The Art And Science Of Dredging Water Treatment

(L-R) Amber Wilson and Scott Ponstein

Posted on May 4, 2021

A Summary of a WEDA Presentation Prepared Exclusively for DredgeWire

By Amber Wilson and Scott Ponstein

Infrastructure Alternatives, Inc., or IAI, has provided dredged material processing and water treatment services for environmental dredging projects since 2002, utilizing the principles of sanitary wastewater treatment and residuals management. IAI is an active member of the Western Dredging Association (WEDA), and recently presented “The Art and Science of Dredging Water Treatment” for the 2021 WEDA Midwest Chapter Virtual Meeting. The company has been responsible for the treatment and discharge of 2.6 billion gallons (over 9.8 billion liters) of water produced through dredging.

Key topics of the presentation included the circumstances that may require water treatment as part of a dredging project, an overview of treatment processes and equipment, treated water discharge and permits, and ended with three case studies of past water treatment work performed by IAI.

Water treatment often becomes part of a dredging project when contamination is present in the sediment to be dredged, if chemicals will be used in a process to treat the dredged material, or if contact water will be discharged to an environmentally sensitive or protected water body.Sources of water to be treated may include top water from barges transporting dredged material, weir overflow from a CDF, drainage from stockpiled dredge material, storm water collecting in a dredged material handling area, or water produced by a dredged material dewatering process.

Types of Treatment Processes

One way to think about water treatment processes is to classify them based on what they remove from the flow of water. There are processes to remove solids, which are generally found first in a treatment scheme. Solids removal processes are critically important on an environmental dredging project because the contaminants that you are addressing through dredging are generally either chemically-bound, or just have a strong affinity for, solid particles in the sediment. That’s why dredging is an effective means of removing certain types of contamination from a water body, and why solids removal processes are so important in a water treatment system for a dredging project. The solids are often where the majority of contamination is going to be found. Clarification, with or without a chemical aid, and filtration are two effective methods of solids removal.  Clarification is slowing the flow of water to allow gravity to pull solids to the bottom, and filtration is physically straining water through a media, and capturing solids in the media.  There are many variations of filtration, and different types of filter processes may be used in series to capture progressively smaller and smaller solids.

A second category of treatment processes are those targeted to remove dissolved contaminants. Those type of processes are typically found at the end of a treatment scheme, following processes to remove particulates. Three common examples of removing dissolved contaminants are: precipitation by pH adjustment, activated carbon adsorption and ion exchange.  Precipitation by pH adjustment involves changing the pH of the water using a chemical, causing inorganic contaminants to become insoluble, and therefore able to be settled or filtered out.  A good example of dissolved inorganics that can be removed this way is metals. Adsorption and ion exchange processes also target dissolved contaminants, but in a different way; adsorption and ion exchange remove dissolved organics such as PCBs, PAHs, PFAS, etc. In those processes, water is passed through a specialized media, like activated carbon, that attracts and binds dissolved contaminants to itself. Adsorption and ion exchange require contact between active sites on the media and the dissolved contaminant in the flow of water, so it is very important to remove all solids, or as much as practical, ahead of these processes. Excessive solids in the water can blind the media and render such a process ineffective – another reason why up-front solids removal is so critical.

Variations on System Installations

Water treatment systems for dredging projects can be constructed in a semi-permanent or temporary fashion, dependent upon project length and scale. There may be certain site or project conditions for which a semi-permanent plant makes most sense – for example, a multi-year, large-scale project; but most projects can be well-served by a temporary installation. Temporary installations have lower capital and maintenance costs, they are easier to modify or expand, if needed, and they produce less waste when the project is complete.

Discharge Modes

There two modes of discharge from a water treatment system: batch mode and continuous discharge. For dredging projects, a combination of the two modes is typically utilized.  In batch mode, the contractor proceeds in a step-wise fashion: first, water is treated; second, the treated water is stored; third, the treated water is tested to demonstrate compliance with discharge requirements; and fourth, the water is discharged. In batch mode, each step must be completed before the next can begin.In a continuous discharge mode, water is treated and discharged simultaneously, in a flow-through process.  Water is tested periodically as it is discharged, to verify permit compliance.We have found that batch discharge is well suited to start up or initial operations, and then transitioning to a continuous discharge after the start up period is most efficient and allows the water treatment system to support much higher dredge production.

