Semi-continuous Feeding Research Articles

Product evaluation of hydrothermal carbonization of biomass: semi-continuous vs. batch feeding

Interest in hydrothermal carbonization (HTC) is increasing worldwide due to its outstanding capabilities. This study introduces a novel semi-continuous feeding system to feed the biomass slurry against the high pressure of an HTC reactor. As expected in a semi-continuous operation, the biomass would be instantaneously exposed to the desired pressure and temperature, instead of a gradual increase to the desired temperature. Higher degrees of carbonization, lower cellulose crystallinity, improved hexagonal graphene sheets, surface hydrophobic characteristics, and higher thermal stability were observed at elevated temperatures in products from semi-continuous HTC when compared with products from batch operation. Reaction pathways were proposed for both batch and semi-continuous operations on the basis of physical and chemical characterization results. For HTC at 260 °C, the proposed semi-continuous system showed approximately 3% higher carbon content in the obtained hydrochar and a 25-min reduction in required time to reach 260 °C. The promising conclusion of these results is that by introducing continuous systems in HTC, not only will the time of the process cycle decrease, it will also achieve higher degrees of carbonization.

Methane production potential and anaerobic treatability of wastewater and sludge from medium density fibreboard manufacturing

This research evaluates the suitability of anaerobic digestion of different waste streams derived from Medium Density Fibreboard (MDF) manufacturing. Methane production potential and anaerobic treatability were firstly assessed in batch assays for raw wastewater, physical-chemical pre-treated wastewater, as well as for sludge generated by current wastewater treatment at an industrial facility. Single-feeding and multi-feeding (semi-continuous feeding) experiments were conducted to quantify methane production and methanogenic toxicity. Digestion of the raw wastewater produced 4.0 L CH4/L, which required dilution of the wastewater to 30–40% because severe toxicity was observed at 40% raw wastewater. Digestion of current process sludge produced 21.4 mL CH4/g raw sludge, but it also required dilution to avoid toxicity. Working with these substrates, the necessary dilution led to a maximum concentration of soluble chemical oxygen demand (COD) of 4.2–4.6 g/L in the digestion medium, while the conversion of total COD to methane remained below 22%. Pre-treated wastewater in the current industrial process did not show inhibition, but gave a low methane production of only 0.8 L CH4/L. An alternative treatment scheme consisting of simple sedimentation of raw wastewater followed by pre-aeration showed enhanced methane production of 4–4.8 L CH4/L at higher concentrations of up to 80% wastewater in the assay medium. Sedimentation allowed increasing the soluble COD in the digestion medium up to 6.9 g/L, while the additional pre-aeration further increased the soluble COD to 12.9 g/L without inhibition of methane production. Conversion of total COD to methane during the exponential phase increased to 64%. Simple sedimentation also removed 82% of suspended solids, which would help prevent clogging and improve anaerobic treatability in continuous digesters.

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.

Effect of substrate concentrations on methane and hydrogen biogas production by anaerobic digestion of a cassava starch-based polymer

This study aims to evaluate the two-phase anaerobic digestion process of a cassava starch-based polymer, determining the ideal organic load to obtain the best results in terms of removal of solids and production of methane and hydrogen. Two reactors were used: one with an agitation and mixing system for the acidogenic phase, and the other was a tubular reactor for the methanogenic phase, both with ascending flow and semi-continuous feeding, with the effluent from the acidogenic reactor conducted from the methanogenic reactor. The loads were defined as concentrations of 8, 10, 12, and 14 g L−1 (on a wet basis), having HRT of 5 days for the acidogenic reactor and 20 days for the methanogenic. The analysis of the results showed that the concentration of 10 g L−1 had the best results for hydrogen and methane production in acidogenic and methanogenic phases, presenting 19.9 mL gVS-1 and 249.1 mL gVS−1, respectively. This treatment also showed the highest levels of gases in the biogas (43.2 % hydrogen in the acidogenic phase, and 76.6 % methane in the methanogenic phase). In addition, it showed the highest volatile solid removal means, reaching 84.0 % in the methanogenic phase. Thus, the anaerobic biodigestion process in two phases for the degradation of the polymer based on cassava starch presents technical feasibility, with high methane and hydrogen production; and concentrations above 10 g L−1, under the test conditions, cause destabilization in the reactors by the great formation of volatile acids and consequent reduction in the stability of microbiological cultures.

