Algal Ponds Research Articles

Agricultural fertiliser from brewery effluent – the recovery of nutrients from the biomass of activated sludge and high rate algal pond treatment systems

Abstract The disposal of waste biomass generated from biological wastewater treatment plants is a costly process and poses environmental threats to the receiving environment. This study aimed to determine the suitability of algae and waste activated sludge (WAS) produced from a brewery effluent treatment system as a fertiliser in agriculture. The change in soil characteristics and the growth of a crop fertilised with algae or WAS was compared with a conventional inorganic fertiliser. Swiss chard plants (Beta vulgaris) fertilised with anaerobically digested (AD) algae or WAS had a significantly higher mean biweekly yield (5.08 ± 0.73 kg/m2) when compared with the inorganic fertiliser control (3.45 ± 0.89 kg/m2; p < 0.0001). No difference was observed in the soil's physical fertility when algae or WAS were applied to the soil (p > 0.05). The nitrogen applied to the soil from algae and WAS biomass appeared to leach out of the soil less than the nitrogen supplied by inorganic fertilisers. The application of WAS or algae on soil increased the soil's sodium concentration and sodium absorption ratio from 774.80 ± 13.66 mg/kg to 952.17 ± 34.89 mg/kg and 2.91 ± 0.04 to 3.53 ± 0.13, respectively. Regulations on the application of algae or WAS on agricultural soils should be altered to consider the limit values for sodium.

Real-time monitoring and prediction of water quality parameters and algae concentrations using microbial potentiometric sensor signals and machine learning tools

The overarching hypothesis of this study was that temporal microbial potentiometric sensor (MPS) signal patterns could be used to predict changes in commonly monitored water quality parameters by using artificial intelligence/machine learning tools. To test this hypothesis, the study first examines a proof of concept by correlating between MPS's signals and high algae concentrations in an algal cultivation pond. Then, the study expanded upon these findings and examined if multiple water quality parameters could be predicted in real surface waters, like irrigation canals. Signals generated between the MPS sensors and other water quality sensors maintained by an Arizona utility company, including algae and chlorophyll, were collected in real time at time intervals of 30 min over a period of 9 months. Data from the MPS system and data collected by the utility company were used to train the ML/AI algorithms and compare the predicted with actual water quality parameters and algae concentrations. Based on the composite signal obtained from the MPS, the ML/AI was used to predict the canal surface water's turbidity, conductivity, chlorophyll, and blue-green algae (BGA), dissolved oxygen (DO), and pH, and predicted values were compared to the measured values. Initial testing in the algal cultivation pond revealed a strong linear correlation (R2 = 0.87) between mixed liquor suspended solids (MLSS) and the MPSs' composite signals. The Normalized Root Mean Square Error (NRMSE) between the predicted values and measured values were <6.5%, except for the DO, which was 10.45%. The results demonstrate the usefulness of MPSs to predict key surface water quality parameters through a single composite signal, when the ML/AI tools are used conjunctively to disaggregate these signal components. The maintenance-free MPS offers a novel and cost-effective approach to monitor numerous water quality parameters at once with relatively high accuracy.

Primitive techno-economic study of bio-diesel and bio-active compound production from microalgae

BackgroundMicroalgae cells can be identified as a potential source for new and renewable energy. The economic investigation for biodiesel and bio-active compound production from the microalgae community (Bloom), which are collected from the high rate algal pond (HRAP) constructed to treat municipal wastewater at Zenin wastewater treatment plant, Giza, was the main target of study.ResultsThe microscopical examination showed that Scenedesmus obliquus is the dominant species. The total carotenoids were extracted using jojoba oil and determined by high-performance liquid chromatography (HPLC) to reach 81.44 μg/g. The biodiesel production through acid transesterification reaction recorded 70.6% of fatty acid methyl ester content with high cetane number (44) and low acid value. Such results prove that the obtained biodiesel has better ignition quality. The total phenolic and flavonoid compounds have been derived from the remaining biomass to give 5.36 ± 0.03 and 1.50 ± 0.19 mg/g respectively. Finally, total proteins and carbohydrates content in algal cells were recorded 54.3 and 1.5 mg/g successivelyConclusionThe preliminary economic evaluation showed that the production of biodiesel and carotenoids from the microalgae growing in municipal wastewater can be considered, as a techno-economic feasible process.

