Activated Sludge Basins Research Articles

Biological phosphorus removal and its microbial community in a modified full-scale activated sludge system under dry and wet weather dynamics

Enhanced biological phosphorus removal (EBPR) performance and microbial community dynamics during dry and wet-weather conditions of a full-scale treatment plant was evaluated by converting a section of activated sludge basins using low-cost operational modifications into an anoxic/anaerobic zone to promote EBPR. Two trains of the activated sludge system at the Des Moines, Iowa Metropolitan Wastewater Reclamation Facility were used for the study with one train modified for EBPR, and the other remained as nitrification-only for comparison.In addition to measuring the modification effectiveness for phosphorus removal, performance was compared during dry and wet weather conditions over the course of two summer seasons to improve understanding of wet and dry weather dynamics for EBPR. DNA sequencing and qPCR tests were conducted to develop an understanding of microbial population changes between control and modified basins and wet and dry weather conditions.Basin hydraulic retention times varied from 2.6 to 12.7 hours with an average of 8.9 hours. EBPR activity was successfully established in the modified basins with average phosphorus content of the return activated sludge 0.032 ± 0.002 compared to 0.016 ± 0.001 mg TP/mg TSS (95% confidence) in the control basins. Phosphorus removal was significantly decreased by prolonged wet weather conditions, particularly in year two of the study, however the modified basin maximum removal of 96% and average of 43.7 ± 5.3% remained significantly higher than the maximum of 46% and average 12.6 ± 2.4% removal in the control basins.DNA sequencing showed a significant increase in relative abundance of phyla Chloroflexi, Nitrospirae, and Verrucomicrobia in the modified basins, but no correlation to EBPR performance. qPCR indicated significant increase in relative quantity of Accumulibacter, but not for Actinetobacter-like phosphorus accumulating organisms (PAOs), which includes the PAO Tetrasphaera. Significant abundance of some Accumulibacter clades found within the modified basins was contrary to previous literature which focused on small-scale and batch studies. A higher than expected dominance of clade I and increased relative quantities of clades IIB and IIC during extended wet weather was observed which may have contributed to rapid recovery of phosphorus removal when dry weather resumed. The abundance of PAOs did not significantly correlate with changes in phosphorous removal performance, contrary to reports from previous small-scale and batch studies.

ASPECTS REGARDING THE ENERGITICAL VALORIFICATION OF URBAN SLUDGE TREATMENT

This paper highlights through the issues addressed, the application aspects regarding the approach of the concept of recovery of sewage sludge from an urban wastewater treatment plant (SEAU), which is in a stage of technological expansion and modernization. Thus, it is considered the need to process the potential of biological mass generated by anaerobic stabilization in the SEAU, with a population of about 10,000 equivalent inhabitants, which requires on one hand, the technological design of activated sludge basins and on the other hand, the design of a sewage sludge fermentation tank, in order to produce biogas, necessary for its recovery for energy purposes.

Oxygen Transfer in Two-Stage Activated Sludge Wastewater Treatment Plants

Aeration is an energy-intensive process of aerobic biological treatment in wastewater treatment plants (WWTP). Two-stage processes enable energy-efficient operation, but oxygen transfer has not been studied in depth before. In this study, α-factors were determined with long-term ex situ steady-state off-gas measurements in pilot-scale test reactors (5.8 m height, 8.3 m3) coupled to full-scale activated sludge basins. A two-stage WWTP with more than 1 Mio population equivalent was studied over 13 months including rain and dry weather conditions. Operating data, surfactant concentrations throughout the two-stage process, and the effect of reverse flexing on pressure loss of diffusers were examined. The values of αmean, αmin, and αmax for design load cases of aeration systems were determined as 0.45, 0.33, and 0.54 in the first high-rate carbon removal stage and as 0.80, 0.69, and 0.91 in the second nitrification stage, respectively. The first stage is characterized by a distinct diurnal variation and decrease in α-factor during stormwater treatment. Surfactants and the majority of the total organic carbon (TOC) load are effectively removed in the first stage; hence, α-factors in the second stage are higher and have a more consistent diurnal pattern. Proposed α-factors enable more accurate aeration system design of two-stage WWTPs. Fouling-induced diffuser pressure loss can be restored effectively with reverse flexing in both treatment stages.

