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Desludging Tanks, Lagoons and Pits

Desludging is the removal of accumulated solids from a structure that is meant to hold liquid. The structure might be a settlement tank, a storm tank, a balancing tank, a septic tank, a lagoon, a sump or a below-ground pit. In each case the usable volume has been reduced by material that has settled out, and the job is to take that material out without damaging the structure or causing a discharge.

The settled layer is rarely uniform. Sludge at the bottom of a tank can be denser and older than the layer above it, and there may be a transition zone of partially settled material. A single suction pass from one position often leaves material behind. Where access allows, working across the floor of the structure in stages gives a more complete result.

Lagoons bring their own problems. Banks may be soft, the water level may need managing during the work, and the sludge can be mixed with silt, vegetation and debris. Lagoon desludging is usually planned around how the liquid is to be controlled and where the removed material will go.

For smaller structures, tank sludge removal covers the practical decisions about access, suction points and how to confirm the tank has been cleared to the agreed level.

Dewatering and Volume Reduction

Many sludges contain a high proportion of water. Dewatering reduces water content and can reduce the quantity that needs to be transported or sent for further treatment.

Methods can include gravity thickening, belt or screw presses, centrifuges, filter presses and geotextile dewatering bags or tubes. Suitability depends on the sludge, available space, site services and the acceptance requirements of the receiving route. Any liquid released during dewatering also needs a suitable route.

Dewatering produces a liquid stream as well as a solid one. That liquid still needs a lawful route, whether back into the treatment process or off site as liquid waste. Sludge dewatering sets out the main options and the questions that decide between them.

Disposal and Recovery Routes

Once classified, sludge can go to a range of destinations. These include landfill, incineration, anaerobic digestion, composting, land spreading under the relevant rules, and recovery of materials such as metals or oils where the sludge supports it. Each route has its own acceptance criteria, and a route that suits one sludge will reject another.

Land spreading, for example, is governed by controls on soil loading, nutrient content and contaminants, and is not available for every material. Sludge to land explains the general principles and the limits. Where a material is hazardous, the requirements are stricter and the number of accepting facilities is smaller.

Dewatering does not change the classification of a waste. A sludge that is hazardous before dewatering remains hazardous afterwards, even though it is drier and easier to handle.

Specialist and Industrial Sludges

Some sludges need particular care. Oil sludges from interceptors, separators and tank cleaning contain hydrocarbons and often water and grit as well. Paint sludges from spray booth water curtains and paint detackification systems behave differently again, and can be difficult to pump without pre-treatment. Drilling muds and cuttings from construction and groundworks vary with the drilling fluid used.

Food and drink processing produces sludges with high organic loading that can turn septic quickly if left. Wastewater treatment sludges range from primary and secondary biological sludges to digested and thermally treated material, each with different handling characteristics.

Where a material may be hazardous, the work should be planned as competent specialist work. That includes assessment before entry to any confined space, control of vapours and gases, and suitable emergency arrangements. The Confined Spaces Regulations 1997 set out the duties that apply where entry cannot be avoided.

Choosing Between Service Areas

If you are unsure which service applies, tell us what the material is, where it is held, the approximate volume and any known analysis. Interceptors and grease traps may need emptying and cleaning, while treatment tanks and lagoons can require a planned desludging project. Where composition is uncertain, sampling or classification may be needed before a receiving route is confirmed.

It also helps to know the national context. The duty of care for waste in England and Wales is set out in the waste duty of care code of practice. Scotland and Northern Ireland have their own codes and regulators, and requirements can differ. The joint agency WM3 waste classification guidance is used across the UK, although the regulatory procedures and terminology around hazardous or special waste differ by nation.

Quotations depend on the material, volume, water content, classification, access, receiving route and transport requirements. Sludge removal cost explains the main factors we need to price a job.