Phosphorus control in passive wastewater treatment and retention works using water treatment residual solids

Abstract

Water treatment residual solids (WTRS) were characterized and studied as a potential phosphorus (P) adsorbent for application in sewage lagoons and stormwater runoff retention ponds. Three conventional WTRS sludge types (mixed Fe(OH)3-CaCO3, mixed Al(OH)3-CaCO3, and Al(OH)3) were collected from the Saskatoon, Prince Albert, and Buffalo Pound water treatment plants (WTP), respectively. The WTRS were collected in slurry form (i.e. not dried) from WTP clarifiers. Samples were collected during the summer and fall in an effort to observe seasonal effects.

WTRS characterization involved determining selected chemical parameters which included pH, ammonium oxalate-extractable aluminum and iron content, and ammonium acetate-extractable calcium content. The pH of the WTRS samples ranged from approximately 6 to 8. Saskatoon WTRS samples had Feox content in the range of 55.2-55.8 g kg-1 of dry WTRS solid. Prince Albert WTRS samples had Alox in the range of 41.8-46.7 g kg-1 of dry WTRS solid. Buffalo Pound had Alox content in the range of 56.0-67.2 g kg-1 of dry WTRS solid. Saskatoon and Prince Albert WTRS samples had Ca content ranging between 34.3-38.1 g kg-1 of dry WTRS solid due to lime softening. Typically the fall WTRS samples had higher Al, Fe, and Ca content than the summer WTRS samples.

Phosphorus adsorption behaviour and the maximum adsorption capacity of the WTRS were investigated through batch adsorption and settling experiments of WTRS in P-spiked deionized water. The Langmuir isotherm model best described the P adsorption behaviour of the WTRS (R2 = 0.97-1.00 linearized transformed data). The Freundlich isotherm model had not as good a fit with R2 ranging from 0.63 to 0.87 for the WTRS. The summer WTRS samples achieved maximum adsorption capacities (Qmax) in the following order: Buffalo Pound (78.1 mg P/g solid) > Prince Albert (70.4 mg P/g solid) > Saskatoon (7.37 mg P/g solid). The fall WTRS samples resulted in similar Qmax results in the following order: Buffalo Pound (82.0 mg P/g solid) > Prince Albert (70.4 mg P/g solid) > Saskatoon (6.41 mg P/g solid). Seasonal variations appeared to have minor impact on WTRS P adsorption.

Phosphorus removal from sewage lagoons and stormwater runoff retention ponds was examined through batch adsorption and settling experiments of WTRS. Municipal primary wastewater effluent from the Saskatoon wastewater treatment plant (WWTP) was used as a surrogate for lagoon effluent during spring discharge. Stormwater runoff was collected from an agricultural runoff pond outside Saskatoon. Aluminum and iron based WTRS were effective at adsorbing phosphorus from municipal primary wastewater effluent in batch adsorption treatment. WTRS dosages removed P to within 6.4% of their target final P concentrations. However, the WTRS were not effective at adsorbing P from agricultural runoff water. After remixing the settled WTRS and doubling the dosage in the agricultural runoff water the WTRS only removed approximately 20-25% P.

Re-suspension and resettling of WTRS after an initial cycle of P adsorption and settling had negligible effect upon the P concentration in the water column. The WTRS had a negligible effect on the pH of the wastewater solutions at the dosed concentrations. Short term (14 days) desorption of P from the WTRS utilized in P adsorption tests was low, typically less than 2% and reaching as high as 10.6% of the total P adsorbed.

WTRS were found to be an effective P adsorbent from municipal primary wastewater effluent. The WTRS had high adsorption capacities compared to other WTRS and P adsorbents in the literature. The high adsorption capacities of the Al-based WTRS make them more practical than Fe-based WTRS for application.