Environmental Journeys 16: SediMite Part 1 — the concept for an in-situ sediment remediation technology

My work involves diagnosing causes of environmental harm and quantifying health and ecological risks of chemical, physical, and biological stressors. The findings inform decisions about how to correct or reduce actual or potential harm, often by remediation of contaminated sediments. The goals of risk-reduction through remediation include protecting human health and the lives of myriad animals and plants living in the marshes, lakes, rivers, estuaries, and coastal areas.

The clean-up of contaminated sediment has evolved over the past three decades and a number of options can be implemented individually or in combination. These range along a gradient of intervention from “monitored natural recovery” to “­in-situ treatment or enhanced monitored natural recovery” to “capping” to “dredging”. In medical terms these can be thought of as ranging from “bedrest” to “medications” to “dressings” to “surgery”.

While many remedial decisions were initially narrowly focused on removal of contamination, the understanding of what it takes to leave the environment whole while including neighboring communities as part of that outcome has matured. A holistic approach to sediment management involves discussions with the public, trustees, companies, and regulatory agencies. This type of approach focuses on human wellbeing, restoration and reduction of damages to natural resources, while achieving net environmental benefits. For me, this includes selecting remedies that reduce the risks adequately with as little collateral harm or damage as possible. Taking in the totality of the problem, this involves lower impact remedial options such as monitored natural recovery and in-situ treatment whenever possible and reserving higher impact options such as dredging for situations where “surgery” is needed.

That said, all remedial technologies have their place and matching the remediation tool to the situation requires good understanding, communication, and regulatory policy. I am a biologist who has had the good fortune of working symbiotically with engineers throughout my professional life to design remediation plans. That symbiosis and the desire to limit remediation-related collateral harm led to the development of SediMite.

What people remember most about SediMite (trademarked) is the name. In 2006, we celebrated our work to create a low-impact technology to deliver treatment amendments to sediments contaminated with PCBs, PAHs, other organic chemicals, mercury and heavy metals. All of these can pose risks to fish and wildlife and to humans because they bioaccumulate in animal tissues and PCBs and mercury are passed up the food chain.

The new technology we developed involved engineering a system and a formula to produce treatment amendment delivery packages in the form of pellets that could carry powdered activated carbon (PAC) and other fine-grained treatment materials to the sediments. The pellets are “packed” with treatment amendments, that make up about 50% to as much as 85% of the SediMite product.

The novel aspect of this technology is that the pellets are designed to sink through the water column and the fine-grained amendments are released on and within the sediments. Prior methods for introducing finer-particle treatment amendments involved placing the amendments in mats or physically mixing the amendments into the sediments. The photograph below shows one of several older physical mixing technologies used to introduce amendments into sediments.

This was an early physical mixing method for introducing amendments into sediments. Photo from the Hunters Point, San Francisco remediation demonstration with activated carbon carried out by Dick Luthy, Upal Ghosh and others.

The U.S. EPA recognized a need for an alternative way of treating sediments with amendments and funded our effort to develop a new delivery system through a Small Business Innovative Research (SBIR) grant. With that funding and our research, the Menzie-Cura / UMBC team successfully created pellets with the desired mixture of treatment amendments, binders, and weighting agents to sink the pellets through the water and into the sediments. The delivery method of using an agglomerate to introduce PAC and other amendments to sediments was unique and we successfully patented the technology (the UMBC/Menzie patent) What we came up with is shown below.

SediMite pellets are shown in the upper right. The pellets are comprised of powdered activated carbon (thus the black color). The pellets are held together by inert binders and weighted so that when spread on water, they sink to the sediments. The pellets can contain a broad range and mixtures of treatment amendments depending on the contaminants that need treatment.

The name SediMite was arrived at during intellectual discourse and beer at the Cat’s Eye Pub. Situated in Fells Point in Baltimore, this is one of my favorite music establishments. A crowd of us and our friends had gathered for the celebration of our efforts to produce the new technology.

The Cat’s Eye Pub in Fells Point where SediMite was named. Photo credited to the Cats Eye Pub.

Various names for the pellets and technology were discussed. I had suggested the name Mud Scrub because it seemed that was what we had accomplished — a low-impact cost-effective technology that “scrubbed the exposure from the sediment contaminants.” But my suggestion did not impress the others around the table. We went around with different ideas until someone said “Why don’t we call it SediMite, kind of like Vegemite.” I was very fond of the Australian rock group Men at Work and their song Land Down Under with its delivery of a Vegemite sandwich. I was sold on the name SediMite; it embodied sediment (Sedi) and strength (Mite), had three syllables, and a fun origin.

The ideas and conceptual approach

The idea of using powdered activated carbon (PAC) to reduce the bioavailability (i.e., exposure) of PCBs and other hydrophobic chemicals began with the work of Dick Luthy and his then post doc Upal Ghosh. They observed that the availability of chemicals in sediments for exposure to aquatic animals and plants, water, and thus people was reduced when the sediments “naturally” contained small amounts of black carbon including tiny bits of coal and products of incomplete combustion. This early work explained why exposures to aquatic animals were less than predicted in some sediments. Dick and Upal found that the contaminants were preferentially adsorbed to the black carbon particles within the sediments. The following picture shows black carbon particles commonly found in harbor sediments.

