Microalgal biomass grown in wastewater treatment raceway ponds may be valorised producing bioenergy through anaerobic digestion. However, pretreatment techniques seem to be necessary for enhancing microalgae methane yield. In this study, hydrothermal pretreatment was studied prior to batch and continuous reactors. The pretreatment increased organic matter solubilisation (8-13%), anaerobic digestion rate (30-90%) and final methane yield (17-39%) in batch tests. The highest increase was attained with the pretreatment at 130 degrees C for 15 min, which was attested in a laboratory-scale continuous reactor operated at a hydraulic retention time of 20 days with an average organic loading rate of 0.7 g VS/L center dot day. The methane yield increased from 0.12 to 0.17 L CH4/g VS (41%) in the pretreated digester as compared to the control. Microscopic images of microalgal biomass showed that pretreated cells had unstructured organelles and disrupted cell wall external layer, which may enhance the hydrolysis. Indeed, images of the pretreated reactor digestate showed how cells were more degraded than in the control reactor. (C) 2014 Elsevier Ltd. All rights reserved.
Les llacunes d'alta càrrega (LAC) es caracteritzen per ser una tecnologia de tractament d'aigua residual econòmica i ambientalment sostenible. Dins les LAC, les microalgues i les bactèries heteròtrofes creixen en relació simbiòtica de manera que l'oxigen que les bactèries necessiten per a la degradació de la matèria orgànica és produït durant la fotosíntesi de les microalgues. Així doncs, en contrast amb els sistemes convencionals de fangs activats, les LAC no requereixen d'aireig mecànic, principal consum energètic de la planta. D'altra banda, la biomassa microalgal, recollida dels decantadors secundaris, pot ser valoritzada en digestors anaerobis alhora que es produeix biogàs. Aquest procés de digestió, però, es veu limitat per la hidròlisi de les macromolècules retingudes dins la paret cel·lular de les microalgues, impedint que aquestes es trobin fàcilment a l'abast dels microorganismes anaerobis. L'objectiu d'aquesta tesi doctoral és doncs investigar la digestió anaeròbia de la biomassa algal cultivada en una LAC i alhora, estudiar possibles mètodes de pretractament per tal de millorar la producció de biogàs.Inicialment, la digestió anaeròbica de la biomassa algal va ser avaluada en una LAC (escala pilot) i en un reactor (escala laboratori) sense aplicar-hi cap pretractament. Quan el reactor fou operat a 15 dies de temps de retenció hidràulica (TRH), la productivitat de metà mitjana fou de 0.13 L CH4/g SV i quan el TRH fou de 20 dies, de 0.17 L CH4/g SV. La biodegradabilitat anaeròbica de les microalgues fou variable i baixa al comparar-la amb d'altres substrats orgànics. Les especies microalgals existents dins la LAC variaren també al llarg del temps, com a conseqüència de les condicions ambientals, la composició de l'aigua residual i les interrelacions amb d'altres microorganismes. Per últim, els resultats suggeriren que la biodegradabilitat de les microalgues depenia de les característiques de les cèl·lules que composaven la biomassa en cada període i que, per tant, era específica segons l'espècie.Com a conseqüència d'aquests resultats, i per tal de millorar la digestió anaeròbica de la biomassa, es van avaluar quatre tècniques de pretractament (microones, tèrmic, hidrotèrmic i ultrasons) utilitzant testos en discontinu. En cada cas, diversos paràmetres operacionals (temperatura, temps d'exposició, potencia consumida i energia específica aplicada), van combinar-se i, posteriorment, les millors condicions s'estudiaren utilitzant reactors en continu. En tot els casos estudiants, el pretractament augmentà tant la solubilització de la biomassa com la productivitat de metà. El pretractament termal a 95 ºC (0.31 L CH4/g SV), fou el que presentà un augment superior (70%) en relació a les microalgues no pretractades. Els anàlisis amb el microscopi indicaren que els mètodes de pretractament estudiats no van aconseguir trencar les parets cel·lulars de les microalgues; tot i això, la majoria de les cèl·lules queden malmeses, augmentant la biodegradabilitat i la bioaccessibilitat de les molècules orgàniques. Les espècies de microalgues que contenien proteïnes i carbohidrats a les parets cel·lulars resultaren en un augment de la productivitat de metà superior a aquelles especies amb parets cel·lulars més resistents, tals com les diatomees. El balanç energètic del procés suggerí que els pretractaments calorífics (tèrmic i hidrotèrmic), més que els elèctrics (microones), foren més eficients a nivell energètic. Els millors resultats s'aconseguiren amb l'aplicació del pretractament tèrmic a 75 i 95 ºC, assolint un balanç energètic positiu (es produí un 30% més d'energia de la consumida). Finalment, la realització d'un balanç energètic estimat del sistema complet, mostrà com les plantes de tractament d'aigua residual basades en les microalgues serien neutrals a nivell energètic amb una producció mitjana de biomassa de 15 g SST/m2¿d en les LAC i amb una producció de biomassa de 10 g SST/m2·d si s'apliqués el pretractament tèrmic.
