Previous PSTA Research Sample Clauses

Previous PSTA Research. Modification of the benthic surface (muck removal or limerock addition) to support periphyton growth is expected to be the costliest aspect of construction of a full-scale PSTA facility. Many of the early PSTA studies were designed to examine the effects of substrate character on outflow TP concentration. This points to a key knowledge gap: 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? The previous studies that we examined were conducted across a variety of scales (2 m2 - 40 ha) and tested substrates including muck, sand, local limerock and shell rock, and imported limerock. Across all studies by all investigators, muck-free, calcitic substrates produced long-term (i.e., life of the project) outflow concentrations at or below 13 µg/L in only 8 of 20 treatment groups. Of course, this general assessment pools studies using diverse inflow waters, hydraulic loading rates and operating depths, but it does indicate that the removal of muck alone does not guarantee acceptable performance. The prior PSTA investigations (not including those with the STA-3/4 PSTA Cell) also produced other useful information, particularly by characterizing construction challenges (the importance of elevation control during construction) and operational issues (seepage and flow quantification, types of macrophyte and periphyton) pertaining to PSTA community development.

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 (Xxxxxx and XxXxxx, 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 (XxXxxx 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. Xxxxxxxx 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.