Behavior of UV Filters, UV Blockers and Pharmaceuticals in High Rate Algal Ponds Treating Urban Wastewater

The present study evaluated the efficiency of a high rate algal pond (HRAP) at pilot scale to remove pharmaceuticals and personal care products (PPCPs) from urban wastewater, including UV-filters and parabens (10), benzotriazoles (4), antibiotics (15), anti-inflammatories (3) and other pharmaceuticals (3). A total of 35 compounds were targeted, of which 21 were detected in the influent wastewater to the HRAP. Removals (RE%) for pharmaceuticals were variable, with efficient eliminations for atenolol (84%) and sulfathiazole (100%), whereas the anti-inflammatories naproxen and ketoprofen were only partially removed &lt;50%. Benzotriazoles showed elimination rates similar to those of conventional WWTPs, with RE% ranging from no elimination to 51% for the UV filter benzophenone-3 (BP3) and 100% for 4-methylbenzilidenecamphor (4MBC). Hazard quotients (HQs) were estimated for those compounds not fully eliminated in the HRAP, as well as the cumulative ecotoxicity in the resulting effluent. The majority of the compounds yielded HQs &lt; 0.1, meaning that no environmental risk would be derived from their discharge. Overall, these results clearly indicate that HRAPs are a reliable, green and cost-effective alternative to intensive wastewater treatment, yielding promising results removing these contaminants.

Environmental factors controlling algal species succession in High Rate Algal Pond

In the last decade, studies have focused on identifying microalgal species responsible for wastewater treatment in high rate algal pond (HRAP). In this study, investigation of the microalgal community composition in Pilot-scale Race way-type high rate algal pond through study period of 24 months, chlorophyll “a” content was estimated, Physico-chemical parameters were analyzed, light intensity and temperature also measured, and the biomass productivity was calculated. It was found from the recorded results that, more than 19 genera of phytoplankton belonging to 4 divisions were identified with dominance variation throughout the study period. The presence of the species inside the pond is dependent on seasonal variation. Chemical analysis data exhibit a significant increase of COD, BOD, and TSS concentration with the temperature increase. There is an obvious impact of light intensity and temperature on the growth rate, where the highest growth response of 3.5 mg/l Chl. (a) was achieved at the highest light intensity of 1900 lux with a temperature degree of 39 °C. The average algal biomass produced from HRAP 0.99 kg algae/m3/d. This study demonstrates that the predominance of algal community structure affected by seasonal variations, and there are clear light intensity and temperature impact on the growth rate, as well as there was a promising production of algal biomass can be used in different aspects.

Microalgal potential for nutrient-energy-wastewater nexus: Innovations, current trends and future directions

Agricultural, domestic and industrial activities contribute in releasing several organic and inorganic substances into the water streams that result in environmental pollution. Biological treatment of industrial and domestic wastewater using Activated Sludge Nutrient Removal (ASNR), the conventional process, is well known; however, it is relatively expensive due to the requirement for high energy inputs. Microalgal applications have been gaining interest as they offer potential cost-effective measures for the treatment of wastewater in the peri-urban and rural areas. Such systems provide an interesting tertiary biological treatment method where valuable biomass is produced with simultaneous uptake of nutrients such as nitrogen and phosphorous with reduction in coliform bacteria, heavy metals, chemical and biochemical oxygen demand (COD &amp; BOD) and the removal/degradation of xenobiotic compounds etc. This paper provides a systematic review on the current microalgal applications (phycoremediation) for wastewater treatment with advanced information on their role towards nutrient recovery and energy (biogas) production under the third generation biorefinery concept. The use of advanced algal pond systems for wastewater treatment including pollutant degradation, microalgal cultivation and employing such facilities for biogas production in view of technology applications is emphasized. This inter-linked network indicating microalgal role into the Nutrient-Energy-Wastewater nexus with future directions and concluding remarks are discussed.

Changes of viral and prokaryote abundances in a high rate algal pond using flow cytometry detection.

High rate algal ponds (HRAPs) are shallow, mixed systems for wastewater treatment, which use sunlight exposure for disinfection. Little is known regarding the relationships between the bacteria and viruses within HRAP systems. Uniquely, flow cytometry permits the rapid identification of bacterial and viral populations in wastewater samples, separating populations based on genome and particle size. Treated wastewater samples were collected from an HRAP at Kingston on Murray, South Australia. Flow cytometry analysis detected bacterial populations and discriminated virus-like particles (VLP) and large VLP (LVLP). Rapid, short term, fluctuations in the abundance of all three populations were observed. Changes in the abundance of these populations was compared; wastewater composition was used as metadata for the comparisons. Linear regression determined relationships in abundances between bacteria and LVLP (R2 0.2985); LVLP and VLP (R2 0.5829) and bacteria and VLP (R2 0.5778) all with p-values of <0.001. Bacterial, LVLP and VLP abundance positively correlated with each other, indicating potential microbial interactions. Overall, the results suggest a parasitic relationship was occurring and driving the abundances of bacteria and viruses within the system.

