Phosphorus recovery from process waste water made by the hydrothermal carbonisation of spent coffee grounds

Journal Pre-proofs Phosphorus recovery from process waste water made by the hydrothermal carbonisation of spent coffee grounds Oliver P. Crossley, Rex B. Thorpe, Judy Lee PII: DOI: Reference:

S0960-8524(19)31893-0 https://doi.org/10.1016/j.biortech.2019.122664 BITE 122664

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Bioresource Technology

Received Date: Revised Date: Accepted Date:

31 October 2019 18 December 2019 20 December 2019

Please cite this article as: Crossley, O.P., Thorpe, R.B., Lee, J., Phosphorus recovery from process waste water made by the hydrothermal carbonisation of spent coffee grounds, Bioresource Technology (2019), doi: https://doi.org/ 10.1016/j.biortech.2019.122664

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Phosphorus recovery from process waste water made by the

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hydrothermal carbonisation of spent coffee grounds

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Oliver P Crossley, Rex B Thorpe and Judy Lee*

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Department of Chemical and Process Engineering, University of Surrey, Guildford, Surrey, United Kingdom, GU2 7XH

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*Corresponding author

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Tel: +44 (0)1483 682618

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Email: [email protected]

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Abstract

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This study investigates the recovery of phosphorus from the process water obtained through

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hydrothermal carbonisation (HTC) of a ‘wet’ biomass waste, namely spent coffee grounds. HTC

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was shown to liberate more than 82% of the total phosphorus in the grounds in the form of

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dissolved ortho-phosphate. Nanofiltration was used to concentrate the inorganic nutrients of

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the HTC process water, achieving a mass concentration factor of 3.9 times. The natural

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stoichiometry of phosphorus, magnesium and ammoniacal nitrogen in the nanofiltration

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retentate was favourable for struvite precipitation. 92.8% of aqueous phosphorus was

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recovered as struvite through simple pH adjustment, yielding a total phosphorus recovery of

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75% from the feedstock spent coffee grounds.

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Keywords: Hydrothermal carbonisation; nanofiltration; phosphate; recovery

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1. Introduction

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Hydrothermal Carbonisation (HTC) is considered to be one of the most effective and promising

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thermochemical upgrading technologies for biomass wastes (Titirici, Thomas and Antonietti,

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2007; Zhao et al., 2014). One major benefit of HTC is in energy densification of ‘wet’ biomass,

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which allows the production of solid char materials with improved dewaterability, increased fuel

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value and greater stability on storage (Hoekman, Broch and Robbins, 2011; Bach and Skreiberg,

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2016). However, HTC produces substantial volumes of process water with significant

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concentrations of dissolved inorganic species and high levels of organic carbon (Mihajlović et al.,

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2018). Effective utilisation of this process by-product continues to be a challenge for the

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application of HTC at the industrial scale (Kambo and Dutta, 2015). Careful control over the HTC

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reaction conditions, however, allows for the intentional release of inorganic biological nutrients

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into the process water, making it a valuable resource for nutrient recovery (Escala et al., 2013).

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In particular, HTC is known to facilitate the conversion of bound organic phosphorus (organic-P)

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into the more available form of dissolved ortho-phosphate (ortho-P) during the decomposition

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of organic biomass (Huang et al., 2017).

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Phosphorus (P) is considered to be a non-renewable resource, as it is predominantly mined from

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phosphate rock, with global reserves found mainly in Morocco, China and the Middle East

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(Loganathan et al., 2013). Consequently, many parts of the world, e.g. the European Union, rely

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almost entirely on imported phosphorus. As current estimates suggest that existing reserves will

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last for just 100-300 years, phosphate rock was designated a critical raw material in 2014 by the

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European Commission (European Commission, 2014). Despite the decreasing purity of P rock

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reserves, there is an increasing global demand for phosphorus, driven predominantly by

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fertiliser consumption in the agricultural industry. Although vital for the growth of biomass,

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excessive release of phosphorus into natural watercourses can have significant environmental

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implications such as eutrophication. As such, regulatory requirements have long governed the

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levels of phosphorus discharged from wastewater treatments plants (WWTPs). The wastewater 2

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industry has traditionally relied on the formation of highly insoluble aluminium/iron phosphate

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precipitates or biological P removal methods to meet the regulatory requirements for P

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discharge (Ye et al., 2016, 2017). Although these techniques remain dominant in the P removal

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practices of current WWTPs, there is an emerging interest within science and industry to recover

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P as a ‘renewable’ product suitable for direct reintroduction into the phosphorus market.

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Precipitation of ortho-P with calcium and magnesium has received significant attention in recent

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years, due to the excellent recovery potential of phosphorus and the high bioavailability of the

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solid products. Struvite (magnesium ammonium phosphate hexahydrate - MgNH4PO4.6H2O) is

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an attractive form of recovered phosphorus as it is a regulated fertiliser in many European

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countries (Anonymous, 2019). On an industrial scale Ostara Nutrient Recovery Technologies are

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market leaders in aqueous phase P recovery and have recently obtained an end-of-waste

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certificate allowing the recovered struvite product, Crystal Green®, to enter the phosphate

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market as a high-quality, slow-release fertiliser. Research has shown the fertiliser potential of

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recovered struvites to equal or better the performance of current commercial fertilisers derived

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from virgin phosphates (Kataki et al., 2016; Li et al., 2019). Therefore, there are significant

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environmental and regulatory motivations for recovering phosphate using the above chemical

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precipitation techniques.

