Turbidity Control and the Concentration of Suspended Particles

Turbidity is the cloudiness of a solution. Turbidity control is directly related to the concentration of suspended particles. A nephelometer is used to measure turbidity, and the unit of measure is the nephelometric turbidity unit (ntu). More particles in a sample of water correspond to a higher ntu value.

Turbidity varies greatly and is commonly measured from 0 to 5,000 ntu. Typical bottled drinking water may have an ntu measurement in the single digits. Muddy water usually contains above 1,000 ntu.

While turbidity itself is not typically regulated, it indicates the overall cleanliness of water, and may indicate the presence of suspended contaminants. This field-friendly measurement indicates the presence of any of a number of contaminants coloring a sample. Turbidity measurements from different locations within a treatment stream can help track improved water quality as water moves through the treatment process.

Turbidity control and reduction may be the goal of treatment, or it may be a pretreatment step within a more complex treatment system. Reducing turbidity ensures final water streams do not contain too much total suspended solids (TSS). These suspended contaminants may damage downstream equipment or render downstream treatment processes ineffective. Many treatment technologies function best at levels below 10 ntu.

Various approaches are used to keep turbidity levels low. Four of these approaches are (in order of complexity):
1. Prevention
2. Sedimentation
3. Filtration
4. Coagulation and flocculation

The simplest method to control turbidity is to reduce the dirt or contaminants allowed to enter a body of water. This starts with selecting or building a well-designed holding area before water is initially collected. Shore conditions should be considered. Silt fences or other erosion control prevents adding turbidity-increasing contaminants to a water body. Rust prevention and proper maintenance in standing facilities also helps control turbidity. Selecting and evaluating holding bodies typically occurs before actual water treatment.

When treatment of a given body of water is needed, appropriate water quality controls should be considered as water is drawn into a system. Influent pump placement and selection and suction requirements are commonly matched to a facility’s holding water body to avoid adding more turbidity. Inlet screens and strainers may be incorporated into treatment system influent hoses and manifolds, as well.

To remove turbidity already present in a contaminated stream, sedimentation may be a primary treatment. This involves physical settling of particles within a holding tank. The effectiveness of sedimentation depends on the nature of the particles, the turbulence of the water, and the allowed for settling to occur. Turbulence is related to system flowrate, and settlement time is related to the available holding space. Often, sedimentation is done by incorporating weir tanks, frac tanks, or other readily available holding tanks. Unaided, one frac tank can successfully settle out 25-micron particles at flow rates of up to 100 gpm.

While sedimentation remains a low-maintenance treatment step well-suited to systems operating at low flow rates, filtration can augment a system’s turbidity control. Sand and bag filters are usually easy to incorporate into a treatment system. Flowrate and particle size are important considerations in choosing whether to use a sand filter or a bag filter, or both.

Sand filters are typically filled with silica sand and gravel matched to each internal distribution system. These filters typically remove solids with diameters of 25 microns or more. These filters. Effectiveness is a function of filter bed size and system flowrate. Flowrate increases as filter diameter increases. Sand filters handle significantly higher flows than sedimentation alone.

In addition to sand filters, bag filters may be added to a treatment stream. Filter pores are various sizes – those ProAct typically uses include 10, 5, 1, and 0.5-micron nominal filters. Bag filters can operate at flow rates of 50 to 100 gpm per bag. These filters are simple to use and very effective at removing solids.

In addition to sedimentation and filtration, turbidity can be controlled by adding flocculants. Polyacrylamide (PAM) flocculant logs or alum-based passive treatment add weight to suspended solids to increase their settling propensity. Adding coagulants and flocculants to an active treatment stream allows precise targeting of turbidity, as injection rates can be fine-tuned to specific influent turbidity levels.

Field measurements allow the user to assess the effectiveness of each turbidity control process. Properly calibrated field nephelometers may be used. Many systems incorporate in-line detectors and automated system response.

Turbidity control is important for certain downstream treatment processes.

Adsorbing media can be clogged by suspended particles. Granular activated carbon (GAC) or zeolite treatment processes’ function depends on pore availability in the media type. Pre-treatment and control of turbidity levels prevents premature media clogging, effectively extending the lifespan, efficacy, and treatment abilities of both GAC and zeolite.

Dissolved iron-targeting media is inhibited by muddy or otherwise dirty water. Heavy metal treatment processes benefit from removing suspended metals that may be contributing to the turbidity. Reducing suspended solids reduces the load on exchange resins. As each media vessel’s influent ntu is lowered, the likelihood that active cation or anionic site are inhibited decreases, thereby allowing each exchange type to function properly and efficiently.

Turbidity control is common within a range of industrial settings:

During construction activities in areas with high water tables, turbid waters are commonly encountered during excavation or rainfall events. In order to prevent discharge of muddy water to surrounding water bodies, turbidity must be monitored and reduced.

Pipeline water treatment systems also benefit highly from turbidity control and reduction. The combination of commonly-used tracer dyes and residual rust buildup result in increasingly turbid water passed through a given pipeline. Removal of these contaminants returns the water to a clearer, cleaner state ready for reuse or discharge to the environment.

ProAct maintains a fleet of units readily available for automatic or manual monitoring and reduction of turbidity. ProAct can design and build the most effective system to meet your turbidity control needs. ProAct’s experienced technicians can operate your system to maximize the treatment effectiveness. Call a ProAct specialist today to discuss how best to meet your turbidity control needs.