Innovative hybrid system for wastewater treatment: High-rate algal ponds for effluent treatment and biofilm reactor for biomass production and harvesting

The use of algal biomass still faces challenges associated with the harvesting stages. To address this issue, we propose an innovative hybrid system, in which a biofilm reactor (BR) operates as an algal biomass production and harvesting unit connected to a high-rate algal pond (HRAP), a wastewater treatment unit. BR did not interfered with the biomass chemical composition (protein = 32%, carbohydrates = 11% and total lipids = 18%), with the wastewater treatment (removals efficiency: chemical oxygen demand = 59%, ammonia nitrogen = 78%, total phosphorus = 16% and Escherichia coli = 1 log unit), and did not alter the sedimentation characteristics of the biomass (sludge volume index = 29 mg/L and humidity content = 92%) in the secondary settling tank of the hybrid system. On the other hand, the results showed that this technology achieved a biomass production about 2.6x greater than the conventional system without a BR, and the efficiency of harvesting of the hybrid system was 61%, against 22% obtained with the conventional system. In addition, the BR promoted an increase in the density (~1011 org/m2) and diversity of microalgae in the hybrid system. Chlorella vulgaris was the most abundant species (>60%) from the 4th week of operation until the end of the experiment. Hence, results confirm that the integration of BR into a wastewater treatment plant optimised the production and harvesting of biomass of the hybrid system, making it a promising technology. The importance of economic and environmental analysis studies of BR is highlighted in order to enable its implementation on a large scale.

High-rate algal pond for removal of pharmaceutical compounds from urban domestic wastewater under tropical conditions. Case study: Santiago de Cali, Colombia.

This study evaluated the capacity of a pilot-scale high-rate algal pond (HRAP) to remove pharmaceutical compounds (PCs) from domestic wastewater in the city of Santiago de Cali, Colombia. The compounds analyzed included antiepileptics, hypolipidemic drugs, tranquilizers and analgesics, and anti-inflammatory drugs. The HRAP operated under a continuous water flow of 0.2 m3d-1 and a 3-day hydraulic retention time (HRT). Removal efficiencies were high (>70%) for fenofibric acid, ibuprofen, and paracetamol; medium (30-70%) for gabapentin, lamotrigine, fenofibrate, gemfibrozil, diclofenac, ketoprofen, naproxen, and pentoxifylline; and low (<30%) for carbamazepine and its metabolite 10,11-Dihidro-10,11-dihidroxicarbamazepine (CBZ-Diol). The findings herein are similar to other studies, but were obtained with a shorter HRT. These results show that tropical environmental conditions favor photodegradation and contribute to the development of microalgae and the biodegradation process. Twenty microalgae species were identified, with the phylum Chlorophyta as the most abundant, particularly due to its natural introduction. The removal of the PCs also reflected a percentage reduction (>50%) in the ecological hazard posed by most of the compounds, although it is important to note that the hazard from gemfibrozil and ibuprofen remained high even after treatment, indicating the need for complementary treatment.

Spectroradiometric detection of competitor diatoms and the grazer Poteriochromonas in algal cultures

To address challenges in early detection of pond pests, we have extended a spectroradiometric monitoring method, initially demonstrated for measurement of pigment optical activity and biomass, to the detection of algal competitors and grazers. The method relies upon measurement and interpretation of pond reflectance spectra spanning from the visible into the near-infrared. Reflectance spectra are acquired every 5 min with a multi-channel, fiber-coupled spectroradiometer, providing monitoring of algal pond conditions with high temporal frequency. The spectra are interpreted via numerical inversion of a reflectance model, in which the above-water reflectance is expressed in terms of the absorption and backscatter coefficients of the cultured species, with additional terms accounting for the pigment fluorescence features and for the water-surface reflection of sunlight and skylight. With this method we demonstrate detection of diatoms and the predator Poteriochromonas in outdoor cultures of Nannochloropsis oceanica and Chlorella vulgaris, respectively. The relative strength of these signatures is compared to microscopy and sequencing analysis. Spectroradiometric detection of diatoms is then further assessed on beaker-contained mixtures of Microchloropsis salina with Phaeodactylum tricornutum, Thalassiosira weissflogii, and Thalassiosira pseudonana, respectively, providing an initial evaluation of the sensitivity and specificity of detecting pond competitors.

