Energy and carbon footprints of sewage treatment methods.

The paper presents energy and carbon footprints of sewage treatment plants (STPs) operating at different scales and using different technology options based on primary data from 50 STPs operating in India and the UK. The study used a combination of fundamental mass-balance approach for energy consumption and the methodology defined by IPCC for the carbon emissions. Small-scale institutional STPs consume twelve times the energy consumed by large-scale municipal STPs, the corresponding energy intensities being 4.87 kWh/m(3) and 0.40 kWh/m(3) respectively. Embodied energy from construction material and chemicals accounted for 46% and 33% of the total energy intensity of the municipal and institutional STPs respectively. The average carbon footprint of large-scale STPs is 0.78 kgCO2eq/m(3) and for small-scale STPs it is 3.04 kgCO2eq/m(3). However, fugitive emissions from large-scale STPs constituted 74% of the total carbon emissions whereas the figure was only 0.05% for small-scale STPs. Average electrical energy intensity in STPs in India is much lower (0.14 kWh/m(3)) than that in the UK (0.46 kWh/m(3)). This is due to the reason that STPs in India do not have resource recovery processes and use solar heat for sludge drying. The paper offers information and insights for designing low carbon strategies for urban waste infrastructure.

Determination of changes in wastewater quality through a treatment works using fluorescence spectroscopy.

Fluorescence spectroscopy was used to characterize municipal wastewater at various stages of treatment in order to understand how its fluorescence signature changes with treatment and how the signal relates to biochemical oxygen demand (BOD) and chemical oxygen demand (COD). The impact of size fractionation on the fluorescence signal was also investigated. Fluorescence measurements were taken for unfiltered and filtered (0.45 and 0.20 microm) samples of crude, settled and secondary treated wastewater (activated sludge and trickling filter), and final effluent. Good correlations were observed for unfiltered, diluted wastewater samples between BOD and fluorescence intensity at excitation 280 nm, emission 350 nm (Peak T1) (r = 0.92) and between COD and Peak T1 intensity (r = 0.85). The majority of the T1 and T2 signal was found to be derived from the <0.20 microm fraction. Initial results indicate that fluorescence spectroscopy, and changes in Peak T1 intensity in particular, could be used for continuous, real-time wastewater quality assessment and process control of wastewater treatment works.

A novel laboratory method to determine the biogas potential of iron-dosed activated sludge.

Sludge biogas potential is often reduced by iron-dosing, the extent of the reduction being related to the nature of the sludge and the dosing process. The aim of this research was to develop a rapid laboratory method to measure the impact of iron-dosing on the biogas potential of activated sludge, taking into account the mechanisms that may be decreasing biogas yield. To validate the method, sequential extraction (SE) was used to fractionate iron and phosphorus in the sludge before and after iron-dosing. The laboratory-dosing regime increased total iron and phosphorus in the sludge but decreased their bioavailability, producing sludge with a similar inorganic composition to full-scale chemical P removal (CPR) sludge. Laboratory-dosed sludge produced 12-20% less biogas and 9-21% less methane when anaerobically digested, in comparison to the same undosed sludge. This method should help water companies and academics to more closely simulate iron-dosing in the laboratory.

The digestibility of iron-dosed activated sludge.

The impact of chemical phosphorus (P) removal on anaerobic digestion (AD) has long been debated, possibly because there is no general consensus of the definition of impaired AD, but also because of the different assessment methods used. This research surveyed 12 wastewater treatment plants to compare the relative digestibility of iron-dosed with undosed activated sludge during two batch test trials. Results showed that iron-dosed sludge negatively impacted AD by reducing the volume of biogas (12%) and methane (5.5%) produced from the same amount of volatile solids fed. Possible reasons for reduced biogas production include lower levels of bioavailable P and iron in iron-dosed sludge, which may hinder the ability of micro-organisms to metabolise organic substrate.

Can fluorescence spectrometry be used as a surrogate for the Biochemical Oxygen Demand (BOD) test in water quality assessment? An example from South West England.

The fluorescence intensities of tryptophan-like, tyrosine-like and humic-like materials were determined using excitation-emission-matrices (EEMs) for a wide range of samples including natural surface waters, sewage and industrial effluents and waters that have experienced known pollution events from the South West of England (n=469). Fluorescence intensities reported in arbitrary fluorescence units (AFU) were correlated with standard five day Biochemical Oxygen Demand (BOD(5)) values which were used as an indicator of the amount of biodegradable organic material present. Tryptophan-like fluorescence, which has been found to relate to the activity of the biological community, showed the strongest correlation with BOD(5). Fluorescence analysis of the tryptophan-like peak (excitation/emission wavelength region 275/340 nm) is found to provide an accurate indication of the presence, and relative proportions of bioavailable organic material present (natural or anthropogenic). It therefore provides an insight relating to its oxygen depleting potential. Thus fluorescence spectroscopy is recommended as a portable or laboratory tool for the determination of the presence of biodegradable organic matter with intrinsic oxidising potential in natural waters. The novel application of Geographically Weighted Regression (GWR) to the data illustrates that strong local relationships exist between the two parameters and that site specific character may be a strong factor in the strength of the tryptophan-like fluorescence/BOD(5) relationship.