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Mean cell residence time (MCRT) is a major operational parameter in all biological treatment systems because of its relationship to growth rate and thus to metabolic activity. Due to their mode of operation, submerged anaerobic membrane bioreactors (SAnMBR) offer a homogenous system in which MCRT can be simply controlled through volumetric wastage. Although a number of studies using SAnMBR have been reported, however, little information is available regarding the effect of MCRT on operational performance, mixed liquor characteristics and the influence of these on membrane performance. In this research an innovative SAnMBR using gravity-induced transmembrane pressure to maintain flux was developed and tested for first time. This configuration was then used to evaluate the impact of MCRT on membrane flux, mixed liquor characteristics and overall performance of SAnMBR treating low-to-intermediate strength wastewater. Long-term experimental periods of more than 240 days allowed steady-state conditions under different MCRTs, in which the mixed liquor suspended solids (MLSS) adjusted to the applied load, making possible to assess the influence of this growth and metabolism-dependent kinetic parameter. The SAnMBRs were monitored for membrane flux, overall process efficiency and mixed liquor characteristics when operating at 36 oC and 20 oC. The results of this work showed that at both operational temperatures, the MCRT has a significant effect on the mixed liquor characteristics, particularly on the filterability which was higher at short MCRTs. This resulted in improved membrane flux at relatively short MCRT, although no advantages were observed if the MCRT was further reduced. Higher specific methane production was observed at longer MCRT, most probably due to a higher fraction of carbon incorporated into biomass as a result of higher microbial growth rates. Overall, the results of this research showed that the MCRT has a considerable effect on the mixed liquor characteristics and thus on the membrane fouling and overall reactor performance. It is clear that there is a trade-off to be made between enhanced membrane performance, specific methane production and sludge yield when considering the most suitable operational MCRT. Further studies are required to identify the optimum MCRT for a wider range of wastewater and other operational parameters and to fully understand the causes of these effects.

The aim of this research was to produce an integrated modelling platform in which an anaerobic digester could be linked to the other unit operations which serve it, both in maintaining the physical-chemical conditions in the digester and in transforming the digestion products to useful fuel and nutrient sources. Within these system boundaries an accurate mass and energy balance could be determined and further optimised, particularly where the desired energy products are a mix of heat, power, and biomethane. The anaerobic digestion of food waste was choosen as the subject of the research because of its growing popularity and the availability of validation data. Like many other organic substrates, food waste is potentially a good source of renewable energy in the form of biogas through anaerobic digestion. A number of experimental studies have, however, reported difficulties in the digestion of this material which may limit the applicability of the process. These arise from the complexity of the biochemical processes and the interaction between the microbial groups that make up the anaerobic community. When using food waste there is a tendency to accumulate intermediate volatile fatty acid products, and in particular propionic acid, which eventually causes the pH to drop and the digester to fail. Two factors are important in understanding and explaining the changes in the biochemical process that leads to this condition. The first is due to the differential in sensitivity to free ammonia of the two biochemical pathways that lead to methane formation. The acetoclastic methanogenic route is inhibited at a lower concentration than the hydrogenotrophic route, and methane formation therefore occurs almost exclusively via acetate oxidation to CO2 and H2 at high free ammonia concentrations. The accumulation of propionic acid is thought to be because formate, a product of its degradation, cannot be converted to CO2 and H2 as the necessary trace elements to build a formate dehydrogenase enzyme complex are missing. The Anaerobic Digestion Model No. 1 (ADM1) was modified to reflect ammonia inhibition of acertoclastic methanogenesis and an acetate oxidation pathway was added. A further modification was included which allowed a 'metabolic switch' to operate in the model based on the availability of key trace elements. This operated through the H2 feedback inhibition route rather than creating a new set of equations to consider formate oxidation in its own right: the end result is, however, identical in modelling terms. With these two modifications ADM1 could simulate experimental observations from food waste digesters where the total ammoniacal nitrogen(TAN) concentration exceeded 4 gN l-1. Under these conditions acetate accumulation is first seen, followed by proprionate accumulation, but with the subsequent decrease in acetate until a critical pH is reached. The ADM1 model was implemented in MATLAB with these modifications incorporated. The second part of the research developed an energy model which linked ADM1 to the mechanical processes for biogas upgrading, Combined Heat and Power (CHP)production, and the digester mixing system. The energy model components were developed in the framework of the Aspen Plus modelling platform, with sub-units for processes not available in the standard Aspen Package being developed in Fortran, MS Excel or using the Aspen Simulation Workbook (ASW). This integration of the process components allows accurate sizing of the CHP and direct heating units required for an anaerobic digestion plant designed for fuel grade methane production. Based on the established model and its sub-modules, a number of case studies were developed. To this end the modified ADM1 was applied to mesophilic digestion of Sugar Beet Pulp to observe how the modified ADM1 responded to different substrate types. Secondly, to assess the capability of adding further mechanical processes the model was used to integrate and optimise single stage biogas upgrading. Finally, the digestion of food waste in the municipal solid waste stream of urban areas in Vietnam was considered.

