Assessing the life-cycle sustainability of algae and bacteria-based wastewater treatment systems: High-rate algae pond and sequencing batch reactor.

High Rate Algae Ponds (HRAPs) are a promising technology for the treatment of municipal wastewater in locations with sufficient space and solar radiation. Algae-based processes do not require aeration, and thus have the potential to be less energy-intensive than activated sludge processes. We used a combination of LCA and LCCA analysis to evaluate the sustainability of HRAP systems, using data from the construction and operation of two demonstration-scale systems in Almería and Cádiz, Spain. As a reference for comparison, we used data from an activated sludge-based Sequencing Batch Reactor (SBR) treatment system in operation in Leppersdorf, Germany, which has comparable removal rates for a similar inflow. We focused solely on the actual wastewater treatment aspect of these technologies, excluding sludge treatment from this analysis. Based on our analysis, the current HRAP technology is more energy-efficient than activated sludge-based SBRs and requires only 22% of its electricity consumption. In addition, HRAP is more advantageous both economically (0.18 €/m3 versus 0.26 €/m3) and environmentally, with both lower global warming and eutrophication potentials (146.27 vs. 458.27 × 10-3 kg CO2 equiv./m3; 126.14 vs. 158.01 × 10-6 kg PO4 equiv./m3). However, the Net Environmental Benefit of SBR was slightly more favorable than of HRAP because of the higher removal rate for nutrients of SBR.

Understanding the biological activity of high rate algae ponds through the calculation of oxygen balances.

Microalgae culture in high rate algae ponds (HRAP) is an environmentally friendly technology for wastewater treatment. However, for the implementation of these systems, a better understanding of the oxygenation potential and the influence of climate conditions is required. In this work, the rates of oxygen production, consumption, and exchange with the atmosphere were calculated under varying conditions of solar irradiance and dilution rate during six months of operation in a real scale unit. This analysis allowed determining the biological response of these dynamic systems. The rates of oxygen consumption measured were considerably higher than the values calculated based on the organic loading rate. The response to light intensity in terms of oxygen production in the bioreactor was described with one of the models proposed for microalgae culture in dense concentrations. This model is based on the availability of light inside the culture and the specific response of microalgae to this parameter. The specific response to solar radiation intensity showed a reasonable stability in spite of the fluctuations due to meteorological conditions. The methodology developed is a useful tool for optimization and prediction of the performance of these systems.

Biostimulant Capacity of Chlorella and Chlamydopodium Species Produced Using Wastewater and Centrate.

The aim of the present study was to assess the potential of producing four microalgal strains using secondary-treated urban wastewater supplemented with centrate, and to evaluate the biostimulant effects of several microalgal extracts obtained using water and sonication. Four strains were studied: Chlorella vulgaris UAL-1, Chlorella sp. UAL-2, Chlorella vulgaris UAL-3, and Chlamydopodium fusiforme UAL-4. The highest biomass productivity was found for C. fusiforme, with a value of 0.38 ± 0.01 g·L-1·day-1. C. vulgaris UAL-1 achieved a biomass productivity of 0.31 ± 0.03 g·L-1·day-1 (the highest for the Chlorella genus), while the N-NH4+, N-NO3-, and P-PO43- removal capacities of this strain were 51.9 ± 2.4, 0.8 ± 0.1, and 5.7 ± 0.3 mg·L-1·day-1, respectively. C. vulgaris UAL-1 showed the greatest potential for use as a biostimulant-when used at a concentration of 0.1 g·L-1, it increased the germination index of watercress seeds by 3.5%. At concentrations of 0.5 and 2.0 g·L-1, the biomass from this microalga promoted adventitious root formation in soybean seeds by 220% and 493%, respectively. The cucumber expansion test suggested a cytokinin-like effect from C. vulgaris UAL-1; it was also the only strain that promoted the formation of chlorophylls in wheat leaves. Overall, the results of the present study suggest the potential of producing C. vulgaris UAL-1 using centrate and wastewater as well as the potential utilisation of its biomass to develop high-value biostimulants.

Photosynthesis Monitoring in Microalgae Cultures Grown on Municipal Wastewater as a Nutrient Source in Large-Scale Outdoor Bioreactors.

Microalgae cultures were used for a WW treatment to remediate nutrients while producing biomass and recycling water. In these trials, raceway ponds (RWPs; 1 and 0.5 ha) were located next to a municipal (WW) treatment plant in Mérida, Spain. The ponds were used for continuous, all-year-round microalgae production using WW as a source of nutrients. Neither CO2 nor air was supplied to cultures. The objective was to validate photosynthesis monitoring techniques in large-scale bioreactors. Various in-situ/ex-situ methods based on chlorophyll fluorescence and oxygen evolution measurements were used to follow culture performance. Photosynthesis variables gathered with these techniques were compared to the physiological behavior and growth of cultures. Good photosynthetic activity was indicated by the build-up of dissolved oxygen concentration up to 380% saturation, high photochemical yield (Fv/Fm = 0.62-0.71), and relative electron transport rate rETR between 200 and 450 µmol e- m-2 s-1 at midday, which resulted in biomass productivity of about 15-25 g DW m-2 day-1. The variables represent reliable markers reflecting the physiological status of microalgae cultures. Using waste nutrients, the biomass production cost can be significantly decreased for abundant biomass production in large-scale bioreactors, which can be exploited for agricultural purposes.

