ~100mg/L maintained low pond water zooplankton densities, while a continuous concentration of ~180mg/L killed all microcrustaceans and rotifers present. As biocontrol agents, the cladoceran Moina tenuicornis at ~2000 individuals/L reduced average rotifer densities by 90% while the ostracod Heterocypris incongruens at ~1000 individuals/L removed all rotifers. Mechanical filtration using 300μm and 500μm filters eradicated M. tenuicornis after one and four filtration periods, respectively. Mechanical hydrodynamic stress killed up to 100% of microcrustaceans, and ~50% of larger rotifers. Furthermore, phototaxis-induced migration promoted higher densities of M. tenuicornis in the upper layer of the water column in an 8m3 HRAP during periods of low solar radiation, suggesting that mechanical treatments should be performed at night and to the upper layer of the pond water. Overall, CO2 asphyxiation appeared to be the most reliable, versatile, and effective zooplankton control treatment.

Screening of potential zooplankton control technologies for wastewater treatment High Rate Algal Ponds

Zooplankton taxa including cladocerans and rotifers are one of the greatest challenges for effective management of High Rate Algal Ponds (HRAPs) for wastewater treatment as they can establish and rapidly consume beneficial microalgae. Harmful zooplankton need to be controlled using cost effective treatments, and here we tested under controlled laboratory conditions the efficacy of chemical CO2 asphyxiation, biological control of rotifers using competitor species expected to graze only moderately on colonial microalgae, and mechanical disruption using hydrodynamic shear stress. CO2 asphyxiation caused acute mortality of all zooplankton species (t<10min). Increasing the cladoceran Moina tenuicornis to densities >2500individuals/L was associated with a decrease in rotifer populations (~23% of the population in the control). The ostracod Heterocypris incongruens at densities >1000 individuals/L were also associated with a decrease in rotifer densities that were ~27% of the population in the control. Hydrodynamic shear stress killed 100% of M. tenuicornis and ~80% of the rotifer Brachionus calyciflorus after a single pass. Furthermore, M. tenuicornis was concentrated in the upper 50mm of a 300mm deep water column using vertical migration induced by CO2 concentrations of between 25 and 180mg/L. All of these treatments have potential for use to control zooplankton blooms in WW HRAPs.

Potential of five different isolated colonial algal species for wastewater treatment and biomass energy production

Wastewater treatment high rate algal pond (WWT HRAP) performance and the potential for low-cost biofuel production from its biomass are affected by the composition of the algal community. While several unicellular algal species have been evaluated extensively for their wastewater treatment and biofuel production potential, there has been little focus on the colonial species that typically predominate in high rate algal ponds and which have the benefit of being easily harvested by cost-effective, simple gravity settling. This study investigates the wastewater treatment performance of five wastewater colonial algal species that are common in high rate algal ponds: Mucidosphaerium pulchellum, Micractinium pusillum, Coleastrum sp., Desmodesmus sp. and Pediastrum boryanum and their potential value for biofuel production in terms of their biochemical composition and biomass energy yield. The algae were isolated from pilot-scale WWT HRAPs and used in batch monoculture experiments with pre-frozen and pre-filtered primary settled sewage that were conducted over 10days under simulated New Zealand summer and winter conditions. Under summer conditions, all species showed similar efficient nutrient removal (>95% of NH4+-N and >85% of PO43−-P within 4days) whereas, under winter conditions, only the Mucidosphaerium pulchellum and Micractinium pusillum cultures showed efficient nutrient removal by the end of the experiment. Algal biomass yield, lipid and energy content varied with species and were higher in summer than winter (respectively 991–1700mg/L, 14.4–48.2wt% and 19.1–25kJ/g in summer, and 158.7–584mg/l, 15.1–31.4wt% and 17.7–21.1kJ/g in winter). The Mucidosphaerium pulchellum and Micractinium pusillum cultures had the highest biomass yield, lipid and energy content under both summer and winter conditions. However the settleability of Micractinium pusillum is much better than Mucidosphaerium pulchellum suggesting that of the colonial algal species tested, Micractinium pusillum has the greatest potential for both wastewater treatment and low-cost energy production in WWT HRAP.

