What is the lowest-cost way to reduce sludge in a lagoon?

Where the accumulation has a substantial volatile organic fraction and the objective is to recover capacity and extend the interval before a clean-out rather than restore full design volume, biological reduction in place is the lower-cost route: an annual operating cost, with no downtime and nothing to dispose of. Where the full design volume has to be restored, or the accumulation is largely inert grit and sand, physical dredging is the only method that removes it, at capital cost.

Can a treatment product remove all the sludge from a lagoon?

No. Bioaugmentation digests the volatile organic fraction of the blanket only. Sand, grit, silt, mineral precipitates and plastics are not biodegradable and remain at any dose, so clearing a lagoon of everything, organic and inert alike, is a job for hydraulic or mechanical dredging. No biological method removes the inert fraction.

Do I have to take the lagoon offline to deal with the sludge?

It depends on the method. Mechanical excavation requires the cell to be taken out of service and dried enough to carry equipment. Hydraulic dredging pumps a slurry while the cell stays in service, but creates a dewatering and disposal task. Biological reduction needs no downtime and produces nothing to dispose of, but acts only on the organic fraction and will not restore volume lost to inert material.

Lagoon sludge removal methods

The methods available for removing or reducing sludge in a wastewater lagoon, how they differ mechanically, and how to choose between them.

The short answer

Lagoon sludge is dealt with in one of three ways: physical removal, biological reduction in place, or a combination. Physical removal covers mechanical dredging, hydraulic dredging with dewatering, and taking a cell out of service to excavate it dry — all of which remove both organic and inert material. Biological reduction uses microbial products to digest the volatile organic fraction in place, which recovers capacity without downtime but cannot act on inert material. Which approach is appropriate depends on how much material is present, what proportion of it is biodegradable, whether a cell can be taken offline, and whether the requirement is to restore full design volume or to extend the interval before that becomes necessary.

Physical removal

Hydraulic dredging pumps a sludge slurry from the lagoon floor while the cell stays in service, then dewaters it — with geotextile tubes, a press, or drying beds — before disposal. Mechanical excavation requires the cell to be taken offline and dried sufficiently to support equipment, which is disruptive but produces a much lower-moisture product to dispose of. Both remove everything, which is their advantage, and both create a solids handling and disposal problem, which is their cost.

Biological reduction

Bioaugmentation introduces microbial cultures — predominantly Bacillus spore-formers, which produce the proteases, lipases, amylases and cellulases that hydrolyse organic solids — to digest the volatile fraction of the blanket in place. It requires no downtime and no disposal, and the cost profile is an annual operating line item rather than a capital event.

The limitation is mechanical rather than biological. A lagoon may be six to fifteen feet deep with the blanket at the bottom, and product applied at the surface has to reach it. This is why several manufacturers have engineered dense sinking or carrier-based formats specifically to place organisms at the benthic layer rather than relying on settling and mixing to carry them down.

Format	Where it ends up	Best suited to
Sinking solid — tablets, pucks, carriers	Falls through the water column and releases at or within the sludge interface.	Accumulated bottom sludge in deeper or poorly mixed lagoons.
Dry soluble pouch or powder	Dissolves at or near the surface, with the same water-column distribution as liquid.	General organic load and maintenance dosing where mixing is adequate.
Liquid	Disperses into the water column, reaching the blanket only through settling and mixing.	Nitrification, surface FOG, and continuous upstream dosing through metering equipment.

Nitrifying bacteria do not survive drying, so ammonia and nitrification can only be addressed by a liquid product.

Choosing between them

If design volume has to be fully restored — because freeboard is critical or the cell is close to failing hydraulically — that is a dredging decision, and biological treatment will not substitute for it.
If the requirement is to recover capacity and extend the interval before a clean-out, and the accumulation has a substantial volatile fraction, biological reduction is the lower-cost route.
If the lagoon is poorly mixed, address that first. Biological treatment performs poorly without adequate mixing, and improving it benefits the whole process.
If the accumulation is largely inert, biological treatment has very little to act on and the money is better spent elsewhere.

Common questions

What is the lowest-cost way to reduce sludge in a lagoon?: Where the accumulation has a substantial volatile organic fraction and the objective is to recover capacity and extend the interval before a clean-out rather than restore full design volume, biological reduction in place is the lower-cost route: an annual operating cost, with no downtime and nothing to dispose of. Where the full design volume has to be restored, or the accumulation is largely inert grit and sand, physical dredging is the only method that removes it, at capital cost.
Can a treatment product remove all the sludge from a lagoon?: No. Bioaugmentation digests the volatile organic fraction of the blanket only. Sand, grit, silt, mineral precipitates and plastics are not biodegradable and remain at any dose, so clearing a lagoon of everything, organic and inert alike, is a job for hydraulic or mechanical dredging. No biological method removes the inert fraction.
Do I have to take the lagoon offline to deal with the sludge?: It depends on the method. Mechanical excavation requires the cell to be taken out of service and dried enough to carry equipment. Hydraulic dredging pumps a slurry while the cell stays in service, but creates a dewatering and disposal task. Biological reduction needs no downtime and produces nothing to dispose of, but acts only on the organic fraction and will not restore volume lost to inert material.

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