Two Decades Constructed Wetland Experience in Treating Municipal Effluent for Power Plant Cooling at the Shand Power Station, SaskPower Part V: The Effect of Seasonal Changes in Temperature, Rainfall

Emmanuel K. Quagraine

 

ABSTRACT

The paper is partof publication series on 2-decade constructed wetland (CW) operation at SaskPower’s Shand Power Station. It highlights influence of some climatological factors and PO43--P load on itsremoval efficiency. Influent PO43--P was influenced by temperature and rainfall in concentration-dependent manner. Therespective effects were estimated as ~3.3% PO43--P reductionof the (at 0oC) background concentrationsper degree rise and 0.7% NO3-N reduction of (at zero rainfall) background concentrationsper mm depth of rainfall.Rainfalleffect in reducing PO43--P is attributed to dilution and was typically noticed after one-month lag period.Its immediate impact was usually adverse leading to ~2.3% increase of the WWTP output PO43--P (at zero rainfall) and attributed to releasesfrom sediment perturbation. Seasonal variation of influent PO43--P loadsubsequently affected effectiveness in its removalby the CW. Regression analysis was used to estimate influent PO43--P, temperature and rainfalleffects on monthly removal efficiency of PO43--P. Temperature was the most consistent and statistically significant influencing factor (at least, at 80% confidence level) across the years, causing release of PO43--P. Its magnitude of effect was shown in either of two main ways: primarily as 3.4 ± 0.9% or otherwise as10.9 ±0.3% PO43--P release (over the inlet concentration) per degree rise in temperature. Rainfall effect was erratic both in direction (positive or negative) and inmagnitude (extent) of influence. The influent concentration effect was consistent in direction (net removal) butvariable in magnitude suggesting co-dependence on other variables. On the whole, exceptthe first spring season where effluent PO43--P of only 0.09 mg/L was displayed with ~99% removal, the CW was incapable of producing effluent PO43--P of the ≤0.33 mg/L required toprevent Ca3(PO4)2 scale formation during condenser cooling process.

Keywords: Wetlands; wastewater; temperature; rainfall; phosphate; cooling; scaling; regression

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doi:10.11912/jws.2018.8.1.1-24