Microalgae have been intensively studied as a source of biomass for replacing conventional fossil fuels in the last decade. The optimization of biomass production, harvesting and downstream processing is necessary for enabling its full-scale application. Regarding biofuels, biogas production is limited by the characteristics of microalgae, in particular the complex cell wall structure of most algae species. Therefore, pretreatment methods have been investigated for microalgae cell wall disruption and biomass solubilization before undergoing anaerobic digestion. This paper summarises the state of the art of different pretreatment techniques used for improving microalgae anaerobic biodegradability. Pretreatments were divided into 4 categories: (i) thermal; (ii) mechanical; (iii) chemical and (iv) biological methods. According to experimental results, all of them are effective at increasing biomass solubilization and methane yield, pretreatment effect being species dependent. Pilot-scale research is still missing and would help evaluating the feasibility of full-scale implementation. (C) 2014 Elsevier Ltd. All rights reserved.
High rate algal ponds are an economic and sustainable alternative for wastewater treatment, where microalgae and bacteria grow in symbiosis removing organic matter and nutrients. Microalgal biomass produced in these systems can be valorised through anaerobic digestion. However, microalgae anaerobic biodegradability is limited by the complex cell wall structure and therefore a pretreatment step may be required to improve the methane yield. In this study, ultrasound pretreatment at a range of applied specific energy (16-67. MJ/kg. TS) was investigated prior to microalgae anaerobic digestion. Experiments showed how organic matter solubilisation (16-100%), hydrolysis rate (25-56%) and methane yield (6-33%) were improved as the pretreatment intensity increased. Mathematical modelling revealed that ultrasonication had a higher effect on the methane yield than on the hydrolysis rate. A preliminary energy assessment indicated that the methane yield increase was not high enough as to compensate the electricity requirement of ultrasonication without biomass dewatering (8% VS).
The Research Group on Cooperation and Human Development of the Technical University
of Catalonia (UPC) has long promoted Bachelor, Master and PhD Thesis in the framework
of sustainable energy projects in low income countries. In this way, students combine their
work at UPC with tasks in the low income countries. The aim of this paper is to present and
evaluate the program through the experience of several students who participated in
sustainable energy projects in Central and South America.
This paper describes the anaerobic digestion of microalgal biomass from high rate ponds in continuous anaerobic reactors for biogas production. With hydraulic retention times (HRT) of 15 and 20 days, the volatile solids (VS) removal did not exceed 30%, and the methane production rate ranged between 0.12 and 0.14 L CH4/L day. To improve process performance, microwave irradiation at 900 W for 3 min (specific energy similar to 70,000 kJ/kg VS) was applied as a pretreatment step. The VS removal increased to 40 and 45% at 15 and 20 days HRT, respectively. Consequently, the methane production rate increased to 0.16 and 0.20 L CH4/L day at 15 and 20 days HRT, respectively. Microscopic analysis confirmed cell wall damage, although generally without lysis, after irradiating microalgal biomass. However, the energy consumption was much higher than the extra energy production of the process. Indeed, microalgal biomass should not only be thickened but also dewatered if microwave irradiation was to be applied as a pretreatment to anaerobic digestion for biogas production. (C) 2013 Elsevier Ltd. All rights reserved.
This work studied the anaerobic conversion of neutralized volatile fatty acids (VFA) into biogas under Autogenerative High Pressure Digestion (AHPD) conditions. The effects of the operating conditions on the biogas quality, and the substrate utilisation rates were evaluated using 3 AHPD reactors (0.6 L); feeding a concentration of acetate and VFA (1-10 g COD/L) corresponding to an expected pressure increase of 1-20 bar. The biogas composition improved with pressure up to 4.5 bar (>93% CH4), and stabilized at 10 and 20 bar. Both, acetotrophic and hydrogenotrophic methanogenic activity was observed. Substrate utilisation rates of 0.2, 0.1 and 0.1 g CODCH4/g VSS/d for acetate, propionate and butyrate were found to decrease by up to 50% with increasing final pressure. Most likely increased Na+-requirement to achieve CO2 sequestration at higher pressure rather than end-product inhibition was responsible. (C) 2013 Elsevier Ltd. All rights reserved.
