No abstract

Public concern over odours from sewage treatment works is increasing. More people are being exposed to odours, due to development around existing works or the construction of new works. Raised awareness of both the environment and individual rights has meant people are now more likely to complain. Odourabatement and control is a major issue for sewage works operators. To control odours, they must first be measured. This is no easy task as response to odours is subjective and our understanding of the sense of smell is incomplete. In assessing an odour nuisance, odour formation, emission, dispersion and perception must be considered. There is no single measure that is suitable for this purpose. Odour measurements fall into two classes. Analytical measurements characterise odours in terms of their chemical composition and are more suited to formation, emission and dispersion models. Unfortunately, they tell us little about the perceived effect of the odour. Sensory measurements employ the human nose and characterise odours in terms of their perceived effect. A link between analytical and sensory measurements is clearly needed. In this paper data collected from odour surveys at 17 different wastewater treatment sites are evaluated to explore possibilities for linking analytical and sensory measures. The relationship between hydrogen sulphide concentration and odour concentration as measured by threshold olfactometry is explored. Correlations suggest power-law relationships between hydrogen sulphide and odour concentration, with r2 values as high as 0.69. This paper will assess the use of hydrogen sulphide (H2S) and olfactometry measurements for a range of unit operations and abatement technologies.

An integrated appraisal of five technology scenarios for the co-combustion of biosolids in the UK energy and waste management policy context is presented. Co-combustion scenarios with coal, municipal solid waste, wood, and for cement manufacture were subject to thermodynamic and materials flow modeling and evaluated by 19 stakeholder representatives. All scenarios provided a net energy gain (0.58-5.0 kWh/kg dry solids), having accounted for the energy required for transportation and sludge drying. Cocombustion within the power generation and industrial (e.g., cement) sectors is most readily implemented but provides poor water utility control, and it suffers from poor public perception. Co-combustion with wastes or biomass appears more sustainable but requires greater investment and presents significant risks to water utilities. Incongruities within current energy and waste management policy are discussed and conclusions for improved understanding are drawn.

In the field of odour assessment, much attention has been paid to the measurement of odour concentration. Whilst the concentration of an odour at a receptor is a useful indicator of annoyance, the concentration at the source tells only half the story. The emission rate - the product of odour concentration and air flow rate - is required to appreciate the significance of odour sources. Knowledge of emission rates allows odour sources to be ranked in terms of significance and facilitates appropriate selection and design of odour control units. The emission rate is also a key input for atmospheric dispersion models. Given the increasing importance of odour to sewage treatment works operators, there is a clear need for predictive methods for odour emission rates. Theory suggests that the emission of odorants from sewage to air is controlled by mass transfer resistances in both the gas and liquid phase. These are in turn controlled by odorant and emission source characteristics. The required odorant characteristics are largely known, and mass transfer from many different types of emission sources have been studied. Sewage treatment processes can be described by one or more of six characteristic emission sources, these being quiescent surfaces, channels, weirs and drop structures, diffused aeration, surface aeration and flow over media. This paper describes the development of odorant mass transfer models for these characteristic emission types. The models have been applied in the form of spreadsheet models to the prediction of H2S emissions and the results compared with commercial VOC emission models.

Sewage treatment works are subject to a range of parameters governing the quality of effluent and sludge produced. An additional product from treatment plants is odorous air. The causes, source, formation and measurement of odour are widely reported and reasonably understood. An important factor in the design and management of works is the prediction of such odours. The importance of this work is explained by the possibility of future legislation controlling odour at wastewater plants. Odour dispersion modelling involves the on-site measurement or prediction of the emission rate of an odorous compound, oftenhydrogen sulphide, and the subsequent prediction of the atmospheric concentrations of that compound downwind of the source. This paper used the USEPA models SCREEN3 and ISCST to determine hydrogen sulphide contour concentrations emitted from unit processes at two different sewage treatment works in the United Kingdom. Results indicated that the first site, locatedin an urban catchment, emitted hydrogen sulphide at varying rates. The predicted downwind concentrations using “urban” dispersion coefficients correlated well with measured concentrations. At the second site, emission rates were less variable. Results from the second site produced the best correlation using “rural” dispersion coefficients. Results from both sites suggest that the definition of the surrounding land use is critical in predicting odour dispersion. The problem of determining land use is highlighted and the importance of correct meteorology is stressed. Both sites were predicted to be capable of producing hydrogen sulphide concentrations at a detectable level outside the site boundary. Odour complaints were therefore anticipated. The operational performance of a unit treatment operation is proposed as a major influence on hydrogen sulphide emission. The idea of a large database of expected emission rates from individual unit treatment processes is proposed as an input for dispersion modelling and as an aid to future design.