The ‘uncontrolled’ formation of struvite (MgNH4PO4.6H2O) is one of the major challenges facing most wastewater treatment plants (WWTPs). It is particularly prevalent around post-anaerobic digestion (AD) processes (treating sewage sludge) where NH3, PO4 and pH levels can be quite high. Common struvite-related problems include the clogging of pipes, pump and dewatering equipment downstream of the AD; resulting in poor performance, increased maintenance and energy costs. This study aimed at utilising the combined effect of varying pH and Mg2+ doses in digesters to pre-precipitate PO4 and NH4-N (as struvite) thereby reducing their availability in postAD processes. Bench-scale AD-reactors were fed with pre-pasteurised sewage sludge with varying Mg-doses and pH, to control struvite formation by reducing PO4 and NH4-N availability in post-AD processes for ‘uncontrolled’ struvite formation. Post-AD Mg2+-dosing tests (with pH variation) were also conducted to compare removal rates as a function of the location of Mgaddition. More than 40% PO4 and NH4-N-removal efficiencies were observed in post-AD tests. Methane production was also seen to increase with increasing Mg2+ .
Keywords: Ammonium, Anaerobic Digestion, Magnesium, Phosphate, Struvite
The high cost of treating nutrient-rich wastewaters (especially digested sludge supernatants); coupled with the stringent regulations limiting nitrogen (N) and phosphorus (P) discharge into aquatic environments have stimulated research in the development of anaerobic digestion processes capable of producing reduced P and N levels in the effluent (Capdevielle et al. 2013). About 60 – 70% of sewage sludge in the UK is treated by mesophilic anaerobic digestion (AD). Most of these AD processes produce liquors (following sludge dewatering) with elevated pH and high soluble P and N concentrations. These conditions favour the precipitation of struvite in digested sludge processing equipment, further resulting in significant financial (increased maintenance costs) and operational problems (e.g. pipeline blockages, fouling of pumps, interrupted plant operations) (Neethling and Benisch 2004; Barat et al. 2005; Marti et al. 2008). The situation is even worsened by the accumulation of P in systems (with little or no P-removal technology in place) resulting from the recycling of liquors (following digested sludge dewatering) back to the head of the WWTP for treatment. Struvite (MgNH4PO4.6H2O) is a white orthorhombic crystal that forms when magnesium, ammonium and phosphorus react in equimolar amounts as presented in the following chemical reaction:
Mg2+ + NH4 + + PO4 3- + 6H2O MgNH4PO4.6H2O(s)
Struvite component ions commonly get resolublized during the AD process making them available for struvite formation (Mamais 1994; Parsons and Smith 2008; Marchi et al. 2015). Struvite formation and subsequent precipitation occurs when the concentrations of soluble magnesium, ammonium and phosphate exceed supersaturation levels. As the supersaturation ratio (i.e. the ratio between the product of the component ions and the solubility product (Ksp)) increases, the potential for struvite formation also increases (Doyle and Parsons 2002). Other factors which have been observed to influence struvite formation in water resource recovery systems include; pH, mixing rate, temperature, and the concentrations of other ions (e.g. Ca2+) (Bhuiyan et al. 2007). The potential for struvite formation has been found to increase with increasing pH (between 6 and 10); commonly in areas where pH increases in response to carbon dioxide stripping or pressure drop (Tansel et al. 2014). Such is the case with anaerobically digested sludge with pH commonly between 7.2 and 8.5. The solubility constant (pKsp) of struvite was also observed to increase with increasing pH, thereby increasing the potential for struvite precipitation at higher pH (with the highest struvite precipitation observed at pH 9.5) (Doyle and Parsons 2002; Sharp et al. 2013). Struvite has been observed to be a slow release fertilizer (Demirer et al. 2010).
In order to control struvite-related problems it is important to understand the underlying factors responsible for its formation. Parsons et al. (2007) proposed pH to be a very important factor in struvite control. Some studies also suggested the use of metal salts (e.g. ferric chloride, magnesium chloride, magnesium hydroxide, magnesium oxide, alum etc.) to control the formation of struvite (Bergmans et al. 2014). Most technologies developed to recover P (as struvite) in WWTPs have generally been applied to the aqueous phase (i.e. dewatering liquors from sludge lines). Although, recent studies (Bergmans et al. 2014; Geerts et al. 2015; Marchi et al. 2015) have shown improved dewaterability of digested sludge as one of the benefits of P-recovery in the sludge phase, a lot still needs to be done to cost-effectively minimise available P and N levels in processes downstream of anaerobic digesters (ADs).
This study investigated the feasibility of recovering P (as struvite) in Anaerobic Digesters (ADs) through the addition of Mg2+ (as MgCl2) and pH adjustment. The study also provided analysis on the effect of Mg2+ addition and pH adjustment on the performance of the ADs (in terms of methane production and volatile solids (VS) removal). Experiments involving Mg2+ addition and pH adjustment in post-AD sludge (i.e. digested sludge) were also conducted to compare P- and N-removal rates with that of the P-recovery experiments in the ADs. It was observed that both methods reduced the available soluble P and N levels in the dewatering liquor which could not only minimise the potential for scaling resulting from ‘uncontrolled’ struvite precipitation in downstream sludge lines, but could also potentially reduce any associated energy costs involved in treating these liquors via activated sludge treatment processes (as is currently being done when the liquors get recycled to the ‘head’ of most WWTPs for treatment).
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