This research finding by Upal Ghosh and others led to the idea of black carbon addition as a means of reducing bioavailability of contaminants and associated human health and environmental risks. Note that coke is a solid fuel made by heating coal in the absence of air so that the volatile components are driven off.

Dick and Upal then reasoned that adding a small amount of activated carbon to contaminated sediment would further reduce exposure and thus reduce environmental and health risks. A demonstration project conducted with powdered activated carbon (PAC) for PCB-contaminated sediments at Hunters Point in San Francisco combined with laboratory studies of bioavailability reductions proved that this was the case. They patented the sediment remediation technology of using activated carbon through Stanford (the Stanford Patent).

For the Hunters Point demonstration project, physical mixing with machinery (as shown in the photograph above) was used to work the activated carbon into the sediments. It occurred to me that there might be an alternative to using heavy machinery. I began discussing this idea with Upal Ghosh who had been having similar thoughts. The physical mixing process then in use resulted in sediment disturbance and likely resuspension and possible remobilization of contaminants.

We also recognized that the physical mixing process could impact sensitive biological habitats, especially those supporting aquatic vegetation or marsh grasses. Putting heavy layers of clay and other amendments on top of such areas would also be undesirable in circumstances where habitat preservation was an important consideration because these types of remedies could smother the area. (Capping with and without amendments is a valuable remedial alternative in other circumstances.) We concluded that the ability to deliver a thin layer of pelletized amendments could reduce potential impact associated with the remedy itself and still provide the needed treatment for contaminants in the biologically relevant zone.

The depths of biologically relevant sediment zones vary among water bodies. Thus, the amount of amendment used at a site should be optimized to minimize impact on the environment while achieving the appropriate amendment concentration for treatment. This requires some level of site-specific knowledge which can be obtained through measurement and observation. Important for this idea was figuring out the optimum amount of treatment necessary to adequately reduce exposure to contaminants. We wanted not only to reduce exposures to organisms living in and on the sediments, but also to reduce the transport of contaminants to the water above the sediments, and exposure to fish, wildlife, and humans. How much treatment amendment mixed into the biologically relevant zone was necessary to accomplish that? It turned out it depended mainly on the amount of native organic matter present.

The SediMite delivery process would depend on natural processes for mixing and obviate the need for mixing equipment and thus eliminate that source of sediment disturbance. When I began giving talks about our concept and the SediMite technology, I often used the slide below to acknowledge “the workers” who accomplished this mixing through a process known as bioturbation.

These animals live on or within the sediments. Their movement and feeding activities create bioturbation which is what mixes the surface layer of the sediments. The depth of this zone is commonly used to guide sampling activities and to inform risk assessments. Fine materials such as powdered activated carbon (PAC) are entrained down into the sediments which completes the treatment delivery process.

The third member of our team is Ben Amos. He brings practical knowledge and has a good mind for experimentation. Over the course of 6 months while Ben worked with graduate students at Upal’s lab at UMBC, we collectively produced pellets with the desired consistency and characteristics. Using laboratory aquaria with sediment, we studied how the SediMite pellets behaved and how they released amendment as they became wetted; we then examined how the animals living in the sediment mixed the amendment into the sediment. To that end, we tagged the SediMite pellets with fluorescent particles which visually showed the bioturbation process as the animals living on and within the sediment mixed the treatment amendment through the biologically relevant zone.

Looking through the side of a laboratory aquaria, with water above and darker sediment below. SediMite pellets with fluorescent particles were added to the sediment surface. These released amendments along with fluorescent tracers. As the tracers show, the amendment is mixed into the biologically-relevant sediment zone by the animals in a short period of time. Various passages by worms and other small animals are evident.

This established the proof for the concept for mixing of PAC into sediment by natural processes. The report on this work concluded that the agglomerate represents the first delivery method for remedial materials to sediment that does not require mechanical mixing into sediment. It is, therefore, applicable in areas where current in situ treatment practices are problematic, such as in deep water, through vegetation, or over large areas. The agglomerate can be designed to carry a number of remedial materials to sediment, allowing for in situ treatment of a variety of contaminants.

Part 2 of this story describes the demonstration projects that established the utility of SediMite for remediating contaminated sediments, bringing the technology to a commercial application for contaminated sediment sites, and for addressing a broader range of environmental matters. In Part 2, I acknowledge many of the individuals and companies who have used and/or helped advance the SediMite technology.

Acknowledgement to reviewers

Parts 1 and 2 of the SediMite story benefitted from editorial reviews by people who are scientists specializing in the field as well as friends who are not employed as scientists but provided important insight into the structure and readability of the story. Among these, a special thanks to Sue Kane Driscoll, Betsy Henry, and Jill Brooks. I especially appreciate my colleagues at Exponent and Sediment Solutions.