Full-scale trials to achieve low total phosphorus in effluents from sewage treatment works

The tightening of phosphorus (P) standards has forced the UK water industry to identify options to improve P removal in sewage treatment works. This article presents results of 12-month trials testing three technologies at full-scale: a membrane bioreactor (MBR), a continuous backwash up-flow sand filter (CBUSF) and a high-rate compressible media filter (HRCMF). The aim of the trial was to determine the lowest total phosphorus (TP) concentrations that these systems can consistently achieve under stable operating conditions. MBR is a mature technology combining biological treatment with membrane filtration for solids removal. CBUSF uses granular media deep filtration as removal process while HRCMF uses a synthetic fibre compressible media to retain solids and both technologies are typically applied as tertiary treatments in combination with chemical P removal. The lowest TP effluent concentration was recorded for CBUSF (0.22 mgP/L) while the highest TP removal was recorded for MBR (95 ± 3 %). HRCMF delivered the poorest performance (32 ± 26 % TP removal, 1.7 mgP/L average TP effluent), attributed to limited chemical TP binding and solids removal under the conditions tested. Factors affecting P removal performance during the trial were the identification of the optimal coagulant dose to bind soluble P, the formation of solids that can be removed by the separation unit, insufficient reaction time for the coagulation process and the carryover of coagulant into the final effluent. The trials gave valuable insights to address the implementation of new technologies or upgrade existing assets' performance

Biological carbon dioxide utilisation in food waste anaerobic digesters

Carbon dioxide (CO2) enrichment of anaerobic digesters (AD) was previously identified as a potential on-site carbon revalorisation strategy. This study addresses the lack of studies investigating this concept in up-scaled units and the need to understand the mechanisms of exogenous CO2 utilisation. Two pilot-scale ADs treating food waste were monitored for 225 days, with the test unit being periodically injected with CO2 using a bubble column. The test AD maintained a CH4 production rate of 0.56 ± 0.13 m3 CH4·(kg VSfed d)−1 and a CH4 concentration in biogas of 68% even when dissolved CO2 levels were increased by a 3 fold over the control unit. An additional uptake of 0.55 kg of exogenous CO2 was achieved in the test AD during the trial period. A 2.5 fold increase in hydrogen (H2) concentration was observed and attributed to CO2 dissolution and to an alteration of the acidogenesis and acetogenesis pathways. A hypothesis for conversion of exogenous CO2 has been proposed, which requires validation by microbial community analysis

Evaluation of anaerobic digestibility of energy crops and agricultural by-products

Abundant wastes from the food and drink supply chain are valuable and infrequently used as anaerobic digestion (AD) substrates. This study quantifies their biomethane potential to contribute to solid waste reduction and energy production. 29 organic materials were evaluated: energy crops (6), pre-treated agricultural by-products (5), livestock slurries (3), agro-industrial wastes (7), fruit and vegetable wastes (4) and co-digestion mixtures of chicken litter (CL) and fruit wastes (4). Results showed highest biogas yields for rendered fat washings (1379 ± 125 mL/g VSfeedstock), fish waste (898 ± 107 mL/g VSfeedstock) and potato waste (768 ± 27 mL/g VSfeedstock). Synergistic benefits of co-digestion were evidenced. CL (20%) with avocado pulp (80%) led to 84% higher biogas than expected from contribution of single substrates

Methods to modify supersaturation rate in membrane distillation crystallisation: control of nucleation and crystal growth kinetics (including scaling)

While water vapour flux is often regarded as the critical parameter in membrane distillation crystallisation (MDC), there are multiple factors that will determine the kinetics of nucleation and crystal growth. A Nývlt-like equation is therefore introduced that can relate how multiple conditional parameters (membrane area, flux, temperature difference, crystalliser volume, magma density) independently modify nucleation rate and supersaturation, enabling a normalising approach for the characterisation of nucleation and crystal growth kinetics within MDC. Each parameter can be modified to increase supersaturation rate, which reduced induction time and broadened the metastable zone width (MSZW) at induction. An increase in supersaturation mitigated scaling and favoured bulk nucleation. This is due to the increase in volume free energy provided by the elevated supersaturation that reduces the critical energy requirement for nucleation to favour a homogeneous primary nucleation mechanism. An increase in temperature difference or magma density narrowed the MSZW. For each parameter, either supersaturation rate, supersaturation or induction time were fixed, while the other two factors were amended. While higher supersaturation rates generally favoured larger crystal sizes with broader size distributions, a high level of supersaturation at a low supersaturation rate increased particle size and narrowed the size distribution. In practice, these factors may be applied collectively and synergistically to deliver strict control over crystal growth, which remains a challenge for current evaporative technology. This was illustrated when facilitating an increase in supersaturation rate with membrane area, where an identical nucleation order was identified between membrane systems, from which it can be implied that MDC affords an inherently scalable solution for crystallisation.European Union funding: 71408

