Pilot High Rate Research Articles

Impact of the algal-bacterial community structure, physio-types and biological and environmental interactions on the performance of a high rate algal pond treating biogas and wastewater

This study investigated the algal and prokaryotic community structure in an outdoors pilot High Rate Algal Pond (HRAP) that was simultaneously used for the treatment of high-strength wastewater and biogas upgrading. The algal and prokaryotic diversity was analyzed and correlated with biological and environmental parameters, and process quality indicators. The results obtained showed that the dominance of a versatile and resistant algae, such as Chlorella vulgaris, which is tolerant to high NH4+ and CO2 concentrations, concomitant to a specific prokaryotic population allowed for the obtention of biomethane with 95.2 ± 0.7% CH4, 1.4 ± 0.1% CO2 and no H2S. The bacterial community, fostered by the algal population, the light hours, and photoperiod had an important role on both biogas upgrading, through O2 and H2S elimination, and wastewater treatment. While the algal population was involved in the elimination of the ammonium of the wastewater through assimilation, the bacterial population was the main factor responsible for organic carbon, nitrate and phosphate removal, as well as nitrification. The low oxygen concentrations at night promoted bacterial denitrification, increasing the nitrite concentration in the treated wastewater. Through these findings, this research encourages photosynthetic biogas upgrading combined with wastewater treatment in HRAPs using an inoculum composed of a dominant microalgae, and an enriched prokaryotic population of aerobic chemo-organoheterotrophs, aerobic nitrifiers, and sulfur bacteria. Unlike the bacterial consortium in this study, this designed consortium can avoid the accumulation of nitrite in the cultivation broth and reduce the content of oxygen in the produced biomethane.

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 (N2O) emissions during real domestic wastewater treatment in an outdoor pilot-scale high rate algae pond

In order to determine if nitrous oxide (N2O) emissions could affect the sustainability of microalgae-based pond systems, N2O emissions were recorded from an outdoor 900 L pilot high rate algal pond (HRAP) fed primary wastewater over 1 year. The HRAP was mixed using a paddle wheel and operated at a hydraulic retention time (HRT) of 7.5–10 days. Direct N2O emissions ranged from 4.1 to 6400 μg N-N2O·m−2·d−1 (median of 560 μg N-N2O·m−2·d−1, n = 28) at 10 days HRT, and 70–18300 μg N-N2O·m−2·d−1 (median of 4200 μg N-N2O·m−2·d−1, n = 22) at 7.5 days HRT. Using 25–75% of the data, we estimated that HRAPs designed for nitrogen removal operated at 7.5 days HRT (i.e. 9.5 m2·capita-1 required) would generate 12–53 g N2O·capita·yr-1 which is 4–17 fold higher than the default value of 3.2 g N2O·capita·yr-1 given by the Intergovernmental Panel on Climate Change for centralized wastewater treatment plants with controlled nitrification and denitrification steps. When indirect N2O emissions (via nitrogen discharge and ammonia volatilization) are included, a HRAP operated at 7.5 days HRT could generate total emissions equivalent to 21–138 g N2O·capita−1·yr−1. When expressed as a % of the nitrogen input load into the system, the HRAP direct emissions (i.e. 0.13–0.57%) and total (i.e. 0.23–1.5%) where within the range of 0–14.6% reported in the literature for centralized wastewater treatment.

Simultaneous biodegradation of tetrahydrofuran, 3-buten-1-ol and 1,4-butanediol in real wastewater by a pilot high-rate UASB reactor