Anaerobic digestion is a simple and energetically efficient way in comparison to some other biofuel methods of producing renewable energy from a range of biomass types. Although digestion of micro-algal biomass was first suggested in the 1950s, only a few studies have been conducted for assessment of its performance. This work assessed the potential for energy recovery from microalgae via anaerobic digestion for both freshwater and marine species. This research screened seven laboratory-grown marine and freshwater microalgal species (Nannochloropsis. oculata, Thalassiosira . pseudonana, Dunaliella. salina, Rhododomas sp, Isochrysis. galbana, Chlorella. vulgaris and Scenedesmus sp) and two samples from large-scale cultivation systems for their suitability as a substrate for anaerobic digestion. Biochemical methane production and a theoretical maximum growth yield of each species were employed to offer a means of comparing methane productivity per unit of cultivation under standard conditions. The data generated were useful in determining suitable species to culture and digest under continuous operation. A review of the literature highlighted a gap in the knowledge for the continuous digestion of different marine micro-algal species, as well as the potential inhibitory effect of high salinities on the anaerobic digestion process to non-acclimatised systems run under continuous operation. Addition of total salt ≥ 10g L-1 caused reactor failure, supporting the findings of the literature review. It was possible, however, to gradually adapt the inoculum to marine concentrations of chloride salts (31.1 g L-1) with <7% difference in specific methane production of controls. Addition of sulphate showed competition between methanogens and sulphate-reducing bacteria with further minor losses in methane yield. There was up to 60% reduction in SMP for the highest sulphate loaded reactors, however, the population successfully adapted to sulphate concentrations above those typically found in seawater and showed gaseous H2S productivity in proportion to the applied sulphate load. This suggests that the effects of marine concentrations of chloride and sulphate salts can be overcome by a gradual acclimatisation. The selected algal species I. galbana and D. salina were continuously cultivated in a photobioreactor under low and high sulphate media and continuously digested using the salt adapted inoculum. The specific methane production for I. galbana and D. salina was 0.19 and 0.23 L CH4 g-1 VS, with a VS destruction of 32% and 50% respectively. Addition of a high SO4 grown D. salina as a feed resulted in a reduction of SMP to 0.19 L CH4 g-1 VS with an increase in H2S production. Loses in total solids and sulphur were observed under continuous study due to oxidation of H2S and struvite precipitation within the reactors, which was not observed under batch analysis. This highlights the importance in conducting continuous studies over batch, as these effects can be overlooked.

