A total of six people were arrested during dawn raids following a nationwide Environment Agency probe into illegal dumping of waste.
The arrests were made on 10 March following a nationwide Environment Agency probe into the illegal dumping of potentially hazardous waste at illegal waste sites in Bristol, Scunthorpe and Newark in Nottinghamshire.
The investigation is also looking at links to an abandoned waste transfer site in Orpington run by operators Waste4Fuel.
The West Midlands Government Agency Intelligence Network (WMGAIN) was involved in the arrest phase of the operation and facilitated the enforcement resources for the day of action.
More than 40 Environment Agency investigators, supported by Humberside, Nottinghamshire, Cambridgeshire and Hertfordshire Police, swooped on addresses as part of Operation Encore.
Environment Agency crime staff from across England, including a number in specialist breathing apparatus, were brought in to support the operation due to its large scale.
Police were deployed to support with arrests, access properties and assist with collecting evidence.
The Home Office Immigration Enforcement and Social Services were also involved as part of this multi-agency operation, due to a suspected number of illegal workers at the sites.
The six people arrested have all been released on conditional bail pending further enquiries The investigation is ongoing and expected to take many more months to complete.
WMGAIN is an intelligence sharing and problem solving network. It was set up so that all law enforcement agencies could formally share intelligence and information helping to frustrate and identify the activities of organised criminal groups.