A stream of wastewater from a synthetic-fibers manufacturing plant with high amount of tetrahydrofuran (THF), 3-buten-1-ol (BTO) and 1,4-butanediol (BDO) results in high chemical oxygen demand (COD). A pilot high rate up-flow anaerobic sludge blanket (UASB) reactor, which recirculation rate was set at three times of feed flowrate, was conducted on-site and continuously run for 108 days. Volumeric organic loading rates were gradually adjusted from to 1.3 ± 0.2 to 21.5 ± 1.8 kg-COD·m−3 d−1 while corresponding COD in the influent increased from 1704 to the maximum up to 30700 mg L−1. Average removals of COD were 75.8%, while temperature, pH, dissolved oxygen, and oxidation reduction potential were intensively monitored throughout the pilot study. Average THF removals of 89.1% and complete BTO removals showed successful biodegradation in the UASB reactor. Meanwhile, average BDO removals increased significantly from 43.2% to 95.8%. In addition, increasing of biomass and food-to-microorganism ratios were performed well by rational regressions. Anaerobic granules were also developed well to the final size of 3–4 mm. Moreover, total biogas production rates were increased by operation days. The main biogas component (CH4) increases five times while the other two (CO2 and N2) were kept steady. In general, start-up of the UASB for biodegradting real wastewaters containing high concentration of C4 chemicals was firstly demonstrated in the pilot study.

Microalgae and bacteria dynamics in high rate algal ponds based on modelling results: Long-term application of BIO_ALGAE model

The mechanistic model (BIO_ALGAE) for microalgae-bacteria based wastewater treatment systems simulation was validated in the long-term (months) using experimental results from a pilot high rate algal pond (HRAP) treating municipal wastewater. Simulated results were compared with data gathered during two different seasons (summer and winter), and with the HRAP operating at different hydraulic retention times (HRT, 4 and 8 days, respectively). The model was able to simulate with a good degree of accuracy the dynamics of different components in the pond, including the total biomass (bacteria and microalgae). By means of practical study cases, the influences of different HRT operating strategies and seasonal variations of temperature and irradiance were investigated for the relative proportion of microalgae and bacteria, and biomass production over a year cycle. Model predictions show that the proportion of microalgae in the microalgal/bacterial biomass is quite similar in warmer months if the pond is operated with 8-day HRT (76–78%) or 4-day HRT (60–75%). Significant differences were observed in colder months (4-day HRT (27–33%) and 8-day HRT (65–68%)). The model identified a scenario in which overall microalgae production and ammonium removal efficiency were optimized. By operating the HRAP with lower HRT (4 days) in warmer months and higher HRT (8 days) in colder months, the average annual microalgae production increased up to 14.1 gTSS m-2d-1, in contrast with 10.2 gTSS m-2d-1 and 9.2 gTSS m-2d-1 operating with constant HRAP (4 and 8 days, respectively) over a year cycle.

Integral (VOCs, CO2, mercaptans and H2S) photosynthetic biogas upgrading using innovative biogas and digestate supply strategies

The performance of a pilot high rate algal pond (HRAP) interconnected with a biogas absorption column during the simultaneous upgrading of biogas and treatment of digestate was evaluated under two innovative biogas and nutrient supply strategies. Process operation with biogas supply during the night at a liquid recirculation/biogas ratio of 0.5 to prevent N2 and O2 stripping resulted in a biomethane complying with most international regulations for injection into natural gas grids (99.1 ± 1% CH4, 0.5 ± 0.2% CO2, 0.6 ± 0.5% N2 and 0.07 ± 0.08% O2). The potential of this technology to remove methyl mercaptan (MeSH), toluene and hexane from biogas (typically present at trace levels) was assessed, for the first time, with removal efficiencies under steady-state correlating with pollutant hydrophobicity (7 ± 7% for hexane, 66 ± 4% for MeSH and 98 ± 1% for toluene). Finally, the supply of digestate during the dark period shifted both microalgae population structure and biomass composition in the HRAP without a significant impact on biomethane quality. Overall, the removal of nitrogen and phosphorous from digestate in the HRAP was almost complete (96–99%) regardless of the nutrient supply strategy.

