A Struvite Control and Phosphorus Removal Process for Centrate: Full-Scale Testing

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ABSTRACT
The North End Water Pollution Control Centre (NEWPCC) in Manitoba, Canada serves a population of approximately 380,000. The dry weather flow to the NEWPCC is approximately 175 ML/d (46 MGD). The plant uses anaerobic digestion and centrifuge dewatering in its solids treatment train. The centrate stream comprises approximately 20 percent of the plant’s phosphorus load. The plant suffers from severe struvite (magnesium ammonium phosphate) deposition: centrifuges and centrate piping have to be regularly cleaned. The Province requires that the plant control the phosphorus load recycled in the centrate: the total phosphorus (TP) load must not exceed 119 kg/d, as determined by a thirty day rolling average. In order to meet the requirements of the Province, and to control struvite, a metal salt will be dosed into the digested sludge, upstream of the centrifuges. Prior to design of a metal salt dosing system, a full-scale trial was initiated. Two potential metal salts were identified; ferric chloride (FeCl3) and aluminum sulphate or alum (Al2(SO4)3·14H2O).

Examination of struvite crystals in the digested sludge indicated that nearly complete disappearance of crystals occurred at all dosing rates used in these trials. The trials demonstrated that alum and ferric chloride were highly effective in meeting the removal objectives and controlling struvite crystal formation in the centrate.

INTRODUCTION
Manitoba Conservation issued a new licence in June, 2005 which provides the operating requirements of the NEWPCC in Winnipeg, Manitoba. The licence stipulates that the centrate from the sludge dewatering facility should have a TP load that does not exceed 119 kg/d, as determined by a thirty day rolling average. In order to meet the requirements of the licence, it is proposed that phosphorus be removed from the centrate stream and directed to the biosolids stream, by the dosing of a metal salt into the sludge. Two potential metal salts were identified; ferric chloride (FeCl3) and aluminum sulphate or alum (Al2(SO4)3·14H2O).

Struvite is magnesium ammonium phosphate (MAP) and has the chemical formula of MgNH4PO4·6H2O. Struvite is commonly found in wastewater treatment plants that have anaerobic digesters. Struvite forms when the chemical constituents reach levels of supersaturation. Struvite formation requires nucleation sites, and is particularly prevalent in
areas of turbulence where CO2 is released, causing an increase in pH.

The City has experienced severe struvite formation in the dewatering facility, which has led to some operational difficulties. Struvite has been found to coat the centrifuges and the centrate piping.

Since phosphate is a key constituent of struvite, removal of phosphate could reduce the potential for struvite precipitation. Therefore, the removal of phosphorus from the centrate stream, could reduce the quantity of struvite deposited in the piping and centrifuges.

Due to the risk that struvite deposition in the centrate pipes could get worse with the implementation of BNR, phosphorus removal options for the centrate treatment project should consider the benefits of phosphorus removal in a manner that reduces unwanted struvite formation.

In order to verify that a metal salt could remove phosphorus from the centrate, it was agreed that jar-tests would be undertaken on the liquid digested sludge. Preliminary jar-tests were undertaken using ferric chloride and alum prior to the full-scale trials. After addition of the chemical, each sample was centrifuged and filtered. Results were positive indicating that the phosphorus concentration in filtered sludge samples could be reduced by metal salt dosing. It was found that at an alum dose rate of approximately 130 mg/L (as Al), approximately 70 percent of the phosphate was removed. For ferric chloride, it was found that at a dosage of approximately 360 mg/L (as Fe), approximately 75 percent of the phosphate was removed. Although jar-tests are valuable in evaluating metal salt dosing rates they are often not directly comparable to the full-scale environment due to differing conditions (e.g. mixing intensity, reaction time and dewatering efficiency).

This paper presents the results of the full-scale trials using ferric chloride and alum conducted in April and May of 2005 in order to estimate the optimal dose rates and to assess which metal salt is better suited for phosphorus removal at the NEWPCC. An additional goal was to investigate if struvite formation in the centrate could be controlled with the addition of these metal salts.

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