The Cost Benefit of Refinery Effluent Pretreatment Upstream of Membrane Bioreactors.

The established classical method of treating oil refinery effluent is flotation followed by biological treatment. Membrane bioreactors (MBRs) offer more advanced treatment, producing a clarified and potentially reusable treated effluent, but demand robust pretreatment to remove oil and grease (O&G) down to consistent, reliably low levels. An analysis of a full-scale conventional oil refinery ETP (effluent treatment plant) based on flotation alone, coupled with projected performance, energy consumption and costs associated with a downstream MBR, have demonstrated satisfactory performance of flotation-based pretreatment. The flotation processes, comprising an API (American Petroleum Institute) separator followed by dissolved air flotation (DAF), provided ~90% removal of both total suspended solids (TSS) and O&G coupled with 75% COD (chemical oxygen demand) removal. The relative energy consumption and cost of the pretreatment, normalised against both the volume treated and COD removed, was considerably less for the API-DAF sequence compared to the MBR. The combined flotation specific energy consumption in kWh was found to be almost an order of magnitude lower than for the MBR (0.091 vs. 0.86 kWh per m3 effluent treated), and the total cost (in terms of the net present value) around one sixth that of the MBR. However, the nature of the respective waste streams generated and the end disposal of waste solids differ significantly between the pretreatment and MBR stages.

A Brief Review of the Status of Low-Pressure Membrane Technology Implementation for Petroleum Industry Effluent Treatment.

Low-pressure membrane technology (ultrafiltration and microfiltration) has been applied to two key effluents generated by the petroleum industry: produced water (PW) from oil exploration, a significant proportion being generated offshore, and onshore refinery/petrochemical effluent. PW is treated physicochemically to remove the oil prior to discharge, whereas the onshore effluents are often treated biologically to remove both the suspended and dissolved organic fractions. This review examines the efficacy and extent of implementation of membrane technology for these two distinct applications, focusing on data and information pertaining to the treatment of real effluents at large/full scale. Reported data trends from PW membrane filtration reveal that, notwithstanding extensive testing of ceramic membrane material for this duty, the mean fluxes sustained are highly variable and generally insufficiently high for offshore treatment on oil platforms where space is limited. This appears to be associated with the use of polymer for chemically-enhanced enhanced oil recovery, which causes significant membrane fouling impairing membrane permeability. Against this, the application of MBRs to onshore oil effluent treatment is well established, with a relatively narrow range of flux values reported (9−17 L·m−2·h−1) and >80% COD removal. It is concluded that the prospects of MBRs for petroleum industry effluent treatment are more favorable than implementation of membrane filtration for offshore PW treatment.

Sorptive removal of disinfection by-product precursors from UK lowland surface waters: Impact of molecular weight and bromide.

The current study compared the impact of three different unit processes, coagulation, granular activated carbon (GAC), and a novel suspended ion exchange (SIX) technology, on disinfection by-product formation potential (DBPFP) from two UK lowland water sources with medium to high bromide content. Specific attention was given to the influence of the organic molecular weight (MW) fraction on DBPFP as well as the impact of bromide concentration. Whilst few studies have investigated the impact of MW fractions from Liquid Chromatography with Organic Carbon Detection (LC-OCD) analysis on dissolved organic carbon (DOC) removal by different processes, none have studied the influence of DOC MW fractions from this analysis on DBP formation. The impact of higher bromide concentration was to decrease the total trihalomethane (THM) and haloacetic acid (HAA) mass concentration, in contrast to previously reported studies. Results indicated that for a moderate bromide concentration source (135 µg/L), the THM formation potential was reduced by 22% or 64% after coagulation or SIX treatment, respectively. For a high bromide content source (210 µg/L), the THM formation potential removal was 47% or 69% following GAC or SIX treatment, respectively. The trend was the same for HAAs, albeit with greater differences between the two processes/feedwaters with reference to overall removal. A statistical analysis indicated that organic matter of MW > 350 g/mol had a significant impact on DBPFP. A multiple linear regression of the MW fractions against DBPFP showed a strong correlation (R2 between 0.90 and 0.93), indicating that LC-OCD analysis alone could be used to predict DBP formation with reasonable accuracy, and offering the potential for rapid risk assessment of water sources.

