Toward gas-phase controlled mass transfer in micro-porous membrane contactors for recovery and concentration of dissolved methane in the gas phase

A micro-porous hollow fibre membrane contactor (HFMC) operated in sweep-gas mode has been studied to enable the recovery of dissolved methane from water in concentrated form. At high sweep-gas flow rates, up to 97% dissolved methane removal efficiency is achievable which is sufficient to achieve carbon neutrality (around 88%). An increase in methane composition of the recovered sweep-gas was achievable through two primary mechanisms: (i) an increase in liquid velocity which improved dissolved methane mass transfer into the gas phase; and (ii) a reduction in gas flow which lowered dilution from the receiving gas phase. It was posited that further refinement of the methane content was provided through counter-diffusion of the nitrogen sweep-gas into the liquid phase. Within the boundary conditions studied, the methane composition of the recovered gas phase exceeded the threshold for use in micro-turbines for electricity production. However, reducing the gas-to-liquid ratio to enhance gas phase methane purity introduced gas-phase controlled mass transfer which constrained removal efficiency. Whilst this reduction in removal efficiency can be compensated for by extending path length (i.e. more than one module in series), it is suggested that the gas-phase controlled conditions encountered were also a product of poor shell-side dispersion rather than an approach toward the limiting theoretical gas-to-liquid ratio. This implies that further optimisation can be ascertained through improved membrane contactor design. Importantly, this study demonstrates that micro-porous hollow fibre membrane contactors provide a compact process for recovery of dissolved methane in sufficient concentration for re-use

Biogas upgrading by chemical absorption using ammonia rich absorbents derived from wastewater

The use of ammonia (NH3) rich wastewaters as an ecological chemical absorption solvent for the selective extraction of carbon dioxide (CO2) during biogas upgrading to ‘biomethane’ has been studied. Aqueous ammonia absorbents of up to 10,000 gNH3 m−3 demonstrated CO2 absorption rates higher than recorded in the literature for packed columns using 20,000–80,000 g NH3 m−3 which can be ascribed to the process intensification provided by the hollow fibre membrane contactor used in this study to support absorption. Centrifuge return liquors (2325 g m−3 ionised ammonium, NH4+) and a regenerant (477 gNH4+ m−3) produced from a cationic ion exchanger used to harvest NH4+ from crude wastewater were also tested. Carbon dioxide fluxes measured for both wastewaters compared reasonably with analogue ammonia absorption solvents of equivalent NH3 concentration. Importantly, this demonstrates that ammonia rich wastewaters can facilitate chemically enhanced CO2 separation which eliminates the need for costly exogenic chemicals or complex chemical handling which are critical barriers to implementation of chemical absorption. When testing NH3 analogues, the potential to recover the reaction product ammonium bicarbonate (NH4HCO3) in crystalline form was also illustrated. This is significant as it suggests a new pathway for ammonia separation which avoids biological nitrification and produces ammonia stabilised into a commercially viable fertiliser (NH4HCO3). However, in real ammonia rich wastewaters, sodium bicarbonate and calcium carbonate were preferentially formed over NH4HCO3 although it is proposed that NH4HCO3 can be preferentially formed by manipulating both ion exchange and absorbent chemistry

Quantifying the loss of methane through secondary gas mass transport (or 'slip') from a micro-porous membrane contactor applied to biogas upgrading

Secondary gas transport during the separation of a binary gas with a micro-porous hollow fibre membrane contactor (HMFC) has been studied for biogas upgrading. In this application, the loss or ‘slip' of the secondary gas (methane) during separation is a known concern, specifically since methane possesses the intrinsic calorific value. Deionised (DI) water was initially used as the physical solvent. Under these conditions, carbon dioxide (CO2) and methane (CH4) absorption were dependent upon liquid velocity (VL). Whilst the highest CO2 flux was recorded at high VL, selectivity towards CO2 declined due to low residence times and a diminished gas-side partial pressure, and resulted in slip of approximately 5.2% of the inlet methane. Sodium hydroxide was subsequently used as a comparative chemical absorption solvent. Under these conditions, CO2 mass transfer increased by increasing gas velocity (VG) which is attributed to the excess of reactive hydroxide ions present in the solvent, and the fast conversion of dissolved CO2 to carbonate species reinitiating the concentration gradient at the gas-liquid interface. At high gas velocities, CH4 slip was reduced to 0.1% under chemical conditions. Methane slip is therefore dependent upon whether the process is gas phase or liquid phase controlled, since methane mass transport can be adequately described by Henry's law within both physical and chemical solvents. The addition of an electrolyte was found to further retard CH4 absorption via the salting out effect. However, their applicability to physical solvents is limited since electrolytic concentration similarly impinges upon the solvents' capacity for CO2. This study illustrates the significance of secondary gas mass transport, and furthermore demonstrates that gas-phase controlled systems are recommended where greater selectivity is required

Controlling shell-side crystal nucleation in a gas-liquid membrane contactor for simultaneous ammonium bicarbonate recovery and biogas upgrading

