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Indigenous microalgae-activated sludge cultivation system for wastewater treatment
Mälardalen University, School of Business, Society and Engineering, Future Energy Center. (ACWA)ORCID iD: 0000-0002-0137-2194
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The municipal wastewater is mainly composed of water containing anthropogenic wastes that are rich in nutrients such as carbon, nitrogen and phosphorous. The cost for biological treatment of wastewater is increasing globally due to the population growth in urban cities. In general, the activated sludge (AS) process is a biological nutrient removal process used in wastewater treatment plants (WWTPs). The AS is composed of different microorganisms in which bacteria play a crucial role in wastewater treatment (WWT). During the process, air is bubbled to supply oxygen and methanol is added to improve nitrogen removal, which is released as a gas. Phosphorous is removed in the expense of precipitation chemicals. Altogether, the current process requires electrical energy, precipitation chemicals, handling of excess sludge and it emits carbon dioxide (CO2) as a by-product. This process is still in practise in the WWTPs since 1914 although numerous modifications are implemented to meet the stringent regulations in the European Union and globally.

Microalgae are microorganisms that perform photosynthesis like plants. They are green and reproduce fast using available nutrients (nitrogen and phosphorous) and CO2 from their environment in the presence of light. As a result of photosynthesis, oxygen is released as waste gas. The synthesised oxygen during this process can be implemented to support the AS bacteria that leads to the microalgae activated sludge (MAAS) process. The main advantage is combined removal of nutrients.

The vision of the research is to implement the indigenous microalgae cultivation in activated sludge process to consume CO2 and recover the nutrients from wastewater. This study is performed to improve the understanding of the process such as: light utilisation, nutrient removal and recovery of the biomass from wastewater in closed photo-bioreactors. Photo-bioreactors are vessels where the cultivation is carried out in the presence of light. At first, the influence of the light spectrum on micro-algal cultivation is investigated for photosynthetic growth. This is followed by operational challenges of the microalgae cultivation during the AS process. The process is experimentally performed in the photo-bioreactors with different treatment time of the raw wastewater. The results showed that 2 - 6 days of treatment time can be used for reducing nutrients in wastewater if the process is optimised further. Also, nutrient ratio is analysed for the availability of the micro-algal growth. Furthermore, the biogas potential of MAAS showed a biogas yield of about 60-80% within 5 to 9 days.

At last, the experimental verification of chemically precipitated wastewater showed limitation of phosphorous for micro-algal growth. Additionally, the optimal oxygen supply through light response is verified for photo-bioreactors. The outcome of this study shows that knowing the right conditions can lower the treatment time. By doing so, a stable nutrient removal and reduction of precipitation chemicals can be established as well as a better recovery of valuable nutrients as phosphorous and nitrogen.

Abstract [sv]

Avloppsvatten innehåller allt det ni spolar ner i avloppet, med höga halter av viktiga näringsämnen som fosfor, kväve och kol. Kostnaden för att rena avloppsvatten ökar i världen på grund av ökad urbanisering och ökade krav på reningen.

Reningsprocessen domineras av aktivslamprocessen, i vilken mikroorganismer (främst bakterier) renar vattnet från kol och kväve medan det mesta av fosforn fälls ut med kemikalier. Processen är mycket energikrävande på grund av den stora mängden luft som pumpas in för att förse bakterierna med syre. Tillsammans utgör alla de här funktionerna en energi- och resurskrävande process som dessutom kräver tillskott av mer kol, oftast som ren alkohol, för att kvävereningen ska fungera.

I den här avhandlingen har ett alternativ till konventionell, bakteriell avloppsvattenrening studerats; mikroalgbaserad aktivslamprocess (MAAS). Bakgrunden är att gröna mikroalger i likhet med växter utnyttjar fotosyntesen. Genom i fotosyntesen kan algerna snabbt föröka sig om det finns näringsämnen, ljus och koldioxid. Eftersom avloppsvattnet innehåller alla nödvändiga näringsämnen kan det räcka med att tillsätta koldioxid under rätt ljusförhållanden för att en livskraftig algkultur ska växa till. Algerna producerar även syre i processen som i sin tur kan användas av bakterierna i MAAS processen i en intressant symbios. Mest intressant är mikroalgbaserad vattenrening ur ett resursutvinningsperspektiv eftersom näringsämnena assimileras i algerna och därmed förblir i slammet och kan sedan utvinnas som en gödselprodukt.

Målet med MAAS-forskningen har varit att använda koldioxid, näringsämnen och en algkultur från svenska förhållanden, i det här fallet Mälaren, för att bygga upp en effektiv kultur för vattenrening. Samtidigt vill vi i forskningen optimera processen utifrån hur mycket ljus som behöver tillföras, hur snabbt mikroorganismerna kan rena vattnet och vilka andra ämnen som kan störa processen.

I den här avhandlingen har olika ljuskällor undersökts, med särskilt fokus på effektiva LED-lampor. Därefter har målet varit att optimera processen så att uppehållstiden, dvs den tid det tar för en viss volym vatten att renas, skulle kunna sänkas från 6 dagar (vilket anses vara internationell standard) till 4 eller till och med 2 dagars uppehållstid. Slutligen studerades effekterna av fällningskemikalier i mikroalgkulturen med slutsatsen att mikroalger blir begränsade av fällningskemikalier men att mikroalgerna däremot har goda förutsättningar att rena vattnet från betydande mängder fosfor och därmed minska mängderna fällningskemikalier som måste till sättas.

