Factors affecting phosphorus release from waste activated sludge
Thesis DisciplineCivil and Natural Resources Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Waste activated sludge (WAS) derived from municipal wastewater treatment plant contains relatively high amounts of P and is a potential source of recoverable P. Phosphorus release into the aqueous phase is an important step for its recovery. The thesis investigated key questions related to engineering systems for P release from WAS using batch experiments.
Aqueous P forms measured were Dissolved Reactive (Ortho) P (DRP) and Total Dissolved P (TDP). An extraction protocol was used to find five solid P fractions: (1) Total P (TP), (2) Inorganic P (IP), (3) Organic P (OP), (4) Non-Apatite Inorganic P (NAIP) and (5) Apatite P (AP). In all cases, the phosphate concentration of extracted solutions was determined spectrophotometrically using a HACH DR/2500 spectrophotometer.
Samples were placed in air-tight, 20 litre plastic containers, transported to the laboratory, and analysed for initial parameters of WAS within one hour of collection. In the same day, any treatments (e.g., pH adjustment with acids and base, incubation different temperatures and maintaining aerobic/anaerobic condition) were applied and the P release tests started.
Five replicate reactors were run for 11 days to test the reliability of results. The coefficient of variation (standard deviation/mean) of the DRP and TDP of the five replicates over the eight sampling days averaged approximately 5%. The solid fractionation was conducted for four replicates of biosolids. The coefficient of variations were all 10% or less, showing high replicability.
P release was found to be increased by use of (1) anaerobic rather than aerobic conditions, (2) temperature of 35 °C rather than ambient temperature, and (3) a pH of 4 to 6 rather than unmodified pH of near 7. pH depression to 6 was sufficient to release NAIP. A lower pH of 4 released effectively all AP. Organic P was found to be more difficult to release from WAS under the range of conditions tested.
P release under favourable conditions (pH4, 35 °C and anaerobic) was studied for WAS from three treatment plants with different designs: activated sludge (AS), carousel biological nutrient removal (CBNR), and sequencing batch nutrient removal (SBNR). The varying levels of P release from sludge solids can be explained by differences in the P fractionation of the WAS, which in turn arise from differences between wastewater treatment processes. Under favourable conditions, total P release from the AS, CBNR, and SBNR sludge solids were 52, 75 and 48 %, respectively, in 21 days, with the CBNR and SBNR releases the most rapid.
It was observed that both inorganic and organic acids effectively released phosphorus from WAS samples. No evidence was found that acetic acid is better than hydrochloric acid for P release. Visual identification of poly-P in staining test results indicated that poly-P was present and not greatly affected by the treatments tested. Testing with glutaraldehyde indicated that P release was not from cell lysis and indicated a biological process was involved in P release.
A series of preliminary tests considered broader issues related to any future development of P release reactors. There was no large adverse effect of using hydrochloric acid on settleability and filterability. COD increase was observed after acid addition, which could improve subsequent gas production. Zn was the only trace metal found to increase significantly during P release treatment. Fe, Ca, Mg also increased, which could have implications for P recovery depending on what process is used.
The findings of this thesis will benefit future work to design reactors to optimise P release from WAS, and provide increased knowledge for the further development of P recovery technology.