Thermo-chemical pretreatment of a microbial biomass: influence of sodium hydroxide addition on solubilization and anaerobic biodegradability

The influence of sodium hydroxide addition during the thermo-chemical pretr eatment of a microbial biomass has been evaluated in terms of particulate c hemical oxygen demand (COD) solubilization and anaerobic biodesadability. B oth COD solubilization and total solid elimination rates increased with the dose of NaOH added: COD solubilization reached 63%, and total solid elimin ation was 33% when 5 g Na/l were added. Additional sodium hydroxide additio n did not increase COD solubilization further. The observed COD solubilizat ion was due mainly to protein hydrolysis that was directly linked to pH var iations. Heating emphasized these pH effects. The higher sodium hydroxide a ddition (26.1 g/l) led to 85% COD solubilization when heated to 140 degrees C for 30 min instead of 53.2% which was at ambient temperature. Biodegrada bility and biotoxicity tests were run with pretreated samples. Both methane production and acetate degradation in the presence of pretreated samples w ere affected as 5 g NaOH/l or more were added. Sodium cation was first susp ected to account for the limitations observed. Tests run at pH = 12 with ot her alkali agents (KOH, Mg(OH)(2), or Ca(OH)(2)), led to the same conclusio n: COD solubilization was enhanced but the biodegradability performances we re limited. Additional tests run with acetate and increasing NaCl concentra tions asserted that sodium cation was not at the origin of the limitations that were attributed to OH- additions: refractory compounds were formed as a consequence of pH modifications. (C) 1999 Elsevier Science Inc. All right s reserved.