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The efficiency of phosphate recovery is vital for commercial application, however, the values

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reported by existing technologies are often unclear and mis-interpreted. Phosphate recovery

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efficiencies of above 80% are regularly reported from aqueous systems (Ye et al., 2017). This

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value refers to the recovery efficiency of dissolved ortho-P in solution and does not consider the

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mass balance for total-P. If total-P is considered for the same systems then the overall P recovery

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efficiency is reduced to less than 25% (Egle et al., 2016). The low total-P recovery is due to the

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high proportion of organic and insoluble inorganic phosphorus, in typical wastewaters, which is

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not accounted for in the former analysis. The conversion of organic-P into ortho-P would,

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therefore, significantly improve the overall phosphorus recovery potential of these processes. 3

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In addition to increasing the relative proportion of ortho-P, increasing the absolute

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concentration of ortho-P can also significantly improve the total-P recovery yield of chemical

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precipitation processes (Stratful, Scrimshaw and Lester, 2001). Nanofiltration (NF) is a filtration

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technique which can be used to reject ortho-P ions in aqueous solutions mainly through

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electrostatic repulsion by negatively charged semi-permeable membranes (Visvanathan and

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Roy, 2010; Mohammad et al., 2015). Rejection of ortho-P through nanofiltration is highly pH

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dependant and relies on the interplay between dissolved inorganic phosphate species (H3PO4,

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H2PO4-, HPO42-, PO43-) and the surface charge of the membrane active layer. At pH less than the

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membrane isoelectric point, a positive membrane surface allows ortho-P to penetrate into the

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permeate (Niewersch et al., 2014; Thong et al., 2016), whilst at pH greater than the membrane

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isoelectric point, when the membrane supports a net negative charge, efficient ortho-P rejection

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is observed (Niewersch et al., 2010; Dolar, Košutić and Vučić, 2011). In the pH range of typical

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lignocellulosic HTC process water (pH 3 to 6), effective ortho-P rejection (above 95%) was

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observed by Visvanathan and Roy (2010) through a Desal-5 membrane. Only a minor reduction

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in ortho-P rejection (1.3%) was recorded, when a complex fertiliser factory wastewater was

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directly compared to synthetic salt solution, thus demonstrating the potential of NF to maintain

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high phosphate rejection for real wastewater systems (Dolar, Košutić and Vučić, 2011). One of

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the main drawbacks of nanofiltration for complex wastewaters, however, is membrane fouling

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which often limits its use if adequate pre-treatment processes are not implemented (Schäfer et

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al., 2004).

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Spent coffee grounds (SCG) were chosen as a model ‘wet’ biomass waste for this study. Coffee

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is the second largest traded commodity after crude oil and its production is increasing annually

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- around 8 Mt per year (Codignole Luz et al., 2018). Only 10wt% of the coffee bean is utilised in

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drink production, meaning that 90% of the dry weight is wasted as SCG. The substantial annual

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production of SCG causes significant issues surrounding landfill accruement, greenhouse gas

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emissions and ecotoxicity. Previous research has examined the potential use of SCG in 4

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production of activated carbon (Lamine et al., 2014; Querejeta et al., 2018) and biofuels (Vardon

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et al., 2013; Codignole Luz et al., 2018) as well as its application as an NPK fertiliser substitute

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(Kasongo et al., 2011). Despite the known fertiliser value of SCG, targeted recovery of these

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nutrients in a pure form is yet to be explored (Mussatto et al., 2011).

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This project investigates a process combination of HTC, nanofiltration and chemical precipitation

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to maximise total-P recovery from SCG. It was hypothesised that the conversion of organic-P to

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ortho-P during HTC combined with an increase in dissolved ortho-P concentration by

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nanofiltration, would significantly improve the potential for total-P recovery from ‘wet’ biomass

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streams.

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2. Materials and methods

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2.1 Chemicals and feedstock

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SCG (Matthew Algie, Tiki espresso beans) were obtained from a local coffee shop on the

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University of Surrey campus. Sodium hydroxide pellets (NaOH, 97%), hydrochloric acid (HCl,

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37%), nitric acid (HNO3, 70%), potassium phosphate monobasic (KH2PO4, ReagentPlus®, >99%),

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calcium chloride (CaCl2, 98%), potassium chloride (KCl, >99%), ammonium sulphate ((NH4)2SO4,

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BioXtra, ≥ 99.0%), magnesium sulphate hexahydrate (MgSO4, >98%), anhydrous magnesium

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chloride (MgCl2, >98%) and vanadate-molybdate reagent were sourced from Sigma Aldrich and

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used as received. COD and high range ammonia test vials were sourced from DelAgua Water

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Testing Ltd.

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2.2 Hydrothermal carbonisation

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SCG were dried to completeness on the day of collection in a fan oven at 90°C before being

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stored in an air-tight container at 4°C until further use. 60g of dried SCG was added to 540g of

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water to produce a slurry at 10wt% DS content. The DS content was varied with a fixed total

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mass of 600g to analyse the extent of phosphorus extraction. A 1000mL autoclave (Columbia

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International Technology Equipment and Supplies, LLC) with a PTFE liner was charged with the

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slurry and the carbonisation reaction was conducted under typical HTC conditions of 200°C for

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5 hours at autogenic pressure. On completion, the reactor was left to cool overnight before

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discharging the slurry and separating the aqueous fraction using vacuum filtration through 11µm

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filter paper (Whatman cellulose fibre filter paper, Grade 1). The filtrate was stored at 4°C until

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further use.