Start-up of dry semi-continuous OFMSW fermentation for methane production

This work proposes a strategy for start-up of dry semi-continuous reactors for methane production without using adapted inoculum. Using OFMSW as substrate, three reactors operating at 35 °C were used. Experimentation consisted in start-up, adjusting operational parameters (80 days) and, after start-up was successful, semicontinuous feeding under different organic loading rates (another 80 days). After the first 22 days, to control acidification, feeding was stopped. On day 42, slow feeding was restarted and, after day 50, the reactors produced methane under stable conditions and, after day 70, semicontinuous maximum feed was achieved with maximal methane production. Although the values for methane production vary between 339 and 333 NL kgVS−1, the statistical analysis indicates indeed significant differences. With increasing organic load, methane production rate increased from 1.89 to 2.50 m3 m−3 d−1 and volatile solids removal from 50 to 59%. According to the results, a successful start-up is possible under slow gradual feeding increase using alkalinities and VFA as monitoring parameters.

Identification of potential waste seeds of wildly growing tree species for the production of biogas.

In this experimental study, seeds of wild tree species namely Acacia nilotica, Prosopis juliflora, Albizia lebbeck, and Leucaena leucocephala were explored as potential feedstocks for anaerobic digestion and compared with cattle manure which is a commonly used feedstock. These seeds occur abundantly as waste biomass in tropical and subtropical parts of Asia, Africa, and the USA. An experimental investigation was carried out in large 300-L anaerobic digesters under semi-continuous feed mode for 90days. The average specific methane production yield observed was 0.208Nm3/kg volatile solids (VS) for A. nilotica, 0.227Nm3/kg VS for P. juliflora, 0.219Nm3/kg VS for A. lebbeck and 0.210Nm3/kg VS for L. leucocephala which was found to be higher than cattle manure's yield of 0.146Nm3/kg VS. Experimental analysis revealed an average methane content of more than 52% for all the seeds and a total volatile solid mass removal efficiency of 41.60% for A. nilotica, 44.19% for P. juliflora, 43.76% for A. lebbeck, and 41.41% for L. leucocephala which was higher than 29.7% for cattle manure. The experimental investigations showed that they have a higher biogas production potential than cattle manure indicating their huge scope and suitability as alternative feedstocks, and their use can also mitigate the ecological risk seeds pose by growing into invasive trees.

Complementing energy production with nutrient management: Anaerobic digestion system for algal turf scrubber biomass

Algal turf scrubbers (ATS) and anaerobic digesters (AD), when combined, remove nutrients from eutrophic water while producing renewable energy through biogas production. Algal biomass was produced from an ATS system that used tipping buckets to introduce riverine water in pulses onto a flow-way with a mesh screen installed to facilitate algal attachment. This study was the first to determine methane (CH4) production from AD of a micro- and macro-algae polyculture, harvested from an ATS, using lab-scale reactors under batch and semi-continuous feeding regimes. The lab-scale study was followed by a pilot-scale (7 m3) ATS-AD study, the largest scale study of biogas production from ATS-derived algae. Batch digestion of ‘wet’ algae (93% moisture) produced 158 ± 13 L CH4/kg VS, which was higher than algae with a lower moisture content (≤62% moisture). Continuing use of ‘wet’ algae as the substrate, the 2 L semi-continuously-fed lab system yielded a maximum CH4 production value of 144 ± 7 L CH4/kg VS, with 60%–62% CH4 in the biogas. The 7 m3 pilot-scale system produced 107 ± 15 L CH4/kg VS, with 66.4% CH4 in the biogas. Scale-up calculations showed that a 1 ha ATS system could capture 8.73 kg N, 1.18 kg P, and 52.5 kg C/ha-d, with the resulting algae being used in AD to produce biogas for direct heating (2.8*108 J) or to power a 1.13 kW generator. While the total energy output from the 1 ha ATS-AD system is relatively low, energy production from the AD system is a supplemental benefit to an ATS system that captures nutrients from eutrophic water. This ATS-AD system could contribute to the overall Chesapeake Bay cleanup, especially in nutrient hot-spots, like the Baltimore Harbor, where high nutrient concentrations support high algal productivity, which would provide substrate for on-site energy production from AD.