Towards energy neutral microalgae-based wastewater treatment plants

The aim of this study was to assess the energy balance of a hypothetical microalgae-based wastewater treatment plant (10,000 PE) located in the Mediterranean Region, where harvested microalgal biomass and primary sludge would be co-digested to produce biogas and bioenergy. The assessment was based on experimental results obtained over one year in pilot high rate algal ponds followed by anaerobic digesters for biogas production from harvested microalgal biomass and primary sludge. The energy balance compared four scenarios: 1) anaerobic co-digestion of microalgal biomass and primary sludge, and cogeneration from biogas in a combined with heat and power (CHP) unit; 2) co-digestion with thermal pretreatment of microalgal biomass, and cogeneration from biogas in a CHP unit; 3) co-digestion and heat generation from biogas in a boiler; and 4) co-digestion with thermal pretreatment of microalgal biomass, and heat generation from biogas in a boiler. According to the results, when biogas was used to cogenerate electricity and heat (scenarios 1 and 2), the electricity balance was always positive, and the best results were obtained with pretreated microalgal biomass (scenario 2). Similarly, the heat balance was always positive when biomass was thermally pretreated (scenario 2). On the other hand, when biogas was only used to produce heat (scenarios 3 and 4), heat requirements were covered during the whole year. The sensibility analysis of the scenarios with pretreatment (2 and 4) confirmed that the microalgae-based WWTP would be energy neutral or even net energy producer.

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.

Thermal pretreatment and bioaugmentation improve methane yield of microalgal mix produced in thermophilic anaerobic digestate

Liquid fraction produced in anaerobic digestion (AD) of biodegradable waste can be treated on-site with microalgae, which can be recycled back as substrate to the biogas plant. For this research, a pilot high rate algal pond (HRAP) was set with connections to a full scale biogas plant that enabled the use of waste heat and CO2 from a combined heat and power gen-set (CHP). The microalgal mix produced in the thermophilic anaerobic digestate supernatant was tested as a substrate for biogas production in the thermophilic AD (i.e. untreated, bioaugmented with anaerobic bacteria Clostridium thermocellum, and thermally pretreated, respectively). The methane potential of the untreated microalgal mix was low (157.5 ± 18.7 mL CH4/g VS). However, after the thermal pretreatment of the microalgae, methane production increased by 62%, while in the bioaugmentation with C. thermocellum under thermophilic conditions (T = 55 °C) it was elevated by 12%. The outcome of our pilot trial suggests that microalgae produced in the thermophilic biogas digestate represent a prospective alternative AD feedstock. At the same time, microalgae reduce the digestate nitrogen and COD to the level sufficient for the outflow to meet the quality required by the sewage system (ammonia-nitrogen max 200 mg/L, nitrite max 10 mg/L).

Photosynthetic biogas upgrading to bio-methane: Boosting nutrient recovery via biomass productivity control

A pilot high rate algal pond (HRAP) interconnected to an external CO2–H2S absorption column via settled broth recirculation was used to simultaneously treat a synthetic digestate and to upgrade biogas to a bio-methane with sufficient quality to be injected into natural gas grids. An innovative HRAP operational strategy with biomass recirculation based on the control of algal-bacterial biomass productivity (2.2, 4.4 and 7.5gm−2d−1) via settled biomass wastage was evaluated in order to enhance nutrient recovery from digestate at a constant hydraulic retention time. The influence of the recycling liquid to biogas (L/G) ratio on the quality of the upgraded biogas was assessed. The bio-methane composition under a L/G ratio of 1 (0.4±0.1% CO2, 0.03±0.04% O2, 2.4±0.2% N2 and 97.2±0.2% CH4) complied with the technical specifications of most European bio-methane legislations regardless of the biomass productivity established. The HRAP operational strategy applied allowed increasing the N and P recovery from 19 and 22% to 83 and 100%, respectively, when the biomass productivity was increased from 2.2 to 7.5gm−2d−1. Finally, the dynamics of microalgae and bacteria population structure were characterized by morphological identification and Denaturing Gradient Gel Electrophoresis analysis.

A case study of a pilot high rate algal pond for the treatment of fish farm and domestic wastewaters