Industrial effluent treatment with immersed MBRs: treatability and cost.

A comprehensive OPEX analysis for both municipal and industrial wastewaters has been conducted encompassing energy, critical component (membrane) replacement, chemicals consumption, waste disposal and labour. The analysis was preceded by a review of recent data on industrial effluent treatability with reference to published chemical oxygen demand (COD) removal data for four effluent types: food and beverage, textile, petroleum and landfill leachate. Outcomes revealed labour costs to be the most significant of those considered, contributing 50% of the OPEX for a 10,000 m3/day capacity municipal wastewater treatment works. An analysis of the OPEX sensitivity to 12 individual parameters (labour cost, flux, electrical energy cost, membrane life, feed COD, membrane cost, membrane air-scour rate, chemicals cost, waste disposal cost, mixed liquor suspended solids (MLSS) concentration, recirculation ratio, and transmembrane pressure) revealed OPEX to be most sensitive to labour effort and/or costs for all scenarios considered other than a large (100,000 m3/day capacity) works, for which flux and electrical energy costs were found to be slightly more influential. It was concluded that for small- to medium-sized plants cost savings are best made through improving the robustness of plants to limit manual intervention necessitated by unforeseen events, such as electrical/mechanical failure, foaming or sludging.

An empirical determination of the whole-life cost of FO-based open-loop wastewater reclamation technologies.

Over the past 5-10 years it has become apparent that the significant energy benefit provided by forward osmosis (FO) for desalination arises only when direct recovery of the permeate product from the solution used to transfer the water through the membrane (the draw solution) is obviated. These circumstances occur specifically when wastewater purification is combined with saline water desalination. It has been suggested that, for such an "open loop" system, the FO technology offers a lower-cost water reclamation option than the conventional process based on reverse osmosis (RO). An analysis is presented of the costs incurred by this combined treatment objective. Three process schemes are considered combining the FO or RO technologies with membrane bioreactors (MBRs): MBR-RO, MBR-FO-RO and osmotic MBR (OMBR)-RO. Calculation of the normalised net present value (NPV/permeate flow) proceeded through developing a series of empirical equations based on available individual capital and operating cost data. Cost curves (cost vs. flow capacity) were generated for each option using literature MBR and RO data, making appropriate assumptions regarding the design and operation of the novel FO and OMBR technologies. Calculations revealed the MBR-FO-RO and OMBR-RO schemes to respectively offer a ∼20% and ∼30% NPV benefit over the classical MBR-RO scheme at a permeate flow of 10,000 m3  d-1, provided the respective schemes are applied to high and low salinity wastewaters. Outcomes are highly sensitive to the FO or OMBR flux sustained: the relative NPV benefit (compared to the classical system) of the OMBR-RO scheme declined from 30% to ∼4% on halving the OMBR flux from a value of 6 L m-2. h-1.

Impact of CO2 concentration and ambient conditions on microalgal growth and nutrient removal from wastewater by a photobioreactor.

The increase in atmospheric CO2 concentration and the release of nutrients from wastewater treatment plants (WWTPs) are environmental issues linked to several impacts on ecosystems. Numerous technologies have been employed to resolves these issues, nonetheless, the cost and sustainability are still a concern. Recently, the use of microalgae appears as a cost-effective and sustainable solution because they can effectively uptake CO2 and nutrients resulting in biomass production that can be processed into valuable products. In this study single (Spirulina platensis (SP.PL) and mixed indigenous microalgae (MIMA) strains were employed, over a 20-month period, for simultaneous removal of CO2 from flue gases and nutrient from wastewater under ambient conditions of solar irradiation and temperature. The study was performed at a pilot scale photo-bioreactor and the effect of feed CO2 gas concentration in the range (2.5-20%) on microalgae growth and biomass production, carbon dioxide bio-fixation rate, and the removal of nutrients and organic matters from wastewater was assessed. The MIMA culture performed significantly better than the monoculture, especially with respect to growth and CO2 bio-fixation, during the mild season; against this, the performance was comparable during the hot season. Optimum performance was observed at 10% CO2 feed gas concentration, though MIMA was more temperature and CO2 concentration sensitive. MIMA also provided greater removal of COD and nutrients (~83% and >99%) than SP.PL under all conditions studied. The high biomass productivities and carbon bio-fixation rates (0.796-0.950 gdw·L-1·d-1 and 0.542-1.075 gC·L-1·d-1 contribute to the economic sustainability of microalgae as CO2 removal process. Consideration of operational energy revealed that there is a significant energy benefit from cooling to sustain the highest productivities on the basis of operating energy alone, particularly if the indigenous culture is used.

