Sulfate reducing processes at extreme salinity and temperature. extending its application window

The characteristics of various sulfate-rich wastewaters, such as temperature, pH and salinity, are determined by the (industrial) process from which they originate, and can be far from the physiological optima of the sulfur cycle microorganisms. The main goal of the research described in this thesis was to investigate and develop high rate sulfate reducing wastewater treatment processes for the treatment of inorganic sulfate-rich wastewaters under extreme conditions, i.e. high temperature and high salinity. In this thesis, several simple organic bulk chemicals were tested as electron donor, viz. lower alcohols (methanol and ethanol) and volatile fatty acids (<span class=SpellE>formate</span>, acetate and propionate).<o:p></o:p></span>With respect to the start-up of anaerobic sludge bed (UASB) reactors at high salinity or high temperature, the results obtained in this investigation indicate that the appearance of a targeted metabolic property (sulfate reduction at high salinity or at high temperature) is independent of the strategy for biomass acclimation (direct exposure vs. stepwise exposure).The stepwise adaptation of thermophilicsulfidogenic methanol degrading biomass to a high<span class=SpellE>osmolarity environment, both at 55C or at 70C, likely does not occur in UASB reactors, as probably no methanol halotolerant thermophilic sulfate reducing bacteria (SRB) were present in the thermophilic</span><span class=SpellE>inoculum</span>sludge used in the investigations described in this thesis. Exposing the sludge directly to a very high salinity (50 g NaCl.L ^-1 ) stimulated the growth of a<span class=SpellE>mesophilic</span>(30C) propionate- and ethanol-utilizing<span class=SpellE>halotolerant</span>SRB population, which supported high rate sulfate reduction (up to 3.6 g SO ₄^2- .L ^-1 .day ^-1 ) in a UASB reactor. The start-up of<span class=SpellE>thermophilic</span>(55 to 65C) and extreme<span class=SpellE>thermophilicC or higher) anaerobic bioreactors inoculated with<span class=SpellE>mesophilic</span><span class=SpellE>sludges</span>at the targeted temperature proceeded fast and stable, as it provoked the rapid selection of (extreme)<span class=SpellE>thermophiles</span>. Therefore, the key for the successful treatment of high salinity or hot wastewaters is to invest enough time for the growth of the targeted microorganism in the biomass.<o:p></o:p></span>The results of this investigation show that the competition between SRB, methane producing<span class=SpellE>archaea</span>and<span class=SpellE>acetogenic</span>bacteria for substrate is highly dependent of the type of substrate and operational conditions imposed to the bioreactor. This thesis describes a situation where the production of acetate and methane was completely suppressed in methanol-fed sulfate reducing UASB reactors operated at 70C. As a result, for the first time a fully sulfate reducing granular sludge has been cultivated in a methanol-fed<span class=SpellE>thermophilic</span>sulfate reducing reactor (with sulfate reduction rates as high as 14.4 g SO ₄^2- .L ^-1 .day ^-1 ), provided that an operational temperature of 70C is kept. The production of methane can be easily suppressed in<span class=SpellE>thermophilic</span>methanol fed reactors, either by running the reactor at temperatures equal or higher than 65C or by exposing 55C operated reactors to a short (2 days) temperature (65 - 70C) shock.<span class=SpellE>Methanogenesis</span>can also be easily suppressed in<span class=SpellE>mesophilic</span>propionate- and ethanol-fed reactors, provided high salinity conditions prevail (e.g. above 50 mS.cm ^-1 ). It seems, however, that the production of acetate, with the exception of methanol-fed reactors operated at 70C, is unavoidable both in<span class=SpellE>thermophilic</span>and<span class=SpellE>mesophilic</span>reactors.</span><span lang=EN-US></span>This thesis also describes the use of specialized microorganisms, the halophilicDesulfobacter</span><span class=SpellE>halotolerans</span> , in bioreactors for the treatment of saline sulfate-rich wastewaters. Very high specific sulfate reduction rates (up to 6.6 g SO ₄^2- .gVSS ^-1 .day ^-1 ) can be obtained in completely mixed tank reactors where the biomass grows in suspension and can be efficiently retained by membranes which are submerged in the reactor system. This investigation showed that anaerobic membrane bioreactors can be operated over extended periods of time at a fixed flux, if this flux is substantially below the nominal critical flux determined experimentally (18-21 L.m ^-2 .h ^-1 ). Chemical cleaning of the membranes will be required only at about 106 days, as long a low constant flux is imposed (4.7 L.m ^-2 .h ^.1 ) and intermittent<span class=SpellE>backflush</span>(e.g. 1 minute each 10 minutes) is adopted as operational strategy