Biogas production from thin stillage : exploring the microbial response to sulphate and ammonia

Sammanfattning: The biogas plant in Norrköping (Tekniska verken i Linköping AB, publ.), Sweden, operates with thin stillage, a residue from bio-ethanol fermentation, as the main feedstock. Thin stillage is energy-rich due to its high protein content, but due to its high nitrogen and sulphate content is a somewhat complicated feedstock. The high nitrogen concentration results in inhibition of the microbial process and also selects for nitrogen-tolerant, but slow-growing, syntrophic acetate-oxidising bacteria (SAOB). The high sulphate concentration in the feedstock results in production of toxic and inhibitory sulphides through the activity of sulphate-reducing bacteria (SRB). Measures currently applied at Norrköping biogas plant to optimise the degradation of thin stillage include: i) use of mesophilic temperature and addition of hydrochloric acid, ii) use of long hydraulic retention time and iii) addition of iron and trace elements. This thesis investigated how to obtain a more efficient biogas process treating thin stillage, with Norrköping biogas plant as the model plant. It also explored the role of SRB in the anaerobic process at high nitrogen content and sought to identify optimal conditions for ammonia-tolerant methane-producing microorganisms. This was done by measuring SRB abundance in several large-scale biogas processes to identify conditions resulting in reduced numbers. In parallel, the effects of increasing temperature and organic load, calcium addition and a two-stage strategy were evaluated in laboratory studies. The results showed a correlation between high ammonia level and temperature with decreased abundance of SRB, but none of the operating strategies tested proved successful in repressing sulphate reduction. However, increasing ammonia and/or organic loading rate influenced both the acetogenic and methanogenic community, including potential SAOB. Moreover, increasing the temperature to 44 ºC resulted in increased abundance of thermotolerant SAOB and their partner methanogen and higher biogas yield (+22%). A maximum ammonia threshold concentration of approximately 1.1 g L-1 was identified. Application of the findings reported in this thesis has resulted in increased process stability in biogas plants in Sweden.