Full-scale sludge reduction in the water line of municipal wastewater treatment plant

Nowadays, sludge management represents one of the most critical challenges in the field of sewage treatment for economic and environmental impacts. Therefore, the reduction of sludge has become a major issue for the operators of municipal wastewater treatment plants. In the present paper, a new system, whose acronym is MULESL (MUch LEss SLudge), is proposed and tested at full scale for reducing the quantity of sludge in the water line of the sewage treatment plant. MULESL system takes the advantage of maintenance metabolism to significantly reduce the sludge production. The effectiveness of MULESL system in removing the typical pollutants and reducing sludge production was evaluated at full scale by using 3500 PE unit located in Putignano's WWTP (Puglia, Italy). This unit was obtained by retrofitting an existing activated sludge basin. The results obtained over 1-year period, during which MULESL unit treated the effluent of the preliminary treatment step, have indicated that it was characterized by a specific sludge production as low as 0.13 kg of dry sludge per kg of COD removed; 77% lower than that recorded for primary and secondary treatments of the conventional plant during the same period. This sludge reduction was obtained with a plant volume 27% smaller than that of the conventional water line. Furthermore, the organic matter of the sludge was already stabilized, thus allowing to save investment costs for digestion process facilities. Finally, MULESL unit guaranteed a mean removal efficiency higher than 95% for COD, BOD5, TSS, TKN, NH3 and TN.

Assessment and risk modeling of airborne enteric viruses emitted from wastewater reused for irrigation

Reclamation of wastewater (WW) for irrigation, after treatment represents a challenge that could alleviate pressure on water resources and address the increasing demand for agriculture. However, the risks to human health must be assessed, particularly those related to human enteric viruses that resist standard treatments in most wastewater treatment plants (WWTP). The risks associated with exposure to viral bioaerosols near WWTP and near agricultural plots irrigated with WW are poorly documented. The objectives of this study were to 1) better characterize human enteric viruses found in bioaerosols near a “standard WWTP” and over fields irrigated with treated WW and 2) propose a numeric model to assess the health risk to populations located close to the irrigated areas, with particular attention to norovirus, which is responsible for most viral gastroenteritis in France. Water and air samples were collected at various locations in the largest French WW-irrigated site near Clermont-Ferrand, at the WWTP entrance and after treatment, in the air above activated sludge basins, and above fields irrigated with WW. Various enteric viruses were found in the water samples collected both before and after treatment. Norovirus was the most abundant with >10e4 genome copies/l (GC/L) before treatment and ~10e3 GC/L after treatment. Low quantities (<10e3GC/m3) were detected in the air above active sludge pools and irrigated plots. Hepatitis E virus was detected in all sampled compartments. A quantitative microbial risk assessment (QMRA) approach, including a simplified atmospheric dispersion model, allowed assessment of norovirus infection risk. The Bayesian QMRA approach considered wind speed measurements over 21years, and the variability and uncertainty of all measurements throughout the chain up to the risk. The probability of infection within one year for the most exposed WWTP employees was >10e-4 for strong wind speed (≥3m/s) and a constant emission rate of 8e3 GC/m3/s. This probability decreases by 3 log when the distance to the emission source is doubled. This information can aid development of safe water reuse policies in terms of local setback distance and wind conditions for wastewater reuse.

CFD for wastewater treatment: an overview

Computational fluid dynamics (CFD) is a rapidly emerging field in wastewater treatment (WWT), with application to almost all unit processes. This paper provides an overview of CFD applied to a wide range of unit processes in water and WWT from hydraulic elements like flow splitting to physical, chemical and biological processes like suspended growth nutrient removal and anaerobic digestion. The paper's focus is on articulating the state of practice and research and development needs. The level of CFD's capability varies between different process units, with a high frequency of application in the areas of final sedimentation, activated sludge basin modelling and disinfection, and greater needs in primary sedimentation and anaerobic digestion. While approaches are comprehensive, generally capable of incorporating non-Newtonian fluids, multiphase systems and biokinetics, they are not broad, and further work should be done to address the diversity of process designs. Many units have not been addressed to date. Further needs are identified throughout, but common requirements include improved particle aggregation and breakup (flocculation), and improved coupling of biology and hydraulics.

N2O emissions from secondary clarifiers and their contribution to the total emissions of the WWTP.

