The purpose of this research was to address the anoxic oxidation of metallic iron and stainless steel powder by nitrate, nitrite, and anaerobic mixed cultures. In sterile batch reactors, both nitrate and nitrite (10 mg N/L) could chemically oxidize metallic iron, with a concomitant reduction to ammonium. Nitrate or nitrite reduction coupled to metal corrosion was not observed in the case of stainless steel powder. Combination of an anaerobic mixed culture and metallic iron led to (cathodically produced) H2 consumption and a complete nitrate or nitrite reduction (mainly to NH4 +). This caused a slightly enhanced metal oxidation. In the case of stainless steel, corrosion caused by denitrifying microorganisms was evidenced by data on nitrate/nitrite ions and solubilized iron. Experiments with increasing nitrite concentrations indicated that nitrite in the range of 50 mg of NO2 - -N/L inhibited the corrosion processes. Moreover, at concentrations above 140 mg NO2 - -N/L, a significant production of nitric oxide (NO) was detected. Differences between iron and stainless steel powder at low concentrations of nitrate or nitrite are most probably due to differences in kinetics:  metallic iron exhibited faster chemical than biological reactions as opposed to stainless steel. It is postulated that the inhibitory effect of higher nitrite concentrations could partly be due to the chemical formation of NO and its toxic effect on the microorganisms acting at the steel surface.

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Two filamentous bacteria causing bulking in two activated sludges were examined. Investigations using morphological features, staining techniques, and fluorescent in situ hybridization identified both filaments as type 021N. However, an examination of the effect of chlorine on the sludges revealed a chlorine-susceptible type 021N in one sludge and a chlorine-resistant type 021N in the other.The performance of the activated sludge process is often determined by the gravity separation between the treated water and the sludge in the final clarifier. The proliferation of filamentous bacteria in the microbial community of the sludge, referred to as "filamentous bulking," has often been reported to hamper this solid-liquid separation (7,18). Filamentous bulking results in less dense and less settleable sludge flocs, and in severe cases the operation of the plant can be totally compromised. Addition of disinfectants (mostly chlorine based) to the sludge to selectively kill off the causative filamentous bacteria is often used in practice as a short-term and cost-effective solution (7). Jenkins et al. (7) presented sludge chlorination as a method of choice in the United States to combat filamentous bulking. They reported several case histories of successful control of bulking at full-scale treatment plants using chlorine and described the method as a "universal success" with any filamentous bacteria. Yet cases of partial success and cases of failure can be found in the literature. Lakay et al. (11) obtained only a partial elimination of Microthrix parvicella bacteria at a high chlorine dose. Hwang and Tanaka (6) found in batch tests that M. parvicella remained intact at very high chlorine doses, while the microbial flocs were completely destroyed. Fontaine (3) could not overcome filamentous bulking in a pilot plant using chlorination. More recently, Madoni et al. (12) reported in a survey in Italy that use of chlorination was successful in only 63% of the cases. Most often, no relevant explanation was offered for these unsuccessful cases of sludge chlorination. Thoroughly examined unsuccessful cases have been ascribed to the presence of wellknown resistant filamentous bacteria with hydrophobic cell walls such as M. parvicella and Nostocoida limicola (6, 18). This paper presents and discusses the effects of chlorine on two sludges, one dominated by a chlorine-vulnerable type 021N bacterium and the other dominated by a chlorine-resistant type 021N bacterium, as a relevant explanation of the discrepancies noted in the effectiveness of chlorine in the cure of bulking sludge.Experimental procedures. The two types of sludges (sludges A and B) involved in this study originated from two industrial wastewater treatment plants (plants A and B) located in the region of Ghent, Belgium. Plant A employed a two-stage activated sludge system fed continuously with the wastewater resulting from the manufacture of mayonnaise and soups. The sludge loading rate (SLR) of sludge A was 0.4 g of COD (g of VSS ⅐ day) Ϫ1 (where COD is the chemica...

Aims: To investigate the bactericidal in¯uence of copper-alloying of stainless steel on microbial colonization. Methods and Results: Inhibition of bacterial adherence was investigated by monitoring (192 h) the development of a multi-species bio®lm on Cu-alloyed (3á72 wt%) stainless steel in a natural surface water. During the ®rst 120 h of exposure, lower numbers of viable bacteria in the water in contact with copper-containing steel relative to ordinary stainless steel were observed. Moreover, during the ®rst 48 h of exposure, lower colony counts were found in the bio®lm adhering to the Cu-alloyed steel. No lower colony or viable counts were found throughout the remainder of the experimental period. Conclusions: The presence of Cu in the steel matrix impedes the adhesion of micro-organisms during an initial period (48 h), while this bactericidal effect disappears after longer incubation periods. Signi®cance and Impact of the Study: The application of Cu-alloyed stainless steels for bactericidal purposes should be restricted to regularly-cleaned surfaces.