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2.3 Nanofiltration

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A flat sheet Dow-Filmtec NF270 nanofiltration membrane was selected for this study due to the

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high rejection efficiency for ortho-P under mildly acidic conditions and its high permeability

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compared to other nanofiltration membranes (Mänttäri, Pihlajamäki and Nyström, 2006). The

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membranes were soaked in deionised water for 24 hours before use to remove any

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manufacturing chemicals. The membrane was compacted overnight at 18 barg hydraulic

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pressure to ensure a stable pure water flux across the membrane. Rejection of MgSO4 was

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compared to the manufacturers specification to confirm the performance of the membrane

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prior to experimentation. A 17 mil Sepa CF low foulant spacer was used on the feed side for

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membrane support. NF was conducted at hydraulic transmembrane pressures (TMP) of 15 barg

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with a membrane active surface area of 140 cm2. Volumetric flow rates of 2 L/min and 8 L/min

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were used to achieve cross flow velocities (CFV) of 0.82 and 3.27 m/s respectively.

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Membrane fouling was investigated under recirculation of both retentate and permeate streams

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to ensure constant feed conditions (A schematic of the experimental setup can be found in

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Supplementary Information). The contribution of organic matter to the observed fouling was

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investigated by comparing results from HTC process water to that of a synthetic salt solution,

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which lacked organic content. The synthetic salt solution was prepared to mimic an average

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inorganic composition of the real process water as shown in Table 1. The composition of the

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synthetic salt solution differs slightly from the process water sample, however, remains within

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the natural variation caused by the feedstock SCG.

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Concentration of dissolved inorganic species was achieved by reconfiguring the nanofiltration

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setup to allow permeate collection. An 8L volume of HTC process water was filtered at a CFV of

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3.27 m/s and a hydraulic pressure of 15 barg. The mass of both the recirculating feed and

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collected permeate were measured to determine the permeate flux and the mass concentration

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factor. Feed and permeate samples (5 mL) were taken at regular intervals for analysis.

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2.4 Chemical precipitation

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Struvite precipitation from sample volumes of 50mL was performed on: NF concentrate (CP1),

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NF concentrate with ammonia dosing to achieve a P:N molar ratio of 1:3.8 (CP2) and

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unconcentrated HTC process water (CP3). For chemical precipitation, CP1 was prepared at a CFV

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of 3.27 m/s under a hydraulic TMP of 15 barg to a concentration factor of 2.53. To induce

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precipitation, the samples CP1, CP2 and CP3 were slowly adjusted to pH 9 with 5M NaOH and

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left to stand for 12 hours. The solid precipitate was separated from the solution using

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centrifugation at 3200 ×g for 10 minutes, dried in an oven at 70°C for 10 hours, and then stored

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at room temperature prior to analysis.

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2.5 Characterisation

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Total-P content of SCG was determined by the molybdovanadate method (APHA Method 4500-

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P), following digestion in 70% nitric acid. Phosphorus speciation of HTC process water samples

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produced at different DS contents was conducted according to APHA method 4500-P with

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appropriate pre-digestion steps. It was confirmed that HTC process water samples contained

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only ortho-P, as the ortho-P concentration obtained by the APHA method 4500-P matched that

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of total-P. With this knowledge, inductively coupled plasma optical emission spectroscopy

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(Agilent 5110, ICP-OES) was used to determine the total inorganic composition of all aqueous

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samples, including ortho-P. Routine analysis of chemical oxygen demand (COD) (SM 5220 D) and

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ammoniacal nitrogen (Hach method 10031) were also performed on aqueous samples.

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The composition of solids formed during chemical precipitation were determined by ICP-OES

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analysis of samples dissolved in a volume of 5% v/v nitric acid. The physical and chemical traits

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of the solid precipitate samples were examined using a scanning electron microscope (SEM)

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coupled with energy-dispersive X-ray spectroscopy (EDX) on a Jeol 7100F instrument. Powder X-

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ray diffraction (XRD) data were collected by a PANalytical X’Pert Powder diffractometer using

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monochromatized Cu-Kα radiation from 2θ = 10° to 45°.

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3. Results and discussion

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3.1 Phosphorus concentration and speciation in HTC process water

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The total-P content of the SCG feedstock was on average 1.42 mg P/g with an experimental error

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of 0.01 mg P/g and a natural variation of 0.10 mg P/g, in accordance with the value of 1.48 mg

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P/g reported by Mussatto et al. (2011). Phosphorus speciation and percentage P extraction into

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HTC process water is given in Figure 1. The concentration of ortho-P was shown to increase with

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dry solid (DS) loading of SCG, increasing the process water concentration from 2.10 mM, for HTC

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conducted at 5wt% DS, to 15.53 mM for a DS loading of 30wt%. The concentration of ortho-P

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was found to equal that of total-P highlighting the hydrolysis of organic-P species into dissolved

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ortho-P under hydrothermal conditions (Huang et al., 2017). The percentage P extraction values

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reported in Figure 1 were calculated using measured P contents, of both the solid and aqueous

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phases following HTC treatment, to avoid effects of sample variation in the coffee feedstock. P

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extraction from the SCG feedstock exceeded 82% across all tested DS loadings. Such high P

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extraction rates have been observed for hydrothermal treatment of algae biomass at 250°C

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(Valdez et al., 2012), however, extracting more than 60% of phosphorus into the aqueous phase

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typically requires addition of mineral acids (Ekpo et al., 2016; Dai et al., 2017). The conversion 8

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of total-P into ortho-P, combined with the fact that P extraction rates are maintained across the

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DS range applicable to HTC, highlights that HTC treatment of SCG is an effective pre-cursor for P

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recovery processes.