Synthesis and characterization of size-tunable core-shell structural polyacrylate-graft-poly(acrylonitrile-ran-styrene) (ASA) by pre-emulsion semi-continuous polymerization

The core-shell polymeric polyacrylate-graft-poly(acrylonitrile-ran-styrene)(ASA) has been explored for the polymer industry due to its excellent properties. However, it has been a longstanding challenge to synthesize large size-tunable particles by the conventional batch polymerization. In this paper, an accurate method, i.e. pre-emulsion semi-continuous polymerization, is applied to prepare ASA polymeric particles, which makes a series ofdifferent particle sizes (100–450 nm) of the crosslinked poly(butyl acrylate) (PBA) inner core coated with a hard shell using the poly(acrylonitrile-ran-styrene) (AS) copolymer. Specifically, the feed modes and proportion of seedare investigated to tune the particle size.The results show that the PBA seed microsphere in pre-emulsion semi-continuous polymerization is enlarged by accurate feed rate, but not in batch polymerization. A possible mechanism for the growth of the PBA core is proposed that secondary particles from homogeneous nucleation in semi-continuous feed mode is less than that of batch mode, which attenuate the impact on the growth of core particle. The size and morphology of resulting particles are characterized using various analytical techniques including the transmission electron microscopy (TEM), the scanning electron microscopy (SEM) and the dynamic light scattering (DLS). Core-shell separates phase corresponding to different glass transition temperatures is evaluated using differential scanning calorimeter (DSC) analyses indirectly. Moreover, the mechanical properties including impacts and tensile strengths are also analyzed. This study thus highlights a detailed strategy to tune polymeric particles related to different properties and the nucleating mechanism of seed emulsion polymerization that governs structure, particle size and distribution of ASA polymer with a bespoke structure for various application.

Semi-continuous feeding combined with traditional domestication improved anaerobic performance during treatment of cassava stillage

The effects of feeding pattern were studied during anaerobic digestion of cassava stillage. Continuous feeding and semi-continuous feeding, were adopted in two internal circulation (IC) reactors (A and B, respectively). The reactors showed different performance in the anaerobic digestion process. The maximum difference, was observed for the soluble chemical oxygen demand (SCOD) removal rate and the biogas production, which were 23.2% and 95.7 L/2 d higher in reactor B than reactor A, respectively. The overall VFAs level of reactor A was higher than that of reactor B. Microbial community analyses indicated that the abundances of dominant bacteria and methanogens became higher in the reactor B than in reactor A as the digestion process progressed. Hence, semi-continuous feeding showed superior performance than continuous feeding for SCOD removal rate, biogas production, and the relative abundances of methanogens in the case of high OLR.

Holistic determination of suitable conditions for biogas production from Pennisetum purpureum x Pennisetum americanum liquor in anaerobic baffled reactor

Holistic approach had been undertaken to determine the suitable conditions for biogas production from Pennisetum purpureum x Pennisetum americanum (Napier Pak Chong1) liquor using the anaerobic baffled reactor (ABR). Effects of factors, i.e. organic loading rates (OLRs), feeding schemes, trace element additions and effluent recirculation rates, on ABR performance were systematically investigated. Increase of OLRs and effluent recirculation rates adversely affected methane yields when capability of ABR in containing microorganisms was deteriorated. High stability of ABR performance was detected under the semi-continuous feeding scheme with trace element additions. The suitable condition for ABR was found at the OLR of 4.0 kg COD/m3.d under the semi-continuous feeding scheme with trace element additions at the effluent recirculation rate of 0.5 (QR/Q). At this condition, high methane yield (0.49 ± 0.05 Nm3/kg VSadded) could be achieved using the economical ABR at relatively high OLR of 4.0 kg COD/m3.d.

Pre-treatment of filter cake for anaerobic digestion in sugarcane biorefineries: Assessment of batch versus semi-continuous experiments

Anaerobic digestion (AD) of sugarcane filter cake (SFC) was investigated by comparing the performance of pre-treatment methods in biochemical methane potential (BMP) tests and semi-continuous experiments. For that, SFC was pre-treated by autoclaving the substrate alone or with sodium hydroxide (NaOH). Experimental data from BMP tests were fitted to a kinetics model and further used for simulating the AD process in a continuous stirred-tank reactor (CSTR). BMP tests showed differences (p < 0.05) in total methane potential (SBMP), which have affected methane yields during simulation in a CSTR. Untreated produced 185 mL CH4 gVS−1, autoclaved pre-treatment 174 mL CH4 gVS−1 and autoclaved with NaOH pre-treatment 222 mL CH4 gVS−1. Interestingly, such higher performance of autoclaved with NaOH pre-treatment was only observed at earlier stages during semi-continuous feeding experiment. At steady-state no significant differences (p > 0.05) in terms of methane yield were observed among the reactors (average of 224 mL CH4 gVS−1). These results demonstrate that the benefits of pre-treatment could only be observed in BMP tests, which is likely explained by a better adaptation of the microbial community to the substrate during long term semi-continuous experiment, making SFC pre-treatment ineffective in a single-stage CSTR and under this feeding regime.