Recent studies have indicated that the emissions of nitrous oxide, N2O, constitute a major part of the carbon footprint of wastewater treatment plants (WWTPs). Denitrification occurring in the secondary clarifier basins has been observed by many researchers, but until now N2O emissions from secondary clarifiers have not been widely reported. The objective of this study was to measure the N2O emissions from secondary clarifiers and weigh the portion they could represent of the overall emissions at WWTPs. Online measurements over several days were carried out at four different municipal WWTPs in Finland in cold weather conditions (March) and in warm weather conditions (June-July). An attempt was made to define the conditions in which N2O emissions from secondary clarifiers may occur. It was evidenced that large amounts of N2O can be emitted from the secondary clarifiers, and that the emissions have long-term variation. It was assumed that part of the N2O released in secondary clarification was originally formed in the activated sludge basin. The emissions from secondary clarification thus seem to be dependent on conditions of the nitrification and denitrification accomplished in the denitrification-nitrification process and on the amount of sludge stored in the secondary clarifiers.

Toxicity of the micropollutants Bisphenol A, Ciprofloxacin, Metoprolol and Sulfamethoxazole in water samples before and after the oxidative treatment

The amount of organic micropollutants detected in surface waters increases steadily. Common waste water treatment plants are not built to remove these substances. Thus there is a need for new technologies. A promising technology is the use of advanced oxidation processes through which organic micropollutants can be removed from waste water. However, the formation of oxidation by-products is likely and needs to be investigated since the by-products not only differ from their parent compounds in regard to their chemical and physical properties but they can also differ in toxicity. Therefore this study was designed to combine chemical and toxicological analyses of the advanced oxidation (O3 [5mg/L] or UV/H2O2 [Hg-LP lamp; 15W; 1g/L H2O2]) of waste water treatment plant effluents and pure water. Effluent samples from conventional activated sludge waste water treatment (mechanical treatment, activated sludge basin, and primary as well as secondary treatment steps) and high-purity deionized water (pure water) were spiked with Bisphenol A, Ciprofloxacin, Metoprolol or Sulfamethoxazole and treated with O3 or UV/H2O2. For the toxicological analyses mammalian cells (CHO-9, T47D) were exposed to the water samples for 24h and were tested for cytotoxicity (MTT Test), genotoxicity (Alkaline Comet Assay) and estrogenicity (ER Calux(®)). The results indicate that the oxidative treatment (O3 or UV/H2O2) of Bisphenol A, Metoprolol, Sulfamethoxazole or Ciprofloxacin in waste water did not result in toxic oxidation by-products, whereas the UV/H2O2 treatment of Bisphenol A and Ciprofloxacin in pure water resulted in by-products with cytotoxic but no estrogenic effects after 60min.

The Effects of Solids Retention Time in Full‐Scale Activated Sludge Basins on Trace Organic Contaminant Concentrations

Although pharmaceuticals and personal care products (PPCPs) and endocrine disrupting compounds (EDCs) are largely unregulated, water resource recovery facilities are increasingly using advanced chemical/physical treatment technologies (e.g., advanced oxidation and reverse osmosis) to remove or destroy these trace organic contaminants (TOrCs). This can both reduce potential adverse human health effects in reuse applications and mitigate environmental effects on aquatic ecosystems. Unfortunately, advanced treatment technologies are typically energy intensive and costly to implement, operate, and maintain. The goal of this study was to determine whether optimization of solids retention time (SRT) provided sufficient benefits to warrant such operational strategies for TOrC mitigation. Specifically, SRTs of 5.5, 6, and 15 days were evaluated to determine the effects on several standard wastewater parameters (e.g., nitrite, nitrate, and ammonia concentrations) and the degradation of TOrCs. The experimental SRTs were operated simultaneously in parallel, full-scale activated sludge basins. The results indicate that it can be beneficial to implement biological process optimization strategies using existing infrastructure while reducing reliance on advanced treatment technologies. This study also identified potential operational issues that might arise in activated sludge systems operating at extended SRTs.