Enhancement of CO2 biofixation and lipid production by Chlorella vulgaris using coloured polypropylene film.

Chlorella vulgaris was cultivated with light at different wavelengths (λmax) and irradiation intensities (I) by applying a coloured tape (CT) as a simple, inexpensive light filter. C. vulgaris was cultivated in a standard medium using blue (CTB), green (CTG), red (CTR), yellow (CTY) and white (CTW) CT to filter the light, as well the unfiltered light (U). The influence of λmax and I on specific growth rate (µ), nutrient removal efficiency (% RE of total nitrogen, TN, and phosphorus, TP), CO2 fixation rate (RC) and lipid productivity (Plipid) were evaluated. The highest biomass concentration Xmax of 2.26 g L-1 was measured for CTW with corresponding µ, TN and TP RE, RC and Plipid values of 0.95 d-1, 92% and 100%, 0.67 g L-1 d-1 and 83.6 mg L-1 d-1, respectively. The normalised µ and Plipid for U were significantly lower than in CTW of 33-50% and 75%, respectively. The corresponding non-normalised parameter values for CTB were significantly lower at 0.45 d-1, 0.18 g L-1, 15% and 37%, 0.03 g L-1 d-1 and 1.2 mg L-1 d-1. Results suggest a significant impact of I and λmax, with up to a 50% increase in growth and nutrient RE from optimising these parameters.

Influence of polyelectrolyte architecture on the electrokinetics and dewaterability of industrial membrane bioreactor activated sludge.

Improvement of sludge dewaterability is greatly hindered by the presence of large amounts of interstitial water molecules trapped in the sludge as a result of strong hydrophilic characteristics. This study has investigated the influence of six different polyacrylamide (PAM) flocculants with different molecular architecture (linear, slightly and highly branched), charge density (CD) and molecular weight (MW) on the electro-kinetics and dewatering of highly stable industrial membrane bioreactor (MBR) sludge. The impact of PAM on flocculation is manifested in the supernatant turbidity, particle zeta potential, sludge capillary suction time (CST), floc size and settleability. Turbidity removal and reduction in zeta potential are used to identify the optimum polymer dose. An optimum dose of 70 mg.L-1 has been determined for linear PAM of 40% CD. However, a highly-branched PAM, with the same CD, has shown an optimum value of 30 mg.L-1. In all cases, a turbidity removal of more than 99% and CST reduction of 51-64% is attained; the linear PAMs have resulted in the highest CST reduction. Higher PAM doses have resulted in larger flocs and the maximum particle size is observed at the saturation point. The reduction in sludge volume relates with the floc size and PAM dose. For sludge conditioning and dewaterability, highly branched PAM with low MW has shown superior performance over linear high MW PAM. Enhancement of flocculation and dewatering is correlated with the surface charge neutralization and bridging mechanisms.

The Impact of Mechanically-Imposed Shear on Clogging, Fouling and Energy Demand for an Immersed Membrane Bioreactor.

