Enumeration of sulphate-reducing bacteria for assessing potential for hydrogen sulphide production in urban drainage systems.

Urban drainage structures have increasing demands which can lead to increasing hydrogen sulphide related problems forming in places where they have not previously been prevalent. This puts pressure on the methods currently used to monitor and diagnose these problems and more sophisticated methods may be needed for identifying the origin of the problems. Molecular microbiological techniques, such as quantitative polymerase chain reaction, offer a potential alternative for identifying and quantifying bacteria likely to be causing the production of hydrogen sulphide, information that, when combined with an appropriate sampling programme, can then be used to identify the potentially most effective remediation technique. The application of these methods in urban drainage systems is, however, not always simple, but good results can be achieved. In this study bacteria producing hydrogen sulphide were quantified in three small combined sewer overflow storage tanks. Bacterial counts were compared between wastewater, biofilms and sediments. Similar numbers were found in the wastewater and biofilms, with the numbers in the sediments being lower. If remediation methods for hydrogen sulphide are deemed necessary in the tanks, methods that target both the wastewater and the biofilms should therefore be considered.

Comparison between InfoWorks hydraulic results and a physical model of an urban drainage system.

Urban drainage systems are frequently analysed using hydraulic modelling software packages such as InfoWorks CS or MIKE-Urban. The use of such modelling tools allows the evaluation of sewer capacity and the likelihood and impact of pluvial flood events. Models can also be used to plan major investments such as increasing storage capacity or the implementation of sustainable urban drainage systems. In spite of their widespread use, when applied to flooding the results of hydraulic models are rarely compared with field or laboratory (i.e. physical modelling) data. This is largely due to the time and expense required to collect reliable empirical data sets. This paper describes a laboratory facility which will enable an urban flood model to be verified and generic approaches to be built. Results are presented from the first phase of testing, which compares the sub-surface hydraulic performance of a physical scale model of a sewer network in Yorkshire, UK, with downscaled results from a calibrated 1D InfoWorks hydraulic model of the site. A variety of real rainfall events measured in the catchment over a period of 15 months (April 2008-June 2009) have been both hydraulically modelled and reproduced in the physical model. In most cases a comparison of flow hydrographs generated in both hydraulic and physical models shows good agreement in terms of velocities which pass through the system.

Experimental and numerical investigation of interactions between above and below ground drainage systems.

This paper presents the results of the experimental and numerical investigation of interactions between surface flood flow in urban areas and the flow in below ground drainage systems (sewer pipes and manholes). An experimental rig has been set up at the Water Engineering Laboratory at the University of Sheffield. It consists of a full scale gully structure with inlet grating, which connects the 8 m(2) surface area with the pipe underneath that can function as an outfall and is also further connected to a tank so that it can come under surcharging conditions and cause outflow from the gully. A three-dimensional CFD (Computational Fluid Dynamics) model has been set up to investigate the hydraulic performance of this type of gully inlet during the interactions between surface flood flow and surcharged pipe flow. Preliminary results show that the numerical model can replicate various complex 3D flow features observed in laboratory conditions. This agreement is overall better in the case of water entering the gully than for the outflow conditions. The influence of the surface transverse slope on flow characteristics has been demonstrated. It is shown that re-circulation zones can form downstream from the gully. The number and size of these zones is influenced by the transverse terrain slope.

Investigating and modelling the development of septic sewage in filled sewers under static conditions: a lab-scale feasibility study.

This paper describes a lab-scale study of the physical and bio-chemical processes associated with the development of septic conditions in sewer pipes filled with static sewage. The study has concentrated on the uptake of oxygen (OUR) and the subsequent changes in chemical oxygen demand (COD), sulphate, sulphide and nitrate concentration and the formation of volatile fatty acids (VFA). OUR of raw sewage ranged from 2 to 13 mg L(-1) h(-1). Apparent nitrate uptake and sulphide generation rates in static sewage varied between 0.2-0.7 mgNO(3) L(-1) h(-1) and 0.02-0.05 mgH(2)S-S L(-1) h(-1), respectively. A logistic function was used to simulate the sulphide generation process in static sewage. It was found that total COD (COD(total)) influenced the apparent sulphide generation rate while nitrate concentrations greater than 4 mg L(-1) controlled the onset of sulphide production in experiments without added sediment phase. Introducing a sediment phase appeared to accelerate hydrolysis and fermentation processes as evidenced by 5-14 times greater dissolved COD generation rates in the bulk water phase.