Wastewater treatment high rate algal pond biomass for bio-crude oil production

This study investigates the production potential of bio-crude from wastewater treatment high rate algal pond (WWT HRAP) biomass in terms of yield, elemental/chemical composition and higher heating value (HHV). Hydrothermal liquefaction (HTL) of the biomass slurry (2.2wt% solid content, 19.7kJ/g HHV) was conducted at a range of temperatures (150–300°C) for one hour. The bio-crude yield and HHV varied in range of 3.1–24.9wt% and 37.5–38.9kJ/g, respectively. The bio-crudes were comprised of 71–72.4wt% carbon, 0.9–4.8wt% nitrogen, 8.7–9.8wt% hydrogen and 12–15.7wt% oxygen. GC–MS analysis indicated that pyrroles, indoles, amides and fatty acids were the most abundant bio-crude compounds. HTL of WWT HRAP biomass resulted, also, in production of 10.5–26wt% water-soluble compounds (containing up to 293mg/L ammonia), 1.0–9.3wt% gas and 44.8–85.5wt% solid residue (12.2–18.1kJ/g). The aqueous phase has a great potential to be used as an ammonia source for further algal cultivation and the solid residue could be used as a process fuel source.

Microalgae recycling improves biomass recovery from wastewater treatment high rate algal ponds

Microalgal biomass harvesting by inducing spontaneous flocculation (bioflocculation) sets an attractive approach, since neither chemicals nor energy are needed. Indeed, bioflocculation may be promoted by recycling part of the harvested microalgal biomass to the photobioreactor in order to increase the predominance of rapidly settling microalgae species. The aim of the present study was to improve the recovery of microalgal biomass produced in wastewater treatment high rate algal ponds (HRAPs) by recycling part of the harvested microalgal biomass. The recirculation of 2% and 10% (dry weight) of the HRAPs microalgal biomass was tested over one year in an experimental HRAP treating real urban wastewater. Results indicated that biomass recycling had a positive effect on the harvesting efficiency, obtaining higher biomass recovery in the HRAP with recycling (R-HRAP) (92–94%) than in the control HRAP without recycling (C-HRAP) (75–89%). Microalgal biomass production was similar in both systems, ranging between 3.3 and 25.8 g TSS/m2d, depending on the weather conditions. Concerning the microalgae species, Chlorella sp. was dominant overall the experimental period in both HRAPs (abundance >60%). However, when the recycling rate was increased to 10%, Chlorella sp. dominance decreased from 97.6 to 88.1%; while increasing the abundance of rapidly settling species such as Stigeoclonium sp. (16.8%, only present in the HRAP with biomass recycling) and diatoms (from 0.7 to 7.3%). Concerning the secondary treatment of the HRAPs, high removals of COD (80%) and N-NH4+ (97%) were found in both HRAPs. Moreover, by increasing the biomass recovery in the R-HRAP the effluent total suspended solids (TSS) concentration was decreased to less than 35 mg/L, meeting effluent quality requirements for discharge. This study shows that microalgal biomass recycling (10% dry weight) increases biomass recovery up to 94% by selecting the most rapidly settling microalgae species without compromising the biomass production and improving the wastewater treatment in terms of TSS removal.