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3.2 Characterisation of HTC process water produced at a DS loading of 10wt%

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The concentration of the main components in HTC process water, produced from a SCG loading

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of 10wt%, are summarised in Table 1. As is typically the case following HTC of lignocellulosic

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material, the process water was acidic, with a pH of 4.2. ICP-OES analysis showed that potassium

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was the most abundant ion in the process water, with a concentration of 17.96 mM. Magnesium

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and calcium were also solubilised from the biomass during HTC producing process water

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concentrations of 5.77 mM and 1.95 mM, respectively. Ortho-P and ammoniacal nitrogen (NH4-

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N) were measured at concentrations of 3.43 mM and 5.79 mM, respectively, meaning that the

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natural composition of Mg, NH4-N and PO4-P (ortho-phosphate as phosphorus) in the HTC

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process water exceeded the equimolar stoichiometry (Mg:N:P of 1:1:1) required for the

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formation of struvite. This, coupled with the relatively low concentration of calcium, makes

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struvite precipitation an attractive method of phosphorus recovery from this process water.

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In addition to inorganic species, the HTC process water contained a significant quantity of

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dissolved organic material (measured in this study as chemical oxygen demand (COD)). The COD

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at 10wt% DS loading was 29 g O2/L which, as with phosphate levels, increased with DS loading

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(from 15 g O2/L at 5wt% DS loading to 57 g O2/L at 30wt% DS loading). Although not a focus of

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this study, COD was considered due to its potential implications on the nanofiltration and

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chemical precipitation processes.

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3.3 Nanofiltration of HTC process water 3.3.1

Performance of NF and assessment of membrane fouling

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Process water filtration was conducted on two independent NF270 membrane sheets. Although

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there was a slight difference in absolute flux, due to membrane variation, the relative flux was 9

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reproducible as shown in Figure 2. The extent of membrane fouling during NF of HTC process

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water treatment was assessed under constant feed conditions by way of relative flux decline.

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The composition of HTC process water used to investigate membrane fouling can be found in

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Table 1. Nanofiltration of HTC process water at a CFV of 0.82 m/s suffered from an average flux

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reduction of 44% after just 2 hours, as displayed in Figure 2A. Increasing the CFV to 3.27 m/s

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significantly limited the rate of flux decline to only 16% over the same time period. The slower

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decline in flux observed at a higher CFV was attributed to the significantly higher shear force

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experienced at the membrane surface preventing the build-up of a fouling layer (Schäfer et al.,

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2004; Lu and Liu, 2010). This was evidenced by a significant reduction in visual fouling on the

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membrane following filtration at a CFV of 3.27 m/s compared to that at 0.82 m/s

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(Supplementary Information). CFV was found to be the most appropriate of the studied fouling

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mitigation parameters (temperature, pH, cross-flow velocity) to allow concentration of the

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inorganic components of HTC process water. Although the increase in CFV did not eliminate

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membrane fouling completely, a high CFV allowed for a significant increase in the total volume

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of permeate that could be collected before the flux was reduced below a reasonable level.

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According to the experimental protocol used here, 6 L of permeate was required to achieve a

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concentration factor of 4 from the initial HTC process water volume of 8 L. Therefore, due to an

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active membrane area of 140 cm2, a relative permeate volume of 42.9 L/m2 was required. For

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this permeate volume a flux reduction of 37% was observed for a CFV of 0.82 m/s, whereas that

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for a CFV of 3.27 m/s was just 8% (Figure 2B).

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To determine whether the observed fouling was organic or inorganic in nature, a synthetic salt

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solution was prepared to mimic the concentration of the dominant inorganic components of the

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HTC process water (Table 1). No flux decline was observed when the synthetic salt solution was

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examined under identical filtration conditions to above (Figure 2). The significant difference in

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the observed fouling could not be explained by the minor difference in inorganic composition

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and, as such, it was confirmed that the fouling was predominantly organic in nature. Despite the 10

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build-up of organic fouling during nanofiltration, pure water flushing of the fouled membrane at

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15 barg hydraulic pressure was sufficient to restore the pure water permeability of the fresh

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membrane. It was therefore confirmed that observed flux decline was due to temporary organic

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fouling.