Performance evaluation of novel attached-growth high rate algal pond system with additional artificial illumination for wastewater treatment and nutrient recovery.

Domestic wastewater containing a high proportion of organic matter and nutrients is a serious pollution problem in developing countries. This study aimed to evaluate the performance of a novel attached-growth high rate algal pond (AG-HRAP) employing attached-growth media and artificial light sources for treating domestic wastewater and enhancing nutrient recovery. Light intensities in the range of 40-180 μmol/m2/s were used in the AG-HRAPs. The experimental results showed that the highest chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) removal efficiencies of 88, 62 and 69%, respectively, were found at the hydraulic retention time (HRT) of 15 days and the average light intensity of 180 μmol/m2/s. Moreover, the effluent COD concentrations could meet Thailand's national discharge standard. The highest biomass and protein productivities of 54 ± 4 and 37 ± 8 g/m2/d, respectively, were found in the AG-HRAPs, which were higher than in previous studies of HRAPs. The Stover-Kincannon kinetic values for COD, TN and TP removals of the AG-HRAPs (R2 = 0.9) were higher than those of the conventional systems. Additionally, the novel AG-HRAP system could provide a highly cost-effective operation when compared to other microalgal systems.

Estimating the biomass density of macroalgae in land-based cultivation systems using spectral reflectance imagery

Sub-sampling large-scale, land-based macroalgal cultures to estimate stocking densities is time-consuming, labour-intensive, and inaccurate. Therefore, the development of innovative methods to monitor stocking densities are required to maximise macroalgal productivity. In this study the spectral reflectance of a range of biomass densities (0.5–12.0 g L−1) of Ulva ohnoi was measured using a spectroradiometer (ASD) and a multispectral camera (REMX). A two-band normalised difference vegetation index (NDVI), which was standardised by depth of the culture system, was quantified from both sets of data, and used to establish biomass density sensing models. The models developed using the ASD and REMX data had strong piecewise linear relationships between the standardised NDVI and biomass density (R2 > 0.85, residual mean standard error < 1.31, and mean standard error < 0.99), but differed in breakpoint and slope of the regression lines. Subsequently, both models were validated using data collected in 4000 L high-rate algal ponds, representing practical conditions of the commercial land-based production of macroalgae. Notably, the REMX model had a better fit with the validation data, and the mean difference in the actual versus predicted densities was ≤0.76 g L−1, with a maximum difference of 1.73 g L−1. Therefore, the REMX biomass density model is a useful management tool for differentiating between low (<2 g L−1), medium (2–4 g L−1), and high densities (>4 g L−1) using the standardised NDVI. This study has demonstrated that there is a predictive relationship between spectral reflectance and the biomass density of U. ohnoi. Notably, the REMX model will inform decision making around the frequency of harvest, as well as the quantity of biomass to be harvested, to maintain an optimal stocking density and, therefore, high productivities at a commercial scale.

A coupled RTD and mixed-order kinetic model to predict high rate algal pond wastewater treatment under different operational conditions: Performance assessment and sizing application

A coupled residence time distribution (RTD) and mixed-order model was developed to simulate the performance of a pilot high rate algal pond (HRAP) for wastewater treatment under different operational conditions requiring simple influent and effluent wastewater characteristics as input data. The model showed potential in general assessment of the HRAP system performance, successfully capturing the magnitude as well as general trend of COD, TKN, TN and TP concentrations with acceptable accuracy (Nash-Sutcliffe Efficiency (NSE) value up to 0.99). However, due to the common limitation of black box model type, the detailed agreement between model and experimental data was highly varied, sometime unsatisfactory performance (NSE < 0) was obtained, especially for highly complex processes like TKN or TN removal. Besides, general information on the dynamics of algal bacterial processes under different operational conditions could be revealed by analyzing calibrated reaction order n and reaction rate constant kn values with higher n value corresponding to higher complexity of the process and higher kn values associating with higher removal rate. Moreover, the model could also be used as a quick and convenient support for HRAP design and construction by providing estimated hydraulic retention time (HRT) values required for single HRAP system treating wastewater in different case studies. Further validation with outdoor HRAP system is necessary to increase the accuracy of the model prediction.

Nitrous oxide emissions during microalgae-based wastewater treatment: current state of the art and implication for greenhouse gases budgeting.