This thesis evaluates the technical potential of using microalgae as a substrate for anaerobic digestion. Investigating the control and operation of different reactors, under different operating conditions (OLR, SRT, HRT) to determine potential of microalgae as a feedstock and determine whether improvements in performance can be achieved. Thermophillic digestion offers higher methane yields compared to mesophilic digestion in simple reactor systems at 25 day SRT, being able to cope with higher organic loading rates. Low C:N ratio in microalgae has the potential to result in high levels of ammoniacal nitrogen within anaerobic systems with levels as high at 754 mgTAN/L observed at maximum loading rates. No apparent inhibition was observed in any reactor, with free ammoniacal nitrogen levels of 100 mg/L achieved without any drop in methane yield. While a UAnMBR system offered improved yields compared to CSTR systems, its performance was still relatively poor compared to theoretical maximum yields. The UAnMBR system did however cope with high hydraulic throughput (low HRT) without a significant drop in methane yield demonstrating that this system is potentially suitable for simultaneous harvesting and digestion. The microalgal biomass was inherently resistant to degradation, and over the duration of a lengthened growth cycle, can change its intracellular and cell membrane structures, changing its susceptibility to enzymatic attack and subsequent methane yield. Nutrient depletion in batch microalgae culture results in intracellular lipid and carbohydrate accumulation, which potentially could have resulted in a higher methane yield of 0.283 LCH4/gVSin (equivalent to 0.184 - 0.201 iii LCH4/gCODin) when compared to microalgae harvested during nutrient replete conditions. Allowing cultures to mature for longer periods in the stationary phase of growth under nutrient depleted conditions resulted in a significant reduction in methane yield to 0.174 LCH4/gVSin (0.124LCH4/gCODin). The selection of microalgal species appears to significantly affect the methane potential and degradation rates, with methane yield as high as 0.313 LCH4/gVSin (0.222 LCH4/gCODin) and as low as 0.130L CH4/gVSin (0.092 LCH4/gCODin) found in different pure cultures. The difference in yield was considered to stem from a wide variability in intracellular and cell wall structures. Poor correlation existed between gross biochemical content (protein, lipid, carbohydrate) and the methane yield, and confirms that variability in methane yield is not solely dependent on the biochemical composition (e.g. lipid content).

Phosphorus (P) is an important constituent for living organisms as an energy carrier and a component of important biomolecules, which cannot be substituted with another element. Also, it is considered as the main element inducing eutrophication in freshwater bodies. In order to control eutrophication, wastewater treatment works (WWTW) remove P from their final effluent mostly through chemical precipitation and biological bacterial uptake. However, in order to achieve nutrient recovery from wastewaters, it has been suggested alternative biological processes based on biological algal uptake. Microalgae can be used to recover P from wastewaters as they can assimilate high amounts of P for their growth and store any excess as polyphosphate (i.e., luxury P uptake). This study is aimed at examining the potential use of microalgae cultivation for P recovery from wastewater and to identify the implications for its implementation at large WWTW. Chlamydomonas reinhardtii 11/32C was chosen as a model organism for this study. With regard to the role of the Nitrogen (N) source (ammonium v. nitrate), results show that there is little impact on P uptake when either ammonium or nitrate is used as the N source; however, by using a mix of ammonium and nitrate, P uptake increases as a result of stress conditions as ammonium is consumed faster than nitrate. Controlling environmental factors for microalgae growth showed that luxurious intracellular P uptake can range between 0.3 and 3.6% dry weight; optimum conditions for algae growth and P uptake where controlled by N concentration (200 mg N L-1, 50:50 NH4+:NO3-), phosphate concentration (100 mg P L-1 ), light intensity (250 μEm-2s-1) and photoperiod (16 hr light: 8 hr dark), resulting in a maximum algal biomass production of 148 mg VSS L-1d-1 and intracellular P uptake of 2.8 mg P L-1d-1. This study proved that C.reinhardtii 11/32C able to store the excess of P as polyphosphate granules located in cell’s vacuole. A continuous flow mixotrophic microalgae cultivation system was operated to investigate its performance as a novel biological nutrient control and recovery process. Under tested conditions, the mixotrophic system was able to remove both nitrogen and phosphorus at 97% and 50% respectively, with average P uptake of 2.03 mg P L-1 d-1 and algal biomass production of 248 mg VSS L-1 d-1. This system in operation generated on average 2.6 g L-1 of harvested algal biomass per day with a P content of 2.1% dry weight. These results show that a mixotrophic microalgae system has the potential to be implemented as a tertiary wastewater treatment process for biological nutrient removal and hence, a desktop case of study was conducted using data from a full-scale WWTW to assess the feasibility to implement this approach for nutrient recovery.