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3.3.2

Concentration of HTC process water using NF

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The NF system was reconfigured to concentrate the inorganic species in the HTC process water

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by collecting the permeate in a separate vessel. During NF, the concentration of PO4-P and Mg

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in the retentate increased with increasing mass concentration factor, as shown in Figure 3. At a

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mass concentration factor of 3.9, the concentrations of PO4-P and Mg in the retentate were

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increased to 11.2 mM and 18.4 mM, respectively. Rejection of the above ions was maintained

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at greater than 96.6% for the duration of NF concentration. The observed PO4-P rejection is

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higher than that normally reported for NF270 at pH 4 with simple salt solutions (Niewersch et

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al., 2010). This could be due to the high conductivity, which exceeds 4000 µS/cm, and the high

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COD levels which were increased from 29 to 70 g O2/L during concentration. dos Santos, Ribeiro

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and Ribau Teixeira (2014) demonstrated that an increase in both feed conductivity, and

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dissolved organic matter concentration, caused an increase in phosphate rejection by NF. Feed

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conductivity was said to improve rejection due to the electroneutrality requirements of the

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membrane whilst dissolved organic carbon was thought to complex with dissolved P reducing

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its ability to migrate through the membrane.

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Divalent aqueous calcium ions were also effectively rejected by the NF270 membrane with an

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overall rejection efficiency of 97.3%. This allowed for an increase in the calcium concentration

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from 1.95 mM in HTC process water to 5.95 mM in the NF retentate (Error! Reference source

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not found.Figure 3). The initial rejection of NH4-N was much lower than that of P, Mg and Ca at

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just 45.7% (Supplementary Information). This increased significantly over the course of the

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filtration, reaching a rejection efficiency of 80.0%. The increase in rejection may appear

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somewhat counterintuitive but is explained by the existence of a critical TMP for ammonium 11

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rejection by NF270. A critical TMP exists due to the combination of surface (electrostatic and

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friction) forces, which act to retain the ammonium ion in the membrane pores, and permeate

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drag forces, which act to drive the ions through the membrane. An increase in pressure typically

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improves solute rejection due to a relative increase in solvent flux. However, if an increase in

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pressure increases the drag forces to such a level that the surface forces (which remain constant)

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are overcome, then the rate of solute transport increases and a reduction in rejection is

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observed (Paugam et al., 2004). Therefore, if a system is in a state above this critical point, then

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a reduction in the TMP would act to reduce the drag forces such that rejection begins to

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increase. At pH5, Cancino-Madariaga, Hurtado and Ruby (2011) reported a critical TMP of 6 bar

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for ammonium retention. Despite operating at pH4, it is highly likely that with a constant

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hydraulic pressure of 15 barg, the critical TMP was exceeded for the duration of this study.

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During NF, a decrease in the effective TMP is a consequence of an increase in the feed osmotic

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pressure. Therefore, according to the above theory, as the effective TMP was reduced by

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increasing the osmotic pressure, an increase in ammonium rejection was observed.

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The concentration of P, Mg and Ca in the permeate were observed to increase slowly with

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increasing concentration factor. The initial permeate concentrations of P, Mg and Ca were 0.11,

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0.06 and 0.03 mM, but these rose steadily to 0.36, 0.39 and 0.16 mM, respectively, at a

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concentration factor of 3.9. The slight increase in permeate concentration was explained by a

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reduction in water flux, and an increase in concentration gradient across the membrane at high

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concentration factors, both of which favour a relative increase in solute permeation (Pérez-

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González et al., 2015).

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3.4 Phosphorus recovery by chemical precipitation 3.4.1

Nutrient content of NF concentrate

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The nutrient composition following NF of HTC process water from SCG favours the recovery of

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phosphate in the form of struvite (MgNH4PO4.6H2O). As demonstrated in Figure 4, the initial

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molar ratio of Mg2+ and NH4-N to PO4-P naturally exceed that required for the theoretical 1:1:1 12

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molar ratio of struvite. Due to the similar rejection efficiencies of magnesium and phosphate,

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the Mg:P molar ratio was maintained throughout NF. The slightly lower rejection efficiency of

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ammonium caused a reduction in the N:P molar ratio during NF, however, the ratio remained

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favourable for struvite precipitation to at least a concentration factor of 3.9. Struvite is an

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effective slow-release fertiliser with a performance comparable to existing commercial

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fertilisers and is therefore a desirable product from phosphate recovery processes (Kataki et al.,

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2016). Due to the low Ca:P molar ratio of the process water studied here (Figure 4), precipitation

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of calcium phosphates such as hydroxyapatite (Ca10(PO4)6(OH)2) would require significant

309

calcium dosing to achieve the required stoichiometry for efficient phosphorus recovery. As

310

solution pH strongly influences the level of solution supersaturation and the related solubility of

311

phosphate compounds, pH regulation can be used to control the purity of the precipitated

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phosphate species. The optimal pH range for struvite precipitation is often found between 7-9.5

313

whereas a higher pH range of 9-11.5 is commonly required for Ca-P precipitation (Peng et al.,

314

2018). Considering the above, struvite precipitation was conducted at a pH of 9, to limit the

315

formation of Ca-P impurities.

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3.4.2

Characterisation of the supernatant following struvite precipitation

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Chemical precipitation at pH 9 coincided with a reduction in solution pH which is characteristic

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of struvite formation according to Equation 1 (Tansel, Lunn and Monje, 2018).

