Competition for substrate and space in biofilms was studied using a microelectrode technique and a micro‐slicing technique. Three different kinds of biofilms were cultured using laboratory‐scale, rotating drum biofilm reactors fed with synthetic wastewater. The measured concentration profiles provide direct experimental evidence of the competition in multispecies biofilms for substrates. Increases in organic loading or ammonium‐nitrogen loading cause more consumption of oxygen, which results in competition for oxygen between heterotrophs and nitrifiers. Even in a pure nitrification system, heterotrophs, supported by soluble microbial products or metabolic products, could exist in the nitrification biofilm. Nitrifiers, however, have difficulty existing in the heterotrophic biofilms, and their populations were always 4 or 5 orders lower than those of heterotrophs. It was found that the value of criterion for transition between oxygen and ammonium in nitrifying biofilms (SN/SDO) was between 0.77 and 1.2, and the value decreased with an increase of glucose loading. The competition for substrate in biofilms resulted in a stratified structure with non uniform spatial distributions of biofilm properties, such as density, porosity, and effective diffusivity. This stratified structure in turn affects the substrate transfer and substrate competition within the biofilm. It was found that a certain biofilm system may not have only one penetration depth, corresponding to the critical thickness, for the whole range of biofilm thicknesses.

The objective of this study was to evaluate the effects of major factors such as PAC dosage, pH, contact time, mixing energy, alum dosage, and enhanced coagulation on the effectiveness of atrazine removal. Jar tests and response surface methodology were used to simulate conditions found in different treatment facilities. The time course of atrazine concentration with an initial atrazine concentration of 12 /JLgfL and initial (Acticarb) PAC of 16 mg/L indicated that it took approximately 5 days to reach equilibrium with the maximum atrazine removal of about 73%. Therefore, in treatment facilities, the adsorption of atrazine with this kind of PAC will be less than the removal achieved at equilibrium, due to the short retention time in a dynamic process. Mixing energy is a major factor affecting atrazine absorption. With jar test times ranging from 30 to 60 min, increasing rpms from 5 to 100 (G = 4 to 321 s_1) resulted in atrazine removals ranging from 34 to 59%. Without addition of PAC, neither lime softening nor alum coagulation (conventional or enhanced dosages ranging from 6 to 18 mg/L) demonstrated atrazine removal. A synergistic relationship appears to exist between PAC dosage and enhanced coagulation (with pH adjusted to about 5.8); neither PAC nor enhanced coagulation resulted in as high a removal rate of atrazine as the two did together (greater than 60%). The results of this study are useful for evaluation of different PAC application points in conventional drinking water treatment plants.

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The comparison of one-phase and two-phase anaerobic digestion processes in the characteristics of substrate degradation and the bacterial population levels was investigated by using the chemostat-type reactors to which starch was fed as substrate when both processes were operated under the same experimental conditions. By decreasing the SRTs of both systems from 10.2 d to 5 d, 2.5 d and 1.75 d. it was found that the two-phase system was more stable to the change in pH than one-phase system. The CH4 recovery rates and COD removal rates in the two-phase system increased by 4 to 9% and 3 to 10%. respectively, although the CH4 recovery rate and the COD removal rate in the one-phase system were slightly higher than those in the two-phase system at the SRT of 10.2 d. The concentration of propionate in the effluent of the one-phase system was 30 to 50% higher than that in the two-phase system; while the concentrations of acetate and butyrate in the one-phase system were slightly lower than those in the two-phase one. The enumeration of the bacteria was performed by the MPN method. The population levels of acidogenic bacteria in both systems were in the same order (108 to 1010 MPN/ml). the population levels of hydrogenotrophs were also in the same order as the acidogenic bacteria in the two-phase system, while the population levels of hydrogenotrophs were 10 to 100 fold less than that of acidogenic bacteria in the one-phase system. The number of HAc-utilizing methanogens in the methanogenesis of the two-phase system were 2 to 10 times higher than that in the one-phase system. Therefore, the one-phase system cannot be regarded simply as the sum of acidogenesis and methanogenesis.