Biodiesel production potential of wastewater treatment high rate algal pond biomass

This study investigates the year-round production potential and quality of biodiesel from wastewater treatment high rate algal pond (WWT HRAP) biomass and how it is affected by CO2 addition to the culture. The mean monthly pond biomass and lipid productivities varied between 2.0±0.3 and 11.1±2.5gVSS/m2/d, and between 0.5±0.1 and 2.6±1.1g/m2/d, respectively. The biomass fatty acid methyl esters were highly complex which led to produce low-quality biodiesel so that it cannot be used directly as a transportation fuel. Overall, 0.9±0.1g/m2/d (3.2±0.5ton/ha/year) low-quality biodiesel could be produced from WWT HRAP biomass which could be further increased to 1.1±0.1g/m2/d (4.0ton/ha/year) by lowering culture pH to 6–7 during warm summer months. CO2 addition, had little effect on both the biomass lipid content and profile and consequently did not change the quality of biodiesel.

Pyrolysis of wastewater treatment high rate algal pond (WWT HRAP) biomass

This study investigates the potential of pyrolitic bio-oil production from wastewater treatment high rate algal pond biomass. The pyrolitic bio-oil produced at different temperatures was assessed in terms of yield, chemical and elemental composition, and energy content. Dry biomass was pyrolysed in a semi-batch pyrolysis reactor at three different temperatures (300°C, 400°C and 500°C) for 30min at atmospheric pressure while the reactor was purged continuously with N2. Condensable gases were removed using two condensers (80°C and 0°C) installed in series. Thermal decomposition behaviour of the biomass was confirmed using thermogravimetric analysis (TGA). Stepwise pyrolysis and TGA were employed to determine the bio-oil conversion at different temperature intervals. TGA results indicated that a maximum of 50±2wt.% of the initial biomass was pyrolysed at 500°C (to 20±2wt.% gas, 30±3wt.% liquid) which was in agreement with the product conversion results in pyrolysis. The highest yield of the liquid fraction was obtained at 500°C, although the stepwise pyrolysis showed that a major portion (50±2wt.%) of the liquid fraction was produced at temperatures below 300°C and the remaining 30±1wt.% and 20±1wt.% portions were produced at 300–400°C and 400–500°C, respectively. At <400°C, the liquid fraction was mainly dominated by an aqueous phase, while the bio-oil phase was mainly produced at 400–500°C. Elemental analysis indicated that the bio-oil contained >65wt.% carbon, 6-9wt.% nitrogen, 8–10.2wt.% hydrogen and had an energy content of 34.4-37kJ/g, all with the higher values at higher temperature except for nitrogen. GC–MS analysis showed high complexity of the liquid fraction in which aromatics and acids were dominant in the bio-oil and aqueous phases, respectively. Energy balance on system indicated that using the non-condensable gases and bio-char as fuel to supply the process energy demand could make algal-based bio-oil feasible from energy point of view. However, further research is required to make bio-oil production economical.

Partitioning of wastewater treatment high rate algal pond biomass and particulate carbon

The partitioning of algae, bacteria, grazers and detritus in two wastewater treatment high rate algal ponds (HRAPs) was investigated in relation to particulate carbon (PC) over a year. Algae dominated the pond biomass accounting for ~61% of the PC. Changes in algal biomass correlated with changes in PC with both varying seasonally and having spring or summer maxima. The algal biomass itself was dominated by larger cells or colonies in the 20–200μm size fraction with Pediastrum sp. prominent. Bacterial biomass, in contrast, only accounted for ~13.5% of the PC and varied less seasonally. Grazer biomass was lowest at ~4% of the PC on average and was dominated by either zooplankton or microzooplankton. Grazer biomass however, varied the most and reached ~14% of the PC during a spring zooplankton bloom that markedly reduced algal biomass. The remaining ~21.5% of the PC was made up of dead algal, detrital matter, and mucilage that tended to aggregated into bio-floccs. The PC of an efficiently operating HRAP is shown to be driven by high algal biomass with low bacterial and grazer biomass. If this balance is lost grazers may grow to levels that enable them to reduce algal biomass and productivity compounding pond instability.