'Subsurface-flow constructed wetlands have become a very popular cost effective and green technology for treatment of waste water throughout Europe and the rest of the world. Original predictions over the longevity of constructed wetlands were approximately 50 -100 years (Conley et al 1991). However, it has become disappointingly apparent that these systems are clogging and have on average a lifetime of less than 10 years (Griffin et al 2008). Currently when a wetland becomes clogged, the whole site has to be refurbished and the reeds regrown, which takes several years and has significant economic consequences for the operators.
Our project ARBI aims to develop and trial an Autonomous Reed Bed Installation containing a magnetic resonance probe, can be deployed in several locations within a wetland to give measurements at different depths. By measuring the relaxation times of MR, sufficient information can be obtained to determine the clog state of the gravel bed of a wetland. This would enable the operators to isolate those areas of the bed where the problem resides and make a partial intervention, without the need to remove and re-plant the whole reed bed. When the system is developed, we will have potential for application in other water treatment systems based on subsurface flows like: slow rate sand filters, and river bank filtration.
The project will have major benefits for those organisations who would like to install reedbeds but have resisted doing because of concerns over performance and maintenance costs. These will increase the potential size of the market for reedbed installers and benefit the SMEs in the consortium who currently are operating in a constrained market which has not achieved its true potential.'
The aim of this study was to evaluate the effect of microwave pretreatment on the solubilisation and anaerobic digestion of microalgae-bacterial biomass cultivated in high rate algal ponds for wastewater treatment. The microwave pretreatment comprised three specific energies (21,800, 43,600 and 65,400. kJ/kg TS), combining three output power values with different exposure times. Response surface analysis showed that the main parameter influencing biomass solubilisation was the applied specific energy. Indeed, a similar solubilisation increase was obtained for the same specific energy, regardless of the output power and exposure time (280-350% for 21,800. kJ/kg TS, 580-610% for 43,600. kJ/kg TS and 730-800% for 65,400. kJ/kg TS). In biochemical methane potential tests, the initial biogas production rate (27-75% increase) and final biogas yield (12-78% increase) were higher with pretreated biomass. A linear correlation was found between biomass solubilisation and biogas yield.
This study was carried out in response to the growing interest on household biogas digesters in Latin America, particularly in rural Andean communities. The aim was to compare traditional and novel digester designs, in terms of biogas production, cost and environmental impact. Three household digesters were compared: 1) fixed dome – Chinese model; 2) fixed dome – Camartec model and 3) plastic tubular – Taiwanese model. Data concerning design and operational parameters, construction materials and costs were obtained from our previous results on plastic tubular digesters and from the literature on fixed dome digesters. The biogas production rate
ranged around 0.07-0.5 m3 biogas m-3 digester d-1 in plastic tubular digesters and around 0.35-0.7 m3 biogas m-3 digester d-1 in fixed dome digesters. The novel design of the Camartec model reduced construction materials and capital cost by 27 % compared to the Chinese model. The cost of plastic tubular digesters was mainly attributed to the geomembrane. The life cycle assessment showed how the Camartec model caused the lowest impact, while the plastic tubular digester had the highest impact; which was mainly attributed to plastic materials (greenhouse and biogas reservoir) and geomembrane (digester). In the Chinese model, the main impact was caused by concrete and bricks, which were respectively 38 and 22 % higher than in the Camartec system. The highest impact of the tubular digester may be explained by the lifespan of construction materials. From an economic and environmental perspective, the Camartec model seems the best option; the main advantage of plastic tubular digesters being its ease of implementation and handling.