Discharge Permits

Whatever type of treatment system is utilized, a contractor must have some kind of authorization to discharge, issued by regulatory authorities. Usually, that’s a discharge permit.  Three typical types of discharge permits utilized on dredging projects are a National Pollutant Discharge Elimination System (NPDES) General Permit, NPDES Individual Permit, or an Industrial Pretreatment Permit (IPP).  NPDES permits are for discharge to surface water and IPP are for discharge to sanitary sewer.Just like it sounds, a general NPDES permit is written for a certain category of work, such as dredging, and it contains general requirements, not specific to the project at hand.  An individual NPDES permit takes longer to obtain, it has more strenuous application process, but it is also specific to the project at hand, and in some scenarios, it is preferable to have that specific permit that takes into account the unique characteristics of the particular project.  An IPP can be advantageous because it’s considered pretreatment; you’re discharging to sanitary sewer and that water will be treated again at a municipal wastewater facility.  That can mean that discharge requirements are less stringent. However, the IPP typically is most restrictive in terms of flow rates, and may not accommodate the flow volume produced by a large-scale or high-production dredging effort.

No matter what type of permit is ultimately issued for a project, it will establish discharge monitoring requirements (sampling and analysis), reporting requirements and schedules. The permit will define the outfall location and possibly, structure, and it will tell you whether or not an Operator of Record is required, and what qualifications are required.

Residuals Management

As particulates and other contaminants are removed from the flow of water, they don’t disappear – typically, they are settled out in light sludges, or physically or chemically trapped in filter media. Residuals can be difficult to treat or otherwise handle, and it is best to limit the volume of residuals created as much as possible. A plan must be worked out for collecting and disposing of all treatment residuals, at least once prior to demobilization of a treatment system.

Operations and Maintenance

An experienced operator is required to properly run any water treatment system. The operator must have a thorough understanding of the treated water discharge criteria, i.e., permit limitations, the design and capacity of the treatment system, and the chemical and physical processes utilized. Treatment systems are not plug and play – particularly in a continuous discharge mode, or on a dredging project where conditions may change suddenly. Any treatment system requires constant attention and adjustment to perform optimally. Some of the tasks an operator may be required to perform include: balancing flow through the system; jar testing for appropriate chemical addition; process control and permit-required sampling and analysis; equipment maintenance, including backwashing and bag filter change-out; and recordkeeping.

Communication

In our experience, it is critically important to foster strong relationships and communicate regularly with all contractors involved on a project, as well as with the regulatory agencies and analytical laboratories.Each stakeholder has their own area of expertise and particular set of deliverables and constraints. Having open communication among all parties ensures efforts are coordinated and allows the opportunity to address issues early-on, before they become problems.

Case Studies

We’ve selected three dredging water treatment projects that we feel are unique and provide valuable lessons for future work. On these particular projects, IAI provided water treatment services as a subcontractor, and other teammates were responsible for dredging and compliance monitoring.

Case Study #1 – Water Treatment for Hydraulic Dredging Project

Water flows over the weirs of IAI’s inclined plate clarifier. In this unit, suspended solids in the water settle to the bottom of the clarifier, and clearer water flows over the weirs and on to the next treatment step.

IAI’s water treatment system is shown, with the river behind. The treatment system included a retention pond (not shown), inclined plate clarifiers, multi-media filters, and bag filters. The system focused on suspended solids removal and pH adjustment to remove metals.

IAI designed, installed and operated a temporary water treatment system on this site, capable of processing 3,500 gallons of water per minute, from a hydraulic dredge removed lead-impacted sediment from brackish water. The dredge slurry was dewatered in geotextile tubes, and the water released from the tubes, plus any storm water that collected on the pad, was treated. The project was issued an individual NPDES permit by the state environmental agency. One unique feature of this project was there were two discharge limits for lead in the treated water: a daily maximum concentration limit and a daily mass loading limit. We realized that the mass loading limit was much more restrictive than the concentration limit, and that we had to achieve a much lower concentration of lead than the concentration limit implied, in order to meet the loading limit. Had we not done the math, and took the time to carefully review the discharge permit, we might have violated our permit before it became clear. Another challenge we faced at this site was a strong hydrogen sulfide (rotten egg) odor in the dredged material. We were able to add a chemical to the dredge slurry prior to the dewatering process which chemically bound the sulfides and eliminated the odor issue. A total of 101.5 million gallons were treated over the course of this project, with an excellent record of compliance with discharge permit limitations.