Place, publisher, year, edition, pages
Västerås: Mälardalen University , 2016.
Series
Mälardalen University Press Licentiate Theses, ISSN 1651-9256 ; 240
National Category
Water Treatment
Research subject
Energy- and Environmental Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-31543ISBN: 978-91-7485-269-1 (print)OAI: oai:DiVA.org:mdh-31543DiVA: diva2:927015
Presentation
2016-06-16, Paros, Mälardalens högskola, Västerås, 09:15 (English)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2016-05-10 Created: 2016-05-10 Last updated: 2016-06-07Bibliographically approved
List of papers
1. Influence of light emitting diodes on indigenous microalgae cultivation in municipal wastewater
Open this publication in new window or tab >>Influence of light emitting diodes on indigenous microalgae cultivation in municipal wastewater
2015 (English)In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 75, 786-792 p.Article in journal (Refereed) Published
Abstract [en]

In this study, the effect of light emitting diodes (LEDs) on microalgae cultivation in municipal wastewater was examined in comparison to the fluorescent light. Two kinds of wastewater were evaluated: first one with low concentration of total phosphorous (TP) and second one with high TP concentration. The nutrient removal and biomass production using LEDs is efficient at photo-synthetically active radiation (PAR) intensity of 107-112 mu mol m(-2) s(-1) which is slightly higher than fluorescent light. Furthermore, this study demonstrates the applicability and distribution of light in wastewater where the environment is not defined. More importantly, winter and rainy periods contribute to dark condition and dilution of wastewater, intense LED light offers a feasible option for the functioning of closed micro algae based activated sludge (MAAS) process for recovery and reuse of nutrients. 

National Category
Environmental Sciences
Identifiers
urn:nbn:se:mdh:diva-29322 (URN)10.1016/j.egypro.2015.07.125 (DOI)000361030001037 ()2-s2.0-84947065292 (Scopus ID)
Conference
7th International Conference on Applied Energy (ICAE), MAR 28-31, 2015, Abu Dhabi, U ARAB EMIRATES
Available from: 2015-10-15 Created: 2015-10-15 Last updated: 2017-12-01Bibliographically approved
2. Influence of hydraulic retention time on indigenous microalgae and activated sludge process
Open this publication in new window or tab >>Influence of hydraulic retention time on indigenous microalgae and activated sludge process
2016 (English)In: Water Research, ISSN 0043-1354, E-ISSN 1879-2448, Vol. 91, 277-284 p.Article in journal (Refereed) Published
Abstract [en]

Integration of the microalgae and activated sludge (MAAS) process in municipal wastewater treatment and biogas production from recovered MAAS was investigated by studying the hydraulic retention time (HRT) of semi-continuous photo-bioreactors. An average total nitrogen (TN) removal efficiency (RE) of maximum 81.5 ± 5.1 and 64.6 ± 16.2% was achieved at 6 and 4 days HRT. RE of total phosphorous (TP) increased slightly at 6 days (80 ± 12%) HRT and stabilized at 4 days (56 ± 5%) and 2 days (55.5 ± 5.5%) HRT due to the fluctuations in COD and N/P mass ratio of the periodic wastewater. COD and organic carbon were removed efficiently and a rapidly settleable MAAS with a sludge volume index (SVI_10) of less than 117 mL g-1 was observed at all HRTs. The anaerobic digestion of the untreated MAAS showed a higher biogas yield of 349 ± 10 mL g VS-1 with 2 days HRT due to a low solids retention time (SRT). Thermal pretreatment of the MAAS (120 °C, 120 min) did not show any improvement with biogas production at 6 days (269 ± 3 (untreated) and 266 ± 16 (treated) mL gVS-1), 4 days (258 ± 11(untreated) and 263 ± 10 (treated) mL gVS-1) and 2 days (308 ± 19 mL (treated) gVS-1) HRT. Hence, the biogas potential tests showed that the untreated MAAS was a feasible substrate for biogas production. Results from this proof of concept support the application of MAAS in wastewater treatment for Swedish conditions to reduce aeration, precipitation chemicals and CO2 emissions. 

Keyword
Activated sludge process, Anaerobic digestion, Hydraulic retention time, Microalgae, Nutrient removal, Thermal pretreatment
National Category
Environmental Biotechnology
Identifiers
urn:nbn:se:mdh:diva-31345 (URN)10.1016/j.watres.2016.01.027 (DOI)000371189100028 ()2-s2.0-84960332248 (Scopus ID)
Available from: 2016-03-24 Created: 2016-03-24 Last updated: 2017-11-30Bibliographically approved
3. Influence of light intensity and phosphorous on microalgae activated sludge in phosphate precipitated condition
Open this publication in new window or tab >>Influence of light intensity and phosphorous on microalgae activated sludge in phosphate precipitated condition
(English)Manuscript (preprint) (Other academic)
National Category
Water Treatment
Identifiers
urn:nbn:se:mdh:diva-31545 (URN)
Available from: 2016-05-10 Created: 2016-05-10 Last updated: 2016-07-05Bibliographically approved

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