Effects of in-sewer processes: a stochastic model approach.

Transformations of organic matter, nitrogen and sulfur in sewers can be simulated taking into account the relevant transformation and transport processes. One objective of such simulation is the assessment and management of hydrogen sulfide formation and corrosion. Sulfide is formed in the biofilms and sediments of the water phase, but corrosion occurs on the moist surfaces of the sewer gas phase. Consequently, both phases and the transport of volatile substances between these phases must be included. Furthermore, wastewater composition and transformations in sewers are complex and subject to high, natural variability. This paper presents the latest developments of the WATS model concept, allowing integrated aerobic, anoxic and anaerobic simulation of the water phase and of gas phase processes. The resulting model is complex and with high parameter variability. An example applying stochastic modeling shows how this complexity and variability can be taken into account.     

The effect of sulphide and organic matter on the nitrification activity in a biofilm process

Septic wastewater, characterised by the appearance of sulphide, is known to cause problems in sewage systems (corrosion and odour), at treatment plants (e.g. inhibition, sludge bulking) and for human beings (toxicity). Sulphide formation in sewers may be prevented by increasing the redox potential, either by oxygen/air injection (aerobic conditions) or dosage of nitrate (anoxic conditions). The effect on the nitrification capacity in a biofilm process of an anoxic wastewater as compared to a septic wastewater has been studied. The main change in wastewater quality as a result of nitrate dosage is reduced concentrations of organic matter and insignificant sulphide concentrations. The results show that a sulphide concentration of 0.5 mg/1 had a considerable negative effect on the nitrification activity. The sulphide and the higher concentrations of organic matter in the septic wastewater caused together a 30–40% reduction of the nitrification capacity as compared to the anoxic wastewater, even with pre-aeration and pre-precipitation with Fe³⁺. The removal of organic matter in the sewer as a result of the anoxic conditions created by the addition of nitrate, resulted in a maximum nitrification capacity when particulate organic matter was removed by pre-precipitation.     

Simulation of the wastewater temperature in sewers with TEMPEST.

TEMPEST is a new interactive simulation program for the estimation of the wastewater temperature in sewers. Intuitive graphical user interfaces assist the user in managing data, performing calculations and plotting results. The program calculates the dynamics and longitudinal spatial profiles of the wastewater temperature in sewer lines. Interactions between wastewater, sewer air and surrounding soil are modeled in TEMPEST by mass balance equations, rate expressions found in the literature and a new empirical model of the airflow in the sewer. TEMPEST was developed as a tool which can be applied in practice, i.e., it requires as few input data as possible. These data include the upstream wastewater discharge and temperature, geometric and hydraulic parameters of the sewer, material properties of the sewer pipe and surrounding soil, ambient conditions, and estimates of the capacity of openings for air exchange between sewer and environment. Based on a case study it is shown how TEMPEST can be applied to estimate the decrease of the downstream wastewater temperature caused by heat recovery from the sewer. Because the efficiency of nitrification strongly depends on the wastewater temperature, this application is of practical relevance for situations in which the sewer ends at a nitrifying wastewater treatment plant.     

Spatial variability of characteristics and origins of urban wet weather pollution in combined sewers.

An experimental on-site observatory of urban pollutant loads in combined sewers was created in the centre of Paris to quantify and characterise the dry and wet weather flow in relation to spatial scale. Eight rainfall events were studied from April 2003 to May 2004. Samples were analysed for suspended solids, organic matter, nitrogen and heavy metals. Results confirm the extent of wet weather pollution. They have shown the relative homogeneity of SS and organic matter characteristics from one urban catchment area to another. Two groups of heavy metals were identified. The first one concerns Cu, which has a higher concentration in wet weather flow (WWF) than in dry weather flow (DWF), and runoff. The second includes Cd, Pb and Zn, where higher concentrations were measured in urban runoff than in WWF and DWF. A first evaluation of contribution of wastewater, urban runoff and sewer deposit erosion sources to wet weather pollution was established and has highlighted the contribution of wastewater and sewer deposits to this pollution. However, it has shown that sewer deposit erosion remains an important source of wet weather pollution at different spatial scales.     