Zooplankton community influence on seasonal performance and microalgal dominance in wastewater treatment High Rate Algal Ponds

High Rate Algal Ponds (HRAPs) with artificial addition of CO2 provide improved tertiary-level wastewater treatment over conventional HRAPs. One of the greatest challenges for performance and management of HRAPs with CO2 addition is the establishment of zooplankton grazers that can consume much of the algal biomass within a few days. High food availability and a near neutral pH provide optimal conditions for the establishment of zooplankton taxa and control strategies require an understanding of their population dynamics in HRAPs. Unfortunately, available literature lacks long-term assessment of such dynamics in HRAPs with CO2 addition. Here, we assess the environmental and biological parameters of two wastewater HRAPs with CO2 addition over a period of 14months, in relation to HRAPs performance and operation. Eight species of zooplankton established in the HRAPs, with higher water temperatures and longer detention times promoting larger populations. Grazing pressure was associated with changes in: 1) the dominance of microalgal species; 2) large and rapid reductions of productivity; 3) reduced nutrient removal; 4) increased colony size, number of cells in colonies and formation of protective spines in microalgae; and 5) higher biomass settleability. Maintaining a dominance of colonial microalgae, operating with short retention times, and facilitating competition among zooplankton species showed greatest potential for reducing and controlling grazer populations.

Settling velocity distribution of microalgal biomass from urban wastewater treatment high rate algal ponds

The aim of this study was to evaluate the settling velocity distribution of microalgal biomass with and without flocculant (Tanfloc SG). Microalgal biomass was obtained from two experimental wastewater treatment high rate algal ponds (HRAPs) operated with 4 and 8days of hydraulic retention time. Two sets of dynamic sedimentation tests were carried out using a water elutriation apparatus. In the first set, most of the biomass of the 8days-HRAP (63%) had settling velocities between 16.5 and 4m/h, while most of the biomass of the 4days-HRAP (65%) had settling velocities between 16.5 and 1m/h. In the second set, most of the biomass from both HRAPs (60% from the 8days-HRAP and 80% from the 4days-HRAP) had settling velocities between 6.5 and 0.4m/h. In this second set, settling velocities of <0.4m/h were reached by 20% and 40% of the biomass from 4days-HRAP and 8days-HRAP, respectively. The addition of flocculant at optimal doses ranging from 20 to 40mg/L had impressive effects on the settling velocity distribution in this second set. 70% and 84% of biomass reached velocities of >6.5m/h, compared to 10% and 14% of microalgal biomass without flocculant for the 8days- and 4days-HRAPs, respectively. With flocculant, a very small amount of biomass (3% for the 4days-HRAP and 8% for the 8days-HRAP) had settling velocities of <0.4m/h. Microscopic examination of samples from sedimentation tests showed how an important amount of microalgae settled in the system. Indeed, <1500microalgaeindividuals/mL were found in all outlet samples from the elutriation apparatus (inlet samples of >105microalgaeindividuals/mL). According to our results, a settler designed with a critical settling velocity of 1m/h would reach biomass recoveries as high as 90–94% with flocculant compared to 77–88% without flocculant.