Correlation of Community Dynamics and Process Parameters As a Tool for the Prediction of the Stability of Wastewater Treatment

Wastewater treatment often suffers from instabilities and the failure of specific functions such as biological phosphorus removal by polyphosphate accumulating organisms. Since most of the microorganisms involved in water clarification are unknown it is challenging to operate the process accounting for the permanent varying abiotic parameters and the complex composition and unrevealed metabolic capacity of a wastewater microbial community. Fulfilling the demands for water quality irrespective of substrate inflow conditions may emit severe problems if the limited management resources of municipal wastewater treatment plants are regarded. We used flow cytometric analyses of cellular DNA and polyphosphate to create patterns mirroring dynamics in community structure. These patterns were resolved in up to 15 subclusters, the presence and abundances of which correlated with abiotic data. The study used biostatistics to determine the kind and strength of the correlation. Samples investigated were obtained from a primary clarifier and two activated sludge basins. The stability of microbial community structure was found to be high in the basins and low in the primary clarifier. Despite major abiotic changes certain subcommunities were dominantly present (up to 80% stability), whereas others emerged only sporadically (down to 3% stability, both according to equivalence testing). Additionally, subcommunities of diagnostic value were detected showing positive correlation with substrate influxes. For instance blackwater (r(s) = 0.5) and brewery inflow (both r(s) = 0.6) were mirrored by increases in cell abundances in subclusters 1 and 6 as well as 4 and 8, respectively. Phosphate accumulation was obviously positively correlated with nitrate (r(s) = 0.4) and the presence of denitrifying organisms (Rhodacyclaceae). Various other correlations between community structure and abiotic parameters were apparent. The bacterial composition of certain subcommunities was determined by cell sorting and phylogenetic tools like T-RFLP. In essence, we developed a monitoring tool which is quick, cheap and causal in its interpretation. It will make laborious PCR based technique less obligatory as it allows reliable process monitoring and control in wastewater treatment plants.

Impact of Silver Nanoparticles on Wastewater Treatment

Silver nanoparticles (AgNPs, nanosilver) are a frequently used nanomaterial with a wide range of industrial and consumer applications, including fiber coating, detergents, and hydrogels and plastics to prevent bacterial and fungal growth. Nanoparticles released from various nanotechnology-enhanced consumer products will inevitably enter our sewers and wastewater treatment plants (WWTPs). This project evaluated how silver nanoparticles would affect wastewater treatment systems and anaerobic digestion. The researchers set up several lab-scale wastewater treatment modular units using activated sludge processes designed to remove organic matter and nutrients in wastewater. The results demonstrated that nitrifying bacteria were especially susceptible to inhibition by silver nanoparticles. At a concentration of 0.4 mg/L total Ag, a mixture of positively charged silver ions and AgNPs (50:50 in mass ratio, average size =15−21 nm) inhibited the growth of nitrifying bacteria from the modified Ludzack-Ettinger bioreactor by 11.5 percent. In an experiment on shock loading of 100% AgNPs (lasting for 12 hours), a peak concentration of 0.75 mg/L total Ag in the activated sludge basin (more than 95% associated with biomass) was detected, and about 50% nitrifying bacterial growth inhibition (or nitrification inhibition) accompanied with a slight accumulation of nitrite concentration in wastewater effluent was observed. Studies of anaerobic digestion, a commonly used solid stabilization process in wastewater treatment plants, indicate that AgNPs at concentrations of 19 mg/L (19,000 ppb) or above in biomass might inhibit anaerobic microbial activities. Most of the silver particles are in the activated sludge. After considering concentration factor and safety factor, the suggested threshold concentration of total silver including nanosilver in wastewater influent is 0.1 mg/L. This study suggests that accumulation of silver in activated sludge could have a detrimental effect on wastewater treatment, if the concentration reaches threshold levels.This title belongs to WERF Research Report Series.ISBN: 9781843393993 (eBook)

The effect of iron and aluminium for phosphorus removal on anaerobic digestion and organic sulfur generation

The addition of iron or aluminium into activated sludge basins for phosphorus removal is likely to impact both the efficiency of the anaerobic digestion process and the generation of odor-causing compounds following digestion and dewatering. In this study, the impact of iron and aluminium addition on digestion and odor-causing compounds was investigated by using batch digestion of combined primary and waste activated sludge. It was found that aluminium addition resulted in a decrease in volatile solids destruction by anaerobic digestion of approximately 2%. Of the 7 sludges tested, 5 showed a small increase in volatile solids destruction after iron addition. With regard to the generation of organic sulfur odors from the dewatered sludge cakes, both iron and aluminium reduced odor-causing gases except for one sludge that had already received iron for phosphorus control in the full-scale process. It appears that iron and aluminium addition will benefit odor control.