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𝑀𝑔2 + (𝑎𝑞) + 𝑁𝐻4+ (𝑎𝑞) + 𝐻𝑃𝑂24 ― (𝑎𝑞) + 6𝐻2𝑂 → 𝑀𝑔𝑁𝐻4𝑃𝑂4.6𝐻2𝑂(𝑠) + 𝐻 + (𝑎𝑞) Equation 1

321

Adjusting the NF concentrate (CP1) to pH 9 resulted in the precipitation of 92.8% of aqueous

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phosphorus as shown in Table 2. A large molar excess of ammonium has been shown elsewhere

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to maximise struvite purity (Stratful, Scrimshaw and Lester, 2001). To assess the influence of

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ammonium concentration on the precipitation of struvite in this system, the ammonium content

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of the NF concentrate was increased using ammonium sulphate (CP2). The overall phosphorus

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recovery potential was increased to 97.0%, 4.2% higher than observed from CP1. The enhanced 13

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phosphate removal at elevated ammonium concentration was due to a shift in the struvite

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association/dissociation equilibrium, in favour of solid struvite, according to the common ion

329

effect. In addition, a larger molar reduction of ammonium was recorded for CP2 which is likely

330

due to an increase in the relative proportion of ammonium struvite, compared to potassium

331

struvite (magnesium potassium phosphate - MgKPO4.6H2O) as discussed below. Despite the

332

small increase in the P recovery potential from CP2, the reduction in chemical costs (Munir et

333

al., 2017), and increased simplicity of direct precipitation was considered favourable and

334

therefore precipitation from CP1 formed the focus of this study.

335

Figure 5 depicts the aqueous concentrations of selected species before and after precipitation

336

from CP1 and CP3. Precipitation from CP1 reduced the concentration of PO4-P, NH4-N and Mg2+

337

by 92.8%, 61.0% and 55.5%, respectively. The residual concentrations of Mg2+ and NH4-N remain

338

relatively high, however, their removal is clearly controlled by the limiting concentration of PO4-

339

P. This is demonstrated by the molar reduction ratios of 0.949 for Mg:P and 0.828 for N:P. The

340

molar ratio of N:P was slightly lower than the theoretical value of 1, however, this is likely due

341

to the competitive incorporation of potassium in the crystal structure of struvite. As alluded to

342

previously, monovalent cations, such as potassium, can directly and stoichiometrically

343

substitute for the ammonium cation, when present in a molar excess, producing potassium

344

struvite (Xu et al., 2015). The high concentration of potassium in the precipitation liquor (37.13

345

mM), compared to ammonium (11.35 mM), increases the probability of potassium struvite

346

formation (Huang et al., 2019). However, due to the greater solubility of the potassium

347

analogue, ammonium struvite remains the dominant product. The clear prevalence of struvite

348

formation over calcium phosphates was highlighted by the significantly lower Ca:P molar

349

reduction ratio of just 0.326.

350

The concentration of phosphorus in the precipitation supernatants of CP1 and CP3 were

351

comparable at 0.65 mM and 0.67 mM, respectively. When the concentrations of Mg2+, NH4+ and

352

HPO42- fall below those required to achieve supersaturation, according to the solubility product 14

353

of struvite, precipitation ceases as the condition of supersaturation no longer exists. Therefore,

354

considering the similar initial molar ratios of Mg:N:P in CP1 (1.64:1.25:1) and CP3 (1.68:1.73:1),

355

it is not surprising that the Mg2+, NH4+ and HPO42- concentrations in the residual solutions are

356

also similar as, theoretically, they are governed by the same solubility product. As expected, the

357

calculated struvite solubility products (KSP) for CP1 and CP3 were comparable at 1.813x10-8 and

358

1.043x10-8 mol3/L3, yielding solubility constants (pKSP = -log(KSP)) of 7.74 and 7.98, respectively

359

(Table 2). The measured solubility constants were lower than that commonly reported for pure

360

struvite solutions of between 9.1 and 13.2 (Hanhoun et al., 2011). This is because the conditional

361

solubility constants reported here, calculated using equilibrium solution concentrations,

362

assumes an activity coefficient of 1 for the dissolved ions. However, the high ionic strength of

363

the solution, and the potential ion pairing of Mg2+, NH4+ and HPO42- with ions such as Cl-, SO42-,

364

and K+ reduces the activity of the dissolved ions and thus reduces the struvite precipitation

365

potential (Lahav et al., 2013). The similarity in residual phosphate concentrations meant that

366

the percentage phosphate recovery was significantly increased, from 78.0% in CP3 to 92.8% in

367

CP1. NF concentration of the aqueous nutrients in HTC process water, therefore, has a clear

368

benefit on the yield of struvite precipitation (Stratful, Scrimshaw and Lester, 2001).

369 370

3.4.3

Characterisation of the recovered struvites

371

The inorganic composition of CP1 and CP2 precipitates are compared to commercially

372

available Crystal Green® and theoretical results for pure struvite in Table 3. The phosphorus

373

(3.982 mol/kg) and magnesium content (4.596 mol/kg) from CP1 show good correlation with

374

Crystal Green® and the theoretical value for pure struvite. The higher NH4-N concentration in

375

CP2 caused an increase in the solid P:K molar ratio, from 4.42:1 to 7.87:1, suggesting a greater

376

selectivity for ammonium struvite precipitation. The solid produced from CP2 had a

377

magnesium (5.944 mol/kg) and phosphorus (5.016 mol/kg) content that was apparently

378

greater than expected for pure struvite. These values are based on the molecular mass of

15

379

struvite (hexahydrate) and as such are highly dependent on the hydration of the crystal

380

structure. At temperatures above 55°C, struvite begins to dehydrate to form dittmarite

381

(MgNH4PO4.H2O) (Ramlogan and Rouff, 2016; Tansel, Lunn and Monje, 2018). Therefore,

382

theoretical Mg contents up to 6.04 mol/kg, as is true for dittmarite, could be explained by

383

partial dehydration. This highlights that, although dehydration does not result in loss of

384

nutrients, the drying process must be strictly controlled if the formation of a single crystal

385

structure is desired.