The impact of the application of mechanically-imposed shear on the propensity for fouling and clogging (or "sludging"-the agglomeration of sludge solids in the membrane channel) of an immersed flat sheet (iFS) membrane bioreactor (MBR) was studied. The bench-scale test cell used contained a single flat sheet fitted with a crank and motor to allow the membrane to be oscillated (or reciprocated) vertically at a low rate (20 RPM). The membrane was challenged with sludge samples from a local MBR installation treating petroleum industry effluent, the sludge having previously been demonstrated as having a high sludging propensity. Sludging was measured by direct visual observation of membrane surface occlusion by the agglomerated solids, with fouling being notionally represented by the rate of transmembrane pressure increase. Results demonstrated membrane reciprocation to have a more beneficial impact on sludging amelioration than on suppressing fouling. Compared with the stationary membrane, sludging was reduced by an average of 45% compared with only 13% for fouling suppression at the reference flux of 15 L·m-2·h-1 applied. The specific energy demand of the mechanical shear application was calculated as being around 0.0081 kWh·m-3, significantly lower than values reported from a recent pilot scale study on a reciprocated immersed hollow fibre MBR. Whilst results appear promising in terms of energy efficiency, it is likely that the mechanical complexity of applying membrane movement would limit the practical application to low flows, and a correspondingly small number of membrane modules.

Carbonaceous and nitrogenous disinfection by-product formation from algal organic matter.

Seasonal algal blooms in drinking water sources release intracellular and extracellular algal organic matter (AOM) in significant concentrations into the water. This organic matter provides precursors for disinfection by-products (DBPs) formed when the water is subsequently chlorinated at the final disinfection stage of the potable water treatment process. This paper presents results of AOM characterisation from five algal species (three cyanobacteria, one diatom and one green) alongside the measurement of the DBP formation potential from the AOM of six algal species (an additional diatom). The character was explored in terms of hydrophilicity, charge and protein and carbohydrate content. 18 DBPs were measured following chlorination of the AOM samples: the four trihalomethanes (THMs), nine haloacetic acids (HAAs), four haloacetonitriles (HANs) and one halonitromethane (HNM). The AOM was found to be mainly hydrophilic (52 and 81%) in nature. Yields of up to 92.4 µg mg-1 C carbonaceous DBPs were measured, with few consistent trends between DBP formation propensity and either the specific ultraviolet absorbance (SUVA) or the chemical characteristics. The AOM from diatomaceous algae formed significant amounts of nitrogenous DBPs (up to 1.7 µg mg-1 C). The weak trends in DBPFP may be attributable to the hydrophilic nature of AOM, which also makes it more challenging to remove by conventional water treatment processes.

Synergistic effects and optimization of nitrogen and phosphorus concentrations on the growth and nutrient uptake of a freshwater Chlorella vulgaris.

The synergistic effects and optimization of nitrogen (N) and phosphorus (P) concentrations on the growth of Chlorella vulgaris (CCAP 211/11B, CS-42) and nutrient removal have been investigated under different concentrations of N (0-56 mg/L) and P (0-19 mg/L). The study showed that N/P ratio has a crucial effect on the biomass growth and nutrient removal. When N/P=10, a complete P and N removal was achieved at the end of cultivation with specific growth rate (SGR) of 1 d-1 and biomass concentration of 1.58 g/L. It was also observed that when the N content <2.5 mg/L, the SGR significantly reduced from 1.04 to 0.23 d-1 and the maximum biomass produced was decreased more than three-fold to 0.5 g/L. The Box-Behnken experimental design and response surface method were used to study the effects of the initial concentrations (P, N and C) on P and N removal efficiencies. The optimized P, N and C concentrations supporting 100% removal of both P and N at an SGR of 0.95 were 7, 55 and 10 mg/L respectively, with desirability value of 0.94. The results and analysis obtained could be very useful when applying the microalgae for efficient wastewater treatment and nutrient removal.

Acidified and ultrafiltered recovered coagulants from water treatment works sludge for removal of phosphorus from wastewater.