Biogas production by two-stage thermophilic anaerobic co-digestion of food waste and paper waste: Effect of paper waste ratio

Abstract The effects of paper waste (PW) ratio on biogas production by two-stage thermophilic anaerobic co-digestion (coAD) of food waste (FW) and PW, two main components of organic fraction of municipal solid waste, were evaluated in lab-scale continuously stirred tank systems with semi-continuous feeding. Four PW ratios (0, 20, 40, and 50%) were studied in the test. The experimental results showed the two-stage thermophilic coAD could operate stably after an adjustment period at all PW ratios. Although the increase of PW ratio decreased methane content, production and yield, reaction rates of four AD steps, it could increase the methane production of substrate when the FW was fixed. The pH, total volatile fatty acids, total alkalinity, and total ammonium of the sludge in the second stage of AD, also decreased with the increase of PW ratio in the test. The change of the substrate's organic composition, increase of carbohydrate and decrease of protein and lipid, were the main reason for the decrease of methane content, production and yield and changes of the second stage sludge characteristics. An apposite PW ratio (PW-20%) could enhance the degradation of matters (TS, VS and TCOD).

Glycerine supplementation as a recovery strategy of long-chain fatty acids inhibition on anaerobic digestion

In anaerobic digestion, less attention has been focused on the digestion of grease trap waste as a single substrate may be due to high lipid content that may cause inhibition effects resulted from long-chain fatty acids accumulation during the degradation of lipid. The inhibition results in the low and slow production of methane. This study investigates the influence of glycerine supplementation on the continuous stir tank reactor anaerobic treatment of grease trap waste. The operating procedures began with a start-up process and continued with a semicontinuous feeding process in an increasing organic loading rate of 1.3–3.6 gCOD/L day. The results showed that the resistance toward grease trap waste inhibition in a glycerine-supplemented reactor (RGS), measured as lag phase, can be shortened to 4 days during the start-up as compared to the control reactor (Rcontrol). With the reduction in lag-phase occurrence, the performance of RGS reactor affected the overall methane production. At standard temperature and pressure, RGS reactor performance of methane composition was 67%, with methane production rate of 0.376 LCH4/L day and methane yield of 0.19 LCH4/gCODremoved. These results were observed at an organic loading rate of 2.2 gCOD/L day. The overall methane production enhancement from the glycerine-supplemented reactor was 25%. Palmitate accumulation indicated the possible main inhibitor during the lag-phase experience. Furthermore, the system was able to recover methane production performance during semicontinuous feeding stage. The use of glycerine supplementation is a reliable strategy to induce the start-up of anaerobic digestion process of high-lipid substrate.

Year-round biogas production in sugarcane biorefineries: Process stability, optimization and performance of a two-stage reactor system

The concept of year-round biogas production to increase the capacity factor of anaerobic digestion (AD) plants in sugarcane biorefineries was investigated for the first time in semi-continuous feeding mode. To simulate the use of sugarcane vinasse during the sugarcane season and sugarcane filter cake (SFC) during the off-season period, a two-stage reactor system based on an acidogenic continuous stirred-tank reactor (1st stage) followed by solid–liquid separation and an upflow anaerobic sludge blanket (UASB) reactor (2nd stage) to convert the COD-rich liquid fraction into biogas was operated. Additionally, to optimize the biogas production from SFC, the effects of its thermo-chemical pre-treatment on AD were investigated in a parallel reactor set-up. The saponification effect provided by autoclaving the substrate with sodium hydroxide improved the hydrolysis/fermentation of SFC in the acidogenic reactor, which in turn resulted in a 28% higher volumetric methane production in the methanogenic reactor (p < 0.05). However, the methane yields observed during operation of the two-stage reactor system were markedly lower than previously found in biochemical methane potential tests using SFC. In this case, the feed-in with low suspended solids required by UASB reactors prevented the utilization of the non-hydrolyzed/fermented solid fraction of SFC (> 60% of the substrate’s methane potential). Nevertheless, the capacity factor of the AD plants in sugarcane biorefineries could be increased from 0.55 up to 0.69 when considering a 200 d a−1 sugarcane season (0.66–0.83 for a longer season of 240 d a−1), representing an increase of 25.7%. The average capacity factor for biogas combined heat and power and upgrading units of around 0.91 (8000 h a−1) would be reached if further developments could improve the solubilization of non-hydrolyzed/fermented solids or alternatively allow their direct use in the methanogenic reactor.

