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Kadmiumreduktion för att möjliggöra återföring av näringsämnen från rötade alger till jordbruksmark
KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Industrial Ecology.
2015 (Swedish)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesisAlternative title
Reduction of cadmium in anaerobic digested algae for the recirculation of nutrients to farmland (English)
Abstract [sv]

På flera ställen i norden, bland annat i Trelleborg kommun i södra Sverige har man försökt skapa hållbara kretsloppsprojekt som syftar till att frigöra badstränder från uppspolade illaluktande alger genom att samla in dem för att producera biogas. Därefter vill man återcirkulera näringsämnen från hav till land genom att avsätta rötresten på jordbruksmark som biogödsel. Tyvärr riskerar då halterna av kadmium i algerna att utgöra ett hot mot en trygg livsmedelsproduktion då kadmiumnivåerna i den jordbruksmark som är ämnad att bli gödslad, i många fall kommer att ackumuleras upptill oacceptabla nivåer. Tidigare har reduktionsförsök av kadmiumnivåer i alger gjorts innan rötning genom extraktion med syra. I denna studie prövades om vanligt sötvatten kunde användas som extraktionslösning. Dessutom prövades om kalciumhydroxid kunde reducera kadmiuminnehållet antingen genom jonbyte mellan kadmium och kalcium eller genom extraktion i egenskap av bas. Ingen av dessa metoder visade sig reducera innehållet kadmium med mer än 10 %. Extraktion med myrsyra användes som referensmetod och reducerade innehållet med 60 % vilket är vad tidigare studier visat. På grund av mätosäkerhet bör inte resultaten tolkas för exakt. Därefter undersöktes om metanpotentialen i algerna påverkades av behandlingarna. Tyvärr stötte utrötningsexperimentet på problem vilket gör resultaten högst osäkra. Ingen behandlingsmetod verkade dock ge en negativ påverkan på metanpotentialen. Myrsyrametoden verkar snarare öka metanutbytet genom att algerna kan ha förhydroliserats. Slutligen tog studien ett större perspektiv kring kadmiumflöden i samhälle och natur samt lyfte blicken mot andra tekniker som har potential att göra näringsåterföring från alg till jordbruksmark möjlig. Vikten av förståelse för massbalansen av kadmium i det aktuella området understryks. Tekniker för utvinning av näringsämnen ifrån en rejektvätska och ifrån aska från rötresten kan göra det möjligt att samröta alger med annat lämpligt substrat för att sedan ta ut näringsämnen som fosfor ur rötresten, rent från föroreningar.

Abstract [en]

Beach cast algae that accumulates and creates thick layers of degrading biomass makes beaches almost inaccessible for tourists with its consistence and bad odor. In several places in Scandinavia there is an interest to clean the beaches from smelly algae and at the same time exploit the potential of the algae as a source of plant nutrients and also of energy. The municipality of Trelleborg in the south of Sweden has been trying to find a sustainable way to use the algae as substrate for production of biogas and then dispose the digested residuals onto farmland. However, the relatively high cadmium content in the algae threaten to make cadmium levels in agricultural soil to accumulate over time and thus putting food production at risk. Techniques involving acid solutions have earlier been proven to reduce the cadmium content with up to 77 % from algae through extraction. In full scale this would demand great volumes of acid and thus would not be likely to be cost effective, considering the relatively low bio methane potential (BMP) around 94 – 290 Nml CH4/g VS and low content of nutrients (e.g. 2,4 – 3,4 g P/kg TS) in algae.


The purpose of this study was to find a potentially more cost effective process to reduce cadmium content in algae by trying extraction of cadmium from algae with A) fresh water and B) a base of sodium hydroxide at pH 11. Formic acid has been used in earlier studies and got to serve as a reference method in this study. The algae from a beach on the southernmost tip of Sweden, was mixed with freshwater and for the other two methods, freshwater and acid or base. After 24 hours, the fluid was extracted from the mix and the dry substrate and the fluid, as well as untreated algae was analyzed for the concentration of cadmium. Results showed that the harvested algae had a concentration of 2 mg Cd/ kg DW which is similar to earlier analyzes. What was also shown was that neither the fresh water nor the sodium hydroxide managed to extract more than approximately 10 % of the total content. The Formic acid did extract approximately 60 % of the total content which is comparable to results of earlier studies. All results are however coupled with great uncertainties of ±20 – 44 %.


Furthermore the aim was to examine whether the extraction experiments had any effect on the bio methane potential of the algae. So, both treated and untreated substrate was mixed with inoculum and put into anaerobic digestion in 37 °C for 44 days in total together with a blank and a control with sodium acetate. Unfortunately the BMP test had suffered problems resulting in great uncertainties that make any interpretation almost pointless. Still, the extraction experiments did not seem to have a negative effect on the BMP (≈ 80 ml CH4/g VS) when comparing with the BMP results from the non-treated substrate. The treatment with formic acid showed to improve the BMP (≈ 140 ml CH4/g VS) but some of that methane came from the acid itself. But it is recommended to see these results only as vague indications, if anything.


Additionally a final aim was to take a broader scope and discuss further solutions on how to proceed with the concept of algae-biogas-nutrient recovery. In order to do this the background part of this work was extended with additional literature studies of the natural and anthropogenic sources and flows of cadmium in the biosphere, earlier experiments with algae to biogas/bio fertilizer and finally other techniques for removal of cadmium from different substrate. The basis for this was the Skåne region and the algae to biogas projects that have been running in the municipality of Trelleborg, even if the discussion reaches out over the society as a whole as well.


It became clear that before any algae are disposed it is of great importance to have a good, holistic picture of the mass balance of cadmium in the area that has been assigned to receive the residuals from digested algae. Important factors are natural content of cadmium in the soils and the loads of atmospheric deposition of cadmium in the area. Algae are not fit to serve as the single substrate in a biogas process so co-digestion of algae and other substrate is desired to achieve the right conditions and to optimize BMP. It will also dilute the cadmium rich algae and help the Cd/P ratio and thereby reducing the load of cadmium on to farmland.


In the experiments of this study as well as in earlier studies, the approach has been to place the cadmium reduction step before digestion because a co-digestion would dilute the cadmium, thus making any extraction much more ineffective. But if the algae are to be co-digested with a substrate with a similar profile of contamination, it would be logical to instead put the cadmium reduction step after the digestion. This could potentially enhance the reduction since the algae cell structure has been degraded and hopefully are more likely to release cadmium through extraction. A different approach, still after co-digestion, is to consider technologies where phosphor and nitrogen can be extracted from a fluid reject of the residuals or from the ashes of combusted residuals. This would allow the co-digestion of substrate with completely different profiles of contamination, like algae and sewage sludge, since uncontaminated nutrients can be extracted in form similar to commercial fertilizers. These alternative ideas would be interesting as an approach for further studies. 

Place, publisher, year, edition, pages
, TRITA-IM-EX 2015, 15
National Category
Energy Systems Environmental Management
URN: urn:nbn:se:kth:diva-170737OAI: diva2:839636
Available from: 2015-07-03 Created: 2015-07-03 Last updated: 2015-07-03Bibliographically approved

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