Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
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.