FUNDAMENTAL CHEMISTRY AND CONTROL OF STRUVITE PRECIPITATION

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FUNDAMENTAL CHEMISTRY AND CONTROL OF STRUVITE
PRECIPITATION James Doyle School of Water
Sciences, Cranfield University England
Struvite Formation
Introduction

Struvite forms according to the general reaction
shown below Mg2 NH4 PO43-
MgNH4PO4.6(H20) However this equation is a
simplification of the chemistry involved in
struvite precipitation. Struvite precipitation is
controlled by pH, degree of supersaturation,
temperature and the presence of other ions such
as calcium and can occur when the concentrations
of magnesium, ammonium and phosphate ions exceed
the solubility product (often denoted as Ksp)
for struvite. The relationship between Ksp and pH
indicates that struvite solubility decreases with
increasing pH, which in turn leads to an increase
in the struvite precipitation potential of a
solution.
Struvite (MgNH4PO4.6H2O) formation in wastewater
treatment has gained greater consideration with
the introduction of the EEC Urban Waste Water
Treatment Directive (UWWTD, (91/271/EEC). The
changes in legislation regarding the removal of
nitrogen and phosphorus from wastewaters has led
to operational problems particularly when
treating the sludges derived from Biological
Nutrient Removal (BNR) processes. Sludges wasted
from BNR if anaerobically digested will
re-hydrolyse poly-phosphates previously formed in
the aerobic treatment stage, magnesium and
phosphate ions will also be released. Since
ammonium ions will be high in concentration
struvite precipitation can occur.
Struvite crystals formed from a real sludge
liquor
150mm diameter pipe reduced to 60mm after 12
weeks by struvite precipitation.
Impact of pH upon the pKsp value and the Ion
Activity Product/Ksp ratio.
Results
Results
A computer model predicting struvite
precipitation potential (SPP) was compared to
struvite precipitation in real and synthetic
liquors. The pH values ranged from 6.5 to 8.8
with masses of struvite formed ranging from 0mg
l-1 at pH values below 7.5 to values exceeding
350mg l-1 at pH values above 8.8. Data from the
jar tests was compared with a crystalline deposit
taken from Coleshill SDP operated by Severn Trent
Water plc. X-ray diffraction (XRD) and
dissolution experiments were used to identify the
purity of struvite precipitates formed in both
real systems and the simulated precipitation
experiments using jar tests.
The precipitates recovered were tested for
magnesium, phosphorus and calcium concentrations
via dissolution experiments. The initial
concentrations of magnesium and phosphorus having
been measured prior to struvite precipitation
were then compared to the concentrations of
magnesium and phosphorus remaining in solution
following precipitation. From these data the
mass of struvite formed can be calculated
assuming that a molar removal of magnesium
equates to the precipitation of a mole of
struvite. The concentrations of magnesium and
phosphorus in the precipitates were calculated as
weight percentages of the theoretical weight
percentages for struvite.
.
Mass of struvite precipitated and the
precipitation potential calculated by a model
with increasing pH.
The percentage of the theoretical concentration
of phosphorus and magnesium measured following
dissolution of various masses of struvite.
Conclusions
Struvite precipitation is predominantly governed
by pH. Application of models can determine a
solutions struvite precipitation
potential. Manipulation of solution chemistry can
increase or decrease struvite precipitation
potential. Struvite precipitation can be
influenced by other ions in solution e.g. Calcium
ions.
Further Work
Research into the kinetics of struvite
precipitation has been initiated. Using modified
impellers to which different materials may be
attached, the kinetics of struvite precipitation
has been investigated. Struvite formation upon
stainless steel, teflon and acylic has generated
data suggesting that surface roughness has a
profound impact upon struvite fouling. This data
can be used to assess materials for use in
struvite reactors.
Stainless steel impeller blades before and after
experiments to assess the kinetics of struvite
precipitation.
The authors would like to express appreciation
for the support of the sponsors EPSRC and
Severn Trent Water Ltd