Nitrogen Removal from Wastewater Using a Hybrid Membrane‐Biofilm Process: Pilot‐Scale Studies

The hybrid membrane biofilm process (HMBP) is a new approach to achieving total nitrogen removal from wastewater. Air-filled, hollow-fiber membranes are placed into an activated sludge basin and bulk aeration is suppressed. A nitrifying biofilm develops on the membranes, exporting nitrate and nitrite to the bulk liquid. The nitrate and nitrite are reduced by suspended biomass using influent BOD as the electron donor. Previous research demonstrated the HMBP concept at the bench scale and explored process fundamentals. This research explored the HMBP at the pilot scale, with a 120-L reaction tank, real wastewater, and a potentially scalable configuration. Nitrification rates averaged 0.5 g N m(-2)/d(-1), which were lower than found at the bench scale, and lower than predicted by a mathematical model, but still allowed effluent total nitrogen concentrations below 6 mg N/L with an average influent total nitrogen concentration of 25 mg N/L and a hydraulic retention time of 12 hours. More than 75% of the produced nitrate and nitrite was reduced with an average influent sCOD of only 68 mg/L and an average C:N ratio of 3.1. Mass balances on carbon and nitrogen suggest that nitrogen removal via nitrite occurred. This research confirms that the HMBP process is effective for BOD and nitrogen removal from wastewater, and suggests that the grid configuration is viable for scale-up.

The Effects of Solids Retention Time in Full-Scale Activated Sludge Basins on Trace Organic Contaminant Concentrations

The Effects of Solids Retention Time in Full-Scale Activated Sludge Basins on Trace Organic Contaminant Concentrations

Bio-Oxidation of Air-Borne Hydrophobic VOCs in Activated Sludge Aeration Tanks-Effects of Activated Sludge Concentration and Gas Diffuser

An activated sludge aeration basin (20 cm i.d., 140 cm height) equipped with either a coarse air diffuser (a plastic pipe perforated with 56 orifices of 2 mm in diameter) or a fine diffuser (porous plastic type with 100-micrometer pores) was utilized to treat an air-borne hydrophobic VOC (toluene, 700-800 mg/m^3). The purposes of this study were to test the influences of both MLSS and diffuser type on the VOC removal efficiency. Results show that higher MLSS (mixed liquor suspended solids) such as 10,000-40,000 mg/L in the mixed liquor did not enhance greatly the transfer and removal of the introduced toluene. Instead, activated sludge basins with a normal MLSS (e.g., 2000-4000 mg/L) in the mixed liquor and an efficient gas diffusion system with volumetric VOC transfer coefficient of around 10-15 1/h can be used for the removal of hydrophobic VOCs from the introduced gas. For achieving a removal of over 95% of the introduced toluene or similar hydrophobic VOCs, commercial air diffusers for aerobic biological wastewater treatment basins can be used with a submerged liquid depth of over 0.40 m over the diffusers and an aeration intensity (air flow rate/basin cross-sectional area) of lower than 5.0 m^3/m^2.h.

Non-Steady State Oxygen Transfer Testing of New High Purity Oxygen Dissolution and Mixing Device for Activated Sludge Aeration

Non-Steady State Oxygen Transfer Testing of New High Purity Oxygen Dissolution and Mixing Device for Activated Sludge AerationAir Products and Chemicals, Inc. (APCI) and Aqua-Aerobic Systems, Inc. (AAS) have jointly developed an oxygen injection and mixing system, OxyMixTM, for the activated sludge wastewater treatment process. The OxyMixTM concept utilizes a high-speed, direct-drive floating mixer fed with high purity oxygen to provide all the necessary aeration and mixing for an activated sludge basin. Clean water...Author(s)Annemarie O. WeistMichelle A. FitzgeraldBulent OzbasChristopher A. HarveyBob ColehourTerry ReidLloyd JohnsonDavid T. RedmonSourceProceedings of the Water Environment FederationSubjectSession 105 - Innovative Technology Forum IIDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2009ISSN1938-6478SICI1938-6478(20090101)2009:8L.6954;1-DOI10.2175/193864709793957832Volume / Issue2009 / 8Content sourceWEFTECFirst / last page(s)6954 - 6964Copyright2009Word count167Subject keywordsOxygen transferhigh purity oxygenactivated sludgeaerationkLadissolved oxygencomputational fluid dynamics