Microalgae have recently drawn attention as a potential resource for biofuels generation. In the short term, anaerobic digestion seems the most viable downstream process. Still, microalgae cell wall restrains the hydrolysis, hampering the methane production potential. This work aimed at improving the anaerobic biodegradability of microalgae through different pretreatment methods: microwave, low temperature and high temperature. For the microwave pretreatment, the specific
energy was the main parameter affecting biomass solubilisation, increasing up to 800 % compared to untreated biomass. For the low and high temperature pretreatments, temperature was the most
important factor, increasing biomass solubilisation up to 1190 and 2140 % in each case. BMP tests showed an increased methane yield after all pretreatments (40-80 %), with the highest results for the microwave pretreatment. In semi-continuous reactors operated at 15 days HRT, the methane production was increased by 30 and 33 % after microwave and low temperature pretreatment, respectively. When increasing the HRT to 20 days, the methane yield was further enhanced by 60 and 52 % after microwave and low temperature pretreatment, respectively. Therefore, initial results showed pretreatment potential for microalgae solubilisation and methane yield
The aim of this study was to investigate the effect of low temperature pretreatment on the anaerobic digestion of microalgal biomass grown in wastewater. To this end, microalgae were pretreated at low temperatures (55, 75 and 95. C) for 5, 10 and 15. h. Biomass solubilisation was enhanced with the pretreatment temperature and exposure time up to 10. h. The methane yield was improved by 14%, 53% and 62% at 55, 75 and 95. C, respectively; and was correlated with the solubilisation increase. The pretreatment at 95. C for 10. h increased VS solubilisation by 1188%, the initial methane production rate by 90% and final methane yield by 60% compared to untreated microalgae. With diluted biomass (~1% VS) positive energy balance was not likely to be attained. However, with concentrated biomass (2% VS) energy requirements may be covered and even surplus energy generated.
'The here proposed NAwATech Europe proposal is closely interconnected with the partner project NaWaTech India. In order to reach the maximal impact the two projects have formed one common work plan for both projects, targeting the same objectives, will present their results at the same web-site and formed a joint management team. Providing adequate water supply and sanitation, particularly in urban areas, is a challenging task for governments throughout the world. This task is made even more difficult due to predicted dramatic global changes. In order to cope with water shortages in urban areas, there is a need for a paradigm shift from conventional end-of-pipe water management to an integrated approach. This integrated approach should include several actions such as: (i) interventions over the entire urban water cycle; (ii) optimisation of water use by reusing wastewater and preventing pollution of freshwater source; (iii) prioritisation of small-scale natural and technical systems, which are flexible, cost-effective and require low operation and maintenance. Natural water systems, such as manmade wetlands and sub-soil filtration and storage via soil aquifer treatment and bank filtration, are such systems. NaWaTech stands for natural water systems and treatment technologies to cope with water shortages in urbanised areas in India. The concept is based on optimised use of different urban water flows by treating each of these flows via a modular natural system taking into account the different nature and degree of pollution of the different water sources. Thus, it will cost-effectively improve the water quality of urban surface water and restore depleting groundwater sources. Due to the multi-barrier approach, these systems will also be able to treat heavily polluted water (i.e. wastewater) in order to reuse them and to supplement traditional sources to cope with water shortages today and in the future.'
Marianna Garfi'; Ferrer-Martí, L.; Velo, E.; Ferrer, I. Renewable and sustainable energy reviews Vol. 16, num. 10, p. 575-581 DOI: 10.1016/j.rser.2011.08.023 Date of publication: 2012-01 Journal article
The aim ofthis research was to improve the anaerobic digestion of cow and guinea pig manure in low-cost unheated tubular digesters implemented at high altitude, by comparing different operating conditions and codigesting both manures
During the last years, the implementation of new Wastewater Treatment Plants has lead to a significant increase of sludge production. As a response, sludge valorisation in agriculture is the preferred option nowadays, ensuring the return of organic constituents, nutrients and microelements to crop fields. In practice, sludge treatments should provide a final product suitable for land application, with reasonable investment as well as operational and maintenance costs. In this sense, sludge treatment wetlands (STW) appear as a suitable technology for sludge management from an economic and environmental point of view.
The main objective of this research work was to assess the suitability of STW for sludge management; with special focus on small communities of the Mediterranean Region. To this end, technical, environmental and economic aspects of the treatment were studied in 3 full-scale systems and in a pilot plant located in Catalonia. A comparison with conventional treatments for sludge management is presented in order to establish the most favourable solution for the Catalan context. As the final result from this work, design and operation criteria are proposed as a guide for STW implementation in small Mediterranean communities.