Microalgae can synthesise the ozone depleting pollutant and greenhouse gas nitrous oxide (N2O). Consequently, significant N2O emissions have been recorded during real wastewater treatment in high rate algal ponds (HRAPs). While data scarcity and variability prevent meaningful assessment, the magnitude reported (0.13-0.57% of the influent nitrogen load) is within the range reported by the Intergovernmental Panel on Climate Change (IPCC) for direct N2O emissions during centralised aerobic wastewater treatment (0.016-4.5% of the influent nitrogen load). Critically, the ability of microalgae to synthesise N2O challenges the IPCC's broad view that bacterial denitrification and nitrification are the only major cause of N2O emissions from wastewater plants and aquatic environments receiving nitrogen from wastewater effluents. Significant N2O emissions have indeed been repeatedly detected from eutrophic water bodies and wastewater discharge contributes to eutrophication via the release of nitrogen and phosphorus. Considering the complex interplays between nitrogen and phosphorus supply, microalgal growth, and microalgal N2O synthesis, further research must urgently seek to better quantify N2O emissions from microalgae-based wastewater systems and eutrophic ecosystems receiving wastewater. This future research will ultimately improve the prediction of N2O emissions from wastewater treatment in national inventories and may therefore affect the prioritisation of mitigation strategies.

Microalgae: A Renewable Source for Wastewater Treatment and Feedstock Supply for Biofuel Generation

The search and exploitation of renewable clean energy sources have become crucial, because of the developing day by day interest for clean water and energy affected by the improvement of the economy, population, industrialization, urbanization, insufficient energy, climate abnormalities, and environmental pollution. The major cause of emissions of harmful gases into the environment is due to the high utilization of petroleum derivatives. In this way, it is paramount to explore environmentally sustainable energy sources for feasible advancement, to satisfy these expanding energy demands and to secure the environment. To mitigate these global problems, academic, industrial, and governmental sectors have engaged in a lot of brainstorming and research to surmount these difficulties, which have brought a steady flow of new information in the area of cultivation of microalgae in innovative technologies including photobioreactors and high rate algal ponds. In this respect, biomass generation from aquatic plants in enriched aquatic environments like wastewater has received considerable. Therefore, this review article provides comprehensive information on recent accounts on applications of microalgae in wastewater treatment using different technologies and their potentials in feedstock generation for biofuels applications. However, in the course of this study, high rate algal ponds (HRAP) and photobioreactors were found to be a reliable system for the cultivation of microalgae and wastewater treatment. Hence, the two-step method of dewatering methods is found to be the most effective approach for microalgae cultivation for wastewater treatment and generation of algal-based feedstock, taking into account high yield potentials and economic factors such as cost of operation and energy required for large scale algal biomass processing for biofuel generations.

Elaboration of an algae-to-energy system and recovery of water and nutrients from municipal sewage.

Increasing pressure is being exerted on the peri‐urban space that has elevated the demand for electricity, affects the global water resource, and impacts the potential to produce food, fiber, and commodity products. Algae‐based technologies and in particular algae‐based sewage treatment provides an opportunity for recovery of water for recycle and re‐use, sequestration of greenhouse gases, and generation of biomass. Successful coupling of municipal sewage treatment to an algae‐to‐energy facility depends largely on location, solar irradiance, and temperature to achieve meaningful value recovery. In this paper, an algae‐to‐energy sewage treatment system for implementation in southern Africa is elaborated. Using results from the continued operation of an integrated algal pond system (IAPS), it is shown that this 500‐person equivalent system generates 75 kL per day water for recycle and re‐use and, ∼9 kg per day biomass that can be converted to methane with a net energy yield of ∼150 MJ per day, and ∼0.5 kL per day of high nitrogen‐containing liquid effluent (>1 g/L) with potential for use as organic fertilizer. It is this opportunity that IAPS‐based algae‐to‐energy sewage treatment provides for meaningful energy and co‐product recovery within the peri‐urban space and, which can alleviate pressure on an already strained water–energy–food nexus.

Natural Pigments and Biogas Recovery from Microalgae Grown in Wastewater.