This study used a range of treated water treatment works sludge options for the removal of phosphorus (P) from primary wastewater. These options included the application of ultrafiltration for recovery of the coagulant from the sludge. The treatment performance and whole life cost (WLC) of the various recovered coagulant (RC) configurations have been considered in relation to fresh ferric sulphate (FFS). Pre-treatment of the sludge with acid followed by removal of organic and particulate contaminants using a 2kD ultrafiltration membrane resulted in a reusable coagulant that closely matched the performance FFS. Unacidified RC showed 53% of the phosphorus removal efficiency of FFS, at a dose of 20 mg/L as Fe and a contact time of 90 min. A longer contact time of 8 h improved performance to 85% of FFS. P removal at the shorter contact time improved to 88% relative to FFS by pre-acidifying the sludge to pH 2, using an acid molar ratio of 5.2:1 mol H(+):Fe. Analysis of the removal of P showed that rapid phosphate precipitation accounted for >65% of removal with FFS. However, for the acidified RC a slower adsorption mechanism dominated; this was accelerated at a lower pH. A cost-benefit analysis showed that relative to dosing FFS and disposing waterworks sludge to land, the 20 year WLC was halved by transporting acidified or unacidified sludge up to 80 km for reuse in wastewater treatment. A maximum inter-site distance was determined to be 240 km above the current disposal route at current prices. Further savings could be made if longer contact times were available to allow greater P removal with unacidified RC.

Coagulant recovery and reuse for drinking water treatment.

Coagulant recovery and reuse from waterworks sludge has the potential to significantly reduce waste disposal and chemicals usage for water treatment. Drinking water regulations demand purification of recovered coagulant before they can be safely reused, due to the risk of disinfection by-product precursors being recovered from waterworks sludge alongside coagulant metals. While several full-scale separation technologies have proven effective for coagulant purification, none have matched virgin coagulant treatment performance. This study examines the individual and successive separation performance of several novel and existing ferric coagulant recovery purification technologies to attain virgin coagulant purity levels. The new suggested approach of alkali extraction of dissolved organic compounds (DOC) from waterworks sludge prior to acidic solubilisation of ferric coagulants provided the same 14:1 selectivity ratio (874 mg/L Fe vs. 61 mg/L DOC) to the more established size separation using ultrafiltration (1285 mg/L Fe vs. 91 mg/L DOC). Cation exchange Donnan membranes were also examined: while highly selective (2555 mg/L Fe vs. 29 mg/L DOC, 88:1 selectivity), the low pH of the recovered ferric solution impaired subsequent treatment performance. The application of powdered activated carbon (PAC) to ultrafiltration or alkali pre-treated sludge, dosed at 80 mg/mg DOC, reduced recovered ferric DOC contamination to <1 mg/L but in practice, this option would incur significant costs. The treatment performance of the purified recovered coagulants was compared to that of virgin reagent with reference to key water quality parameters. Several PAC-polished recovered coagulants provided the same or improved DOC and turbidity removal as virgin coagulant, as well as demonstrating the potential to reduce disinfection byproducts and regulated metals to levels comparable to that attained from virgin material.

Algal remediation of CO2 and nutrient discharges: A review.

The recent literature pertaining to the application of algal photobioreactors (PBRs) to both carbon dioxide mitigation and nutrient abatement is reviewed and the reported data analysed. The review appraises the influence of key system parameters on performance with reference to (a) the absorption and biological fixation of CO2 from gaseous effluent streams, and (b) the removal of nutrients from wastewaters. Key parameters appraised individually with reference to CO2 removal comprise algal speciation, light intensity, mass transfer, gas and hydraulic residence time, pollutant (CO2 and nutrient) loading, biochemical and chemical stoichiometry (including pH), and temperature. Nutrient removal has been assessed with reference to hydraulic residence time and reactor configuration, along with C:nutrient ratios and other factors affecting carbon fixation, and outcomes compared with those reported for classical biological nutrient removal (BNR). Outcomes of the review indicate there has been a disproportionate increase in algal PBR research outputs over the past 5-8 years, with a significant number of studies based on small, bench-scale systems. The quantitative impacts of light intensity and loading on CO2 uptake are highly dependent on the algal species, and also affected by solution chemical conditions such as temperature and pH. Calculations based on available data for biomass growth rates indicate that a reactor CO2 residence time of around 4 h is required for significant CO2 removal. Nutrient removal data indicate residence times of 2-5 days are required for significant nutrient removal, compared with <12 h for a BNR plant. Moreover, the shallow depth of the simplest PBR configuration (the high rate algal pond, HRAP) means that its footprint is at least two orders of magnitude greater than a classical BNR plant. It is concluded that the combined carbon capture/nutrient removal process relies on optimisation of a number of process parameters acting synergistically, principally microalgal strain, C:N:P load and balance, CO2 and liquid residence time, light intensity and quality, temperature, and reactor configuration. This imposes a significant challenge to the overall process control which has yet to be fully addressed.

