Previous PSTA Research Sample Clauses

Previous PSTA Research. The original research on PSTA was initiated at a time when the expected outflow TP target concentration from the technology would be 10 µg/L. While the current WQBEL target for the STAs is slightly higher (outflow average of 13 µg/L TP three out of five years), it is clear that to achieve an annual mean value of 13 µg/L, the wetland must be capable of providing sub-13 µg/L outflow levels for a good portion of the operational period. This concentration range appears to be outside the long-term capability of SAV cells established on previously farmed lands (▇▇▇▇▇▇ and ▇▇▇▇▇▇, 2011). A key question, when reviewing results from previous PSTA platforms, is “Does the establishment of a limerock or shell rock substrate in the treatment wetland provide a strong likelihood of attaining annual mean outflow TP levels of 13 µg/L and lower?” Prior to the establishment of the STA-3/4 PSTA Cell, numerous investigations were performed at mesocosm, test cell and field scale using a number of inert substrates (e.g., limerock, shell rock and acid-rinsed sands), and these results are described below. Eighteen mesocosm-scale PSTA treatments have been investigated at STA-1W, 14 of which contained an “inert” substrate, such as sand or limerock. Of these, only three achieved average outflow concentrations below 13 µg/L: a very shallow raceway (9 cm water depth) using local limerock reduced inflow levels of 18 µg/L to 10 µg/L over an 18-month period (▇▇▇▇▇▇ et al., 2004); a slightly deeper treatment (30 cm water depth) with an acid-rinsed sand substrate reduced inflow TPs of 19 µg/L to 11 µg/L (over 12 months) (CH2M-Hill 2003), and; a 60 cm depth shell rock substrate treatment reduced inflows from 19 µg/L to 13 µg/L over 12 months (CH2M-Hill 2003). Two STA-1E mesocosm treatments, one with a 12” Ft. ▇▇▇▇▇▇▇▇ limestone substrate and one with 6” of local limestone over 6” sand, performed relatively well, reducing inflow TP levels of ~25 µg/L to 13 and 9 µg/L, respectively, over 27 months. It is unclear why the ~13 µg/L TP “target” was achieved in only 21% (5 of 14) of the “inert substrate” mesocosm treatments, but it indeed demonstrates that the presence of limerock does not guarantee that ultra-low P levels will be achieved.