The Performance Of High Rate Anaerobic Reactor

Water pollution has become a serious problem in China.Anaerobic biotechnology is of particular concern in organic pollution control due to its low operational cost and energy saving.Bioreactor is the core of anaerobic biotechnology,which marks the level of anaerobic biological treatment systems.It has been reported that the maximum organic loading rate(OLR) of full-scale anaerobic digesters is higher than 40 kgCOD/(m³Â·d).Thus,the OLR of 40 kgCOD/(m³Â·d) can serve as a standard for super high rate anaerobic bioreactors(SHRAB).It is helpful to take SHR as model and to make clear the working characteristics for the optimization and control of anaerobic bioreactors.The acute toxicity of 4 typical intermediates in anaerobic process,i.e., ethanol,acetate,propionate and butyrate,has been determined;and the performances of three SHRABs fed with sucrose,ethanol and acetate,respectively,have been investigated in this study.Major results are as follows:1) Different biological toxicity of the four typical intermediates was observed.The results of single toxicity experiments showed that the IC₅₀ values(half inhibition concentration for 15 min) of ethanol,acetate,propionate and butyrate were 27.0 g/L,3.96 g/L,2.49 g/L and 6.00 g/L,respectively.The toxicity descended in the order of propionate,acetate,butyrate and ethanol.The inhibition of VFAs(volatile fatty acids) in their unionized forms was stronger than that in their ionized forms.The IC₅₀ values of unionized acetic acid, propionic acid and butyric acid were 23.4 mg/L,19 mg/L,37 mg/L, respectively,which were 234,190,370 times of the IC₅₀ value of standard toxicity reference substrate HgCl₂ respectively.The combined toxicity experiments showed that the combined effects of ethanol and acetate,ethanol and propionate,ethanol and butyrate,acetate and propionate,acetate and butyrate,propionate and butyrate were all additive effect.The combined effect of four anaerobic digestion intermediates was also additive effect.2) Anaerobic digestion process can be divided into three stages:the hydrolysis fermentation stage,the hydrogen and acetate producing stage and methane producing stage.Reactorâ… was fed with sucrose which is the substrate for hydrolysis fermentation,reactorâ…¡was fed with ethanol which is the substrate for hydrogen and reactorâ…¢was fed with acetate which is the substrate for methane production.The OLRs of reactorâ… ,â…¡,â…¢were as high as 75,100 and 100 kg/(m³Â·d),respectively.Fitting by Monod kinetics,the maximal volumetric loading rates of reactorsâ… ,â…¡,â…¢were 89.0,170.6 and 227.5 kg/(m³Â·d), respectively.It is obvious that the volumetric loading rate could be enhanced by shortening the food chain of anaerobic digestion.3) The activity of the biomass taken from reactorsâ… ,â…¡andâ…¢after long-term operation(namely sludge A,B and C) showed significant difference.The glucose conversion rates(V_max) of sludge A,B,C were 55.53,29.05 and 22.26 kg/(m³Â·d),respectively.Taking V_max of sludge A as 100,the relative activity of sludge A,B,C were 100,52.3 and 40.1,respectively.The ethanol conversion rates(V_max) of sludge A,B,C were 45.57,86.47,36.56 kg/(m³Â·d),respectively. Taking V_max of sludge A as 100,the relative activity of sludge A,B,C was 100, 189.8 and 80.2,respectively.The propionate conversion rates(V_max) of sludge A,B,C were 34.58,24.64,14.87 kg/(m³Â·d),respectively.Taking V_max of sludge A as 100,the relative activity of sludge A,B,C was 100,71.3 and 43.0, respectively.The butyrate conversion rates(V_max) of sludge A,B,C were 42.25, 25.17,20.61 kg/(m³Â·d),respectively.Taking V_max of sludge A as 100,the relative activity of sludge A,B,C was 100,59.3 and 48.6,respectively.The acetate conversion rates(V_max) of sludge A,B,C were 22.41,34.21,63.78 kg/(m³Â·d),respectively.Taking V_max of sludge A as 100,the relative activity of sludge A,B,C was 100,152.7 and 284.6,respectively.Based on the conversion rates of sludge A for hydrolysis fermentation substrate,hydrogen and acetate producing substrate and methane producing substrate,methane producing stage was judged as the rate-limiting step of the anaerobic digestion.However, evaluated by the conversion rates of three sludges to the five substrates,the hydrogen and acetic acid producing stage might be the rate-limiting step if the main product of hydrolysis fermentation stage was propinate or butyrate. However,the hydrolysis fermentation stage would be the rate-limiting step if the primary product of hydrolysis fermentation stage was ethanol.4) The relationship of volumetric loading rate to alkalinity and VFA concentrations in anaerobic bioreactor was established.The total effluent alkalinity and effective alkalinity declined with the increase of volumetric loading rate.The effective alkalinity was 1540,1150 and 1082 mg/L in reactorâ… ,â…¡andâ…¢,respectively,when the reactors reached their maximal volumetric loading rates;and the corresponding ALK_com/COD_rem ratios were 0.12,0.11, 0.10,respectively.At the maximal volumetric loading rate and gas production rate,the corresponding effluent VFA concentrations were 1507,908,1145 mg/L, respectively,which were much higher than those in normal operation periods; the corresponding ratios between effluent alkalinity and VFA were 1.25,1.40, 1.13,respectively,which were much lower than those in normal operation periods.