Target Turbidity Limits for Passive Treatment Systems

Richard A. McLaughlin, Ph.D.

Executive Summary

Current erosion and sediment control practices on construction sites have been demonstrated to provide a range of water quality improvements in stormwater discharges, but considerable turbidity still re mains. A number of recent studies are cited which provide some insight to the possibilities of using passive treatment systems (PTS) to introduce flocculants to address the turbidity problem. The use of these systems can pr oduce dramatic reductions in turbidity to levels <200 NTU, as demonstrated for numerous storm events on many sites.
This is 1-2 orders of m agnitude lower than discharges from sites using standard practices alone, such as silt fences and sediment basins. However, PTS have been found to be highly susceptible to dist urbances from construction activities which can greatly reduce their effectiveness. In the studies cited, however, compliance with a turbidity limit wa s not required and so disturbances compromising the PTS were not actively avoided. As was demonstrated at several sites, it is often possibl e to quickly mitigate needed construction
disturbances if the skill and motivation are in place. The following are the main findings to date:

• Erosion and sediment control sys tems need to be designed to include PTS from the beginning.

• Successful PTS requires sufficient tr aining of construction site staff, primarily those involved in grading and utility installation, in PTS functions and problems to avoid. It is likely a person trained in PTS will be needed on staff, or as a contractor.

• In most cases, the key factor in successful PTS use is routing all water from disturbed areas into the water conveyance with PTS installed. Bypass flows of untreated runoff is o ften the cause of turbidity spikes.

• The area immediately around the sedim ent basins needs to be stabilized
at all times so as to not contribute unt reated, turbid runoff into the basin.

• A great deal of diligence will be requi red to avoid spikes above any target turbidity level.

• One suggested strategy for LID projects is to site the sediment basins just outside the construction envelope to avoid compromising the basin functions during construction activities. While this will disturb a slightly larger area (5-10%), the benefits will likely be substantial.

Recent Results: Successful Deployment of PTS

The use of Passive Treatment Systems (PTS) has been proven to significantly reduce turbidity in construction site r unoff on a variety of sites. We haveestablished PTS on construction sites and co llected samples at the outlets using automatic samplers, which collect samples over the course of storm events. The data is fully representative of the storms , as opposed to single samples taken by hand or by single-stage samplers. Only ev ents with at least three samples were
included in this discussion.

One example of successful PTS use wa s in our study of road improvement projects in the North Carolina mountains 1 . These projects involved widening existing gravel roads and paving. On each project, the site was divided into two sections which were hydrologically separated by diversions into culverts, which discharged off the project. The standard practice in the ditch was to place rock
check dams with small excavations in front of them (Fig. 1). Our practice involved fiber check dams (FCD) insta lled at grade (no excavations) with polyacrylamide applied to them (FCD+PA M). Runoff from both systems was
monitored for 9-11 months. The results (Table 1) show two points: 1. standard practices produce high turbidity dischar ges, and 2. with little additional effort, a passive dosing system results in much lower turbidities. The FCD+PAM system is actually less expensive than the standard practices. When the FCD+PAM system is intact and undisturbed, the results are even better (Table 2).

1 McLaughlin, R. A., S. E. King, and G. D. Jennings. 2009. Impr oving construction site runoff with fiber check dams and polyacrylamide. J. Soil and Water Cons. (accepted)

Figure 1. Standard practice (left) and fiber check dam (right). The ditch on the
left was also lined with excelsior matti ng soon after this picture was taken.Table 1. Results of sampling runoff from all storms at two construction sites with two different systems for erosion and sedi ment control. FCD + PAM is fiber
check dams made of coir or straw wattl es with polyacrylamide power applied to the top surface where water passes over them.