ABSTRACTBACKGROUNDMicroalgae‐based technologies have emerged as a promising approach for simultaneous wastewater treatment and fish food production able to overcome the main limitations in the aquaculture sector. The current study focused on the mechanisms of carbon and nutrient removal from fish farm and domestic wastewaters in a 180 L outdoors pilot high rate algal pond (HRAP) at different hydraulic retention times (HRTs) considering water evaporation losses.RESULTSMaximum chemical oxygen demand, total Kjeldahl nitrogen and total phosphorus removal efficiencies of 77 ± 9%, 83 ± 10% and 94 ± 6%, respectively, were recorded during the treatment of fish farm wastewater at 10 d of HRT. Carbon and nitrogen were removed by assimilation into biomass (52 ± 12% and 74 ± 22%, respectively) and stripping, while phosphorus was mainly assimilated into biomass (69 ± 23%). Carbon stripping, along with the low carbon and nutrient loading rates supplied, resulted in low biomass productivities (maximum of 5 g m−2 d−1). A successful solids removal was achieved in the settler (82 ± 18%), which entailed effluent solid concentrations below the maximum permissible discharge limit.CONCLUSIONDespite the successful wastewater treatment supported by the HRAP, the high water evaporation losses (up of 15 L m−2 d−1) could compromise the technical and environmental viability of this green wastewater treatment technology. © 2014 Society of Chemical Industry

Dynamics of Nitrogen Transformation and Removal in a Pilot High Rate Pond

The transformation and removal of nitrogen was studied in a pilot high rate pond with a surface area of 10.2 m2 and water depth of 60 cm. The pilot unit received wastewater from an existing field scale primary facultative pond at the University of Dar es Salaam. Wastewater samples were collected from the influent and effluent of high rate pond and were analyzed for physical-chemical parameters in the laboratory and in situ. An appropriate model complexity was selected, from which a conceptual model was then developed to model various processes in the system using STELLA 6.0.1 software. The study demonstrated that dominant nitrogen transformation processes in HRP were nitrification and denitrification, which transformed 0.95 and 0.87 gN/m2·d, respectively. These were followed by mineralization (0.37 gN/m2·d), ammonia uptake by microorganisms (0.34 gN/m2·d), volatilization (0.30 gN/m2·d), sedimentation (0.24 gN/m2·d), and regeneration (0.15 gN/m2·d). Uptake of nitrate was not observed because of microorganisms preference for ammonia, which was abundant in the pond. The major nitrogen transformation mechanisms in high rate pond were denitrification, net sedimentation and volatilization, which accounted for 69.1%, 7.1% and 23.8% of the total permanent removal mechanisms of nitrogen in High Rate Pond.

Microalgal-Biotechnology As a Platform for an Integral Biogas Upgrading and Nutrient Removal from Anaerobic Effluents

The potential of a pilot high rate algal pond (HRAP) interconnected via liquid recirculation with an external absorption column for the simultaneous removal of H2S and CO2 from biogas using an alkaliphilic microalgal-bacterial consortium was evaluated. A bubble column was preferred as external absorption unit to a packed bed column based on its ease of operation, despite showing a comparable CO2 mass transfer capacity. When the combined HRAP-bubble column system was operated under continuous mode with mineral salt medium at a biogas residence time of 30 min in the absorption column, the system removed 100% of the H2S (up to 5000 ppmv) and 90% of the CO2 supplied, with O2 concentrations in the upgraded biogas below 0.2%. The use of diluted centrates as a free nutrient source resulted in a gradual decrease in CO2 removal to steady values of 40%, while H2S removal remained at 100%. The anaerobic digestion of the algal-bacterial biomass produced during biogas upgrading resulted in a CH4 yield of 0.21-0.27 L/gVS, which could satisfy up to 60% of the overall energy demand for biogas upgrading. This proof of concept study confirmed that algal-bacterial photobioreactors can support an integral upgrading without biogas contamination, with a net negative CO2 footprint, energy production, and a reduction of the eutrophication potential of the residual anaerobic effluents.

Chlorophyll and carotenoid composition of the algal flora developed under immobilized conditions—Influence of the support

The amount and type of pigments of the algal microflora developed in four pilot high rate waste water treatment ponds have been studied. The conditions for culture development included: immobilization supports of silica type, such as sepiolite and montmorillonite, as well as culture development in the absence of support, under different environmental conditions. Qualitative differences were not found between the pigments present in any of the biomass studied. However, individual HPLC quantification of the main pigments showed clear differences depending on the source of the algal microflora. In consequence, very small changes in the culture conditions produced significant changes in the pigment content, without drastic changes in other parameters such as dry matter content, due to the selective changes of algal species.