Biomethanation of Red Algae from the Eutrophied Baltic Sea

In the semi-enclosed Baltic Sea, excessive filamentous macro-algal biomass growth as a result of eutrophication is an increasing environmental problem. Drifting huge masses of red algae of the genera Polysiphonia, Rhodomela, and Ceramium accumulate on the open shore, up to five tones of algae per meter beach. During the aerobic decomposition of these algal bodies, large quantities of red colored effluents leak into the water what are toxic for the marine environment. In this study, feasibility of anaerobic conversion of red algae Polysiphonia, rich in nitrogen and phosphorous, was investigated. Biogas and methane potential of Polysiphonia, harvested in two different seasons [October and March], was investigated through three different batch digestion experiments and laboratory scale CSTR [continuous stirred tank reactor] at mesophilic (37oC) condition. Autoclavation [steam and heat] and ultrasound pretreatments were applied in order to enhance the biodegradation. In STR, anaerobic codigestion of algal biomass with SS [sewage sludge] was applied with a gradual increase in organic loading rate [1.5-4.0 g VS/L/day] and operated for 117 days at 20days HRT [hydraulic retention time]. Reactor digestate was analyzed four times over the period to determine the nutrients and heavy metals content. It is concluded that the methane potential of algae harvested in October is almost two-fold than that of algae harvested in March, probably due to it’s higher [more than double] nitrogen richness. An increase in biogas yield was observed upto 28% and VS reduction was increased from 37% to 45% due to autoclave pretreatment. Ultrasound pretreatment had no effect on digestion. In batch digestion, maximum methane yield 0.25 m3/kg VS added at 273oK, was obtained from algae [harvested in October] pretreated in autoclave. Codigestion of algae with SS worked well in STR with a comparatively lower OLR. At a higher OLR, methanogens were inhibited due to increased VFAs accumulation and decreased pH. A maximum biogas yield 0.49 m3/kg VS added at 310oK , was obtained from algae [harvested in October] pretreated with autoclave. The methane content of the produced biogas was 54%. Average [over a short period, day 99-107, reactor showed steady performance] maximum biogas yields from untreated algae obtained 0.44 m3/kg VSadded at 310oK and the VS reduction was calculated 32%. Digestate, to be used as a fertilizer, was found NH4-N, N, P, K, S and Na rich and only Cadmium level was above the maximal limit among the heavy metals. The sand content in algae during harvesting was considered as a factor to disrupt the operation. Codigestion of Polysiphonia algal biomass with substrate with higher C:N ratio like paper mill waste should be more appropriate to increase the methane and biogas yield. It is inconclusive whether AD process is a good method to dewater redalgae or not but large scale harvesting of algae will definitely contribute to curb eutrophication of the Baltic Sea through decreasing N and P level