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Encapsulation of whole microbial cells in microtubes for use in bioremediation of pollutants in water systems was the main focus of this investigation. Coelectrospinning of a core polymeric solution with bacterial cells and a shell polymer solution using a spinneret with two coaxial capillaries resulted in microtubes with porous walls. The ability of the microtube's structure to support cell attachment and maintain enzymatic activity and proliferation of the encapsulated microbial cells was examined. The results obtained show that the encapsulated cells maintain some of their phosphatase, β-galactosidase and denirification activity and are able to respond to conditions that induce these activities. This study demonstrates electrospun microtubes are a suitable platform for the immobilization of intact microbial cells.

The sensitivity of nitrifying bacteria to acidic conditions is a well-known phenomenon and generally attributed to the lack and/or toxicity of substrates (NH 3 and HNO 2 ) with decreasing pHs. In contrast, we observed strong nitrification at a pH around 4 in biofilms grown on chalk particles and investigated the following hypotheses: the presence of less acidic microenvironments and/or the existence of acid-tolerant nitrifiers. Microelectrode measurements (in situ and under various experimental conditions) showed no evidence of a neutral microenvironment, either within the highly active biofilm colonizing the chalk surface or within a control biofilm grown on a nonbuffering (i.e., sintered glass) surface under acidic pH. A 16S rRNA approach (clone libraries and fluorescence in situ hybridizations) did not reveal uncommon nitrifying (potentially acid-tolerant) strains. Instead, we found a strongly acidic microenvironment, evidence for a clear adaptation to the low pH in situ, and the presence of nitrifying populations related to subgroups with low K m s for ammonia (Nitrosopira spp., Nitrosomonas oligotropha, and Nitrospira spp.). Acid-consuming (chalk dissolution) and acid-producing (ammonia oxidation) processes are equilibrated on a low-pH steady state that is controlled by mass transfer limitation through the biofilm. Strong affinity to ammonia and possibly the expression of additional functions, e.g., ammonium transporters, are adaptations that allow nitrifiers to cope with acidic conditions in biofilms and other habitats.Chemolithoautotrophic nitrifying bacteria, i.e., ammoniaoxidizing bacteria (AOB), catalyzing the first oxidation step of ammonia to nitrite and nitrite-oxidizing bacteria (NOB) completing the oxidation of the intermediate nitrite to nitrate are known to be sensitive to low pHs. Optimum growth occurs under neutral to moderately alkaline conditions (pH 7.5 to 8.0). In liquid pure culture, growth is usually restricted to a lower pH of 5.8 (AOB) or 6.5 (NOB) (62) and activity ceases typically below pH 5.5 (28, 31). The failure of AOB to cope with acidic conditions is thought to be primarily based on the unavailability of a substrate: with decreasing pHs, ammonia, the substrate of AOB (58), is increasingly protonated. Nitrite, the substrate of NOB, undergoes protonation to nitric acid, which disproportionates to nitrate and gaseous nitric oxide at low pHs (6). Furthermore, when present at elevated concentrations under low pHs, free nitric acid negatively affects the growth and activity of nitrifying bacteria (4).Despite these limitations, autotrophic nitrifying bacteria have been isolated from, or nitrifying activity has been demonstrated in, acidic environments, such as soils, activated sludge, and biofilms. Numerous nitrifying isolates have been obtained from soils with pHs around 4 (for a review, see reference 18 and references therein) to even as low as 2.5 (45). However, the majority of such isolates do not show nitrifying activity in acidic mineral medium (18). In contrast, autotrophic nitr...

This article reports on high-rate nitrification at low pH in biofilm and suspended-biomass reactors by known chemolithotrophic bacteria. In the biofilm reactor, at low pH (4.3 ؎ 0.1) and low bulk ammonium concentrations (9.3 ؎ 3.3 mg ⅐ liter ؊1 ), a very high nitrification rate of 5.6 g of N oxidized ⅐ liter ؊1 ⅐ day ؊1 was achieved. The specific nitrification rate (0.55 g of N ⅐ g of biomass ؊1 ⅐ day ؊1 ) was similar to values reported for nitrifying reactors at optimal pH. In the suspended-biomass reactor, the average pH was significantly lower than that in the biofilm reactor (pH 3.8 ؎ 0.3), and values as low as pH 3.2 were found. In addition, measurements in the suspended-biomass reactor, using isotope-labeled ammonium ( 15 N), showed that in spite of the very low pH, biomass growth occurred with a yield of 0.1 g of biomass ⅐ g of N oxidized ؊1 . Fluorescence in situ hybridization using existing rRNA-targeted oligonucleotide probes showed that the nitrifying bacteria were from the monophyletic genus Nitrosomonas, suggesting that autotrophic nitrification at low pH is more widespread than previously thought. The results presented in this paper clearly show that autotrophic nitrifying bacteria have the ability to nitrify at a high rate at low pH and in the presence of only a negligible free ammonia concentration, suggesting the presence of an efficient ammonium uptake system and the means to cope with low pH.

A new concept for ammonium removal from secondary effluent by zeolite followed by bio-regeneration has been studied. In contrast to other studies of hybrid biological-ion exchange multireactor systems, the proposed process uses the ion exchange material, zeolite, as the carrier for the nitrifying biomass. This enables the two mode process to be carried out in a single reactor. In the first mode (ion exchange), secondary effluent is passed through an ion exchange column where ammonium is concentrated in the zeolite. During the second mode (bioregeneration), the absorbed ammonium is released gradually and converted to nitrate by the active biomass residing on the zeolite. Nitrification is carried out batchwise and in a small volume reactor where optimal conditions can easily be maintained. Moreover, the addition of chemicals for the desorption of ammonium is minimal due to regenerant reuse during several cycles of nitrification. As a result, operational costs and production of large volumes of brine are minimized. Batch and breakthrough experiments showed that the amount of ammonium adsorbed on the chabazite is strongly affected by the presence of competing cations present in secondary effluent. A reduction of about 75% was observed when using a typical Israeli sewage ion composition. The attached biomass did not significantly effect the efficiency of the ion exchange column. Ammonium desorption experiments showed that regeneration with 10,000 mg/L Na+ is much faster than with 2440 mg/L (more than 90% ammonium recovery after 40 and 70 bed volumes, respectively). A nitrification rate of 6 g NH4-N/(L reactor *day) was obtained in a fluidized bed reactor with chabazite as the carrier. Although this rate is in the high range of reported values for biofilm reactors, desorption experiments proved that nitrification will be the process's rate limiting step, rather than the desorption rate when regenerant solutions as low as 2440 mg/L Na+ are used.