A laboratory assessment of the impact of brewery wastewater discharge on sulfide and methane production in a sewer

The impact of brewery wastewater discharge on sulfide and methane production in a sewer was assessed. Experiments were carried out on laboratory scale sewer reactors consisting of both an experimental and a control reactor. The control reactor was intermittently fed with real fresh sewage while the experimental reactor was fed with a mixture of brewery and domestic wastewater at two different proportions (10 and 25% v/v). 10% v/v discharge of brewery wastewater increased the H2S and CH4 production rates in the sewer reactor by 40% and 30%, respectively. When the brewery wastewater fraction was increased to 25% v/v, the H2S production rate of the experimental reactor decreased to the level of the control reactor. In contrast, the CH4 production rate maintained at a level that was 30% higher than that in the control reactor. These results indicate that the discharge of brewery wastewater into sewers can give negative impacts in relation to odour and corrosion management of the systems and will increase the greenhouse gas emissions from sewers. The study also reveals that the impact of trade waste on the biological reactions in sewers is complex, and requires careful experimental assessment in each case.     

An integrated approach for improving the wastewater discharge and treatment systems

Since treatment plants have been built all over Germany during the last decades, the water quality of receiving streams has been improved remarkably. But there are still a lot of quality problems left, which are caused e.g. by combined sewer overflows (CSO), treatment plant effluents or rainwater discharges from separate sewer systems. At present different efforts are undertaken to control sewer systems in order to improve the operation of urban drainage systems or more generally, design processes. The Emschergenossenschaft and Lippeverband (EG/LV) are carrying out research studies, which are focusing on a minimization of total emissions from sewer systems both from wastewater treatment plant (WWTP) effluents and from CSO. They consider dynamic interactions between rainfall, resultant wastewater, combined sewers, WWTP and receiving streams. Therefore, in an advanced wastewater treatment, a model-based improvement of WWTP operation becomes more and more essential, and consequently a highly qualified operational staff is needed. Some aspects of the current research studies are presented in this report. The need and the use of an integrated approach to combine existing model components in order to optimize dynamic management of combined sewer systems (CSS) with a benefit for nature are outlined.     

Upgrading wastewater treatment plants by the use of biofilm carriers,oxygen addition and pre-treatment in the sewer network

In upgrading of treatment plants for enhanced nitrification there are three strategies (1) the existing process may be optimised with respect to the biological activity, (2) the biological activity may be increased by changing to a process with a higher capacity, and (3) a combination of 1 and 2. The potential of the first strategy in a KMT moving bed biofilm process (the kaldnes process) is presented in this paper. The nitrification capacity was enhanced by using pure oxygen resulting in an increased oxygen concentration, and the specific biomass activity was increased by removing organic matter and sulphide in the sewer network by addition of nitrate to the wastewater. The results show that the nitrification capacity may be increased by 30–40% by pre-treatment of wastewater in the sewer network and increased by 40–50% by using an increased oxygen concentration in the first step of the nitrification reactor. Oxygen concentrations below 12 mg O₂/l and/or the amount of biomass were found to be the limiting factors at ammonium concentrations above 5–7 mg NH₄-N/l.     

Recent findings on sinks for sulfide in gravity sewer networks.

Sulfide buildup in sewer networks is associated with several problems, including health impacts, corrosion of sewer structures and odor nuisance. In recent years, significant advances in the knowledge of the major processes governing sulfide buildup in sewer networks have been made. This paper summarizes this newly obtained knowledge and emphasizes important implications of the findings. Model simulations of the in-sewer processes important for the sulfur cycle showed that sulfide oxidation in the wetted biofilm is typically the most important sink for dissolved sulfide in gravity sewers. However, sulfide emission and thereby potential hydrogen sulfide buildup in the sewer atmosphere is of particular importance in sewers constructed with large diameter pipes, in sewers constructed with steep slopes and in sewers conveying low pH wastewater. Precipitation of metal sulfides is only important when the sulfide concentration in the wastewater is low; i.e. less than 1 g Sm(-3).     

Influence of biofilm composition on the resistance to detachment.