This study assessed the recovery of natural pigments (phycobiliproteins) and bioenergy (biogas) from microalgae grown in wastewater. A consortium of microalgae, mainly composed by Nostoc, Phormidium, and Geitlerinema, known to have high phycobiliproteins content, was grown in photobioreactors. The growth medium was composed by secondary effluent from a high rate algal pond (HRAP) along with the anaerobic digestion centrate, which aimed to enhance the N/P ratio, given the lack of nutrients in the secondary effluent. Additionally, the centrate is still a challenging anaerobic digestion residue since the high nitrogen concentrations have to be removed before disposal. Removal efficiencies up to 52% of COD, 86% of NH4+-N, and 100% of phosphorus were observed. The biomass composition was monitored over the experimental period in order to ensure stable cyanobacterial dominance in the mixed culture. Phycocyanin and phycoerythrin were extracted from harvested biomass, achieving maximum concentrations of 20.1 and 8.1 mg/g dry weight, respectively. The residual biomass from phycobiliproteins extraction was then used to produce biogas, with final methane yields ranging from 159 to 199 mL CH4/g VS. According to the results, by combining the extraction of pigments and the production of biogas from residual biomass, we would not only obtain high-value compounds, but also more energy (around 5–10% higher), as compared to the single recovery of biogas. The proposed process poses an example of resource recovery from biomass grown in wastewater, moving toward a circular bioeconomy.

Surfactant removal and biomass production in a microalgal-bacterial process: effect of feeding regime.

The influence of the feeding regime on surfactant and nutrient removal and biomass production was evaluated in three high rate algal ponds for primary domestic wastewater treatment. Feeding times of 24, 12 and 0.1 h d-1 were studied in each reactor at a similar hydraulic retention time of 7.0 days and organic load of 2.3 mg m-2 d-1. Semi-continuous feeding at 12 and 0.1 h d-1 showed better microalgal biomass production (0.21-0.23 g L-1) and nutrient removal, including nitrogen (74-76%) and phosphorus (80-86%), when compared to biomass production (0.13 g L-1) and nitrogen (69%) and phosphorus (46%) removals obtained at continuous feeding (24 h d-1). Additionally, the removal efficiency of surfactant in the three reactors ranged between 90 and 97%, where the best result was obtained at 0.1 h d-1, resulting in surfactant concentrations in the treated effluent (0.3 mg L-1) below the maximum freshwater discharge limits.

The effect of CO2 addition and hydraulic retention time on pathogens removal in HRAPs.

The influence of CO2 addition and hydraulic retention time (5 and 7 days) on removal of Pseudomonas aeruginosa, Clostridium perfringens, Staphylococcus sp., Enterococcus sp., and Escherichia coli was evaluated in a system with three parallel 21 L high rate algal ponds. Both the addition of CO2 and an increase in HRT had no significant influence on bacterial removal, but bacterial removal was higher than found in previous studies. The removal was 3.4-3.8, 2.5-3.7, 2.6-3.1, 2.2-2.6 and 1.3-1.7 units log for P. aeruginosa, E. coli, Enterococcus sp., C. perfringens, and for Staphylococcus sp., respectively. Although CO2 addition did not increase disinfection, it did significantly increase biomass productivity (by ≈60%) and settleability (by ≈350%). Additionally, even at the lower 5-day hydraulic retention time, CO2 addition improves removal of chemical oxygen demand (COD), total organic carbon (TOC), total organic nitrogen and phosphorus by 97, 91, 12 and 50%, respectively.

Escherichia coli removal during domestic wastewater treatment in outdoor high rate algae ponds: long-term performance and mechanistic implications.

Escherichia coli (E. coli) first-order decay rates ranging from 3.34 to 11.9 d-1 (25-75% data range, N = 128) were recorded in two outdoor pilot-scale (0.88 m3) high rate algal ponds (HRAPs) continuously fed primary domestic wastewater over two years (influent E. coli cell count of 4.74·106 ± 3.37·106 MPN·100 mL-1, N = 142). The resulting removal performance was relatively constant throughout the year (log10-removal averaging 1.77 ± 0.54, N = 128), apart from a significant performance drop during a cold rainy period. E. coli removal performance was not strongly correlated to any of the meteorological or operational parameters recorded (e.g. sunlight intensity, pH, temperature). Hourly monitoring of E. coli cell count evidenced that E. coli removal, pH, dissolved oxygen (DO) and pond temperature peaked in the late afternoon of sunny summer days. Such improved daytime removal was, however, not evidenced in spring, even under sunny conditions causing milder increases in pH, DO and temperature. Overall, the data confirm the potential of HRAPs to support efficient E. coli removal during secondary domestic wastewater treatment and suggests E. coli decay was mainly caused by dark mechanisms episodically enhanced by indirect sunlight-mediated mechanisms and/or high pH toxicity.