Bioconversion of carbon dioxide in anaerobic digesters for on-site carbon capture and biogas enhancement - A review

Energy consumption of the water sector presents an increasing energy demand, contrary to GHG mitigation aims. As a result, research aimed at capturing emitted CO2 and at developing treatment technologies with a low energy demand and increased renewable energy production has increased, leading to a surge in implementation of anaerobic digestion (AD). Valorization of the biogenic CO2 emitted with biogas AD (estimated at over 1 MtCO2 per annum for the UK water and organic waste sectors), presents an opportunity to further reduce carbon footprint and support energy supply decarburization. This paper reviews bioconversion of CO2 into CH4 in ADs (without addition of H2) as a means to valorize CO2 emissions. The review has concluded this to be a promising solution to reduce carbon footprint and uplift renewable energy production. However, in order to increase readiness for implementation (1) the mechanisms of CO2 utilization need to be elucidated, including the sources of additional H2 needed, (2) studies need to report more thoroughly the conditions of CO2 injection and (3) trials where ADs are integrated with gas to liquid mass transfer technologies need to be performed

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

An exploratory study of the impact and potential of menstrual hygiene management waste in the UK

An estimated 15 million people in the UK menstruate over the span of approximately 37.5 years, using every year around 3.3 billion units of single-use menstrual management products (MMPs) (i.e. pads and tampons). A more circular design and sustainable management of these products could greatly reduce their waste and environmental impacts. This research is an exploratory study into the current menstrual products, waste and systems in the UK. The study found that an estimated 28,114 tonnes of waste is generated annually from menstrual products, 26,903 tonnes from disposable products of which about 4% (3,363 tonnes) is lost in the environment by flushing. The less sustainable products within those studied are disposable pads, which are the main contributors to menstrual waste volumes in the UK (21,094 t/y) and produce around 6,600 tCO2 eq. of GHG. Replacing disposable MMPs with reusable would reduce waste production by 22,907 t/y and avoid about 7,900 tCO2 eq. of GHG. In addition, even a simple better waste management process, such replacing landfill with thermal treatment, would further reduce emissions by around 5,000 tCO2 eq. of GHG and produce every year approximately 5,500 MKh with incineration and 18,000 MKh with gasification

Influence of innate sludge factors and ambient environmental parameters in biosolids storage on indicator bacteria survival: A review

The potential health risks associated with sludge cake application to agricultural land are managed by controlling the levels of Escherichia coli (E. coli) bacteria which indicate the risk of pathogen transfer. Analyses undertaken following post-digestion sludge dewatering have shown unpredictable levels of E. coli increase in stored sludge cake. Presently there is limited understanding on environmental parameters controlling the indicator bacteria density in storage and the contributory effects dewatering may have. This review aims to establish the state of current knowledge on innate and environmental factors influencing E. coli dynamics and survival in biosolids. A key factor identified is the effect of mechanical dewatering processes, which transform the sludge matrix environmental conditions through the increased availability of growth factors (e.g. nutrient and oxygen). Examples of storage practices from the agricultural and food industries are also discussed as successful methods to inhibit bacterial growth and survival, which could be extrapolated to the biosolids sector to regulate E. coli concentrations

Performance and stability of sewage sludge digestion under CO2 enrichment: a pilot study

Carbon dioxide (CO2) injection in anaerobic digestion has recently been proposed as an interesting possibility to boost methane (CH4) recovery from sludge and organic waste by converting a greenhouse gas into a renewable resource. This research assessed the effects of exogenous CO2 injection on performance and process stability of single-phase continuous anaerobic digesters. Two pilot scale reactors treating sewage sludge were operated for 130 days. One reactor was periodically injected with CO2 while the other acted as control. Two injection frequencies and injection devices were tested. The results indicated that CO2 enrichment allowed an increase in CH4 production of ca. 12%, with a CH4 production rate of 371 ± 100 L/(kgVSfed⋅d) and a CH4 concentration of ca. 60% when dissolved CO2 levels inside the test reactor were increased up to 1.9-fold. Results also indicated an improvement in process resilience to temporary overloads and no impacts on stability parameters

Combining unmanned aircraft systems and image processing for wastewater treatment plant asset inspection

Wastewater treatment plants are essential for preserving the water quality of freshwater and marine ecosystems. It is estimated that, in the UK, as much as 11 billion liters of wastewater are treated on a daily basis. Effective and efficient treatment of wastewater requires treatment plants to be maintained in good condition. Recent studies have highlighted the potential of unmanned aircraft systems (UASs) and image processing to be used in autonomous and automated monitoring systems. However, the combined use of UASs and image processing for wastewater treatment plant inspections has not yet been tested. This paper presents a novel image processing-UAS framework for the identification of failures in trickling filters and activated sludge facilities. The results show that the proposed framework has an accuracy of 95% in the detection of failures in activated sludge assets, with the accuracy ranging between 55% and 81% for trickling filters. These results are promising and they highlight the potential use of the technology for the inspection of wastewater treatment plant