A gas–liquid hollow fibre membrane contactor (HFMC) process has been introduced for carbon dioxide (CO2) separation from biogas where aqueous ammonia (NH3) is used to chemically enhance CO2 absorption and initiate heterogeneous nucleation of the reaction product ammonium bicarbonate at the membrane–solvent interface. Aqueous ammonia absorbents (2–7 M) were initially used in single pass for CO2 separation from a synthetic biogas where nucleation of ammonium bicarbonate crystals was observed at the perimeter of the micropores. Recirculation of the aqueous ammonia absorbent encouraged the growth of ammonium bicarbonate crystals on the shell-side of the membrane that measured several microns in diameter. However, at high aqueous NH3 concentrations (3–7 M), lumen side crystallisation occurred and obstructed gas flow through the lumen of the HFMC. The suggested mechanism for lumen-side crystallisation was absorbent breakthrough into the lumen due to pore wetting which was promoted by low absorbent surface tension at high NH3 concentration. Preferential shell-side nucleation can therefore be promoted by (1) raising surface tension of the absorbent and (2) selection of a membrane with a more regulated pore shape than the PTFE membrane used (d/L 0.065) as both actions can diminish solvent ingress into the pore. This was evidenced using 2 M NH3 absorbent where shell-side crystallisation was evidenced without the onset of lumen side crystallisation. Raising surface tension through the inclusion of salt into the chemical absorbent also promoted greater CO2 flux stability. Importantly, this study demonstrates that chemically enhanced HFMC are an attractive prospect for gas–liquid separation applications where reaction product recovery offers further economic value

Repairing Type Errors in Functional Programs

Type systems for programming languages can be used by compilers to reject programs which are found to be unsound and which may, therefore, fail to execute successfully. When a program is rejected the programmer must repair it so that it can be type-checked correctly and then executed safely. Diagnostic error messages are essential to help the programmer repair the program. Hindley-Milner type systems give the programmer a great deal of flexibility (polymorphism and implicit typing) while ensuring type safety. As a consequence of this flexibility repairing mistakes can be difficult and programmers have previously observed that type error messages produced by compilers are not helpful enough. This thesis examines the problem of producing more helpful error messages for ill-typed programs written in programming languages with a Hindley-Milner typing discipline. Three main results are described. Firstly, type inference algorithms which infer types in different orders are described, and the ability of these to produce more meaningful error messages is investigated. Secondly, the results of several other authors on helping to explain type inference are condensed into a single generalisation. Thirdly, error messages which suggest concrete changes to the program to remove type errors are produced using the theory of linear type isomorphisms. This theory is implemented as an extension to the MLj compiler. Finally, extensions to Hindley-Milner are explored, taking the type system of MLj as an example

Dissolved methane recovery from anaerobic effluents using hollow fibre membrane contactors

Hollow fibre membrane contactor (HFMC) systems have been studied for the desorption of dissolved methane from both analogue and real anaerobic effluents to ascertain process boundary conditions for separation. When using analogue effluents to establish baseline conditions, up to 98.9% methane removal was demonstrated. Elevated organic concentrations have been previously shown to promote micropore wetting. Consequently, for anaerobic effluent from an upflow anaerobic sludge blanket reactor, which was characterised by a high organic concentration, a nonporous HFMC was selected. Interestingly, mass transfer data from real effluent exceeded that produced with the analogue effluent and was ostensibly due to methane supersaturation of the anaerobic effluent which increased the concentration gradient yielding enhanced mass transfer. However, at high liquid velocities a palpable decline in removal efficiency was noted for the nonporous HFMC which was ascribed to the low permeability of the nonporous polymer provoking membrane controlled mass transfer. For anaerobic effluent from an anaerobic membrane bioreactor (MBR), a microporous HFMC was used as the permeate comprised only a low organic solute concentration. Mass transfer data compared similarly to that of an analogue which suggests that the low organic concentration in anaerobic MBR permeate does not promote pore wetting in microporous HFMC. Importantly, scale-up modelling of the mass transfer data evidenced that whilst dissolved methane is in dilute form, the revenue generated from the recovered methane is sufficient to offset operational and investment costs of a single stage recovery process, however, the economic return is diminished if discharge is to a closed conduit as this requires a multi-stage array to achieve the required dissolved methane consent of 0.14 mg l−1.Yorkshire Water; Severn Trent Water; Anglian Water; Northumbrian Water; EPSR

Diagnosis of an anaerobic pond treating temperate domestic wastewater: An alternative sludge strategy for small works