Case Study #2 – Water Pretreatment for Mechanical Dredging Project

Black bag filter canisters are staged next to larger, gray activated carbon vessels. An enclosed trailer in the background contains more process units, comprising a second treatment train.

The system IAI designed, installed and operated for this project processed contact water from mechanical dredging operations, ultimately treating 13.5 million gallons and achieving a near-perfect record of compliance with discharge criteria. The dredged material was impacted with PCBs, and the discharge limit was virtually zero (non-detectable by laboratory analysis). This meant there was very little room for error, and solids needed to be aggressively removed early in the treatment system, so that the activated carbon process utilized to remove the PCBs, could be most effective.Geotextile tubes were used as the first process in the treatment system, to rough polish the contact water prior to mechanical treatment units, which included: multi-media filters, bag filters and granular activated carbon.

Geotextile tubes were used as aninitial filtration process, ahead of the mechanical treatment units in this system, to reduce the solids loading on the rest of the plant. This photo shows a geotextile tube being filled with dredged material contact water, as clearer water sheds off the outside of the tube.

Case Study #3 – Water Treatment for Project with Mechanical and Hydraulic Dredging Phases

The last step in this treatment system was activated carbon adsorption, to remove dissolved contaminants. The carbon vessels are shown on the right of this photo, with the treated water discharging into the creek via an energy-dissipating diffuser, on the left of the photo.

This aerial view of the dredged material dewatering pad shows stacked geotextile tubes, filtrate collection pond, and mechanical water treatment process units.

This photo takes samples collected at the site, from hydraulic dredge slurry on the far left, to final treated water discharge on the far right. The jars between show samples of the water after each treatment step, getting progressively clearer.

IAI utilized two separate water treatment systems for the mechanical and hydraulic dredging phases of this project; each system had an identical process flow design, but the system for the hydraulic dredging phase had a much larger capacity. The contaminant of concern at this site was again PCBs, and the discharge limitation was again virtually zero (non-detectable by laboratory analysis). The discharge authorization issued for this project was a Wisconsin Pollutant Discharge Elimination System (WPDES) permit equivalency, and also included limits for total suspended solids (TSS) and oil/grease. Efficient solids removal was critical to ensuring compliance with both the TSS and PCBs limitations. 232.9 million gallons were treated in total, with no permit equivalency discharge limit violations recorded.

Summary

Four main points that we hope you’ll take away from this article:

  1. There is both an art and science to water treatment. By “art,” we mean the feel for operating a system that a person can only really gain by experience. By “science”, we mean designing a proven system based on the contaminants of concern and the specific permit limitations issued.
  2. The ability to modify a treatment scheme to meet changing conditions is very important to success on a dredging project.  The system must not only keep pace with dredge production and water flow, but also maintain permit compliance, which is so critical to high-profile environmental projects.  Influent characteristics can change quickly and drastically, and the operator must be agile and able to adapt on-the-fly.
  3. Solids removal processes are absolutely key to meeting permit limits.
  4. And lastly, the operator must know their permits and their requirements very well, understanding that permits can be complicated and requirements can pile up.  Achieving and maintaining compliance requires constant effort and attention to detail.

About the Authors

Amber Wilson is a licensed wastewater treatment operator in the state of Michigan, and has been with IAI for 18 years. She has supported environmental dredging projects throughout the Great Lakes region and eastern U.S., helping to develop work plans and submittals for water treatment systems.

Scott Ponstein has 11 years of experience, managing environmental dredging, dewatering and water treatment projects and serving as Lead Estimator for IAI. He has been instrumental in the design of water treatment systems for numerous sediment remediation projects in Great Lakes Areas of Concern.

DredgeWire thanks Wilson and Ponstein and IAI, and WEDA, for this useful paper.

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