Molecular Diversity of a North Carolina Wastewater Treatment Plant as Revealed by Pyrosequencing

We report the results of pyrosequencing of DNA collected from the activated sludge basin of a wastewater treatment plant in Charlotte, NC. Using the 454-FLX technology, we generated 378,601 sequences with an average read length of 250.4 bp. Running the 454 assembly algorithm over our sequences yielded very poor assembly, with only 0.3% of our sequences participating in assembly of significant contigs. Of the 117 contigs greater than 500 bp long that were assembled, the most common annotations were to transposases and hypothetical proteins. Comparing our sequences to known microbial genomes showed nonspecific recruitment, indicating that previously described taxa are only distantly related to the most abundant microbes in this treatment plant. A comparison of proteins generated by translating our sequence set to translations of other sequenced microbiomes shows a distinct metabolic profile for activated sludge with high counts for genes involved in metabolism of aromatic compounds and low counts for genes involved in photosynthesis. Taken together, these data document the substantial levels of microbial diversity within activated sludge and further establish the great utility of pyrosequencing for investigating diversity in complex ecosystems.

Savings with upgraded performance through improved activated sludge denitrification in the combined activated sludge–biofilter system of the Southpest Wastewater Treatment Plant

The purpose of the experiments was to increase the rate of activated sludge denitrification in the combined biological treatment system of the Southpest Wastewater Treatment Plant in order to gain savings in cost and energy and improve process efficiency. Initial profile measurements revealed excess denitrification capacity of the preclarified wastewater. As a consequence, flow of nitrification filter effluent recirculated to the anoxic activated sludge basins was increased from 23,000 m3 d(-1) to 42,288 m3 d(-1) at an average preclarified influent flow of 64,843 m3 d(-1), Both simulation studies and microbiological investigations suggested that activated sludge nitrification, achieved despite the low SRT (2-3 days), was initiated by the backseeding from the nitrification filters and facilitated by the decreased oxygen demand of the influent organics used for denitrification. With the improved activated sludge denitrification, methanol demand could be decreased to about half of the initial value. With the increased efficiency of the activated sludge pre-denitrification, plant effluent COD levels decreased from 40-70 mg l(-1) to < 30-45 mg l(-1) due to the decreased likelihood of methanol overdosing in the denitrification filter.

Pilot-Scale Testing of the Hybrid Membrane Biofilm Process (HMBP) for Total Nitrogen Removal from Municipal Wastewater

Pilot-Scale Testing of the Hybrid Membrane Biofilm Process (HMBP) for Total Nitrogen Removal from Municipal WastewaterThe hybrid membrane biofilm process (HMBP) is a novel technology for removing total nitrogen (TN) from municipal wastewater. Air-filled, hollow-fiber membranes are placed in an activated sludge basin, and the bulk liquid is left anoxic. Membranes support an aerobic, nitrifying biofilm, while produced nitrate is exported to the bulk liquid and reduced with BOD as the electron donor. Bench scale...Author(s)Leon S. DowningKyle BibbyKathleen EspositoThomas FascianellaRobert NerenbergSourceProceedings of the Water Environment FederationSubjectSession 82: Membrane ProcessesDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2008ISSN1938-6478SICI1938-6478(20080101)2008:9L.6236;1-DOI10.2175/193864708790893440Volume / Issue2008 / 9Content sourceWEFTECFirst / last page(s)6236 - 6244Copyright2008Word count173

Behaviour of pathogenic and indicator bacteria during urban wastewater treatment and sludge composting, as revealed by quantitative PCR

Two enteric pathogens, Salmonella spp. and Campylobacter jejuni, and two bacteria commonly used as indicators, Escherichia coli and Clostridium perfringens, were monitored using quantitative real-time PCR during municipal wastewater treatment and sludge composting. The results were compared with those obtained using standard culture methods. A reduction of all bacteria was observed during wastewater treatment and during the thermophilic phase of composting. However, the bacterial groups studied behaved differently during the process, and the main differences were observed during biological treatment in activated sludge basins. In particular, Salmonella spp. and C. jejuni survived better during activated sludge treatment than E. coli. C. jejuni was the most resistant to wastewater treatment among the four bacterial groups. Overall, differences in survival were observed for all bacteria studied, when submitted to the same environmental pressure. This holds both for differences between indicators and pathogenic bacteria and between pathogenic bacteria. These results show the difficulty in defining reliable indicators.