Heavy metal & hydrocarbon contaminated sludge


Courtesy of Soilutions Ltd

Solidification of Heavy Metal & Hydrocarbon Contaminated Sewage Sludge

Client: North Lanarkshire Council

Project Managers: Jacobs

Challenge: The former sewage treatment works were decommissioned in the 1970’s. It is thought that during the closure process, demolition material originating from the site’s operational buildings was placed into five digester tanks in an attempt to backfill them.

The tanks measured 12.2m in diameter and were 9m deep from ground level. The top three tanks, which were the most easily accessible, had double walls with a cavity void, the size of which was unknown at the outset of the contract. The lower two tanks were less accessible, but with limited storage and working space.

The material: In total the five tanks contained approximately 4,700 m3 of material with a chemical profile resembling that of a typical sewage sludge with elevated concentrations of hydrocarbons, zinc and rich in organics. Unfortunately, during a second round of sampling, the sludge and demolition material were also found to contain high quantities of bonded asbestos.

The solution: This presence of asbestos prevented the material from being simply land farmed or disposed of to a sewage treatment works and the high levels of organics would normally prohibit the material from being disposed of into a suitably licensed landfill site. Some lateral thinking and discussions with SEPA were required to ensure that the Best Environmental Technique could be found. To ensure that this exercise was carried out properly several options needed to be explored in depth to ascertain their validity and merit.

The first option considered was that of using a thermal pre-treatment, to remove the high moisture content and organics, followed by a simple solidification process to bind up the resultant material which in turn would allow for its subsequent disposal into an asbestos cell.

The second option was to use a drying solidification technique of either adding binders or applying centrifuges/belt presses to remove excess moisture to allow for its subsequent disposal into an asbestos cell with prior agreement with SEPA derogating the organic content.

A third option considered was to solidify the material with a cementitious binding agent to allow for the material’s retention and reuse as backfill for the tanks.

While the third option had very good BET merits, the complexities of waste regulations and the technical challenges of ensuring the bound material’s future strength as a suitable backfilling aggregate negated this option as a worthwhile solution.

Option 1 was then appraised against the merits and varying costs of option 2. During dialogue with SEPA BET, logistics and commonality of the problem were discussed. It would not have been viable to conduct a full BET appraisal on these two options due to their underlying complexities of issues such as the heat source for the thermal treatment versus the energy requirement for processing cement. However, through using reasoned assumptions in conjunction with consideration of real time rates, option 2, with the use of a cementitious binding agent, was seen as the BET, the most economic and the one with the least risk whilst considering the unknown nature and quantity of the demolition material and asbestos contained therein. SEPA granted approval of the risk-based approach based on disposal ot the organically rich material into a discrete asbestos cell providing it was treated sufficiently to transform it from a liquid to a solid state.

Contractual issues had delayed the proposed start date by nearly six months, with the unfortunate consequence that the proposed blending plant was no longer available. However, during the option appraisal process this eventuality had been factored in; and the site-specific working plan, submitted to SPEA under the requirements of Soilutions Ltd’s Mobile Plant License, had allowed for mixing the sludge in the tanks not only though the use of a blending plant but also by the use of a long reach excavator.

The solidification process involved placing a known volume of material into an emptied tank together with known amounts of cementitious binding agents. The material was then simply mixed together and then moved into storage bunds, adjacent to the tanks, to allow for the hydration process. Once the material had become solidified to a strength as previously agreed with the landfill site it was then transported off site for subsequent disposal.

During the works a steel lid within tank 2 was found approximately 3.5 m below ground level which had allowed the tank to function in much the same manner as a gas manometer. Whilst it was fortunate that the lid had prevented any cross contamination of the sludge below it with the asbestos demolition material above, it posed a major problem in how to safely remove the steel structure. Also a delay to the contract would be inevitable whilst the ‘sealed sludge’ below the lid was tested to ascertain its chemical composition.

The Challenges: Throughout the contract a number of other unknown obstacles and challenges were encountered. These included the discovery of a a partial steel lid and associated structures at the base of tank three, a cavity void found to be 600mm wide not 50mm as assumed, and a further three 9m deep chambers adjacent to the top three tanks. During all these challenges Soilutions continued to deliver the contract within the revised programme dates and contract sum.

In total 4,300 tonnes of sludge were processed and disposed of to landfill.
780 tonnes of cementitious binding agent were used.
8,000 tones of material were used for backfilling the empty tanks.

The Future of the Site: The site of the former sewage treatment works at Luggie Glen has been remediated in support of a major regeneration programme by Fusion Assets to transform the three acres of former brownfield land into a new Drumpelier Business Park, with a range of owner-occupied office space and mixed commercial development. Fusion Assets is a joint venture company formed by North Lanarkshire Council and Scottish Enterprise Lanarkshire.

Client recommendation: “This was the first contract delivered for the Council by Soilutions, in what was a challenge to all parties involved including Jacobs, Consultants for North Lanarkshire Council on the project. The enforced changes to the originally agreed treatment method were tackled head –on by the Project Team, minimising the delay as well as setting a number of challenges throughout the duration of the contract. The knowledge and experience from both Contractor & Consultant ensured that these issues reached a successful conclusion, providing a brownfield site available for new development.” Kevin Lawlor of North Lanarkshire Council.

Length of Project: 23 Weeks

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