Bacillus cereus and Pseudomonas fluorescens were used to develop monoculture biofilms in a bioreactor rotating system using a stainless steel cylinder for biofilm formation. The biofilms were allowed to grow for 7 days, exposed continuously to a Reynolds number of agitation (ReA) of 2,400. Afterwards, the biofilms were characterised in terms of respiratory activity, amount of biomass, cellular density, cellular size and total and extracellular proteins and polysaccharides. The biofilm mechanical stability was assessed by sequential submission of the biofilms to increasing ReA, respectively, 4,000, 8,100, 12,100 and 16,100. The results showed that P. fluorescens biofilms were five times more active, had a higher amount of biomass, cellular density, a reduced cellular size and a four-fold higher amount of extracellular proteins and polysaccharides than B. cereus biofilms. The application of shear stress forces higher than the one under which the biofilm was formed (ReA = 2,400) caused biomass removal. The high percentage of removal occurred with the implementation of a ReA of 8,100 for both B. cereus and P. fluorescens biofilms. The total series of ReA did not give rise to total biofilm removal, as only about 76% of P. fluorescens biofilm mass and 53% of B. cereus biofilm mass were detached from the cylinders. This latter result evidences that B. cereus had a higher mechanical stability than P. fluorescens biofilms. The overall results demonstrate that P. fluorescens and B. cereus formed physiologically distinct biofilms, B. cereus biofilms mainly being constituted by cells and P. fluorescens biofilms largely constituted by extracellular proteins and polysaccharides. B. cereus biofilms had a substantially higher mechanical stability than P. fluorescens biofilms.     

Biofilm early stage development in two nutrient‐rich streams with different urban impacts

The aim of this study was to describe the colonization of the biofilm during its early stages under different concentrations of nutrients and organic matter, specifically in urban streams influenced by agriculture and urbanization. We hypothesized that in a stream with higher concentrations of nutrients and organic matter, the initial biomass growth would be faster, and the changes in the structure of the community would be greater. Sterile glass substrates were placed in 2 urban streams that differed in nutrient and organic matter concentrations; samples were collected during their first week of colonization to measure total biomass, bacterial biomass, chlorophyll a, activity of the electron transfer system, and the community composition. Results show that biofilm development in both streams began within a few hours and differed under different conditions of nutrients; in the stream with a better water quality, the colonization dynamics consisted of 2 increments of bacterial biomass linked with an increase of algal biomass. In the urban stream with higher nutrient and organic matter concentrations, biofilm development was slower and consisted of a simultaneous increase of bacteria and algae, consistent with a lower electron transfer system activity. Therefore, the dynamics of the colonization process in addition to those characteristics of the fully developed biofilm could have potential applications in water monitoring of urban streams.     

Impact of water source management practices in residential areas on sewer networks - a review

Prolonged drought which has occurred everywhere around the world has caused water shortages, leading many countries to consider more sustainable practices, which are called source management practices (SMPs) to ensure water availability for the future. SMPs include the practices of water use reduction, potable water substitution and wastewater volume reduction such as water demand management, rainwater harvesting, greywater recycling and sewer mining. Besides the well known advantages from SMPs, they also contribute to the alteration of wastewater characteristics which finally affect the process in downstream infrastructure such as sewerage networks. Several studies have shown that the implementation of SMPs decreases the wastewater flow, whilst increasing its strength. High-strength wastewater can cause sewer problems such as sewer blockage, odour and corrosion. Yet, not all SMPs and their impact on existing sewer networks have been investigated. Therefore, this study reviews some examples of four common SMPs, the wastewater characteristics and the physical and biochemical transformation processes in sewers and the problems that might caused by them, and finally the potential impacts of those SMPs on wastewater characteristics and sewer networks are discussed. This paper provides sewer system managers with an overview of potential impacts on the sewer network due to the implementation of some SMPs. Potential research opportunities for the impact of SMPs on existing sewers are also identified.     

白洋淀养鸭废水对生物膜生物量和胞外酶活性的影响

为了对应用生物膜进行湖泊的生态监测与评价提供理论依据,通过微宇宙试验研究了不同浓度梯度养鸭废水对生物膜生物量和胞外酶活性的影响。结果表明:养鸭废水会显著增加生物膜的生物量;在短期暴露中,生物膜的生物量与养鸭废水浓度线性相关;而在长期暴露中则符合Logistic方程,养鸭废水对生物膜生物量的促进具有饱和效应。养鸭废水对生物膜胞外酶活性的效应受暴露时间的影响:短期暴露(24 h)时养鸭废水会促进碱性磷酸酶、β-葡萄糖苷酶和亮氨酸肽酶活性的增加,3种酶活性均随养鸭废水浓度的升高而呈对数增加;长期暴露(60 d)时,养鸭废水对3种酶活性均产生抑制作用,酶活性随养鸭废水浓度升高而呈对数降低。     

High-rate wastewater treatment combining a moving bed biofilm reactor and enhanced particle separation.