Although wastewater reuse presents numerous benefits, wastewater-borne pathogens, especially human enteric viruses, may pose risks to human health. Wastewater treatment processes have been shown to remove bacterial pathogens more effectively than they do viral pathogens, and in aquatic environments, levels of traditional faecal indicator bacteria (FIB) do not appear to correlate consistently with levels of human viral pathogens. There is, therefore, a need for novel viral indicators of faecal pollution and novel surrogates of viral pathogens. Potential candidates for this role include enteric bacteriophages (phages), viruses capable of infecting enteric bacteria.

The operation of a health-care facility, such as a cholera or Ebola treatment centre in an emergency setting, results in the production of pathogen-laden wastewaters that may potentially lead to onward transmission of the disease. The research presented here outlines the results of field and laboratory studies devised to inform the design and operation of a novel full-scale treatment protocol to disinfect pathogen-laden hospital wastewaters in situ, thereby eliminating the need for potentially hazardous road haulage and disposal of human excreta or wastewater to poorly-managed waste facilities. The approach investigated has the potential to provide an effective barrier to disease transmission by means of a novel but simple sanitary intervention. During Phase I of this research, a fieldwork study in Haiti focused on the design and operation, at short notice and within a disaster setting, of a new treatment technology that aimed to obviate the transport of untreated human excreta from emergency cholera treatment centres (CTC) to poorly-managed waste facilities. The results of this fieldwork period were validated and further optimised during Phase II: a detailed laboratory-based study in the UK that assessed the performance of the novel treatment technology in order to improve its efficacy. The performance of two physico-chemical protocols was monitored, first in the field (Port-au-Prince, Haiti), by means of both bench-scale and full-scale batch treatment of real highly-contaminated faecal waste from a cholera treatment centre (Phase I), and subsequently during more detailed laboratory studies (Phase II) using a ‘faecal-waste matrix’ that was created by mixing various municipal wastewaters and sludges in a proportion that aimed to mimic the composition of wastewaters produced at health-care facilities in emergency settings. The two investigated protocols achieved coagulation/flocculation and disinfection by exposure to high– or low–pH environments, using thermotolerant coliforms, intestinal enterococci, and somatic coliphages as indices of disinfection efficacy, and several physico-chemical parameters as indicators of treatment performance. In the high–pH treatment protocol, the addition of hydrated lime resulted in wastewater disinfection and coagulation/flocculation of suspended solids. In the low-pH treatment, disinfection (and partial colloidal destabilization followed by sedimentation) was achieved by the addition of hydrochloric acid, followed by pH neutralisation. A potential further step in this 4 second protocol was the coagulation/flocculation of suspended solids using aluminium sulphate. During Phase II, removal rates achieved for the high pH treatment protocol, in terms of physico-chemical parameters, were: COD > 80%; suspended solids > 85%; turbidity > 85%. Removal rates in terms of microbiological parameters were: thermotolerant coliforms > 5 Log10, intestinal enterococci >2 Log10 and somatic coliphage > 2 Log10. Removal rates achieved for the low-pH treatment protocol in terms of physico-chemical and microbiological parameters were: COD > 80%; thermotolerant coliforms between 0.2 and 1.2 Log10, with a mean removal of 0.75 Log10 and > 3 Log10 removal for intestinal enterococci. The removal of somatic coliphage was in excess of 4 Log10. The quantity and density of the sedimented sludge and several other physicochemical parameters (such as total nitrogen, total phosphorous, ammonia and ammonium, etc.) for the analysis of the supernatant were also monitored. This study represented the first known successful attempt to disinfect wastewater in a disease outbreak setting without resorting to the alternative, untested, approach of ‘super-chlorination’ which, it has been suggested, may not consistently achieve adequate disinfection. In addition, a basic costs analysis demonstrated significant savings in the use of reagent compared with super-chlorination. The approach to sanitation for cholera treatment centres and other disease outbreak settings presented here offers a timely response to a UN call for in situ disinfection of wastewaters generated in such emergencies. Further applications of the method to other emergency settings have been actively explored in discussion with the World Health Organization (WHO) in response to the ongoing Ebola outbreak in West Africa, and with the UK-based non-governmental organization (NGO) Oxfam.

