David de Haas

Nitrous oxide generation in full-scale biological nutrient removal wastewater treatment plants.

International guidance for estimating emissions of the greenhouse gas, nitrous oxide (N(2)O), from biological nutrient removal (BNR) wastewater systems is presently inadequate. This study has adopted a rigorous mass balance approach to provide comprehensive N(2)O emission and formation results from seven full-scale BNR wastewater treatment plants (WWTP). N(2)O formation was shown to be always positive, yet highly variable across the seven plants. The calculated range of N(2)O generation was 0.006-0.253 kgN(2)O-Nper kgN denitrified (average: 0.035+/-0.027). This paper investigated the possible mechanisms of N(2)O formation, rather than the locality of emissions. Higher N(2)O generation was shown to generally correspond with higher nitrite concentrations, but with many competing and parallel nitrogen transformation reactions occurring, it was very difficult to clearly identify the predominant mechanism of N(2)O production. The WWTPs designed and operated for low effluent TN (i.e. <10 mgN L(-1)) had lower and less variable N(2)O generation factors than plants that only achieved partial denitrification.

Comprehensive life cycle inventories of alternative wastewater treatment systems.

Over recent decades, the environmental regulations on wastewater treatment plants (WWTP) have trended towards increasingly stringent nutrient removal requirements for the protection of local waterways. However, such regulations typically ignore other environmental impacts that might accompany apparent improvements to the WWTP. This paper quantitatively defines the life cycle inventory of resources consumed and emissions produced in ten different wastewater treatment scenarios (covering six process configurations and nine treatment standards). The inventory results indicate that infrastructure resources, operational energy, direct greenhouse gas (GHG) emissions and chemical consumption generally increase with increasing nitrogen removal, especially at discharge standards of total nitrogen <5 mgN L(-1). Similarly, infrastructure resources and chemical consumption increase sharply with increasing phosphorus removal, but operational energy and direct GHG emissions are largely unaffected. These trends represent a trade-off of negative environmental impacts against improved local receiving water quality. However, increased phosphorus removal in WWTPs also represents an opportunity for increased resource recovery and reuse via biosolids applied to agricultural land. This study highlights that where biosolids displace synthetic fertilisers, a negative environmental trade-off may also occur by increasing the heavy metals discharged to soil. Proper analysis of these positive and negative environmental trade-offs requires further life cycle impact assessment and an inherently subjective weighting of competing environmental costs and benefits.

Dynamics and dynamic modelling of H2S production in sewer systems

Accurate and reliable predictions of sulfide production in a sewer system greatly benefit formulation of appropriate strategies for optimal sewer management. Sewer systems, rising main systems in particular, are highly dynamic in terms of both flow and wastewater composition. In order to get an insight in sulfide production as a response to the dynamic changes in sewer conditions, several measurement campaigns were conducted in two rising mains in Gold Coast, Australia. The levels of various sulfur species and volatile fatty acids (VFAs) were monitored through hourly sampling for periods ranging from 8 to 29 h. The results of these field studies showed large temporal as well as spatial variations in sulfide generation. A dynamic sewer model taking into account the hydraulics and the biochemical transformation processes was formulated and calibrated and validated using the data collected during the four measurement campaigns at the two sites. The model was demonstrated to reasonably well describe the temporal and spatial variations in sulfide, sulfate and VFA concentrations. Application of the model was illustrated with a case study aimed to optimize oxygen injection to one of the two mains studied, which is being used as a means to control sulfide production on this site. The model predicted that, moving the current oxygen injection point to a location close to the end of the sewer line could achieve the same degree of sulfide control with only 50% of the current oxygen use. This study highlighted that the location at which oxygen is injected plays a major role in sulfide control and a dynamic model could be used to make a proper choice of the location.

Long-term trends and opportunities for managing regional water supply and wastewater greenhouse gas emissions

Greenhouse gas emissions are likely to rise faster than growth in population and more than double for water supply and wastewater services over the next 50 years in South East Queensland (SEQ), Australia. New sources of water supply such as rainwater tanks, recycled water, and desalination currently have greater energy intensity than traditional sources. In addition, direct greenhouse gas emissions from reservoirs and wastewater treatment and handling have potentially the same magnitude as emissions from the use of energy. Centralized and decentralized water supply and wastewater systems are considered for a scenario based upon a government water supply strategy for the next 50 years. Many sources of data have large uncertainties which are estimated following the IPCC Good Practice Guidelines. Important sources of emissions with large uncertainties such as rainwater tanks and direct emissions were identified for further research and potential mitigation of greenhouse gas emissions.

The diverse environmental burden of city-scale urban water systems

Recent years have seen an increase in the use of Life Cycle Assessment (LCA) to inform urban water systems research. The attraction of LCA is its capacity to identify trade-offs across a broad range of environmental issues and a broad range of technologies. However, without some additional perspective on the scale of the results, prioritisation of these concerns will remain difficult. LCA studies at the whole-of-system level are required to identify the diversity of life cycle environmental burdens associated with urban water systems, and the main contributors to these impacts. In this study, environmental impact profiles were generated for two city-scale urban water systems: one typical of many urban centres, with a high reliance on freshwater extraction and the majority of treated wastewater being discharged to the sea; and one that adopts a more diverse range of water supply and wastewater recycling technologies. The profiles were based on measured data for most system components, otherwise best available empirical data from the literature. Impact models were chosen considering the substantial methodological developments that have occurred in recent years. System operations, directly within the sphere of influence of water system managers, play the dominant role in all but one of the 14 life cycle impact categories considered. While energy use is the main cause of changes in the impact profiles when the alternative water supply technologies are included, it is not the only important driver of impacts associated with city-scale urban water systems. Also extremely important are process emissions related to wastewater treatment and dams (notably fugitive gases, wastewater discharges, and biosolids disposal). The results clearly indicate a diverse range of environmental impacts of relevance, extending beyond the traditional concerns of water use and nutrient discharge. Neither energy use, nor greenhouse gas footprints, are likely to be an adequate proxy for representing these additional concerns. However, methodological improvements will be required for certain LCA impact models to support future case study analysis, as will a comprehensive critique of the implications from selecting different impact models.