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 remains. 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 produce dramatic reductions in turbidity 

to levels <200 NTU, as demonstrated for numerous storm events on many sites. 

This is 1-2 orders of magnitude 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 disturbances from construction activities 

which can greatly reduce their effectiveness. In the studies cited, however, 

compliance with a turbidity limit was not required and so disturbances 

compromising the PTS were not actively avoided. As was demonstrated at 

several sites, it is often possible 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 systems need to be designed to include 

PTS from the beginning. 

• Successful PTS requires sufficient training 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 often the cause of turbidity spikes. 

• The area immediately around the sediment basins needs to be stabilized 

at all times so as to not contribute untreated, turbid runoff into the basin. 

• A great deal of diligence will be required 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 runoff on a variety of sites. We have 
established PTS on construction sites and collected 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 events with at least three samples were 

included in this discussion. 

One example of successful PTS use was in our study of road improvement 

projects in the North Carolina mountains1. 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) installed at grade (no excavations) with 

polyacrylamide applied to them (FCD+PAM). Runoff from both systems was 

monitored for 9-11 months. The results (Table 1) show two points: 1. standard 

practices produce high turbidity discharges, 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. Improving 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 matting 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 sediment control. FCD + PAM is fiber 

check dams made of coir or straw wattles 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 construction 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 currently 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 activity 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 wattles and PAM. This is an LID development involving 

three buildings, and due to the small construction 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 system. One option to prevent these 

occurrences would be to temporarily install the PTS, including a sediment basin, 

out of the construction envelope and route all stormwater to it. Once the site is 

stabilized, these areas could be reclaimed or used for post-construction 

stormwater treatment. The slight enlargement of the footprint 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 functional 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 result the ditch treatment system failed 

during the next storm event when the wattles 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 simulated storm flows with soil added, the 

addition of PAM has consistently and significantly reduced outlet turbidity2

. 

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 materials, and basin size. While sediment 

capture rates were >97%, the turbidity remained at >350 NTU at the outlet. The 

only factor which consistently reduced turbidity was the addition of PAM. An 

example is shown in Table 4, showing a reduction of about 700 NTU from the 

inlet turbidity regardless of outlet or buffer type. 

Table 4. Effects of outlet type and buffers 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 significantly different (P < 0.05). 

**One APS 702c log and two APS 730b logs. 
2

McLaughlin, R. A. 2002. Sediment and Erosion Control Practices Evaluation. 

Final Report to North Carolina Sedimentation Control Commission. 

Passive and active dosing of pumped, turbid water has also been investigated 

under controlled conditions at SECREF3,4. 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 treated 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 sediment bag is also effective in 

removing the flocs formed with PAM treatment, 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 stilling 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 stilling basin performance. Transactions 

of the ASABE. 51(5): 1645-1652.