Performance evaluation of a control strategy for photosynthetic biogas upgrading in a semi-industrial scale photobioreactor.

The validation of a control strategy for biogas upgrading via light-driven CO2 consumption by microalgae and H2S oxidation by oxidizing bacteria using the oxygen photosynthetically generated was performed in a semi-industrial scale (9.6 m3) photobioreactor. The control system was able to support CO2 concentrations lower than 2% with O2 contents ≤ 1% regardless of the pH in the cultivation broth (ranging from 9.05 to 9.50). Moreover, the control system was efficient to cope with variations in biogas flowrate from 143 to 420 L h-1, resulting in a biomethane composition of CO2 < 2.4%, CH4 > 95.5%, O2 < 1% and no H2S. Despite the poor robustness of this technology against failures in biogas and liquid supply (CH4 concentration of 67.5 and 70.9% after 2 h of biogas or liquid stoppage, respectively), the control system was capable of restoring biomethane quality in less than 2 h when biogas or liquid supply was resumed.

Urban wastewater photobiotreatment with microalgae in a continuously operated photobioreactor: growth, nutrient removal kinetics and biomass coagulation-flocculation.

The aim of this study was to investigate the growth, nutrient removal and harvesting of a natural microalgae bloom cultivated in urban wastewater in a bubble column photobioreactor. Batch and continuous mode experiments were carried out with and without pH control by means of CO2 dosage. Four coagulants (aluminium sulphate, ferric sulphate, ferric chloride and polyaluminium chloride (PAC)) and five flocculants (Chemifloc CM/25, FO 4498SH, cationic polymers Zetag (Z8165, Z7550 and Z8160)) were tested to determine the optimal dosage to reach 90% of biomass recovery. The maximum volumetric productivity obtained was 0.11 g SS L-1 d-1 during the continuous mode. Results indicated that the removal of total dissolved nitrogen and total dissolved phosphorous under continuous operation were greater than 99%. PAC, Fe2(SO4)3 and Al2(SO4)3 were the best options from an economical point of view for microalgae harvesting.

Technology validation of photosynthetic biogas upgrading in a semi-industrial scale algal-bacterial photobioreactor.

The performance of photosynthetic biogas upgrading coupled to wastewater treatment was evaluated in an outdoors high rate algal pond (HRAP) interconnected to an absorption column at semi-industrial scale. The influence of biogas flowrate (274, 370 and 459 L h-1), liquid to biogas ratio (L/G = 1.2, 2.1 and 3.5), type of wastewater (domestic versus centrate) and hydraulic retention time in the HRAP (HRT) on the quality of the biomethane produced was assessed. The highest CO2 and H2S removal efficiencies (REs) were recorded at the largest L/G due to the higher biogas-liquid mass transfer at increasing liquid flowrates. No significant influence of the biogas flowrate on process performance was observed, while the type of wastewater was identified as a key operational parameter. CO2 and H2S-REs of 99% and 100% at a L/Gmax = 3.5 were recorded using centrate. The maximum CH4 content in the biomethane (90%) was limited by N2 and O2 desorption.

Optimization of pilot high rate algal ponds for simultaneous nutrient removal and lipids production.

Special attention is required to the removal of nitrogen and phosphorous in treated wastewaters. Although, there are a wide range of techniques commercially available for nutrient up-take, these processes entail high investment and operational costs. In the other hand, microalgae growth can simultaneously remove inorganic constituents of wastewater and produce energy rich biomass. Among all the cultivation technologies, High Rate Algae Ponds (HRAPs), are accepted as the most appropriate system. However, the optimization of the operation that maximizes the productivity, nutrient removal and lipid content in the biomass generated has not been established. In this study, the effect of two levels of depth and the addition of CO2 were evaluated. Batch essays were used for the calculation of the kinetic parameters of microbial growth that determine the optimum conditions for continuous operation. Nutrient removal and lipid content of the biomass generated were analyzed. The best conditions were found at depth of 0.3m with CO2 addition (biomass productivity of 26.2gTSSm-2d-1 and a lipid productivity of 6.0glipidsm-2d-1) in continuous mode. The concentration of nutrients was in all cases below discharge limits established by the most restrictive regulation for wastewater discharge.

Urban wastewater treatment by seven species of microalgae and an algal bloom: Biomass production, N and P removal kinetics and harvestability.