The study of the performance of the full-scale STW suggests good treatment efficiency. Moisture content was reduced by 16-30%, reaching efficiencies similar to those of conventional dewatering technologies. On the other hand, VS were reduced up to 30-49%VS/TS, suggesting the progressive sludge stabilisation and mineralisation. Similar values were found at the end of the treatment in the pilot plant. In this case the sludge volume was reduced around 80% and TS increased up to 16-24%. However, the observed VS reduction (up to 50%VS/TS) after 2 months without feeding, indicates that longer resting periods should be applied in order to increase mineralisation of the sludge.
A finite element model able to simulate sludge dewatering in STW was developed by combining the evapotranspiration (ET) and the Terzaghi¿s consolidation theory representing water percolation. The model allows for the determination of the most appropriate feeding frequency as a function of the sludge height stored on the wetland. Similarly, the sludge loading rate is determined as a function of ET, feeding frequency and sludge height. On the whole, the model implemented is a useful tool for the establishment of standardised criteria of STW operation.
The characterisation of the final product from the pilot plant and from three full-scale systems demonstrated the suitability of biosolids as organic fertilisers. DRI values indicated the partial stabilisation of the product. Moreover, the absence of phytotoxicity and the heavy metals concentrations below the legal thresholds confirmed their viability to be reused in agriculture. However, in the pilot plant, pathogens were still present after 2 resting months, confirming the necessity of a longer resting period. Monitoring the stabilisation degree as phytotoxicity, heavy metals and pathogens' concentration during the final resting period would help optimising its duration.
Looking at the environmental aspects, the static chamber method was successfully adapted to the determination of gas emissions from STW. Aerobic conditions before sludge feeding, characterised by low methane emissions and high nitrous oxide emissions, were strongly altered by fresh sludge feeding, which increases CH4 emissions and reduces N2O emissions. According to the measured emissions, the Global warming potential of STW corresponds to 17kgCO2eq/PE¿y, which is from 2 to 9 times lower than that of sludge centrifugation and transport. Besides,the economic and environmental assessment indicates STW with direct land application as the most cost-effective technique, which is also characterised by the lowest environmental impact.Thus STW are the best solution to manage waste sludge in decentralised small communities.
Marianna Garfi'; Ferrer-Martí, L.; Velo, E.; Ferrer, I. International Conference on Micro Perspectives for Decentralized Energy Supply p. 37 Presentation's date: 2011-04 Presentation of work at congresses
Water hyacinth (Eichornia crassipes) is a fast growing aquatic plant which causes environmental problems in continental water bodies. Harvesting and handling this plant becomes an issue, and focus has been put on the research of treatment alternatives. Amongst others, energy production through biomethanation has been proposed
Water hyacinth (Eichornia crassipes) is a fast growing aquatic plant which causes environmental problems in continental water bodies. Harvesting and handling this plant becomes an issue, and focus has been put
on the research of treatment alternatives. Amongst others, energy production through biomethanation has been proposed. The aim of this study was to assess the anaerobic biodegradability of water hyacinth under mesophilic and thermophilic conditions. The effect of a thermal sludge pre-treatment at 80 C was also evaluated. To this end, anaerobic biodegradability tests were carried out at 35 C and 55 C, with raw
and pre-treated water hyacinth. According to the results, the thermal pre-treatment enhanced the solubilisation of water hyacinth (i.e. increase in the soluble to total chemical oxygen demand (COD)) from 4%
to 12% after 30 min. However, no significant effect was observed on the methane yields (150–190 L CH4/kg volatile solids). Initial methane production rates for thermophilic treatments were two fold those of mesophilic ones (6–6.5 L vs. 3–3.5 L CH4/kg COD day). Thus, higher methane production rates might be expected from thermophilic reactors working at short retention times. The study of longer low temperature pre-treatments or pre-treatments at elevated temperatures coupled to thermophilic reactors should be considered in the future.
Sludge management has become a key issue in wastewater treatment, representing some 20–60% of the operational costs of conventional wastewater treatment plants. The high water content of the sludge
results in large daily flow rates to be handled and treated. Thus, the search for methods to improve sludge volume reduction continues to be of major interest. The technology known as sludge treatment wetlands
has been used for sludge dewatering since the late 1980s. Major advantages include its low energy requirements, reduced operating and maintenance costs, and a reasonable integration in the environment. However, the number of plants in operation is still low in comparison with conventional technologies.
This study represents a review of the state of the art of sludge treatment wetlands. The main characteristics and operational aspects of the technology are described, including a summary of the main results reported in the literature. Finally, the efficiency of sludge treatment wetlands versus conventional
treatments is compared.