Many cities around the world are looking for compact wastewater treatment alternatives since space for treatment plants is becoming scarce. In this paper development of a new compact, high-rate treatment concept with results from experiments in lab-scale and pilot-scale are presented. The idea behind the treatment concept is that coagulation/floc separation may be used to separate suspended and colloidal matter (resulting in > 70% organic matter removal in normal wastewater) while a high-rate biofilm process (based on Moving Bed biofilm reactors) may be used for removing low molecular weight, easily biodegradable, soluble organic matter. By using flotation for floc/biomass separation, the total residence time for a plant according to this concept will normally be < 1 hour. A cationic polymer combined with iron is used as coagulant at low dosages (i.e. 1-2 mg polymer/l, 5-10 mg Fe/l) resulting in low sludge production (compared to conventional chemical treatment) and sufficient P-removal.     

Integrated modelling of sewer system and wastewater treatment plant for investigating the impacts of chemical dosing in sewers

Chemicals are often dosed to control the production and accumulation of hydrogen sulfide in sewers. The biological and/or chemical actions of these chemicals have profound impacts on the composition of wastewater entering a WWTP, thereby affecting its performance. In this paper, an integrated modelling methodology for simultaneously investigating the effects of dosing of chemicals in sewer network and N and P removal at the downstream WWTP is reported. The sewer system is modelled using a sewer model (SeweX), and the WWTP is modelled using ASM2d model with some modifications. The importance of integrated modelling in sewer management is also demonstrated.     

Evaluation of biofilm performance as a protective barrier against biocorrosion using an enzyme electrode

Sulfide is known to be an important factor in microbiologically influenced corrosion (MIC) of metals and concrete deterioration in wastewater treatment structures and sewer pipelines. A sulfide biosensor was used to determine the effectiveness of Escherichia coli DH5 alpha biofilm as a protective barrier against MIC. The biofilm was shown to be effective in protecting surfaces from sulfide and helping to reduce MIC using amperometric measurements. The results also indicated that the growth conditions of E. coli DH5 alpha may have an impact on the performance of the biofilm as a sulfide barrier. The simple method provided in this work enables the comparison of several microbial biofilms and selection of the ones with potential to prevent MIC in a relatively short time.     

Integrated modelling for the evaluation of infiltration effects.

The objective of the present study is the estimation of the potential benefits of sewer pipe rehabilitation for the performance of the drainage system and the wastewater treatment plant (WWTP) as well as for the receiving water quality. The relation of sewer system status and the infiltration rate is assessed based on statistical analysis of 470 km of CCTV (Closed Circuit Television) inspected sewers of the city of Dresden. The potential reduction of infiltration rates and the consequent performance improvements of the urban wastewater system are simulated as a function of rehabilitation activities in the network. The integrated model is applied to an artificial system with input from a real sewer network. In this paper, the general design of the integrated model and its data requirements are presented. For an exemplary study, the consequences of the simulations are discussed with respect to the prioritisation of rehabilitation activities in the network.     

Sterols: a tracer of organic matter in combined sewers.

The important organic pollution of combined wet weather flows (WWF), its acute impact on receiving waters have been widely demonstrated. The main three possibly origins for this organic pollution are: runoff water (streets and roofs), wastewater and erosion of sewer sediments in combined sewer system. This work, for tracing the origin of organic particles bound in combined sewer system, has been focused on the innovative use of sterols. So, eight sterols have been selected and analysed for each kind of sample. Results are represented in contents of sterols (microg g(-1)) and in sterol profiles (%).The comparison of contents and profiles leads the separation between two groups: runoff water, characterized by the total absence of coprostanol, epicoprostanol and coprostanone, and the group of sewer deposits (gross bed sediment (GBS), organic layer (OL), biofilms) and wastewater. Moreover, sewer deposits and wastewater can be distinguished by their sterol contents and profiles. To evaluate their contribution to WWF a comparison between sterol signatures is done which shows that these effluents have a strong similarity in profiles and in contents of sterols to the organic layer.