Removal of disinfection by-product precursors by coagulation and an innovative suspended ion exchange process.

This investigation aimed to compare the disinfection by-product formation potentials (DBPFPs) of three UK surface waters (1 upland reservoir and 2 lowland rivers) with differing characteristics treated by (a) a full scale conventional process and (b) pilot scale processes using a novel suspended ion exchange (SIX) process and inline coagulation (ILCA) followed by ceramic membrane filtration (CMF). Liquid chromatography-organic carbon detection analysis highlighted clear differences between the organic fractions removed by coagulation and suspended ion exchange. Pretreatments which combined SIX and coagulation resulted in significant reductions in dissolved organic carbon (DOC), UV absorbance (UVA), trihalomethane and haloacetic acid formation potential (THMFP, HAAFP), in comparison with the SIX or coagulation process alone. Further experiments showed that in addition to greater overall DOC removal, the processes also reduced the concentration of brominated DBPs and selectively removed organic compounds with high DBPFP. The SIX/ILCA/CMF process resulted in additional removals of DOC, UVA, THMFP, HAAFP and brominated DBPs of 50, 62, 62, 62% and 47% respectively compared with conventional treatment.

Experimental evaluation of intermittent aeration of a hollow fibre membrane bioreactor.

Intermittent membrane aeration provides a substantially improved energy efficiency in hollow fibre-based immersed membrane bioreactors (HF iMBRs). The benefits of intermittent aeration have been assessed with respect to sustaining a target flux and/or limiting the fouling rate to a sustainable level based on a small plant using full-scale HF modules. Results show that for the same specific aeration demand per unit of permeate produced (SAD(p)), fouling rates were significantly lower for 10 s filtration, 30 s relaxation ("10:30" intermittent aeration) compared to 10:10 and continuous aeration. At a net flux (J(net)) of 23.3 litres m(-2) h(-1) (LMH), a SAD(p) of 4.6 was found sufficient to sustain operation, this value being up to 75% and 50% lower compared to continuous and 10:10 aeration respectively. This empirical data was compared with heuristic data from 5 large scale HF iMBR plants, which revealed that 10:30 aeration can sustain a relatively high flux (up to 25.3 LMH) under dry weather conditions in warm climates, with the recorded SAD(p) ranging from 5.3-10.9.

BSM-MBR: a benchmark simulation model to compare control and operational strategies for membrane bioreactors.

A benchmark simulation model for membrane bioreactors (BSM-MBR) was developed to evaluate operational and control strategies in terms of effluent quality and operational costs. The configuration of the existing BSM1 for conventional wastewater treatment plants was adapted using reactor volumes, pumped sludge flows and membrane filtration for the water-sludge separation. The BSM1 performance criteria were extended for an MBR taking into account additional pumping requirements for permeate production and aeration requirements for membrane fouling prevention. To incorporate the effects of elevated sludge concentrations on aeration efficiency and costs a dedicated aeration model was adopted. Steady-state and dynamic simulations revealed BSM-MBR, as expected, to out-perform BSM1 for effluent quality, mainly due to complete retention of solids and improved ammonium removal from extensive aeration combined with higher biomass levels. However, this was at the expense of significantly higher operational costs. A comparison with three large-scale MBRs showed BSM-MBR energy costs to be realistic. The membrane aeration costs for the open loop simulations were rather high, attributed to non-optimization of BSM-MBR. As proof of concept two closed loop simulations were run to demonstrate the usefulness of BSM-MBR for identifying control strategies to lower operational costs without compromising effluent quality.

Occurrence and fate of pharmaceutical and personal care products in a sewage treatment works.

