Keystone Environmental Ltd. Selects Cement-based Solidification/Stabilization
Since the early 1950’s the City of Burnaby, B.C. had leased 3 parcels of land, a total of 1.8 hectares, to a private gun club for use as a rifle range. A few years ago the City made the decision to end the gun club leases and return the target ranges to park use. At this time the sites were investigated for contamination and, as to be expected, considerable metal contamination was found, associated with the bullets and shell casings of over 50 years of target practice. Site investigations showed lead, zinc, copper and antimony, with concentrations in some spots high enough to be designated ‘hazardous waste’ under BC’s Environmental Management Act. Options for remediating the contaminated soil included traditional “dig and dump”, but the costs for that approach was substantial.
The City of Burnaby was seeking a more reasonable approach to site remediation and employed Keystone Environmental Ltd. to complete a risk assessment on the site. Keystone took a unique approach, utilizing B.C.’s risk-based regulations for contaminated soils. For risk-based remediation, the Ministry of the Environment certifies compliance with the regulations if the remediation plan shows that (1) the post-remediated contamination levels are no longer a threat, (2) people, plants or animals have no access to the contamination and (3) via pathways the contamination can not get to people, plants or animals. Using a combination of on-site treatment methods, a cement-based solidification and stabilization solution met those criteria. This involved excavating the most highly contaminated 3,500 cubic metres of soil, treating it with Portland cement powder and securing it within a vault built beneath a new parking lot.
The cement-based solidification/stabilization process contributed to the rehabilitation of a 1.8 hectare rifle range along the north side of Burnaby Mountain into safe, green parkland that can be enjoyed as an oasis in this growing urban setting. This process took only 12-weeks to finalize and compared to the conventional ‘dig-and-dump’ method of meeting provincial regulations for polluted soil, the on-site solution saved the City of Burnaby approximately $1 million. Using on-site Soil Stabilization treatment the rehabilitation and redevelopment of the former target ranges cost less than $1.85 million. The original “dig and dump” approach was budgeted at $2.8 million.
What is Cement-Based S/S?
Solidification/stabilization (S/S) is a widely used treatment for the management and disposal of a broad range of contaminated materials and wastes – particularly those contaminated with substances classified as hazardous in the United States. The treatment involves mixing a binding reagent into the contaminated substance. This process protects human health and the environment by immobilizing contaminants within the treated material, preventing them from migrating to plants, animals and humans.
S/S treatment has been used to treat radioactive wastes since the 1950s and hazardous wastes since the 1970s. S/S continues as a cornerstone treatment technology for the management of radioactive and hazardous wastes, as well as site remediation and Brownfield redevelopment. The U.S. Environmental Protection Agency (EPA) considers S/S an established treatment technology. EPA has selected S/S treatment for 24% of its Superfund source-control remedial actions.
How S/S Works
S/S treatment involves mixing a binding reagent into the contaminated substance. Although the terms solidification and stabilization sound similar, they describe different effects that the binding reagents create to immobilize hazardous constituents. Solidification describes changes in the physical properties of a contaminated substance. The desired changes usually include an increase in compressive strength, a decrease in permeability, and encapsulation of hazardous constituents. Stabilization refers to chemical changes of the hazardous constituents in the treated substance. The desired changes include converting the constituents into a form that is less soluble, mobile, or toxic.
Effects of Binding Reagents on Waste – Inorganic & Organic
Commonly used binding reagents include Portland cement, cement kiln dust (CKD), and a number of proprietary reagents. Portland cement is a generic material principally used in concrete for construction. This material is also a versatile S/S binding reagent with the ability to both solidify and stabilize a wide variety of wastes. Portland cement-based mix designs have been the most popular S/S treatments and have been applied to a greater variety of wastes than any other S/S binding reagent.
Cement is frequently selected for the reagent’s ability to:
- chemically bind free liquids
- reduce the permeability of the waste form
- encapsulate waste particles, surrounding them with an impermeable coating
- chemically fix hazardous constituents by reducing their solubility
- help reduce the toxicity of some contaminants
This is accomplished by bringing about physical changes to the waste form and, often, chemical changes to the hazardous constituents themselves. Cement-based S/S has been used to treat wastes that have either or both inorganic and organic hazardous constituents.
Due to the great variation of waste constituents and media, a mix of reagents should be designed specifically for each waste that is to be treated. Mix designs often include by-products or additives in addition to Portland cement. Fly ash is often used to capitalize on the pozzolanic effect of this material when mixed with hydrating Portland cement. CKD and slag have minor cementitious properties and are sometimes used for economy. Lime can be used to adjust pH or to drive off water utilizing the high heat of hydration produced by these S/S binders.
|Owner||The City of Burnaby|
|Engineer||Keystone Environmental Ltd.|