Obtaining green energy from dry-thermophilic anaerobic co-digestion of municipal solid waste and biodiesel waste

This study has been conducted to optimise the dry-thermophilic anaerobic co-digestion of two different types of real waste (municipal solid waste and biodiesel waste) in a semi-continuous feeding regime, under high organic loading rates (OLRs) or low hydraulic retention times (HRTs). For this purpose, different high OLRs (from 10 to 34 g [VS] l−1 d−1) or low HRTs (from 10 to 4.4 d) were investigated in a continuously stirred tank reactor. Optimal conditions (257 ml [CH4] g−1 VS; 6.03 ± 0.43 l [CH4] l−1 d−1; 16.3 ± 2.3 × 10−12 l [CH4] cell−1 d−1) were obtained at 4.4 d HRT (OLR = 23 g [VS] l−1 d−1), in which the average values of the ratios of Eubacteria:Archaea and hydrolytic-acidogenic bacteria:acetogens were 95:5 and 87:8, respectively.

Nitrogen and Phosphorous Removal from Municipal Wastewater Using High Rate Algae Ponds

Eutrophication due to uncontrolled discharge of sewage water rich in nitrogen and phosphorous is one of the most significant water quality problems world-wide. Nitrogen and phosphorous can be removed by activated sludge process, one of the widely applied technologies among other conventional methods for municipal wastewater treatment. However, such technology requires high capital and operational costs, making it unaffordable for many developing nations, including Ethiopia. This study aimed to investigate the performance of two high rate algal ponds (HRAPs) in nitrogen and phosphorous removal from primary settled municipal wastewater under high land tropical climate conditions in Addis Ababa. The experiment was run under semi-continuous feed for 2 months at hydraulic retention times (HRT) ranging from 2 to 8 days and organic loading rates ranging from 44.3 to 9.08 g COD/m2/day using two HRAPs 250 and 300 mm deep, respectively. In this experiment, Chlorella sp., Chlamydomonas sp., and Scenedesmus sp. in the class of Chlorophyceae were identified as the dominant species. The maximum TN and TP removal of 91.70 and 82.81% was achieved in the 300 mm deep HRAP during 8 and 6 day HRT operations, respectively. Increased HRT and pond depth increased nutrient removal but high chlorophyll-a biomass was observed in the 250 mm deep HRAP. Therefore, the 300 mm deep HRAP is promising for scaling up nutrient removal from municipal wastewater at a daily average organic loading rate in the range of 14.3–15.33 g COD/m2/day or 10.34–11.46 g BOD5/m2/day and a 6 day HRT. We conclude that HRAP is a dependable approach to remediate nitrogen and phosphorous from primary settled municipal wastewater in Addis Ababa climate with appropriate control of pond depth, organic loading rates and HRT.

Syngas production from polyethylene and biogas in porous media combustion

The production of syngas from biogas (surrogate of CH4/CO2: 55/45 v/v) and polyethylene in a porous media combustion reactor is experimentally studied. The employed setup is novel and has not been studied before. A semi-continuous feed of solid fuel and a constant filtration velocity of the gaseous reactants of 17 cm/s were considered. Temperature, velocity of propagation, and composition of the syngas produced in the combustion waves were registered in a tubular reactor packed with a ceramic foam porous medium and two solid fuel inlets. In the first part of the study, a baseline determined by the filtration combustion of a biogas/air mixture through the ceramic foam at the equivalence ratio (∅) range 0.7≤∅≤1.6, having transient (upstream and downstream) and stationary combustion wave propagation regimes, is established. In the second part of this work, portions of the ceramic foam in two different separated zones are extracted, leaving space for the semi-continuous supply of polyethylene. In this second part the biogas-air combustion was performed only for ∅=0.8 and ∅=1.6. Although the combustion temperature decreased by the presence of polyethylene, it was found that the syngas (both H2 and CO) yield was larger than for the baseline. The highest degrees of conversion to hydrogen and carbon monoxide was reached under the presence of polyethylene, having 45% and 67% for ∅=0.8, and 45% and 60% for ∅=1.6, respectively. These results are very promising and they demonstrate the capabilities of the presented methodology and experimental setup, which should encourage future attempts of applications of the technology.

