Treatment technologies for recycle liquors :nutrient removal, mass balances, and potential recovery at wastewater treatment plants

Abstract

Recycle liquors (RLs) from anaerobic digesters at wastewater treatment plants (WWTPs) can be a challenge to WWTP operations, but also a source of nutrient recovery. For example, Northumbrian Water Ltd.’s Bran Sands WWTP has historically had elevated NH4-N levels and accumulation of PO4-P within its works, primarily due to the recycling of nutrient-rich RLs from its sludge treatment process to its activated sludge (AS) units. This has led to chronic scaling problems (as struvite), fluctuations in AS process stability, and increased operational costs (i.e. increased maintenance, chemical and energy costs). In order to tackle these issues, this thesis performed a mass balance on NH4-N and PO4-P around the WWTP to understand the material flow across plant operations, which led to various NH4-N and PO4-P treatment and recovery studies. Relative to PO4-P removal and struvite reduction, the addition of MgCl2 (and pH adjustment) were examined to assess their value in removing P and N from the soluble phase (i.e. in RLs) in AD and post-AD units. NH4-N and PO4-P removal/recovery increased as Mg2+ and pH increased when added to both locations. However, NH4-N and PO4-P removal/recovery was most efficient in post-AD applications (i.e. ~49 ± 0.17% and ~83 ± 0.25%, respectively, at pH 8 with 150 mg/L of added Mg2+). A simple cost benefit analysis shows this could potentially offset 83% of the annual cost of dealing with scaling problems on site, and ~£321,000/yr can be generated from the sale of the recovered PO4-P. The AD experiments also showed increase volatile solids removal and biogas production as Mg2+ and pH increased. To further reduce NH4-N in RLs, the thesis investigated the suitability of aerobic, nitrifying granule bioreactors for NH4-N removal. Up to 95% NH4-N removal (mean influent NH4-N = 300 mg/L) was observed at NH4-N loading rate of 0.6 gNH4-N L-1.d-1, which was most efficient when nitrite-oxidising bacteria abundances were comparatively higher in the granules relative to ammonia-oxidizing bacteria. Overall, the work shows that a combination of Mg2+ addition for PO4-P removal and aerobic granule bioreactors for NH4-N removal could reduce nutrient levels in RLs, although work is still needed to translate this potential into actual enhanced NH4-N and PO4-P recovery.