The occurrence and fate of eight pharmaceutical and personal care products (PPCPs) during sewage treatment has been studied in a pilot-scale treatment plant, comprising a primary settler (2.85 m(3)), an aeration tank (1.845 m(3)) and a secondary clarifier (0.5 m(3)), placed on site at a wastewater treatment works in the north west of the UK. It was fed both with raw sewage and the return liquor produced after sludge centrifugation, thus representing the most common configuration for a municipal sewage treatment plant based on the activated sludge process. Samples were taken at six different locations, including the return liquor stream, and analysed for musk fragrances and pharmaceutically active compounds belonging to various therapeutic groups such as anti-inflammatory drugs, tranquillisers and antiepileptics. Mass balances were conducted for those PPCPs that were quantifiable. The fate of the PPCPs was found to differ according to their physical-chemical characteristics. Anti-inflammatories underwent a degradation process and were almost completely removed from sewage during the biological treatment step. Musk fragrances were only partially removed, through adsorption onto the primary suspended solids and the biomass in the aerobic process, due to their strong lipophilic characteristics. The results of this study provide increasing evidence that the partial removal of these substances through the sewage treatment process contribute to the environmental occurrence of PPCPs. Consequently, existing STPs should be upgraded in order to attenuate the release of these substances into the aquatic environment.

The cost of a large-scale hollow fibre MBR.

A cost sensitivity analysis was carried out for a full-scale hollow fibre membrane bioreactor to quantify the effect of design choices and operational parameters on cost. Different options were subjected to a long term dynamic influent profile and evaluated using ASM1 for effluent quality, aeration requirements and sludge production. The results were used to calculate a net present value (NPV), incorporating both capital expenditure (capex), based on costs obtained from equipment manufacturers and full-scale plants, and operating expenditure (opex), accounting for energy demand, sludge production and chemical cleaning costs. Results show that the amount of contingency built in to cope with changes in feedwater flow has a large impact on NPV. Deviation from a constant daily flow increases NPV as mean plant utilisation decreases. Conversely, adding a buffer tank reduces NPV, since less membrane surface is required when average plant utilisation increases. Membrane cost and lifetime is decisive in determining NPV: an increased membrane replacement interval from 5 to 10 years reduces NPV by 19%. Operation at higher SRT increases the NPV, since the reduced costs for sludge treatment are offset by correspondingly higher aeration costs at higher MLSS levels, though the analysis is very sensitive to sludge treatment costs. A higher sustainable flux demands greater membrane aeration, but the subsequent opex increase is offset by the reduced membrane area and the corresponding lower capex.

Model-based energy optimisation of a small-scale decentralised membrane bioreactor for urban reuse.

The energy consumption of a small-scale membrane bioreactor, treating high strength domestic wastewater for community level wastewater recycling, has been optimised using a dynamic model of the plant. ASM2d was chosen as biological process model to account for the presence of phosphate accumulating organisms. A tracer test was carried out to determine the hydraulic behaviour of the plant. To realistically simulate the aeration demand, a dedicated aeration model was used incorporating the dependency of the oxygen transfer on the mixed liquor concentration and allowing differentiation between coarse and fine bubble aeration, both typically present in MBRs. A steady state and dynamic calibration was performed, and the calibrated model was able to predict effluent nutrient concentrations and MLSS concentrations accurately. A scenario analysis (SCA) was carried out using the calibrated model to simulate the effect of varying SRT, recirculation ratio and DO set point on effluent quality, MLSS concentrations and aeration demand. Linking the model output with empirically derived correlations for energy consumption allowed an accurate prediction of the energy consumption. The SCA results showed that decreasing membrane aeration and SRT were most beneficial towards total energy consumption, while increasing the recirculation flow led to improved TN removal but at the same time also deterioration in TP removal. A validation of the model was performed by effectively applying better operational parameters to the plant. This resulted in a reduction in energy consumption by 23% without compromising effluent quality, as was accurately predicted by the model. This modelling approach thus allows the operating envelope to be reliably identified for meeting criteria based on energy demand and specific water quality determinants.