Phosphorous Release and SCVFA production from non-BNR co-thickened sludge using short term fermentation

Phosphorous (P) recovery from wastewater sludge is being researched for the potential to recover P, a valuable resource, in a reusable form. At the North End and South End Water Pollution Control Centres (NEWPCC and SEWPCC) in Winnipeg, concentration of total P in the co-thickened sludge is approximately 400 mg/L. Due to the addition of FeCl3 prior to anaerobic digestion and again before mechanical dewatering the phosphates are strongly bound to iron in the sludge, which makes them unavailable for recovery (Donnert et al., 1999; Strickland, 1999). Recent research involving short term fermentation of primary and secondary sludge for VFA production, demonstrated that P is released during early stages of acid fermentation (Yuan et al., 2010; Tremblay et al., 2005). In addition to P, ammonia is also generated in high concentrations due to the anaerobic metabolism of proteins in the sludge (Rabinowitz et al., 2011). Controlled struvite precipitation is thought to be the most promising way to recover P in a useable form (Jaffer, 2002). Fluidized bed struvite precipitation processes have been shown to successfully remove 70% of P from digester supernatant at PO4 concentrations as low as 40 mg P/L and achieve up to 90% removal at PO4 concentrations of 70 mg P/L with sufficient Mg+ addition (Britton et al., 2005). Short term batch fermentation of the cothickened sludge, harvested from the outlet of the primary clarifier before FeCl3 dosing at the NEWPCC has shown significant P release, with soluble P levels in the fermented sludge between 100 – 200 mg P/L (depending on influent sludge characteristics) after 2 days in the batch fermenter.Additionally, short chain volatile fatty acid (SCVFA) production was measured during the fermentation experiments to determine if there is potential from this process to produce an easily degradable carbon source for other biological processes, such as sidestream centrate treatment in the case of the NEWPCC. It was found that in a semi-continuous batch fermenter, the co-thickened HPO sludge produced on average concentrations of 6200 mg VFA/L and 8300 mg VFA/L from the semi-continuous fermenters with 2d and 3d HRT respectively.