Latif, M. A.

McPhee, P.

Mehta, C. M.

University of Queensland eSpace

  
Benefits of Controlled Struvite Precipitation during Anaerobic 
Digestion 
 
 
C. M. Mehta*, M. A. Latif* and P. McPhee** 
 
*Advanced Water Management Centre, The University of Queensland, St Lucia, QLD 4072, Australia 
(E-mail: c.mehta@awmc.uq.edu.au; a.latif@awmc.uq.edu.au) 
**Queensland Urban Utilities, GPO Box 2765, Brisbane, QLD 4001, Australia 
(E-mail: Paul.McPhee@urbanutilities.com) 
 
 
INTRODUCTION 
Controlled phosphate precipitation during anaerobic digestion could reduce inorganic scaling and 
clogging in the sludge downstream processes, and the phosphate load in the reject water that is 
returned to sewage wastewater treatment (STP) plant inlet (Wu and Bishop 2004). This could 
significantly reduce operational cost for the treatment plant associated with acid flushing and 
overhauling shutdowns (Shu et al. 2006). Currently, phosphate precipitation is mostly applied to 
centrate or reject water post anaerobic sludge dewatering (Mehta et al. 2015). However, the uptake 
of this technology is sluggish due to requirement for a crystallizer, chemical cost and low value 
fertilizer products (struvite or calcium phosphate) recovered from the process. The phosphate 
precipitation before dewatering has the additional advantage such as improvement in sludge 
dewaterability (Bergmans et al. 2014), possibly due to presence of excess Mg. An improvement in 
dewaterability would reduce the volume of the dewatered sludge (low moisture content) and 
polymer requirement, hence additional savings through reduce polymer consumption and 
transport/disposal cost of the sludge. Previous work has reported influence of MgCl2 on controlled 
struvite formation of the digested wastewater sludge at different Mg dosing and pH conditions 
(Bergmans et al. 2014). But, there is a lack of information on precipitation kinetics and influence of 
Mg(OH)2 as Mg source. Hence, further studies are required to validate benefits of this technology 
for STP operators. The aim of this work was to investigate influence of pH, aeration and Mg source 
on phosphate precipitation in the digested sludge and test dewatering of the precipitated sludge.  
 
METHODS 
Batch experiments were performed using existing 1 L reactor in a temperature controlled water bath 
(30oC). The sludge pH was raised through aeration or CO2 stripping using submerged air stone. 
Three different aeration rates (200, 400 and 800 ml min-1) were tested and solution pH was 
recorded. Mg was dosed as MgCl2 and magnesium hydroxide liquor (MHL or Mg(OH)2) in the 
aerated sludge at three different dosing rates, soluble Mg:P molar ratio of 0.5, 1 and 1.5 and at three 
pH conditions, 7.4, 7.7 and 8.0. Each precipitation experiment was conducted for 24 hrs, with 
sampling at fixed intervals. An acid dosing system was installed to control setpoint pH and avoid 
influence low mechanical mixing causing pH increment. The elements (P, K, Mg and Ca) were 
analysed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and P-PO4 
and N-NH4 were measured using Flow Injection Analyser (FIA). The dewatering test was 
performed on the control and the precipitated sludge using centrifuge technique. 
 
RESULTS AND DISCISSION 
Aeration of the digested sludge stripped the dissolved CO2 and increased the pH (Fig. 1). The pH 
rise from 7.0 to 8.3 for all three aeration rates, with maximum pH in the range of 8.2 and 8.3. Linear 
effect of aeration was observed, time required to achieve pH 8.2 reduced from 200, 100 and 50 min 
as the aeration rate increased from 200, 400 and 800 ml min-1 respectively using 1 L sludge.  
 
  
Fig 1: Increment in pH of 1L digested sludge at different aeration rate, 200, 400 and 800 ml/min. 
 