Site 1 (27 storms) Site 2 (9 storms) Turbidity (NTU) Standard
Rock FCD + PAM Standard
Rock FCD + PAM

Average 3813 34 867 115

Median 2488 16 308 45 Single Sample High 14768 335 3419 533

Table 2. Turbidity results only from events with FCD + PAM installed and properly functioning for the two constr uction sites shown in Table 1. Average Site 1 (22 storms) Site 2 (7 storms) Number of Samples 12 15 Rainfall (in) 1.06 0.68 Turbidity (NTU) 20 81 Standard Deviation (NTU) 15 28 Maximum (NTU) 50 125

We have several sites where we are current ly working to implement PTS. These are construction sites where the developer has agreed to either let us install our
devices or in some cases assist in the installation. The challenge on these sites is to maintain PTS on a voluntary basis in the face of typical pressures to maintain construction progress and schedules . As a result, we have a mix of successes and failures depending on the activi ty on the site. An example is given in Table 3, which shows turbidity data from seven storm events for which the PTS was intact and functioning. For an additional 9 events, the activities on the site disrupted the PTS, primarily by disturbing areas which did not go through
the ditch treatment of wattl es and PAM. This is an LID development involving three buildings, and due to the small constructi on site footprint much of the site is under constant disturbance by the various contractors involved. This creates challenges to maintaining the PTS and keeping stormwater flowing through stabilized ditches with the treatment syst em. One option to prevent these occurrences would be to temporarily instal l the PTS, including a sediment basin,
out of the construction envel ope and route all stormwater to it. Once the site is stabilized, these areas could be recl aimed or used for post-construction stormwater treatment. The slight enl argement of the fo otprint may be a good tradeoff for reducing impacts on adjacent waters.Table 3. Turbidity in discharges from a sediment basin with fully functional PTS in place.

Date Rainfall (in) Turbidity (NTU)

Average Standard
Deviation Maximum
9/16/2008 1.15 167 93 339
11/4/2008 0.88 43 27 108
11/14/2008 1.17 50 19 98
11/25/2008 0.35 40 22 80
11/30/2008 1.01 37 8 48
12/25/2008 0.34 38 22 116
1/06/2009 0.68 11 12 46 Average 55 29 119

Another site had one storm event with f unctional PTS in place, with an average turbidity of 39 NTU (std. dev = 39) and a maximum of 156 NTU. The wattles were not properly installed and as a re sult the ditch treatment system failed during the next storm event when the wattl es were undercut. Because of scour, the ditches needed reworking to correct the problem, and this was not done for several months before we finally removed our samplers.

Under controlled conditions with simula ted storm flows wit h soil added, the addition of PAM has consistently and significantly reduced outlet turbidity 2

. These tests were conducted at the NC State Sediment and Erosion Control Research and Education Facility ( SECREF) under a variety of conditions, including different outlets, baffle materi als, and basin size. While sediment capture rates were >97%, the turbidity re mained at >350 NTU at the outlet. The only factor which consistently reduced tu rbidity was the additi on of PAM. An example is shown in Table 4, showi ng a reduction of about 700 NTU from the inlet turbidity regardless of outlet or buffer type.

Table 4. Effects of outlet type and bu ffers on turbidity with and without PAM. Sampling Position No PAM* (NTU) PAM** (NTU) Basin In (untreated) 843a 847a
Rock Outlet 758ab 152b

Skimmer Outlet 353c 162b

Grass Buffer 498bc 70b

Forest Buffer na 108b *Average turbidity weighted by volume. Numbers within a column followed by different letters are signific antly different (P < 0.05). **One APS 702c log and two APS 730b logs.2 McLaughlin, R. A. 2002. Sediment and Eros ion Control Practi ces Evaluation. Final Report to North Carolina Sedimentation Control Commission.

Passive and active dosing of pumped, turb id water has also been investigated
under controlled conditions at SECREF 3,4 . We have shown that turbid water can be treated with PAM by either injecting a solution into the water pump intake or by passing the water pump discharge over a solid block of PAM. The injection method is much more controlled, since you can dose at a known concentration, but is relatively simple. Passing the tr eated water through a stilling basin with at
least one baffle is sufficient for removal of the flocculated sediment and retention times can be relatively short. Using these simple systems we constantly achieved turbidity reductions from 200-400 NTU to 40-70 NTU. The alternative of pumping treated water into a geotextile se diment bag is also effective in removing the flocs formed with PAM tr eatment, but the bags tend to clog and need frequent replacement.

3 Bhardwaj, A. K., and R. A. McLaughlin. 2008. Simple polyacrylamide dosing systems for turbidity reduction in st illing basins. Transactions of the ASABE. 51(5): 1653-1662.

4 Bhardwaj, A. K., R. A. McLaughlin, and *D. L. Babcock. 2008. Energy dissipation and chemical treatment to improve st illing basin performance. Transactions of the ASABE. 51(5): 1645-1652.