Ochre and biochar: technologies for phosphorus capture and re-use
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Date
03/07/2017Author
Shepherd, Jessica Grace
Metadata
Abstract
Despite recent instability in the global supply of phosphate-rock derived fertiliser and the
potential for this to continue into the future, the recovery of phosphorus (P) from wastewater
treatment systems, where P is abundant and accessible, is well below maximum potential.
Considerable resource is spent on removing P from wastewater in order to comply with
environmental standards and to protect aquatic ecosystems from eutrophication, yet there is
little emphasis on capturing the P in a way that is optimised for re-using it as agricultural
fertiliser.
To address this lack of innovation in the face of climate change and food insecurity, a
concept for a material capable of capturing P from wastewater was developed, with an
emphasis on the utilisation of otherwise waste materials and the use of carbon neutral or
negative production technologies. Based on the demonstrated P capture properties of coal
minewater treatment waste (ochre) and biochar made from anaerobically digested
feedstocks, a range of biochars were designed and produced using different mixtures of
ochre (“OC”), sourced from the UK Coal Authority Minto minewater treatment scheme in
Fife, Scotland and anaerobically digested sewage sludge (“AD”), sourced from the
Newbridge wastewater treatment plant in Edinburgh.
A first generation of materials consisting of either AD or a 1:1 mixture (dry weight basis) of
OC and AD were produced in a small-scale batch pyrolysis unit at two pyrolysis highest
treatment temperatures (HTTs) (450 and 550°C) to give the biochars AD450, AD550,
OCAD450 and OCAD550. These were tested for their P capture properties in repeated P-exposure
experiments with pH buffering in comparison to unpyrolysed ochre, activated
carbon and a natural zeolite. After 5 days of repeated exposure to a P solution at a
wastewater-relevant concentration (20 mg P l-1) replenished every 24 h, relatively high
masses of P were recovered by ochre (1.73 ± 8.93×10-3 mg P g-1) and the biochars
OCAD550 (1.26 ± 4.66×10-3 mg P g-1), OCAD450 (1.24 ± 2.10×10-3 mg P g-1), AD450
(1.06 ± 3.84×10-3 mg P g-1), and AD550 (0.986 ± 9.31×10-3 mg P g-1). The biochar materials
had higher removal rates than both activated carbon (0.884 ± 1.69×10-2 mg P g-1) and zeolite
(0.130 ± 1.05×10-2 mg P g-1). To assess the extractability of recovered P and thus potential
plant bioavailability, P exposure was followed by repeated extraction of the materials for 4
days with pH 7-buffered deionised water. The AD biochars retained 55% of the P recovered,
OCAD biochars 78% and ochre 100%. Assessment of potentially toxic element (PTE) concentrations in the biochars against guideline values indicated low risk associated with
their use in the environment.
A second generation of materials were produced to examine the scalability of the concept.
Mixtures of AD and OC were pelletised with a lignin binder (89.1:9.9:1.0 ratio, dry weight
basis) and AD was pelletised with binder (99:1 ratio, dry weight basis). The pelletised
feedstocks were pyrolysed in a bench-scale continuous flow pyrolysis kiln at the same two
HTTs to give the pelletised biochars PAD450, PAD550, POCAD450 and POCAD550.
Analysis of digested biochar samples compared to the previous generation of biochars
showed general similarities between the two groups, apart from the substantially lower Fe
content.
Sub-samples of the pelletised biochars were exposed to a 20 mg l-1 P solution over 6 days,
with the solution replaced every 24 h to give the P-exposed biochars EPAD450, EPAD550,
EPOCAD450 and EPOCAD550. To probe the mechanisms of P capture by these materials
and how feedstock preparation and pyrolysis conditions affected these, spectroscopic
analysis using laser-ablation (LA) ICP-MS, X-ray diffraction, X-ray photo-electron
spectroscopy (XPS) and scanning electron microscopy coupled with energy dispersive X-ray
was performed. The results highlighted the general importance of Fe minerals in P capture
and subsidiary roles for Al, Ca and Si.
A 3-week barley (Hordeum vulgare) seedling growth experiment was conducted using the
pelletised and P-exposed biochars, in comparison with other biochars produced using
feedstock which contained high amounts of PTEs. The biochars were also extracted using a
range of different methods used to assess the bioavailability of PTEs and nutrients in soils,
and the results compared to digests of barley leaves to identify whether any of these could
reliably predict plant bioavailability in biochar. The above ground biomass and its total P
concentration of barley grown in a 5% mixture of EPOCAD550 in sand was significantly
higher than the control (p < 0.05 and p < 0.01, respectively). A significant positive
correlation between mean leaf P mass and dry weight leaf yield (R2 = 0.865, p < 0.001) was
found, indicating that dry weight yield could be used as an indicator for the P fertilising
capability of biochar for barley seedlings. Element concentrations in unbuffered and buffered
and (pH 7) 0.01 M CaCl2 biochar extractions were significantly positively correlated with
plant leaf concentration for 6 of the 18 elements investigated, more than any of the other
extractions. A longer barley growth experiment was conducted, using rhizoboxes, to test the
bioavailability of P in the biochars compared to conventional fertiliser. The pelletised and Pexposed
biochars were applied to a sandy loam soil with P constraints. Biochar application
rates were based on 2% formic acid extractable P, calculated for summer barley using Index
0 soil. Analysis of total leaf length at harvest (12 weeks), dry weight yield, leaf P
concentration and leaf P mass showed no significant differences between the biochar
treatments, NPK fertilised and NK fertilised controls. This shows that biochar, when applied
at low total application rates based on extractable P, is as effective as conventional fertiliser.
Now that AD biochar materials have been shown to have useful phosphorus recycling
properties in laboratory experiments, additional work is required to optimise their use in
wastewater and agricultural systems. The next stage of research should determine their
performance in flow-through filtration systems with simulated and real wastewater effluent,
as well as their performance in field trials with different crops of interest to demonstrate their
potential as viable alternative fertilisers.