Often suitable to larger infestations, mechanical sifting of excavated knotweed lowers the regenerative power of the plant, allowing for on-site burial or relocation.
Excavation and Sifting
The Japanese knotweed is excavated either fully or to a reduced level (see our 'Dig n Cap' page). The excavated spoil is then passed through a mechanical screener. The sifting process crushes and fragments the knotweed reducing its regenerative power. The sifted knotweed materials can then be re-used (see 'Relocation' page) or buried on-site.
Re-use of Sifted Spoil Onsite
The key benefit of this procedure is that the spoil which passes through the screener will only contain very small fragments of knotweed which have lowered regenerative power. This allows for the sifted spoil to be re-used on-site as backfill subject to local Environment Agency Officer approval!
To exploit the low regenerative power of these fragments the sifted spoil is buried, subjected to engineering compaction, and covered with an approved geosynthetic membrane prior to clean backfill being installed above. The burial depth should be suitable as to avoid the risk of accidental disturbance and prevent any small knotweed fragments from producing growth capable of breaking the compaction and reaching the surface.
Alternatively the sifted spoil can be re-used on site at finished levels to areas of Public Open Space landscaping, but some limited re-growth will be expected. The re-use area would therefore need to be subject to a monitoring and herbicide treatment programme until successive years of no-growth have been recorded.
The Knotweed Management Plan (KMP) should be used to record the position of the low level burial or re-use area, with this being marked upon an as-built site drawing. This will help prevent potential future disturbance of these locations. If these works are part of a site development project the KMP should then be included in the Operations and Maintenance (O&M) Manuals.
It is important to note that ANY form of screening/sifting cannot guarantee to remove 100% of the knotweed! Therefore the screened/sifted material cannot be removed from site unless done so as knotweed contaminated (controlled waste).
Land Remediation Tax Relief
Disposing of the excavated knotweed on-site is sustainable option for which Land Remediation Tax Relief (LRTR) can be claimed. A system of LRTR has been in place for several years, but the complexities of it, along with the lack of suitable publicity, have ensured LRTR has rarely been applied for.
In the 2007 Budget, the Chancellor announced a consultation into improving the tax relief system and to help redress this situation, which suggested any savings gained from scrapping Landfill Tax Exemption (1st April 2010) will be transferred into LRTR instead. However, LRTR cannot be applied for if any material is to be disposed of off-site.