This study evaluates the capacity of seven species and a Bloom of microalgae to grow in urban wastewater. Nutrient removal kinetics and biomass harvesting by means of centrifugation and coagulation-flocculation-sedimentation have been also tested. Results show that the best biomass productivities ranged from between 118 and 108 mgSS L(-1) d(-1) for the Bloom (Bl) and Scenedesmus obliquus (Sco). Regarding nutrient removal, microalgae were able to remove the total dissolved phosphorus and nitrogen concentrations by more than 80% and 87% respectively, depending on the species tested. The final total dissolved concentration of nitrogen and phosphorus in the culture media complies with the European Commission Directive 98/15/CE on urban wastewater treatment. Regarding harvesting, the results of coagulation-flocculation sedimentation using a 60 mg L(-1) dose of Ferric chloride were similar between species, exceeding the biomass removal efficiency by more than 90%. The results of centrifugation (time required to remove 90% of solids at 1000 rpm) were not similar between species, with the shortest time being 2.9 min for Sco, followed by the bloom (7.25 min). An overall analysis suggested that the natural bloom and Scenedesmus obliquus seem to be the best candidates to grow in pre-treated wastewater, according to their biomass production, nutrient removal capability and harvestability.

Capability of different microalgae species for phytoremediation processes: wastewater tertiary treatment, CO2 bio-fixation and low cost biofuels production.

Scenedesmus obliquus, Chlorella vulgaris, Chlorella kessleri and a natural Bloom were cultivated in batch experiments, under controlled conditions, in urban wastewater (WW) and synthetic wastewater (SW) under 5% CO2 in air, with the object of estimating their capacity for nutrient removal, carbon dioxide biofixation, and generation of valuable biomass. In both culture media, the Bloom (Bl) and Scenedesmus (Sc) showed higher final biomass concentration (dried weight, dw) than the other species; the maximum yield obtained was 1950 ± 243 mg L(-1) for Bl and the minimum 821 ± 88 mg L(-1) for Cv, both in synthetic wastewater. Maximum specific growth rate values do not show significant differences between any of the 4 strains tested (p ≤ 0.05), nor between the 2 culture media. A new homogeneous method of calculating productivities has been proposed. Nitrogen removal in all the reactors was higher than 90%, except for BlSW (79%), and for phosphorus, the removal was higher than 98% in all trials. Maximum CO2 consumption rates reached were 424.4 and 436.7 mg L(-1) d(-1) for ScSW and ScWW respectively.

Lipid production of microalga Ankistrodesmus falcatus increased by nutrient and light starvation in a two-stage cultivation process.

The aim of this work was to study the stimulation of lipid production on the microalga Ankistrodesmus falcatus by varying cultivation conditions during the stationary phase. The effect of three factors (presence and absence of nitrogen, phosphorus, and light) has been tested once the cultures reached the stationary phase with the aim to increase the value of the biomass for further applications. Lipid content, elemental composition, Nile red fluorescence evolution, and calorific value of microalgal biomass were studied as well as biomass growth. Biomass presented a lipid content of 36.54 % at the end of the first stage, while at the end of the second stage, the experiments with the absence of phosphorus increased their lipid content until 45.94 and 44.55 %, the first with nitrogen and light presence and the second with absence of all factors. The combination of phosphorus absence and nitrogen and light presence achieved the highest lipid productivity (20.27 mg/L/day). The two-stage strategy to culture microalgae is a feasible option to increase the economic or energetic value of biomass.

Comparing the use of different domestic wastewaters for coupling microalgal production and nutrient removal.

The streams from municipal wastewater treatment plants (WWTP) have been considered a valuable medium for mass cultivation of algal biomass. The aim of this work is to test and compare the performance of Chlorella vulgaris on several streams from five stages, from two different WWTP. The results showed biomass yields ranging from 39 to 195mg dry-weightl(-1)days(-1). The best performance as biomass production was obtained with the centrate (effluent from drying the anaerobic sludge). After testing a wide range of N/P ratios with centrate, the highest productivity and growth rates were obtained with the original N/P ratio (2.0) of this stream. The highest removal rates were of 9.8 (N) and 3.0 (P) mgl(-1)days(-1), in the centrate. Finally, this research also suggests that microalgal production seems to be a promising process when coupled to wastewater treatment.

Chlorella stigmatophora for urban wastewater nutrient removal and CO2 abatement.

Batch experiments were performed to study biomass growth rate, nutrient removal and carbon dioxide bio-fixation of the marine microalgae Chlorella stigmatophora. Four different cultures at different salinities were tested: wastewater (WW), synthetic wastewater (SWW), seawater (SW) and diluted seawater (DSW). Experimental results showed that Chlorella stigmatophora grew satisfactorily in all culture media, except in SWW where inhibition occurred. In all cases, biomass experimental data were fitted to the Verlhust Logistic model (R2 > 0.982, p < or = 0.05). Maximum biomass productivity (P(bmax)) and CO2 biofixation (P(vCO2)) were reached in the WW medium, 1.146g SSL(-1)day(-1) and 2.324g CO2L(-1)day(-1) respectively. The order of maximum specific growth rates (micro max) was WW >DSW>SW. In order to compare nitrogen and phosphorous removal kinetics, an estimation of the time required to reach the most restrictive concentration of total N and P in effluents as defined in the Directive 98/1565/CE (10 mg sigmaNL(-1) (T10(N)) and 1 mg sigmaPL(-1) (T1(p)) was performed. In the WW test T10(N) and T1(p) needed were of 45.15 and 32.27 hours respectively and at the end of the experimental the removal was in both 100%.