Why Some Biosolids Geobag Dewatering Projects Succeed — and Others Quietly Fail
- Ben Lewis
- May 6
- 3 min read
A recent project in City of Castlegar (link to article) highlighted something the industry sees repeatedly: expectations were clear, the guidelines were followed, yet the outcome did not meet performance targets.
The plan was to achieve a final dried solids consistency, follow accepted dosing protocols, apply standard geobag practices, and deliver a stable end product within program timeframes.
On paper, that sounds reasonable.
In practice, biosolids rarely behave that way.
The Expectation Gap
Asset owners often assume that if:
Polymer dosing charts are followed
Geotextile pore size is correctly selected
Platform drainage is properly designed
Filling heights are controlled
Then performance should be consistent.
But lagoon sludge and dredged biosolids are not uniform materials.
They are:
Highly compressible
Biologically active
Variable in solids concentration
Sensitive to minor dosing shifts
Affected by temperature and shear
Design guidelines assume relative stability. Biosolids do not provide it.
The Silent Killer: Early Overdosing
Most people assume underdosing is the main risk.
It isn’t.
Overdosing polymer during early filling is far more damaging.
When overdosed:
Flocs become too tight
Geotextile pores blind prematurely
Internal permeability drops sharply
Water becomes trapped within the forming cake
If the first layers inside the bag become hydraulically restrictive, even perfectly dosed sludge added later struggles to drain.
The system can be compromised within hours.
And that impact can persist for months.
The Dredge Line Problem
With lagoon dredging:
Solids concentration fluctuates constantly
“Watery patches” occur unexpectedly
Organic fraction shifts as you move through the pond
Pump shear affects floc structure
Unless dosing is being actively adjusted, polymer rates quickly become misaligned with incoming solids.
Following a static dose curve is not enough.
Biosolids dewatering requires live judgement.
Why Guidelines Aren’t Enough
Specifications typically cover:
Target polymer dose (kg/t dry solids)
Geotextile AOS
Platform layout
Filling limits
They rarely cover:
Real-time solids testing
Pump shutdown protocols
Staged filling discipline
Active dewatering timing
Response to drainage signals
Those decisions sit with the operator.
And this is where projects often diverge.
The Human Factor
In most underperforming programs, the polymer wasn’t fundamentally wrong. The bag wasn’t fundamentally wrong. The platform wasn’t fundamentally wrong.
What was missing was disciplined, active management during filling.
As Kristian Tipovsky of TK Marine explains:
“We never let a bag fill fully before actively dewatering it. If dosing isn’t right, water pockets get trapped — and that can compromise the whole program.”
That level of control is not usually written into specifications. It requires stopping, adjusting, and sometimes slowing down — which can feel counterintuitive on a programmed job.
He adds:
“Some think it’s just turning the dosing on in the morning and rolling out a new bag when the last one is full. In our experience, that approach rarely works.”
That is the difference.
Biosolids dewatering is not a set-and-forget process.
It is dynamic. It is sensitive. And early decisions during the first hours of filling can determine performance months later.
Setting Realistic Expectations
Biosolids are among the most difficult materials to dewater because:
Organic fractions create compressible cakes
Consolidation is slow
Minor dosing shifts cause major permeability changes
Weather and temperature influence drainage behaviour
Expecting rapid transformation into firm, soil-like material can be unrealistic — particularly with lagoon-derived sludge.
Successful programs are structured with:
Flexible polymer adjustment
Staged filling
Active drainage management
Operators experienced in biosolids nuance
Acceptance that downtime is calculated, not wasted
Final Thought
Projects that struggle often followed the guidelines.
Projects that succeed understand the nuance.
If expectations are set around “following the specification”, disappointment is common.
If expectations are set around disciplined operational control, outcomes improve dramatically.
Biosolids dewatering is not simply an engineering design problem.
It is an operational craft.
And experience matters more than most specifications acknowledge.
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