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Influence of hydraulic retention time on indigenous microalgae and activated sludge process
Mälardalens högskola, Akademin för ekonomi, samhälle och teknik, Framtidens energi.ORCID-id: 0000-0002-0137-2194
Mälardalens högskola, Akademin för ekonomi, samhälle och teknik, Framtidens energi.ORCID-id: 0000-0002-5014-3275
Mälardalens högskola, Akademin för ekonomi, samhälle och teknik, Framtidens energi. ABB AB, Corporate Research, Västerås, Sweden.ORCID-id: 0000-0003-0274-4719
Mälardalens högskola, Akademin för ekonomi, samhälle och teknik, Framtidens energi.ORCID-id: 0000-0003-3311-9465
2016 (Engelska)Ingår i: Water Research, ISSN 0043-1354, E-ISSN 1879-2448, Vol. 91, s. 277-284Artikel i tidskrift (Refereegranskat) Published
Resurstyp
Text
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. 

Ort, förlag, år, upplaga, sidor
2016. Vol. 91, s. 277-284
Nyckelord [en]
Activated sludge process, Anaerobic digestion, Hydraulic retention time, Microalgae, Nutrient removal, Thermal pretreatment
Nationell ämneskategori
Miljöbioteknik
Identifikatorer
URN: urn:nbn:se:mdh:diva-31345DOI: 10.1016/j.watres.2016.01.027ISI: 000371189100028PubMedID: 26803263Scopus ID: 2-s2.0-84960332248OAI: oai:DiVA.org:mdh-31345DiVA, id: diva2:914642
Tillgänglig från: 2016-03-24 Skapad: 2016-03-24 Senast uppdaterad: 2018-10-16Bibliografiskt granskad
Ingår i avhandling
1. Indigenous microalgae-activated sludge cultivation system for wastewater treatment
Öppna denna publikation i ny flik eller fönster >>Indigenous microalgae-activated sludge cultivation system for wastewater treatment
2016 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Västerås: Mälardalen University, 2016
Serie
Mälardalen University Press Licentiate Theses, ISSN 1651-9256 ; 240
Nationell ämneskategori
Vattenbehandling
Forskningsämne
energi- och miljöteknik
Identifikatorer
urn:nbn:se:mdh:diva-31543 (URN)978-91-7485-269-1 (ISBN)
Presentation
2016-06-16, Paros, Mälardalens högskola, Västerås, 09:15 (Engelska)
Opponent
Handledare
Forskningsfinansiär
KK-stiftelsen
Tillgänglig från: 2016-05-10 Skapad: 2016-05-10 Senast uppdaterad: 2016-06-07Bibliografiskt granskad
2. A passage to wastewater nutrient recovery units: Microalgal-Bacterial bioreactors
Öppna denna publikation i ny flik eller fönster >>A passage to wastewater nutrient recovery units: Microalgal-Bacterial bioreactors
2018 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt) [Forskning på konstnärlig grund]
Abstract [en]

In recent years, the microalgal–bacterial process has been considered to be a very attractive engineering solution for wastewater treatment. However, it has not been widely studied in the context of conventional wastewater treatment design under Swedish conditions. The technology holds several advantages: as a CO2 sink, ability to withstand cold conditions, ability to grow under low light, fast settling without chemical precipitation, and reducing the loss of valuable nutrients (CO2, N2, N2O, PO4). The process also provides the option to be operated either as mainstream (treatment of municipal wastewater) or side stream (treatment of centrate from anaerobic digesters) to reduce the nutrient load of the wastewater. Furthermore, the application is not only limited to wastewater treatment; the biomass can be used to synthesise platform chemicals or biofuels and can be followed by recovery of ammonium and phosphate for use in agriculture.

In the present study, the feasibility of applying the process in Swedish temperature and light conditions was investigated by implementing microalgae within the activated sludge process. In this context, the supporting operational and performance indicators (hydraulic retention time (HRT), sludge retention time (SRT) and nutrients removal) were evaluated to support naturally occurring consortia in photo-sequencing and continuous bioreactor configuration. Furthermore, CO2 uptake and light spectrum-mediated nutrient removal were investigated to reduce the impact on climate and the technical challenges associated with this type of system.

The results identified effective retention times of 6 and 4 days (HRT = SRT) under limited lighting to reduce the electrical consumption. From the perspective of nitrogen removal, the process demands effective CO2 input either in the mainstream or side stream treatment. The incorporation of a vertical absorption column demonstrated effective CO2 mass transfer to support efficient nitrogen and phosphorus removal as a side stream treatment. However, the investigation of a continuous single-stage process as the mainstream showed a requirement for a lower SRT in comparison to semi-continuous operation due to faster settlability, regardless of inorganic carbon. Furthermore, the process showed an effective reduction of influent phosphorus and organic compounds (i.e. COD/TOC) load in the wastewater as a result of photosynthetic aeration. Most importantly, the operation was stable at the temperature equivalent of wastewater (12 and 13 ˚C), under different lighting (white, and red-blue wavelengths) and retention times (6 and 1.5 d HRT) with complete nitrification. Additionally, the biomass production was stable with faster settling properties without any physiochemical separation.

The outcomes of this thesis on microalgal–bacterial nutrient removal demonstrates that (1) photosynthesis-based aeration at existing wastewater conditions under photo-sequential and continuous photobioreactor setup, (2) flocs with rapid settling characteristics at all studied retention times, (3) the possibility of increasing carbon supplementation to achieve higher carbon to nitrogen balance in the photobioreactor, and (4) most importantly, nitrification-based microalgal biomass uptake occurred at all spectral distributions, lower photosynthetic active radiation and existing wastewater conditions.

Ort, förlag, år, upplaga, sidor
Västerås: Mälardalen University, 2018. s. 66
Serie
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 263
Nyckelord
Microalgae, Bacteria, Carbon, Nitrogen, Light
Nationell ämneskategori
Vattenbehandling Biosanering Bioprocessteknik
Forskningsämne
energi- och miljöteknik
Identifikatorer
urn:nbn:se:mdh:diva-39158 (URN)978-91-7485-387-2 (ISBN)
Disputation
2018-06-19, Delta, Mälardalens högskola, Västerås, 09:00 (Engelska)
Opponent
Handledare
Forskningsfinansiär
KK-stiftelsen
Tillgänglig från: 2018-05-08 Skapad: 2018-05-07 Senast uppdaterad: 2019-10-01Bibliografiskt granskad

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