386

Data obtained from solid state analysis (SEM, EDX and XRD) of the precipitates produced from

387

CP1 can be found in the supplementary information. The recovered precipitates were crystalline

388

in nature with a size range between 10 and 30µm. The crystals adopted either trapezoidal or

389

cube-like morphologies, consistent with reports of other recovered struvite samples (Rahman

390

et al., 2014). The strong correlation between the sample XRD pattern and that of pure struvite

391

further evidenced the formation of struvite crystals. EDX analysis confirmed that the crystals

392

comprised of mainly phosphorus (12.6 at%), magnesium (13.1 at%) and oxygen (48.4 at%). The

393

atomic % ratio of Mg:P obtained by EDX was close to 1:1, characteristic of pure struvite, and the

394

atomic % ratio of P:O was consistent with the presence of the phosphate anion - PO43-. The

395

chemical composition obtained through EDX analysis highlighted a deficiency of nitrogen (6.9

396

at%) for a product of pure ammonium struvite. The nitrogen deficiency, however, coincided with

397

the presence of potassium (3.0 at%) as predicted from the supernatant analysis. An EDX map

398

scan of the recovered crystals shows a strong correlation between the high-density regions of

399

Mg, P, O, N and K suggesting the formation of a mixed ammonia/potassium struvite. The

400

combined nitrogen (6.9 at%) and potassium (3.0 at%) content should, in theory, match that of

401

phosphorus (12.6 at%) if the sample consisted of crystalline struvites (ammonium and

402

potassium) only. However, the analysis demonstrates a slight excess of phosphorus, which may

403

be due to the presence of a small amount of amorphous magnesium phosphate (AMP). A very

404

minor broad peak was observed in the XRD pattern between 30 and 34o which may provide 16

405

evidence for the presence of AMP, and therefore account for the slight excess of Mg and P (Ren,

406

Babaie and Bhaduri, 2018). A much lower potassium content was found in the sample of Crystal

407

Green®. As Crystal Green® is obtained from municipal wastewaters, which typically contain a

408

more favourable ammonium to potassium molar ratio for the formation of ammonium struvite,

409

far less potassium is incorporated into the precipitate (Pastor et al., 2010). To the authors’

410

knowledge, the presence of potassium need not be an issue for the product end use as an

411

agricultural fertiliser but may actually be a benefit due to the addition of ‘K’ to an ‘NP’ product.

412 413

3.5 Overall process

414

The process combination of HTC, nanofiltration and struvite precipitation is considered a viable

415

method of recovering phosphorus in the form of a solid fertiliser product with high efficiency. In

416

this study, 74.6% of the total-P present in the native spent coffee grounds was converted into

417

solid struvite precipitate. Nanofiltration was shown to have a clear benefit on the struvite

418

precipitation yield, however, a cost-benefit analysis of the overall process would be required to

419

assess the applicability of this technology at an industrial scale. Importantly, the largest

420

phosphorus loss occurred during the hydrothermal treatment, which was not optimised in this

421

study. Adjusting the reaction conditions of the hydrothermal treatment to maximise the

422

dissolution of phosphorus into the aqueous phase, for example by mineral acid addition to the

423

reaction, has the potential to unlock an even greater level of phosphorus recovery.

424

425

4. Conclusion

426

HTC converted 82.7% of the total-P present in SCG into dissolved ortho-P. Mg and PO4-P ions

427

were effectively concentrated by nanofiltration with rejection efficiencies exceeding 97%. NH4-

428

N was also rejected by nanofiltration such that the molar ratios of Mg, NH4-N and PO4-P

429

remained favourable for the precipitation of struvite. A dissolved phosphate recovery of 92.8% 17

430

was achieved from the NF concentrate through simple pH adjustment. The process

431

combination of HTC, nanofiltration and precipitation facilitated the conversion of 74.6% of

432

total-P from the native SCG into pure struvite product, a recovery that far exceeds that

433

reported for current state-of-the-art industrial solutions.

434 435

Acknowledgements

436

We would like to thank Dr. Dan Driscoll of the chemistry department at the University of Surrey

437

for conducting the XRD analysis. This work was co-funded by the UK Engineering and Physical

438

Science Research Council (EPSRC) and Antaco UK Ltd.

439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466

Appendix A Supplementary data associated with this article can be found in online version of the paper.