An anaerobic pond (AP) for treatment of temperate domestic wastewater has been studied as a small works sludge management strategy to challenge existing practice which comprises solids separation followed by open sludge storage, for up to 90 days. During the study, effluent temperature ranged between 0.1 °C and 21.1 °C. Soluble COD production was noted in the AP at effluent temperatures typically greater than 10 °C and was coincident with an increase in effluent volatile fatty acids (VFA) concentration, which is indicative of anaerobic degradation. Analysis from ports sited along the AP's length, demonstrated VFA to be primarily formed nearest the inlet where most solids deposition initially incurred, and confirmed the anaerobic reduction of sludge within this chamber. Importantly, the sludge accumulation rate was 0.06 m3 capita−1 y−1 which is in the range of APs operated at higher temperatures and suggests a de-sludge interval of 2.3–3.8 years, up to 10 times longer than current practice for small works. Coincident with the solids deposition profile, biogas production was predominantly noted in the initial AP section, though biogas production increased further along the AP's length following start-up. A statistically significant increase in mean biogas production of greater than an order of magnitude was measured between winters (t(n=19) = 5.52, P < 0.001) demonstrating continued acclimation. The maximum methane yield recorded was 2630 mgCH4 PE−1 d−1, approximately fifty times greater than estimated from sludge storage (57 mgCH4 PE−1 d−1). Anaerobic ponds at small works can therefore enable sludge reduction and longer sludge holding times than present thus offsetting tanker demand whilst reducing fugitive methane emissions currently associated with sludge storage, and based on the enhanced yield noted, could provide a viable opportunity for local energy generation

Gas to liquid mass transfer in rheologically complex fluids

The increase of studies relaying on gas to liquid mass transfer in digested sludge (shear thinning fluid) necessitates a better understanding of the impact of apparent viscosity (μa) and rheology in process performance. Mass transfer retardation due to μa variations was investigated in a pilot scale absorption bubble column for Newtonian and shear thinning fluids with varied superficial gas velocities (UG). A non-linear reduction of mass transfer efficiency with increasing μa was observed, being the impact higher at low μa ranges and high UG. An increase of 114 cPo in μ from 1.01 to 115 cPo in glycerol solutions saturated with UG = 1.73 cm s−1 led to a reduction of 96% in kLa (α = 0.04), while a comparable raise from 115 to 229 cPo implied a reduction of 52% (α = 0.02). Slug–annular flow regime was identified for shear thinning fluids of high μa (1.0% and 1.5% carboxymethyl cellulose sodium salt solutions), where bubble buoyancy was conditioned by the μ of the fluid at rest and the active volume for mass transfer was reduced because of the presence of stagnant areas. Conditions imitating the rheological variability of anaerobically digested sewage sludge were included within those tested, being a reduction in gas transfer efficiency of 6 percentage points (from 7.6 ± 0.3% to 1.6 ± 0.1%) recorded when increasing μa from 130 to 340 cPo. It is thus recommended that rheology and μa variability are accounted for within the design of gas to liquid mass transfer systems involving digested sewage sludge, in order to avoid reductions in process performance and active volume

Using fatty acid markers to distinguish between effects of salmon (Salmo salar) and halibut (Hippoglossus hippoglossus) farming on mackerel (Scomber scombrus) and whiting (Merlangius merlangus)

Presence of coastal aquaculture activities in marine landscapes is growing with impacts on the wild fish that share these habitats. However, it is difficult to disentangle subsequent ecological interactions between these activities and marine fish communities. We evaluated the impact of both salmon and halibut farms on mackerel (Scomber scombrus) and whiting (Merlangius merlangus) sampled near sea cages using condition indices and fatty acid (FA) biomarkers. Results of the stomach content analysis indicated that mackerel and whiting consumed waste feed which was also reflected in their modified FA profiles. Both mackerel and whiting had elevated levels of FAs that are of vegetable oils origin. The use of vegetable oils as replacement for marine oils is a lot more common in salmon farming than halibut farming. Additionally, the overall effects of the two fish farms were more pronounced in whiting than in mackerel sampled near the sea cages. By allowing discrimination between sources of trophic interactions, this method could lead to more informed decisions in managing different farming activities

Sustaining membrane permeability during unsteady-state operation of anaerobic membrane bioreactors for municipal wastewater treatment following peak-flow

In this study, the impact of peak flow on anaerobic membrane bioreactor operation is investigated to establish how system perturbation induced by diurnal peaks and storm water flows will influence membrane permeability. Good permeability recovery was attained through increasing gas sparging during peak flow, which was explained by the transition in critical flux of the suspension at higher shear rates. However, supra-critical fluxes could also be sustained, provided peak flow was for a short duration. We suggest longer durations of supra-critical operation could be sustained through introduction of reactive fouling control strategies (e.g. TMP set-point control). An initial flux below the critical flux, prior to the introduction of peak flow, was advantageous to permeability recovery, suggesting membrane ‘conditioning’ is important in governing recoverability following peak flow. The importance of conditioning was confirmed through analysis of multiple peak flow events in which the loss of permeability following each peak-flow event was increasingly negligible, and can be ascribed to the arrival of a steady-state in membrane surface deposition. Whilst responding to peak flow with increased gas sparging has been shown effective, the energy demand is considerable, and as such a pseudo dead-end filtration strategy was also evaluated, which required only 0.04 kWh m−3 of energy for gas sparging. Comparison of both filtration modes identified comparable fouling rates, and the feasibility of a low energy gas sparging method for peak flow management that has successfully enabled supra-critical fluxes to be achieved over long-periods in other MBR applications. Importantly, membrane area provides the highest contribution toward capital cost of AnMBR. The potential to turn-up flux in response to peak-flow has been identified in this study, which suggests membrane area can be specified based on average flow rather than peak flow, providing substantial reduction in the capital cost of AnMBR for municipal wastewater treatment