Partial nitrification/denitrification can be attributed to the slow response of nitrite oxidizing bacteria to periodic anoxic disturbances

This work aims to assess and model the behavior of both ammonium (AOB) and nitrite (NOB) oxidizing bacteria during the transition from completely anoxic to aerobic conditions. An enhanced aerobically grown culture containing AOB and NOB was subjected to anoxic conditions of varying durations from 1.5 to 12 h before its exposure to aerobic conditions. Experiments were carried out in both continuously stirred tank reactor (CSTR) and batch type reactors. Although the AOB did not exhibit any impact in their performance following the anoxic disturbance, the NOB were seriously inhibited presenting a period of reduced growth rate, which was proportional to the duration of the disturbance. This finding proves the previously postulated mechanism (NOB inhibition under periodic aerobic/anoxic operation) for achieving nitrogen removal via the partial nitrification/denitrification (PND) process as demonstrated in lab- and pilot-scale operating conditions. A mathematical model was developed to describe with sufficient accuracy the performance of AOB and NOB under aerobic, anoxic, and transient conditions in both CSTR and batch type systems. The model is able to describe the inhibitory effect of anoxic exposure to NOB by assuming enzyme deactivation (under anoxic conditions) and reactivation (adjustment of the NOB enzymatic mechanism) under aerobic conditions. The presented kinetic model is quite simple and general and therefore may be used for predicting the performance of mixed growth biological systems operating via the PND process.

Impact of five selected xenobiotics on isolated ammonium oxidizers and on nitrifying activated sludge

Sewage treatment plants (STPs) are usual receptors of xenobiotic compounds that have to be cotreated with municipal wastewaters before being discharged to the water environment. The presence of organic contaminants, such as surfactants, polycyclic aromatic hydrocarbons (PAHs), phthalates, and their primary degradation products in the influents of STPs may inhibit irreversibly sensitive biological processes, such as nitrification. The first step of nitrification, i.e., the oxidation of ammonium to nitrite (nitritification), is particularly sensitive. Inhibition of this step under uncontrolled conditions may completely inhibit biological nitrogen removal. The aim of this work was to study the possible inhibitory effect of five selected xenobiotics on (a) a mixed culture of ammonium-oxidizing bacteria isolated from activated sludge and (b) nitrifying activated sludge directly. The xenobiotics that were tested include nonylphenols (NP), nonylphenolethoxylates (NPEO), linear alkylbenzene sulfonates (LAS), di(2-ethylhexyl) phthalate (DEHP), as a representative phthalate ester, and the PAH phenanthrene. Remarkable inhibitory effects for all tested compounds were observed in this study even at xenobiotic concentrations as low as 1 mg/L. The observed inhibition of xenobiotics on nitrifying activated sludge was less pronounced, because of the masking effect exerted by the sludge flocs, but was still significant for many of the tested substances at concentrations up to 10 mg/L.

Sensitivity analysis of a biofilm model describing mixed growth of nitrite oxidisers in a CSTR

A simple kinetic model has been developed for describing nitrite oxidation by autotrophic aerobic nitrifiers in a CSTR reactor, in which mixed (suspended and attached) growth conditions are prevailing. In this work, a critical dimensionless parameter is identified containing both biofilm characteristics and microbial kinetic parameters, as well as the specific (per volume) surface of the reactor configuration used. Evaluation of this dimensionless parameter can easily provide information on whether or not wall attachment is critical, and should be taken into account either in kinetic studies or in reactor design, when specific pollutants are to be removed from the waste influent stream. The effect of bulk dissolved oxygen (DO) concentration on the validity of this model is addressed and minimum non-limiting DO concentrations are proposed depending on the reactor configuration.

On the occasional biodegradation of pharmaceuticals in the activated sludge process: the example of the antibiotic sulfamethoxazole

Sulfamethoxazole, a common antibiotic, was found to be biodegradable under aerobic conditions. The fate of sulfamethoxazole in the activated sludge process was studied using a Sequencing Batch Reactor (SBR). Aerobic biomass was acclimated to sulfamethoxazole and a series of kinetic experiments were conducted to investigate the impact of other carbon and nitrogen sources on the degradation of the antibiotic. It was found that sulfamethoxazole serves both as carbon and nitrogen source for the enriched consortium. It was degraded whenever there was a depletion of carbon or nitrogen or both in the feeding medium, while in the presence of acetate and ammonium nitrogen (alternative carbon and nitrogen sources, respectively), sulfamethoxazole remained intact.

On the effect of pharmaceuticals on bacterial nitrite oxidation

Pharmaceuticals or their metabolites are partially excreted with urine or faeces ending up in raw sewage. Many of these substances are not biodegradable and their presence in influents of municipal wastewater treatment plants may cause adverse effects to sensitive biological processes such as nitrification, while on the other hand, they may go through the activated sludge process unreacted. The second step of nitrification, i.e. oxidation of nitrite to nitrate is particularly sensitive. Inhibition of this step under uncontrolled conditions may lead to accumulation of nitrite nitrogen in the plant effluent, a form of nitrogen which is particularly toxic. The effects caused by the presence of seven different pharmaceuticals to a culture of nitrite-oxidizing bacteria isolated from activated sludge are presented. These pharmaceuticals were ofloxacin, propranolol, clofibrate, triclosan, carbamazepine, diclofenac and sulfamethoxazole. Different effects were observed for each of the pharmaceuticals tested in this study. In the cases of ofloxacin and sulfamethoxazole significant inhibition was observed. Triclosan presented a substantial inhibitory effect on the substrate (nitrite) reduction rate. The long-term effect of triclosan on nitrite oxidizers was also examined in a CSTR reactor and conclusions were drawn regarding the reversibility of the inhibition caused by this compound.

Removal of phenolics in olive mill wastewaters using the white-rot fungus Pleurotus ostreatus

Olive mill wastewaters (OMW) present a major environmental problem. The large amounts generated, combined with the high phenols and chemical oxygen demand concentrations, are the main difficulties in finding a solution for the management of these wastewaters, which are dangerous for the environment. The phenols, which are contained in the OMW have a structure similar to lignin, which makes them difficult to biodegrade. Lignin can be degraded only by a few microorganisms, such as "white-rot" basidiomycete, which produce manganese (MnPs) and lignin peroxidases (LiPs) and laccases that are responsible for the oxidisation of lignin compounds. The capability of Pleurotus ostreatus to degrade phenols of OMW in different conditions such as in sterilized and thermally processed (at 100 degrees C) wastewater, with and without dilution, is investigated in this work. According to the experimental results P. ostreatus removed phenols from the culture medium, under all different conditions that were examined. The degradation of phenols reached up to 78.3% for the sterilized and 50% diluted OMW, 66.7% and 64.7% for the thermally processed OMW, with and without dilution, respectively. The effect of pre-treatment of OMW on the performance of anaerobic digestion is also assessed, as methanogenic bacteria are seriously affected by the presence of phenol compounds. The pre-treated wastewater was shown to be more amenable to a subsequent anaerobic digestion.