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620 621

Table 1: Composition of process water and synthetic salt solution used to investigate fouling of the NF270 membrane. PO4-P (mM)

NH4-N (mM)

Mg2+(mM)

K+ (mM)

Ca2+ (mM)

COD (g O2/L)

pH

24

HTC process water

3.43

5.79

5.77

17.96

1.95

29

4.2

Synthetic salt solution

3.21

5.98

5.81

18.12

1.77

0.0

4.2

622

25

Table 2: Aqueous concentration of Mg2+, NH4-N and PO4-P of sampled CP1, CP2 and CP3 pre- and post-struvite precipitation as well as the corresponding phosphate recovery potential, solubility products (Ksp) and solubility constants (pKsp). Prior to chemical precipitation

Post chemical precipitation

Concentration (mM)

Mg2+

NH4-N

PO4-P

Mg2+

NH4-N

PO4-P

CP1 CP2 CP3

14.26 14.26 6.33

11.34 33.96 5.99

8.99 8.99 3.05

6.35 5.42 3.79

4.43 17.93 4.08

0.65 0.27 0.67

P recovery potential (%)

Ksp (x108)

pKsp

92.8 97.0 78.0

1.828 2.624 1.036

7.74 7.58 7.98

26

Table 3: Nutrient content (mol/kg) of the precipitates obtained in this study compared to commercially available Crystal Green® and the theoretical values for pure struvite. Composition obtained from ICP-OES data of dissolved samples. Sample

Phosphorus

Magnesium

Calcium

Potassium

Sodium

CP1

3.982

4.596

0.052

0.900

0.132

CP2

5.016

5.944

0.076

0.637

0.363

Crystal Green

4.040

4.656

0.030

0.056

0.034

Pure Struvite

4.075

4.075

0

0

0

27

90

16

80

14

70

12

60

10

50

8

40

6

30

Ortho-P Hydrolysable-P Total-P P extraction into aqueous phase (%)

4 2 0 0

5

10

15 20 Dry solid loading (%)

25

30

20 10

P extraction into aqueous phase (%)

Aqueous P concentration (mM)

18

0 35

Figure 1: Concentration of reactive, hydrolysable and total phosphorus in HTC process water at varying dry solid loading. Percentage extraction of P was calculated using the P contents of the solid and liquid products following HTC treatment.

28

1

A

J/J0

0.8

0.6

0.4

CFV 0.82 m/s - process water trial 1 CFV 0.82 m/s - process water trial 2 CFV 0.82 m/s - synthetic salt solution CFV 3.72 m/s - process water

0.2

0 0

50

100

150

200 250 Time (mins)

300

350

400

450

1

B

J/J0

0.8

0.6

0.4

CFV 0.82 m/s - process water trial 1 CFV 0.82 m/s - process water trial 2 CFV 0.82 m/s - synthetic salt solution CFV 3.27 m/s - process water

0.2

0 0

50

100 150 Cumulative permeate volume (L/m2)

200

250

Figure 2: Comparison of the relative permeate flux (J/J0) against filtration time (A) and cumulative permeate volume (L/m2) (B) to assess fouling potential of HTC process water on NF270 membrane under different CFV. Data from a synthetic salt solution is included to highlight the difference in fouling observed in the absence of dissolved organic matter.

29

20

100

16 Concentration (mM)

90

P retentate Mg retentate Ca retentate P permeate Mg permeate Ca Permeate P % rejection Mg % rejection Ca % rejection

14 12 10 8 6

80 70 60 50 40 30

4

20

2

10

0

0 0

0.5

Rejection (%)

18

1

1.5

2 2.5 3 Mass concentration factor

3.5

4

4.5

Figure 3: Concentration of Mg, PO4-P and Ca in the nanofiltration reject and permeate streams for increasing mass concentration factor. Solute rejection efficiencies are displayed for each sample point.

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2 1.8 1.6

Molar ratio

1.4 1.2 1 0.8 0.6 0.4

Retentate Mg:P molar ratio Retentate N:P molar ratio Retentate Ca:P molar ratio Stiochiometric requirement for struvite precipitation

0.2 0 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Mass concentration factor

Figure 4: Molar ratio of Mg, Ca and NH4-N to PO4-P during HTC process water nanofiltration. The dotted line depicts a Mg:NH4-N:PO4-P molar ratio of 1:1:1 required for struvite formation.

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Element concentration (mM)

14 12

1.2 1.0 0.8

10 8

0.6

6

0.4

4

Molar reduction ratio (X:P)

CP1 initial CP1 supernatant CP3 initial CP3 supernatant CP1 - X:P molar reduction ratio CP3 - X:P molar reduction ratio

16

0.2

2 0

0.0 Magnesium

Phosphorus

Ammoniacal Nitrogen

Calcium

Figure 5: Concentration of aqueous species (X) as well as molar reduction ratios of P:X before and after chemical precipitation for CP1 and CP3. Where X denotes either Mg, NH4-N or Ca.

Highlights    

Phosphorus was recovered from HTC process water with a total-P recovery of 75% Hydrothermal carbonisation converted 83% of total-P into dissolved ortho-P The yield of struvite precipitation was significantly improved by nanofiltration Phosphorus was recovered as a pure struvite precipitate by simple pH adjustment

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Oliver P Crossley: Conceptualization, methodology, validation, formal analysis, investigation, data curation, writing-original draft, writing-Review & editing, visualization Rex B Thorpe: Conceptualization, methodology, writing-Review & editing, visualization, supervision, Project administration, Funding acquisition Judy Lee: Conceptualization, methodology, Resources, writingReview & editing, visualization, supervision, Project administration, Funding acquisition

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