ENHANCEMENT STRATEGY OF METHANE PRODUCTION FROM ANAEROBIC DIGESTION START-UP PROCESS OF GREASE TRAP WASTE

Anaerobic digestion is one of the potential processes of waste lipid recovery for beneficial use to produce methane (CH4). In anaerobic digestion, the utilization of grease trap waste for mono digestion is less reported. This might be due to high lipid content in grease trap waste that may cause inhibition effects which resulted from long chain fatty acids (LCFA) accumulation during the degradation process. This study is intended to investigate the effects of lipid acclimated biomass (LAB) and non-acclimated biomass (NAB) in the anaerobic treatment of grease trap waste over increasing organic loading rate (OLR) in continuous stir tank reactor. The results showed that the resistance of grease trap waste toxicity was higher in LAB while in inhibited reactor (NAB), 9 days lag phase occurred during the start-up process and affected the overall CH4 production. At OLR of 2.2 gCOD/L.day, high CH4 was yielded of 0.22 LCH4/gCODremoved at standard temperature and pressure with 11% CH4 enhancement in LAB. The inoculum acclimatization is one of the strategy to improve CH4 production and the purpose is to provide favorable condition to the methanogens towards build-up of volatile acids and inhibitive components such as LCFA. Thus, the overall enhancement for acclimated to LCFA inoculum strategy was 42%. The induction during the start-up had promoted and enhanced CH4 production when semi-continuous feeding was introduced and remain higher than the other reactors throughout the entire experiment. This indicates that the use of biomass consortium acclimated to long chain fatty acids compounds is a reliable strategy to speed up the start-up of anaerobic digestion process and to enhance the overall CH4 yield.

Thermophilic Digestion of Food Waste by Dilution: Ammonia Limit Values and Energy Considerations

Source segregated domestic food waste (FW) collected by Veolia Environmental Services was used as a test substrate for thermophilic anaerobic digestion in a laboratory-scale trial with semicontinuous feeding at an organic loading rate of 3 g of volatile solids (VS) L–1 day–1, reached after an acclimatization period of 60 days. The FW had a total Kjeldahl nitrogen content of 0.73% on a wet weight basis which, without dilution, gave a digestate total ammonia nitrogen (TAN) concentration of >5.0 g of N L–1. This led to the accumulation of propionic and other longer-chain volatile fatty acids (VFA), followed by a sharp increase in acetic acid, which was sufficient to overcome the ammonia buffering capacity, causing the pH to fall and gas production to cease. In digesters run in parallel, TAN concentrations were regulated by diluting the FW with water. This allowed the determination of critical threshold concentrations for TAN, and for free ammonia nitrogen (FAN) by calculation, and monitoring of the pattern o...

Experimental study for the reduction of CO2 emissions in wastewater treatment plant using microalgal cultivation

Wastewater treatment plants (WWTPs) contribute to CO2 emissions in atmosphere through direct (biological metabolism) and indirect (fuel combustion) oxidation of organic carbon. This detrimental effect of WWTPs operation can be mitigated by integrating the traditional treatment with a microalgae cultivation pond where CO2 is fixed into autotrophic biomass and the positive side effect of removing nutrients also takes place. To test the feasibility of this modified WWTPs configuration, a pilot-scale 200L raceway pond, operating outdoor, was designed and used for biomass cultivation in untreated urban wastewater. Nitrogen gas enriched with 20% CO2, simulating the exhausted gas of biogas combustion, was supplied continuously during daytime at different flowrates. The dynamics of microalgae growth as well as inorganic carbon and nutrients uptake were studied during the pond start-up and semi-continuous feeding conditions. The absorbed bio-available CO2 was monitored during daylight for different gas flowrates (0.2, 0.4 and 1.0L/min) and for wastewater semi-continuous feeding conditions (0.8L/h). The highest efficiency, equal to 83%, of bio-available CO2 fixation was obtained for the lowest gas flowrate of 0.2L/min., whereas the highest CO2 removal rate of 24.6mg/L/min was reached for the highest gas flowrate of 1.0L/min. Furthermore, this operating condition resulted in the highest microalgae biomass productivity of 28.3g/d/m2. Nutrients removal was complete for each operating condition tested.