In the developed world, 75% of the population live in urban areas, a figure projected to rise to nearly 83% by 2030, while in the developing world, the rate of urbanisation is even faster. One of the most important environmental problems associated with urbanisation is the amount of waste that is generated at a rate that outstrips the ability of the natural environment to assimilate it and authorities to manage it. Therefore, if we are to deliver a more sustainable economy, we must do more with less by making better use of resources. The recovery of energy from waste or EfW is an important component of an integrated waste management strategy, as it reduces our reliance on landfill. It is also a low carbon, low cost fuel, which by displacing fossil fuels can help the UK Government in meeting its energy policy and emission targets. Furthermore, EfW can contribute to energy security through diversification of supply it is projected that EfW may supply up 17% of the total UK electricity consumption by 2020. The main objectives of this work are to investigate the appropriate scales and technologies for the production of energy from waste in the urban environment. The suitability and effectiveness of fluidized bed combustion and gasification processes have been studied, together with gas clean-up systems. The most appropriate scales for each of these approaches in relation to system efficiencies and costs were evaluated, so that a sound judgement can be made as to which processes should be used in the urban context. Within this framework, a comprehensive assessment of fluidized bed reactor types and operational process conditions has been presented. Current and future status of these technologies was discussed, as well as the non-technical barriers hampering their development. The assessment concluded with a review of the different emissions and residues generated from the thermal treatment processes, their management, practices and costs. Mass and energy balances of traditional moving-grate combustion plants and key issues regarding the treatment of the output gas stream have been investigated during a five- month placement programme at Germana & Partners Consulting Engineers in Rome (Italy). The aim of the study was to gain an in-depth understanding of design methodologies and engineering principles applied in the detailed design of real industrial energy recovery plants. The study led to the development of a consistent approach for the technical and economic evaluation of more advanced technologies, namely fluidized bed combustion and gasification systems. Two different scale scenarios of 50,000 tpa and 100,000 tpa plant capacities were considered for the generation of electric power using a steam turbine for the combustion process and gas engine & combined cycle gas turbine (CCGT) for the gasification process. Mass and energy balances of the processes were performed and the cost effectiveness of the different waste treatment options was assessed using a discounted cash flow (DCF) analysis, which includes current market-based mechanisms, such as eligibility for Renewables Obligation Certificates (ROCs). A sensitivity analysis was carried out to evaluate the effects of changing system variables on the economic performances of the different waste treatment options. Seventeen system variables have been chosen and the effects of a 10% change in these variables on the levelised costs and gate fees were examined. These variables include waste calorific value, gasifier efficiency, prime mover electrical generation efficiency, as well as electricity and ROC prices and biodegradable fraction of the waste. As part of this study, the techno-economic performances of traditional moving-grate combustions systems was reported and compared against the different fluidized bed systems co-located with Mechanical Biological Treatment (MBT) facilities. The work was subsequently extended to analyse the technical and cost effectiveness of the simultaneous generation of heat and power from EfW fluidized bed combustion and gasification systems, using the same scale scenarios of 50,000 tpa and 100,000 tpa. The study focused on the additional capital and operating costs involved in incorporating combined heat and power (CHP) into EfW facilities. The projected revenues from heat sales and eligibility for ROCs were also evaluated for a range of market penetration levels. Furthermore, the environmental benefits associated with EfW with CHP facilities were assessed and the CO2 savings achieved from displacing fossil fuels in the separate generation of heat and power were also determined.