Variation of biomass energy yield in wastewater treatment high rate algal ponds

Wastewater treatment high rate algal ponds (WWT HRAPs) have been recently highlighted as a potential system for low-cost algal energy production since the algal biomass is essentially a free by-product of the wastewater treatment process. This paper investigates the biomass energy yield potential of WWT HRAP (calculated by multiplying biomass productivity and biomass energy content). We address experimentally, for the first time, the influence of algal species dynamics, zooplankton grazing, environmental conditions, biomass chemical composition and biomass algal proportion on overall biomass energy yield. Two parallel identical pilot-scale HRAPs (8m3 volume, 0.3m depth and 31.8m2 surface area) were operated for one year and monitored each week for biomass productivity and energy content. Pond effluent nutrient concentration, microalgal relative abundance, biomass chemical composition and chlorophyll-a content were all measured and their effect on biomass productivity and energy content was assessed. The algal species composition and algal proportion in the HRAP effluent varied with season and grazing pressure. The highest biomass lipid content (45wt%) was achieved when effluent ammonia concentration was lowest (<1mg·L−1). Biomass productivity depended on season and zooplankton grazing pressure and biomass energy content increased algal proportion and lipid content of the HRAP biomass. The average biomass energy yield in the HRAPs was 113.3kJ·m−2·day−1 (based on the average annual biomass energy content of 19.2kJ·g−1 and the mean annual HRAP biomass productivity of 5.9gVSS·m−2·day−1 during the year). Biomass energy yield increased significantly during summer (175±5kJ·m−2·day−1) compared to winter (68±18kJ·m−2·day−1) since summer environmental conditions were more favorable for biomass growth. Results suggest improving algal proportion and productivity would promote biomass energy yield in WWT HRAP by enhancing biomass energy content and productivity concurrently.

Algal recycling enhances algal productivity and settleability in Pediastrum boryanum pure cultures

Recycling a portion of gravity harvested algae (i.e. algae and associated bacteria biomass) has been shown to improve both algal biomass productivity and harvest efficiency by maintaining the dominance of a rapidly-settleable colonial alga, Pediastrum boryanum in both pilot-scale wastewater treatment High Rate Algal Ponds (HRAP) and outdoor mesocosms. While algal recycling did not change the relative proportions of algae and bacteria in the HRAP culture, the contribution of the wastewater bacteria to the improved algal biomass productivity and settleability with the recycling was not certain and still required investigation. P. boryanum was therefore isolated from the HRAP and grown in pure culture on synthetic wastewater growth media under laboratory conditions. The influence of recycling on the productivity and settleability of the pure P. boryanum culture was then determined without wastewater bacteria present. Six 1 L P. boryanum cultures were grown over 30 days in a laboratory growth chamber simulating New Zealand summer conditions either with (Pr) or without (Pc) recycling of 10% of gravity harvested algae. The cultures with recycling (Pr) had higher algal productivity than the controls (Pc) when the cultures were operated at both 4 and 3 d hydraulic retention times by 11% and 38% respectively. Furthermore, algal recycling also improved 1 h settleability from ∼60% to ∼85% by increasing the average P. boryanum colony size due to the extended mean cell residence time and promoted formation of large algal bio-flocs (>500 μm diameter). These results demonstrate that the presence of wastewater bacteria was not necessary to improve algal productivity and settleability with algal recycling.

Effect of algal recycling rate on the performance of Pediastrum boryanum dominated wastewater treatment high rate algal pond.

Recycling a portion of gravity harvested algae promoted the dominance of a rapidly settling colonial alga, Pediastrum boryanum (P. boryanum) and improved both biomass productivity and settleability in High Rate Algal Pond (HRAP) treating domestic wastewater. The effect of algal recycling rate on HRAP performance was investigated using 12 replicate mesocosms (18 L) that were operated semi-continuously under ambient conditions. Three experiments were conducted during different seasons with each experiment lasting up to 36 days. Recycling 10%, 25%, and 50% of the 'mass' of daily algal production all increased total biomass concentration in the mesocosms. However, recycling >10% reduced the organic content (volatile suspended solids (VSS)) of the mesocosm biomass from 83% to 68% and did not further increase biomass productivity (based on VSS). This indicates that if a HRAP is operated with a low algal concentration and does not utilise all the available sunlight, algal recycling increases the algal concentration up to an optimum level, resulting in higher algal biomass productivity. Recycling 10% of the daily algal production not only increased biomass productivity by ∼40%, but increased biomass settleability by ∼25%, which was probably a consequence of the ∼30% increase in P. boryanum dominance in the mesocosms compared with controls without recycling.

