Operational Impact of Phosphorus Removal Using Ferric Chloride on Anaerobic Digesters and Dewatering

Wastewater treatment plants operating an anaerobic digester typically encounter operational problems i.e. struvite formation and corrosion of several equipment. Winnipeg's North End Water Pollution Control Centre (NEWPCC) is no exception, experiencing frequent and excessive struvite formation on the centrate transfer pipes mainly because of high concentration of phosphorus and severe corrosion in its biogas-fuelled boilers because of high concentration of hydrogen sulphide (H2S) gas in biogas. In May 2007, a ferric chloride dosing system was installed to meet the interim effluent phosphorus license requirement stipulated by a regulator. Ferric chloride (FeCl3) was chosen over other metal salts because in addition to being able to precipitate phosphorus from the centrate stream, it also provides other benefits, including reduced H2S production in the digester gas. The selected dosing locations were upstream of the digesters, upstream of the centrifuges, and into the centrate itself. Currently the City is dosing upstream of the digesters at 55 L/hr and to the centrifuges at 50 L/hr. This dose rate corresponds to iron (Fe) to phosphorus (P) ratio of 3.5 to 1, or 11 kg Fe per tonne of dry solids, which is much less than those rates found in other plants dosing FeCl3 to digesters. The NEWPCC is meeting the effluent phosphorus license requirement, and FeCl3 addition to the digesters and centrifuges has eliminated the high cost of maintenance and repair associated with struvite formation in the pipes and the effects of H2S in the digester gas on the boilers.