Phosphate (soluble P) concentration reduced with increase in pH due to aeration without added Mg 
(Fig. 2), and it was further reduced by adding of excess Mg (soluble Mg:P ratio >1.0 mol/mol). 
Soluble P concentration around 10 mg/l was achieved at pH 8.0 and equimolar Mg:P ratio, that is 
more than 80% removal of phosphate from the digested sludge. The removal was almost half at 
Mg:P ratio 0.5, while no difference was observed at 1 and 1.5 molar ratios. Instantaneous drop in 
phosphate concentration (within 10 min) was observed with addition of MgCl2 (Fig. 2). For all the 
solutions, equilibrium condition was achieved within experimental period, such crystallisation 
kinetics had been observed in previous work (Mehta and Batstone 2013). The phosphate removal 
was comparatively lower with MHL at similar dosing rate and pH conditions (Fig. 2), suggesting 
lower solubility and slower dissolution of MHL. The soluble Mg concentration in the MHL treated 
sludge increased over 24 hr operation and was found dependent on sludge pH (dissolved Mg 
concentration from MHL at pH 7.4>pH 7.7> pH 8.0). The MHL is less expensive compared to 
MgCl2, but it is insoluble in water and require longer disassociation times (Zeng and Li 2006). 
Hence, there is possibility of high proportion of un-dissociated MHL during struvite precipitation 
where it is used as Mg source due to basic pH conditions. The soluble calcium concentration 
reduced above pH 7.4, suggesting formation of calcium phosphates along with struvite (data not 
shown). During struvite crystallization, presence of Ca2+ above 30 ppm in the wastewater have been 
reported to react with soluble P to form different forms of calcium phosphates (Le Corre et al. 
2005).  
 
 
Fig. 2. Soluble P concentration profile at pH 7.4, 7.7 and 8.0 during continuous aeration with a) 
MgCl2 and b) MHL (at Mg:P = 1 or over). 
 
 
An increase in pH without magnesium addition had no effect on the sludge dewaterability. At a 
constant pH of 8.0, achieved by CO2 stripping, the addition of magnesium led to an improvement in 
dewaterability compared to the sludge without any treatment (Fig. 3). This could be due to increase 
in cation concentration in the effluent, leading to improvement in sludge dewatering (Higgins and 
Novak 1997). However, there was no significant difference in dewaterability at different Mg:P 
ratio. This is contrary to previous study where improvement in sludge dewaterability was observed 
due to a decrease in orthophosphate concentration or by precipitant formation (Bergmans et al. 
2014). 
 
 
Fig. 3. Influence of Mg:P ratio on sludge dewatering. 
 
At present, phosphate precipitation is economical feasible for wastewater with a P-PO4
3- 
concentration above 100 mg L-1, as it has the potential to reduce operating costs by reducing energy 
and chemical consumption and minimizing nuisance phosphate precipitants formation in 
piping/equipment. It is reasonable to assume that a sewage treatment plant can recover 1 kg of 
struvite from 100 m3 d-1 of wastewater, and that would cost AUD 1.4 d-1 (Shu et al. 2006). Based on 
that following financial benefits can be achieved (Shu et al. 2006): reduced chemical requirement 
(AUD 1.3 d-1  as alum); reduced sludge handling and disposal cost (AUD 1.1 d-1); reduced cleaning 
cost of deposits (AUD 7.8-39.1 d-1); and reduced cost of sludge landfilling (AUD 0.003 d-1).  
 
CONCLUSIONS 
Phosphate concentration in the digested sludge reduced with increment in pH and Mg-dosing. The 
phosphate concentrated can be reduced to 10 mg/l at pH 8.0 using excess Mg. Mg released was 
slower from MHL compared to MgCl2 and the MHL remained insoluble at alkaline conditions. 
Hence, for similar P removal at pH 8.0, excess amount of MHL will be required compared to 
MgCl2. Mg dosing improved dewatering by 5-7% compared to control. Hence, there is a significant 
savings from recovery of phosphate from sewage digested wastewater. 
 
REFERENCES 
Bergmans, B. J. C., Veltman, A. M., van Loosdrecht, M. C. M., van Lier, J. B. and Rietveld, L. C. 
(2014). Struvite formation for enhanced dewaterability of digested wastewater sludge. 
Environ. Tech., 35(5), 549-555. 
Higgins, M. J. and Novak, J. T. (1997). The effect of cations on the settling and dewatering of 
activated sludges: Laboratory results. Water Environ. Res., 69(2), 215-224. 
Le Corre, K. S., Valsami-Jones, E., Hobbs, P. and Parsons, S. A. (2005). Impact of calcium on 
struvite crystal size, shape and purity. J. Cryst. Growth, 283(3–4), 514-522. 
Mehta, C. M. and Batstone, D. J. (2013). Nucleation and growth kinetics of struvite crystallization. 
Water Res., 47(8), 2890-2900. 
Mehta, C. M., Khunjar, W. O., Nguyen, V., Tait, S. and Batstone, D. J. (2015). Technologies to 
Recover Nutrients from Waste Streams: A Critical Review. Crit. Rev. Environ. Sci. Tech. , 
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Zeng, L. and Li, X. (2006). Nutrient removal from anaerobically digested cattle manure by struvite 
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Benefits of controlled struvite precipitation during anaerobic digestion

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Benefits of controlled struvite precipitation during anaerobic digestion

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