Effects of two different nutrient loads on microalgal production, nutrient removal and photosynthetic efficiency in pilot-scale wastewater high rate algal ponds

When wastewater treatment high rate algal ponds (HRAP) are coupled with resource recovery processes, such as biofuel production, short hydraulic retention times (HRTs) are often favoured to increase the microalgal biomass productivity. However, short HRT can result in increased nutrient load to the HRAP which may negatively impact on the performance of the microalgae. This paper investigate the effects of high (NH4–N mean concentration 39.7 ± 17.9 g m−3) and moderate ((NH4–N mean concentration 19.9 ± 8.9 g m−3) nutrient loads and short HRT on the performance of microalgae with respect to light absorption, photosynthesis, biomass production and nutrient removal in pilot-scale (total volume 8 m3) wastewater treatment HRAPs. Microalgal biomass productivity was significantly higher under high nutrient loads, with a 133% and 126% increase in the chlorophyll-a and VSS areal productivities, respectively. Microalgae were more efficient at assimilating NH4–N from the wastewater under higher nutrient loads compared to moderate loads. Higher microalgal biomass with increased nutrient load resulted in increased light attenuation in the HRAP and lower light absorption efficiency by the microalgae. High nutrient loads also resulted in improved photosynthetic performance with significantly higher maximum rates of electron transport, oxygen production and quantum yield. This experiment demonstrated that microalgal productivity and nutrient removal efficiency were not inhibited by high nutrient loads, however, higher loads resulted in lower water quality in effluent discharge.

Microalgae conversion to biogas: thermal pretreatment contribution on net energy production.

Microalgal biomass harvested from wastewater treatment high rate algal ponds may be valorised through anaerobic digestion producing biogas. However, microalgae anaerobic biodegradability is limited by their complex cell wall structure. Thus, pretreatment techniques are being investigated to improve microalgae methane yield. In the current study, thermal pretreatment at relatively low temperatures of 75-95 °C was effective at enhancing microalgae anaerobic biodegradability; increasing the methane yield by 70% in respect to nonpretreated biomass. Microscopic images showed how the pretreatment damaged microalgae cells, enhancing subsequent anaerobic digestion. Indeed, digestate images showed how after pretreatment only species with resistant cell walls, such as diatoms, continued to be present. Energy balances based on lab-scale reactors performance at 20 days HRT, shifted from neutral to positive (energy gain around 2.7 GJ/d) after thermal pretreatment. In contrast with electricity consuming pretreatment methods, such as microwave irradiation, thermal pretreatment of microalgae seems to be scalable.

Investigating the life-cycle and growth rate of Pediastrum boryanum and the implications for wastewater treatment high rate algal ponds

The colonial alga Pediastrum boryanum has beneficial characteristics for wastewater treatment High Rate Algal Ponds (HRAP) including high biomass productivity and settleability. Our previous work has shown that these characteristics are enhanced when a portion of gravity harvested algae is recycled back to the pond. To help understand the mechanisms behind the improved performance of P. boryanum dominated HRAP with algal recycling, this study investigated the life-cycle of P. boryanum. Experiments determined the exact timing and growth rate of P. boryanum life-cycle stages (‘juvenile’, ‘growth’ and ‘reproductive’) under four combinations of light and temperature (250 or 120 μMol/m2/s; 20 or 10 °C). Single juvenile 16-celled colonies were grown in microcosms on an inverted microscope and photographed every 15 min until reproduction ceased. Two asexual life-cycles and a rarely occurring sexual life-cycle were observed. The time required to achieve asexual reproductive maturity increased from 52 h (high light and temperature) to 307 h (low light and temperature), indicating that the minimum hydraulic retention time or mean cell residence time (MCRT) must be higher than these values to sustain a P. boryanum HRAP culture under ambient conditions. The net growth rate of a P. boryanum colony varied between life-cycle stages (growth > juvenile > reproductive). This suggests that the higher biomass productivity measured in HRAP with algal recycling could be due to both the increased MCRT and an increase in the net growth rate of the HRAP culture by ‘seeding’ with faster growing colonies.