Is Your Study of the Residual Capacity of Aerated Lagoons Up-to-Date?
Water Treatment and Environment, December 10, 2019
The Ministry of the Environment has revised these design criteria for optional aerated ponds. Although this information is not yet available in the “Guide for the Study of Conventional Technologies for the Treatment of Domestic Wastewater”, published by the MELCC, one of the criteria has been revised downwards to offer a better theoretical performance of the ponds during the winter months for the removal of the biochemical oxygen demand (BOD 5 ).
If you have carried out a study of the residual capacity of your aerated ponds since the beginning of the 2000s, you have surely noticed that, although the performance measured by sampling is well below the design criteria, the theoretical verification of the performance of these did not make it possible to achieve the same processing performance. Worse still, several stations, having always respected their discharge requirements, concluded from the calculations that the station had reached its treatment capacity.
To improve the theoretical calculations, the MELCC proposed, since 2015 in the document entitled “Approach for Evaluating the Residual Treatment Capacity of a Wastewater Treatment Plant Exceeding its Design Criteria”, a mathematical approach in order to “calibrate” the calculation coefficients of the aerated ponds in connection with the actual data observed at the station. Given the great variability of the data and operating conditions of the operating stations, this method did not always lead to a favourable result for the municipalities. This method is also limited to 125% exceedance of the design criteria. This solution left several municipalities without the possibility of increasing the connections to its plant, even if the purification performance observed was still below the required discharge criteria.
The MELCC wanted to improve and simplify the correlation between the actual efficiency observed at the pond outlet and the theoretical capacity calculated by the modelling equations. The Teta coefficient of Ekenfelder’s equation was thus revised from 1.07 to 1.04. This modification increases the calculation coefficient Ke, in winter, from 0.099 d-1 to 0.172 d-1. To give you an idea of the extent of this change, in a concrete case of one of our customers, the theoretical BOD5 concentration at the outfall during the winter period went from 35.9 mg/l to 17 .0 mg/l (25 mg/l requirement) for the 30-year tie-in projections. The station has therefore gone from a case where investments of several million were required in the short term so as not to limit development.
So, if your residual capacity studies or master plans for your aerated pond-type treatment plants have not been updated in the last year, you may be planning for unnecessary upgrades, or your home renovations may be sidelined for no good reason. An update is therefore necessary!
François Desjardins, Eng., M.Eng.
Director – Civil and Water Treatment Process